Silicon Tandem Solar Cells: Flat

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Effect of Silicon Surface for Perovskite/ Silicon Tandem Solar Cells: Flat or Textured? Hiroyuki Kanda, Naoyuki Shibayama, Abdullah Uzum, Tomokazu Umeyama, Hiroshi Imahori, Koji Ibi, and Seigo Ito ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08701 • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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ACS Applied Materials & Interfaces

Effect of Silicon Surface for Perovskite/Silicon Tandem Solar Cells: Flat or Textured? Hiroyuki Kanda†, Naoyuki Shibayama†, Abdullah Uzum‡, Tomokazu Umeyama§, Hiroshi Imahori§, ∥, Koji Ibi⊥, and Seigo Ito†* Corresponding author: [email protected] (Seigo Ito)

†

Department of Materials and Synchrotron Radiation Engineering, Graduate School of

Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo, 671-0121, Japan ‡

Department of Electrical and Electronics Engineering, Karadeniz Technical University,

61080, Trabzon, Turkey §

Department of Molecular Engineering, Graduate School of Engineering and

∥

Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Nishikyo-ku, Kyoto, 615-8510, Japan ⊥

Choshu Industry Co., Ltd., 3740 Shin-Yamanoi, Sanyo-Onoda, Yamaguchi, 757-0003,

Japan

Keywords: perovskite, silicon, tandem, solar cell, texturing, optical reflectance

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Abstract Perovskite and textured silicon solar cells were integrated into a tandem solar cell through a stacking method.

In order to consider the effective structure of silicon solar

cells for perovskite/silicon tandem solar cells, the optic and photovoltaic properties of textured and flat silicon surfaces were compared using mechanical-stacking-tandem of 2- and 4- terminal structures by perovskite layers on crystal silicon wafers.

The

reflectance of the texture silicon surface in the range of 750-1050 nm could be reduced more than that of the flat silicon surface (from 2.7% to 0.8%), which resulted in increases in average IPCE values (from 83.0% to 88.0%) and current density (from 13.7 mA cm-2 to 14.8 mA cm-2).

Using the texture surface of silicon heterojunction (SHJ)

solar cells, the significant conversion efficiency of 21.4% was achieved by 4-terminal device, which was 2.4%-up from that of SHJ solar cells alone.

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Introduction Tandem solar cells have been proposed to achieve higher conversion efficiency than single-junction solar cells.1-3 gap solar cells.

A tandem solar cell consists of high and low band

An advantage of the tandem solar cell is that by combining solar cells

suitable for long and short wavelength regions, each wavelength region can be efficiently converted to electric power and a high output can be obtained.4 the bottom cells absorb short and long wavelength spectra, respectively.

The top and Since the

two-junction tandem solar cell5-8 is composed of the top and bottom cells connected in a series9,10, the voltage of the tandem solar cell is enhanced by the sum of the voltage of the top cell and bottom cell.

However, the current density (JSC) of the tandem solar

cell is restricted by the sub cell of the tandem solar cell with the lower JSC, which is referred to as “current matching.”

The theoretical band gaps were calculated to

perform the current matching for high photoenergy conversion efficiency.11 Recently, perovskite/crystalline silicon tandem structures12-16 have received a great deal of attention towards high conversion efficiency using perovskite17-21 as a top cell and crystalline silicon as a bottom cell, because of the low material price of silicon and perovskite to be a cost-effective power generation system and because of the ability of perovskite bandgap tuning for current matching.

According to the Shockley−Queisser

limit, the maximal conversion efficiency as a single-junction silicon solar cell (33.5% for the AM 1.5G spectrum at 25 ˚C) can be improved as a tandem cell using silicon and perovskite (45.1% and 45.3% for the 2- and 4-terminal structures, respectively).22 From a practical point of view, the possible conversion efficiency of the perovskite/silicon-tandem solar cell was estimated to be over 31%.23 Figure 1 shows the structural images of the perovskite and silicon solar cells for 3



the tandem device.

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The short and long wavelength lights of the illuminated spectrum

are absorbed by perovskite and silicon layers, respectively, and the generated electrons from silicon solar cells and holes from perovskite solar cells recombine at the interface between the ITO layer on the perovskite top cell and Au layer on the silicon bottom cell. A transparent conductive oxide was formed on the perovskite solar cell, which is composed of indium tin oxide and thin Au layer.24

Thus, it is important to enhance the

absorption of not only the perovskite top cell, but also of the silicon bottom cell for the current matching.25,26

The bandgap tuning of perovskite solar cells27,28 should be

performed for the purpose of passing through more photons at a long wavelength into the silicon solar cells, in order to increase the JSC of tandem solar cells.

For the current

matching, the absorbed photons in perovskite and silicon should be equal.

Although

the photon-absorption coefficient of the perovskite layer (e.g., CH3NH3PbI3) is quite high (≃104-105 cm-1)28, that of silicon is relatively low (≃101-103 cm-1)29, which requires light trapping engineering to utilize it as a bottom cell in a tandem solar cell. To improve the photon absorption in silicon solar cells, texture and anti-reflection coating are formed on the silicon solar cell to increase the absorption of illumination. The texture of silicon is formed by anisotropic etching by using an alkaline solution, which was developed in the 1980s and is still being used.30

Regarding anti-reflection

coating, magnesium fluoride and silicon nitride, etc. were applied to the silicon solar cell.31,32

From a theoretical point of view, Knipp and co-workers reported the possible

conversion efficiency of the perovskite/silicon-tandem solar cell was prospected to be over 30% considering pyramidal textured silicon surface.25,33-36

The calculation shows

the usability of textured silicon for a two-terminal tandem solar cell. C. Ballif and co-worker reported the textured two-terminal tandem demonstrated by using thermal 4


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evaporation method.37 Regarding the spin-coating processed two-terminal tandem solar cell, it is technically difficult to form the perovskite layer on the textured surface, and thus a textured tandem solar cell cannot be demonstrated by the spin-coating process. This is because the texture size of the silicon and the thickness of the perovskite layer are 5–10 µm and 200–500 nm, respectively.

In this case, the perovskite layer is too

thin to form uniformly on the textured surface, and the performance of the solar cell goes down.

Thus, the effect of texture on the performance of a tandem solar cell was

not verified. The stacking method has applied to fabricate the two-terminal tandem solar cell by combining a top solar cell and a bottom solar cell (Figure 2a).24,38

The feature of this

method is that the tandem solar cell was stacked solar cells fabricated separately.

In

this way, the fabrication processes of each solar cell do not interfere with each other. The advantage of the stacking method is that the performance of each solar cell can be measured, which can clarify the effect of texture on tandem solar cells.

Therefore, by

using this method, it is expected that the effect of the surface profile of the bottom cell can be determined (Figure 2b). In this study, we fabricated the perovskite/textured silicon tandem solar cell by using the stacking method to investigate the effect of texture toward the performance of the tandem solar cell.

It was concluded that a textured surface on silicon solar cells is

effective at reducing the reflection and to increase the JSC of the tandem solar cell, which improved the performance of 2-terminal mechanically stacked perovskite/silicon tandem solar cells.

In order to confirm the significance of the results, optical

simulations were performed on the results of texture and flat silicon solar cells for a tandem device.

Finally, we demonstrated the further improvement of the JSC using 5



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silicon heterojunction (SHJ) solar cells with a 21.4% conversion efficiency by 4-terminal device.

Experimental section Materials All utilized solvents and reagents were were used without any purification. Titanium diisopropoxide bis(acetylacetonate) (75 wt% in isopropanol) was purchased from Sigma-Aldrich.

The TiO2 paste of PST-30NRD was obtained from JGC

Catalysts and Chemicals Ltd. Co., Ltd.

PbI2/MAI(1:1)-DMF Complex was purchased from TCI

Polysilazan (SSH-SD2000-HB) was purchased from Ekususia Co., Ltd.

Alka-Tex was purchased from GP Solar GmbH.

Alka-Tex was produced by GP Solar

GmbH.

Fabrication of perovskite solar cells Perovskite solar cells were fabricated following the same experimental conditions and protocol in the previously published literature.39

The perovskite top solar cell for

the tandem-structure device was shown in Figure 1a.

Fluorine-doped tin oxide (FTO)

glass plates (15 Ω/sq, TEC-15, Pilkington) were used as the substrates.

The FTO glass

plates were cleaned using an ultrasonic bath in pure water with detergent, followed by a rinse using pure water, and ethanol, subsequently. cleaned by using UV/O3 cleaner for 15 min.

The surface of the substrate was

The blocking TiO2 and mesoporous TiO2

layer were deposited on FTO glass in order.40 After TiCl4 treatment,41 the perovskite layer (CH3NH3PbI3) was deposited with a solvent of dimethyl sulfoxide (DMSO) on the mesoporous TiO2 film.42

During the second speed step of the spin-coating (at 5000 6


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rpm for 30 secs), toluene was dropped on the substrate for 15 sec prior to the end of spin coating for anti-solvent treatment39,43

Although the top speed of the spin-coating was

set at 5000 rpm in this work, we changed the thickness of the perovskite layer by changing the spin-coating top speed from 5000 rpm (for thin perovskite layer) to 4000 rpm (for thick perovskite layer).

After that, the sample was annealed using hotplate.

Spiro-OMeTAD was deposited on perovskite layer by spin-coating, which is referred to previous report.39

The thin Au (2.5 nm) was evaporated on the HTM layer by thermal

evaporation in order to form a buffer layer that can prevent HTM from sputtering damages.24

The ITO layer (154 nm) was deposited on the thin Au layer by sputtering

as well, as a transparent conductive oxide layer.

Fabrication of silicon solar cells Silicon solar cells were fabricated following the same experimental conditions and protocol in the previously published literature.39 were fabricated, as shown in Figure 1b,c.

A Flat and textured silicon solar cell

Boron doped CZ-Si wafers (100) with a

thickness of 625 µm, a resistivity of 3 Ω cm, and the effective minority carrier lifetime of over 10 µs, were used.

A mirror etching for silicon wafers were performed by using

HF/HNO3 (1:5 in volume) for 7 min to obtain a thickness of 400 µm.

Silicon wafers

were processed HF treatment, rinse by deionized water, and RCA cleaning, in order. 44 To prevent form texturing on the rear side, the SiO2 layer was coated on the substrate.45 Samples were textured in an alkaline solution using KOH (0.9 M in water, 80 °C, 30 min) with an additive of Alka-Tex (0.28% in volume ratio) to achieve a random pyramid texture on the front surface.46

The height and period of the texture in this study are

consist with commonly used silicon surface as 5－10 µm.30,37 7


After rinsing samples


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with deionized water and dipping in aqueous HF (10% in volume ratio), SiO2 was once again coated on the rear side of the samples using polysilazan as a diffusion barrier for phosphorus doping.

After all, substrate was cut into 5 × 5 mm2 square pieces by a

laser scribing system (Indeco Co., Ltd., Japan). treatment was performed.

To remove the defect on edge, KOH

This KOH treatment was referred to previous work.39

After this etching step, all small substrates were aligned on a quartz boat with front surfaces facing upwards, and phosphorus pre-deposition was performed by N2 flow passing through a POCl3 bottle at 800 °C for 20 min (O2 gas was added after the POCl3 liquid bottle) and phosphorus diffusion was carried out at 890 °C for 20 min under N2 gas flow without POCl3.

After forming n-layer, phosphorus glass and the SiO2 layer

was removed by HF treatment, which was following by published literature.39

Then,

Al paste was printed on the rear side, and the Al contacts were fired at 770 °C by quartz annealing tube for 10 sec at the peak temperature.

ITO layer (121 nm) was deposited

on silicon solar cell by sputtering system in according with the previous report.39

The

silicon heterojunction solar cells , which were provided by Choshu Industry Co., Ltd., had a textured surface on the front and rear as the bifacial solar cells.

Fabrication of tandem solar cells The prepared perovskite and silicon solar cells were stacked so that they contacted the respective ITO layers. The stacked solar cell was fixed using a spring contact probe. The excited electrons and holes were extracted through the FTO and Al electrodes on the perovskite and silicon solar cells, respectively.

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Characterization In order to stack the perovskite top cell and silicon bottom cell, mechanically, each cell was just attached and fixed by scotch tape and a contact needle.

Measured I-V

data of perovskite and tandem solar cell were obtained by forward and reverse scans. In terms of I-V curves of perovskite and tandem solar cell, reverse scan data is shown in this study.

The electrical properties of reverse scan are discussed in this research.

The I-V data of silicon solar cell was measured by forward scans. of I-V measurement was 100 mV/sec.

The scanning speed

The active and the masked area of perovskite,

silicon, and tandem solar cell were 0.25 cm2 and 0.09 cm2, respectively. The stacking pressure was approximately 0.8 N/cm2 by using spring-mounted contact probe (B-06RA-00), fabricated by SK KOHKI CO., LTD.

An AM1.5 solar simulator (with a

500W Xe lamp, YSS-80A, Yamashita Denso Co., Japan) calibrated to 100 mW cm−2 using a reference Si photodiode (Bunkou Keiki Co., Ltd., Japan) was employed for photovoltaic measurements.

Silicon surface structures were observed under a

scanning-electron microscopy (SEM) (JSM-7001F, JEOL Ltd., Japan).

Effective

minority-carrier lifetimes were measured by µ-PCD (WT-1000, Semilab Japan Co., Ltd., Japan).

Measurements of reflectance and transmittance were performed by

ultraviolet-visible spectroscopy (Lambda 750 UV/VIS Spectrometer, PerkinElmer Inc., USA).

Thickness measurements of oxides were managed by ellipsometer (SEMILAB,

Japan).

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Results and discussion Figure 3 shows the cross-sectional SEM image of the flat and textured silicon substrate.

The structure of the texture is pyramids with a height of around 5―10 µm

and an angle of around 51 degrees from the surface direction of the substrate. pyramidal texture induces multiple scattering of the light irradiation.

The

As a result of the

multiple scattering, a higher JSC of the tandem device can be expected than that of the cells with a flat silicon solar cell by enhancement of the light trapping for reduction of the reflectance (Figure 2a and 2b). Figure 4 shows the transmittance and photovoltaic characteristics of a perovskite top cell for discussion about the illumination spectrum which can pass through the perovskite top cell and can reach the silicon bottom cell (Figure 4a).

The

transmittance of the perovskite top cell (Figure 4b) rises at 500 nm and increases rapidly up to 78% at 750 nm.

Thus, the silicon bottom cell receives the spectrum with

wavelength of more than 500 nm filtered by the perovskite top cell.

The effect of the

thin Au layer on optical properties was investigated by comparing the transmittance of glass/Au layer and glass only (Figure S-1).

An optical transmission loss by Au layer

was calculated as the difference of a transmittance of glass/Au layer and glass. The average of optical transmission loss of Au layer between 550－1100 nm is 0.8%. It has reported that the thin Au layer of 2.5 nm does not affect transparency in the range of 550 －1100 nm.24

From this result, it is considered that the Au layer does not influence

into the transparency of perovskite top cell hardly.

In terms of the stand-alone

performance of the perovskite top cell, Figure 4c and Table 1 show the I-V curve and photovoltaic properties, respectively.

From the incident photon to current conversion

efficiency (IPCE) measurement (Figure 4d), it can be confirmed that the perovskite top 10


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cell can convert the photon to electron at a wavelength of up to around 800 nm.

The

current density of perovskite solar cell was 18.3 mA cm-2 which is consisted with integrated IPCE value (17.7 mA cm-2). Figure 5a-d shows the schematics image of the flat or the textured silicon surfaces with 121 nm thickness ITO films and the measured and calculated reflectance spectra using Fresnel equations47 (explained in supplemental section), in order to show correspondence between the measured and the calculated reflectance spectra.

It is not

necessary to consider the thickness and index factors of the top cell in order to calculate the reflectance spectra of the bottom cell, because the top cell and the bottom cell have an air gap between each subcell, which could not cause optical interference between the top and bottom cells.24

The calculated reflectance with flat surface (Figure 5b, black

dashed line) almost corresponded to the measured reflectance (Figure 5b, red line) in the range of 350-1050 nm.

The difference between the calculated and measured

reflectance values in the range of 1050-1200 nm might be due to the reflection from the internal measurement system.

In terms of textured silicon, the calculated reflectance

(Figure 5d, black dashed line) also corresponded to the measured reflectance (Figure 5d, red line) in the range of 350-1050 nm.

These results indicate that the reflectance of

silicon substrate with the flat and texture surfaces can be calculated, which meaningfully corresponds to the measured value. Figure 6 shows the reflectance measurement and photovoltaic characteristics of the surface-textured stand-alone silicon solar cells compared with the flat silicon cell. Figure 6c presents the comparison of the light trapping of the flat (Figure 6a) and textured silicon solar cells (Figure 6b).

The measured average reflectance of the

texture silicon surface in the range of 750-1050 nm reduced more than that of the flat 11



silicon surface (from 2.7% to 0.8%).

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Owing to the textured surface, the reflectance of

the textured silicon solar cell was significantly lower than that of the flat silicon cell, which results in a high JSC of 35.3 mA cm-2 with the textured surface when compared to the cells with the flat surface (30.6 mA cm-2), which can be confirmed in Figure 6d and Table 2. These results consist of the integrated IPCE as 34.5 (texture) and 29.5 (flat) mA cm-2.

In other words, textured silicon solar cells can convert the incident photons to

the electron more efficiently than flat silicon solar cells.

Additionally, it is evident that

from the IPCE spectra measurements in Figure 6e the IPCE value of textured silicon solar cell was increased in the range around 350-1100 nm compared to that of the cell with the flat surface.

In terms of long wavelength (750-1050 nm), the average IPCE

values of textured silicon solar cell (88.0%) is higher than that of flat (83.0%). Figure 7 shows I-V and IPCE spectra of flat and textured silicon solar cells filtered by perovskite solar cells with a transmittance value as shown in Figure 4b.

The JSC of

the silicon solar cell with a textured surface was 14.7 mA cm-2, which is higher than that of the cell with a flat surface (13.5 mA cm-2) (Table 3).

Figure 7b show that the IPCE

value of textured silicon was higher than that of a flat cell in the range of around 750-1050 nm.

And the JSC calculated from IPCE value results in 14.6 mA cm-2 for the

textured silicon solar cell and 13.4 mA cm-2 for the flat cell, respectively.

The

calculated JSC from the IPCE value well matched the measured JSC. Figure 8 shows the I-V curves of 2-terminal perovskite/silicon tandem solar cells. The illuminated spectra at the short wavelength and long wavelength were absorbed into the perovskite and silicon layers, respectively. Owing to the texturing, tandem solar cell improved the JSC up to 14.8 mA cm-2, while the JSC of the tandem cell using flat silicon cell was 13.7 mA cm-2 (Figure 8, Table 4).

The JSC of the tandem cells (Table

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4) corresponds to the JSC of silicon bottom cell filtered by perovskite top cell (Table 3). Since the JSC of light-filtered silicon solar cells (Table 3) was lower than that of perovskite solar cells (Table 1), it was confirmed that the current of the tandem device was more limited by the lower current of the bottom silicon solar cells than the top perovskite solar cells. Besides that, the VOC of the tandem cell (Table 4) can be confirmed as the sum of the perovskite solar cell (Table 1) and silicon bottom cell filtered by perovskite solar cell (Table 3).

The IPCE value of the tandem solar cell

(Table 4) is the sum of the IPCE value of the perovskite solar cell and silicon solar cell. At the 850 nm of wavelength, the IPCE value of tandem solar cell with flat and texture silicon solar cell were 72.7% and 74.6%, respectively.

At the 1000 nm of wavelength,

the IPCE value of tandem solar cell with texture surface was 5% higher than that of flat surface.

These increase of IPCE value by texture result in improve of JSC value.

The

conversion efficiency of a tandem solar cell with silicon subcell with texture achieved 14.6%, which is higher than that of the tandem cell with silicon subcell with the flat surface (12.7%).

From these results, one can confirm that the textured silicon solar

cell is significantly more effective at enhancing the JSC of 2-terminal perovskite/silicon tandem solar cells owing to the improved light trapping. Finally, the silicon heterojunction (SHJ) solar cell was integrated into the perovskite/silicon tandem solar cell.

In order to show the effect of the top solar cell,

the different thickness of the perovskite layer was utilized.

Figure 9a shows the I-V

curve of the tandem solar cell using SHJ with thin (400 nm) and thick (500 nm) perovskite layers, as shown in Figure S-3.

Figure 9b,c shows the IPCE of the tandem

solar cell using SHJ with a thin and thick perovskite layer, respectively.

Tables 5 and 6

show the electrical properties of the tandem solar cells and the SHJ solar cell as 13



stand-alone, respectively.

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In terms of the perovskite solar cells, the JSC with a thin

perovskite layer was 18.3 mA cm-2, which is lower than that of thick perovskite layer (20.7 mA cm-2) since the thick layer can absorb more photons compared to the thinner one.

The integrated IPCE was calculated as 17.7 (thin perovskite) and 18.3 (thick

perovskite) mA cm-2, respectively.

On the contrary, regarding the SHJ solar cell, the

JSC with thin perovskite layer was 15.5 mA cm-2, which is slightly higher than that of thick perovskite layer (15.0 mA cm-2).

These results consist of integrated IPCE which

are 16.0 (thin perovskite) and 15.4 (thick perovskite) mA cm-2, respectively.

It is

considered that the thinner perovskite layer can pass through the illumination more efficiently than the thick perovskite layer, which increases the absorption in the SHJ solar cell.

These results can be explained by IPCE result shown in Table 5.

At the

700 nm of wavelength (Table 5), the IPCE value of thin perovskite solar cell (65.0%) is lower than that of thick perovskite solar cell (66.9%).

On the contrary, IPCE value of

SHJ solar cell with thin perovskite (19.3%) is higher than that of thick perovskite (18.2%) at 700 nm.

These results can be attribute to difference of absorption by

changing thickness of perovskite layer. cell

and

reference

To compare semitransparent perovskite top

perovskite

solar

cell

structured

, I-V properties of reference cell are shown in Figure S-4 and Table S-1.

The JSC of reference cell with thin perovskite

layer was 19.0 mA cm-2, which is slightly higher than that of the transparent perovskite solar cell (18.3 mA cm-2).

This reason is considered that the reflection at the Au layer

lengthens the pass length of light in the perovskite layer.

Regarding the fill factor (FF)

of the reference cell, it was 0.741 that is higher than that of the transparent perovskite solar cell (0.658).

It is considered that some extent of ITO spattering damage still 14


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remains even though utilizing ultra-thin Au buffer layer, which results in lower FF of semitransparent perovskite solar cell than reference cell. to thick perovskite layer as well.

This tendency corresponded

As a result, the 2-terminal tandem solar cell with thin

perovskite layer achieved the JSC of 15.5 mA cm-2 and the conversion efficiency of 15.9%, which are higher than those with the thicker perovskite layer (JSC of 14.8 mA cm-2 and conversion efficiency of 14.6%).

These results indicate that the increase of

the JSC of the bottom cell is important to enhance the JSC of the 2-terminal solar cell. Regarding theoretical JSC of the tandem device, K. Jäger and co-author have calculated considering thickness, the band gap of perovskite layer and its influence on tandem optics.48 The calculated JSC of the tandem solar cell was 17.6 mA cm2 which is higher than that of the measured value (15.5 mA cm-2). The reason for the difference of the JSC is considered as the optical loss at the charge transporting layer. It was calculated that the charge transporting layer loss 2.6 mA cm-2.48 The other reason can be attributed that the light scattering at the interface between perovskite top cell and silicon bottom cell. R. Santbergen and co-worker explained optical loss and simulated JSC as a function of reflectance index of the interface.49 The reflectance index of the interface is 1 due to the air at the interface in this report. These simulated and obtained results indicate that further improvement of JSC can be expected by changing the reflectance index of the interface. In case of the 4-terminal solar cell, the conversion efficiency reached 21.4% (= Top cell: perovskite + Bottom cell: SHJ) with a thick perovskite layer, which is higher than that with a thin perovskite layer.

Hence, it was confirmed that the optimized

thicknesses between 2-terminal and 4-terminal are different since the 4-terminal solar cell does not require the current matching.

Anyway, the perovskite/silicon tandem

15



solar cell using surface-textured SHJ solar cell has the advantage to increase the JSC and the conversion efficiency of the tandem solar cell.

16


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Conclusion The tandem solar cell with perovskite and textured silicon solar cells was demonstrated using the stacking method, which applies the textured silicon solar cell to the two-terminal tandem solar cell.

It was concluded that the structure of the front

surface of a silicon solar cell in a perovskite/silicon tandem solar cell should be a pyramidal texture.

The textured surface of the silicon solar cell reduced the reflectance

more than the flat surface, which improved the IPCE value of the silicon solar cell in the wavelength of 750-1050 nm and the JSC of the tandem solar cell.

The optical

calculation confirmed that the textured silicon solar cell could reduce reflection more effectively than the flat surfaced solar cell.

Therefore, textured silicon solar cell can

convert the incident photons to the electrons more efficiently than flat silicon solar cells.

17



Acknowledgements We would like to express our sincere thanks to the Advanced Low Carbon Technology Research and Development Program, Japan Science and Technology Agency (ALCA-JST, Japan).

H.K. sincere thanks to Nanotechnology Platform

(project NPS17013) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and Prof. T. Tabei, S. Yamada, and K. Okada by Hiroshima University and technology for SEM measurement and etching technics and valuable discussion.

Associated content The Supporting Information is available: [Optical transmission of glass/Au layer and glass only; Fresnel equations to calculate reflectance; SEM image of perovskite layer; Photovoltaic properties of reference and semitransparent solar cell]

18


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Tables

Table 1. The photovoltaic properties of stand-alone transparent perovskite top solar cell for mechanical tandem solar cells.

Scan JSC

VOC

FF

Eff.

(mA cm-2)

(V)

(-)

(%)

Forward

18.1

0.967

0.655

11.5

Reverse

18.3

0.985

0.658

11.9

direction

Top cell: Perovskite

Table 2. Photovoltaic properties of flat and textured silicon solar cells.

Active area

JSC

VOC

FF

Eff.

(cm2)

(mA cm-2)

(V)

(-)

(%)

Flat

0.09

30.6

0.55

0.65

11.0

Texture

0.09

35.3

0.55

0.65

12.6

Table 3. Photovoltaic properties of flat and textured silicon solar cells under the illumination filtered by a perovskite top cell.

Active area

JSC

VOC

FF

Eff.

(cm2)

(mA cm-2)

(V)

(-)

(%)

Flat

0.09

13.5

0.50

0.67

4.5

Texture

0.09

14.7

0.51

0.67

5.0

27



Page 28 of 40

Table 4. The photovoltaic properties of 2-terminal perovskite/silicon tandem solar cells using flat and texture silicon solar cells.

Scan Bottom cell

IPCE at IPCE at IPCE at

JSC

VOC

FF

Eff.

direction

700 nm 850 nm 1000 nm

(mA cm-2)

(V)

(-)

(%)

Forward

13.7

1.49

0.620

12.7

Reverse

13.7

1.50

0.620

12.7

Forward

14.7

1.52

0.648

14.5

Reverse

14.8

1.52

0.648

14.6

Flat

Texture

28


(%)

(%)

(%)

82.7

72.7

49.7

83.2

74.6

54.7

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Table 5. The photovoltaic properties of perovskite/silicon tandem solar cells using SHJ solar cell with thin (400 nm) and thick (500 nm) perovskite layers.

The thickness of the perovskite layers was confirmed by SEM (in Figure S-3).

The bottom cells of SHJ were filtered by the top cell of

perovskite during the photovoltaic measurement.

Thin (400 nm) perovskite layer IPCE at IPCE at Scan direction

JSC

VOC

FF

Thick (500 nm) perovskite layer IPCE at

Eff.

IPCE at IPCE at JSC

VOC

FF

700 nm 850 nm 1000 nm (mA (-)

(%)

(%)

(%)

(%)

cm-2) Forward

(V)

(-)

(%)

(%)

(%)

(%)

66.9

0.0

0.0

18.2

73.7

60.8

84.2

73.7

60.8

-

-

-

cm-2)

18.1 0.967 0.655 11.5

20.1 0.982 0.710 14.0 65.0

Perovskite

700 nm 850 nm 1000 nm (mA

(V)

Top cell:

IPCE at

Eff.

Reverse

18.3

0.985 0.658 11.9

-

15.5

0.616 0.730 7.0

Forward

15.5

1.50 0.683 15.9

0.0

0.0 20.7

1.00 0.712 14.7

15.0

0.615 0.728 6.7

14.8

1.51 0.653 14.6

Bottom cell: 19.3

76.7

62.7

84.3

76.7

62.7

SHJ 2-terminal tandem

Reverse

15.5

-

-

1.51 0.682 15.9

14.8

1.51 0.653 14.6

4-terminal -

-

18.9

-

-

-

tandem

29


-

-

-

21.4


Page 30 of 40

Table 6. The photovoltaic properties of SHJ solar cell stand-alone

JSC

VOC

FF

Eff.

(mA cm-2)

(V)

(-)

(%)

SHJ solar cell 39.1

0.665 0.730

(stand-alone)

30


19.0

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Table of contents

31



Figure 1. Structural image of the (a) top cell: perovskite solar cell, (b) bottom cell: flat silicon solar cell and (c) textured silicon. 268x136mm (150 x 150 DPI)


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Figure 2. Schematic image of the perovskite/silicon tandem solar cells with (a) flat and (b) textured silicon solar cells. 290x176mm (96 x 96 DPI)



Figure 3. SEM image of the front cross-sectional surface of a bottom cell of (a) flat silicon and (b) textured silicon with a slope of 30°. 219x65mm (150 x 150 DPI)


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Figure 4. (a) Schematics image of perovskite top cell, (b) Transmittance of perovskite top cell, (c) I-V curve of stand-alone perovskite top cell, and (d) IPCE spectra of perovskite top cell. 224x208mm (150 x 150 DPI)



Figure 5. Structures and reflectance spectra (measured and calculated) of flat [(a) and (b)] and textured [(c) and (d)] silicon surfaces (thickness of the ITO layer is 121 nm). 278x212mm (150 x 150 DPI)


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Figure 6. Schematics image of (a) flat and (b) textured silicon bottom cell. (c) Reflectance, (d) I-V curve, and (e) IPCE spectra of flat and texture silicon solar cells. 215x271mm (150 x 150 DPI)



Figure 7. (a) I-V and (b) IPCE spectra of flat and texture silicon solar cells under the illumination filtered by perovskite top cell. 216x110mm (150 x 150 DPI)


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Figure 8. I-V curves of the tandem solar cell with flat and textured surface. 109x106mm (150 x 150 DPI)



Figure 9. (a) I-V curves of the tandem solar cell using SHJ solar cell with thin and thick perovskite layer. IPCE spectra of the tandem solar cell with (b) thin and (c) thick perovskite solar cell (black line is a sum of perovskite top cell (blue line) and silicon bottom cell (red dashed line)). 318x105mm (150 x 150 DPI)


Page 40 of 40

Silicon Tandem Solar Cells: Flat

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