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Interface Optoelectronics Engineering for MechanicalStacked Tandem Solar Cells Based on Perovskite and Silicon Hiroyuki Kanda, Abdullah Uzum, Hitoshi Nishino, Tomokazu Umeyama, Hiroshi Imahori, Yasuaki Ishikawa, Yukiharu Uraoka, and Seigo Ito ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07781 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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ACS Applied Materials & Interfaces

Interface Optoelectronics Engineering for Mechanical-Stacked Tandem Solar Cells Based on Perovskite and Silicon Hiroyuki Kanda1, Abdullah Uzum1*, Hitoshi Nishino2, Tomokazu Umeyama3, Hiroshi Imahori3,4, Yasuaki Ishikawa5, Yukiharu Uraoka5 and Seigo Ito1*

3

1

Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan.

2

Energy Technology Laboratories, Osaka Gas Co., Ltd., 6-19-9 Konohana-Ku, Osaka

554-0051, Japan. Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan. 4 Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan. Information Device Science Laboratory Graduate School of Materials Science Nara Institute of Science and Technology 8916-5 Takayama, Ikoma, Nara 630-0192, Japan. 5

E-mails of corresponding authors: [email protected] Keywords: tunnelling junction, anti-reflection coating, point contact, p-type single crystal silicon, CH3NH3PbI3

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Abstract

Engineering of photonics for anti-reflection and electronics for extraction of the hole using 2.5 nm of thin Au layer have been performed for 2- and 4- terminals tandem solar cells using CH3NH3PbI3 perovskite (top cell) and p-type single crystal silicon (c-Si) (bottom cell) by mechanical stacking.

Highly transparent connection multilayers of evaporated-Au and

sputtered-ITO films were fabricated at the interface to be point-contact tunneling junction between the rough perovskite and flat silicon solar cells.

The mechanically stacked tandem

solar cell with an optimized tunneling junction structure of performed the conversion efficiency of 13.7% and 14.4% as 2- and 4-terminals devices, respectively.


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1. Introduction Reducing the costs involved in the solar cell fabrication processes and improving solar cell efficiencies are key points for realizing a wide utilization of photovoltaics in the energy market.

Dominantly used crystalline silicon solar cell technology is already close to its

realizable limit

1

and has already reached an efficiency of 25.6% 2.

Therefore, achieving

higher efficiencies with low-cost processes is becoming increasingly important for photovoltaic applications.

One of the possible solutions features tandem structures which

combine multiple absorbers in order to cover as wide of the range of the solar spectrum as possible.

A dual junction tandem device including a high bandgap top cell and a lower

bandgap bottom cell is a promising alternative for high efficiency solar cells.

A suitable top

cell must be considered for such a structure when a crystalline silicon (c-Si) solar cell is considered for the bottom cell. One of the candidates for the top cell are organic–inorganic lead halide based perovskites [CH3NH3PbX3 (X: I or Br)] solar cell which was found by Miyasaka’s group as a first report in 2009

3

with 3.8% conversion efficiency using iodide liquid electrolyte.

Instead of iodide liquid electrolyte, an organic hole conductor (spiro-OMeTAD) was used to be solid state perovskite devices as which was established by groups of Miyasaka, Grätzel, Snaith, and Park in 2012 with around 10% conversion efficiencies

4,5

.

Significant advantage of the

organic–inorganic lead halide based perovskites [CH3NH3PbX3 (X: Cl, Br, or I)] is tunable bandgaps from 1.55 eV to 2.23 eV, which have emerged as a new class of light absorbers, achieving exceptional progress in solar cell performance in recent years with over 20% conversion efficiency

6–15

.

This rapid growth of perovskite solar cell technology has

appealed researchers to focus more on the application of tandem solar cells as the high bandgap top cell on c-Si solar cells.

Using an optical splitter, which reflects shorter

wavelengths of light onto the high bandgap cell while transmitting longer wavelengths of 3 ACS Paragon Plus Environment


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light to the low bandgap cell, 28%-efficiency tandem solar cells combining a perovskite solar cell and a heterojunction silicon solar cell have been demonstrated to show the possibility of the creation of a high-efficiency tandem devise

16

.

In addition, simulation studies on

perovskite/c-Si tandem solar cells 17–19 have also estimated the possibility of achieving >30% conversion efficiencies, which offer a reproducible low-cost high efficiency solar cell. Moreover, it was prospected that a fully-optimized ultimate device with matched current using perovskite and silicon solar cells could yield up to a 31.6% conversion efficiency 20. In order to apply perovskite solar cells as high bandgap top cells into a tandem device, several studies on two- and four-terminal perovskite/silicon tandem solar cells have been reported 20–23.

However, the connection of the top and the bottom cells is a crucial point for

such tandem solar cells due to the current matching and junction tunneling between two solar cells. Specially, for the two-terminal tandem device, the total photocurrent can be regulated by the value of smaller photocurrents from the top (perovskite) or bottom (silicon) solar cells. For example, a monolithic-deposited tandem structure with a perovskite (CH3NH3PbI3) top cell (single device efficiency: 11.4%) and CZTS bottom cell (single device efficiency: 11.6%) were introduced with an efficiency of a mere 7.66% tunneling is an important issue.

24–27

.

Moreover, junction

In order to improve the tunneling junction in a tandem

device, a TiO2 layer was formed on the tunneling junction of n++/p++ a-Si layers by a heavily doped n++ hydrogenated amorphous silicon (a-Si:H) using plasma-enhanced chemical vapor deposition (PECVD), which was set on the n-type silicon bottom cell to perform with an efficiency of 13.7% as a monolithic-deposited tandem solar device

21

.

Another report

demonstrated tandem device using MoOx as a buffer layer with amorphous/crystalline silicon heterojunction (SHJ) cell fabricated by plasma-enhanced chemical vapor deposition (PECVD) process with 21.2% of conversion efficiency 28.

Since these perovskite solar cells

were fabricated on silicon solar cells as the substrate, it can be named as “monolithic tandem 4 ACS Paragon Plus Environment

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device”. To the contrary, a four-terminal tandem device can extract each photocurrent from a solar cell without consideration for the current matching due to its independence to the other solar cell.

A four-terminal perovskite/silicon mechanical-stacked tandem solar device with

a semi-transparent perovskite top cell mechanically stacked onto a mc-Si solar cell

22

results

in a conversion efficiency of 17%, and a perovskite top cell and c-Si heterojunction bottom cell demonstrated feasibility conversion efficiency of 25.2% 22. In this work, in order to improve the study of tandem solar cells by perovskite and silicon solar cells, we have investigated special mechanical junction by considering photonics and electronics at the interface, which can work for both of tandem cells with 2- and 4terminals.

We have used a tunneling junction of on the

perovskite rear side and an individual on the front side of a c-Si solar cell. Each Au and ITO film was analyzed and optimized to achieve high performance tandem solar cells.



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2. Experimental The structure of the fabricated cell is shown in Figure 1.

Figure 1. Perovskite/c-Si tandem solar cell structure of (a) Schematic diagram of fabricated perovskite solar cells, (b) Schematic diagram of fabricated 25 mm2 p-type CZ-Si solar cells and (c) Schematic image of the interface between ITO layers on the top and bottom solar cells (d), (Figures are not to scale).

For the first step of the tandem device, perovskite solar cells were fabricated (Figure 1a).

Commercially available fluorine-doped tin oxide (FTO) glass substrate (TEC-15, NSG-

Pilkington) was used as a substrate.

The dense (blocking) TiO2 layers were coated on the

cleaned FTO by spray pyrolysis using a solution of titanium di-isopropoxide bis (acetylacetonate) (TAA; 0.3 ml) in ethanol (4 ml) on a hot plate at 450 °C. solution was prepared by pouring acetylacetone into titanium iso-propoxide.

The TAA

Porous TiO2

layer was fabricated by a spin coating method (3500 rpm for 30 s with 5-sec acceleration) on 6 ACS Paragon Plus Environment

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TiO2 paste (TiO2 particle (≃30 nm) fabricated by

the dense TiO2-coated FTO. hydrothermal synthesis

29

) was diluted in a ratio of 1:3.5 (by wt%) with ethanol for the

spin-coating solution. After spin coating, the porous TiO2 layer was annealed at 500 °C. For the deposition of perovskite layer (CH3NH3PbI3) using two-step process 30, 1.2 M of PbI2 was dissolved in N,N dimethylformamide by stirring at 70 °C, deposited on the mesoporous TiO2 film by spin-coating at 6500 rpm for 20 s and dried at 70 °C for 30 min.

Then, the

TiO2/PbI2 film was dipped into a solution of CH3NH3I in isopropanol (10 mg/ml) for 20 s and spun to dry at 4000 rpm for 8 s (2 s of acceleration). film

was

dried

on

a

hotplate

at

Finally, the deposited CH3NH3PbI3 70

°C

for

30

min.

2,2,7,7-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD) was deposited as an organic hole transporting material (HTM) according to the literature 30.

The

organic HTM solution was deposited using the spin-coating method at 4000 rpm 8 seconds. Finally, gold was evaporated on top to form a gold layer with various thicknesses before processing to achieve the tandem structure. As the second part of the tandem solar cell, a crystalline silicon (c-Si) solar cell process starts with the use of boron-doped 30 mm × 30 mm of single crystal silicon polished wafer by Czochralski method (CZ-Si) ( (100) crystal phase, thicknesses of 550 µm and a resistivity of 3-10 cm).

The schematic of the crystalline silicon solar cell without front metal-grid

contact can be seen in Figure 1b. diluted HF dip.

First, all wafers were put through RCA cleaning and

Then, prior to phosphorous diffusion, polysilazane-based SiO2 was coated

on the rear side of the samples as a diffusion barrier by the spin coating method at 1500 rpm for 20 s (accelerating 5 s) and annealed at 500 ºC for 1 h under O2 flow

31,32

.

Phosphorous

diffusion was carried out by annealing of the CZ-Si wafers at 890 °C for 20 min under nitrogen gas flow passed though POCl3 liquid.

After the diffusion process, phosphorus

silica glass and the SiO2 layer on the rear were removed by a diluted HF solution.

Then, Al

paste was deposited by screen printing on the back side of substrates and fired at 800 °C 7 ACS Paragon Plus Environment


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using rapid thermal annealing for 1 s at the peak temperature (ramping up for 60 s).

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Finally,

the substrate was cut into 25 mm2 size pieces. In order to form a tandem structure as shown in Figure 1, the junction between the perovskite solar cell and the c-Si solar cell was maintained by the deposition of an indium tin oxide (ITO) film on evaporated Au on the rear side of perovskite solar cell and directly onto the front side of the c-Si solar cell (Figure 1c).

The systems of sputtering and thermal

evaporation were E-200S (Canon Anelva, Japan) and VPC-250 (ULVAC, Japan), respectively.

Sputtering of ITO film was carried out by RF sputtering deposition at room

temperature while keeping the substrate stage on rotating.

Sputtering was performed under

Ar:O2 (10:0.2 in sccm) with a power of 50 W for 10 to 25 min which resulted in a thickness range of from 82 nm to 181 nm.

In order to stack the perovskite top cell and silicon bottom

cell, each cell were just attached and fixed by scotch tape and spring needle. Measurements and characterizations were mainly performed by ultraviolet-visible spectroscopy (UV/Vis Spectrometry, Lambda 750 UV/VIS Spectrometer, Perkin Elmer) for transmission and reflection analysis, ellipsometry (GES5, Sopra) for the measurement of the thicknesses of the deposited thin Au layer and ITO layers, four point probe measurement (Loresta-EP MCP-T36 by Mitsubishi Chemical Corp.) for resistivity measurements, and an AM1.5 solar simulator (with a 500 W Xe lamp, YSS-80A, Yamashita Denso, Japan) calibrated to 100 mWcm−2 using a reference Si photodiode (by Bunkou Keiki, Japan) for photovoltaic measurements. mV/second, respectively.

The scanning step and the speed were 0.05 V and 400

In this work, results of forward scanning I-V curves and that of

electric properties are shown and discussed basically.

The active area of perovskite solar

cell, silicon solar cell and tandem solar cell were 0.25 cm2.

The silicon solar cell was cut as

0.25 cm2 and the size was measured again after cutting to be correct.


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3. Results and discussion Primarily, the transparent ITO film was considered to be a substitution for Au contact and a tandem cell structure of was fabricated.

The thickness of all ITO films on HTM was fixed to 154 nm.

A distortion of

the I-V curve was observed at around 1.5 V when the top and bottom cells were connected only by ITO films (without Au) (Figure 2, dashed line).

Figure 2. Comparison of tandem solar cells with (w/) and without (w/o) an Au layer beneath the ITO film on the rear side of perovskite top cell (blue dashed line: , solid line: ).

The distortion has been explained as a reversed diode effect defects in p-type Spiro OMe-TAD by sputtering-ITO deposition.

33–35

, due to the damage

Such damage defects in

p-type Spiro OMe-TAD by sputtering was suggested at Mo deposition 36. In order to suppress such sputtering damage in Spiro OMe-TAD, a thin film of Au with a thickness of 6.6 nm (which is around 8 times thinner than that of the conventional



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perovskite rear contact 8) was deposited on hole transporting layer (HTL) prior to ITO sputtering.

In this case, the final structure became: .

By depositing a thin Au layer on the Spiro OMe-TAD prior to

forming the ITO film, the distortion of the I-V curve could be successfully eliminated (Figure 2, solid line).

Owing to the intermediate Au layer which acts as a junction tunnel also, the

fill factor (FF) and conversion efficiency (η) of the tandem solar cell was significantly increased from 57.1% to up to 70.9% and from 8.71% to up to 11.91%, respectively.

In

terms of improvement of I-V curves by thin Au layer, it is considered that the each work function of Spiro OMe-TAD, Au and ITO layer are one of the essential point.

The work

function of Spiro OMe-TAD (ΦHTM), Au (ΦAu) and ITO (ΦITO) are 5.22 eV, 5.1 eV and 4.7 eV, respectively 37,38.

Therefore, interface between Spiro OMe-TAD (ΦHTM = 5.22 eV) and

ITO layer (ΦITO = 4.7 eV) could form Schottky diode due to the deference of work function. The Schottky diode could affect to the I-V curve to be S-shaped curve (shown in Figure 2, blue line) which reduce fill factor.

These phenomena were similar to several reports

explained by the theory of metal-semiconductor Schottky diode 39–41.

On the other hand, the

work function of the Au (ΦAu = 5.1 eV) can be matched to that of Spiro OMe-TAD (ΦHTM = 5.22 eV) which form well ohmic contact and improve S-shaped I-V curve to be ideal I-V curve (shown in Figure 2, red line). et al.

41

This result agrees with that reported by Kouskoussa B

using 0.5 nm of ultra-thin Au layer and reason why thin Au layer can prevent from

I-V degradation.

However, although Schottky barrier could be eliminated by thin Au layer,

it is necessary to be suspect that physical damage of ITO sputtering against Spiro OMe-TAD layer.

It is needed to discuss alternative approaches which avoid physical damage of ITO

sputtering completely.

F. Lang et al. reported perovskite solar cell depositing graphene on

the Spiro-OMeTAD layer, which do not have any physical damage 42. the work function between graphene (Φgraphene = 4.5 eV)

43

Despite deference of

and Spiro OMe-TAD (ΦHTM =

5.22 eV), the I-V characteristics is ideal curve, not S-shaped curve.

In addition to this result, 10

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several reports show similar tendency

24,44,45

.

From these results, therefore, it is considered

that the S-shaped I-V curve could be composed of the physical damage as well as Shottoky junction. For the further optimizations, the thickness of Au layer was varied from 0 to 10 nm and similar tandem solar cells were fabricated with the tandem structure of .

In order to achieve optimum

value of each layer (ITO on top cell, thin Au and ITO on bottom cell) as correctly as possible and exclude interfere effects, each layer have been optimized in advance to some extent, which optimized value was set as initial value in this experiment.

The dependence of the

electrical properties of the tandem cell on the thickness of Au layer is shown in Figure 3.



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Figure 3. Dependence of the electrical properties of solar cells on the thickness of Au layer. (Circles stand for the average values of the three samples and the top and bottom bars mean the maximum and minimum values, respectively.)

Although no significant change was observed in the open circuit voltage (VOC) by decreasing the Au thickness from 10 nm to 2.5 nm, the short circuit current density (JSC) and η were increased from 7.10 mA cm-2 to up to 11.09 mA cm-2, and from 7.74% to up to 11.81%, respectively. The increase of JSC can be attributed to the increased transmittance of the Au layer brought on by decreasing its thickness.

In order to confirm this transparency,

Au was deposited on glass substrates for individual transmittance measurements. Transmittances of Au layers with thicknesses from 1.6 nm to 10 nm are shown in Figure 4a.


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Figure 4.

Transmittance spectra of Au layer with various thicknesses (a) and of

Considering the operating light-wavelength range of c-Si solar cells up to 1100 nm, and the photons that can pass through the perovskite top cell over 550 nm (shown in Figure 4b), the region of the spectrum that can be available to be absorbed by the bottom silicon cell will be between 550 nm to 1100 nm.

The weighted transmittance (weighted with the AM1.5G

spectrum between 550 nm-1100 nm) of the Au layer at 1.6 nm, 2.5 nm, 4.1 nm, 6.6 nm and 10 nm were 99.20%, 99.05%, 88.80%, 81.15% and 77.36%, respectively.

The average of

transmittance of full perovskite top cell is 47.17% (550 nm-1200 nm) shown in Figure 4b.



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From the high transmittance ratio with better electrical properties on the final cells, the optimum value of Au thickness was determined to be 2.5 nm. Sheet resistance of deposited ITO films (t = 154 nm) on perovskite solar cells was measured as 35 Ω/sq, which was deposited on a glass substrate for the measurement. Relatively high sheet resistivity is due to the sputtering process which was performed at room temperature in order to protect temperature-sensitive HTM on the perovskite solar cell. Moreover, the rough perovskite surface provides limited connectability between the two ITO layers (Figure 1c and 1d).

The ITO layer on the Au of the top cell can work as a carrier

collector to the contacting points.

On the other hand, the ITO layer on the n-Si of the

bottom cell can work as an anti-reflection coating (ARC) and carrier collector.

Each ITO

layer should be optimized for further photovoltaic results. Hence, the thickness of the ITO film on Au (in Figure 1c) was optimized after fixing the Au layer to have a thickness of 2.5 nm.

Figure 5 shows the electrical properties of

tandem cells depending on the thickness of the ITO film deposited on the thin Au layer (thickness of the ITO film on a silicon cell was fixed at 154 nm).


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Figure 5. Electrical properties of tandem cells with various thickness of ITO film on 2.5 nm of the Au layer. (Circles stand for the average values of the three samples where the top and bottom bars mean the maximum and minimum values, respectively.)

When the tandem structure was mechanically assembled without an ITO film on the thin Au layer, electrical properties including VOC, FF and η were significantly low, due to the poor conductivity of the Au layer (as Figure 1).

Owing to the deposition of the 154-nm

thickness ITO film on the Au, the tandem-device electrical properties of VOC, FF and η could be improved by up to around 1.5 V, 68% and 11%, respectively. In order to consider the photonic effect of the ITO layer on the Au, the transmittance spectra of the ITO films after individual deposition onto a glass substrate with different thicknesses (from 82 nm to 181 nm) are shown in Figure 4b.

From the measured 15

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transmittance data, the average transmittance (550 nm-1100 nm) of ITO films of thicknesses 82 nm, 121 nm, 154 nm and 181 nm were 88.48%, 88.50%, 89.25% and 89.91%, respectively. The electrical-performance tendency of the tandem solar cells can be attributed to the transmittance variation with various thicknesses of ITO film deposited. Therefore, the best performance of the tandem solar cells was achieved to the 154-nm thickness ITO film. Finally, the dependence of cell performance on the ITO film thickness on the c-Si bottom cell was also investigated.

Figure 6 shows the electrical properties of the tandem

cells fabricated with varying thicknesses of the ITO film (from 82 nm to 181 nm) deposited on the front surface of the c-Si bottom cell.


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Figure 6. Electrical properties of the tandem cells fabricated with varying thicknesses of ITO films deposited on the front surface of the c-Si bottom cell. (Circles stand for the average values of three samples where the top and bottom bars mean the maximum and minimum values, respectively.)

In this case, the thickness of the Au layer on the HTM and that of the ITO film on the thin Au layer were fixed to 2.5 nm and 154 nm, respectively.

The peak of photoenergy

conversion efficiency was obtained at 108 nm of ITO thickness.

An average of the tandem

solar cells fabricated with 108 nm ITO film on c-Si emitter provides a JSC of 12.84 mA cm-2, VOC of 1.54 V, FF of 62.3% and η of 12.34%.

A gain of around 3 mA cm-2 in JSC and 5% of

η were achieved compared to those cells fabricated without ITO film on the front surface of the c-Si.

On the contrary, no significant change was observed on VOC.

The improvements



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of JSC indicate that the existence of an ITO film on an emitter layer of silicon solar cell may provide an antireflection effect.

In order to confirm this effect, reflectance of the ITO films

deposited on silicon wafers (film thicknesses of from 0 nm-181 nm) were measured as shown in Figure 7.

Figure 7. Reflectance of the ITO films deposited on the c-Si substrate with various thicknesses of the ITO films.

In order to estimate the maximum theoretical JSC of bottom cell depend on ITO thickness, the theoretical current density (JSC_calculated) was calculated by transmittance of perovskite layer (shown in Figure 4b) and reflectance of ITO film on silicon substrate (shown in Figure 7).

The current density was calculated using equation (1) show in below:

_

_ !" #$%&'(##%& !"" 1

* $+$#%& !"" -!

(1) Ideal EQE data of silicon solar cell for this simulation was obtained by PC1D simulation 46,47 18 ACS Paragon Plus Environment

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shown in Figure 8b (front and rear surface recombination = 100 cm/s, Bulk recombination 1000 µs, Thickness of substrate = 550 µs, p-type background doping = 1.4*1015 cm-3, n-type doping = 3*1020 cm-3, electrode series resistance = 0.015 Ω, no internal shunt pathways and without any reflection).

The calculated (JSC_calculated) and measured (JSC_measured) results are

shown in Figure 8.

Figure 8. Calculated and measured JSC depend on thickness of ITO on c-Si substrate (JSC_calculated was calculated by reflectance of ITO film on silicon substrate (shown in Figure 8) and transmittance of perovskite layer (shown in Figure 6) (a) and external quantum efficiency of ideal silicon solar cell calculated by PC1D (b).



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The current density of JSC_calculated should be higher than that of JSC_measured over the whole thickness of ITO layer on silicon due to the variation of EQE.

The maximum current

density of JSC_calculated is 13.7 mA cm-2 at 121 nm which is higher 1.9 mA cm-2 than that of JSC_measured.

The tendency of JSC_calculated and JSC_measured variation was same.

Therefore, it

can be confirmed that the variation of JSC_measured is due to the photonic anti-reflection effect for the introduction of light to c-Si, not due to the conducting effects of ITO on c-Si. Morphology of the ITO layer of top cell is shown in Figure 1d (cross sectional image tilted 15 degrees).

Surface condition is uneven which reflect morphology of under layer of Spiro

OMe-TAD layer and gold layer.

The morphology of ITO layer could affect to light

scattering to increase Haze values. Figure 9 shows the I-V curves, external quantum efficiency and optical reflectance 1-R measurements of the best tandem device and its components stand-alone (a perovskite solar cell and c-Si solar cell filtered by a perovskite top cell).

Table 1 summarizes the

photovoltaic properties of the solar cells in Figure 9.


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Figure 9. I-V curve of the stand-alone perovskite top cell (a), best fabricated tandem solar cell and silicon solar cell filtered by the perovskite top cell (b), external quantum efficiency of solar cells (c) and optical reflectance 1-R measurements (d).



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Table 1: Electrical properties of the best perovskite/c-Si tandem cell JSC / mA cm-2

VOC / V

FF / %

η/%

28.5

0.562

72.3

11.6

Perovskite solar cell

17.2

1.051

66.3

12.0

(Au 50 nm)

(17.2)

(1.060)

(75.9)

(13.8)


17.3

0.978

35.2

5.9

(ITO 154 nm / Au 50 nm)

(18.7)

(0.982)

(38.8)

(7.1)


16.8

1.028

58.1

10.0

(Au 2.5 nm/ ITO 154 nm / Au

(17.7)

(1.011)

(59.4)

(10.6)

12.3

0.520

68.9

4.4

Perovskite / c-Si tandem solar

12.3

1.564

71.3

13.7

cell (2-terminals)

(12.3)

(1.568)

(71.8)

(13.8)

Perovskite / c-Si tandem solar

-

-

-

14.4

c-Si solar cell with flat surface and ITO ARC

50 nm) c-Si solar cell without surface texture (flat surface) and ITO ARC (shadowed by perovskite top cell)

cell (4-terminals) (Results of forward scan and back scan are shown in upper and below side, respectively.)

A flat (non-textured) silicon solar cell with an ITO ARC for tandem device exhibited 11.6% conversion efficiency.

This front ITO layer on c-Si cell is necessary for the tandem

device due to conductivity (Figure 1), which can’t be performed by a normal ARC (SiNx or TiO2) layer.

I-V curves of various configurations of perovskite solar cells are shown in Figure 9a.

Thick Au

layer (50 nm) just as contact layer were deposited on each perovskite solar cells by thermal vapor deposition.

Fill factor and efficiency of the reference configuration was 66.3 and 12.0%, respectively, shown in Table 1.

In the case

of , fill factor was decreased from 66.3 to 35.2, and S-shaped I-V curve was confirmed at forward and reverse scanning condition.

By

depositing thin Au layer , fill factor was significantly 22 ACS Paragon Plus Environment

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improved from 35.2 to 58.1, and 10.0% of conversion efficiency was confirmed. Comparing reference cell and , there was less difference of open circuit voltage as 1.051 V and 1.028 V, respectively, which might be equal to or greater than that of MoOx 48.

However, comparing fill factor of reference cell (FF = 66.3)

and (FF = 58.1), it is considered that the physical damage of ITO sputtering may remain some extent.

The resulting electrical properties of

the best 2-terminals tandem solar cell were observed as being a JSC of 12.30 mA cm-2, VOC of 1.564 V, FF 71.3 % of and η of 13.70 %.

It was confirmed that less hysteresis comparing

forward and back scanning for 2-terminals tandem solar cell shown in Figure 9b.

VOC of the

tandem solar cell reached the sum of the VOC yielded by the perovskite top cell and c-Si bottom cell.

The 12.30 mA cm-2 of JSC of tandem solar cell was limited by the JSC of the

c-Si bottom cell solar cell, which was filtered by the top cell.

If we can use 4-terminal

tandem device, the total conversion efficiency can be 14.4% (= 10.0% (perovskite) + 4.4% (c-Si)), which was free from the regulation by low photocurrent of filtered c-Si bottom cell.

5. Conclusions In this paper, a tandem device with a perovskite top solar cell and a c-Si bottom one was introduced.

Au and ITO films were utilized and optimized for junction tunneling

between the top and bottom cells.

The electrical characteristics of the best fabricated

tandem solar cell had a structure consisted of and was measured as JSC of 12.30 mA cm-2, VOC of 1.564 V, FF 71.3 % of and Eff of 13.70 %.

Evaporated Au (t = 2.5 nm, with a

transmittance of 99.15%) and sputtered ITO layer (t = l54 nm on Au, with a transmittance of 89.25%) were found to be the optimum stack layer on the rear side of the perovskite top cell. The direct sputtering of ITO onto HTL can cause damage in HTM.

Hence, a thin film of Au

deposited on HTL prior to ITO sputtering can avoid possible damage, which will be checked 23 ACS Paragon Plus Environment


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Page 24 of 29

in our future paper. The significant point is that these silicon solar cells have been fabricated without plasma-enhanced chemical vapor deposition (PECVD).

Such silicon solar cells fabricated

without PECVD constitute a cost-effective solar generation system in the future.

Moreover,

such 4-tarminal structure can perform the optimization of the perovskite top and c-Si bottom cells separately, including structural engineering and considering optical and material properties, which may lead to a magnificent improvement for such tandem solar cells. Considering the current match of series-connected top and bottom cells and improving the antireflection performance on the bottom cell, the perovskite-silicon tandem cells can be improved further in the near future.


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Bailat, J.; Yum, J.-H.; De Wolf, S.; Ballif, C. Organic–inorganic Halide Perovskite/crystalline Silicon Four-Terminal Tandem Solar Cells. Phys. Chem. Chem. Phys. 2015, 17 (3), 1619–1629.


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