Monolithic Wide Band Gap Perovskite/ Perovskite

Tandem Solar Cells with Organic Recombination

Layers

Rui Shenga†, Maximilian T. Hörantnerb†, Zhiping Wangb, Yajie Jianga, Wei Zhang,c,

Amedeo Agostid, Shujuan Huanga, Xiaojing Haoa, Anita Ho-Bailliea*, Martin Greena, Henry

J. Snaithb*

aAustralian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and

Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia

bDepartment of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, United

Kingdom

cAdvanced Technology Institute, University of Surrey, Guildford GU2 7XH, UK

dDepartment of Materials, ETH Zurich, CH-8093 Zurich, Switzerland

† These authors contribute equally to this work

*Corresponding author: henry.snaith@physics.ox.ac.uk , a.ho-baillie@unsw.edu.au

Abstract

We demonstrate a monolithic tandem solar cell by sequentially depositing a higher-bandgap (2.3 eV)

CH3NH3PbBr3 sub-cell and a lower-bandgap (1.55 eV) CH3NH3PbI3 sub-cell bandgap perovskite

cells, in conjugation with a solution-processed organic charge carrier recombination layer, which

serves to protect the underlying sub-cell and allows for voltage addition of the two sub-cells. Owing

to the low-loss series connection, we achieve a large open-circuit voltage of 1.96 V. Through optical

and electronic modelling, we estimate the feasible efficiency of this device architecture to be 25.9 %,

achievable with integrating a best-in-class CH3NH3PbI3 sub cell and a 2.05 eV wide bandgap

perovskite cell with an optimised optical structure. Compared to previous reported all-perovskite

tandem cells, we solely employ Pb-based perovskites, which although have wider band gap than Sn

based perovskites, are not at risk of instability due to the unstable charge state of the Sn 2+ ion.

Additionally, the bandgap combination we use in this study could be an advantage for triple junction

cells on top of silicon. Our findings indicate that wide band gap all-perovskite tandems could be a

feasible device structure for higher efficiency perovskite thin-film solar cells.

Introduction

Methylammonium lead halide perovskite solar cells have attracted enormous research

interest since the seminal work of CH3NH3PbX3 (X=Br, I) perovskite-sensitized solar cells

reported by Kojima et al. in 2009, and the demonstration of highly efficient solid-state

perovskite solar cells in 20121,2. Tremendous power conversion efficiency enhancements

have been achieved since then3-8, 42. The improved photovoltaic performance has been

attributed to excellent optical properties9, long carrier diffusion lengths 10-13, and high light

emission yields14. Apart from improving the properties of the perovskite solar cell, overall

efficiency gains can be achieved by moving towards multiple junctions, where a series of

solar cells that absorb in different regions of the solar spectrum are stacked on top of each

other. By this means, more energy is extracted from the sun light, which raises the sealing on

the ultimate efficiency achievable once fully optimised. Much work thus far has focused on

developing “hybrid” tandem cells by combining perovskite with established photovoltaics

like silicon. However, compared with the tandem cell structure with a high efficiency silicon

bottom cell and a perovskite top cell15-19, the combination of two large band gap perovskite

cells is capable of providing much higher voltage, which could be very interesting for not just

solar photovoltaic electricity generation, but also enabling direct power conversion from solar

energy to chemical energy such as solar fuels20,21. There are many aspects requiring

optimisation, in order to enable a multi-junction solar cell to operate efficiently. One of which

is the recombination layer between the junctions, which serves both as charge recombination

centre and as a protection layer for the underlying cell during the deposition of second

junction. Many materials have been proposed as the recombination layer for the

perovskite/silicon tandem architecture15,19 and for perovskite/perovskite tandem structures. 20

Recently, Eperon et al. demonstrated that efficient 2-terminal all-perovskite tandem solar

cells could be fabricated by including a dense sputter-coated indium oxide tin oxide (ITO)

layer as the intermediate recombination electrode, which also served to protect the underlying

cell from subsequent processing steps22. However, sputter coating, which is a relatively

aggressive deposition technique, upon the sensitive perovskite cells requires the introduction

of multiple additional “buffer” layers in order to protect the underlying perovskite cells from

sputter damage, and prevent the potential effect of sputter induced unintentional heating. In

addition, the thick ITO layer, with a lower refractive index to the perovskite layers, is likely

to introduce optical interference loses which are non-ideal for maximising the efficiency of a

tandem solar cell. It would be desirable if a printable recombination layer could be developed

which obviated the requirement for the sputter coated ITO. Additionally, we use two

complementary Pb based absorbers which do not suffer from the same oxygen sensitivities of

the lower band gap Sn based perovskite absorbers.

In this study, we introduce a bi-layer of C60 and PEDOT:PSS as the recombination junction,

and demonstrate a device structure of FTO/ compact TiO2/ mesoporous TiO2/ CH3NH3PbBr3/

Spiro-OMeTAD/ PEDOT: PSS/ C60/ CH3NH3PbI3/ Spiro-OMeTAD/ Au, and achieved a high

open circuit voltage of up to 1.96 V. We subsequently perform combined optical and

electronic modelling to reveal where the present major optical loses exist, and identify a

precise device stack which could lead to tandem open-circuit voltages of up to 2.69 V, and

solar cell efficiencies of up to 25.9 %, with realistic device characteristics for the top and

bottom cells.

Results and discussion

Figure 1(a) shows the energy band diagram of the proposed tandem cells. Both top cell and

bottom cell have the regular n-i-p structure with Spiro-OMeTAD as the hole transport

material (HTM). Here we define the cell first receiving the light as the top cell, and the

bottom cell is the second cell to be processed, which is subsequently capped with a metallic

Au electrode. The primary criteria for a recombination layer is for it to have an appropriate

work function to contact the p-type side of one cell and the n-type side of the other cell. In

our current device structure the holes generated from CH3NH3PbBr3 cell and electrons from

CH3NH3PbI3 cell must recombine in the recombination layer. Therefore either a highly doped

“metallic” recombination layer is required, or a layer consisting of a highly doped p- and n-

type tunnel-junction. Another requirement for the recombination layer for all-perovskite

tandem cells is that we do not subject it to excessive thermal treatment, otherwise accelerated

degradation of the underlying bromide cell could occur. As a solution processed

recombination layer, orthogonal solvents are also required for each adjacent layer. We

employ Spiro-OMeTAD as the first p-type layer to transport holes from the bromide top cell,

and C60 as the n-type electron transporter extracting electrons from the iodine bottom cell.

For the recombination layer we simply employ a thin and pinhole-free layer of PEDOT:PSS.

Figure 1. (a) Schematic energy band diagram and (b) device architecture of perovskite/

perovskite tandem structure.

We show the complete tandem device architecture in Figure 1b, and a schematic of the

energy levels of the materials we employ in Figure 1a. For cell fabrication we spin-coated a

compact layer of TiO2 on patterned fluorine doped tin oxide (FTO) substrates, followed by a

thin layer of mesoporous TiO2. Subsequently, we fabricated the CH3NH3PbBr3 film by the

vapour-assisted solution-processing method within the mesoporous structure12,23: We first

spin-coated a PbBr2 solution in DMF on top of mesoporous TiO2. We then exposed the

annealed PbBr2 film to a CH3NH3Br vapour (crucible temperature 175 ⁰C) for 10 min in dry

air. After the bromide cell perovskite deposition we spin-coated Spiro-OMeTAD. For the

conductive charge recombination layer we diluted the PEDOT:PSS (PH1000, Heraeus) in

isopropanol (IPA) and spin-coated the film, followed by a 120 ᵒC anneal for ten minutes in

air. We subsequently deposited the C60 in dichlorobenzene by spin-coating. To increase the

wettability, and also the conductivity of C60, we added 10 mg/ml 4-(1,3-dimethyl-2,3-

dihydro-1H-benzimidazol-2-yl)-N,N-diphenylaniline (N-DPBI) (in dichlorobenzene) to the

C60 solution as an n-dopant, with a concentration of 10 µl/ml24. Encouragingly, we observed

no evidence of the Spiro-OMeTAD being dissolved by the subsequent C60 deposition process,

This implies that the PEDOT:PSS layer is suitably dense and pin-hole free so as to protect the

underlying cell. We also note that this structure is not compatible with solution processing of

the CH3NH3PbI3 film that involves the use of solvent Dimethylformamide (DMF), which we

found dissolves the underlying perovskite structure, indicating that DMF can permeate

through this multi-layer device stack. To overcome this issue we employed a sequential

vapour-solution processing method whereby we deposited a CH3NH3PbI3 film by thermal

evaporation of the PbI2 and then converted to perovskite via inter-diffusion of MAI from an

isopropanol solution25. Although perovskite films can be fabricated via this method, due to

the limited penetration of CH3NH3I, thick PbI2 films results in unreacted residual PbI2.

Therefore we found that the optimized PbI2 thickness for maximizing the single junction

device performance was only 115 nm, leading to ~200 nm thick final perovskite film.

Following this second perovskite layer deposition, we spin-coated the final Spiro-OMeTAD

film and capped the devices with a gold electrode.

We show a scanning electron microscope (SEM) cross-sectional image in Figure 2a of

CH3NH3PbBr3/CH3NH3PbI3 tandem solar cell, with a complete layered structure (with layer

thickness’ in brackets) of FTO (300nm) compact TiO2 (40nm), mesoporous TiO2 (350nm),

CH3NH3PbBr3 (350nm), Spiro-OMeTAD (185nm), PEDOT: PSS (80nm), C60 (30nm),

CH3NH3PbI3 (200nm), Spiro-OMeTAD (250nm) and gold (100nm). Notably, we observe a

rather rough but fully covered C60 film. This may be due to the poor wettability of C60

solution on the PEDOT: PSS film.

Figure 2. (a) Cross-sectional SEM image of tandem cell and (b) JV curves for individual

cell and monolithic tandem. (c) Stabilized efficiency and current output of tandem cell.

We show the J-V curves and photovoltaic performance parameters of the fabricated

individual cells and tandem cells in Figure 2b and in Table 1. The structure for single

junction iodine cell is glass/ FTO/ compact TiO2/ CH3NH3PbI3/ Spiro/ Au. For high band gap

single junction cell is glass/ FTO/ compact TiO2/ mesoporous TiO2/ CH3NH3PbBr3/ Spiro/

Au. We measure an open-circuit voltage of over 1.4 V for the single junction bromide cell,

which as others have observed can be attributed to the large bandgap of CH3NH3PbBr3. We

observe a significantly boosted open-circuit voltage for the tandem solar cell, up to 1.96 V.

However, this tandem cell open-circuit voltage is not the linear sum of both sub cells, which

should be up to 2.44 V, indicating that we have introduced some additional voltage losses in

the tandem cell construction or in the recombination layer. In addition, the J-V curve of

tandem device shows a low fill factor, also indicative of resistive losses in the recombination.

In Figure 2c, we show the power output measured over time for the tandem cell. We observe

that the efficiency of the tandem cell take up to 500 s before it reaches its stabilized value of

5.9%. We did not observe this severe light soaking in either of the single junction iodine and

bromide cells. This indicates that some “light healing” effect is occurring26. Previous study

suggested that the C60 layer would be partially washed away after perovskite deposition,

which would result in inhomogeneous/discontinuous morphology38. We therefore postulate

that we have some regions of direct contact of the perovskite to the PEDOT:PSS

recombination layer through pin-holes in the C60. These regions will induce rapid surface

recombination in the CH3NH3PbI3 layer being detrimental to open-circuit voltage. Upon light

soaking, defects at this interface, through which the surface recombination occurs, may

become temporarily passivated through ion migration.

Table 1. Photovoltaic performance of single junction and tandem cell.

Scan sirection

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

CH3NH3PbI3Reverse 1.04 17.30 71 12.7Forward 1.04 17.66 61 11.1

CH3NH3PbBr3Reverse 1.41 6.16 64 5.2Forward 1.35 5.81 55 4.4

Monolithic tandem

Reverse 1.96 6.40 41 5.1Forward 1.87 6.00 40 4.6

We have constructed working all-perovskite tandem solar cells, however, their operation is

non-optimum and significant improvement is required in order to make them competitive and

eventually superior to the best single junction perovskite cells. However, it is valuable to

understand where the current losses are, and how efficient these wide-band-gap all-perovskite

tandem cells could become, once fully optimized.

For this purpose, we firstly estimate the optical losses in our tandem device stack, via

performing an optical transfer matrix model. We account for the roughness of each layer by

employing an effective medium approximation between the adjacent layers.

We extract the optical constants, refractive index n and extinction coefficient k, of each

individual layer by fitting ellipsometry data as well as transmission spectra over the

wavelength range of interest. We show the measured and modelled transmittance (T) of the

top bromide cell and recombination layer in Figure S2, which show good agreement between

the experimental and modelled data with the discrepancy attributable to the non-uniformity of

the mesoporous layer and the roughness of C60 layer, see Figure 2a for cross-sectional SEM

image, which propagates throughout the whole structure. This fitting also validates the optical

model36, 37.

We used the determined optical constants and thickness of each layer to calculate the total

reflection and absorption occurring within each layer of the device27. Under the assumption

that 100% IQE (Internal Quantum Efficiency) across the measured spectrum could be

achieved28,29, we determined the External Quantum Efficiency (EQE) by assuming all light

absorbed within each perovskite layer leads to photocurrent generation.

In order to estimate the current-voltage characteristic of each sub cell we employed the

detailed balance limit theory, taking into account parasitic power consuming parameters from

an equivalent one-diode circuit model, such as series resistance, shunt resistance, ideality

factor and reverse saturation current30,31. Specifically for determining these device parameters,

we extracted the shunt and series resistances, ideality factor and reverse saturation current by

fitting published experimental data from the best in class 1.55 eV band gap perovskite cells32.

We show the extracted diode parameters in Table S3 of the SI. We calculated the reverse

saturation dark current via integrating our calculated EQE spectrum over the black body

spectrum at 300K, following methods reported in the literature33.

In Figure 3a we present our modeled external quantum efficiency spectrum of the device

stack we used experimentally, which we calculate assuming 100% internal quantum

efficiency for light absorbed in each perovskite junction. It is quite apparent that there are

many optical losses in the region of the spectrum. Because PEDOT:PSS does absorb a

fraction of the solar spectrum itself. As a final optimisation, we reduced the PEDOT:PSS

thickness to zero, and allowed all layer thickness to vary. Although this seems unrealistic, it

is equivalent to swapping PEDOT:PSS for a more transparent highly doped organic

semiconductor, which for instance Bolink et al 47. have shown to work very well in all-

evaporated perovskite multi-junction cells. We fix the band gap of the rear cell to that of

MAPbI3, shift the band gap of the top cell in 0.1eV steps and allow each structure to

optimise. For calculating the efficiency of the MAPbBr3 cell, we now apply the diode

characteristics of the best-in-class lower gap perovskite cell, to estimate the ultimate

efficiency feasible once these wider gap cells can be optimised to work comparably to the

best lower gap cells. From our calculations, the band gap of the top cell which delivers the

overall highest tandem efficiency is 2.05 eV. We note that the band gap of the MAPbBr3 and

FAPbBr3 films are 2.3 eV 12 and 2.25 eV 44. Therefore, there is a requirement to drop this

band gap 2.05eV in order to optimise a top-cell for a wide band gap all-perovskite tandem,

employing only Pb based perovskites. We show the EQE of the device stack with optimum

layer thickness in Figure 3c, thickness FTO (200 nm), compact TiO2 (33 nm), CH3NH3PbBr3

(1280 nm), Spiro-OMeTAD (140 nm), C60 (44 nm), CH3NH3PbI3 (1488 nm), Spiro-OMeTAD

(141 nm) and gold (100 nm). In Figure 3b and Table 2, we show the simulated cell

performance parameters for the top, bottom and tandem junctions for all the calculated

devices. It is apparent that in optimized stack, the maximum short-circuit current density (Jsc)

we could obtain from the front cell is 11.6 mA/cm2, and the maximum Jsc from the rear cell is

12.2 mA/cm2. For the tandem cell, the maximum VOC achievable is 2.69 V, with a

corresponding power conversion efficiency of 25.9 %. Our simulated results reveal that, by

increasing the thickness of CH3NH3PbBr3 capping layer to 395 nm, and CH3NH3PbI3 layer to

313 nm, the tandem structure can reach the maximum output.

A specific challenge with fine-tuning of the band gap for perovskite solar cells is that

this relies on precise compositional control of mixed halides. The Br and I compositions can

segregate in the perovskite layer, which can lead to reduction in the open-circuit voltage 45.

Although this can be suppressed for cells with band gaps around 1.7eV, halide segregation is

especially problematic for high Br contents in the I-Br mixed halide perovskites 46. Here, we

therefor estimate the efficiency achievable in a Pb-perovskite-on-Pb-Perovskite tandem solar

cell, if the band gap of the wide band gap cell has to remain at the band gap of the neat Br

perovskite, FAPbBr3, which is 2.25eV. If we combine the 2.25 eV band gap with a 1.55 eV

rear cell, we determine a maximum efficiency of 21.3%, with Jsc of 8.37 mA/cm2 and VOC of

2.9 V. If we allow the rear-cell band gap to widen, to satisfy current matching conditions, we

combine the 2.25 eV cell with a 1.80 eV rear cell, and determine an efficiency of 22.23%,

with a Jsc of 8.34 mA/cm2, VOC of 3.12 V, and a FF of 0.85. We present the simulated data in

Figure S3, S4 and Table S1, S2. Notably, these latter efficiencies are not significantly higher

than the best single junction perovskite solar cells reported to-date, which have band gaps

between 1.55 and 1.63eV. Therefore, our calculations highlight that FAPbBr3 is not a suitable

top cell material for Pb-perovskite-on-Pb-Perovskite tandem cells. Therefore, an essential

challenge is to achieve good stable efficiency with an absorber material with a band gap

closer to 2eV.

Figure 3. (a) simulated EQE for experimental stack, and simulated (b) JV and (c) EQE of

optimized cell architecture.

Table 2. Photovoltaic performance of single junction and tandem cell for calculated devices.

Voc (V) Jsc (mA/cm2) FF PCE (%)CH3NH3PbI3 1.07 12.2 0.84 10.76CH3NH3PbX3 1.61 11.6 0.83 15.2

Monolithic tandem 2.69 11.6 0.83 25.94

Conclusion

We have demonstrated operational all-perovskite tandem solar cells employing a solution

processed interlayer. Through subsequent optical and electronic modeling we have identified

that increasing the thickness of our rear lower band gap cell is required to further improve the

tandem cells. Of equal importance, improvement to the recombination layer is required in

order to overcome our present electrical losses. Once this has been achieved, our modeling

has revealed that tandem solar cell efficiencies of 25.9 % are feasible with a 2.05 eV band

gap top junction and a 1.55 eV band gap bottom junction, once the PV performance of each

junction reaches that of the current highest efficiency single junction cells. This is in

comparison to 22% efficiency, which is the current world-record for a single junction

perovskite cell. Therefore, future research in this area should focus on developing lossless

recombination junctions, and optimizing perovskite wide gap cells with close to 2.05 eV band

gaps. As a final consideration, these wide band gap perovskite tandem cells are well matched

to sit on top of silicon in a perovskite-perovskite-silicon triple junction cells. Investigating the

prospect of such triple junction cells is the subject of ongoing work.

Experimental Section

CH3NH3Br and CH3NH3I were synthesized following a previously reported method34, by

mixing methylamine (33% in methanol, Sigma-Aldrich) with hydrobromic acid (40% in

methanol, Sigma-Aldrich) or hydriiodic acid (48% in water, Sigma-Aldrich) in a 1:1 molar

ratio in a 250ml round bottom flask under continuous stirring at 0⁰C for 2h. The precipitates

were recovered by rotary evaporation at 60⁰C. To increase the purity of products, the powder

was then dissolved in ethanol, recrystallized from diethyl ether. The final product was

collected after dehydration at 60⁰C and placed in a vacuum chamber for overnight.

Solar cell devices were fabricated on fluorine-doped tin oxide (FTO) coated glass

(Pilkington, 8 Ω/square). FTO was patterned with 2 M HCl and zinc powder. Substrates were

then cleaned in 2% Hallmanex detergent, acetone and isopropanol in ultrasonic bath for

10min in each cleaning agent followed by oxygen plasma treatment for 10min. The compact

TiO2 layer was deposited by spin-coating a mildly acidic solution of titanium isopropoxide in

ethanol at 2500 rpm for 60 s followed by annealing at 500 ⁰C for 30 min. The mp-TiO2 layer

composed of 20-nm-sized particles was deposited by spin-coating at 2000 rpm for 60s using a

commercial TiO2 paste (Dyesol 18NRT, Dyesol) diluted in ethanol (2:7, weight ratio). After

dried at 125 ⁰C, the TiO2 film was heated to 500 ⁰C, annealed at this temperature for 30 min

and gradually cooled to room temperature.

CH3NH3PbBr3 films were deposited using the vapour-assisted method. Firstly, PbBr2

solution in DMF with a concentration of 1 M was spin-coated on the mp-TiO2 at 2500 rpm

for 60s. After annealing at 70 ⁰C for 30 min, the film was treated by CH3NH3Br vapour at

175 ⁰C for 10 min in a closed glass petri-dish with CH3NH3Br powder surrounded on a

hotplate in glovebox, then rinsed in isopropanol at room temperature. HTM was then

deposited by spin-coating at 2000 rpm for 60 s. The solution was prepared by dissolving 72.3

mg (2,2’,7,7’-tetrakis-(N, N-di-p-methoxyphenyl-amine)-9,9’-spirobifluorene) (Spiro-

MeOTAD), 28.8 ml 4- tert-butylpyridine (4-TBP), anniqued 17.5 ml of a stock solution of

520 mg/ml lithium bis(trifluoromethane)sulfonimide (LiTFSI) in acetonitrile in 1 ml

Chlorobenzene. The samples were left overnight in dry air before recombination layer

deposition.

PEDOT: PSS was diluted in IPA with volume ratio of 1:4, solution was spined on oxidized

Spiro-OMeTAD films at 5000 rpm for 45 s, then was annealed at 120 ⁰C for 15 min. C60 was

dissolved in Dichlorobenzene with concentration of 10 mg/ml, solution was kept at 100 ⁰C

before spin-coating, which at 2000 rpm for 30 s. followed by annealing at 100 ⁰C for 10 min.

CH3NH3PbI3 films were deposited by inter-diffusion method. 115 nm PbI2 was evaporated

in a vacuum chamber; CH3NH3I solution in IPA with concentration of 20 mg/ml was then

spin-coated at 6000 rpm for 30s. After annealing at 100 ⁰C for 60 min, Spiro-OMeTAD was

deposited with identical recipe and spin-coating condition as described above. To complete

the devices, 100nm gold contacts were thermally evaporated on the back through a shadow

mask.

The optical and electronic device simulation was carried out by initially applying a transfer

matrix model43 with the input of published complex refractive indices for the layered

materials. The modelled absorption spectrum was assumed to resemble the EQE, if 100%

internal quantum efficiency is present. The EQE for each of the two absorbing layers was

then used for the detailed balance theory39 to calculate JSC and J0 and plugged into the one-

diode equivalent circuit equation:

J (V )=J SC−J 0 ¿

The loss parameters (RSH, RS),ideality factor (m) and reverse saturation current (J0) were

extracted from published currently best performing JV characteristics of the corresponding

perovskite devices. The modelled JV characteristic curves were combined by limiting the

combined JSC to the lower JSC delivering cell and adding the voltages of both devices for each

current step40. The performance parameters, PCE, FF, VOC, JSC were then extracted from the

resulting JV curve. In order to find the optimal thicknesses of the layers, a differential

evolution optimisation algorithm41 was used to vary the thicknesses and maximise the

resulting PCE.

Supporting Information

The supporting information is available free of charge on the ACS Publications website.

Hysteresis of fabricated monolithic tandem solar cell, the measured and modelled

transmittance of the top bromide cell and recombination layer, as well as the calculated JV

for 2.25 eV top cell, and parameters used for simulation are presented in the supporting

information.

Acknowledgement

The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australian-

based activities of the Australia-US Institute for Advanced Photovoltaics (AUSIAPV) and is

supported by the Australian Government through the Australian Renewable Energy Agency

(ARENA). Responsibility for the views, information or advice expressed herein is not

accepted by the Australian Government. This work is part funded by EPSRC, UK, through

EP/M024881/1.

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