Perovskite-kesterite monolithic tandem solar cells with high open-circuit voltageTeodor Todorov, Talia Gershon, Oki Gunawan, Charles Sturdevant, and Supratik Guha Citation: Applied Physics Letters 105, 173902 (2014); doi: 10.1063/1.4899275 View online: http://dx.doi.org/10.1063/1.4899275 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Suns-VOC characteristics of high performance kesterite solar cells J. Appl. Phys. 116, 084504 (2014); 10.1063/1.4893315 Band tailing and efficiency limitation in kesterite solar cells Appl. Phys. Lett. 103, 103506 (2013); 10.1063/1.4820250 Indications of short minority-carrier lifetime in kesterite solar cells J. Appl. Phys. 114, 084507 (2013); 10.1063/1.4819849 Admittance spectroscopy in kesterite solar cells: Defect signal or circuit response Appl. Phys. Lett. 102, 202105 (2013); 10.1063/1.4807585 Copper-phthalocyanine-based organic solar cells with high open-circuit voltage Appl. Phys. Lett. 86, 082106 (2005); 10.1063/1.1871347

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Perovskite-kesterite monolithic tandem solar cells with high open-circuitvoltage

Teodor Todorov, Talia Gershon, Oki Gunawan, Charles Sturdevant, and Supratik Guhaa)

IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA

(Received 5 September 2014; accepted 3 October 2014; published online 28 October 2014)

We report a monolithic tandem photovoltaic device with earth-abundant solution processed absorb-

ers. Kesterite Cu2ZnSn(S,Se)4 and perovskite CH3NH3PbI3 solar cells were fabricated monolithi-

cally on a single substrate without layer transfer. The resulting devices exhibited a high open

circuit voltage (Voc) of 1350 mV, close to the sum of single-absorber reference cells voltages and

outperforms any monolithic tandem chalcogenide device (including Cu(In,Ga)Se2) reported to

date. Ongoing optimization of several device elements including the severely limiting top contact

electrode is expected to yield superior currents and efficiency. Importantly, our device architecture

demonstrates the compatibility and synergistic potential of two of the most promising emerging

photovoltaic materials and provides a path for optimization towards >20% efficiency. VC 2014AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4899275]

Chalcogenide solar cells such as Cu(In,Ga)Se2 (CIGS)

and Cu2ZnSn(S,Se)4 (CZTSSe) can benefit from a partner

material suitable for a tandem photovoltaic device that would

allow broader spectrum light harvesting and enhanced effi-

ciency.1 Researchers have attempted different approaches in

the past including mechanical stacking of separate devices

with different band gap absorbers via external wiring of

CIGS-CdTe,2 CIGS-dye sensitized solar cell,3 CIGS-CGS4

and CIGS-CIGS5 cells with different ratios of In:Ga.

However, the use of multiple transparent conducting layers

with related losses due to transmission and series resistance

as well as the additional interconnections needed for external

wiring of the two separate solar cells limits the practicality

of these concepts. A preferred monolithic design on a single

substrate would theoretically be capable of delivering supe-

rior performance and lower fabrication cost. However, the

processing and materials constraints associated with the

preparation of highest-efficiency chalcogenide devices such

as elevated absorber crystal growth temperatures (above

400 �C), the rapid deterioration of the p-n junction above

220 �C and non-transparent Mo bottom contact have been a

significant barrier to the fabrication of efficient CIGS or

CZTSSe based monolithic tandem devices. One attempt

towards a CIGS-organic solar cell tandem via a sophisticated

layer transfer reached an efficiency of 3.8%.6 With earth-

abundant CZTSSe solar cells, neither monolithic nor mechani-

cally stacked tandem devices have been reported to date.

Recently, perovskite organo-lead halide solar cells have

made rapid progress with the demonstration of solar cells

with over 16% efficiency.7 The low processing temperatures

of metal-organic (MO) perovskite devices (<150 �C) and

tunable band gap from 1.4 to over 2 eV,8 pair ideally with

the lower band gap, and therefore higher short-circuit current

density (Jsc) of the CIGS and CZTSSe families of chalcoge-

nide solar cells for forming a tandem photovoltaic device.

The highest-efficiency CIGS and CZTSSe are achieved at

relatively low band gaps from about 1.1 to 1.25 eV, where

the sulfur content is low and/or the In:Ga ratio is high

(for CIGS). Concomitantly, the open circuit voltage (Voc)

deficit9 in CZTSSe is also significantly reduced when shift-

ing to lower sulfur contents or band gaps closer to 1 eV. At

the same time, perovskites have demonstrated their highest

efficiencies at band gaps of 1.55 eV with high Voc values of

1.1 V (Ref. 10) for NH3CH3PbIxCl3�x. The band gaps can

be further increased to �1.7–1.9 eV (Ref. 8) by Br or Cl

addition for improved current matching with the bottom cell

in tandem structures. The two materials systems (CZTSSe

and MO-perovskites) are, therefore, well matched both in

terms of opto-electronic properties as well as processing

compatibilities for integration in a monolithic tandem solar

cell architecture.

In this paper, we demonstrate the fabrication and opera-

tion of a monolithic CZTSSe-perovskite tandem device

structure consisting of Glass/Mo/Cu2ZnSn(S,Se)4/CdS/ITO/

PEDOT:PSS/NH3CH3PbI3/PCBM/Al, where PEDOT:PSS

(poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) and

PCBM ([6,6]-phenyl C61 butyric acid methyl ester) are organic

hole- and electron-conducting species, respectively, and are

commonly employed in organic photovoltaics.

The scope of this report is a proof of concept of the

chalcogenide-perovskite monolithic architecture and does

not focus on individual optimization of the different ele-

ments, e.g., band gap tuning of the absorbers or transparent

top contact engineering. It does however demonstrate a via-

ble fabrication sequence for producing a working monolithic

tandem device with cumulative Voc. Preliminary results and

modeling indicate that improved transparent conductive con-

tact on our current CZTSSe and perovskite devices (power

conversion efficiency g� 12% each) could produce tandem

devices with efficiency up to 16.0%. Further band gap opti-

mization and integration of state-of-the-art top perovskite

cell with >15% efficiency is projected to reach tandems with

over 20% efficiency.

The device (SEM cross-section shown in Fig. 1) was

built on a molybdenum coated soda-lime glass substrate and

a)Author to whom correspondence should be addressed. Electronic mail:

guha@us.ibm.com

0003-6951/2014/105(17)/173902/4/$30.00 VC 2014 AIP Publishing LLC105, 173902-1

APPLIED PHYSICS LETTERS 105, 173902 (2014)

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comprises a bottom CZTSSe-based solar cell, prepared by a

solution process described in detail previously.11,12 The

CZTSSe absorber used in the study was prepared via a pure

solution deposition approach, where the precursors dissolved

in hydrazine are spin-coated onto a Mo-coated soda lime

glass substrate and subsequently annealed to form a large-

grained kesterite layer with a band gap of approximately

1.1 eV (details in Ref. 13). CdS was then deposited by chem-

ical bath deposition, followed by a sputtered ITO top elec-

trode. On top of this completed device, we prepared a top

cell based on a perovskite absorber in a manner similar to

that described previously.14 A PEDOT:PSS layer was spun

directly on the top ITO contact of the completed CZTSSe

cell, and was then annealed in air at �140 �C to remove

excess water. The samples were then moved to a nitrogen-

filled glove box for subsequent processing. The

NH3CH3PbI3 perovskite absorber was prepared by adapting

the vapor-assisted solution process described by Chen15 con-

sisting of spin-coating a layer of PbI2 and then annealing it

on a hot plate in a vapor of NH3CH3I until conversion to the

perovskite phase was complete. A PCBM layer was then pre-

pared by spin-coating from solution in chlorobenzene on top

of the perovskite layer. A thin Al contact was then deposited

by thermal evaporation on top of the PCBM layer. The Al

thickness (�20–25 nm) was tuned to allow 30%–40% optical

transmission enabling device characterization (Fig. 2). As

we will discuss below, the poorly transparent and highly

resistive (sheet resistance> 200 X/sq) Al layer is the main

performance limiting factor, requiring further optimization

of the transparent top contact. Device area of 0.45 cm2 was

defined by mechanical scribing. We also fabricated separate

stand-alone CZTSSe (glass\Mo\CZTSSe\CdS\ZnO\ITO)

with e-beam evaporated Ni-Al grid contact and superstrate

perovskite devices (Glass/ITO/PEDOT:PSS/NH3CH3PbI3/

PCBM/Al) by identical procedures, with the exception of a

thick (>50 nm) Al contact, for comparison as shown in

Table I. We did not observe any evidence of optical or elec-

trical deterioration of the ITO layer in the superstrate cell as

is most likely the case with the ITO layer in the tandem

device.

A current density-voltage (J-V) curve of a typical stand-

alone CZTSSe device (single-cell) is shown in Figure 3

(blue), and the parameters are tabulated in Table I. The Voc

values of the reference CZTSSe devices prepared identically

as the ones used for the tandem structures had range between

460–490 mV. Overlaid with the CZTSSe J-V curve is a simi-

lar measurement performed on a stand-alone perovskite ref-

erence device fabricated on ITO-coated glass substrate (red)

with parameters tabulated in Table I. Due to the larger band

gap of this material, the Jsc value is smaller and the Voc is

larger (typically 850–960 mV for our devices) than the val-

ues corresponding to the CZTSSe device. From the Tauc

plot shown in Fig. 2(b), we estimate the band gap of the per-

ovskite layer as �1.58 eV. The band gap of our bottom

FIG. 1. Cross-sectional SEM image of a monolythic tandem CZTSSe/

perovskite solar cell structure identifying individual layers.

FIG. 2. (a) Relationship between the average transmission (over the range of

400–1200 nm) and sheet resistance for a given aluminum film thickness. The

Al layer used in our device is expected to be similar to the middle data point.

(b) Tauc plot generated from the transmission curve of the stand-alone per-

ovskite layer assuming the relationship I=I0 ¼ e�ax and a perovskite thick-

ness of �200 nm.

TABLE I. Device parameters of various devices at simulated 1 sun illumina-

tion. RS is the series resistance and n is the ideality factor extracted using

Lambert W function fitting method.16

g FF Voc Jsc RS nDevice % % V mA/cm2 X.cm2

CZTSSe 11.6 68.2 0.477 35.7 0.63 1.66

Perovskite 12.3 76.6 0.953 16.8 2.53 2.76

CZTSSe-shadoweda 4.5 64.4 0.452 15.5 0.66 1.85

Tandem-projectedb 16.0 73.6 1.401 15.5 3.20 4.41

Tandem 4.6 60.4 1.353 5.6 15.70 6.40

a“CZTSSe-shadowed” is a CZTSSe cell shadowed by a perovskite absorber.bThe “tandem-projected” cell is a theoretical estimate based on the CZTSSe-

shadowed cell and the perovskite cell.

FIG. 3. J-V measurements of stand-alone devices (CZTSSe with Ni-Al grid

contacts and a superstrate perovskite on ITO-coated glass), and a tandem

CZTSSe/perovskite device.

173902-2 Todorov et al. Appl. Phys. Lett. 105, 173902 (2014)

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164.76.102.52 On: Fri, 14 Nov 2014 10:07:54

CZTSSe layer is estimated to be �1.1 eV from quantum effi-

ciency measurement.

The J-V characteristics of the tandem CZTSSe/perovskite

device at 1 sun (red) is shown in Fig. 2 and tabulated in Table

I. We note that the Voc value of 1353 mV is nearly equal to

the sum of the individual Voc values of the CZTSSe and per-

ovskite devices, thus confirming that we have effectively

formed a series-connected, monolithic, two-terminal tandem

solar cell, and also confirming the integrated processing com-

patibility of these two materials systems (chalcogenide and

the perovskite) for tandem photovoltaic device fabrication.

The efficiency of our tandem device is still limited by

low current and fill factor with two main contributing ele-

ments. First is the significant optical transmission and reflec-

tion losses in the top Al layer17 (Fig. 2(a)) resulting in

extremely low Jsc. There is a tradeoff between optical trans-

parency and sheet resistance in thin metallic films such as

Al, where films with 30%–40% optical transmission

still have sheet resistances that are too high for optimal per-

formance (>200 X/sq, Fig. 2); this is in contrast to >90%

transmission for indium tin oxide (ITO) with sheet resistance

of <10 X/sq which can be achieved by sputtering. We have

used only Al as the top electrode in these feasibility studies

due to the risk of sputter damage associated with standard

ITO deposition. Future strategies for less aggressive top con-

tact fabrication could be evaporated ITO or solution-

processed alternatives such as silver nanowires.18

The tradeoff between transmittance and conductivity

dictates the additional limitation due to high series resistance

in the tandem cell (15.7 Xcm2) which is significantly higher

than the sum of the individual CZTSSe and perovskite cell.

While contribution of other device elements to the high se-

ries resistance cannot be ruled out at this point, the combina-

tion of drastically reduced illumination in all layers and high

sheet resistance of the Al contact are major contributors to

the low current and fill factor. Optimized top contact fabrica-

tion is currently under development and significant device

performance improvement is expected by enhancing the

transmission and conductivity of this element alone.

In order to assess the effect of the sub-optimal Al trans-

parent electrode and the related performance enhancement

potential, we estimate the upper limit of the efficiency of

our tandem cell based on the individual characteristics of

the stand-alone CZTSSe (g¼ 11.6%) and perovskite

(g¼ 12.3%) cells. We assume that we can have a high

quality transparent conductor on our perovsite top cell (e.g.,

ITO with 90% transmission and sheet resistance of �10 X/

sq) as in our standalone superstrate perovskite devices built

on ITO-coated glass.

To obtain a realistic estimate of the limit in current, we

measure the performance of the stand-alone CZTSSe cell

shadowed by the stand-alone perovskite, labeled as “CZTSSe-

shadowed” (see Table I). We observe a lower Jsc of

15.5 mA/cm2 (and g¼ 4.5%) as expected due to shadowing

(transmission, reflection, and absorption) losses of the top per-

ovskite device. A rough estimate of this shadowing test using

the unshadowed Jsc multiplied by the fraction of the photon

flux in the AM1.5 G spectrum in between the top and bottom

bandgap yields a consistent result of 14.6 mA/cm2. We then

combine the characteristics of the perovskite and the

“CZTSSe-shadowed” device in series to obtain the “tandem-

projected” device characteristics as shown in Table I and plot-

ted in Fig. 3 (dashed-red). In tandem device, we assume that

each individual device maintains the same diode characteris-

tics (ideality factor n, reverse saturation current J0, series

resistance RS, and shunt conductance GS) and the current is

limited by the cell with lower current. The “tandem-

projected” device shows a high efficiency of 16.0%, signifi-

cantly higher than both individual CZTSSe and perovskite

cell pointing to the high potential of this CZTSSe-perovskite

tandem device.

Optimization of the CZTSSe and perovskite bandgaps

for enhanced light harvesting and better current matching

provides another route for efficiency improvement. Figure

4(a) shows a projected calculation of a tandem CZTSSe and

perovskite devices with bandgap ranges of 1.0–1.5 eV and

1.5–2.2 eV, respectively. Each of the devices is assumed to

have a fixed efficiency (e.g., 12% for the CZTSSe and 12%

or 15% for the perovskite)—with device characteristics

(Voc, Jsc, FF, n, J0, RS, GS) based on parameter values that

our devices currently exhibit. We assume the Jsc and Voc of

each device scales proportionally to the bandgap and main-

tains the same efficiency, variable transmission to the bot-

tom cell due to shadowing loss of the top cell and current

matching condition for a series-connected tandem cell. The

transmission to the bottom cell is estimated based on the

fraction of the photon flux in the AM1.5 G spectrum with

energy in between the top and bottom cell bandgap, which

is consistent with the experimental shadowing test as shown

in Table I.

FIG. 4. The efficiency projection of

the CZTSSe/perovskite tandem solar

cells for 1 sun AM1.5 G spectrum with

varying bandgaps based on 12%

CZTSSe and (a) 12% perovskites (b)

15% perovskites devices.

173902-3 Todorov et al. Appl. Phys. Lett. 105, 173902 (2014)

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For 12% CZTSSe and perovskites (Fig. 4(a)), we

observe that an optimum tandem efficiencies (�17%) are

obtained with bandgap combinations stretching almost line-

arly from Eg,BOTTOM and Eg, TOP¼ 1 and 1.65 eV to 1.5 and

2 eV, respectively. For our existing device configuration of

Eg¼ 1.1 eV CZTSSe and Eg¼ 1.58 eV perovskite, a large

efficiency gain can be obtained by improving the perovskite

bandgap to 1.70 eV while keeping the same CZTSSe

bandgap. Note that the current CZTSSe bandgap of 1.1 eV is

close to the empirical optimum bandgap in CZTSSe

(�1.13 eV).12 An example of similar calculation based on

12% CZTSSe and 15% perovskite is shown in Fig. 4(b),

which suggests optimum bottom and top bandgap combina-

tion stretching from 1 and 1.75 eV to 1.5 and 2.1 eV with a

maximum attainable efficiency of over 20%.

We have demonstrated a monolithic tandem photovoltaic

device architecture with solution processed kesterite and per-

ovskite absorbers. To date, this is the most efficient tandem

structure grown on a chalcogenide bottom device of any type,

despite the severely limiting Al top contact with low transmis-

sion and high resistivity. Optimization of the transparent

conductive electrode alone is expected to allow efficiency

enhancement of up to 16.0% with our current devices. Further

improvement potential with band gap engineering is projected.

We thank Professor Franky So and his group,

University of Florida, for helpful advice regarding

perovskite solar cell fabrication methods, Harry Hovel,

Ning Li, Ali Afzali, George Tulevski, and Jeehwan Kim

for useful discussion, Jay S. Chey, Ravin Mankad, and

Liang-yi Chang for their help with sample fabrication and

equipment operations.

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