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A R T I C L E I N F O

Article history:

Received 23 February 2012

Accepted 13 May 2012

Available online 19 May 2012

A B S T R A C T

advantages of being lightweight, cost-effective, highly flexible

interpenetrating networks in the nanoscale range between

p-type and n-type materials of the BHJ structures can maxi-

mize interfacial areas for exciton dissociation and generate

more efficient charge pathways to electrodes [57]. However,

the low charge mobility of organic materials and unoptimized

one-dimensional nanomaterial with high charge mobility,

collecting layers in OPVCs [1214]. However, the use of CNTs

in OPVCs sometimes produces a lower PCE than conventional

OPVCs without CNTs [57]. One possible reason for the low

PCE is the low charge selectivity of CNTs. The metallic por-

tions of CNTs [15], which have no band gap for effective

C A R B O N 5 0 ( 2 0 1 2 ) 4 4 8 3 4 4 8 8

Avai lab le a t www.sc ienced i rec t .com

els* Corresponding author: Fax: +82 42 350 3310.and mass producible via simple solution processing [13]. In

spite of these advantages, the commercialization of OPVCs

is still limited due to their low power conversion efficiency

(PCE) and short life span [4]. Most of the highly efficient

OPVCs have polymer bulk heterojunction (BHJ) structures that

consist of electron donating (p-type) conjugated polymers

and electron accepting (n-type) fullerene derivatives. Highly

such as carbon nanotubes (CNTs). CNTs have a large aspect

ratio and high charge mobility [8,9]. Thus; they can provide

a fast charge pathway in OPVCs. Two distinctive uses of CNTs

in OPVCs have been suggested as a means of increasing the

PCE. One suggestion is to incorporate CNTs into photo-active

polymers to produce charge transport pathways [10,11]. The

other suggestion is to utilize CNTs for the hole transport or1. Introduction

Organic photovoltaic cells (OPVCs) have received considerable

attention as a clean renewable energy source due to their

BHJ structures increases the chance of carriers recombining

or dissipating during the charge transport and collection

states; as a result, the PCE is low.

One way of overcoming these problems is to introduce a0008-6223/$ - see front matter 2012 Elsevihttp://dx.doi.org/10.1016/j.carbon.2012.05.027

E-mail address: jeon39@kaist.ac.kr (S. JeoWe propose a new strategy for improving the charge selectivity of carbon nanotubes (CNTs)

for organic photovoltaic cells (OPVCs). The strategy involves the coating of an ultrathin

layer of titanium suboxide (TiOx) on CNTs by atomic layer deposition (ALD). ALD can facil-

itate that conformal and uniform coating of TiOx on CNT networks while preserving their

nanoporous structure. We used the TiOx-coated CNT networks as an electron transport

layer in inverted OPVCs. TiOx-coated CNTs can provide electrons with an extremely fast

conductive path through CNTs and selectively block the holes by means of the hole-barrier

property of the TiOx in OPVCs. The nanoporous structure of TiOx-coated CNT networks can

improve the device performance of OPVCs due to synergetic effects of the electron selective

transport property of TiOx and the high conductivity of CNTs. In addition, further improve-

ment of device performance can be achieved by adding a hole transport layer (MoO3)

between the active layer and the Au electrode.

2012 Elsevier Ltd. All rights reserved.Conformal coating of titaniumnetworks by atomic layer depophotovoltaic cells

Sung H. Jin, Gwang H. Jun, Soon H. Hong, S

Department of Material Science & Engineering, KAIST Institute for

(KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of

journal homepage: www.er Ltd. All rights reservedn).uboxide on carbon nanotubeition for inverted organic

kwoo Jeon *

Nanocentury, Korea Advanced Institute of Science and Technology

ea

evier .com/ locate /carbon.

4484 C A R B O N 5 0 ( 2 0 1 2 ) 4 4 8 3 4 4 8 8charge selection in OPVCs, can generate an undesired charge

recombination between the holes and electrons during the

charge transportation along the CNTs [16].

Here we propose a new strategy for improving the charge

selectivity of CNTs. The strategy involves the coating of an

ultrathin layer of titanium suboxide (TiOx) on CNTs. TiOx is

an n type material with a large band gap (Eg 3.7 eV) [17],and it can act as a hole injection barrier in OPVCs [18,19].

TiOx-coated CNTs can therefore provide electrons with an ex-

tremely fast conductive path through CNTs and selectively

block the holes by means of the hole-barrier property of the

TiOx coating. For effective charge separation of TiOx-coated

CNTs, the CNTs should first be uniformly coated with TiOxto a precise thickness. Atomic layer deposition (ALD) is a per-

fect tool for controlling the thickness of TiOx at the atomic

scale because the self-limiting nature of ALD [20] helps to pre-

serve the nanostructure of the CNTs. To investigate the effect

of charge separation through the TiOx-coated CNTs, we used

TiOx-coated CNT networks which were fabricated by ALD as

an electron transport layer in inverted OPVCs. Inverted OPVCs

employ the transparent electrode as a cathode andmetal elec-

trode as an anode which show the reverse charge collection

compared to conventional OPVCs. In general, inverted OPVCs

have higher stability than conventional OPVCs in ambient

conditions due to the oxygen barrier property of electron

transporting metal oxides and metal anode with high work

function [2123]. Transmission electron microscopy (TEM)

and atomic force microscopy (AFM) confirm that the coating

of TiOx on the CNT networks is uniform and retains their

nanoporous structures. The PCE value of the inverted OPVC

made with TiOx-coated CNT networks by means of ALD is

30% greater than the PCE value of OPVCs without CNT net-

works; the improvement is due to the effective charge trans-

port of nanoporous CNTs networks. The homogeneous

coating of TiOx on CNT networks is the key factor to improve

the PCE of OPVCs. It is confirmedwhen the results of CNT net-

works coated homogeneously with TiOx by means of ALD are

comparedwith the results of CNT networks coated inhomoge-

neously with TiOx by means of solgel solution; the latter

manifests large leakage currents and poor device perfor-

mance. We also investigated the optimal coating thickness

of TiOx on CNT networks to prevent large leakage currents

from the CNT networks in OPVCs. This study is the first to sys-

tematically investigate the atomic layer-deposited, highly con-

ductive, nanoporous CNT networks with TiOx in OPVCs.

2. Experimental section

2.1. Fabrication of CNT networks on ITO

Thin multi-walled CNTs were supplied by the Hanhwa Nano-

tech Co. The 50 mg of CNTs, which had a diameter of 5 nm to

10 nm, were functionalized by 12 h sonication in a solution of

HNO3 and H2SO4 mixed at a ratio of 1:3 (100 ml) at room tem-

perature. Then, functionalized CNTs were obtained after

being washed with water on the Nylon filter (pore size:

0.2 lm) until the rinsed water becomes fully neutral (pH 7).

This process was designed to form carboxylate functionalgroups on the CNTs. A 1 mg sample of functionalized CNTswas dispersed in 250 ml of water by sonication for 3 h. The

dispersed solution was then centrifuged at 10,000 rpm to re-

move any agglomerates or bundled CNTs. Homogeneously

dispersed CNT solutions were filtered with an alumina mem-

brane (0.2 lm pores, Whatman) to fabricate the CNT net-

works. The CNT networks on the membrane were then

transferred to an ITO substrate by means of poly(dimethylsi-

loxane) (PDMS) stamping.

2.2. Fabrication of TiOx-coated CNT networks

A traveling wave-type thermal ALD system (CN1 Co., Ltd.) was

used for the ALD coating of TiOx on the CNT networks. We

used tetrakis dimethylamino titanium (TDMAT, 99.9999%,

supplied by DNF Co., Ltd.) for the Ti precursor and H2O vapors

for the reactant. One cycle of ALD consisted of a 0.5 s injection

of TDMAT, a 30 s Ar purge, a 0.5 s injection of H2O vapors, and

another 30 s Ar purge (chamber temperature: 200 C). The solgel TiOx was fabricated from the spin coating of tetra isoprop-

oxide (SigmaAldrich 99.999%) precursor solution in an air

atmosphere as referred method [18].

2.3. Fabrication of inverted OPVCs

A 20 mg of poly(3-hexylthiophene) (P3HT, supplied by Rieke

Metals) and a 16 mg of [6,6]-phenyl C61 butyric acid methyl

ester (PCBM, supplied by SigmaAldrich) were dissolved and

stirred in 1 ml chlorobenzene for 12 h, respectively. The

P3HT and PCBM solutions were mixed together and stirred

for an additional 12 h. The blend of P3HT and PCBM was spin

coated on the TiOx-coated CNT networks at 300 rpm for

1 min. The Au electrode was thermally evaporated on top of

the active layer in a vacuum with a pressure of 107 torr. Forthe inverted OPVCs with MoO3 layer, 1 nm thick MoO3 and

Au electrode was sequentially evaporated on top of the active

layer. Finally, the device was placed directly on a digital hot

plate in a glove box filled with N2 gas. It was left on the hot

plate for 5 min at a temperature of 110.

2.4. Characterizations of functionalized CNTs,TiOx-coated CNTs, and inverted OPVCs

The microstructures of the functionalized CNTs and TiOx-

coated CNTswere observedwith a TEM (Tecnai G2 F30 S-Twin)

and AFM (Seiko). The photovoltaic properties of the OPVCs

were characterized under 1 sun illumination from a xenon

lamp with an AM 1.5 global filter. A silicon reference solar cell

certified by the National Renewable Energy Laboratory was

used to calibrate the light and to confirm the measurement

conditions. JV measurements were taken with an electro-

chemical analyzer (Ivium Compactstat).

3. Results and discussion

Fig. 1a shows a schematic of inverted OPVCs with TiOx-coated

CNT networks. The CNT networks were fabricated from a

simple vacuum filtering process of CNT aqueous solutions on

alumina membranes [24]. They were then transferred onto in-dium tin oxide (ITO) substrates by stamping with PDMS [25].

1 2C A R B O N 5 0 ( 2 0Next, we used ALD to deposit TiOx on the CNT networks at

various thicknesses (1, 5, 10, and 15 nm). Then, we used spin

coating to insure the active materials of P3HT/PCBM compos-

ites infiltrated into the CNT networks. Finally, we then

thermally evaporated the Au electrode on top of the active

layer. Due to their energy level in these inverted OPVCs, TiOxcan only transfer electrons and block holes. (Properties of

TiOx by ALD like the X-ray diffraction (XRD), ultravioletvisible

(UVvis) and ultraviolet photoelectron spectroscopy (UPS)

analysis results are shown in Fig. S1.) Furthermore, because

the work function of CNTs is located below the LUMO level of

TiOx and near the work function of the ITO electrode [26,27],

the transfer of electrons is conducted selectively from the TiOxto the ITO electrode via CNTs as shown in Fig. 1b. Therefore,

uniform and conformal TiOx-coated CNT networkswith nano-

porous structures can serve as electrons transport pathways

from an active layer to the ITO electrode in inverted OPVCs.

The coating property of TiOx on CNT networks is investi-

gated by microscopy analysis. Fig. 2. shows the TEM images

of functionalized CNTs (Fig. 2a), CNT networks with an ALD

coating of TiOx (Fig. 2b), and CNT networks with a solgel

coating of TiOx (Fig. 2c). All the CNTs are multi-walled and

have a thickness of 510 nm (Fig. 2a). The CNT networks with

an ALD coating of TiOx are fabricated by a thermal ALD sys-

tem based on TDMAT and H2O gas and CNT networks with

a solgel coating of TiOx are fabricated by spin coating of a tet-

ra isopropoxide precursor solution in an air atmosphere,

respectively [18]. After the ALD coating of TiOx, the outer

Fig. 1 (a) Schematics of TiOx conformally coated CNT network

energy level diagrams in this works.) 4 4 8 3 4 4 8 8 4485walls of the CNTs are coated uniformly and conformally with

a 10 nm thick layer of TiOx (Fig. 2b). The thickness of the ALD

coating of TiOx was also confirmed by a control experiment

on Si wafer (200 cycles: 0.5 A per cycle, Fig. S2). In the case

of the solgel coating of TiOx on the CNT networks, the coat-

ing does not cover individual CNTs conformally; furthermore,

the overall thickness varies even though the layer of TiOxspin-coated from the solgel solution is also 10 nm on a flat

Si surface. Most of the solgel TiOx fills in the pores between

the CNT networks; as a result, surface area of CNT networks

is decreased (Fig. 2c). This outcome, as shown in Fig. 2d, is in

good agreement with the root mean square (RMS) roughness

as measured by AFM (noncontact mode). The nanoporous

structure of the CNT network (RMS roughness: 13.15 nm) re-

mains almost the same after the ALD of a 10 nm thick coating

of TiOx but forms smooth surfaces (RMS roughness: 7.09 nm)

after the solgel deposition of a 10 nm thick coating of TiOx.

Furthermore, the overall coating of TiOx from the solgel solu-

tion is inhomogeneous. The actual thickness of TiOx on pro-

truded CNTs at the edge of the TiOx layer is much thinner

than the average coating thickness of the film (inset image

of Fig. 2c). ALD is superior to solgel in terms of the overall

uniformity and conformability of the TiOx coating on CNT

networks. It can also make the nanoporous structure of

TiOx-coated CNT networks suitable for effective charge collec-

tion in inverted OPVCs.

For infiltrating the active materials into TiOx-coated CNTs,

we modified surface of TiOx-coated CNTs by amphiphilic

s as electron transport layer in inverted OPVC and (b) their

1 24486 C A R B O N 5 0 ( 2 0RuLL 0(NCS)2 (L = 2,20-bipyridyl-4,4 0-dicarboxylic acid; L 0 = 4,4 0-

dinonyl-2,2 0-bipyridine) (Z907) dye to enhance the wetting be-

tween TiOx-coated CNTs and active materials of P3HT/PCBM

composites [28]. The P3HT/PCBM composites obviously infil-

trate into the TiOx-coated CNT networks after surface modifi-

cation. (Detailed information of infiltration of active materials

into TiOx-coated CNT networks is shown in Figs. S3 and S4.)

Fig. 3a shows the currentvoltage (JV) characteristics of

the inverted OPVCs under 1 sun illumination (AM 1.5G condi-

tion) and Table S1 provides detailed information on the photo-

voltaic parameters. The PCE of an inverted OPVC with

TiOx-coated CNT networks on ITO is 1.13%. That value repre-

sents an increase of about 30% over the PCE value of 0.85% ob-

tained in an inverted OPVC with TiOx-coated ITO. This

improvement is mainly due to the nanoporous TiOx-coated

CNT networks and the fact that highly conductive CNTs cause

a decrease in the series resistance (Rs) of inverted OPVCs

(85.4! 18.9 O cm2). Further PCE improvement can be achievedby adding a hole transport layer (namely a 1 nm thick MoO3

Fig. 2 TEM images of (a) functionalized CNTs, (b) TiOx-

coated CNTs by ALD, (c) TiOx-coated CNTs by spin coating of

solgel solution (inset image is protruded CNTs at the edge

of TiOx layer) on Lacey TEM grids and (d) surface height

images (AFM, non-contact mode) of ITO (RMS roughness:

4.89 nm), CNT networks on ITO (RMS roughness: 13.15 nm),

TiOx on CNT networks by ALD (RMS roughness: 13.17 nm)

and TiOx on CNT networks by spin coating of solgel

solution (RMS roughness: 7.09 nm) (all coating thickness of

TiOx is about 10 nm).) 4 4 8 3 4 4 8 8layer) between the active layer and the Au electrode by ther-

mal evaporation of powder typed MoO3 source. MoO3 serves

as a hole transport material and results in enhancing the

PCE of inverted OPVCs. The maximum PCE of an inverted

OPVC with a 10 nm thick coating of TiOx on CNT networks is

about 2.54% (Fig. 3b and Table S2). However, the inverted

OPVCs that use CNT networks without a TiOx coating or a

solgel coating manifest ohmic behaviors and have a very

Fig. 3 JV characteristics of inverted OPVC (a) under 1 sun

illumination, (b) by adding a 1 nm thick MoO3 between

photo-active layer and Au electrode under 1 sun

illumination, (c) without illumination (black line: TiOx, red

line: TiOx-coated CNT networks by ALD, blue line:

TiOx-coated CNT networks by solgel, cyan line: CNT

network without TiOx coating). (For interpretation of the

references to color in this figure legend, the reader is

referred to the web version of this article.)

ing of TiOx on CNT networks and Table 1 gives a summary of

their photovoltaic parameters. A thick TiOx layer can improve

the transport of electrons (through a decrease of Rs) and selec-

tively block the transport of holes (through an increase of

shunt resistance, Rsh) from a photo-active layer to the ITO

electrode. This behavior, as shown in Fig. 4a, produces a high-

er PCE. The PCE increases as the thickness of the TiOx in-

creases because a thicker coating of TiOx tends to block the

holes better. When the thickness of TiOx exceeds 10 nm, the

PCE is saturated or slightly decreased on account of the in-

crease in Rs. TiOx-coated CNT networks behave in a similar

manner, depending on the thickness of the TiOx (Fig. 4b). As

expected from the high electrical conductivity of CNT net-

works, the Rs of OPVCs with CNT networks is much lower

C A R B O N 5 0 ( 2 0 1 2 ) 4 4 8 3 4 4 8 8 4487low PCE. The OPVCwithout TiOx coating exhibits large leakage

currents (Fig. 3c) due to the low charge selectivity of CNTs.

CNT networks coated with TiOx from a solgel solution also

behave in a similar manner due to the inhomogeneous cover-

age of TiOx. Thus, the conformal ALD coating of TiOx on CNT

networks appears to be an effective method of preventing

large leakage currents from the CNT networks in OPVCs.

Inverted OPVCs with ALD coatings of TiOx of varying thick-

nesses (1, 5, 10, and 15 nm) are tested to clarify how TiOx af-

fects the photovoltaic property of inverted OPVCs. Fig. 4

shows the JV characteristics of an inverted OPVC under 1

sun illumination in relation to the thickness of the ALD coat-

with an ALD coating of TiOx for OPVCs. Our promising results

Fig. 4 JV characteristics of inverted OPVCs (a) without, (b)

with CNT networks under 1 sun illumination depend on

thickness of TiOx by ALD.

Table 1 Summary of photovoltaic parameters depend on thick

TiOx1 nm

TiOx5 nm

TiOx10 nm

TiO15

PCE (%) 0.42 0.73 0.85 0.7JSC (mA/cm

2) 4.67 4.70 5.22 5.0VOC (V) 0.25 0.37 0.38 0.3FF 0.36 0.42 0.43 0.4*Rs (O cm

2) 547.4 118.3 85.4 113.0*Rsh (O cm

2) 6.56 103 4.00 105 1.95 105 5.5*Rs and *Rsh are derived from the slope of the JV characteristic curve u

[29,30].suggest that CNT networks with an ALD coating of TiOx can

be used as electron transport and hole blocking layer and

thereby improve the performance of OPVCs.

ness of TiOx by ALD.

x

nmTiOx1 nm onCNTnetworks

TiOx5 nm onCNTnetworks

TiOx10 nm onCNTnetworks

TiOx15 nm onCNT networks

5 0.24 0.69 1.13 0.926 4.40 4.45 6.20 5.566 0.19 0.37 0.38 0.361 0.29 0.42 0.48 0.46

28.8 85.2 18.9 29.60 106 88.2 2.1 103 1.0 104 2.7 105than that of OPVCs without CNT networks (Table 1). The TiOxthickness of about 10 nm is optimal for achieving the highest

PCE value from TiOx-coated CNT networks in inverted OPVCs.

4. Summary

We studied the effect of an ALD coating of TiOx on CNT net-

works for inverted OPVCs. A comparison with solgel deposi-

tion confirms that ALD is a superior method to fabricate the

TiOx uniformly coated CNT networks while preserving the

nanoporous structure of the CNT networks. The PCE of in-

verted OPVCs that use CNT networks with an ALD coating

of TiOx is enhanced by 30% on account of the increase in sur-

face area and high conductive of CNT networks. The maxi-

mum PCE of an inverted OPVC is about 2.54% at the

addition of 1 nm thick MoO3 as an hole transport layer. How-

ever, an inhomogeneous or thin coating of TiOx on CNT net-

works causes leakage currents, which tend to lower the PCE

of inverted OPVCs. The PCE of inverted OPVCs gradually in-

creases as the thickness of TiOx increases up to 10 nm. The

higher PCE value is due to better electron transport and hole

blocking. In the case of inverted OPVCs with TiOx-coated

CNT networks, a TiOx coating of more than 10 nm in thick-

ness is required to prevent any large leakage currents. This

study is the first to fabricate and investigate CNT networksnder dark conditions close to voltage and current axis, respectively

Acknowledgements

This research was supported by the Nano R&D program

through theKorea Science andEngineering Foundation funded

[7] Ma W, Yang C, Heeger AJ. Spatial fourier-transform analysis

[8] Yao Z, Kane CL, Dekker C. High-field electrical transport insingle-wall carbon nanotubes. Phys Rev Lett 2000;84:29414.

[13] Rowell MW, Topinka MA, McGehee MD, Prall H-J, Dennler G,Sariciftci NS, et al. Organic solar cells with carbon nanotubenetwork electrodes. Appl Phys Lett 2006;88:233506-13.

[14] Chaudhary S, Lu H, Muller AM, Bardeen CJ, Ozkan M.

4488 C A R B O N 5 0 ( 2 0 1 2 ) 4 4 8 3 4 4 8 8[9] Durkop T, Getty SA, Cobas E, Fuhrer MS. Extraordinarymobility in semiconducting carbon nanotubes. Nano Lett2004;4:359.

[10] Kymakis E, Amaratunga GA. Single-wall carbon nanotube/conjugated polymer photovoltaic devices. J Appl Phys Lett2002;80:1124.

[11] Kymakis E, Koudoumas E, Franghiadakis I, Amaratunga GAJ.Post-fabrication annealing effects in polymernanotubephotovoltaic cells. J Phys D: Appl Phys 2006;39:105862.

[12] Pasquier AD, Unalan HE, Kanwal A, Miller S, Chhowalla M.Conducting and transparent single-wall carbon nanotubeelectrodes for polymerfullerene solar cells. Appl Phys Lett2005;87:203511-13.of the morphology of bulk heterojunction materials used inplastic solar cells. Adv Mater 2007;19:138790.by the Ministry of Science & Technology (2010-0019132). It was

also partially supported by the Converging Research Center

Program through the National Research Foundation of Korea

(NRF) funded by the Ministry of Education, Science, and Tech-

nology (2010 -1079000).

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/j.carbon.

2012.05.027.

R E F E R E N C E S

[1] Hoth CN, Choulis SA, Schilinsky P, Brabe CJ. High photovoltaicperformance of inkjet printed polymer: fullerene blends. AdvMater 2007;19:39738.

[2] Kim S-S, Na S-I, Jo J, Tae G, Kim D-Y. Efficient polymer solarcells fabricated by simple brush painting. Adv Mater2007;19:44105.

[3] Sndergaard R, Helgesen M, Jrgensen M, Krebs FC.Fabrication of polymer solar cells using aqueous processingfor all layers including the metal back electrode. Adv EnergyMater 2011;1:6871.

[4] Jrgensen M, Norrman K, Krebs FC. Stability/degradation ofpolymer solar cells. Sol Energy Mater & Sol Cells2008;92:686714.

[5] Moon JS, Lee JK, Cho S, Byun J, Heeger AJ. Columnlikestructure of the cross-sectional morphology of bulkheterojunction materials. Nano Lett 2009;9:2304.

[6] Ma W, Yang C, Gong X, Lee K, Heeger AJ. Thermally stable,efficient polymer solar cells with nanoscale control of theinterpenetrating network morphology. Adv Funct Mater2005;15:161722.Hierarchical placement and associated optoelectronic impactof carbon nanotubes in polymerfullerene solar cells. NanoLett 2007;7:19739.

[15] Dresselhaus MS, Dresselhaus G, Saito R, Jorio A. Ramanspectroscopy of carbon nanotubes. Phys Rep 2005;409:4799.

[16] Liu L, Stanchina WE, Li G. Effects of semiconducting andmetallic single-walled carbon nanotubes on performance ofbulk heterojunction organic solar cells. Appl Phys Lett2009;94:233309-13.

[17] Kim JY, Kim SH, Lee H-H, Lee K, Ma W, Gong X, et al. Newarchitecture for high-efficiency polymer photovoltaic cellsusing solution-based titanium oxide as an optical spacer. AdvMater 2006;18:5726.

[18] Park SH, Roy A, Beaupre S, Cho S, Coates N, Moon JS, et al.Bulk heterojunction solar cells with internal quantumefficiency approaching 100%. Nat Photon 2009;3:297303.

[19] Lee JH, Cho S, Roy A, Jung H-T, Heeger AJ. Enhanced diodecharacteristics of organic solar cells using titanium suboxideelectron transport layer. Appl Phys Lett 2010;96:163303-13.

[20] George SM. Atomic layer deposition: an overview. Chem Rev2010;110:11131.

[21] Kim JB, Kim CS, Kim YS, Loo Y-L. Oxidation of silverelectrodes induces transition from conventional to invertedphotovoltaic characteristics in polymer solar cells. Appl PhysLett 2009;95:183301-13.

[22] Kim CS, Lee SS, Gomez ED, Kim JB, Loo Y-L. Transientphotovoltaic behavior of air-stable, inverted organic solarcells with solution-processed electron transport layer. ApplPhys Lett 2009;94:113302-13.

[23] Hau SK, Yip H-L, Baek NS, Zou J, OMalley K, Jen AK-Y. Air-stable inverted flexible polymer solar cells using zinc oxidenanoparticles as an electron selective layer. Appl Phys Lett2008;92:253301-13.

[24] Hu L, Hecht DS, Gruner G. Percolation in transparentconducting carbon nanotube networks. Nano Lett2004;4:25137.

[25] Zhang D, Ryu K, Liu X, Polikarpov E, Ly J, Tompson ME, et al.Transparent conductive and flexible carbon nanotube filmsand their application in organic light-emitting diodes. NanoLett 2006;6:18806.

[26] Landi BJ, Castro SL, Ruf HJ, Evans CM, Bailey SG, Raffaelle RP.CdSe quantum dot-single wall carbon nanotube complexesfor polymeric solar cells. Sol Energy Mater & Sol Cells2005;87:73346.

[27] Kymakis E, Alexandrou I, Amaratunga GAJ. High open-circuitvoltage photovoltaic devices from carbon-nanotubepolymercomposites. J Appl Phys 2003;93:17648.

[28] Hau SK, Yip H-L, Acton O, Baek NS, Maa H, Jen AKY.Interfacial modification to improve inverted polymer solarcells. J Mater Chem 2008;18:51139.

[29] Li G, Shrotriya V, Huang J, Yao Y, Moriaty T, Emery K, et al.High-efficiency solution processable polymer photovoltaiccells by self-organization of polymer blends. Nat Mater2005;4:8648.

[30] Shirland FA. The history, design, fabrication andperformance of cds thin film solar cells. Adv EnergyConversion 1966;6:20122.

Conformal coating of titanium suboxide on carbon nanotube networks by atomic layer deposition for inverted organic photovoltaic cells1 Introduction2 Experimental section2.1 Fabrication of CNT networks on ITO2.2 Fabrication of TiOx-coated CNT networks2.3 Fabrication of inverted OPVCs2.4 Characterizations of functionalized CNTs, TiOx-coated CNTs, and inverted OPVCs

3 Results and discussion4 SummaryAcknowledgementsAppendix A Supplementary dataReferences