jap.93.3693.2003(review of solar cells)

8/3/2019 JAP.93.3693.2003(Review of Solar Cells)

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APPLIED PHYSICS REVIEWSFOCUSED REVIEW

Small molecular weight organic thin-film photodetectors and solar cells

Peter Peumans, Aharon Yakimov,a) and Stephen R. Forrestb)

Department of Electrical Engineering and the Princeton Materials Institute, Center for Photonics

and Optoelectronic Materials (POEM), Princeton University, Princeton, New Jersey 08544

Received 31 July 2002; accepted 29 October 2002

In this review, we discuss the physics underlying the operation of single and multiple heterojunction,vacuum-deposited organic solar cells based on small molecular weight thin films. For singleheterojunction cells, we find that the need for direct contact between the deposited electrode and theactive organics leads to quenching of excitons. An improved device architecture, the doubleheterojunction, is shown to confine excitons within the active layers, allowing substantially higherinternal efficiencies to be achieved. A full optical and electrical analysis of the doubleheterostructure architecture leads to optimal cell design as a function of the optical properties andexciton diffusion lengths of the photoactive materials. Combining the double heterostructure withnovel light trapping schemes, devices with external efficiencies approaching their internal efficiencyare obtained. When applied to an organic photovoltaic cell with a power conversion efficiency of

1.0%0.1% under 1 sun AM1.5 illumination, devices with external power conversion efficienciesof 2.4%0.3% are reported. In addition, we show that by using materials with extended excitondiffusion lengths LD , highly efficient double heterojunction photovoltaic cells are obtained, even inthe absence of a light trapping geometry. Using C60 as an acceptor material, double heterostructureexternal power conversion efficiencies of 3.6%0.4% under 1 sun AM1.5 illumination areobtained. Stacking of single heterojunction devices leads to thin film multiple heterojunctionphotovoltaic and photodetector structures. Thin bilayer photovoltaic cells can be stacked withultrathin (5 ), discontinuous Ag layers between adjacent cells serving as efficient recombinationsites for electrons and holes generated in the neighboring cells. Such stacked cells have open circuitvoltages that are n times the open circuit voltage of a single cell, where n is the number of cells inthe stack. In optimized structures, the short circuit photocurrent remains approximately constantupon stacking thin cells, leading to higher achievable power conversion efficiencies, as confirmedby modelling optical interference effects and exciton migration. A 2.5%0.3% power efficiency

under 100 mW/cm2 AM1.5 illumination conditions is obtained by stacking two 1% efficientdevices. Alternatively, when the contact layers between the stacked cells are eliminated, a multilayerstructure consisting of alternating films of donor and acceptor-type materials is obtained. Since thethicknesses of the individual layers ( 5 ) can be substantially smaller than the exciton diffusionlength, nearly 100% of the photogenerated excitons are dissociated, and the resulting free chargesare detected. In addition, the ultrathin organic layers facilitate electron and hole transport throughthe multilayer stack by tunneling. When these devices are operated as photodetectors under appliedfields 106 V/cm, the carrier collection efficiency reaches 80%, leading to external quantumefficiencies of 75%1% across the visible spectrum in cells containing the thinnest layers. We findthat due to the fast carrier tunneling process, the temporal response of these multilayer detectors isa direct measure of exciton dynamics. Response times of 72050 ps are achieved, leading to a 3 dBbandwidth of 43030 MHz. A summary of representative results obtained for both polymer andsmall molecule photovoltaic cells and photodetectors is included in this review. Prospects for furtherimprovements in organic solar cells and photodetectors are considered. 2003 American Instituteof Physics. DOI: 10.1063/1.1534621

aNow with: General Electric Global Research Center, Niskayuna, NY12309.

bAuthor to whom correspondence should be addressed, electronic mail:[email protected]

JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 7 1 APRIL 2003

36930021-8979/2003/93(7)/3693/31/$20.00 2003 American Institute of Physics

Downloaded 06 Apr 2004 to 155.198.17.114. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp


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CONTENTS

I. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3694II. Single heterojunction devices. . . . . . . . . . . . . . . . . . 3697

A. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3697B. Theory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3697

1. Photodiode quantum efficiency. . . . . . . . . . . 36972. Optical interference effects. . . . . . . . . . . . . . 36983. Exciton diffusion. . . . . . . . . . . . . . . . . . . . . . 3699

4. Model predictions. . . . . . . . . . . . . . . . . . . . . . 3700C. Experimental methods. . . . . . . . . . . . . . . . . . . . . 3701D. Results and discussion. . . . . . . . . . . . . . . . . . . . . 3702

1. Exciton diffusion length measurements.. . . . 37022. Bilayer devices. . . . . . . . . . . . . . . . . . . . . . . . 37023. Double heterostructure devices. . . . . . . . . . . 37034. Light trapping in thin PV cells. . . . . . . . . . . 37055. PV cells using C60 . . . . . . . . . . . . . . . . . . . . . 37066. Cathode induced defect states in the EBL. . 3708

III. Multiple heterojunction devices. . . . . . . . . . . . . . . . 3709A. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3709B. Theory of stacked photovoltaic cells. . . . . . . . . . 3710

1. Series versus parallel cell contribution. . . . . 3710

2. Optimization of stacked solar cells. . . . . . . . 3711C. Theory of donoracceptor multilayers. . . . . . . . 3712

1. Field-induced exciton dissociation. . . . . . . . . 37122. Exciton migration and dissociation

dynamics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3712D. Experimental methods. . . . . . . . . . . . . . . . . . . . . 3713E. Stacked photovoltaic cells: Results and

discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37141. Stacked PV cells with an ultrathin metal

interlayer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37142. Stacked PV cell efficiency optimization. . . . 3714

F. Donoracceptor multilayer photodetectors:Results and discussion. . . . . . . . . . . . . . . . . . . . . 37161. Electrical characteristics in the dark. . . . . . . 37162. Photodetector quantum efficiency. . . . . . . . . 37173. Photodetector temporal response. . . . . . . . . . 3719

IV. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3720

I. INTRODUCTION

The emerging field of organic semiconductor materialsand devices is extending the capabilities and possibilities ofmodern electronic and photonic devices into unexpected do-mains. Among the earliest and potentially most significantdevices to have been studied are photovoltaic PV cells and,to a lesser extent, organic photodetectors PDs. In this re-

view, we discuss small molecular weight, organic, vacuum-deposited thin-film organic electronic devices based onsingle and multiple organic heterojunctions that are opti-mized for both solar energy conversion and photodetectionapplications. Due to the potential for very low cost solarenergy conversion, organic PV cells have attracted attentionsince a 0.95% power efficient thin film organic cell based ona single donoracceptor DA heterojunction was reportedby Tang in 1986.1 After a considerable period with few im-provements over this demonstration, the pace of advanceshas been particularly rapid in the last 5 yr,1 7 with develop-ments in solution-processed polymer blend cells leading tosolar power conversion efficiencies of up to5,7 P2.55%,

as well as in vacuum or vapor-phase deposited small mol-ecule based cells with efficiencies up to6 P3.6%0.3%, both under AM1.5 G 100mW/cm2 solar illumina-tion. In Table I we provide a summary of the best resultsobtained thus far in the field of thin film organic photovoltaiccells based on both small molecules and polymers. It is ap-parent that power efficiencies are still well below 5% a valuewhich is at the lower limit of interest in practical niche

applications, and falling well short of 24% and 17%achieved for crystalline and thin film Si,8 respectively.

Photovoltaic cells are optimized for maximum electricalpower generation under standard illumination conditionsAM1.5 spectral illumination, i.e., for the maximum productof photocurrent times photovoltage. The power conversionefficiency of a PV cell under standard illumination condi-tions depends on the following three parameters see Fig. 1:1 The current density under zero bias, i.e., the short-circuitcurrent density JSC , 2 the photovoltage under open circuitconditions, i.e., the open-circuit voltage VOC , and 3 the fillfactor FFmax(JV)/JSCVOC , which characterizes theshape of the current density versus voltage (JV) curve inthe power-generating fourth quadrant. The power conversionefficiency is then PJSCVOCFF/P inc , where P inc is the in-cident optical power density typically the AM1.5 solar spec-trum is used at an intensity of P inc100 mW/cm

2). In Fig.1, the three PV cell parameters are indicated on the JVcurve of the device reported by Tang.1 This early thin filmcell stands out because it contains a heterojunction analogousto that employed in a conventional pn junction inorganicsemiconductor PV cell. However, the success of this hetero-

junction PV cell, which lies at the basis of all subsequentwork on efficient organic PV cells, was not solely due to theestablishment of a pn-like junction. Instead, it is now recog-

nized that the energy-level offset of the heterojunction alsoplays an essential role.Energy level offsets at heterojunctions are essential to

the operation of organic detectors because of the fundamen-tal nature of the photogeneration process in organic materi-als. As illustrated in Fig. 2, upon optical excitation of anorganic material, localized Frenkel or charge-transfer exci-tons are generated.9,10 For sufficiently thick organic layersi.e., thicker than the optical absorption length, LA1000 ) , the majority of photons is absorbed, leading toan absorption efficiency ofA100%. For electrical detec-tion, the tightly bound excitons with binding energies rang-ing from 0.1 to 2 eV1115 must be dissociated into their

constituent electrons and holes. Such a process can be in-duced by the built-in electric field,16 but the efficiency atelectric fields typically found in organic electronic devices(F106 V/cm) is low (D10%). Once an exciton disso-ciates into a free electron and hole, their collection efficiencyat the opposing electrodes is high (CC100%) due to thepresence of the built-in electric field.

It has been recognized that the most efficient excitondissociation in organic materials occurs at a DA interface.2,3

At such an interface, the donor material with a low ionizationpotential IP forms a heterojunction with an acceptor mate-rial with a high electron affinity EA. Depending on thealignment of the energy levels of the donor and acceptor

3694 J. Appl. Phys., Vol. 93, No. 7, 1 April 2003 Appl. Phys. Rev.: Peumans, Yakimov, and Forrest



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materials, the dissociation of the strongly bound excitons canbecome energetically favorable at such an interface,17,18

leading to a free electron polaron in the acceptor material,and a free hole polaron in the donor, as illustrated in Fig.3a. The dissociation, or charge-transfer CT process, typi-cally occurs over time scales CT of a few hundred femto-seconds or less.19,20 Since CT is much shorter than any othercompeting process, the charge transfer efficiency approachesCT100%. Application of this process to organic PV cellsand PDs has led to orders of magnitude improvement inquantum efficiency over those achieved in single layer de-vices.

To obtain a high photosensitivity, exciton dissociation ata DA interface must be preceded by efficient transport of thephotogenerated excitons toward this interface, and followed

by efficient charge extraction at low bias. Nearly completetransport of excitons to the DA interface is achieved by es-

tablishing an intimate contact between the donor and accep-tor materials by blending,2,5 laminating,3 codeposition,21 orchemical linking.22 The main problem with these approachesis that carrier mobilities are reduced, and charge-trap densi-ties are increased. This appears to be a common property ofblended materials where crystalline order and high purityare difficult to achieve. Research in this area has, therefore,focused on improving the charge transport properties of theblends by controlling the film morphology5 such that the twophases form percolating paths along which the photogener-ated carriers can be readily transported to their respectiveelectrodes. This approach has yielded5,7 solution processedorganic PV cells with P2.5% under 1 sun, AM1.5 spec-

TABLE I. Summary of best organic photovoltaic cell results.

Device structurel JSC (mA/cm2) VOC (V) FF P (% ) Pinc

a (mW/cm2) Aream (cm2) Ref.

Small molecule systemssingle layerAg/merocyanine/Al 0.18 1.2 0.25 0.62 78 1 82,83Au/ZnPc/Al 5.6104 0.59 0.1 3104 0.1 84

Small molecule systemsheterojunctionITO/CuPc/PTCBI/Ag 2.6 0.45 0.65 0.95 75 0.1 1ITO/CuPc/PTCDA/In 2.0 0.55 0.35 1.8 10 5.7104 85

ITO/DMPTCDI/H2Pc/Au 2.6 0.55 0.30 0.77f

100 862.0 0.58 0.23 0.49g 1.6 0.66 0.22 0.41

ITO/DMPTCDI/H2Pc/Au 2.7 0.40 0.56 0.76 78 50ITO/PTCBI/H2Pc/Au 0.18 0.37 0.32 0.08 25 25ITO/PTCBI/DMPTCDI/H2Pc/Au 0.5 0.37 0.27 0.20 ITO/DMPTCDI/CuPc/Au 1.9 0.42 0.41 0.33 100 0.15 87ITO/CuPc/PTCBI/BCP/Ag 4.2 0.48 0.55 1.10.1 100 7.85103 4Same with light trap 2.40.3 20 ITO/PEDOT:PSS/CuPc/C60 /BCP/Al 18.8 0.58 0.52 3.60.2 150 7.8510

3 6Small molecule systems stacked heterojunctions

ITO/DMPTCDI/H2Pc2 /Au 0.55 0.30 0.35 0.08 78 50Same with Au interlayer 0.75 0.80 0.21 0.16 ITO/PEDOTPSS/CuPc/PTCBI2 /Ag

h 5.2 0.93 0.52 2.50.1 100 7.85103 51ITO/PEDOT:PSS/CuPc/PTCBI3 /Ag

h 4.5 1.20 0.43 2.30.1 Polymer systemssingle layerITO/MEHPPV/Ca 6.1103 1.6 0.20 0.03 20j 88

Polymer systemsheterojunctionITO/PPV/C60/Al 3.6103 0.8 0.48 0.55 0.25

j 0.03 31ITO/PEDOT:PSS/MDMO PPV/PCBM/Al 0.96 0.78 0.5 0.5 78 0.081 89ITO/MEHPPV:PCBM/Cad 2 0.8 0.25 1.5 20k 0.115 2Au/PEDOT:PSS/PEOPT:MEHCNPPV/Cae 0.300.35 1.9%c 77 2.5 90ITO/PEDOT:PSS/MDMOPPV:PCBM/LiF/Ald 5.25 0.82 0.61 2.5% 80b 0.1 5ITO/MDMO-PPV:PCBM/Ald 7.2 0.942 0.46 3.1% 100 0.02 74

aSimulated AM1.5 spectral illumination unless otherwised noted.bAM1.5 spectral illumination.cCalculated efficiency based on EQE.dBlended heterojunction.eLaminated heterojunction.fH2 doped.g

NH3 doped.hWith 100 Au interlayer.iWith 5 Ag interlayer.jMonochromatic illumination at 488 nm.kMonochromatic illumination at 430 nm.lZnPczinc phthalocyanine, CuPccopper phthalocyanine, PTCBI3,4,9,10-perylenetetracarboxylic bis-benzimidazole, PTCDA3,4,9,10-perylene tetracar-boxylic acid, DMPTCDI2,9-dimethyl-antra2,1,9-def:6,5,10-defdiisoquinoline-1,3,8,10-tetrone, H2Pcfree base phthalocyanine, BCPbathocuproine, PEDOT:PSSpoly3,4-ethylenedioxythiophene:polystyrenesulfonate, MEH PPVpoly2-methoxy-5-2-ethyl-hexyloxy-1,4-phenylene vinylene, PPVpolyphenylene-vinylene, MDMOPPVpoly2-methoxy-5-3,7-dimethyloctyloxy-1,4-phenylene-vinylene, PCBM6,6-phenyl-C61-butyric acid methyl ester, and PEOPTpoly 3-4-1,4,7-trioxaoctylphenylthiophene.

mActive area of the devices not including the area required for contact pads.

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tral illumination. Organic PDs with an external quantum ef-ficiency EQE defined as the number of carriers producedper incident photon EQE50% at wavelengths of peak de-tector responsivity have been fabricated as well.23 However,the low carrier velocities and high trap densities preclude theuse of this approach to devices with response times 50ns.

Here, we concentrate our attention on approaches basedon vacuum deposited, organic multilayer structures in whichcrystalline order is maintained during growth. These well-defined systems have power conversion efficiencies compa-rable or superior to solution processed organic PV cells. Inaddition, their layer structures can be precisely controlledduring the growth process.24 In this case, exciton rather than

charge transport is the limiting factor. Depending on whetherthe device is employed as a PV cell or a PD, this problem issolved in different ways.

The performance of such vacuum deposited multilayerPV cells has been optimized through the introduction ofdonoracceptor mixed layers grown by codeposition,21 metalnanoclusters,21 multiple heterojunctions,25 staircase PVcells, exciton-blocking layers,4,6 light-trapping,4 andthrough the optimization of optical interference effects.26

Since the latter three techniques are the most successful ap-proaches, they will be the focus of the first part of this reviewSec. II.

Stacking partially transparent bilayer PV cells, each witha high internal power conversion efficiency, is an alternativemeans for achieving higher photodetection efficiencies. Thisapproach yields devices with external power efficiencies ap-proaching the internal power conversion efficiency of asingle cell. When optimized, such a stacked device yields ashort-circuit photocurrent ISC similar to that of a single cell,but has an open circuit voltage EQE70% that is an integermultiple equal to the total number of cells of that of asingle cell. In this approach, transparent, but ohmic contactsmust be placed between the different cells in the stack.

When these ohmic contacts between the stacked cells areomitted, a multilayer structure consisting of alternating ultra-thin (5 ) films of donor and acceptor-type materials isobtained. Previously, it has been shown that long-range crys-talline order can be maintained in such multilayerstructures.24 For sufficiently thin layers, photogenerated ex-citons diffuse to a DA interface with a high probability.When reverse biased, such structures27 exhibit external quan-tum efficiencies EQE75%. Hence, the charge collectionprocess is efficient when an external voltage is applied, de-spite a large number of energy barriers to electron and holetransport arising from the energy level offsets between thehighest occupied molecular orbital HOMO and lowest un-

FIG. 1. Current density vs voltage characteristics of an ITO/copper-phthalocyanine/3,4,9,10-perylenetetracarboxylic bis-benzimidazole/Ag pho-tovoltaic cell under 75 mW/cm2 AM1.5 spectral illumination. The short-circuit current-density JSC , open-circuit voltage VOC , and fill factor FFare indicated. The power conversion efficiency of this cell is P0.95%.Inset Layer structure of the PV cell in cross section. From Ref. 1.

FIG. 2. Schematic illustration of the three consecutive steps in the genera-tion of photocurrent from incident light in a single layer organic PV cell: 1photon absorption with efficiency A , 2 exciton dissociation, where thefraction of excitons dissociating is given by D ; and 3 collection of thecarriers at the electrodes, with efficiency CC . The dips in the energy leveldiagram represent the exciton binding energy.

FIG. 3. a Schematic illustration of the energy level alignment require-ments for efficient charge transfer from the photoinduced state to take place.The energy level diagram for two donoracceptor DA heterojunctions areshown. The donor material has a lower IP given by the highest occupiedmolecular orbital HOMO level, while the acceptor material has a high EAgiven by the lowest unoccupied molecular orbital LUMO level. The exci-ton in the donor material has an energy Eex which is smaller than the

HOMOLUMO gap Egap , by the exciton binding energy typically between0.1 and 2 eV. The charge-transfer state, with the electron in the acceptorand the hole in the donor has an energy IPDEAA the D and A subscriptsrefer to the donor and acceptor materials, respectively. Since for the left-hand DA junction EexIPDEAA , the charge-transfer reactions D*ADA and DA*DA will take place, where D and A aredonor and acceptor ground states, D* and A* are donor and acceptor ex-cited states, and D and A are hole and electron polarons in the donor andacceptor materials. However, for the right-hand DA junction EexIPDEAA , such that the charge-transfer reaction is energetically unfavorable.b Schematic illustration of the four consecutive steps in the generation ofphotocurrent from incident light: 1 Photon absorption with efficiency A .2 Exciton diffusion, where the fraction of excitons reaching the DA junc-tion is ED . 3 The charge-transfer reaction with efficiency CT . 4 Col-lection of the carriers at the electrodes with efficiency, CC .




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occupied molecular orbital LUMO levels at each heteroint-erface.

These multilayer devices can be operated as PDs opti-mized for maximum signal-to-noise ratio, which translatesinto maximizing the external quantum efficiency and mini-mizing the dark current a source of shot noise.28 In thiscase, a voltage can be applied to facilitate charge extraction.These differences in operation between PV cells and PDs are

reflected in their different designs. Indeed, when devices areoptimized for the detection of high bandwidth optical sig-nals, remarkable performance has been reported. For ex-ample, vacuum deposited multilayer photodetectors withEQE75% at dark current densities of Jdark100nA/cm

2,and a 3 dB rolloff frequency of f3 dB43040 MHz, wererecently demonstrated.27 The surprisingly short responsetimes of 72050 ps, make them useful for a range of mo-lecular optoelectronics applications. This performance illus-trates that organic materials can compete with more conven-tional semiconductor material systems for some specializedPV and PD applications. We will discuss the physics under-lying the performance of multiple junction PV cells and PDsin the second part of this review Sec. III.

The purpose of this review is threefold. First, it is ourintention to introduce the principles and limitations of or-ganic thin film photodetectors. Second, we will apply thisunderstanding to the design and realization of several singleand multiple junction, thin film organic detectors which ex-hibit the highest performance of any such devices yet re-ported in the literature. Finally, we will consider the pros-pects for further improvements in the performance of organicthin film detectors based on our current understanding oftheir fundamental properties.

This review is organized as follows: Single heterojunc-

tion devices are discussed in Sec. II. The subject is intro-duced briefly in Sec. II A. In Sec. II B we discuss the theoryunderlying the operation of organic PV cells and model thedependence of the performance of the cells on layer structureand the exciton diffusion lengths of the active organic mate-rials. Section II C is devoted to experimental details of sub-strate preparation, material growth, and electro-optical char-acterization. In Sec. II D, the results are discussed andexplained in terms of the models and predictions of Sec. II B.

Multiple active heterojunction devices are the subject ofSec. III. Following a brief introduction in Sec. III A, weconsider the design of stacked PV cells in Sec. III B. Thetheory of carrier and exciton dynamics and DA multilayer

photodetectors is the subject of Sec. III C. Experimentalmethods are described in Sec. III D. In Sec. III E, we dem-onstrate that stacking n PV cells with ultrathin (5 ) Agfilms as charge recombination layers produces a VOC that is ntimes that of a single cell. We show that optimized stackeddevices can exhibit more than twice the efficiency of singleheterojunction cells. These results are discussed and com-pared to the analysis of Sec. III B. In Sec. III F, the currentdensityvoltage (JV) and capacitancevoltage (CV)characteristics, the external and internal quantum efficien-cies, and the temporal response of multilayer PDs are ex-plained in terms of the theory presented in Sec. III C. Finally,we draw conclusions in Sec. IV.

II. SINGLE HETEROJUNCTION DEVICES

A. Introduction

The photoresponsivity of vacuum-deposited single het-erojunction devices can be accurately modeled by includingthe optical properties of the thin-film multilayer structurewhile simultaneously solving the equations governing exci-ton transport. This framework was developed in Ref. 29 and

was applied to organic PV cells in Ref. 26. This treatment isdeveloped and expanded in Sec. II B, and is applied to amodel small molecule thin film heterojunction system, yield-ing a simple tool for the design of thin-film organic PV cells.We find that the optimal thicknesses of the constituent activeorganic layers are approximately equal to the exciton diffu-sion lengths LD of the respective layers.

Applying this design criterion requires knowledge of theexciton diffusion length LD of the active materials. In Sec. IID 1, we obtain that for the acceptor material 3,4,9,10-perylenetetracarboxylic bis-benzimidazole PTCBI, whereLD

PTCBI303 . In the case of the single heterojunction

Tang bilayer PV cell in Fig. 1, our model suggests that

cathode induced exciton quenching provides a significantlimitation to the quantum efficiency in this architecture Sec.II D 2. To remedy this shortcoming, the double heterojunc-tion architecture incorporating an exciton-blocking layerEBL, is introduced in Sec. II D 3. This architecture,coupled with a light trapping geometry, yields performanceclosely predicted by the model, resulting in the demonstra-tion of a power conversion efficiency ofP7.5% using thesame active materials as the 0.95% efficient Tang typecell1 see Sec. II D 4.

Further improvements in quantum efficiency require theuse of materials with longer exciton diffusion lengths. In-deed, devices based on C60 , with L

D

C60100 , exhibit in-

creased quantum efficiencies Sec. II D 5. The optimizationof C60-based devices results in the demonstration of a devicewith a power conversion efficiency ofP3.6%0.4%.

Finally, in Sec. II D 6, the exceptional conductivity ofthe EBL used in this work is explained in terms of cathode-induced defect states.

B. Theory

The external quantum efficiency is the primary param-eter reflecting the fundamental detection and charge transportproperties of the PV cell materials and structure. In this sec-

tion, we describe a framework for the understanding of thisquantity.

1. Photodiode quantum efficiency

The external quantum efficiency EQE is defined as thenumber of electrons flowing in the external circuit per pho-ton incident on the PV cell. For a device based on excitondissociation by charge transfer at a DA interface, the externalquantum efficiency EQE is the product of the efficiencies offour sequential steps schematically illustrated in Fig. 3b:1 photon absorption leading to the generation of an excitonA , 2 diffusion of the exciton to the DA interface ED , 3




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exciton dissociation by CT at a DA interface CT , and 4collection of the free charge carriers at the electrodes CC .That is

EQEAIQEAEDCTCC , 1

where IQE is the internal quantum efficiency defined as theratio of the number of carriers collected at an electrode to thenumber of photons absorbed in the device. The exciton dif-fusion ED efficiency ED is the probability that the photo-generated exciton diffuses to a donoracceptor interface be-fore it recombines. Since the exciton diffusion length (LD50) 26,30,31 is typically shorter than the optical absorptionlength (LA5001000 ) , this step is often efficiency lim-iting see Fig. 3b. As mentioned above, CT100% istypical, as will also be shown in Sec. III to hold for thematerial systems studied here. For our PV structures, we willassume that under short-circuit conditions, the carrier collec-tion efficiency CC100% c.f. Sec. II B 4. Hence, Eq. 1

simply becomes EQEAED .

2. Optical interference effects

Both A and ED are functions of the optical propertiesof the materials employed, excitation wavelength , layerthicknesses, and the layer configuration. To evaluate AED ,the optical electric field amplitude E(x) is calculated as afunction of position in the multilayer structure in the thinfilm device. We follow the treatment described by Heavens,29

previously applied to organic PV cells by Petterson et al.26

For simplicity, we assume homogeneous and isotropic mate-rials described by a complex index of refraction n. Further-

more, the interfaces are assumed to be optically flat. In thiscase, the propagation of light can be described by 22 ma-trices subject to the continuity of the tangential component ofthe electric field at each interface.29 To calculate the opticalfield due to a plane wave incident on a PV active region, weassume the multilayer is embedded between two semi-infinite layers (j0, jm1) see Fig. 4. The activemultilayer is composed of individual layers, j(j

1,2,...,m) with thicknesses, dj , and wavelength-dependent complex indices of refraction, njnji kj . Weassume that the multilayer is illuminated from the direction xalong the surface normal.

At an interface between layer j and k, the propagation ofthe optical field is described by the interface matrix Ijk

Ej

EjIjkEk

Ek

1

tjk

rjk

tjk

rjk

tjk

1

tjk EkEk , 2

where Ej and Ek

are the components of the optical electricfield propagating in the positive () and negative () di-rections in adjacent layers j and k, respectively. For a planewave propagating along the surface normal, the Fresnel com-plex reflection and transmission coefficients are rjk(nj nk)/( nj nk) and tjk2nj /( nj nk), respectively. Thepropagation through a layer j causes absorption and a phaseshift, as described by the layer matrix Lj

Lj eij dj 0

0 e ijdj , 3where j(2/)nj . The electric field in the two outermostlayers j0 and jm1 are related via the transfer matrix S

E0

E0SEm1

Em1 , 4

where

S

S11 S12

S21 S22

n1

m

I(n1) nL n Im(m1) . 5The reflection and transmission coefficients are then rE0

/E0S21/S11 and tEm1

/E0S11

1 , respectively.The absorption efficiency of a multilayer stack is then A1TR, with T t2nm1 /n0 and R r

2, the trans-missivity and reflectivity, respectively, of the multilayerstructure. The device is typically supported by a substratewith a thickness of 0.11 mm Fig. 4. Hence, the effectof the substrate is included by correcting T and R for reflec-tions at the air/substrate and substrate/multilayer interfacesrather than by including it directly in the transfer matrixcalculation, viz.

FIG. 4. Geometry of the multilayer stack used in the optical electric fieldcalculations. Layers 0 and m1 are the transparent substrate and air, re-spectively. All calculated properties of this multilayer system are correctedfor the air/glass reflections at the back side of the transparent substrate.




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RR*R

1R*R, 6a

TT*T

1R*R, 6b

with

R*1

n

01 n0

2

, 7a

T* 21 n0

2

, 7b

where n0 is the refractive index of the substrate. The absorp-tion efficiency is then A1TR. To obtain the electricfield within layer j, we note that the total multilayer transfermatrix is

SSj

LjSj , 8

with

Sj

n1

j1

I(n1) nL n I(j1)j , 9and

Sj

nj1

m

I(n1) nL n Im(m1) . 10The electric field propagating in the positive direction inlayer j at the left interface is related to the incident planewave by

E

j

E0tj

1

Sj11

1Sj12

Sj21

Sj11

Sj11

e i2j dj

, 11

and similarly for the electric field propagating in the negativedirection at the left interface

Ej

E0tj

tj

Sj21

Sj11

e i2jdj. 12

The total electric field at an arbitrary position inside layer j isgiven in terms of the electric field of the incident wave by

Ejx Ej

x Ej

x tj

e ijxtj

eijx E0 . 13

The time averaged absorbed power as a function of positionis then

Qjx 4c0kjnj

2Ejx

2, 14

where c is the speed of light and 0 is the permittivity of freespace.

3. Exciton diffusion

The optical field generates excitons within the PV cellactive region. Hence, to complete the efficiency analysis, the

resulting exciton population must be considered. For thispurpose, the continuum steady state exciton diffusion equa-tion is solved for every layer j that participates in photocur-rent generation

LDj 2

2p

x2pjGj0, 15

where p is the exciton density, LDj

Djj is the excitondiffusion length of the material of layer j, Dj is the excitondiffusivity, j its lifetime, and Gj(x)(/hc )Qj(x) is theexciton generation rate.

Equation 15 must be solved by taking into account thefollowing types of boundary conditions: 1 ideal non-quenching interfaces with a vanishing surface recombinationvelocity; 2 quenching interfaces with an infinite surfacerecombination velocity; and 3 interfaces with a finite sur-face recombination velocity. The first boundary condition,corresponding to p/x0, applies to excitons in a narrowenergy gap organic semiconductor at a heterointerface whenthe LUMO and HOMO of the narrow energy gap material

are nested within the LUMOHOMO gap of the adjacentmaterial type I heterostructure, prohibiting exciton disso-ciation by CT.

The second boundary condition corresponds to p0,and applies to organic DA interfaces in which case the ex-citons dissociate by CT at the heterojunction and contributeto the photocurrent as well as to other interfaces that areknown to quench excitons efficiently, such as at the organic/metal boundary in which case excitons will recombine non-radiatively and do not contribute to the photocurrent. Thephotocurrent density contribution from a DA interface fol-lows from the exciton diffusion current toward the interface

JjqLD

j 2

j pxxxDA, 16where xDA is the position of the DA junction. The externalquantum efficiency contribution of the interface at xxDA oflayer j, EQE

j , is obtained by normalizing the photocurrent tothe incident photon flux

EQEj

Jj /q

1

2c0E0

2, 17

assuming that CTCC1. Since in Eq. 15 GjE02,

then JjE

0

2 in Eq. 16, and hence EQE in Eq. 17 isindependent of the incident power. To obtain the total EQEof the cell, the contributions EQE

j for each DA interface aresummed. The third boundary condition may apply in caseswhere exciton dissociation by charge transfer is only weaklyexothermic.

In our calculations, Eq. 15 is solved on a discrete gridwith a mesh size (5 ) much smaller than other dimensionsof the PV cell layers, using a linear algebra package. 32 Thesteps involved in calculating EQE are schematically summa-rized in Figs. 5ac for an archetype organic heterojunc-tion HJ PV cell with the layer structure: indiumtinoxide(ITO)/320 3,4-polyethylenedioxythiophene: polystyrene-




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sulfonate (PEDOT:PSS)/150 CuPc/350 C60 /160 ba-thocuproine (BCP)/1000 Al at 600nm.

From the above discussion, the exciton diffusion lengthLD is needed to determine EQE . This quantity can be inde-pendently measured via photoluminescence PL quenchingas follows: Assume that a layer of thickness d has onequenching interface e.g., by CT dissociation of excitons at aDA interface and the other is completely nonquenching.Further, assuming a uniform exciton generation rate G0throughout the thin film, then the boundary conditions are:

px00, 18a

p

x xd0, 18bGx G0 . 18c

The assumption of a uniform generation rate is valid only forfilms sufficiently thin such that interference (d/2n , withn the real part of the index of refraction of the film andabsorption (dLA) effects are negligible. Solving Eq. 15and using Eq. 18, yields the fraction of excitons reachingthe quenching interface

EDLD

d

1exp2d/LD

1exp2d/LD . 19

Assuming negligible self absorption, the PL signal is propor-tional to the average exciton density. Hence, by measuringthe PL signal both in the presence (PL1) and absence (PL2)of a quenching interface, ED1PL1 /PL2 is measured asa function of the film thickness. Then by fitting PL1 /PL2 toEq. 19, one obtains LD .

We note that LD obtained by this means may be a con-volution of the diffusion length and other extrinsic param-eters such as interface morphology, exciton quenching or

trapping sites, multiple randomly oriented grains, etc. Nev-ertheless, the measured LD is of interest since it effectivelydescribes exciton diffusion in films used in the actual hetero-structures themselves.

4. Model predictions

Figure 6 is calculated using the procedures discussedabove, and serves as a design guide for bilayer organic PVcells. The internal quantum efficiency IQE is used to quan-tify performance since it is a measure of the exciton harvest-ing efficiency. In addition, cells with a high IQE can be usedin light trapping configurations c.f. Sec. II D 4 or stacks

c.f. Sec. III, resulting in devices where EQE approachesIQE . Here, IQE is calculated versus the thickness of theactive organic layers with the exciton diffusion length as aparameter. For these calculations the wavelength is fixed at620nm. The PV layer structure is glass/1500anode/x 88060 acceptor/100 spacer/1500 cathode.The optical parameters of the materials are tabulated in Fig.6. The glass, anode, and spacer layers were assumed nonab-sorbing (k0), while for the cathode the parameters for Alwere used. For the donor and acceptor layers, k1.0 wasused, corresponding to 2k/1.0105 cm1 which istypical for * transitions in small molecular weight or-ganic materials commonly used in organic PV cells. The

FIG. 5. Steps involved in calculating EQE of the following multilayer struc-ture: glass/1600 ITO/320 PEDOT:PSS/200 CuPc/400 C 60/120 BCP/1000 Al. The wavelength of the incident light is 600 nm. aCalculation of the electric component of the optical field using the formal-

ism described in Sec. II B 2. The value of the optical intensity E

j(x)2,normalized to E0

2, where E0 is the amplitude of the incident plane wave,

is shown. E(x)21 indicates regions where the optical field is enhanceddue to interference effects. b Exciton generation rate G(x) at 600 nm. The CuPc film is strongly absorbing, yielding high exciton gen-eration rates. The parasitic absorption by the Al cathode is also shown. cAfter solving the exciton diffusion equation LD

2 2p/x2pG0, whereLD is the exciton diffusion length, is the exciton lifetime, and p the excitondensity, the steady state exciton concentration profile is obtained, shownhere by a plot ofp(x) Dp(x) LD

2/. The photocurrent resulting from theexciton current at a DA interface Eq. 17 can be directly read as obtainedin the graph.

FIG. 6. Internal quantum efficiency IQE as a function of the thickness x ofthe photoactive layers for the device structure glass/1500 anode/x donor/x acceptor/100 spacer/1500 cathode. The open and filledsquares correspond to measurements on glass/ITO/CuPc/PTCBI/BCP/Agand glass/ITO/CuPc/PTCBI/Ag, respectively, at 620 nm. The solid tri-angle is a measurement for glass/ITO/PEDOT:PSS/CuPc/C60/BCP/Ag.The solid lines are calculations of IQE for exciton diffusion lengths LD20, 40, 100, 200, and 400 . The dotted lines are solutions to the steady

state diffusion equation for the same range of LD . The data points aremeasurements for several devices discussed in Sec. II D.




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same LD was used for both the donor and acceptor layers.The solid lines are calculations of IQE as a function of thedonor and acceptor layer thicknesses for L

D20400.

The dotted lines represent a simple exciton diffusion modelthat does not take into account interference effects or un-wanted absorption c.f. Eq. 19. The data points are mea-surements for several different organic devices discussed inSec. II D.

From Fig. 6 it is clear that for maximum IQE , the thick-ness of the photoactive layers dLD . Thicker layers sufferfrom unwanted absorption in regions far from the DA inter-face. For layers thinner than LD , absorption in the metalcathode decreases IQE , causing a large deviation from thesimple model dotted lines. This latter effect poses a limit tothe achievable IQE for a given LD .

Accurate modeling of an actual PV cell requires preciseknowledge of the layer thicknesses, optical properties n andk, and LD of the materials used. In our calculations, theoptical constants of the materials used in our experiments:e.g., Al, Ag, and C60 , were taken from the literature.

26,29 Theoptical constants of the other materials used were determinedby spectroscopic ellipsometry.33 The absorbance of thephotoactive materials is shown in Fig. 7a. The exciton dif-fusion lengths LD of the active materials are either deter-mined from PL quenching measurements c.f. Sec. II D 1, orby fitting of the experimental EQE data to model predictionsc.f. Secs. II D 3 and II D 5.

Once satisfactory agreement between model and experi-

ment is obtained, the calculations can be used to optimize thePV layer structure for a given materials combination. Sincethe open circuit voltage VOC and the fill factor

34 FF arenearly independent of the layer thicknesses and materialsused, optimizing the short-circuit current density JSC alsooptimizes the power conversion efficiency P of the PV cell.Now, JSC is obtained by the overlap ofEQE with the solarspectrum

JSCq d EQE S , 20where S() is the spectral shape of the illumination source.Starting from layer thicknesses dj

init, the optimal values

djopt are obtained using a NelderMead nonlinear optimi-

zation algorithm.32 All calculations use the AM1.5 illumina-tion spectrum with a total intensity of 100 mW/cm2. In Fig.7, the AM1.5 solar spectrum is compared to the absorptionspectra of the photoactive materials used in this work.

C. Experimental methods

The multilayer PV cell structures fabricated to test themodels were deposited on glass substrates precoated with a1500 thick, transparent, conducting ITO anode sheet re-sistance 40 /). For optical measurements, quartz sub-strates were used. The substrates were cleaned immediatelyprior to transferring them into vacuum.35 In some cases, theITO was spin coated with a 320 thick layer of 3,4-PEDOT:PSS from solution at 4000 rpm for 40 s, followed bydrying at 90180 C for 1560 min in vacuum or N2 . Theuse of PEDOT:PSS results in a significantly higher yield, and

in the case of PV cells based on C60 , its use increases theopen circuit voltage and hence the power conversion effi-ciency of the PV cells. Where indicated, the PEDOT:PSSfilms was treated with an Ar plasma 10 W, 30 s, 100 mTorr,100 sccm Ar.6

The organic materials were commercially obtained andpurified using thermal gradient sublimation.24 Growth of theorganic layers was either by ultrahigh vacuum UHV or-ganic molecular beam deposition24 base pressure 11010 Torr) from a BN crucible or by high vacuum HVthermal evaporation base pressure 1107 Torr) from atungsten boat. This was followed by the deposition of themetal cathode through a shadow mask by thermal evapora-tion in a separate vacuum chamber with a base pressure of1106 Torr. Contact diameters of 0.1, 0.4, and 1 mmwere employed. For devices incorporating a layer of C60 ,atmospheric exposure of the organic layers prior to cathodedeposition was avoided. The layer thickness was determinedby a quartz microbalance and calibrated using a high resolu-tion scanning electron microscope.

The 540 nm line of an Ar ion laser was used as anexcitation source for the PL measurements. The PL signal ofthe organic films was recorded using a spectrometer afterremoving the pump signal with an optical filter.

Electrical characterization was performed in air or in a

N2 -filled glovebox (1 ppm O2 and H2O) using a semicon-ductor parameter analyzer for currentvoltage (IV) mea-surements. For photovoltaic power efficiency measurements,the devices were illuminated through the substrate with a 150W Oriel solar simulator36 equipped with an AM1.5 filter. Theintensity was measured using a calibrated broadband opticalpower meter36 in the position of the sample, and was variedusing neutral density filters. For measurements of the exter-nal quantum efficiency, a monochromatic beam of variablewavelength light chopped at 400 Hz 50% duty cycle, wasfocused onto a 1 mm diam device. The photocurrent wasmeasured using a lock-in amplifier referenced to the chopperfrequency.

FIG. 7. a Absorbance of the photoactive materials CuPc, PTCBI, and C60)used in this work. The photoluminescence spectrum of PTCBI is alsoshown. Note that nearly 50% of the solar photons are at 750 nm, whichis only weakly absorbed in the materials used in this study. b AM1.5 solarspectral density S() with an integrated intensity of 100 mW/cm2.




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D. Results and discussion

1. Exciton diffusion length measurements

As discussed in Sec. II B, the exciton diffusion lengthLD limits EQE in a bilayer cell configuration. We have mea-

sured LD in the acceptor material PTCBI using the donormaterial, copperphthalocyanine CuPc, to provide excitonquenching at the CuPc/PTCBI interface.1 The energy leveldiagram of the heterojunction is shown in the inset of Fig.8a. For this measurement, quartz substrates were partiallycoated with a 30 thick film of CuPc. Next, PTCBI wasgrown across the entire substrate with a thickness tPTCBI (0 tPTCBI160 ) . Finally, the PL signals of the PTCBI filmon top of CuPc (PL1) and quartz (PL2) were measured asfunctions of tPTCBI . No photoluminescence was detectedfrom the CuPc films under these conditions. A linear depen-dence of PL2 on thickness is observed, indicating that thequartz/PTCBI interface is nonquenching, and that the films

are sufficiently thin for the light absorption to be a linearfunction of thickness. The ratio PL1 /PL2 for a given filmthickness is then a measure of the exciton quenching at theinterface c.f. Sec. II B 3. This ratio is plotted versus tPTCBIin Fig. 8a filled squares. The data can be fit to Eq. 19solid line, yielding LD

PTCBI303 . This is comparable to

the values measured for other disordered molecular andpolymeric materials, as summarized in Table II.

No PL signal was detected from films of either CuPc or

C60 . The LD of these materials were estimated by fitting ofEQE versus wavelength for devices incorporating thesecompounds to experimental values c.f. Sec. II D 5. Fromour fits, we obtain LD

CuPc8030 and L

D

C6040050

see Table II.

2. Bilayer devices

As shown in Sec. II B 4, the ideal bilayer device consistsof active layers whose thicknesses are LD . This allowsmost photogenerated excitons to reach the DA interface, suchthat IQE1. Several possible schemes can then be em-ployed where A1, such that EQEIQE , leading to

higher external power conversion efficiencies.To test this possibility, we have fabricated bilayer cells

with the layer structure: ITO/CuPc/PTCBI/1000 Ag Fig.9, inset, where the thicknesses of the PTCBI layer was var-ied. In Fig. 9, EQE filled squares measured at the absorp-tion peak of PTCBI (540 nm) is plotted as a function ofthe thickness of the PTCBI layer tPTCBI for a CuPc thicknessof tCuPc300 . For tPTCBI300, EQE decreases astPTCBI increases due to absorption in regions farther thanLD from the DA interface. For tPTCBI300, however,EQE rolls offbefore the film thickness approaches LD .

In Fig. 6, IQE is plotted for bilayer cells with tCuPctPTCBI filled squares, showing a large deviation from the

TABLE II. Reported exciton diffusion lengths.

Materiala LD () Technique Ref.

Small molecule systemsPTCBI 303 PL quenching This workPTCDA 88060 from EQE 30PPEI 700 PL quenchingb 91CuPc 10030 from EQE This work

680200 from EQE 92

ZnPc 300

100 from EQE 93C60 40050 from EQE This work141 from EQE 26

Alq3 200 94200 95

Polymer systemsPPV 7010 from EQE 31

12030 from EQEc 92

PEOPT 47 from EQE 2650 PL quenching 96

aPPEIperylene bisphenethylimide, Alq3tris8-hydroxyquinolinealuminum. Other abbreviations are defined in Table I.

bUsing the result for the SnO2 quenching surface and assuming infinitesurface recombination velocity. The results leading to LD

PPEI2. 50. 5 m

are likely influenced by quencher diffusion and morphological changes

during solvent vapor assisted annealing.cOptical interference effects not considered.

FIG. 8. a The PL intensity ratio PL1 /PL2 of PTCBI vs the thickness, t ofa PTCBI film filled squares grown on 50 of CuPc. The solid line is a fitusing Eq. 19, yielding LD303 . Inset Energy level diagram of theCuPc/PTCBI heterojunction. The HOMO levels or IP values were ob-tained by ultraviolet photoelectron spectroscopy Ref. 37 with an uncer-tainty of0.1 eV. The LUMO levels or EA values were estimated as IPEopt , where Eopt is the film optical energy gap (Eopt1.70.1 eV forboth CuPc and PTCBI, as determined from their respective optical absorp-tion spectra. Efficient charge transfer of PTCBI excitons is expected if theexciton energy Eex is larger than the difference between the ionization po-tential of the donor IPD5.20.1 eV, and electron affinity of the acceptorEAA4.40.1 eV. In this case IPDEAA0.80.1 eV see Fig. 3. Giventhat the optical energy gap of PTCBI is EgapEexEB1.70.1 eV,charge transfer will take place for exciton binding energies EB0.9 eV. bThe PL ratio PL1 /PL2 of a 30 thick PTCBI film vs the thickness t of aBCP film sandwiched between a 50 thick CuPc and a PTCBI layer filled

squares. The solid line is a guide to the eye, drawn between the two limitingcases dashed lines. For t0 , PL1 /PL20.240.02 corresponds to thevalue in a for tPTCBI30 . The upper limit is PL1 /PL21 for t,when exciton quenching is completely suppressed. Inset: Schematic en-ergy level diagram of the CuPc/BCP/PTCBI system. The large HOMOLUMO energy gap of BCP prevents quenching of PTCBI excitons viacharge transfer between CuPc and PTCBI.




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trends predicted by the model at t100 . This discrepancyis due to the assumption that the PV cell employs a doubleheterostructure architecture of Sec. II D 3, in contrast to thesingle HJ devices measured here. As will be shown, thedouble heterostructure is particularly effective at increasingthe efficiency as t0.

The rolloff in EQE is partially due to the decreasingoptical field intensity resulting from the proximity of themetal cathode to the DA heterojunction which decreases A .This effect is modeled as described in Secs. II B 2 and II B3, assuming that the ITO/CuPc interface is nonquenching,

and that the exciton recombination velocity is infinite at theCuPc/PTCBI and PTCBI/Ag interfaces, with only the CuPc/PTCBI interface contributing to the photocurrent model A.Agreement between this model of EQE and the data is ob-tained for tPTCBI100 Fig. 9, solid line. The discrepancyat tPTCBI400 is attributed to a decrease in CC for thesethick films due to their high series resistance. The built-involtage is, in this case, no longer sufficient to drive the car-riers toward their respective electrodes prior to recombina-tion or trapping.

For tPTCBI100 the experimentally determined valuesof C60/120 are consistently lower than those predicted bymodel A, which is attributed to chemical or physical modi-

fication of the topmost layers of the PTCBI film,37 and dif-fusion of Ag atoms into the PTCBI upon deposition of theAg cathode Fig. 9, inset, leading to significant excitonquenching. Assuming that complete exciton quenching takesplace in the PTCBI film up to 80 from the PTCBI/Aginterface model B, agreement between the data and themodel is obtained for EQE over the experimental range oftPTCBI300 Fig. 9, dashed line.

3. Double heterostructure devices

The internal quantum efficiency of bilayer devices islimited due to the deposition of the metal cathode directly

onto the active organic layers. This problem can be reducedby the insertion of a layer between the active layers and themetal cathode with the following properties: 1 It musttransport electrons from the acceptor layer to the metal cath-ode with minimal increase in the total cell series resistance;2 it should provide a barrier to exciton transport, confiningthe excitons to the active organic layers and thereby prevent-ing them from recombining at the cathode/organic interface;

3 the material should absorb damage during cathode depo-sition; and 4 it must be transparent to the incident radiation.In addition, the layer must be sufficiently thick to result inincreased optical absorption in the photoactive layers byphysically separating them from the metal cathode.38 Thislayer is referred to as the EBL.

Previous reports39 have indicated that 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline BCP is an electron transportmaterial with a large HOMOLUMO energy gap EG3.5 eV, the latter property suggesting that BCP can blockexcitons while ensuring transparency to solar radiation. Fur-thermore, it has been shown that BCP prevents the underly-

ing organic layers from being damaged during metaldeposition.40,41 As a disadvantage, BCP thin films readilycrystallize, especially in the presence of moisture, yieldingmicron sized domains. This process can be eliminated bydoping with a larger molecule such as PTCBI. Hence, fordevices that require exposure to intense light or long-termstability, the BCP layer was doped with 5%15% byweight of PTCBI by coevaporation.

To verify the exciton blocking function of BCP, we de-posited layers of varying thicknesses ( tBCP) onto the surfaceof a 50 thick layer of CuPc. This structure was then coatedwith an additional 30 thick layer of PTCBI. The PL signalfrom this PTCBI film (PL1) was then measured as a function

of tBCP and normalized to the PL signal from a 30 thickfilm of PTCBI on quartz (PL2). The resulting fraction PL1 /PL2 is a measure of the exciton blocking capabilitiesof the BCP layer. In Fig. 8b, we plot as a function oftBCP . In the absence of BCP (tBCP0 ), equals that ob-

tained in Fig. 8a for tPTCBI30 . In this case, uninhibitedcharge transfer of excitons takes place at the DA interface,leading to nearly complete quenching of the PL of the thinPTCBI layer (0.240.02). This lower limit is indicatedby a dashed line. For tBCP20 , the PL of the PTCBI layeris recovered (1.0), indicating that the excitons generatedin PTCBI are effectively confined by a BCP layer of thick-

ness tBCP20.As shown in Fig. 10, the introduction of a 100 thicklayer of BCP into the CuPc/PTCBI bilayer device leads to amonotonic increase in EQE measured at 540nm astPTCBI filled squares is reduced to 90 . This is in strikingcontrast to the results for the single heterojunction device inFig. 9, where the efficiency is reduced as the layer thicknessis decreased to 300 . The values for EQE are in agree-ment with model calculations of the EQE solid line, assum-ing that exciton quenching is absent at the PTCBI/BCP in-terface. Furthermore, an increase in IQE is observed, from amaximum of 10%1% in the case of a bilayer device to25%1% for a double HJ DHJ device with tPTCBI

FIG. 9. External quantum efficiency EQE filled squares for ITO/CuPc/PTCBI/Ag devices with a CuPc layer thickness oftCuPc300 and varyingtPTCBI . The vertical error bars are smaller than the markers. Also shown aretwo model calculations. Model A solid line assumes that the PTCBI layeris continuous and that exciton quenching only takes place at the PTCBI/Aginterface. Model B dashed line assumes that the exciton recombination ratein the PTCBI film, up to 80 away from the PTCBI/Ag interface, is infi-nite. Inset: Schematic energy level diagram of an ITO/CuPc/PTCBI/Agdevice. The damage induced by the deposition of the Ag cathode on thePTCBI film is indicated by the shaded region in the diagram.




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90 . The calculations ofIQE dashed line are also inagreement with the data.

The measured IQE of DHJ devices for which tCuPc tPTCBI and tBCP150 are plotted in Fig. 6 opensquares. The data follow the expected trend for LD30,consistent with our measurements for PTCBI in Fig. 8a.The thinnest devices (tCuPctPTCBI60 ) exhibit IQE33%3% approximating the predicted optimal perfor-mance for LD30.

Figure 8b suggests that a 20 thick film of BCPwould be sufficient to prevent excitons from quenching at themetal cathode. However, the penetration depth of the damage

induced by the metal deposition is 20. Therefore, toprevent exciton quenching, a thicker film of BCP is required.In addition, thicker BCP films increase the optical field, andhence the absorption in the active layers see Sec. II B 4.

Fitting EQE versus wavelength for the device structureITO/150 CuPc/150 PTCBI/175 BCP/800 Ag, usingthe procedure of Sec. II B 4 with LD

PTCBI30 , yields

LDCuPc8030 . This is shown in Fig. 11, where the ex-

perimental values ofEQE open squares are compared withthe model fit solid line. The calculated relative contribu-

tions of CuPc and PTCBI to the EQE are also indicateddashed lines. The discrepancy between the model and ex-periment is attributed to slight inaccuracies in the opticalparameters of the materials measured by ellipsometry, andthe nanometer scale roughness of the active interface.24

While the peak value of EQE is not significantly im-proved by inclusion of the EBL, the EBL nevertheless allowsvery thin active regions to be used, leading to a reduced cellseries resistance, and hence to efficient operation under in-tense illumination. This is shown in Fig. 12, where currentdensity versus voltage characteristics of a double hetero-structure device cell A: ITO/150 CuPc/60 PTCBI/150 BCP/800 Ag incorporating an EBL, are

shown.4 The illumination intensity of the AM1.5 spectral il-lumination was varied from a fraction of a sun (2 mW/cm2)to 13suns(1300mW/cm2). The device surface tempera-ture remained 35 C under all illumination intensities. The

FIG. 10. External quantum efficiency EQE filled squares and internalquantum efficiency IQE open squares for ITO/CuPc/PTCBI/BCP/Ag de-vices with a CuPc layer thickness oftCuPc300 , tBCP100 , and vary-ing tPTCBI . The vertical error bars are smaller than the markers. Also shownare model calculations ofEQE and IQE . The model assumes no quenchingat the ITO/CuPc and PTCBI/BCP interfaces. Inset: Schematic energy leveldiagram of an ITO/CuPc/PTCBI/BCP/Ag device. The damage induced bythe deposition of the Ag cathode on the BCP film is indicated by the shadedregion in the diagram.

FIG. 11. External quantum efficiency EQE of a device with the followinglayer structure: ITO/150 CuPc/150 PTCBI/175 BCP/800 Ag opensquares. The model calculation is shown by the solid line, with the calcu-lated contributions from the CuPc and PTCBI layers shown as dotted anddashed lines, respectively.

FIG. 12. Current density vs voltage characteristics of an ITO/150 CuPc/60 PTCBI/150 BCP/800 Ag PV cell under varying AM1.5simulated solar illumination intensities of up to 13 suns.

FIG. 13. a Short circuit current ISC as a function of the incident opticalpower density of the AM1.5 illumination source for cell A (ITO/150 CuPc/150 PTCBI/150 BCP/800 Ag and cell B (ITO/300 CuPc/300 PTCBI/150 BCP/800 Ag. b The power conversion effi-ciency P of cell A and B and the open circuit voltage VOC and fill factor FFof cell A as a function of the incident optical power density of the AM1.5illumination source.




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shunt resistance, measured in the dark, is 202 k cm2, andthe series resistance, measured at an illumination intensity of1300 mW/cm2, is 3010 cm2. The dark current followsthat of a conventional C60 junction diode

34 with an idealityfactor of n1.41.7 and a rectification ratio of 105 at1 V.

In Fig. 13, several performance parameters of this deviceare plotted as functions of the illumination intensity. The

short-circuit current cell A, shown in Fig. 13a filledsquares, is linear with illumination intensity. We concludethat even at the highest intensity of 13 suns, no significantspace charge buildup occurs which would lead to charge re-combination and hence a rolloff in JSC from its linear depen-dence on optical power. As shown in Fig. 13b, the opencircuit voltage VOC filled triangles increases monotonicallyuntil it reaches a plateau of VOC0.54 V for illuminationintensities10 suns. As discussed in Sec. III, the maximumVOC is a measure of the separation between the CuPc andPTCBI Fermi levels under flatband conditions. The FF filledcircles approaches 0.57 at low intensities, and exceeds 0.35even at the highest illumination intensities considered. Theexternal power conversion efficiency P filled squares ofthis device reaches a maximum ofP1.1%0.1% over abroad plateau extending from 0.1to 10 suns.

Also shown in Fig. 13b is the P of a device with athicker photoactive region cell B: ITO/300 CuPc/300 PTCBI/150 BCP/800 Ag open circles. The lowermaximum efficiency is in agreement with the trend in peakexternal quantum efficiency Fig. 9, filled squares. The effi-ciency rolloff for this thicker device for illumination intensi-ties 1 sun is attributed to an increased internal series resis-tance, leading to space charge buildup and increased carrierrecombination. This effect is also seen in the plot of ISC

versus illumination intensityFig. 13

a

, open circles

, whichis sublinear for this thicker cell.

4. Light trapping in thin PV cells

In the thinnest double heterostructure PV cells, only afraction of the incident light is absorbed in a single reflectionfrom the cathode contact. However, higher efficiencies havebeen attained in a light-trapping configuration,4 where inprinciple, external efficiencies approaching the internal effi-ciency can be achieved. To demonstrate this principle, weplaced a reflective Ag layer with a small aperture with1% of the area of the entire cell on the substrate surface

of the cell. We then focussed a near normal incidence beamof concentrated radiation (10 suns at AM1.5 on the aper-ture. Light trapped between the reflective surface and thecathode passes multiple times through the photoactive het-erojunction, undergoing additional absorption with eachpass. For a cell with tCuPctPTCBI60 and tBCP100,this approach increases the peak external power efficiencyfrom P1.0%0.1% to P2.4%0.3%. Due to thesmall top electrode, not all of the incident radiation wastrapped, and hence the measured P represents a lower limit.

The improvement introduced by the EBL and the simpleaperture-type light trapping structure demonstrated above isshown in Fig. 6. Here, the internal quantum efficiency IQE

at 620 nm is shown for devices with a variable tCuPctPTCBIx and with tBCP100 . Double heterostructuredevices open squares, which incorporate an EBL, exhibit asubstantially higher IQE up to 33% for tCuPctPTCBI60 ) as compared to single heterostructures solidsquares. Through the use of a light trap, EQE approachesIQE , allowing an improvement in P by more than a factorof 2 over a device without an EBL. It is clear that furtherimprovement ofIQE, and hence P , is hampered by para-sitic absorption in the cathode. To achieve IQE40%,therefore, requires materials with LD40 . We infer that

for LD200, an optimal cell exhibits IQE70%. Withthe addition of light trapping and assuming VOC0.5V andFF0.5, we obtain P6% .

An efficient and practical implementation of light trap-ping in a thin film PV cell is shown in Fig. 14a. It consistsof an array of Winston42 solar collectors, each collector fo-cusing the incident solar radiation through an aperture at itsapex. The light is subsequently trapped between the reflec-tive undercoating of the collectors and the partially reflectiveorganic PV cell, as shown in Fig. 14a. The escape probabil-ity is low due to the small area occupied by the apertures7.3% of the total area, ensuring a large number of reflec-tions. For maximum efficiency, we note that such an assem-

bly may require solar tracking.This structure can be fabricated by molding the collector

shapes from a flexible material43 e.g., an elastomeric poly-mer such as polydimethylsiloxane, followed by the reflec-tive coating of the collector and its reverse surface. Thecoated collector array is then laminated onto the substrateside of a PV cell. The principle can be applied to any thin-film PV cell on a transparent substrate.

Ray-tracing calculations were carried out to evaluate theefficiency of this particular light trapping scheme. We as-sume that the structure is illuminated with normally incidentlight at an intensity of 1 sun, that the reflectivities of themirror coatings and cathode are 95% (R0.95, correspond-

FIG. 14. a Geometry of the light trapping configuration discussed in thetext. b Spatial distribution of the absorbed optical power density in the thinfilm organic PV cell under 1 sun illumination, displayed in a sixth of thehexagonal unit cell. The values at other points in the unit cell can be ob-tained by symmetry operations. Note that the absorbed power density is1.4 suns at all points.




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ing to that of a Ag coating over most of the visible spec-trum, and the thin PV cell absorbs a fraction A of theincident light in a single reflection. Note that the angle de-pendence of the PV cell absorption and mirror reflectivitywas not taken into account. The rays were traced in a hex-agonal unit cell c.f. Fig. 14b of the array, and periodicboundary conditions were assumed to simulate the full col-lector structure. Uniform illumination was simulated using adense array of rays incident on the collector structure.

The resulting spatially resolved absorbed power densityin the thin PV cell is shown in Fig. 14b for a cell withA0.10. Only a sixth of the unit cell is shown due to the

hexagonal symmetry of the array. The absorbed power den-sity averages 0.60 suns compared to 0.10 suns in the ab-sence of light trapping, with a maximum of 0.81 suns. Thegeometrical features in the spatial distribution of the ab-sorbed power are a consequence of the focusing effect of thecollectors and multiple reflections from the cathode and re-flector rear surfaces.

The results of the ray-tracing calculations are shown inFig. 15. The efficiency of absorption of the incident opticalpower in the PV cell (A

LT), the dissipation by mirror loss(ML), and losses by escape from the structure (EL), areplotted as functions of the absorption A of the PV cell inFig. 15a. For a PV cell with a very low absorption of A0.1, 60% of the light is absorbed by the PV cell (ALT0.6) when used with the light trap, while mirror and escapelosses each consume 20% (MLEL0.2) of the incidentoptical power.

Applying the light-trapping structure, therefore, signifi-cantly improves the efficiency of this thin PV by at least afactor of 6. This improvement, or light-trapping efficiencyLT , is plotted versus the absorption A of the PV cell inFig. 15b. As materials are chosen with larger LD , the layerscan be made thicker without incurring a decrease in quantumefficiency see Fig. 6. For thicker, more absorbing layers,however, the advantage obtained by employing a light trap isreduced, as is apparent from Fig. 15b.

5. PV cells using C60

To increase the efficiency beyond that obtainable usingCuPc/PTCBI DHJ cells, a material combination with an in-creased LD is required see Fig. 6. For example, thereported26 exciton diffusion length of the acceptor materialC60 (LD80140) is substantially longer than that ofPTCBI. This is attributed44 to the rapid intersystem crossingof the singlet excited state ( 1C60* ) to the long-lived tripletexcited state (3C60* ). Because of the large spinorbit interac-tion in these nearly spherical molecules, intersystem crossingis more rapid (ISC650 ps) than the natural lifetime of thesinglet state (S16 ns), resulting in nearly unity intersys-tem crossing quantum yields (ISC96%) at room

temperature.

45

The resulting triplet excitons have a lifetimeT1 s, leading to the large LD observed. The smallsinglettriplet splitting (EST0.15 eV

46, and hence largetriplet energy may aid in ensuring efficient charge transfer ofthe triplet excitons at a DA interface. As a guideline, Fig. 6predicts that internal efficiencies of IQE60% can beachieved at LD100.

By replacing PTCBI with C60 , an increase in EQE isexpected for wavelengths within the C60 absorption bandi.e., 550nm, c.f. Fig. 7. The EQE for a device with thelayer structure ITO/320 PEDOT:PSS/150 CuPc/350 C60/160 BCP/1000 Al cell C is shown in Fig. 16 opensquares. Here EQE peaks at P , coincident with the

430 nm absorption peak of C60 , and EQE45%1% atthe CuPc 630nm absorption peak. This appears to beinconsistent with the values of EQE obtained for CuPc/PTCBI-based devices with a maximum of only EQE15%1% . Fitting of the quantum efficiency model Sec. II B 4to the data for EQE yields for LD

CuPc10030 and L

D

C60

40050 , the latter value being substantially larger thanthat reported by Petterson et al.26 We attribute this to the highpurity of our source material obtained by thermal gradientsublimation.24 The high current densities of 200mA/cm2

achieved under a forward bias of only 1 V are also consistentwith this inference c.f. Fig. 19. The value ofLD

CuPc is iden-tical, within experimental error, to that

FIG. 15. a Fractions of the optical power incident on the light trappingstructure of Fig. 14 absorbed in the thin PV cell (A

LT), dissipated by mirrorloss (ML), and loss by escape from the structure (EL), as functions of theabsorption efficiency A of the thin PV cell. Note that A

LTMLEL

1. All other parameters are as shown in Fig. 14. b The light-trappingefficiency LT or ratio of the optical power absorbed in the thin PV cell inthe presence of the light-trapping structure over that in its absence, as afunction of the absorption A of the thin PV cell. All other parameters are asshown in Fig. 14.

FIG. 16. External quantum efficiency measured in a N2 ambient of a devicewith layer structure ITO/320 PEDOT:PSS/150 CuPc/350 C60/160 BCP/1000 Al open squares. The model calculation is shown as a solidline, with the calculated contributions from the CuPc and C60 layer asdashed lines.




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obtained when fitting EQE data of CuPc/PTCBI devices.The internal quantum efficiency IQE at 620nmIQE(620nm)56% is also plotted in Fig. 6 solid tri-angle. The average of the donor and acceptor layer thick-nesses was used as the abscissa.

The higher EQE of the C60 cells as compared to thoseemploying PTCBI is a consequence of a maximization of theoptical field intensity near the CuPc/C60 heterojunction. The

CuPc/C60 system is advantageous as compared to CuPc/PTCBI because: 1 The use of an acceptor material with alarge LD allows the acceptor to act as a spacer layer, movingthe CuPc donor layer farther away from the metal cathodeand into regions of higher optical field intensity. 2 A mate-rial such as C60 with a large index of refraction n allowsthinner acceptor layers to be used since the position of themaximum optical field intensity is at a distance /(4n)from the metal cathode (nC602.1 over the wavelength range400nm800nm). 3 The largest improvements are ex-pected when the absorption spectra of the donor and acceptormaterials do not overlap c.f. Fig. 7. 4 To maximize theexciton generation near the DA interface, it is preferable to

have the layer that is in closest proximity to the metal filmabsorb on the blue side of the absorption of the second layerforming the heterojunction. Using C60 as an acceptor layersatisfies all of these four requirements.

The optical field intensity maximization is illustrated inFig. 17a, where the absorption of 1400 ITO/200 CuPc/400 C60/200 BCP PV cell on quartz is calculatedusing the methods of Sec. II B 2, in transmission solid lineand in reflection dotted line from an Al mirror. We observethat the reflective geometry increases the absorption in theCuPc by approximately a factor of 4 because of the com-bined effects of the double pass of the light, and constructiveinterference of the optical field c.f. Fig. 17b.

The optimal device layer structure was also determinedusing the methods discussed in Sec. II B. The dependence ofthe short-circuit current density on the thickness of the CuPcand C60 layers, for a cell with layer structure 1500 ITO/320 PEDOT:PSS/x CuPc/ y C60/120 BCP/1000 Al, is shown in Fig. 18. This calculation sug-gests a maximum response for xMAX14020 and y MAX46040 . The strong dependence of JSC on y resultssince C60 acts as a spacer layer, pushing the CuPc/C60 inter-face into regions of high optical field intensity. The lessereffect of x on J

SCfor xx

MAXis mainly due to optical fil-

tering, which only affects the CuPc contribution to JSC sincethe CuPc and C60 absorption spectra are substantially non-overlapping. The data points cells C and D indicate theperformance of actual cells. Cell C is the cell of Fig. 16(ITO/320 PEDOT:PSS/150 CuPc/350 C60/160 BCP/1000 Al. The experimentally obtained short-circuit

FIG. 17. a Calculation of absorption of a PV cell on glass in the absencesolid line and presence dotted line of an Al cathode versus wavelength ofthe incident light. b Model calculation of the optical field intensity nor-malized to the incident optical intensity as a function of the position in the1400 ITO/200 CuPc/400 C

60 /200 BCP PV cell in the absence

solid line and presence dotted line of an Al cathode.

FIG. 18. Calculation of the short-circuit current density JSC contours under100 mW/cm2 AM1.5 spectral illumination for a device with layer structure1500 ITO/320 PEDOT:PSS/x CuPc/y C60 /120 BCP/1000 Al. The CuPc layer thickness is varied from x40 to 400 , and the C60layer thickness is varied from y40 to 800 . Bold contour lines are drawnwhere JSC is at integer multiples of 1 mA/cm

2. The data points filledcircles indicate the performance of the following layer structure:ITO/320 PEDOT:PSS/150 CuPc/350 C60 /160 BCP/1000 Alcell C and ITO/320 PEDOT:PSS(Ar treated)/200 CuPc/400 C

60/120 BCP/1000 Al cell D.

FIG. 19. Current density vs voltage characteristics of the following opti-mized device structure: ITO/320 PEDOT:PSS(Ar treated)/200 CuPc/400 C60 /120 BCP/1000 Al under AM1.5G simulated solar il-lumination of variable intensity. The maximum power rectangles are indi-cated for illumination intensities of150 mW/cm2 gray areas.




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current density at 100mW/cm2 AM1.5 illumination wasJSC10.6 mA/cm

2, while the model predicts JSC7.6mA/cm2.

Cell D is an optimized cell with the following layerstructure: ITO/320 PEDOT:PSS/200 CuPc/400C60/120 BCP/1000 Al. The IV characteristics of cell Dunder a range of illumination intensities with an AM1.5spectral source are shown in Fig. 19. The P , VOC , and FFof this device are shown in Fig. 20 as functions of the opticalpower density. The power conversion efficiency P reaches amaximum of 3.7%0.2% at an incident power of44mW/cm2 0.44 suns and rolls off at higher illuminationintensities due to cell series resistance R S6.21.2 cm

2.At an illumination intensity of 150 mW/cm2, P3.6%0.2%, JSC18.8 mA/cm

2, VOC0.58V, and FF0.52,while at 1200 mW/cm2, JSC138mA/cm

2. The effect ofR S

can be clearly seen in Fig. 19. At intensities 150 mW/cm2,the maximum electrical output power is achieved at lowervoltages, and hence lower P . In contrast to previous resultsfor CuPc/PTCBI/BCP cells,4 no broad plateau of maximumpower conversion efficiency is observed as a consequence ofthe higher overall efficiency which amplifies the series resis-tance effects.

6. Cathode induced defect states in the EBL

As mentioned in Sec. II D 3, a minimum thickness of theBCP EBL is required to prevent damage to the active organiclayers during cathode metal deposition. For this purpose, a

thickness tBCP100 is typically used. In many cases, it isbeneficial to increase the thickness of the EBL to optimizeEQE through optical interference effects, as discussed inSec. II B 2. However, we have previously shown6 that fortBCP150, EQE decreases exponentially with tBCP due toa reduction in carrier collection efficiency CC . This issue isnow addressed in more detail.

The ability of BCP to act as an EBL and hole blockinglayer is expected from the energetic positions of the HOMOand LUMO levels of this material. However, if we assumethat the difference between the ionization potential and opti-cal energy gap is an accurate measure of the position of theLUMO level, it appears that the LUMO of BCP lies 1.0

0.2 eV above that of C60 ,6 and 0.90.2 eV above that of

PTCBI.4 Hence, its ability to transport electrons from thePTCBI or C60 LUMO to an Ag or Al cathode despite the

large barrier formed by the BCP LUMO without incurring avoltage drop, even at high illumination levels of up to 13suns, is surprising. It was suggested4,6 that electron transportin the BCP EBL occurs mainly through states below theLUMO induced during deposition of the metal cathode47 asshown in the proposed energy level diagram in the inset ofFig. 21.

This hypothesis is supported by the data in Fig. 21.Here, JSC of cells with layer configurationITO/PEDOT:PSS/200 CuPc/400 C60/t BCP/1000Al is plotted as a function of the BCP thickness x for incidentpower densities of 2.9 mW/cm2 open circles, 21mW/cm2

filled circles, 185 mW/cm2 open squares, and

1480 mW/cm2 filled squares. At every incident power den-sity, JSC is approximately constant until a critical BCP thick-ness t0 is reached, beyond which JSC drops exponentiallywith the BCP thickness t. The critical thickness is t0150 for an incident power density of 1480 mW/cm2,and increases to t0300 at 2.9mW/cm

2 c.f. Fig. 21.This is explained as follows: the electron mobility in theBCP layer is proportional to the defect state density48 Ndefectnear the C60/BCP interface, which is assumed to be an ex-ponentially decreasing function of the distance d to BCP/Alinterface: viz. Ndefect(d)N0 exp(d/d0). Here N0 is the trapdensity at the BCP/Al interface, and d0 characterizes thedepth of the film damage during cathode deposition. As the

FIG. 20. Power conversion efficiency P , FF, and open circuit voltage VOCof the optimized device with layer structure: ITO/320 PEDOT:PSS(Ar treated)/200 CuPc/400 C60/120 BCP/1000 Al asa function of the incident optical power from an AM1.5 illumination source. FIG. 21. Short-circuit current density JSC of photovoltaic cells with layer

configuration ITO/PEDOT:PSS/200 CuPc/400 C60/t BCP/1000 Al as a function of the BCP thickness tBCP for incident power densities of2.9 mW/cm2 open circles, 21 mW/cm2 filled circles, 185 mW/cm2 opensquares, and 1480 mW/cm2 filled squares. The solid lines are guides tothe eye and indicate the slopes for JSCexp(t /50 ) and JSCexp(t /30 ). The dashed line indicates the dependence of the criticalBCP thickness t0 on the illumination level. Inset: Proposed energy leveldiagram of PEDOT:PSS/CuPc/C60 /BCP/Al devices. The Fermi-level ener-gies of the electrodes PEDOT:PSS and Al, and HOMO and LUMO levelsfor CuPc, C60 and BCP, are indicated. The PEDOT:PSS work function datawere taken from Ref. 79. The C60 ionization potential and electron affinityare taken from Ref. 80 and Ref. 81, and are used for CuPc and BCP. Thedata for PTCBI are courtesy of I. G. Hill and A. Kahn. The electrode workfunctions and HOMO levels were obtained by ultraviolet electron spectros-copy, and have an error of0.1 eV. The LUMO levels were estimated fromthe difference of the HOMO energy and the optical energy gap.




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thickness t of the BCP is increased, its resistance increases

exponentially until the voltage drop across it equals VOC . Atthis point, the bias across the BCP layer is pinned at VOC andJSC decreases exponentially with t, as observed in Fig. 21,where we estimate d04010.

In Fig. 22, the square root of the current density J1/2 isplotted versus the applied voltage V for a varying illumina-tion intensity. The linear dependence of J1/2 on V for illu-mination levels 56mW/cm2 suggests that the current isspace-charge limited. For the lowest illumination intensity of2.9mW/cm2, the photocurrent is sufficiently low such that ata reverse bias of1 V, the current density is limited by thephotocurrent, and hence saturates.

Following these observations, the current density versus

voltage behavior of the BCP EBL is derived by assuming0 exp(d/d0), where 0 is the mobility determined bythe defect density at the BCP/Al interface.48 The space-charge limited current density is then

JV20 i0

8d03

exp td0

, 21which reproduces the observed voltage and thickness depen-dence, as shown in Figs. 21 and 22. The linear fits in Fig. 22reveal that 0 is a weakly increasing function of the illumi-nation level and hence current density. At the illuminationlevel of 2.9 mW/cm2, 0(1.40.1)10

7 cm2/V s,

while at 1480mW/cm2, 0(2.90.1)10

7 cm2/V s.These values are typical for amorphous small molecularweight materials such as BCP.49 The weak dependence of0on J may be due to trap filling at high J, reducing the prob-ability that charge trapping will impede carrier transport.

III. MULTIPLE HETEROJUNCTION DEVICES

A. Introduction

In Sec. II, it was shown that when the thickness of a DAbilayer in a double heterostructure organic PV cell ap-proaches LD of the active materials, one obtains cells with ahigh internal quantum efficiency, IQE . However, because

J1/2 is typically one order of magnitude less than the opticalabsorption length LA , the absorption efficiency A is typi-cally low. Hence, the external quantum EQEAIQE andpower conversion P efficiencies are also low because mostphotons are left unabsorbed. This problem was solved byemploying very thin cells ( 100 thick with a high IQE33%3% in a light trapping geometry in which lightpasses multiple times through the same bilayer cell to

achieve substantial absorption c.f. Sec. II D 4. For an op-timized light trapping cell, A1, and EQE approachesIQE . When applied to a thin cell with P1.0%0.1%,this approach leads to4 P2.4%0.3%.

The limitations encountered in thin cells can also beovercome by placing a series of such cells in a stack. In thiscase, the individual cells are sufficiently thin to allow for alarge fraction of the photogenerated excitons to reach a DAinterface high IQE), while the complete stack is sufficientlythick to absorb most of the incident photons. This principle,which is analogous to the multiple optical pass topologyemployed in the light trap, was demonstrated by Hiramoto etal.50 There, it was shown that a stack of two Tang singleheterojunction PV cells1 in a series configuration separatedby a thin (100 ) Au layer, yielded a PV cell with nearlytwice the open circuit voltage VOC of a single cell. Unfortu-nately, it was also found that the power conversion efficiencywas considerably lower than for an equivalent single-

junction cell. This was attributed to attenuation of light inci-dent on the back cell i.e., the cell farthest from the transpar-ent anode contact by absorption in the front cell and metalinterlayer, and to increased series resistance of the multilayerstructure.

Further improvement of this appro

jap.93.3693.2003(review of solar cells)

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