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RESEARCH ARTICLE

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Minimizing Interfacial Recombination in 1.8 eV Triple-Halide

Perovskites for 27.5% Eﬃcient All-Perovskite Tandems

Fengjiu Yang,* Philipp Tockhorn, Artem Musiienko, Felix Lang, Dorothee Menzel,

Rowan Macqueen, Eike Köhnen, Ke Xu, Silvia Mariotti, Daniele Mantione, Lena Merten,

Alexander Hinderhofer, Bor Li, Dan R. Wargulski, Steven P. Harvey, Jiahuan Zhang,

Florian Scheler, Sebastian Berwig, Marcel Roß, Jarla Thiesbrummel, Amran Al-Ashouri,

Kai O. Brinkmann, Thomas Riedl, Frank Schreiber, Daniel Abou-Ras, Henry Snaith,

Dieter Neher, Lars Korte, Martin Stolterfoht, and Steve Albrecht*

All-perovskite tandem solar cells show great potential to enable the highest

performance at reasonable costs for a viable market entry in the near future.

In particular, wide-bandgap (WBG) perovskites with higher open-circuit

voltage (VOC) are essential to further improve the tandem solar cells’

performance. Here, a new 1.8 eV bandgap triple-halide perovskite

composition in conjunction with a piperazinium iodide (PI) surface treatment

is developed. With structural analysis, it is found that the PI modiﬁes the

surface through a reduction of excess lead iodide in the perovskite and

additionally penetrates the bulk. Constant light-induced magneto-transport

measurements are applied to separately resolve charge carrier properties of

electrons and holes. These measurements reveal a reduced deep trap state

density, and improved steady-state carrier lifetime (factor 2.6) and diﬀusion

lengths (factor 1.6). As a result, WBG PSCs achieve 1.36 V VOC, reaching 90%

of the radiative limit. Combined with a 1.26 eV narrow bandgap (NBG)

perovskite with a rubidium iodide additive, this enables a tandem cell with a

certiﬁed scan eﬃciency of 27.5%.

F. Yang, P. Tockhorn, A. Musiienko, D. Menzel, R. Macqueen, E. Köhnen,

K. Xu, S. Mariotti, B. Li, D. R. Wargulski, J. Zhang, F. Scheler, S. Berwig,

M. Roß, A. Al-Ashouri, D. Abou-Ras, L. Korte, S. Albrecht

Division Solar Energy

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH

12489 Berlin, Germany

E-mail: [email protected];[email protected]

F. Yang

National Renewable Energy Laboratory

Golden, Colorado 80401, USA

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

can be found under https://doi.org/10.1002/adma.202307743

GmbH. This is an 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/adma.202307743

1. Introduction

Metal-halide perovskites have raised great

interest for the application in tandem

solar cells since they oﬀer the ability to

greatly exceed the eﬃciency of today’s in-

cumbent photovoltaics (PVs) technologies,

which are based on single junctions of

silicon (Si), or thin-ﬁlm semiconductors.

One of the key features of perovskites is

that the bandgap can be tuned through

composition engineering.[1–3]Choosing

an optimum combination of bandgaps

allows to eﬃciently utilize sunlight in

two (or more) absorber layers, such as

in the combination of perovskite/Si,[4–6]

perovskite/CIGS (copper indium gallium

selenide),[7–9]perovskite/organic,[10–12]and

perovskite/perovskite[13–30]tandem solar

cells. Particularly, all-perovskite tandems

are of great interest since they have the

F. Lang, D. Neher, M. Stolterfoht

Institute of Physics and Astronomy

University of Potsdam

14476 Potsdam-Golm, Germany

D. Mantione

POLYMAT

University of the Basque Country UPV/EHU

Av. Tolosa 72, Donostia-San Sebastián 20018, Spain

D. Mantione

IKERBASQUE

Basque Foundation for Science

Bilbao 48009, Spain

D. Mantione

POLYKEY s.l.

Av. Tolosa 72, Donostia-San Sebastián 20018, Spain

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potential to be manufactured at lower costs and CO2emis-

sions than silicon-containing tandem technologies.[31]Typically,

an all-perovskite tandem solar cell (APTSC) combines a narrow-

bandgap (NBG, ≈1.25 eV) mixed tin-lead (Sn-Pb) perovskite for

the bottom cell, and a wide-bandgap (WBG, ≈1.8 eV) lead (Pb)

perovskite for the top cell.[13–15,19,26,27,29,32–35]The highest certiﬁed

power conversion eﬃciency (PCE) of APTSC is 29.1%, which

largely surpasses the best single-junction perovskite solar cells

(PSCs, 26.1% PCE).[36,37]In addition, the PCE of all-perovskite

tandems increased rapidly over the last years due to signiﬁcant

improvements in the NBG Sn-Pb perovskite subcell. This orig-

inates from combined research eﬀorts to improve crystallinity,

suppress the oxidation of Sn2+, increase carrier lifetime and

diﬀusion length, and reduce trap density by employing a close-

space annealing strategy,[16]passivating molecules,[25,38]and

additives’ application.[14,26,27,32,39–44]

On the other hand, the performance of WBG top cells has to

be improved, as these cells face signiﬁcant recombination losses

due to halide-segregation, composition- and bromide (Br)-related

defects, and non-ideal contact layers.[2,45,46]In addition, there are

pronounced non-radiative recombination losses at the interface

of the perovskite with electron transport materials (ETMs, such

as fullerene-C60) that can reduce the VOC of WBG PSCs even

further, especially for bandgaps above 1.7 eV.[47]Post-treatment

of perovskite surfaces has been demonstrated as an eﬀective ap-

proach to reduce non-radiative recombination losses and achieve

higher VOC.[21,48,49]Despite these eﬀorts, most reported VOCsof

WBG PSCs are still below 90% of the radiative limit for bandgaps

≈1.80 eV, lagging behind the performance of perovskites with

bandgaps of 1.5–1.6 eV, which reach over 95% of the radiative

limit.[50]

L. Merten, A. Hinderhofer, F. Schreiber

Institute of Applied Physics

University of Tübingen

72076 Tübingen, Germany

S. P. Harvey

Materials, Chemical and Computational Sciences (MCCS)

National Renewable Energy Laboratory

Golden, CO 80401, USA

J. Thiesbrummel, H. Snaith

Clarendon Laboratory

Department of Advanced Materials and Interfaces for Photovoltaic Solar

Cells

University of Oxford

Parks Road, Oxford OX1 3PU, UK

K. O. Brinkmann, T. Riedl

Institute of Electronic Devices

University of Wuppertal

42119 Wuppertal, Germany

K. O. Brinkmann, T. Riedl

Wuppertal Center for Smart Materials & Systems

University of Wuppertal

42119 Wuppertal, Germany

M. Stolterfoht

Electronic Engineering Department

The Chinese University of Hong Kong

Hong Kong SAR, China

S. Albrecht

Faculty of Electrical Engineering and Computer Science

Technische Universität Berlin

Berlin Germany

2. Results and Discussion

In this work, we developed a 1.80 eV triple-halide WBG-

perovskite with piperazinium iodide (PI) as a surface

treatment.[50]Our study expands on our recent work[51]on

PI applied to perovskite in perovskite/silicon tandem solar cells

and deepens the understanding of the PI working mechanism.

We ﬁnd that PI is also exceptionally well suited to reduce non-

radiative recombination losses for perovskites with a bandgap

of 1.8 eV. With this approach, we reduced excessive lead iodide

in the perovskite ﬁlm, achieved lower non-radiative recombi-

nation losses, and increased carrier lifetime. In addition, we

study the charge carrier dynamics, with constant light-induced

magneto-transport (CLIMAT)[52,53]measurements to resolve

ﬁngerprints of free electrons and holes separately and ﬁnd that

the PI treatment increases the steady-state carrier lifetime and

diﬀusion lengths under one-sun illumination by a factor of 2.6

and 1.6, respectively. Quasi-Fermi level splitting (QFLS) analysis

demonstrated that PI entirely eliminated recombination losses

at the modiﬁed perovskite/C60 interface, reaching 1.45 eV, which

marks a signiﬁcant improvement over the reference (1.37 eV).

The PI-modiﬁed PSC achieved a maximum PCE of 20.3% (VOC:

1.34 V) and a maximum VOC of 1.36 V (PCE: 20.0%), which is

equal to 90% of the radiative limit, being among the highest

VOC for a WBG perovskite that can be integrated as the top cell

in APTSCs. Combining this superior WBG PSC with an NBG

Sn-Pb perovskite which utilizes rubidium iodide additive, we

achieved a tandem cell with a certiﬁed eﬃciency of 27.5% and

27.2% maximum power point (MPP) tracked eﬃciency. Further-

more, as evidenced through absolute PL measurements, the

potential PCE of these tandems was 30.4% with a VOC potential

of 2.18 V.k.

2.1. Wide-Bandgap Perovskite Study

We developed a triple halide WBG perovskite composition with

a bandgap of 1.80 eV, suitable for integration into all-perovskite

tandem solar cells.[55,56]For this, we determined bandgap, pho-

tovoltaic performance, and QFLS for diﬀerent variations of Br

(15% to 30%), and Cl contents (5% to 10%). As reported by Xu

et al., perovskite compositions comprising I, Br, and Cl show

a non-monotonous development of the bandgap in response to

variations at the X-site[56]and therefore require a careful opti-

mization, as detailed in Note I and Figures S1–S3 (Supporting

Information). In all cases, a self-assembled monolayer (2-(9H-

cabazol-9-yl)ethyl)phosphonic acid (2PACz) was employed as the

hole transport material (HTM). After optimization, we selected

triple-halide perovskite with a bandgap of 1.80 eV and a compo-

sition of FA0.78Cs0.22Pb(Br0.3I0.7)3+10%MAPbCl3.

To further improve the VOC of the WBG PSCs, we post-treated

the annealed WBG perovskite with the spin-coated ionic liq-

uid piperazinium iodide (PI), following a previously reported

procedure.[50,51]This leads to a signiﬁcant alteration of the per-

ovskite surface and of the selected ﬁlm properties as we describe

below. In addition, the PI molecule may interact with the per-

ovskite surface via its diﬀerent surface terminating ends, po-

tentially passivating diﬀerent defects such as vacancy defects

(such as VIand VMA/VFA) forming undercoordinated Pb2+and

halide sites.[50,57]Diﬀerent PI concentrations, ranging from 0.5

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Figure 1. a,b) ToF-SIMS depth proﬁle of reference and PI-treated perovskite ﬁlms with the ionic traces of MA, FA, Pb, I, In, and piperazinium. Note, that the

strong changes in signals at the near-surface region are due to surface artifacts inherent to SIMS data until surface contamination is removed and sputter

equilibrium is achieved. Additional ionic traces determined from the TO-SIMS measurements are displayed in Figure S7 (Supporting Information). c)

Peak area ratio of PbI2to perovskite (110) as a function of probing depth calculated from GIWAXS data at grazing angles ranging from 0°to 0.3°.d,e)

Top-view SEM-CL images of the reference and PI-modiﬁed perovskites, respectively. The PbI2can be distinguished by overlaying the SEM secondary

electron image (grey) with an SEM-CL intensity image (yellow).

to 2.0 mg mL−1, were employed to determine the optimum con-

ditions for the best performance, as detailed in Figures S4, S5

(Supporting Information). Solar cells employing 1 mg ml−1PI

showed the highest PCE, which can mostly be attributed to an in-

crease. This suggests that, for multiple perovskite compositions,

PI seems to be a very promising and unique molecule for surface

treatment in high-eﬃciency top cells for tandem applications. In

this study, we attempt to investigate the working mechanism of

PI on the WBG perovskite and expand the understanding of its

inﬂuence on structural properties, defect density, and charge car-

rier dynamics of the perovskite.

First, we investigated the inﬂuence of PI on the structural

properties of the WBG perovskite. For this, we performed atten-

uated total reﬂection Fourier-transformed infrared (ATR-FTIR,

Figure S6, Supporting Information) spectroscopy and time-of-

ﬂight secondary ion mass spectrometry (ToF-SIMS, Figure 1a,b)

measurements to prove the existence of PI after treatment. Figure

S6 (Supporting Information) displays the vibrational spectra of

PI powder, perovskite, and PI-modiﬁed perovskite. The PI modi-

ﬁcation leads to a reduction of transmission in the ﬁngerprint re-

gion (1050–1350 cm−1), which is challenging to deconvolute, due

to the multitude of diﬀerent vibrational modes at low wavenum-

bers, which can also be observed in the spectrum of PI powder.

Yet, no clear change in the amidic (N-H) vibration mode (≈3000–

3500 cm−1)[59]could be observed, possibly due to the low concen-

tration of PI and the occurrence of N-H bonds in the perovskite

itself. These measurements indicate the presence of the piper-

azinium cation on the perovskite surface, without evidence of

strong interaction with the organic framework of the perovskite.

To further explore the integration of PI into the perovskite, ToF-

SIMS measurements were performed. As expected, the ionized

mass corresponding to piperazinium is in the noise level for the

reference (Figure 1a), while for the PI-modiﬁed sample, the trace

of a piperazinium cation expanding well into the perovskite was

detected (Figure 1b). These results also demonstrate that the PI

treatment does not alter the concentration of other constituents

in the perovskite such as methylamine or formamidinium. A set

of more device-relevant ionic species is displayed in Figure S7

(Supporting Information).

To understand the inﬂuence of PI on the WBG perovskite’s

crystal structure X-ray diﬀraction (XRD) in Bragg–Brentano and,

grazing incidence (GIXRD, 0.2°GI angle) conﬁguration, and

grazing-incidence wide-angle X-ray scattering (GIWAXS) have

been employed, with results shown in Figure 1c,FiguresS8, S9

(Supporting Information). As seen in Figure S8a (Supporting

Information), the crystal structure of perovskite ﬁlms was not

changed before and after PI modiﬁcation. The GIXRD showed

the changed diﬀraction intensity of PbI2at the surface of the

PI-modiﬁed sample compared to the reference (Figure S8b,Sup-

porting Information). To quantify the reduction of PbI2, the GI-

WAXS measurements were employed by tuning the incidence

angles between 0°and 0.3°, enabling us to collect signals from

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various integrating depths in GIWAXS, as shown in Figure 1c.

The PbI2content decreased from 28.7% to 19.3% on average af-

ter PI modiﬁcation at the surface, which remains to inﬂuence the

integrated signal over the whole considered probing depth.

The decrease of PbI2upon PI treatment and surface features of

perovskite ﬁlms were further conﬁrmed by scanning electron mi-

croscopy cathodoluminescence (SEM-CL) with 500 nm CL emis-

sion and 500 ±50 nm band pass ﬁlter, and SEM images. The

PbI2-to-perovskite coverage ratio reduced (i.e., 12.6% for the ref-

erence, and 8.1% for the PI-modiﬁed samples) upon PI modiﬁ-

cation, as evident from the overlay of the SEM secondary electron

image (grey) with an SEM-CL intensity image (yellow), as shown

in Figure 1d,e and Table S1 (Supporting Information). To exclude

the solvent eﬀect, we also conducted a control experiment, in

which we rinsed the surface with IPA only. As expected, the SEM-

CL analysis (Figure S10, Supporting Information) only shows a

minor reduction of PbI2which is further conﬁrmed by top-view

and cross-sectional SEM images (Figure S11, Supporting Infor-

mation). The role of surﬁcial PbI2and its inﬂuence on optoelec-

tronic properties was extensively investigated in previous studies

and is still under debate.[60]Typically, it is found that an optimum

surface coverage of PbI2enhances the device’s performance.[55]

Macroscopically, the PI treatment does not alter the optical prop-

erties of the perovskite absorber as evidenced by reﬂection and

transmission measurements (Figure S12, Supporting Informa-

tion).

Time-dependent steady-state PL spectra of the neat perovskite

on quartz revealed that the PI-modiﬁed perovskite featured an

improved photostability when compared with the reference sam-

ple (Figure 2a,b). The PI-modiﬁed sample showed a slight light

soaking eﬀect with increasing PL intensity over time, while it

declined over time and suﬀered from the emergence of a sec-

ond PL emission feature for the reference, possibly due to halide

segregation.[10,61,62]To evaluate the initial non-radiative recom-

bination losses, absolute PL measurements were conducted to

determine the QFLS values (Figure S10b,c, Supporting Informa-

tion). The QFLS constitutes the upper limit for the VOC and can

thus enable valuable insights into the optoelectronic quality of

a photovoltaic absorber. The best QFLS of the PI-modiﬁed per-

ovskite reached 1.45 eV and was therefore slightly higher than

for the untreated reference (1.42 eV, see inset of Figure 2a,b).

This indicates that the PI modiﬁcation reduced the perovskite’s

non-radiative recombination losses, suggesting a passivation of

the surface by, for example, a reduced defect density. This is dif-

ferent from our previous study, where even a decrease in QFLS

after PI treatment of the absorber layer was found and hence no

indication of a chemical passivation.[51]A detailed analysis of the

inﬂuence of the charge transport materials (CTM) on QFLS fol-

lows below.

Transient PL (trPL) characterizations were conducted at one-

sun equivalent charge carrier generation to evaluate the carrier

lifetime (𝜏) and recombination losses, as shown in Figure 2c

and Table S2 (Supporting Information). The carrier lifetime of

the neat perovskite ﬁlm on quartz improved signiﬁcantly up to

3.32 μs after PI modiﬁcation compared to 0.96 μs of the reference

sample.

To further study the inﬂuence of the PI on the surface prop-

erties of the, we conducted near-UV photoelectron spectroscopy

(PES) measurements with varying excitation energy in constant

ﬁnal state mode (CFSYS) for the perovskite with a layer stack

of glass/ITO/2PACz/perovskite (without/with PI). Under the as-

sumption of a constant dipole matrix element, CFSYS allows us

to trace the density of occupied states over a large dynamic range

in the valence band region and bandgap up to the Fermi edge.

The CFSYS spectra shown in Figure 2d are aligned at the mod-

eled valence band maximum (VBM) to allow for a direct compari-

son of the density of occupied defects in units of the internal pho-

toelectron yield.[63]The integrated density of defect states of the

PI-modiﬁed perovskite was signiﬁcantly decreased compared to

the reference sample as indicated by the grey arrow in Figure 2d.

This supports our results obtained by PL, suggesting chemical

passivation as the origin of the increased charge carrier lifetime

and enhanced QFLS of the PI-modiﬁed sample. Interestingly, a

reduced defect density and increase in QFLS were not found for

the PI-treated perovskite absorber in our above-mentioned study

on a diﬀerent perovskite composition.[51]The herein-found de-

fect states are likely to be attributed to PbI2on the surface,[63]and

their decrease is in line with the reduced PbI2concentration as

discussed in the structural analysis above. Our results agree well

with the chemical working mechanism of PI recently described

by Li et al. as acting as both, an electron donor and acceptor while

interacting with PbI2on the perovskite surface.[50]

Besides the inﬂuence of the defect density at the perovskite

surface, also the energy level alignment with the CTMs can play

a crucial role in the device’s performance. The energetic posi-

tions of the VBM and work function (WF) with respect to the

surface Fermi level (EF*) were determined from the CFSYS and

UPS (h𝜈=6.5 eV) spectra as shown in Figure S14 (Supporting

Information) and summarized in Figure 2e according to our pre-

viously established ﬁtting procedure.[64]We found that the Fermi-

level of the PI-modiﬁed sample shifted slightly further away from

the VBM as compared to the reference: EV−EF* increased from

−1.45 to −1.55 eV. Considering the optical bandgap of the per-

ovskite (1.8 eV), the conduction band minimum (CBM) of the

reference and PI-modiﬁed perovskites was determined to 0.35

and 0.25 eV above the Fermi-level, respectively, indicating an in-

creased electron concentration at the perovskite surface after PI

treatment. The WF of the reference and PI-modiﬁed samples is

found to be very similar with 4.45 and 4.48 eV (Figure S14c,Sup-

porting Information), respectively. Hence, the ionization energy

of the PI-modiﬁed perovskite was increased by ≈130 meV com-

pared to the reference, which might be due to the formation of

a positive dipole on the surface. A downward oﬀset between the

perovskite conduction band minimum and the C60 LUMO can be

reduced by such a dipole and thereby lead to a beneﬁcial energy

level alignment.[51,64]

The dipole obtained here is roughly half the size of the dipole

found in our previous study, which might be caused by the dif-

ferent perovskite composition and/or PI concentration. To fully

reveal the chemical and physical mechanisms, a broader survey

study is required. Here, our results suggest that both the dipole

and decrease in surface defects contribute to the improved inter-

face.

To gain more insight into the defect physics, we applied a

unique version of four-point probe photo-Hall measurements,

based on a combination of light illumination with magnetic ﬁeld:

CLIMAT (Figure S15a,b, Supporting Information, see the Exper-

imental Section, Supporting Information for more details).[52,53]

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Figure 2. a,b) Steady-state PL spectra recorded as a function of time (11 min duration) for reference and PI-modiﬁed perovskites on a quartz substrate.

The inset shows the initial QFLS of the reference and PI-modiﬁed perovskites. c) PL transients of the reference and PI-modiﬁed perovskites on quartz

substrates. The noted lifetimes assigned to radiative recombination in the perovskite are ﬁtted with an exponential function over the ranges marked in

light red and grey. d) Valence band spectra measured by constant ﬁnal state yield spectroscopy (CFSYS) of the perovskite with and without PI modiﬁcation.

The spectra are aligned to the modeled VBM and the Fermi level at the surface (EF*) is indicated by small black and red arrows. The grey arrow indicates

the reduced density of defect states within the bandgap. e) Energetic positions of the perovskites’ valence and conduction band edges as well as

the vacuum level determined by near-UV PES. f) Electron n(I) and hole p(I) concentrations as a function of the generation rate (I) as extracted from

CLIMAT measurements. The simulated concentrations are included as dashed lines and measured data as symbols. g,h) Carrier lifetime and diﬀusion

length of the reference and PI-modiﬁed perovskites calculated from the concentration of electrons and holes under various illumination intensities. The

marked “1 sun” values correspond to a photogeneration equal to standard test conditions (AM1.5G spectrum, irradiance 100 mW cm−2). i) Deep trap

concentrations of the reference (Ref.) and PI-modiﬁed perovskites as simulated from the CLIMAT results.

This method allows to disentangle concentrations, lifetimes, and

diﬀusion lengths of electrons and holes and ultimately deter-

mines the bulk defect density by probing charge transport in

the plane parallel to the substrate. Illumination parameters in

a steady-state regime closely resembling “one sun” conditions,

reveal charge carrier transport that is essential to solar cell oper-

ation conditions.

First, we investigate the conductivity, Hall coeﬃcient, and

Hall mobility under various steady-state illumination conditions

(Figure S15d,e, Supporting Information), which can be extracted

by measuring the longitudinal voltage induced by the electrical

current without applied magnetic ﬁeld and transverse Hall volt-

age under oscillating the magnetic ﬁeld. With this, the electron

and hole concentration (nand p), mobilities (μeand μh), lifetimes

(𝜏eand 𝜏h), and diﬀusion lengths (Leand Lh) can be determined.

The reference sample showed a signiﬁcant p-type conductivity

for low generation rates, whereas the PI-modiﬁed sample ex-

hibited n-type conductivity (Figure 2f; Figure S15f, Supporting

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Information). This trend toward higher electron concentration

for PI-modiﬁed layers is consistent with the Fermi level ap-

proaching the CBM as observed by CFSYS. Note, that the ab-

solute values are however very diﬀerent, which is reasonable to

be expected. We attribute this to ﬁrst, the diﬀerent probing at-

mospheres and second, the diﬀerent probing depths of the mea-

surement techniques. The probing atmosphere has been found

to strongly impact the sample work function with typically much

lower work function under UHV conditions for PES as compared

to ambient or inert atmosphere (for CLIMAT).[65]Furthermore,

PES measurements are highly surface sensitive and only probe

the topmost few nm, while CLIMAT measurements are bulk sen-

sitive and hence over all much less sensitive to surface defects

or band bending. As the reference and PI-modiﬁed perovskites

are p-orn-type, respectively, the free hole (μh) and electron (μe)

mobilities (0.53 and 0.55 cm2V−1s−1) can be directly assigned

to the dominant carrier type at low illumination conditions (see

Equation SH3, Figure S15d,f, Supporting Information). With in-

creased illumination, the Hall mobility decreased to the diﬀer-

ence of electron and hole mobilities, as predicted by Equation

SH3 (Supporting Information). Note that Hall mobility includes

the contribution of both free electrons and holes (𝜇2

ppand 𝜇2

enin

Equation SH3, Supporting Information) and thus can decrease

when μhand μebecome similar.

Electron and hole concentrations (nand p) can be calculated by

solving the equations for semiconductor conductivity (Equation

SH1, Supporting Information) and for the Hall coeﬃcient (Equa-

tion SH2, Supporting Information) with mobilities found in the

low illumination regime. The charge carrier lifetimes and diﬀu-

sion lengths as functions of the generation rate were calculated

according to Equations SH4–SH7 (Supporting Information) and

are visualized in Figure 2g,h. The lifetimes of electron and hole

become similar as the illumination intensity is increased while

they are distinctly diﬀerent at low and moderate illumination con-

ditions, as shown in Figure S15g,h (Supporting Information). To

the best of our knowledge, such a diﬀerence between hole and

electron lifetimes was not revealed previously by other methods

due to natural limitations (e.g., in photoluminescence and pho-

toconductivity transient methods only one type of carrier is de-

tected) or unjustiﬁed simpliﬁcations (𝜏e=𝜏h).[66–68]

For device-relevant illumination conditions, one sun inten-

sity was considered. Since high injection conditions are reached

at this intensity, both densities and lifetimes of electrons and

holes become equal. For the PI-modiﬁed sample, the charge car-

rier lifetimes, and consequently the diﬀusion lengths of holes

and electrons signiﬁcantly increased compared to the reference

(Figure 2g,h). The charge carrier lifetime was increased from 0.48

to 1.24 μs and the diﬀusion length from 0.81 to 1.30 μm (Table S3,

Supporting Information). These lifetimes show the same trend

as determined by trPL but are slightly lower in absolute num-

bers, which might be due to diﬀerent measurement conditions

(steady-state vs pulsed illumination). To the best of our knowl-

edge, these steady-state diﬀusion lengths of the electrons and

holes are among the longest values compared with previous re-

ports on polycrystalline metal-halide perovskites.[66–68]

The knowledge of hole and electron concentration as a func-

tion of intensity allows us to develop charge transport simula-

tions (more details in Note II, Supporting Information) with the

theoretical model considering non-radiative and radiative recom-

bination. Performing our simulations in a steady-state condition

allows us to further understand the charge losses (more details

in Note II, Supporting Information). For reproducing the exper-

imental CLIMAT data, diﬀerent models with various trap state

characteristics (deep vs shallow) were tested. Only the model with

deep trap states could explain the measured data for the transport

of electrons and holes (Table S4, Supporting Information), hence

they dominate the charge recombination processes. By this mod-

eling, we ﬁnd that the deep trap concentration is reduced from

24 ×1013 cm−3to 4.2 ×1013 cm−3by the PI treatment (Figure 2i;

Figure S15g,h, Table S4, Supporting Information). The observed

decrease of deep trap concentration is consistent with the ToF-

SIMS results above, which proved the penetration of the piper-

azinium cation well into the bulk and can thereby chemically pas-

sivate the perovskite.

Next, we study the photovoltaic performance of single-junction

WBG solar cells with a layer stack of glass/ITO/2PACz/WBG

perovskite/C60/SnO2/Cu (Figure 3a). Prior to the analysis of full

solar cells, we performed a layer-by-layer loss analysis with abso-

lute PL measurements adding none, one, or both CTMs but leav-

ing out the SnO2buﬀer layer and Cu electrode. The calculated

QFLS values (Figure 3b) show that the neat perovskite reached

1.45 eV after PI modiﬁcation with 2.2% PL quantum yield (Figure

S17, Supporting Information), which marked an improvement

over the reference sample (1.42 eV). When only adding C60 on top

of the perovskite, the QFLS of the reference sample was strongly

reduced to 1.37 eV, while the PI-treated sample retained its high

QFLS of 1.45 eV. This indicates that the PI modiﬁcation success-

fully inhibited non-radiative recombination losses that occurred

at the perovskite/C60 interface. Upon the addition of 2PACz,

the QFLS of the reference and PI-modiﬁed samples were only

slightly reduced, which indicated low recombination losses at

that interface.[4,9]When combined with both, 2PACz and C60, the

PI-modiﬁed perovskite achieved a QFLS of 1.42 eV as compared

to 1.32 eV for the reference (Figure 3b;FiguresS13, S17,Sup-

porting Information).

Analogous to the analysis of the absolute PL, we studied

the recombination of charge carriers with transient PL (Figure

S18a–c, Supporting Information) and computed diﬀerential life-

times to separate the charge transfer process from trap-assisted

recombination,[69]asshowninFigureS18d–f (Supporting In-

formation). After depositing C60, trPL transients indicated a

much longer decay time for the PI-modiﬁed perovskite com-

pared to the reference sample on quartz substrates (8.78 μsvs

0.89 μs, Figure S18a, Table S2, Supporting Information). Fur-

thermore, on a glass/ITO/2PACz substrate, the PI-modiﬁed per-

ovskite also had a longer trPL transient compared to the reference

sample, as shown in Figure S18b (Supporting Information). In

the complete layer stack (glass/ITO/2PACz/perovskite/C60), the

trPL transients of the PI-modiﬁed perovskite, yield a remarkable

mono-exponential decay lifetime >10 μs compared to <1μsfor

the reference sample, as shown in Figure S18c (Supporting Infor-

mation). These remarkable improvements in the trPL decay life-

time of the PI-modiﬁed perovskites, both with and without C60

can be attributed to the signiﬁcant reduction of the non-radiative

recombination losses and likely to the reduction of trap states.

This analysis revealed a signiﬁcantly improved interface between

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Figure 3. a) Schematic stack of the WBG PSC. b) Layer-by-layer analysis of WBG perovskite QFLS with and without PI modiﬁcation in the following

layer stacks quartz/perovskite, glass/ITO/2PACz/perovskite, quartz/perovskite/C60, glass/ITO/2PACz/perovskite/C60. c) Reverse scan J–Vcurves of

the best reference and PI-modiﬁed PSCs, measured with an aperture of 0.12 cm2. The corresponding J–V parameters are displayed in Table 1. Inset:

PCE during 6 min MPP tracking. d,e) Statistic performance of reference (26 devices) and PI-modiﬁed PSCs (28 devices) under illumination. The average

J–V parameters of PI PSCs are presented in the ﬁgures. The middle horizontal line of the boxes represents the mean value, and the boxes span over the

standard deviation.

the perovskite and CTMs with PI modiﬁcation by reducing non-

radiative recombination losses.

Subsequently, we carried out J–V measurements to determine

the photovoltaic performance. Figure 3c displays J–V character-

istics of the best PI-modiﬁed and reference device. The PV per-

formance parameters of PSCs with and without PI modiﬁcation

were characterized and are shown in Figure 3d,e and Table 1.The

best PI-modiﬁed PSC achieved a VOC of 1.34 V, an FF of 83.9%,

and a PCE of 20.3%, which is a clear improvement over the ref-

erence device (VOC: 1.20 V, FF: 77.6%, PCE:16.6%). To the best

of our knowledge, this result marks one of the ﬁrst PSC with

a bandgap of ≈1.8 eV surpassing a PCE of 20%.[34,35,40,45,47,70,71]

The photogenerated current densities (Jph) calculated from exter-

nal quantum eﬃciency (EQE) spectra (Figure S19a, Supporting

Information) agree well with the JSCs determined from J–V char-

acterization.

The MPP tracking (inset of Figure 3c) demonstrates that the PI

modiﬁcation strongly improved the steady-state operational sta-

bility of the PSC. This is in line with the improved photostability

discussed above and the suppression of degradation in grain mor-

phology as evidenced by SEM images before and after anneal-

ing (Figure S20, Supporting Information). In addition, the PSCs

stability was also improved in longer MPP tracking and shelf-

storage in the glovebox (Figure S19b,c, Supporting Information).

The PI-modiﬁed PSCs exhibited a signiﬁcantly improved per-

formance compared to reference samples, with mean values of

1.33 V (VOC), 82.7 % (FF), 19.6% (PCE), and 17.8 mA cm−2(JSC),

Table 1. Photovoltaic performance parameters of the best WBG and NBG

single-junction PSCs, and APTSCs. The measurement apertures of cham-

pion WBG- and NBG-PSCs, and APTSCs are 0.12, 0.10, and 0.12 cm2,re-

spectively. Reverse (forward) scan direction refers to scanning from open-

circuit to short-circuit conditions (or vice versa).

Sample Scan

direction

VOC

[V]

JSC

[mA

cm−2]

[%]

PCE

[%]

MPP PCE

[%]

WBG Ref Reverse 1.20 17.8 77.6 16.6 14.6

Forward 1.19 17.8 77.1 16.4

WBG PI Reverse 1.34 18.2 83.9 20.3 19.9

Forward 1.33 18.2 81.3 19.6

NBG Ref Reverse 0.78 31.5 74.2 18.3 16.3

Forward 0.77 31.4 70.2 17.0

NBG RbI Reverse 0.85 31.4 76.7 20.6 18.8

Forward 0.84 31.4 73.2 19.3

Certiﬁed Tandem Reverse 2.113 16.0 81.4 27.4 27.2

Forward 2.111 16.1 80.9 27.5

respectively (Figure 3d,e). Remarkably, the maximum VOC with

PI-modiﬁed PSC reached 1.36 V with a 20.0% PCE (Figure S19d,

Supporting Information). This marks an Eg/q−VOC deﬁcit of

only 0.44 V, which marks the highest PCE at a bandgap of 1.80 eV

and the highest VOC achieved for a PSC with a bandgap <1.85 eV

(Figure S19e,f, Supporting Information).[10,15,21,34,35,54,72,73]The

www.advancedsciencenews.com www.advmat.de

QFLS still surpasses the best VOC, of which the possible reason

will be discussed below.

Additionally, light-intensity-dependent J–V curves reveal a re-

duction of the ideality factor from 1.48 to 1.35 after PI modi-

ﬁcation (Figure S21, Supporting Information), which is indica-

tive of reduced non-radiative recombination and thus in good

agreement with the ﬁndings presented above. To further eval-

uate the performance potential of the WBG perovskite, a layer-

by-layer analysis analogous to the results above (Figure 3b)was

performed by means of intensity-dependent absolute PL mea-

surements (Figure S22, Table S5, Supporting Information). With

the PI-modiﬁcation, the drop of the pseudo-FF (pFF) invoked

by the CTMs is less (from 88.1 to 87.4%) than for the refer-

ence sample (88.4% to 86.4%). As a result of the improved

pVOC and pFF, the pseudo-PCE (pPCE) of the PI-modiﬁed sam-

ple reached 21.8% as compared to 19.4% for the reference de-

vice (assuming a JSC of 17.8 mA cm−2from the EQE opti-

mized device as above). In view of previous studies on diﬀerent

compositions[50,51]our results demonstrate that the PI treatment

can enhance the VOC for a wide range of triple halide perovskite

compositions.

2.2. Tandem Integration

To realize all-perovskite tandems, a bottom cell with an

NBG tin-lead (Sn-Pb) perovskite was developed. For this, the

performance of an NBG perovskite with the composition

Cs0.1FA0.6MA0.3Pb0.5Sn0.5I3[43]was optimized with rubidium io-

dide (RbI) additive according to our previous report for a diﬀerent

composition.[74]The details of the optimization process and so-

lar cell results are described in Note III, Figures S23–S26,and

Table S6 (Supporting Information). The RbI-optimized NBG so-

lar cells achieved a VOC of 0.85 V, which resulted in an improved

PCE of 20.6% (reference: 18.3%), as shown in Table 1and Figure

S26b (Supporting Information). This demonstrates that RbI is

an eﬃcient additive for various NBG perovskite compositions for

bottom-cell utilization.

We combined the optimized WBG and NBG single junctions

into APTSC. The schematic layout of the monolithic tandem cell

is displayed in Figure 4a. The tandem cell consists of the op-

timized WBG top cell with PI modiﬁcation, which was electri-

cally coupled to the optimized NBG bottom cell with RbI addi-

tive through a 100 nm thick ITO recombination contact, which

protects the WBG subcell from damage during fabrication of the

NBG subcell and reduces reﬂection losses (see Figure S27,Sup-

porting Information). To improve the optical performance of the

device, an anti-reﬂection foil was glued to the substrate glass. A

detailed optical analysis is shown in Note IV, Figures S28, S29 and

Table S7 (Supporting Information). The cross-sectional morphol-

ogy of the tandem device in Figure 4b shows layer thicknesses

of ≈400 nm (WBG) and ≈1000 nm (NBG), respectively, both

demonstrating good crystallinity in the ﬁnal tandem stack.

Figure 4c displays the J–V curves and 5 min MPP tracking

(inset ﬁgure) of a tandem cell that was certiﬁed at the Japan

Electrical Safety & Environmental Technology Laboratories (JET,

the full set of certiﬁcation measurements is displayed in Figure

S30, Supporting Information). The certiﬁed cell exhibits a VOC

of 2.11 V (2.11 V), an FF of 80.9% (81.4%), and a PCE of 27.5%

(27.4%) in the forward (reverse) scan. The certiﬁed device sus-

tained a PCE of 27.2% over 5 min MPP tracking. The certi-

ﬁed values for the integrated photogenerated current density for

the top and bottom subcell (Jph, WBG,andJph, NBG) reached 16.2

and 16.1 mA cm−2, which are consistent with integrated val-

ues extracted from EQE spectra measured in-house, as shown

in Figure 4d. The tandem cell exhibited a low current loss in the

top cell, while the bottom cell still suﬀered from relatively high

internal and reﬂection losses due to the lower absorption coef-

ﬁcients in the band-edge region as compared to Pb-based per-

ovskites (for a summary of optical losses, refer to Table S7,Sup-

porting Information).[13,75]The thickness of our NBG perovskite

layer is limiting the absorption of infrared photons compared

with a previous report.[27]The PV parameters of this certiﬁed

tandem sample were also measured in-house before certiﬁcation

(see Figure S31a,b, Supporting Information), and agree well with

the certiﬁed values. The best VOC of all fabricated tandem de-

vices was measured to be >2.15 V with a negligible hysteresis,

as shown in Figure S27c and Table S8 (Supporting Information).

This tandem VOC value is better or comparable to values reported

for the best APTSCs.[14–16,20,22,27,28,34,35,42,58,76]The statistical distri-

bution of the performance parameters is displayed in Figure 4e

and Figure S31d–f (Supporting Information). They show a nar-

row distribution with an average VOC of 2.11 ±0.02 V and PCE

of 27.2%. To evaluate the stability of our tandem cells, a long-term

MPP track under atmospheric conditions and room temperature

was performed after encapsulation (Figure 4f). The tandem de-

vice showed a rise in eﬃciency during the ﬁrst 50 h, which is

in line with the PL results also showing enhanced emission over

time (Figure 2b). After 264 h of MPP tracking, the tandem sus-

tained 88.5% of its initial PCE, which is comparable to the stabil-

ity reported for an APTSC with a similar PCE.[30]

To further study the eﬃciency potential of our tandem

cells, pseudo J–Vcurves were acquired from illumination-

intensity dependent J–V measurements,[55]absolute-injection

dependent electroluminescence (EL),[4,5,58,77–79]and absolute-

intensity-dependent PL.[58,77,78]For this purpose, a structurally

identical tandem cell with a PCE of 27.5%, (performance param-

eters in Table S9) was used. As shown in Figure 4g and Table

S9 (Supporting Information), the pseudo J–V parameters of this

cell achieved a pFF of 83.4% and 28.7% pPCE (assuming JSC of

16.3 mA cm−2) when extracting values from intensity-dependent

J–V curves (suns-VOC), which agrees well with the pseudo J–V

generated by EL (inset table of Figure 4g). A pVOC of 0.815 V

for the NBG subcell was lower than expected from the single-

junction J–V characterization, which could be due to the adapted

annealing temperature of PEDOT: PSS in the tandem applica-

tion to reduce the thermal load for the WBG perovskite (120 °C

for 10 min vs 150 °C for 20 min). Interestingly, the pVOC of

the NBG perovskite determined from absolute PL measurements

was 52 mV higher than that obtained from EL. According to pre-

vious reports, this signiﬁcant discrepancy could be evoked by

an energetic mismatch in the band structure of perovskite and

CTMs[58,78]or more generally due to a poor selectivity of a CTM

(i.e., a low majority/minority carrier conductivity ratio next to the

limiting interface).[80]In contrast, the pVOC of the WBG from PL

was 1.31 V and thus only slightly higher than the 1.29 V deter-

mined from EL. As a result, the pVOC of the tandem device ex-

tracted from PL achieved 2.18 V with a pseudo PCE of 30.4%,

www.advancedsciencenews.com www.advmat.de

Figure 4. a) Sketch of the all-perovskite tandem solar cell stack. b) Cross-sectional SEM image of the tandem solar cell. The scale bar is 400 nm. c)

J–Vcurves, and 5 min MPP tracking (inset) of the certiﬁed tandem solar cell. The aperture for the certiﬁcation measurement is 0.1212 cm2.d)EQE

spectra of the certiﬁed tandem solar cell, including the 1-reﬂection (1-R), internal optical losses, and the cumulated EQE of the two subcells (light

green). e) Statistical VOC distribution of 28 tandem cells. The measurement aperture is 0.1225 cm2. The middle horizontal line of the boxes represents

the mean value, and the boxes span over the standard deviation. f) Long-term MPP stability of an encapsulated all-perovskite tandem solar cell recorded

in ambient air at 25 °C, with no humidity control. The initial eﬃciency was 26.8% from J–Vscans. The 80% (T80) of peak eﬃciency of tandem solar cells

is included according to the ISOS standards. g–i) J-–V and pseudo J–V(pJVsuns, only in g) curves of the tandem, NBG, and WBG solar cell calculated

from intensity-dependent sun simulator measurements, as well as pseudo J–V curves calculated from intensity-dependent EL (pJVEL)andPL(pJVPL)

measurements on the same sample.

which sets a realistic limit, if the series resistance losses could

be further reduced. To understand the losses in more detail, the

single-junction J–V and pseudo J–V curves (PL and EL) of the

NBG and WBG PSCs were also measured (Figure 4h,i). Similar

to the results obtained for the tandem cell, also the characteriza-

tion of the subcells shows lower pVOCs from EL than PL (NBG:

40 mV, WBG: 30 mV; see inset tables of Figure 4h,i). We thus

conclude that for further improvements of the performance, it is

necessary to develop new HTMs with better energetic alignment

to the NBG perovskite. Overall, the results from the pseudo J–V

measurements highlight that the PCE of the all-perovskite tan-

dem solar cells could surpass 30%. In addition, improvements in

the thermal stability are another key to further increase the VOC

of the WBG PSCs (see Note V and Figure S32, Supporting Infor-

mation) and enable >30% eﬃcient all-perovskite tandem solar

cells.

3. Conclusion

We developed a new triple-halide perovskite with a bandgap of

1.80 eV and implemented a piperazinium iodide (PI) surface

treatment, which virtually eliminated all non-radiative recombi-

nation losses at the perovskite/C60 interface. This study for the

ﬁrst time demonstrates the successful application of PI to WBG

triple-halide perovskite absorbers and thereby shows the versa-

tility of PI to improve the interface properties of diﬀerent metal-

www.advancedsciencenews.com www.advmat.de

halide perovskites. Moreover, this study combines detailed struc-

tural analysis with optoelectronic investigations and modeling to

further understand the inﬂuence of PI on device performance.

The structural analysis with ATR-FTIR and ToF-SIMS proved

the presence of PI at the surface and its penetration into the

bulk. Furthermore, XRD measurements demonstrated that the

PI modiﬁcation decreases the PbI2fraction at the surface. Subse-

quently, near-UV PES measurements with varying excitation en-

ergy in constant ﬁnal state mode (CFSYS) were employed to char-

acterize the surface energetics. Here, we found that both chem-

ical passivation by the decrease of surface and bulk defect states

and an electrostatic dipole contribute to reduced non-radiative

recombination and potentially improved energy level alignment

to the ETM, which is distinctively diﬀerent from earlier stud-

ies. Moreover, we use CLIMAT measurements to further under-

stand the PI’s eﬀect on charge transport by separately resolving

and quantifying electron and hole charge transport properties

under varying illumination conditions. These measurements re-

vealed that the PI modiﬁcation signiﬁcantly increased the steady-

state carrier lifetime and diﬀusion length compared to the ref-

erence samples. We devised a model to simulate charge trans-

port, which suggested that the PI treatment decreases the de-

fect concentration and thus leads to better charge transport and a

boostinPCE.ArecordVOC of 1.36 V of WBG-PSCs was achieved

with 20% PCE after PI modiﬁcation. Combining this superior

WBG-PSCs with a narrow bandgap perovskite for monolithic all-

perovskite tandems, we achieved certiﬁed PCEs of 27.5% and

27.2% from J–V scans and MPP tracking, respectively. Encapsu-

lated tandem cells sustained >88% of the initial eﬃciency after

264 h of continuous MPP tracking in air. To estimate the PCE

potential, intensity-dependent absolute photoluminescence mea-

surements were performed and demonstrated that our tandem

design can potentially achieve >30% PCE. Overall, our study pro-

vides important scientiﬁc insights to accelerate the development

toward commercialization of this technology.

Supporting Information

Supporting Information is available from the Wiley Online Library or from

the author.

Acknowledgements

The authors are grateful to C. Ferber, T. Lußky, H. Heinz, and J. Beckedahl

for daily technical assistance in the laboratory, C. Klimm for SEM support,

and M. Härtel for help with optimizing the TCO sputtering. The authors

further thank E. Neumann and P. Husemann (Department of Chemistry,

TU Berlin, Germany) for ATR-FTIR measurements. The authors acknowl-

edge funding from the Deutsche Forschungsgemeinschaft (DFG, German

Research Foundation) within the SPP 2196 (HIPSTER-PRO 424709669).

M.S. further acknowledges the Heisenberg program from the Deutsche

Forschungsgemeinschaft (DFG, German Research Foundation) as well

as the Vice Chancellor Early Career Professorship Scheme from CUHK

for funding, Project No. 498155101. This work was partly funded by EP-

SRC, project number EP/S004947/1. Funding was provided by the Fed-

eral Ministry of Education and Research (BMBF) through Young Inves-

tigator Group Perovskite Tandem Solar Cells within the program “Ma-

terialforschung für die Energiewende” (grant no. 03SF0540) as well as

the project PEROWIN (grant no. 03SF0631) and by the Helmholtz As-

sociation within the projects HySPRINT Innovation lab, the EU Partner-

ing project TAPAS and the project “Zeitenwende – Tandem Solarzellen”.

A.M. and F.S. acknowledge ﬁnancial support from the German Science

Foundation (DFG) in the framework of the priority program SPP 2196

and funding from the European Union HORIZON-MSCA-2021-PF-01-01

under grant agreement no. 101061809 (HyPerGreen). A.M. gratefully ac-

knowledges Danny Kojda and Klaus Habicht for their assistance with the

Hall setup. The authors further acknowledge HyPerCells, a joint graduate

school of the University of Potsdam and the Helmholtz-Zentrum Berlin.

The authors gratefully acknowledge DESY, Hamburg for the granting of

beamtime and support by the beamline staﬀ of beamline P08, especially

F. Bertram. F.L. acknowledges funding from the Volkswagen Foundation

via the Freigeist Program. K.B. and T.R. acknowledge funding by HIPSTER

PRO (DFG, SPP2196, RI 1551/15-2), and MUJUPO2(DFG, RI 1551/12-2).

The research leading to these results had received partial funding from

the European Union’s Horizon 2020 Programme under grant agreement

no. 951774 (FOXES). This work was authored in part by the National Re-

newable Energy Laboratory, operated by Alliance for Sustainable Energy,

LCC, for the U. S. Department of Energy (DOE) under contract no. DE-

AC3608GO28308. We acknowledge the support on perovskite ﬁlms’ prepa-

ration from the Advanced Perovskite Cells and Modules program, and the

support on ToF-SIMS analysis from the Hybrid Tandem Core program of

the National Center for Photovoltaics, funded by the U. S. Department

of Energy Oﬃce of Energy Eﬃciency and Renewable Energy, Solar Energy

Technologies Oﬃce. The views expressed in the article do not necessarily

represent the views of the DOE or the U. S. Government.

Open access funding enabled and organized by Projekt DEAL.

Conﬂict of Interest

The authors declare no conﬂict of interest.

Author contributions

F.Y. and S.A. coordinated the project and designed the experiments. F.Y.,

S.A., and P.T. co-designed the all-perovskite tandem solar cells and their

characterizations. F.Y. executed experiments of samples’ preparation and

protocol modiﬁcation and conducted the characterizations of the XRD, PL,

trPL, solar cells, and the data analysis with the help of S.A., P.T., E.K., S.M.,

K.X., A.A., R.M., and L.K.P.T. conducted ideality factor measurement for

the WBG PSCs and supported EQE measurement for tandem solar cells.

A.M. designed and built the CLIMAT setup and method, conducted the

CLIMAT study, results analysis, model design, and simulation of charge

transport. D.M. contributed to the PES characterization and analysis. F.L.

and M.S. conducted the PL and EL measurements of tandem devices. R.M.

supported trPL measurement and setup modiﬁcation. E.K. conducted sun

simulator intensity-dependent measurements for pseudo J–V curves. K.X.

introduced the 1.68 eV bandgap perovskite. S.H. performed ToF-SIMS

measurements. S.M. introduced the PI processes. D.M. synthesized the

PI ionic liquid molecules. L.M., A.H., and F.S. carried out the GIWAXS

characterization and analysis. J.Z. conducted the power dependence PL

of perovskite ﬁlms for pJV. B.L. conducted the stability evaluation of tan-

dem solar cells. F.S. supported the PI optimization. M.R. supported the

evaluation of ATR-FTIR measurements. S.B. prepared additional samples

for ATR-FTIR measurements. J.T. supported the discussion of NBG per-

ovskite. F.Y., P.T., A.M., D.M., L.K., and S.A. created and designed the ini-

tial manuscript. All authors contributed to the data discussion, manuscript

writing, and revision. S.A. supervised this project.

Data Availability Statement

The data that support the ﬁndings of this study are available from the

corresponding author upon reasonable request.

Keywords

all-perovskite tandem solar cells, piperazinium iodide, recombination

losses, triple-halide wide-bandgap perovskite

www.advancedsciencenews.com www.advmat.de

Received: August 2, 2023

Revised: November 6, 2023

Published online: December 6, 2023

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