1 Title Deep -level transient spectroscopy of the charged defects in p-i-n perovskite solar cells induced by light -soaking

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Title:

Deep-level transient spectroscopy of the charged defects in p-i-n perovskite solar cells induced

by light-soaking

Authors:

Vasilev A.A.1,2, Saranin D.S.2*, Gostishchev P. A.2, Tuhova M.P.2 Didenko S.I1,2, Polyakov A.Y.1,

and Di Carlo A.3

1. Department of Semiconductor Electronics and Semiconductor Physics, National

university of Science & Technology MISIS, 4 Leninsky Ave., Moscow, 119049

2. Laboratory of Advanced Solar Energy (LASE), National university of Science &

Technology MISIS, 6 Leninsky Ave., Moscow, 119049

3. Department of Electronic Engineering, University of Rome Tor Vergata, via del

Politecnico 1, 100, 00133 Rome, Italy

* Corresponding author Dr. Saranin.D.S.* email: saranin.ds@misis.ru

Keywords

P-i-n perovskite solar cell, deep level transient spectroscopy, defect engineering

Highlights

• The evaluation of the defect parameters of the p-i-n perovskite solar cells under light

soaking stress.

• Analysis of impact of the charged defects on the performance and long-term stability of the

CsFAPbI3 based devices with Cl-doping.

• Cl-doping suppressed the formation of the antisite defect (IPb, IFA) and iodine interstitials (Ii).

• Cl-doping increase efficiency and improve the light soaking stability.

• The presence of the defect vacancies (VI, VCs) remains the critical factor for the long-term

stable operation of the PSCs based on double cation compositions.

Abstract

The long-term stability of halide perovskite solar cells (PSCs) remains the critical problem of this

photovoltaic technology. Different structural defects formed in the thin-film perovskite films were

considered as a main trigger for the decomposition of the absorber and corrosion of the interfaces

in the device structure. The changes in the stability performance of the PSCs require a detailed

analysis of the defects generated under external stress (light and heat). Using admittance, deep-

level transient spectroscopy (DLTS) and reverse DLTS we determined the evolution of the defect

energy levels in p-i-n PCS under continuous light soaking stress. We compared the impact of the

charged defects on the performance and long-term stability of the CsFAPbI3 based devices with

and without Cl-doping. Despite the gain in the output performance of the PCSs, the devices with

CsFAPbI3-xClx showed improved light soaking stability. The T80 (time required to reduce initial

efficiency by 20%) for Cl-doped PSCs was 1280h, while for pure CsFAPbI3 based devices only

650h. Three different defect energy levels were determined for different device configurations.

We found that Cl-doping suppressed the formation of the antisite defects (IPb, IFA) and iodine

interstitials (Ii). The changes in the defect’s energy levels after continuous light soaking stress

were analyzed and discussed. The present work provides new insights for the defect behavior of

PSCs under continuous external stress, revealing the physical-chemical impact of the Cl-additive

strategy.

Introduction

Over the past decade, the halide perovskite (HP) gained significant interest from the scientific

community because of unprecedented progress in photovoltaics applications[1]. Perovskite solar

cells (PSCs) showed rapid growth in the increment of power conversation efficiency(PCE), with a

recent record at the level of 25.8%[2]. However, the degradation processes induced by

compositional and structural defects in the bulk of thin-films of interfaces of the devices limit the

long-term operation of PSCs with stable output performance[3]. Most promising chemical

compositions of the HP-based light absorbers have a hybrid organic-inorganic nature. The

general formula of the perovskite molecular structure is ABX3, where A- cation is typically

presented with methyl ammonium (MA+), formamidine (FA+) or cesium (Cs+); B-cation is typically

lead (Pb2+), and X-anion is represented with halides - iodine (I-), bromine (Br-) or chlorine (Cl-).

PSCs could be fabricated with a low-cost solution processing methods[4]–inkjet printing[5], slot-

die[6] or blade coating[7] which could significantly reduce capital expenditures of the industrial

production[8]. On the other hand, using of solution-based crystallization methods at relatively low-

temperatures induces the formation of different charged defects in microcrystalline HP-based

absorbers. The defects in HP thin-films are associated to X-anion vacancies, A – and B- site

cation interstitials, as well as with products of perovskite molecule decomposition – volatile MA+

or FA+; metallic lead Pb0; Iodine (I2); HI, etc [9]. Defects form “shallow” states (for example for

iodine vacancies), or «deep» states (for examples for MA interstitials or free MA ions) which could

form traps or recombination centers. The ionic defects in HP thin-films could trigger the

electrochemical interaction at the interfaces with charge-transporting layers in PSCs, start the

decomposition processes with presence of oxygen and moisture at the grain boundaries and

contribute to the charge accumulation/depletion in the devices. The strategies for passivation and

defect-healing for PSCs require specific methods for the rational identification of the defect-level

parameters such as activation energy, cross-section, and concentration with quantitative

assessment. Deep-level Transient Spectroscopy (DLTS) and Admittance Spectroscopy were

widely regarded as standard techniques for the characterization of defects parameters in

semiconductor-based device structures. The combination of both methods gives us an insight

into the processes underlying the efficiency and stability of perovskite solar cells. Accessing deep-

states through high-frequency capacitance transients or low-frequency conductivity is the shortest

path to awareness of inner material properties such as charge carriers concentration, mobile ions

distribution and built-in field screening.

The quantitative control of the defect parameters regarding the methods of the fabrication, doping

levels and external stress factors is one of the key factors for the development of the stable

performing solar cells. To date, the DLTS method is used more and more for the identification of

defects in PSCs[10–13]. However, the evaluation of the defect characteristics in PSCs and its

evolution with continuous influence of the stress factors (heat, high-intensity light) is still an open

issue. Typically, the identification of the defect parameters performed only for “as fabricated”

PSCs to distinguish the benefits of different crystallization methods[14], interface modification[15]

etc.

In order to close this gap and shed light on the impact of accelerated stress tens on defect

evolution, we made the attempt to analyze the changes of the defect parameters of PSCs under

light soaking stress. We performed a DLTS investigation for p-i-n PSCs based on СsFAPbI3

absorber with Cl-doping and analyzed the evolution of the defect parameters induced by external

stress factor. We focus on the double cation HP with MA – and bromine free composition

considering that multication HP is an effective strategy to improve photo and thermal stability of

PSCs[16] and that CsFAPbI3 engaged the interest of scientific community[17–20] thanks to the

suppressed phase segregation[21] and thermal diffusion of the ionic defects[22].

We performed the stability tests following to the ISOS-L-2 protocol at open circuit conditions (Voc)

under continuous light soaking. The CsFAPbI3-xClx based PSCs demonstrated higher stability with

respect to CsFAPbI3 with a T80=1280h (T80=time required to reduce initial efficiency by 20%)

compared to T80=650h for CsFAPbI3. The difference in the charged defect behavior before/after

light soaking was evaluated by performing Admittance Spectroscopy and DLTS measurements

with “classic” and reversed biasing conditions (RDLTS)[10,23]. We performed two DLTS sessions

for the PCSs at the initial conditions and after 650h of stability tests. The results showed that

fabrication of the pure triiodide CsFAPbI3 tends to formation of the deep traps possibly related to

the iodine (VI) and cesium vacancies (VCs); antisites IPb - IFA and iodine interstitials (Ii). Cl-doping

suppressed the formation of the antistes and interstitials, even though VFA and VI were obtained.

The presence of the Ii after continuous light soaking in CsFAPbI3 PCS was considered as one of

the main instability factors.

Results & Discussion

Perovskite solar cells were fabricated in the p-i-n planar configuration with the following

architecture: Glass [1.1 mm]/ITO [330 nm]/NiOX [20 nm]/Perovskite/C60 [40 nm]/Bathocuproine

(BCP)[8 nm]/Cu [100 nm] encapsulated with a glass coverslip (see experimental for the details).

The absorber layer was manufactured in two configurations – СsFAPbI3 (Reference, here and

after Reference sample) and CsFAPbI3-xClx (Cl-doped, here and after Cl-sample). Briefly, the

absorber films were fabricated with solution processing and using solvent engineering method for

the crystallization. The molar ratio between Cs and FA cation was 1:4 and specific stoichiometry

was Cs0.2FA0.8PbI3. The Cl-doping was realized through the partial replacement of CsI used for

the ink preparation to CsCl. The use of chlorine for anion substitution was considered as tool for

the improvement of charge-carrier lifetimes [24,25] and diffusion lengths[26] in perovskite

photoactive layers. The thickness of fabricated thin-films was 470 nm (measured with stylus

profilometry) and didn’t change after doping. We measured the performance of the fabricated

PSCs under standard illumination conditions 1.5 AM G (Xe light simulator (AAA class)) (see

fig.1a). Reference samples showed open circuit voltage (Voc) values of 1.001 V, short-circuit

current density (Jsc) of 22.5 mA/cm2, filling factor (FF) equal to 0.755 and PCE=17.06%. The Cl-

doping boosts the performances of PSC with an increase of Voc up to 1.012 V(+ 1% with respect

to undoped device), Jsc up to 25.2 mA/cm2(+12%) and PCE up to 19.39%(+13.6%). Following to

the initial characterization, we set up the stability tests according to the ISOS-L-2[27] with a

continuous light soaking at Voc conditions (see experimental for the details). The temperature of

the samples was 63.5±1.5 ˚C under light exposure. The accelerated stress induced a degradation

of the PSC characteristics as shown in Fig.1b-d. However, the T80 period for the reference sample

was equal to 650h, while with the Cl-doping T80 increased up to 1280h (Fig. 1d).

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1Title:Deep-leveltransientspectroscopyofthechargeddefectsinp-i-nperovskitesolarcellsinducedbylight-soakingAuthors:VasilevA.A.1,2,SaraninD.S.2*,GostishchevP.A.2,TuhovaM.P.2DidenkoS.I1,2,PolyakovA.Y.1,andDiCarloA.31.DepartmentofSemiconductorElectronicsandSemiconductorPhysics,NationaluniversityofScienc...

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