This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg)Nanyang Technological University, Singapore.

Highly efficient semitransparent perovskite solarcells for four terminal perovskite‑silicon tandems

Dewi, Herlina Arianita; Wang, Hao; Li, Jia; Thway, Maung; Sridharan, Ranjani; Stangl, Rolf;Lin, Fen; Aberle, Armin G.; Mathews, Nripan; Bruno, Annalisa; Mhaisalkar, Subodh Gautam

2019

Dewi, H. A., Wang, H., Li, J., Thway, M., Sridharan, R., Stangl, R., . . . Mhaisalkar, S. G. (2019).Highly efficient semitransparent perovskite solar cells for four terminal perovskite‑silicontandems. ACS Applied Materials & Interfaces, 11(37), 34178‑34187.doi:10.1021/acsami.9b13145

https://hdl.handle.net/10356/142481

https://doi.org/10.1021/acsami.9b13145

This document is the Accepted Manuscript version of a Published Work that appeared infinal form in ACS Applied Materials & Interfaces, copyright © American Chemical Societyafter peer review and technical editing by the publisher. To access the final edited andpublished work see https://doi.org/10.1021/acsami.9b13145

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1

Highly Efficient Semi-Transparent Perovskite Solar

Cells for Four Terminal Perovskite-Silicon Tandems

Herlina Arianita Dewi a#, Hao Wang a#, Jia Li a, Maung Thway b,c, Ranjani Sridharan b, Rolf

Stangl b, Fen Lin b, Armin G. Aberle b,c, Nripan Mathews a,d, Annalisa Bruno a*, Subodh

Mhaisalkar a,d

a. Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, Singapore

637553

b. Solar Energy Research Institute of Singapore (SERIS), National University of Singapore,

Singapore 117574

c. Department of Electrical and Computer Engineering, National University of Singapore,

Singapore 117583

d. School of Materials Science & Engineering, Nanyang Technological University, Singapore

639798

KEYWORDS

perovskite, semi-transparent perovskite solar cell, silicon solar cell, tandem solar cell, efficiency

2

ABSTRACT

Tandem solar cells (SCs) based on perovskite and silicon represent an exciting possibility for a

breakthrough in photovoltaics, enhancing solar cell power conversion efficiency (PCE) beyond

the single junction limit while keeping the production cost low. A critical aspect to push the tandem

PCE close to their theoretical limit is the development of high-performing semi-transparent

perovskite top-cells which also allow suitable near-infrared transmission. Here, we have developed

highly efficient semi-transparent perovskite solar cells (PSCs) based on both mesoporous and

planar architectures, employing Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 and FA0.87Cs0.13PbI2Br

perovskites with bandgap of 1.58 eV and 1.72 eV respectively which achieved PCEs well above

17% and 14% by detailed control of the deposition methods, thickness and optical transparency of

the interlayers and the semi-transparent electrode. By combining our champion 1.58 eV PSCs

(PCE of 17.7%) with an industrial-relevant low cost n-type Si SCs, a 4 terminals (4T) tandem

efficiency of 25.5% has been achieved. Moreover for the first time, 4T tandem SCs performances

have been measured in the low light intensity regime achieving a PCE of 26.6%, corresponding to

a revealing a relative improvement above 9% compared to standard 1 sun illumination condition.

These results are very promising for their implementations under field-operating conditions.

INTRODUCTION

Silicon solar cells (Si-SCs) dominate the photovoltaic market with their high power conversion

efficiencies (PCE), long term stability, and continuous improvements in the production processes

and costs 1. Heterojunction Si-SCs have recently achieved a record PCE of 26.6% 2-3, close to

their theoretical Auger recombination limit of 29.4% 4. Novel photovoltaic devices are needed to

3

overcome the 30% efficiency ‘barrier’ without significant increase in the cost of production.

Tandem solar cells, which combine multiple cells absorbing complimentary spectral regions, can

exceed 30% efficiencies by minimizing thermalization and absorption losses. Although state-of-

the-art multi-junction solar cells based on III-V semiconductor materials have demonstrated record

efficiencies of 39.2% under 1 sun illumination, their expensive production costs limit the

possibility of their widespread development 5-6.

Instead, tandem SCs based on perovskite and Si-SCs represent a real possibility for breakthrough

photovoltaic performance, enhancing the efficiency of the existing Si-SCs. Indeed, low cost

perovskite solar cells (PSCs) can deliver high PCE 7-8 and good bandgap tenability 9-11, maximizing

bandgap matching with silicon solar cells. Over the last few years, perovskite/Si tandem SCs 12-14

have made impressive progresses reaching record published PCEs of 25.5% 15 (announced of 28%

16) and 27.1% for 2-terminals (2T) and 4-terminals (4T) tandem configurations 17. The primary

difference between these two configurations is that top and bottom cells are series connected in 2T

tandem while they have independent electrical connections in 4T tandem. The main advantage of

a 4T architecture is to allow a broader top cell bandgap selection and independent optimization of

both top and bottom SCs 12, 14, 18.

The development of high performance semi-transparent PSCs is critical to push the tandem

efficiency well above that of a single junction Si-SCs and close to their theoretical efficiency of

43% 12, 19. Highly transparent interfacial layers, i.e. electron transport layer (ETL) and hole

transport layer (HTL) with suitable band alignment, able to guarantee effective charge collection,

are key elements to achieve high-performing PSCs. To date, high-efficiency semi-transparent

PSCs are often realized in n-i-p architecture (glass/Fluorine doped Tin Oxide

(FTO)/ETL/perovskite/HTL/electrode) based on metal-oxide ETLs such as mesoporous TiO2 (m-

4

TiO2) and planar SnO2 (p-SnO2) 20-24. Both these ETLs have significant advantages. The highly

porous thick m-TiO2 scaffold allows reproducible nucleation of the perovskite layer, mitigates

active layer defects, enhance charge collection by decreasing the carrier transport distance, and

prevent electrical shunts 25-27. p-SnO2 has a deep conduction band that facilitate fast electron

extraction and promotes UV-stability 28-30. Moreover, low temperature processed p-SnO2

architecture improves overall SC transparency and is compatible with flexible substrate and 2T

tandem configurations 31-32. HTLs such as 2,2′,7,7′-Tetrakis-(N,N-di-4-methoxyphenylamino)-

9,9′-spirobifluorene (Spiro-OMeTAD) and poly (triarylamine) (PTAA) are extensively used as

hole transporting materials with efficient hole transport and electron-blocking properties 8, 33-35,

resulting in the highest efficiencies. Another challenge to achieve high-PCE is the transmission at

wavelengths longer than the perovskite bandgap that determines current generation in the bottom

Si-SCs. Top PSCs in 4T tandem have to utilize two layers of transparent electrodes, which are the

main cause for parasitic absorption losses. Perovskites with ~1.55 eV bandgap only allow NIR

light to reach the bottom Si-SCs, whereas PSCs with higher bandgap (>1.55 eV) result in a wider

transparency window but yield lower efficiencies. Thus, a balance between overall transparency

and efficiency is a critical consideration. Recent results clearly indicate that for 4T tandem

perovskite-silicon cells, top cells make a major contribution towards higher efficiencies (Table

S114, 17, 21, 24, 36-48).

In this work, we have developed highly efficient, semi-transparent PSCs using two different

bandgaps and different architectures to investigate the best performing top cell for a 4T tandem

configuration. Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 and FA0.87Cs0.13PbI2Br perovskites with

bandgap of 1.58 eV and 1.72 eV respectively, were implemented in both m-TiO2 and p-SnO2

architectures. The semi-transparent PSCs demonstrate efficiencies above 17% and above 14% for

5

1.58 eV and 1.72 eV band-gap devices in both m-TiO2 and p-SnO2. Combining 1.58 eV and 1.72

eV semi-transparent PSCs with bottom Si-SCs, the 4T tandem SCs achieved PCEs as high as

25.5% and 22.7% respectively. Silicon and perovskite single junctions and 4T tandem SCs have

been also investigated in intensity regime between 1 and 0.1 sun in order to emulate realistic

operating conditions. 4T tandem PCEs showed a relative 9.7% PCE improvement at the lowest

illumination respect to the standard (1 sun) conditions.

RESULTS AND DISCUSSION

Perovskite Thin Film Characterization

Varying the perovskite bandgap in PSCs is a straightforward approach to examine the effect of

top cell transparency on the 4T tandem PCE. Indeed, aiming at unveil the trade-off between

transparency spectral range and efficiency in pursuit of best performing 4T tandem SCs,

perovskites with different bangdap have been implemented. Perovskites with mixed cations and

halide compositions have been synthetized - Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)349 and

FA0.87Cs0.13PbI2Br - to attain bandgaps of 1.58 eV and 1.72 eV respectively (Figure 1a). These

compositions have been chosen for suitable bandgap match with Si-SC, stability, and fabrication

reproducibility 14, 19, 49. The valence band (VB) levels for these perovskites have been measured

using photoelectron spectroscopy in air (PESA), whereas the conduction band (CB) levels were

derived by combing them with their optical bandgaps Figure S1a. The corresponding energy levels

are shown in Figure S1b. The CB values of 1.58 eV perovskite and 1.72 eV perovskite are

estimated to be at -4.22 eV and -4.15 eV, which show a favourable energy level alignment with

both m-TiO2 and p-SnO2 layers, suggesting a good and efficient charge injection to the electron

transport layers.

6

Both 1.58 eV and 1.72 eV perovskites thin films display continuous and compact morphologies

as indicated in the Field Emission Scanning Electron Microscopy (FESEM) images (Figure 1b).

Specifically, the 1.58 eV perovskite film shows a homogenous distribution of large grains while

in the 1.72 eV a larger spread of grain sizes can be observed. The crystal structures of both 1.58

eV and 1.72 eV perovskites have been further characterized by X-Ray diffraction (XRD) (Figure

S2). Both perovskite compositions show a PbI2 excess at 12.4° which is beneficial to both passivate

the perovskite surface and to increase the charge mobility and consequently the device

performance 50-52.

Carrier lifetime and injection properties of 1.58 eV and 1.72 eV perovskite thin films with and

without the electron quenching layer have been studied by steady state and time resolved photo-

luminescence (PL) measurements. The steady state PL spectra peak at 760 nm and 710 nm for the

1.58 eV and 1.72 eV perovskites respectively and they both composition show a significant

quenching (above 90%) when the thin film is deposited on top of the m-TiO2 and p-SnO2 layers

confirming the efficient charge injection (Figure 1c and Table S2) from the perovskite to the ETL.

Time resolved PL (TRPL) measurements of the (Figure 1d). Pristine 1.58 eV and 1.72 eV

perovskites reveal long fluorescence lifetimes of 488 ns and 292 ns respectively, which confirms

their good optoelectronic properties and low defect densities. Moreover, both ETLs effectively

quench the fluorescence emission, reducing the 1.58 eV perovskite excitons lifetimes to 28 ns and

41 ns for m-TiO2 and p-SnO2 respectively. Using a similar structure, 1.72 eV perovskite lifetime

has been reduced to 23 ns and 60 ns on m-TiO2 and p-SnO2 respectively. The substantial PL

quenching is an evidence of efficient charge separation necessary for good performance in SCs.

Slightly higher quenching for m-TiO2 is hypothesized to be due to the larger surface area contact

with perovskite films as compared to p-SnO2.

7

Figure 1. 1.58 eV and 1.72 eV perovskites film optical and morphological characterizations. (a)

Tauc plot curves of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 (deep red circles) and

(FA0.87Cs0.13PbI2Br) (purple circles) perovskite compositions showing the 1.58 eV and 1.72 eV

bandgap respectively. (b) 1.58 eV and 1.72 eV perovskite thin films on FTO/glass substrates

FESEM Top-view images reveal continuous and compact perovskite surface with largest grains

around 400 nm for both compositions, (c) steady-state PL and (d) time resolved PL decays of 1.58

8

eV and 1.72 eV perovskite thin films on glass and on top of m-TiO2 and p-SnO2 layers, obtained

with excitation wavelength of 405 nm, <2 J/cm2, 40 MHz.

Semi-Transparent Perovskite Solar Cells

The selection of top, bottom electrode contacts and charge extraction materials is critical to

minimizing the overall optical losses, maximizing light transmission to the bottom Si-SC and

enabling high performance 4T tandem 57, 58. The PSCs structures employed in this work are shown

on Figure 2a. PSCs were implemented on top of high quality glass/FTO substrates with good

conductivity (sheet resistance of 15 Ω/cm) and high transparency over visible and infrared spectra

(Figure S3a), guaranteeing a good quality bottom electrode. Indeed, other FTO substrates with

lower sheet resistance (7 Ω /cm) have a much lower transparency in the infrared range, which

could significantly hamper the overall 4T performance (Figure S3b). As ETLs, wide bandgap

semiconductors (TiO2 and SnO2) can provide high transparency 59, 60 and good mobility 30, 61. Both

these properties allowed us to implement efficient cells in both mesoporous and planar

architectures.

The perovskite deposition process has been optimized for both band gaps to obtain large grains

and continuous films (Figure 1b, and details in the Experimental section). Tuning of perovskite

thickness is also necessary to resolve the trade-off between the transparency and efficiency of the

perovskite absorber 62. Different precursors’ concentrations have also been screened to tune the

film thickness for achieving the highest efficiency (Figure S4). The precursors’ concentration of

1.5 M leads to a film thickness of ~500 nm and to the best performing cells. We kept thicknesses

constant for both mesoporous and planar architectures (Figure 2b). Spiro-OMeTAD was chosen

as the HTM due to its good band alignment (Figure S1b) and efficient charge transport. Although

9

Spiro-OMeTAD present a non-negligible parasitic absorption in the UV region (300-400 nm) 45,

63, it does not affect the overall 4T tandem performance since the top cells are illuminated through

the glass/FTO side and the UV light component is absorbed by the perovskite layer before reaching

the Spiro-OMeTAD layer.

Semi-transparent PSCs employed sputtered indium tin oxide (ITO) as top transparent conducing

electrode. The ITO thickness has been carefully optimized, through varying the ITO sputtering

power (Figure S5). The highest transparency corresponds to the lowest power of 25 W and to ITO

film thickness of around 220 nm. Thinner film could improve the transparency but at the cost of

higher sheet resistance. For each sputtering conditions, semi-transparent PSCs have been

fabricated and their photovoltaic performances are reported in Table S3. Details on deposition

conditions are reported in Methods section. In addition of reduced total transparency of the semi-

transparent PSCs, the high power sputtering condition (30 W) also induces damage on under-layer

as shown by the lower device performances. Although our deposition process has been optimized

to maximize the transparency at reduced sputtering power, the best operating PSCs still required a

thin thermally evaporated Ag film as buffer layer to protect the under-layer and guarantee good

ITO crystallinity, and minimally reduces the total transmission. The effect of Ag thickness on the

total cell transparency has been also studied previously 63. The optical transmittance of individual

layers and top electrode of the semi-transparent PSCs, with 1.58 eV perovskite as absorber

material, are shown in Figure S6. Although the transparency for each layer has been carefully

optimized, the total device transparency could be further improved by minimizing reflectance

between device sub-layers.

10

Figure 2. 1.58 eV or 1.72 eV Semi-transparent PSCs. (a) Schematic of semi-transparent PSCs:

FTO/ ETL (m-TiO2 or p-SnO2) / perovskite (1.58 eV or 1.72 eV) / HTL (Spiro-OMeTAD) /Ag/

ITO, (b) Cross section FESEM of 1.58 eV bandgap PSCs with m-TiO2 (top panel) and p-SnO2

(bottom panel) ETLs. The different individual layers have been tinted according to the colour

scheme of the device schematic shown in (a). 1.58 eV PSCs and 1.72 eV PSCs J-V curves using

11

both m-TiO2 and p-SnO2 configurations (c, d). (e) PCE statistics and (f) shelf-life stability of the

1.58 eV and 1.72 eV PSCs. p-SnO2 PSCs: 1.58 eV (filled deep red), 1.72 eV (filled purple). m-

TiO2 PSCs: 1.58 eV (open deep red) and 1.72 eV (open purple). For shelf-life stability, the PSCs

were stored inside the glovebox (argon controlled environment, in dark) without any encapsulation

and measured periodically in ambient air with over 70% RH.

Using the optimized conditions for each layers, 1.58 eV and 1.72 eV semi-transparent PSCs are

fabricated. The perovskite thicknesses for both bandgaps have been kept the same to let

transparency variation be solely due to bandgap tuning. Both 1.58 eV and 1.72 eV semi-transparent

m-TiO2 based PSCs showed transparency slightly lower than 75% in the spectral region above the

perovskite bandgaps (Figure S7a and Figure S7b), while semi-transparent p-SnO2 PSCs achieved

transparency above 75%, making them more promising for 4T tandem integration. The similarity

of observed in both 1.58 eV and 1.72 eV PSCs confirmed that parasitic absorption of the sputtered

ITO and bottom FTO substrate are the main limitations in the near infrared regions.

Our best-performing semi-transparent 1.58 eV PSCs achieved a record PCEs of 17.7% and

17.1% in p-SnO2 and m-TiO2 architectures respectively (Figure 2c and Table 1). To the best of our

knowledge, our semi-transparent 1.58 eV PSCs are among the highest ever reported for similar

bandgaps 20, 24, 46. Likewise, the 1.72 eV champion PSCs J-V curves (Figure 2d and Table 1) show

PCEs of 14.1% and 14.7% in p-SnO2 and m-TiO2 architectures. These PCEs are comparable with

the state-of-art efficiencies of other high bandgap perovskite (>1.70 eV) PSCs 17, 20-21, 23, 42. By

shifting to higher bandgap of 1.72 eV, the PSCs suffer from unavoidable Jsc reduction of ~3

mA/cm2, which is in consistent with their Shockley-Queisser characteristic for different bandgaps

12

53-54, whereas, loss in both Voc and FF, may indicate presence of more carrier recombination and

non-uniformity in the absorber grain sizes distribution.

Table 1. Photovoltaic parameters of semi-transparent 1.58 eV and 1.72 eV PSCs with and as p-

SnO2 and m-TiO2 electron transport layer.

Perovskite ETL Voc

(V)

Jsc

(mA/cm2)

FF

(%)

PCE champ

(%)

PCE ave

(%)

1.58eV

p-SnO2 1.06 21.52 77.5 17.7 17.2 ± 0.5

m-TiO2 1.09 21.08 74.6 17.1 16.8 ± 0.3

1.72eV

p-SnO2 1.06 17.89 74.1 14.1 13.7 ± 0.4

m-TiO2 1.10 18.07 74.1 14.7 14.3 ± 0.4

Both 1.58 eV and 1.72 eV PSCs fabrication processes have good reproducibility as illustrated

by the low PCEs spread of ~ 0.4% for both p-SnO2 and m-TiO2 architectures (Figure 2e). In

addition, the PSCs also exhibit very good shelf-life stability, with a PCE drop smaller than 10%

over 80 days (1.58 eV) and 50 days (1.72 eV). The PSCs have been stored without any

encapsulation in controlled environment and been measured in >70% RH ambient air (Figure 2f).

These data further confirms the good stability of the perovskite layer and the effectiveness of the

sputtered ITO electrode to protect underlying layers from moisture 44. Both mesoporous and planar

architectures have shown comparable high performances when the active layer film morphology

is properly optimized, but the higher transparency of the planar SnO2 based PSCs make them

more promising for tandem integration.

13

4T Perovskite-Silicon Tandem

Both 1.58 eV and 1.72 eV semi-transparent PSCs with p-SnO2 ETL have been implemented in

a 4T tandem configuration with small-size (1cm2) n-type monoPolyTM bottom Si-SCs (insets of

Figure 3a and Figure 3b) 55. The 1cm2 localized grown Si-SCs has been specifically designed to

eliminate the effect of edge recombination losses as compared to conventional laser cutting process

from full size silicon wafer. The schematic of silicon bottom cell is shown in Figure S8. Fabrication

and characterization details of the bottom Si-SCs are reported in the experimental section and will

be further described in a future work. As shown in Table 2, the stand-alone Si-SC has a PCE of

21.1% under the standard AM1.5G spectrum. As the Si-SCs under tandem configurations only

receive the light filtered by the top PSCs, they effectively perform under a much lower injection

level compared to standard AM1.5G conditions, resulting in much lower Voc and Jsc values. The

effective add-on efficiency from the Si-SC under the 1.58 eV PSCs is 7.8% (Figure 3a). A 4T

tandem efficiency of 25.5% was achieved by combining the 1.58 eV PSC with the Si-SC.

Instead, when in the 4T tandem configuration the incident light is filtered by the 1.72 eV PSCs,

a larger fraction light can reach the bottom Si SCs and concomitantly its PCE contribution rises to

8.3%. This higher value is mostly driven by the lower Jsc reduction (25 mA/cm2 instead 23.8

mA/cm2) (Figure 3b). Although Si-SCs contribute a higher effective add-on efficiency, the 1.72

eV PSCs PCE is still much lower than the 1.58 eV PSCs and the total 4T tandem efficiency

achieved in this configuration was only f 22.4%. Interestingly, the Jsc values for the 1.72 eV PSCs

and the Si-CSs filtered by 1.72 eV PSCs are comparable, indicating the potential of this perovskite

composition for future 2T monolithic integration where current matching between the top and

bottom cells is required. The EQE of both 1.58 eV and 1.72 eV semi-transparent PSCs show a

very sharp edge at their particular bandgap indicating the good absorption of the thick perovskite

14

layers (Figure 3c and Figure 3d). The Si-SCs EQEs reach around 75% under the perovskite filter

in the infrared range closely matching the transparency value of the top PSCs. Although 4T tandem

efficiency is higher than each of the stand-alone sub-cells, the EQEs clearly show presence of

parasitic absorption of the top PSCs which limits the efficiency of the filtered Si-SC. The 4T

tandem SCs performances of m-TiO2 1.58 eV and 1.72 eV PSCs show a similar trend to the p-

SnO2 PSC. Indeed, 4T tandem PCEs reached of 24.6% and 22.7% for 1.58 eV and 1.72 eV PSCs

respectively (Figure S9a and Figure S9b). The photovoltaic parameters are summarized in Table

S4. These less transparent top PSCs, as compared to the p-SnO2 counterpart, resulted in lower Jsc

in the filtered Si-SCs as clearly shown in the Si-SCs EQE (Figure S9c and Figure S9d).

For both mesoporous and planar architectures, the absolute PCE gains from stand-alone Si-SCs

to a 4T perovskite-on-Si tandem are ~5% and ~2% with 1.58eV and 1.72eV top PSCs, respectively,

highlighting the advantage of deploying a multi-junction configuration to better utilize the solar

irradiance spectrum.

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Figure 3. 4T tandem based on 1.58 eV and 1.72 eV p-SnO2 PSCs. (a) J-V curve of 1.58 eV PSCs

(deep red circles), stand-alone Si-SC (blue circles), ‘1.58 eV filtered’ Si-SC (green circles); (b) J-

V curve of 1.72 eV PSCs (purple circles), stand-alone Si-SC (blue circles), ‘1.72 eV filtered’ Si-

SC (green circles). Insets in (a) and (b) show the schematic of mechanical stack 4T tandems using

their respective top PSCs bandgaps. EQE curves of 4T tandems: (c) EQEs for 1.58 eV PSC (deep

red), ‘1.58 eV filtered’ Si-SCs (green), stand-alone Si-SCs (blue); (d) 1.72 eV PSC (purple), ‘1.72

eV filtered’ Si-SCs (green), stand-alone Si-SCs (blue).

16

Table 2. IV parameters of p-SnO2 1.58 eV and 1.72 eV semi-transparent PSCs, Si-SCs with and

without PSCs filtering, and 4T perovskite/silicon tandems

SCs Voc

(V)

Jsc

(mA/cm2)

FF

(%)

PCE

(%)

Si unfiltered 0.66 40.2 79.0 21.1

Si filtered (1.58 eV) 0.63 15.2 81.1 7.8

Perovskite (1.58 eV) 1.06 21.5 77.5 17.7

4T tandem (1.58 eV) 25.5

Si filtered (1.72 eV) 0.63 16.4 81.4 8.3

Perovskite (1.72 eV) 1.06 17.9 74.1 14.1

4T tandem (1.72eV) 22.4

In this work, we deployed a rear-side contact passivated silicon bottom cell (monoPolyTM

technology), which is considered most industrial relevant and cost-effective for n-type high-

efficiency silicon SCs. This is particularly important for future commercialization

implementations due to its cost effectiveness and easy process ability 55, but here we are still

currently compromising on PCE as compared to the more costly silicon heterojunction (SHJ) and

interdigitated-back-contact (IBC) n-type silicon bottom cells (compare Table S1). Contrarily, most

of the previously published tandem works employed more advanced high-efficiency bottom Si

SCs architectures (i.e. SHJ and IBC), which have higher PCE, due to superior values of both Voc

and Jsc 17, 43-44, 48.

We evaluate the feasibility to maximize our 4T tandem performance by substituting the

monoPolyTM Si SC (PCE: 21.1%) with a previously reported small area high efficiency IBC Si-

17

SC (PCE: 24.4% 56) in conjunction with our 17.7% p-SnO2 1.58 eV ST-PSC. Assuming the same

relative drop in efficiency for the 24.4% IBC Si SCs as the one observed for the monoPolyTM

SCs, a PCE of 9.1% would be achieved, when filtered with our 1.58 eV ST-PSC. This value is

significantly higher to the 7.8% PCE measured for monoPolyTM Si SC. The 4T tandem

configuration deploying a 1.58 eV ST-PSCs as top cell and an IBC Si SC as bottom cells could

then lead to a PCE of 26.8%.

Moreover light management strategies could also be implemented to further improve our 4T

tandem cell efficiency. These are likely to arise from enhancement in top perovskite cell

transparency by minimizing overall reflectance losses through proper coatings and reduction of

parasitic absorption 46, 56-57.

4T Perovskite-Silicon Tandem under Low Light Intensity

We also explore the feasibility of 4T tandem SCs for realistic operation, when both spectral

distribution and intensity of the sunlight radiance reaching the SCs can vary during the daytime

and the season. Therefore, understanding the PCE trend under different light conditions is critical

for field-operating conditions. Here, we investigate both perovskite and silicon single junction SCs

and 4T tandem SC photovoltaic parameters under different light intensities ranging from 0.1 to 1

Sun. The 4T tandem PCEs are reported in Figure 4a and b, while the detailed Jsc, Voc, and FF are

shown in Figure S10a, Figure S10b, and Figure S10c. Even if theoretical studies have shown the

simulated trend of both 2T and 4T tandem SCs under different illuminations 58-59, to the best our

knowledge these are the first experimental data showing the trend of a perovskite-silicon tandem

solar cells under low intensity illumination.

18

Silicon SCs, either under direct illumination (stand-alone) or filtered by the perovskite, exhibit

very marginal PCE reduction as the illumination intensity decreases, with the highest relative drop

around 5% at 0.1 sun (Figure 4a). The detailed photovoltaic characteristics for stand-alone and

filtered Si SC, Figure S10, are consistent with previous works 60. On the other hand, the PCE of

1.58 eV semi-transparent perovskite SCs remains almost constant in the light intensity region

between 1-0.5 sun, while it slightly increases at even lower intensities (0.5-0.1 sun) reaching the

maximum value of 19.5% at 0.1 sun (18% relative improvement) (Figure 4a). The perovskite SC

Jsc and FF mainly drive the PCE increase. The Jsc linear trend with light intensity of single junction

perovskite SC has been also previously reported, indicating good charge separation at the

perovskite and transport layers interface regardless of different light intensity 61-63 (Figure S10a).

The increase of FF with the intensity decrease suggests a reduced charge recombination under

lower light intensity (Figure S10b). At the same time Voc only slightly decreases from 1 to 0.1 sun

in agreement with the hypothesis of a reduced number of electron generated (Figure S10c) 64-65.

As a combined effect of the behaviour of both sub-cells, the 4T tandem SCs exhibits a stable

PCE also under lower intensities reaching 26.6% of PCE and a 9.7% relative improvement of the

PCE when operated at 0.1 sun (Figure 4b), suggesting that 4T tandem configuration is versatile to

be implemented in different illumination conditions. Our experimental 4T results under different

light intensity are also in line with theoretical simulations which have shown that 4T tandem SCs

under practical condition can achieve higher PCE, which does not critically depends on solar

radiance intensity, as compared to 2T counterpart 66-67. Although our studies also justify good

performance of single junction perovskites and silicon SCs under low light intensity regime, the

presented 4T tandem is able to outperform the single junction due to better spectrum light utilized

and less thermalisation losses.

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Figure 4. PCE of sub-cells and 4T tandem based on 1.58 eV p-SnO2 PSCs under different light

intensity. (a) PCE of sub-cells. 1.58 eV PSCs (deep red), ‘1.58 eV filtered’ Si-SCs (green), stand-

alone Si-SCs (blue), (b) PCE of 4T tandem (dark yellow)

CONCLUSION

In conclusion, here we have investigated the effect of perovskite bandgap and SCs structure on

a 4T perovskite-on-Silicon tandem SCs. The optimized semi-transparent 1.58 eV PSCs achieved

PCE higher than 17% for both m-TiO2 (17.1 %) and p-SnO2 (17.7%) architectures, leading to 4T

tandem PCEs of 25.5%. Whereas, 4T tandem based on the 1.72 eV PSCs reached 22.7% PCE,

mostly due to the lower efficiency of the top cells. Our results highlight the importance of

perovskite bandgap selection and PSCs devices architecture to understand best performing

conditions for 4T tandem SCs. We have also explored for the first time the 4T tandem SCs

performances under low intensity illumination proving a relative 9.7% increase of the

performances at low intensities as compared to standard 1 sun illumination. These results confirm

20

the potential of the 4T tandem SCs in field-operating conditions, as predicted from previous

theoretical simulations.

EXPERIMENTAL SECTION

Perovskite deposition

The 1.58 eV perovskite (Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3) precursor solution, was prepared

following ref. 49 and was spin-coated at 1000 rpm for 10 s followed by 6000 rpm for 17 s. The 1.72

eV perovskite (FA0.83Cs0.17PbI2Br) was spin-coated at 2000 rpm for 10 s followed by 6000 rpm

for 30 s. Both perovskites utilized chlorobenzene as secondary solvent and undergo 100 °C for 1

h post-annealing inside argon-filled glovebox.

Perovskite film characterization

Photo Electron Spectroscopy in Air (PESA) measurements was recorded with a Riken Keiki

AC-2 PESA spectrometer with a power setting of 800 nW and a power number of 0.5. Absorbance

(A) and Transmission (T) spectra were recorded using UV-Vis-NIR Spectrophotometer (UV3600,

Shimadzu) equipped with an integrating sphere (300-1200 nm wavelength). Morphological

characterization for both top view perovskite film and cross section of the complete devices were

recorded using a Field Emission Scanning Electron Microscope (FESEM, JEOL JSM-7600F, 5

kV and 10 mA). X-Ray Diffraction (XRD) patterns were recorded on (Bruker D8 Advance).

Steady state photoluminescence spectra had been measured using a top-table Fluoromax-5 system.

TRPL decays were collected using a micro-PL setup and a Picoquant PicoHarp 300 time-correlated

single photon counting (TCSPC) system and a laser diode (Picoquant P-C-405B, λ = 405 nm, <2

J/cm2, 40 MHz) as excitation source 68.

21

Semi-transparent perovskite top cell

Substrate preparation: Fluorine doped tin oxide (FTO) glass substrates (TEC-15) were cleaned

in decon soap and treated under UV ozone for 15 min prior to usage. ETLs: High temperature m-

TiO2 was fabricated following the procedure in ref 16. Low temperature p-SnO2: 0.1 M SnCl2.2H2O

(Sigma Aldrich) with 8% InCl3 (Sigma Aldrich) doped was dissolved in ethanol and stirred for 12

hours. The dissolved SnCl2 solution was spun coated using a two-step program at 1500 rpm for 10

s followed by 5000 rpm for 10 s. The substrate then was preheated at 80°C for 10 min and

subsequently heated to 180°C for 1h in air to completely oxidize the SnCl2 to SnO2. HTL: Spiro-

OMeTAD (70 mg/mL in chlorobenzene) with addition of 4-tert-butylpyridine, lithium

bis(trifluoromethylsulphonyl)imide (520 mg/mL in acetonitrile (ACN)) and FK209

(37.6mg/100 L of ACN) was spin-coated at 5000 rpm for 30 s inside glovebox. Back contact:

Indium tin oxide (ITO) was deposited using DC sputter at 25 W for 1.5 hour to form transparent

top electrode. Prior ITO deposition, a 1 nm thin Ag was thermally evaporated as buffer layer.

Silicon bottom cell

The Si bottom cells were fabricated on 6-inch n-type pseudo-square silicon wafers (180 µm, 2

ohm-cm) using industrially relevant tools. The fabrication procedure followed the standard

fabrication steps of monoPoly cells 55. The polycrystalline silicon layer used in this work was

deposited using low pressure chemical vapour deposition (LPCVD) instead of plasma enhanced

chemical vapour deposition (PEVCD) method. Furthermore, a localized small-area diffused front

emitter was realized to limit the active cell area to 1 cm2. The finished cells were passivated by

AlOx/SiNx stack on the front and SiNx on the rear using PECVD, with screen-printed H-pattern

22

metal grids on both sides. The H-pattern grids were custom-designed within the active area to

minimize the dark current loss through non-illuminated areas.

Characterizations

The J-V measurements were measured using (Wavelabs Sinus-220 LEDs IV tester). The external

quantum efficiency (EQE) of both perovskite and silicon solar cells were determined using a small

beam spectral response measurement system (Bentham PVE300) in DC mode using dual

xenon/quartz halogen light source without light bias. An aperture mask with dimension of 0.3 cm

x 0.3 cm was used for perovskite J-V measurement, while the silicon bottom cell area is 1 cm2.

The Si-SCs measured efficiency is taking care of the shading loss.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Figure

S1: PESA and band diagram. Figure S2: XRD pattern. Figure S3: Glass/FTO transparency. Figure

S4: Perovskite concentration variation. Figure S5: Transparency curves with various ITO sputter

power. Figure S6: Transparency curves of individual layers. Figure S7: Semi-transparent

perovskite transparency curves. Figure S8: silicon cell schematic. Figure S9: 4T tandem J-V curves

and EQE. Figure S10: Photovoltaic under different illumination intensity. Table S1: Summary of

4T tandem efficiencies. Table S2: Photoluminescence quenching efficiency. Table S3: J-V curves

different sputter power. Table S4: m-TiO2 device photovoltaic summary

23

AUTHOR INFORMATION

Corresponding Author

Email: annalisa@ntu.edu.sg

ORCID

Maung Thway: 0000-0003-4904-0639

Armin G. Aberle: 0000-0003-0456-2070

Nripan Mathews: 0000-0001-5234-0822

Annalisa Bruno: 0000-0002-6963-1682

Subodh Mhaisalkar: 0000-0002-9895-2426

Author Contributions

# These authors contributed equally.

HAD, HW, JL, NM, AB and SM developed and characterized the semitransparent perovskite solar

cells. MT, RS, RS FL, and AA developed and characterized the Silicon solar cells. The manuscript

was drafted by HAD and AB and finalized with contributions of all the authors. This work was

coordinated by AB and SM. All the authors have given approval to the final version of the

manuscript.

Notes

The authors declare no competing financial interest.

24

ACKNOWLEDGMENT

This research is supported by the National Research Foundation, Prime Minister’s Office,

Singapore under Energy Innovation Research Program (Grant number, NRF2015EWT-

EIRP003-004 and NRF-CRP14-2014-03 and Solar CRP: S18-1176-SCRP)

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