Document [original]

27.9% Efﬁcient Monolithic Perovskite/Silicon Tandem

Solar Cells on Industry Compatible Bottom Cells

Eike Köhnen, Philipp Wagner, Felix Lang, Alexandros Cruz, Bor Li, Marcel Roß,

Marko Jošt, Anna B. Morales-Vilches, Marko Topiˇc, Martin Stolterfoht, Dieter Neher,

Lars Korte, Bernd Rech, Rutger Schlatmann, Bernd Stannowski,* and Steve Albrecht*

1. Introduction

Today's photovoltaic market is dominated

by crystalline silicon-based solar cell tech-

nology. With a record power conversion

efﬁciency (PCE) of 26.7%,

[1]

silicon

single-junction solar cells are approaching

their theoretical limit of 29.4%.

[2]

To over-

come this limit, silicon solar cells can be

combined with wider bandgap materials

into multijunction solar cells, where each

photovoltaic active material converts a spe-

ciﬁc part of the spectrum efﬁciently into

electrical power. With two active materials

(commonly termed tandem solar cells), the

theoretical PCE limit is 46% based on

detailed balance arguments.

[3]

The excel-

lent optoelectronic properties as well as

the tunable bandgap and potentially

low-cost fabrication make metal halide per-

ovskites suitable candidates for the top cell

material in tandem solar cells.

[4–13]

Only

3 years after the ﬁrst realization of a

p–i–n tandem solar cell by Bush et al.,

[14]

E. Köhnen, Dr. P. Wagner, B. Li, M. Roß, Dr. L. Korte, Prof. S. Albrecht

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH

Young Investigator Group Perovskite Tandem Solar Cells

12489 Berlin, Germany

E-mail: steve.albrecht@helmholtz-berlin.de

Dr. F. Lang, Dr. M. Stolterfoht, Prof. D. Neher

Institute of Physics and Astronomy

University of Potsdam

14476 Potsdam, Germany

Dr. A. Cruz, Dr. A. B. Morales-Vilches, Prof. R. Schlatmann,

Prof. B. Stannowski

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH

PVcomB

12489 Berlin, Germany

E-mail: bernd.stannowski@helmholtz-berlin.de

The ORCID identiﬁcation number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/solr.202100244.

open access article under the terms of the Creative Commons Attribution

License, which permits use, distribution and reproduction in any medium,

provided the original work is properly cited.

DOI: 10.1002/solr.202100244

Dr. M. Jošt, Prof. M. Topiˇc

Faculty of Electrical Engineering

University of Ljubljana

Tržaška 25, 1000 Ljubljana, Slovenia

Prof. B. Rech

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH

Scientiﬁc Management

12489 Berlin, Germany

Prof. B. Rech, Prof. S. Albrecht

Faculty IV –Electrical Engineering and Computer Science

Technical University Berlin

10587 Berlin, Germany

Prof. R. Schlatmann

Faculty 1 School of Engineering –Energy and Information

HTW Berlin–University of Applied Sciences

12459 Berlin, Germany

Prof. B. Stannowski

Faculty II –Mathematics, Physics, Chemistry

Beuth University of Applied Sciences Berlin

13353 Berlin, Germany

Monolithic perovskite/silicon tandem solar cells recently surpass the efﬁciency of

silicon single-junction solar cells. Most tandem cells utilize >250 μm thick,

planarized ﬂoat-zone (FZ) silicon, which is not compatible with commercial

production using <200 μm thick Czochralski (CZ) silicon. The perovskite/silicon

tandem cells based on industrially relevant 100 μm thick CZ-silicon without

mechanical planarization are demonstrated. The best power conversion efﬁciency

(PCE) of 27.9% is only marginally below the 28.2% reference value obtained on

the commonly used front-side polished FZ-Si, which are about three times

thicker. With both wafer types showing the same median PCE of 27.8%, the thin

CZ-Si-based devices are preferred for economic reasons. To investigate

perspectives for improved current matching and, therefore, further efﬁciency

improvement, optical simulations with planar and textured silicon have been

conducted: the perovskite's bandgap needs to be increased by 0.02 eV when

reducing the silicon thickness from 280 to 100 μm. The need for bandgap

enlargement has a strong impact on future tandem developments ensuring

photostable compositions with lossless interfaces at bandgaps around or above

1.7 eV.

RESEARCH ARTICLE

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the highest scientiﬁcally reported efﬁciency of 29.15% is close to

the theoretical limit of silicon single-junction solar cells.

[15]

With

a certiﬁed efﬁciency of 29.52%, Oxford PV surpassed this limit

but did not disclose any further details.

[16]

These high efﬁciencies are achieved on rather thick

front-side polished ﬂoat-zone (FZ) silicon heterojunction solar

cells, which are industrially not relevant for three reasons:

1) chemical–mechanical polishing (CMP) is time consuming

and expensive. Therefore, it is desirable to use either chemical

polishing as it is used in passivated emitter and rear cell (PERC)

industry or double-side textured wafers. The latter approach is

favored because such textures can be produced in a standard

batch process and they provide optical advantages. Perovskite/

silicon tandem processing on such wafers is addressed in recent

publications.

[4,9,17–21]

However, solution processing of high-

efﬁciency tandem solar cells using such textures still remains

challenging due to the difﬁculties of processing very thin perov-

skite layers on micrometer-sized pyramid structures. 2) FZ-sili-

con is not used for PV mass production. Instead, Czochralski

(CZ) silicon will remain the main method to fabricate silicon

ingots,

[22]

mainly because of lower costs. 3) The absorption

of Si for photon energies just above the bandgap, i.e., in the

infrared (IR) part of the spectrum, is relatively poor. For tandem

cells, however, where the top cell will absorb most of the higher

energy photons, the IR response of the bottom cells is

crucial.

[5,23]

Therefore, the bottom cell thickness in most

publications on perovskite/silicon tandem solar cells is 260 to

300 μm, whereas according to current market forecasts, the

industrially relevant thickness for n-type monocrystalline

silicon is just 140 to 150 μm (as cut) in 2022.

[24]

In this article, we demonstrate for the ﬁrst time monolithic

perovskite/silicon tandem solar cells based on thin non-CMP

n-type CZ-silicon bottom cells. The reduced response in

the IR region for thinner bottom cells will shift the optimal

top cell bandgap for standard test conditions toward larger

energies.

2. Solar Cells

We use (100)-oriented 130 μm thick (as cut) n-type CZ-silicon

wafers with random pyramids on the rear side and a speciﬁed

resistivity of 5Ωcm. The front side of these CZ-based bottom

cells was chemically polished using standard etching procedures

in PERC industry but using a more aggressive treatment, remov-

ing up to 20 μm, to obtain a surface compatible with the top-cell

processing.

[25,26]

Tandem solar cells with these type of bottom

cells are termed “CZ-based.”As a reference, we use

(100)-oriented 280 20 μm thick FZ wafers with random pyra-

mids on the rear side, a CMP front side, and a resistivity of

3Ωcm (in the following termed “FZ-based”). The front and

rear side of all wafers are passivated with intrinsic amorphous

silicon ((i)a-Si:H) layers. On the rear side, p-doped a-Si:H is

deposited on the passivating layer. N-doped nanocrystalline sili-

con oxide (nc-SiO

:H) optimized in refractive index for optimum

NIR incoupling on the front passivating layer serves as

electron-selective contact.

[5]

All silicon layers were deposited by

plasma-enhanced chemical vapor deposition (PECVD). On top

of the (n) nc-SiO

:H layer, an In

-based transparent conduct-

ing oxide (TCO) is deposited, whereas the rear contact consists of

a layer stack of aluminum-doped zinc oxide (AZO) and silver.

More details can be found in the Materials and methods section

in the Supporting Information. After processing the bottom cells,

the measured thicknesses of the CZ- and FZ-based bottom cells

are 100 and 280 μm, respectively. Figure 1 shows photographs of

the polished and nonpolished bottom cell front surfaces and the

topography of the respective wafers acquired via confocal 3D

laser scanning microscope (CLSM). For the CMP surface of

the FZ silicon, horizontal artifacts appeared during acquisition

Figure 1. A,B) Photographs of the CZ-based silicon bottom cell and chemical–mechanical polished FZ-based silicon bottom cell. C,D) The topography of

the CZ-based bottom cell and FZ-based bottom cell is acquired with a confocal laser scanning microscope and similar z-scalebars. E) Schematic stack of

the monolithic perovskite/silicon tandem solar cell used in this work.

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leading to falsiﬁed roughness values (see Figure S2, Supporting

Information, for an adjusted scale). Therefore, atomic force

microscopy (AFM) is used to image and analyze the surface

of the FZ-based bottom cell (Figure S3, Supporting

Information). The root mean square roughness values (Sq) are

extracted from the areas as shown in Figure S3, Supporting

Information. They amount to 1 and 736 nm for the FZ-based

and CZ-based bottom cells, respectively. The maximum height

values (Sz) are 9 nm and 7.7 μm for the FZ-based and CZ-based

bottom cells, respectively. Although the Sz for CZ silicon is very

high, the lateral dimension of the features is large enough to

enable complete coverage during spin coating. This is evident

for the saw mark visible for the CZ silicon in the CLSM image

(Figure 1C): The step height is 5–6μm (see Figure S4,

Supporting Information), but since it extends over 100 μm, it

should not be problematic for solution-processed perovskite layers.

To investigate the inﬂuence of the different wafer types (i.e.,

thickness and topography) on the optical properties, we mea-

sured reﬂection of the bare wafers. The reﬂection spectra shown

in Figure S5, Supporting Information, demonstrate that the

reﬂection for wavelengths below 950 nm is not affected by the

difference in topography or thickness. For longer wavelengths,

the reﬂection is higher for the thin CZ silicon. Less light is

absorbed due to the thinner silicon. Thus, the amount of light

arriving at the rear side of the cell is increased, which conse-

quently increases the amount of light reﬂected at the rear side,

too. For the same reason, the light reﬂected at the rear side of the

cell is absorbed less while being transferred back in the thinner

silicon bottom cell, leading ultimately to an increased outcou-

pling at the front side and thus reﬂection.

For the p–i–n top cell which is identical on both types of bot-

tom cells (Figure 1E), a self-assembled monolayer, 2PACz, is

used as a hole-selective layer (HSL). In addition to its electrical

advantages, it enables conformal coverage on the nonpolished

bottom cell.

[27]

The nominal perovskite composition is

0.05

(MA

0.23

0.77

)

0.95

Pb(Br

0.23

0.77

)

yielding a bandgap of

1.68 eV. On top of the perovskite, LiF and C

are deposited

via thermal evaporation and SnO

is deposited via atomic layer

deposition. After the sputter deposition of zinc-doped indium

oxide (IZO) as transparent conductive oxide, Ag and LiF are ther-

mally evaporated as split ring-type bus bar electrode and antire-

ﬂective coating, respectively. The active area of the resultant

tandem solar cells is 1 cm

. A detailed description can be found

in the Materials and Methods section in the Supporting

Information. To monitor the process, opaque perovskite single-

junction solar cells with an active area of 0.16 cm

were fabricated

together with the tandem solar cells. The median performance

values (10 devices) for opaque perovskite single-junctions are

78.5% for the ﬁll factor (FF), 20.3 mA cm

2

for the short-circuit

current density (J

), 1.2 V for the open-circuit voltage (V

and 19.3% for the PCE (see Figure S6, Supporting

Information). A maximum efﬁciency of 19.9% with a V

1.21 V was obtained in this p–i–ntypeconﬁguration, which is

among the highest PCE and V

values for perovskite cells as

typically used in two-terminal tandem solar cells.

[28]

Figure 2A shows the external quantum efﬁciency (EQE) and

reﬂection spectra of two-terminal (monolithic) tandem solar cell

champion devices based on thin CZ and thick FZ bottom cells.

In the short-wavelength range, a minor difference in reﬂection

occurs. We account this difference to very slight variations of layer

thicknesses in the front contact, leading to altered interference pat-

terns. The difference in the long wavelength range is a result of a

difference in the bottom cell thickness, as described previously.

For both tandem solar cells, the EQE spectra of the perovskite sub-

cells (top cells) are very similar. Consequently, the photogenerated

current densities (J

) under 100 mW cm

2

AM1.5 G illumina-

tion are also similar in both perovskite subcells (19.56 and

19.44 mA cm

2

for the CZ and FZ cells, respectively). The main

difference between the tandem cells occurs in the EQEs of the

silicon subcells (bottom cells). The J

in the silicon bottom cell

of the thick FZ-based tandem solar cell is 19.08 mA cm

2

.The

reduced bottom cell thickness in the CZ-based tandem solar cell

causes a lower response in the near-IR region leading to a reduced

photogenerated current density of 17.81 mA cm

2

.Therefore,the

cumulative photogenerated current density decreases from

38.52 to 37.37 mA cm

2

.AstheJ

of tandem solar cells is

mainly determined by the J

of the limiting subcell, a lower

for the thin CZ-based tandem solar cell is expected. In contrast,

Figure 2. A) Comparison of the EQE spectra and reﬂection spectra (denoted as 1-R) between champion tandem solar cells based on thick FZ-silicon and

thin CZ-silicon. B) J–Vmeasurement of the tandem solar cells shown in (A) including the photovoltaic parameters and values obtained by 5 min MPP-

tracking as shown in Figure S9, Supporting Information.

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the current mismatch between the subcells increases. As we have

reported previously, the tandem's FF is affected by the current

mismatch.

[7]

Generally, the FF increases with larger mismatch.

In addition, thinner silicon wafers lead to higher V

values

due to an decreasing total recombination current density.

[2]

To esti-

mate the gain in V

, we simulated silicon single-junction solar

cells (illumination spectrum as in the tandem) with CZ- and FZ-

silicon as used for tandem solar cells with the program Quokka3

(see Figure S7, Supporting Information, and the Materials and

Methods section in the Supporting Information for more details).

enhancement of 17 mV is expected when using 100 μm

CZ-silicon instead of 280 μm FZ-silicon. Even though the FF of

the bottom cell also depends on the thickness and fabrication

method, the simulations reveal that the conﬁgurations investi-

gated in this work, both cell types, FZ and CZ, should deliver

the same FF of 82.5% to 83% (see Figure S7, Supporting

Information). Summarizing, the thinner CZ-based tandem solar

cell is expected to have a lower J

,higherFF(duetolargercurrent

mismatch), and higher V

The J–Vcurves shown in Figure 2B conﬁrm these expecta-

tions. The best reference device based on thick FZ wafers has

of 19.13 mA cm

2

of 1.89 V, and FF up to 78.01%

and as a result a PCE of up to 28.15%. This value is in very good

agreement to our previous results for similar tandem layer

stacks.

[15]

For the thin CZ tandem solar cell, the high FF of

80.89% partially compensates the lower J

of 17.81 mA cm

2

Combined with a higher V

of 1.94 V, the PCE of this cell is

27.89%. This value is just 0.26%

abs

below the PCE of the

front-side polished, thick FZ reference cell. Note that another

J–Vscan of the same cell led to a similar but yet slightly higher

FF of 81.15% which is to the best of our knowledge the highest

FF presented for perovskite/silicon tandem solar cells to date (see

Figure S8, Supporting Information). The improvement of the FF

per mismatch is higher than what we reported previously,

[7]

but

as elaborated by Boccard et al., the improvement in FF depends

strongly on the performance of the individual subcells.

[29]

Stable

operation of the herein presented tandem solar cells is

highlighted by 5 min maximum power point (MPP)-tracks as

shown in Figure S9, Supporting Information. After 300 s

MPP-tracking, PCE values of 28.05% and 27.81% are measured

for the FZ- and CZ-based device, respectively, which is well in

line with the J–Vcurve-derived efﬁciency. The illuminated

J–Vresults for three CZ and four FZ tandem solar cells are sum-

marized in Figure S10, Supporting Information. They reveal the

same median PCE of 27.8% for both CZ- and FZ-based tandem

solar cells. The V

improvement by 30 to 40 mV for the best

devices is slightly more than expected from simulations.

Therefore, we measured absolute photoluminescence of the

top and bottom cell for both conﬁgurations to extract the

quasi-Fermi level splitting (QFLS or implied V

[30,31]

The

intensity of the laser was set to match the current density gener-

ated within each subcell under AM1.5 G illumination. The PL

spectra, QFLS values, and radiative limits are shown in

Figure S11, Supporting Information. For the perovskite subcell,

the QFLS values are the same on both the FZ- and CZ-based tan-

dem solar cells. They amount to 1.20 eV, which is consistent

with the V

of the perovskite single-junction solar cells (see

Figure S6, Supporting Information). The QFLS of the silicon

subcell in the FZ-based tandem solar cell is 690 meV.

Consequently, the sum of the perovskite and silicon QFLS for

the FZ-based cell is 1.89 eV, which is in very good agreement

with its V

extracted from the J–Vcurve (1.90–1.91 V for this

speciﬁc sample). For the CZ-based tandem cell, a QFLS of

710 meV in the Si wafer was calculated. The enhancement of

19 meV compared to the FZ-based cell matches well with

the simulated V

enhancement of 17 mV. We ﬁnd well-agree-

ing values of the cumulative QFLS (1.910 eV) and the measured

(1.92–1.93 V for this sample) for the CZ-based tandem cells.

Therefore, we account the previously mentioned V

improve-

ment of up to 40 mV to a sample to sample variation.

To exclude any structural changes in the perovskite due to dif-

ferent surface topographies of the bottom cells, X-ray diffraction

(XRD) measurements were conducted. The XRD patterns

acquired for the HSL/perovskite stack deposited on the different

bottom cells reveal similar crystallization of the perovskite ﬁlms

on both surfaces (see Figure S12, Supporting Information). We

attribute the additional peak around 32.8for the FZ-sample to

stem from the In

-based recombination layer.

To analyze the effect of the bottom cell in more detail, we mea-

sured the J–Vcurve of the CZ-based tandem solar cell in a way that

the J

values of both subcells are equal to the respective J

values

in the FZ tandem solar cell (i.e., same mismatch conditions for

CZ- and FZ-based tandem solar cells). This required to increase

the illumination intensity only in the IR region of the spectrum,

which can be easily done with the utilized light-emitting diode

sunsimulator. In Figure S13, Supporting Information, this J–V

measurement with adjusted J

values is compared to the J–V

of the FZ tandem solar cell under AM1.5G conditions. In addition

to the increased V

, just a slight deviation occurs at voltages just

below the MPP. The FF values of both measurements are very

similar, demonstrating that the increased FF of the CZ tandem

solar cell under AM1.5G conditions arises mainly from the

increased current mismatch.

[7]

The long-term stability of one CZ- and two FZ-based tandem

solar cells (nonencapsulated) is shown in Figure S14, Supporting

Information. The initial PCE values are 27.6% (CZ), 28.15%

(FZ), and 27.4% (FZ). The cells were held at 25 C in air, the

relative humidity (RH) was not actively controlled. In addition

to the J

MPP

, the resulting PCE, and the normalized

PCE, we show time series of the cell temperature and RH.

The latter one ranges from 11% to 26%. After 1000 h continuous

tracking, the cells were still performing at 67% (CZ), 70% (FZ),

and 67% (FZ) of their respective initial PCE values, where the

main parameter driving the reduction in PCE is J

MPP

These efﬁcient monolithic perovskite/silicon tandem solar cells

demonstrate that it is not mandatory to use chemical-mechanical

polishing for spin-coated perovskite ﬁlms. Instead, chemical pol-

ishing as it is already deployed in industry, is sufﬁcient for solution

processing such as spin coating. Furthermore, it shows that indus-

try relevant, almost threefold thinner CZ-silicon wafers can enable

the same performance as the thick, CMP FZ-silicon wafers stan-

dardly used in lab-scale devices.

3. Optical Simulation

The reduced photogenerated current density for thinner silicon

bottom cells necessitates adjustments to achieve current

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matching or power matching conditions. Although the afore-

mentioned experiment and previous reports demonstrate that

the tandem solar cells are not highly sensitive to current

mismatch conditions,

[7,32–34]

the highest PCE values might be

achieved with current or power matching conditions.

Moreover, there are various effects affecting the mismatch con-

ditions, as we will discuss later. Each of the effects needs to be

controlled to obtain ultimately a current or power matched tan-

dem solar cell.

Apart from increasing the IR response of the bottom cell by

optical improvements, which is not the focus of this study, current

matching can be achieved by reducing the perovskite thickness

and/or widening the perovskite bandgap. To shed light on this

aspect, we used GenPro4 to simulate the optical performance

of tandem solar cells.

[35]

We simulated tandem solar cells with

a silicon bottom cell thickness of 100 and 280 μm. For both bottom

cells, the perovskite thickness and its bandgap were varied. The

lower limit of the thickness of 700 nm represents a realistic case

as this thickness can be easily achieved with solution process-

ing.

[9,10,17,18,21,36–38]

As the J

saturates toward thicker ﬁlms

(Figure 3A), an upper limit of 1500 nm was chosen. For each thick-

ness, the bandgap of the perovskite was varied by shifting the

refractive index nand extinction coefﬁcient k(measured via spec-

tral ellipsometry for E

¼1.63 eV) along the wavelength axis to

cover a bandgap range of 1.63 to 1.78 eV (Figure S15,

Supporting Information).

[39,40]

The bandgap is taken as the inﬂec-

tion point of the perovskite absorption edge as shown in

Figure S16, Supporting Information. All other layers were kept

as in the experiment. An example of simulated EQE spectra with

various perovskite bandgaps is shown in Figure S17, Supporting

Information. Figure 3B shows the ideal top cell bandgap

g,top,matched

as a function of the top cell's thickness when utilizing

a280μm and 100 μm thick silicon bottom cell (see also Table S3,

Supporting Information). The photogenerated current density of

the perovskite top cell J

Ph,Perovskite

increases with thicker perovskite

layers. As a consequence, the photogenerated current density of

the silicon bottom cell J

Ph,Silicon

decreases with thicker perovskite

layers (Figure S18, Supporting Information). Thus, for thicker

perovskite layers, it is necessary to widen the top cell bandgap

if current matching is desired. When increasing the thickness

from 700 to 1500 nm, the top cell bandgap needs to be increased

by 0.047 eV for both bottom cell thicknesses. In the best case, this

would improve the V

. As evident from Table S3, Supporting

Information, the matched photogenerated current density

Ph,matched

stays almost constant. In addition, the bottom cell thick-

ness alters current matching conditions. We found that the reduc-

tion of the bottom cell thickness from 280 to 100 μm requires to

widen the top cell bandgap by 0.02 eV, regardless of the perov-

skite's thickness. However, simultaneously J

Ph,matched

decreases

from 19.64 to 19.14 mA cm

2

(average values). Therefore, for a

perovskite thickness of 700 nm, its bandgap needs to increase

from 1.69 to 1.71 eV if the bottom cell thickness is reduced.

The higher V

from both top and bottom cell together

(40–50 mV) will exactly compensate the reduced J

(when assum-

ing that J

Ph,matched

¼J

). Obviously, the FF of the perovskite top

cells needs to remain the same regardless of the perovskite thick-

ness to maintain the high PCE. Ultimately, a trade-off between

high J

(thick silicon wafer and narrow perovskite bandgap)

and high V

(thin silicon wafer and wide perovskite bandgap)

needs to be made to yield the highest efﬁciency. Finding this opti-

mum bottom cell thickness will be work for the future. These sim-

ulations do not include any optimization of other (e.g., contact)

layer thicknesses. The adjustment of these layer thicknesses

can reduce the interference patterns appearing, especially in the

NIR wavelength range for the silicon subcell (see Figure S18,

Supporting Information). This would require an optimization

for each individual top cell bandgap and thickness.

The same simulations were performed for double-side tex-

tured tandem solar cells. As previously simulated and experimen-

tally demonstrated,

[4,6,19,20]

the additional front-side texture

reduces reﬂection and removes interference patterns, enabling

higher J

and J

values (see Figure S19, Supporting

Figure 3. A) Simulated photogenerated current densities J

of perovskite/silicon tandem solar cells as a function of the perovskite thickness. The

thickness of the silicon bottom cell is 100 μm and the perovskite bandgap is 1.73 eV. The rear side of the tandem cells is textured. The front side

is either ﬂat (denoted as “Flat”) or textured (denoted as “Textured”). The corresponding EQE spectra are shown in Figure S18, Supporting

Information. B) Ideal top cell bandgap which is needed to obtain current matching conditions as a function of the perovskite's thickness. Reducing

the thickness of silicon from 280 to 100 μm increases the ideal top cell bandgap by 0.02 eV for both textured and ﬂat front sides. The values including

the photogenerated current density are shown in Table S3 (Flat) and Table S4 (Textured), Supporting Information.

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Information, for simulated EQEs). The same trends appear as for

planar devices: With thinner silicon, a larger perovskite bandgap

is needed to ensure current matching conditions (See Table S4,

Supporting Information and Figure 3). When reducing the sili-

con thickness from 280 to 100 μm, the perovskite bandgap

should increase by 0.019 eV to maintain current matching at

the same perovskite thickness. However, this comes along with

a reduction of J

ph,matched

by 0.5 mA cm

2

Ultimately, the optimum bandgap not just depends on the

thickness of the perovskite layer and the thickness of the silicon

wafer. Luminescence from the perovskite top cell into the silicon

bottom cell will relax the requirement for current matching con-

ditions.

[41]

Furthermore, it was previously shown that higher

operational temperatures and respective optical changes in top

and bottom cells will lead to different optimum perovskite top

cell bandgaps around 1.63 eV for highest energy yield with thick

bottom cell wafers.

[20]

The transition from a monofacial to a bifa-

cial tandem solar cell reduces the bandgap as well, if current

matching should be maintained.

[21]

Therefore, the ideal device

design cannot be easily derived from the performance under

standard test conditions but needs to be derived for each case

individually considering realistic outdoor conditions.

4. Conclusion

We demonstrated perovskite/silicon tandem solar cells based on

industrially relevant silicon bottom cells, namely, 100 μm thin

CZ-wafer with an industrial deployed chemical polishing for

the front side and a textured rear side. For comparison, we fab-

ricated tandem cells based on 280 μm thick FZ-wafers with a

chemical–mechanical polished front side, which is standardly

used in lab-scale devices. The CZ-based tandem cells have a

PCE of up to 27.89%, which is just slightly below the value of

28.15% for FZ-based tandem cells. However, the median PCE

of 27.8% indicates equal performance for both bottom cell types.

The median V

increases from 1.89 V (max. 1.91 V) for the

FZ-based cells to 1.92 V (max. 1.94 V) for the CZ-based cells,

explained by the higher V

of the thin CZ bottom cell.

Simultaneously, thinner silicon bottom cells present a lower

EQE in the IR region, leading to a lower photogenerated current

density and, thus, a lower J

(19.1 vs 17.8 mA cm

2

). The

increased mismatch, when using an identical top cell, results

in improved FF values (77.2% vs 80.9%). After 1000 h continu-

ous MPP-tracking, the nonencapsulated cells still performed at

67% (CZ) and 67 to 70% (FZ) of their initial PCEs. We performed

optical simulations to ﬁnd current matching conditions for the

100 and 280 μm silicon bottom cells with planar and textured

front sides. The perovskite bandgap needs to be increased by

0.02 eV when using a 100 μm thin silicon wafer instead of

the commonly used thickness of 280 μm. Simultaneously, the

expected J

reduces by 0.5 mA cm

2

. The higher V

from

both top and bottom cell together (40 to 50 mV) can exactly com-

pensate the reduction in J

for thinner wafers. Thus, to achieve

highest PCE values with industrial bottom cells, the perovskite's

bandgap needs to be widened to values well over 1.71 eV. The

precise optimum top cell bandgap in this region is highly impor-

tant, as these wide bandgap compositions are typically prone

to phase segregation or are limited by nonradiative

recombination.

[42,43]

Therefore, this work highlights that further

investigation is needed to enable highly efﬁcient and stable wider

bandgap compositions and with that, highest tandem PCE values

when using industry relevant bottom cells.

5. Experimental Section

Detailed information about the fabrication and characterization is given

in the Supporting Information.

Supporting Information

Supporting Information is available from the Wiley Online Library or from

the author.

Acknowledgements

The authors acknowledge help in technical assistance by T. Lußky, H. Heinz,

M. Gabernig, and C. Ferber, Institute for Silicon Photovoltaics and

M. Muske, T. Henschel, K. Mayer-Stillrich, H. Rhein J. Kleesiek, PVcomB.

The authors acknowledge the support of Thorsten Dullweber and Silke

Dorn (both ISFH) for the chemical polishing of CZ wafers. The authors

acknowledge funding from HyPerCells (Hybrid Perovskite Solar Cells,

http://www.perovskites.de) joint Graduate School, as well as from the

German Federal Ministry for Economic Affairs and Energy (BMWi) through

the “PersiST”project (grant no. 0324037C) as well as ProTandem (grant no.

0324288C). Further funding was provided by the Federal Ministry of

Education and Research (BMBF) for funding of the Young Investigator

Group Perovskite Tandem Solar Cells within the program

“Materialforschung für die Energiewende”(grant no. 03SF0540) and by

the Helmholtz Association within the projects HySPRINT Innovation lab

and TAPAS (Tandem Perovskite and Silicon solar cells—Advanced optoe-

lectrical characterization, modelling, and stability). F.L. acknowledges ﬁnan-

cial support from the Alexander von Humboldt Foundation via the Feodor

Lynen program. The authors also acknowledge ﬁnancial support by the

Federal Ministry for Economic Affairs and Energy within the framework

of the 7th Energy Research Programme (P3T-HOPE, grant no.

03EE1017C). M.J. and M.T. acknowledges ﬁnancial support from the

Slovenian Research Agency (ARRS) within the grants P2-0197 and J2-1727.

Open access funding enabled and organized by Projekt DEAL.

Conﬂict of Interest

The authors declare no conﬂict of interest.

Data Availability Statement

The data that supports the ﬁndings of this study are available in the

supplementary material of this article or from the corresponding authors

upon reasonable request.

Keywords

industry, perovskites, silicon, tandem solar cells

Received: April 2, 2021

Published online: May 5, 2021

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