Role of PC60BM in defect passivation and improving degradation behaviour in planar perovskite solar cells

Role of PC60BM in defect passivation and improving degradation behaviour in planar perovskite solar cells

Solar Energy Materials & Solar Cells 207 (2020) 110335 Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal home...

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Solar Energy Materials & Solar Cells 207 (2020) 110335

Contents lists available at ScienceDirect

Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat

Role of PC60BM in defect passivation and improving degradation behaviour in planar perovskite solar cells Rahul Ranjan a, Belal Usmani c, Sowjanya Pali c, Sudhir Ranjan a, Anand Singh b, ***, Ashish Garg c, **, Raju Kumar Gupta a, d, * a

Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, UP, India Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, 208016, UP, India Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, UP, India d Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, Uttar Pradesh, India b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Perovskite solar cells PC60BM Passivation Aging Degradation

Titanium oxide (TiO2) is the most commonly employed electron transport layer (ETL) in planar perovskite solar cells (PSCs) structure (FTO/c-TiO2/CH3NH3PbI3/Spiro-OMeTAD/Ag). However, it suffers from defects such as oxygen vacancies and structural defects due to the formation of Ti3þ ions. While defects in TiO2 could play a role in determining the device performance, perovskite layer in PSCs also exhibits traps at interfaces and grain boundaries causing increased recombination losses lowering the power conversion efficiencies (PCE) of the devices. In this manuscript, we demonstrate the use of a thin interlayer of [6, 6]-phenyl-C60-butyric acid methyl ester (PC60BM) as a passivation layer between TiO2 and perovskite layer in planar PSC structure, as analysed using aging and stability studies. The study reveals that the quality of the interface between the constituent layers improves after introduction of PC60BM as passivation layer thus causing an enhancement in the performance of devices. The performance of the devices showed a peculiar behaviour when measured at different times with ~40% enhancement in PCE after 50 h followed by a drop when measured after 150 h. The initial improvement in the device performance was studied using photoluminescence (PL) spectroscopy which showed a blue shift in the emission while impedance spectroscopy measurements showed a significant reduction in charge transfer resis­ tance after 50 h, aided by an improved band alignment as suggested by Kelvin probe force microscopy (KPFM). Further, we show that initiation of degradation in perovskite starts from interface instead of degradation in the bulk of perovskite layer.

1. Introduction

affect the performance of PSCs is the electron and hole transporting layers, device configuration i.e. P–I–N or N–I–P etc. In the N–I–P ar­ chitecture, the two common configurations are those using a thick mesoporous TiO2 layer as ETL or a compact TiO2 in planar configura­ tion. While mesoporous configuration has yielded higher efficiencies than those of planar ones, processing of mesoporous TiO2 is far more complex than that of compact TiO2 in planar configuration. Planar PSCs typically employ TiO2 and ZnO as electron transport layer (ETL) while more recent works have also utilized SnO2 with SpiroOMeTAD (N2,N2,N20 ,N20 ,N7,N7,N70 ,N70 -octakis(4-methoxyphenyl)-9,90 spirobi[9H-fluorene]-2,20 ,7,70 -tetramine), P3HT(poly-3-hexylthiophen e) or PTAA((Poly(triaryl amine)) as hole transport layers (HTL)

Organic-inorganic hybrid perovskite solar cells (PSCs) are currently of extensive research interest because of their excellent properties e.g. high absorption coefficient, suitable direct band gap, high carrier mobility, long charge carrier diffusion length, potentially low cost of materials and a solution-based low-cost fabrication process. Rapid progress has been made almost in a decade yielding power conversion efficiencies of ~23%, close to those of crystalline silicon solar cells [1–7]. However, presence of Pb and inferior environmental stability has prevented the technology from achieving its commercial potential and it is an area which has received substantial attention. The factors which

* Corresponding author. Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, UP, India. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (A. Singh), [email protected] (A. Garg), [email protected] (R.K. Gupta). https://doi.org/10.1016/j.solmat.2019.110335 Received 3 September 2019; Received in revised form 13 November 2019; Accepted 2 December 2019 Available online 16 December 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic representation of device structure (a) a normal structure of FTO/c-TiO2/MAPbI3/SpiroOMeTAD/Ag without PC60BM interlayer between TiO2 and perovskite layer, (b) band alignment of a normal structure of FTO/c-TiO2/MAPbI3/Spiro-OMe­ TAD/Ag without PC60BM interlayer between TiO2 and perovskite layers, (c) a normal structure of FTO/cTiO2/MAPbI3/Spiro-OMeTAD/Ag with PC60BM inter­ layer between TiO2 and perovskite layer, (d) band alignment of a normal structure of FTO/c-TiO2/ MAPbI3/Spiro-OMeTAD/Ag with PC60BM interlayer between TiO2 and perovskite layer, (e) Normalized PCE of FTO/c-TiO2/MAPbI3/Spiro-OMeTAD/Ag without PC60BM after different time interval of 0 h, 25 h, 50 h, 100 h, 150 h and 200 h and (f) Normalized PCE of FTO/c-TiO2/MAPbI3/Spiro-OMeTAD/Ag with PC60BM after different time interval of 0 h, 25 h, 50 h, 100 h, 150 h and 200 h.

Table 1 Summary of average photovoltaic performance at AM1.5G of six devices after 0 h, 50 h and 150 h without PC60BM interlayer between TiO2 and perovskite layers. Time

Jsc (mAcm

2

)

Voc (V)

Forward sweep direction 0h 16.6 � 1.5 0.80 � 0.0 50 h 15.1 � 2.6 0.82 � 0.04 150 13.4 � 2.7 0.81 � h 0.40 Reverse sweep direction 0h 18.6 � 1.7 0.82 � 0.04 50 h 18.2 � 2.1 0.90 � 0.0 150 17.6 � 2.1 0.88 � h 0.04

PCE (%)

RS (Ωcm2)

RSh (Ωcm2)

0.38 � 0.01 0.34 � 0.04 0.32 � 0.03

5.1 � 0.6 4.3 � 1.1 3.6 � 1.1

23.8 � 3.2 33.7 � 20.1 39.9 � 16.3

163.1 � 21.3 128.4 � 25.8 111.4 � 8.1

0.44 � 0.04 0.38 � 0.06 0.35 � 0.04

6.8 � 1.3 6.4 � 1.6 5.7 � 1.5

19.2 � 4.2 23.7 � 6.4 29.3 � 10.9

303.7 � 92.2 258.6 � 123.1 196.7 � 74.0

FF (%)

Table 2 Summary of average photovoltaic performance at AM1.5G of ten devices after 0 h, 50 h and 150 h with PC60BM interlayer between TiO2 and perovskite layers. Time

Jsc (mA cm 2)

Voc (V)

Forward sweep direction 0h 16.2 � 0.98 � 1.7 0.03 50 h 17.7 � 0.96 � 1.6 0.03 150 16.5 � 0.97 � h 2.1 0.03 Reverse sweep direction 0h 18.2 � 1.0 � 1.6 0.02 50 h 19.1 � 1.06 � 1.5 0.04 150 18.6 � 1.02 � h 1.9 0.04

2

FF (%)

PCE (%)

RS (Ωcm2)

RSh (Ωcm2)

0.32 � 0.04 0.43 � 0.03 0.40 � 0.05

5.1 � 0.7 7.20 � 1.0 6.32 � 1.4

58.8 � 16.4 20.3 � 3.30 27.2 � 7.10

139.0 � 33.0 283.5 � 73.2 190.7 � 38.7

0.43 � 0.06 0.58 � 0.04 0.52 � 0.07

7.9 � 1.0 11.73 � 1.7 9.8 � 1.4

40.7 � 12.5 14.2 � 2.9 16.2 � 4.2

724.0 � 350.0 2431.0 � 1327.0 1328.0 � 827.0

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Fig. 2. Schematic showing (a) the formation of Pb–I antisite defect and formation of trimer and (b) bonding between PC60BM and defective halides.

such as fullerenes and its derivatives [27]. In another study, annealing of PC60BM coated perovskite layer showed PC60BM diffusion into the perovskite layer which passivated the traps at grain boundaries of perovskite films [17]. In addition, coating of compounds such as fullerene self-assembled monolayer (C60-SAM), N-phenyl[60]full­ eropyrrolidines (PNP) or triblock fullerene derivative (PCBB–2CN–2C8) over TiO2 have been demonstrated to enhance charge transfer and hence device performance by passivating the traps on crystalline TiO2 [17, 29–32]. In this work, we have studied the effect of PC60BM ([6,6]-phenyl-­ C60-butyric acid methyl ester) interlayer between TiO2 and perovskite layer and its effect on the aging characteristics of the perovskite photovoltaic devices. We demonstrate that addition of PC60BM layer between TiO2 and perovskite layers enhances the charge extraction process from perovskite layer besides showing good compatibility with perovskite layer. Aging studies revealed that PC60BM passivated PSC devices exhibit a maximum power conversion efficiency (PCE) of ~14.3% after 50 h.

Table 3 Photovoltaic performance at AM1.5G of the best device for different time with PC60BM interlayer between TiO2 and perovskite layers (reverse sweep). Time

Jsc (mA cm 2)

Voc (V)

FF (%)

PCE (%)

RS (Ωcm2)

RSh (Ωcm2)

0h 50 h 150 h

20.4 20.4 19.6

1.06 1.10 1.00

0.39 0.64 0.59

8.5 14.3 11.5

50 10.8 12.8

555 3571 1428

Table 4 Photovoltaic performance of PSC device at AM1.5G after different time intervals in dark and light conditions. Time Jsc (mA cm 2) Light Soaking 0h 15.8 50 h 17.8 150 h 14.3 Without Light Soaking 0h 16.2 50 h 21.2 150 h 20.5

Voc (V)

FF (%)

PCE (%)

1.0 1.02 1.0

0.42 0.58 0.50

6.7 10.5 7.1

1.0 1.03 1.03

0.35 0.58 0.43

5.7 12.3 8.7

2. Experimental methods 2.1. Materials

[8–10]. In planar PSC structure, there is an imbalance between extraction of electrons and holes due to low conductivity of TiO2. One of the ways to balance it by increasing the conductivity of TiO2 layer which can be achieved by doping TiO2 with ions such as Yttrium, Niobium, Tantalum, Lithium, and Aluminium [11–14]. Second popular method is to coat a thin organic ETL film such as that of a fullerene and its derivative above TiO2 layer; thus, achieving good fill factor and PCE of PSCs. For example, fullerene derivatives, a well-known acceptor materials used extensively in organic solar cells, have been incorporated into PSCs leading to improved stability and reduction in the device hysteresis without affecting other device pa­ rameters [15–19]. Zhang et al. used PC60BM as an interlayer and re­ ported that the grain boundary vacancies in perovskite film were filled by α-bis- PC60BM, causing enhanced crystallization of perovskite film as well as enhanced electron extraction with better performance, resistance to moisture and improved device stability [20]. Presence of PC60BM interlayer is also reported to reduce the interfacial potential loss and hysteresis effects [19]. The high electron affinity of PC60BM enhances the charge transportation from perovskite to TiO2 layer [21]. During thermal annealing of perovskite (CH3NH3PbI3) layer, va­ cancies are generated at the grain boundaries due to desorption of methylammonium (MA) [22–24]. Also, Pb–I antisite defects form due to occupation of Pb sites with iodine atoms and forms trimer with another neighbouring atom. These defects and vacancies acts as trap states causing charge accumulation and recombination leading to reduced open circuit voltage (Voc) and FF [25–27]. In addition, TiO2 is also believed to consist of inherent surface trap states leading to carrier recombination. Surface and interface passivation in perovskite solar cells is generally done to reduce the density of defects and trap states using a functional layer or thin films of organic/inorganic layer [28]

Methyl ammonium iodide (MAI) was purchased from Osilla while lead acetate trihydrate (Pb(OAc)2), fluorine-doped-tin oxide (FTO) coated glass sheet, Spiro-OMeTAD, anhydrous dimethylformamide (DMF), titanium isopropoxide (TTIP) were purchased from SigmaAldrich [6,6].-phenyl-C61-butyric acid methyl ester (PC60BM) was procured from nano-C. All the materials were used as received without any further purification. 2.2. Device fabrication and measurements FTO coated glass substrates were patterned using Zn dust and 2 M HCl solution. The patterned substrates were cleaned in an ultrasonic bath, using soap solution employing DI (deionized) water, acetone and isopropanol (10 min for each step). TiO2 film (~50 nm) was deposited on cleaned and patterned substrate using spin coating technique at 3000 rpm for 1 min TiO2 precursor solution was prepared by mixing 350 μL of TTIP in 5 mL of ethanol in slightly acidic medium. The deposited sub­ strate was annealed at 120 ̊C for 10 min followed by at 500 ̊C for 30 min. 20 mg mL 1 solution of PC60BM in chlorobenzene was spin coated on compact TiO2 coated layer at 4000 rpm inside the glove box to get ~20 nm thin layer. PC60BM layer coated substrate was annealed at 140 ̊C for 10 min. 40 wt% perovskite solution was prepared after mixing of MAI and Pb(OAc)2 in 3:1 ratio in DMF solvent and was deposited at 2500 rpm employing spin-cast technique to get ~ 300 nm thick perovskite layer. The sample was kept for bench dry for 10 min and then annealed at 100 C̊ for 5 min on hot plate inside the glove box. The hole transport material (HTM) was then deposited at 3000 rpm for 40 s through spin-coating a 72.3 mg mL 1 solution of Spiro-OMeTAD in chlorobenzene doped with 28.8 μL of tert-butylpyridine and 17.7 μL of lithium bis(tri­ fluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg in 1 mL of 3

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Fig. 3. J-V curve of the best device having structure FTO/c-TiO2/MAPbI3/Spiro-OMeTAD/Ag without PC60BM for different time intervals (a) in light, (c) in dark and with PC60BM for different time intervals (b) in light at AM1.5G and (d) in dark.

acetonitrile). The substrates were taken out from the glove box and kept overnight for oxidation. Finally, the substrates were transferred into a high vacuum chamber to thermally evaporate ~100 nm thick Ag top contact. The device area was 0.09 cm2. Devices were characterized for 0 h, 25 h, 50 h, 100 h, 150 h and 200 h inside the glove box and were also kept inside the glove box during the characterizations.

3. Results and discussion 3.1. Device performance Planar n-i-p perovskite photovoltaic device structures FTO/c-TiO2/ FTO/c-TiO2/PC60BM/MAPbI3/SpiroMAPbI3/Spiro-OMeTAD/Ag, OMeTAD/Ag and corresponding band diagrams are schematically shown in Fig. 1(a–d). The thicknesses of TiO2, PC60BM and perovskite layers were ca. 50 nm, 20 nm and 300 nm, respectively. In this device, ntype TiO2 and p-type organic molecular Spiro-OMeTAD are used as ETL and HTL, respectively while in Fig. 1(c), a layer of PC60BM has been inserted as interlayer between TiO2 and perovskite layers. The PCE of devices were measured after 0 h, 25 h, 50 h, 100 h, 150 h and 200 h. Interestingly, the device having PC60BM as interlayer showed gradual increase in efficiency till 50 h, remained constant till 150 h and started decreasing after 150 h whereas device without PC60BM showed continuous decrease in PCE for the same time interval. Fig. 1(e–f) show normalized PCE of devices with and without PC60BM layer. There was almost ~40% enhancement in PCE after 50 h whereas device without PC60BM showed no increment suggesting that this is mainly due to the presence of fullerene as an interlayer. This is also corroborated by a previous report showing enhancement in the PCE when PC60BM was annealed after coating on perovskite and was attributed to PC60BM passivating the traps in the perovskite [17]. From the above data, it can be inferred that device efficiency first increases and reaches a maxima after 50 h and then starts showing deterioration after 150 h. For further analysis of device and film char­ acterizations, we chose 0 h, 50 h and 150 h time intervals. Table 1 summarizes the electrical performance of planar perovskite photovoltaic devices without PC60BM interlayer between TiO2 and perovskite layer (FTO/TiO2/Perovskite/Spiro-OMeTAD/Ag) as measured after 0 h, 50 h,

2.3. Characterization X-ray diffraction spectra (θ-2θ scan) was collected on single and multilayers of samples deposited on substrates using X-ray diffractom­ eter of X’Pert Pro PANanalytical in thin film mode with monochromatic CuKα (λ ¼ 1.5405 Å) radiation source. Scans were performed with 0.2� / min and X-ray generator settings of 40 kV and 30 mA. To understand the optical response of single and multilayer of the devices, absorbance and PL spectra were collected employing UV–Vis (Cary 7000) spectropho­ tometer and photoluminescence spectroscopy of Agilent (excitation at 470 nm), respectively. For Field Emission scanning electron microscopy (FESEM), Advance Scientific, Tescan Mira-3 was employed. Atomic force microscopy (AFM, Asylum MFP-3D) was used to study surface morphology and Kelvin-Probe force microscopy (KPFM) mea­ surement of the layers. Profilometer (Bruker’s DektakXT stylus) was used to measure thickness of single and multilayer films. Impedance spectroscopy measurement was carried out using Potentiostat/Galva­ nostat (Autolab 302 N, Metrohm, Netherlands). J–V characteristics of the photovoltaic devices were performed using a Keithley 2420 source unit under a simulated AM 1.5G spectrum. With an Oriel 9600 solar simulator, the light intensity was calibrated by a KG-5 Si diode. I–V measurements were carried out in a glove box.

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Fig. 4. (a) X-ray diffraction pattern of FTO/c-TiO2/PC60BM/MAPbI3 structure for 0 h, 50 h and 150 h. (b) Magnified x-ray diffraction spectra at 2θ ¼ 12.6� shows no peak and suggests the absence of PbI2 even after 150 h.

and 150 h. The results show that the PCE of the devices starts decreasing with time i.e. 6.8%, 6.4% and 5.7% for 0 h, 50 h and 150 h (in reverse sweep, Table 1), respectively. This drop in performance can be related to various surface defects present innately on TiO2 layer [33] as well as also to the degradation of CH3NH3PbI3 to form PbI2 which gave rise to the dangling bonds and thus, charge traps and midgap states [17]. We further explore the microscopic reasons which are discussed below using structural, optical and electrical analysis. Next we look at the performance of devices with PC60BM as passiv­ ation layer summarized in Table 2 (average of ten devices), measured in similar time intervals. As observed, the PCE of the devices for 0 h was 7.9%, increased to 11.73% after 50 h, which, in a sharp contrast to devices without PC60BM and started decreasing after 150 h. Also, series and shunt resistances changed drastically from 40.70 Ω cm2 to 14.2 Ω cm2 and 734.6 Ω cm2 to 2431.0 Ω cm2 respectively, after 50 h (in reverse sweep) compared to 0 h before they start decreasing after 150 h as shown in Table 2. It is interesting to note that while PCEs of device at 0 h for PC60BM was slightly higher, which is attributed to passivation of charge traps at the interface between TiO2 and perovskite after insertion of PC60BM layer. It has been suggested that the defects associated with excess halide at the grain boundaries of perovskite and trimer formed due to presence of Pb–I antisite defect are the primary reasons behind presence of electronic traps in lead based methylammonium iodide which lead to charge recombination [27]. After introducing PC60BM layer, PC60BM adsorbs on Pb–I antisite defects at the grain boundaries during perovskite film crystallization/formation and passivates the halide induced traps. Researchers have theoretically studied that after inserting PC60BM near Pb–I antisite defects, the ground state wave

function hybridizes between the PC60BM and perovskite surface and the bonding between PC60BM and the defective halide is thermodynami­ cally favoured suppressing the formation of traps enabling the elec­ tron/hole transfer [27]. Fig. 2 shows the schematic of Pb–I antisite defect, due to which iodine comes in place of Pb site and forms trimer with neighbouring iodine atom and when fullerene is introduced, it gets suppressed. Table 3 summarizes photovoltaic performance of the best device in reverse sweep direction for different times with PC60BM as interlayer. It has also been suggested that light irradiation can lead to annihi­ lation of some of the defects by trapping light induced electron and hole at the positive or negative interstitial iodine defect pair [34,35]. How­ ever, since the performance was improved after 50 h in devices stored in both dark as well as light conditions, the effect of light soaking can be ruled out (Table 4). The best devices with and without PC60BM for the same time interval along with the dark data are shown in Fig. 3. The device without PC60BM for reverse bias does not show much improvement in leakage current as the time passes. For the device with PC60BM, at 2 V, the leakage cur­ rent value is ~0.8 mA/cm2 at 0 h and it improves by one order to 0.04 mA/cm2 for 50 h [36]. This confirms the fact that after 50 h, PC60BM starts interacting with the traps and reduces the dark current leakage and hence, improvement in shunt resistance [37]. Also, the rectification ratio was improved by 2 orders (100 times) for the same time interval which shows that diode behaviour of device is also improving. In addition, in forward bias region after 1 V, the injection current density for 50 h increases thus confirming the fact that injection barrier between perovskite and PC60BM reduces [38,39]. This fact was further supported 5

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Fig. 5. a(i), a(ii), & a(iii) of column 1st show AFM Surface morphology with root mean square (Rrms) roughness of structure FTO/c-TiO2 for 0 h, 50 h and 150 h b(i), b(ii), & b(iii) of column 2nd show the AFM surface morphology with root mean square roughness of structure FTO/c-TiO2/PC60BM for 0 h, 50 h and 150 h c(i), c(ii), & c(iii) of column 3rd show the AFM surface morphology with root mean square roughness of structure FTO/c-TiO2/PC60BM/MAPbI3 for 0 h, 50 h and 150 h.

by Kelvin probe force microscopy (KPFM) measurements, as shown in subsequent sections which showed changed energy levels of constituent layers leading to improved band alignment for the devices at 50 h. Since performance of the devices with PC60BM showed maxima at 50 h and started decreasing subsequently with faster deterioration after 150 h, as shown in Fig. 1(f), the question arises that whether this degradation is due to interfacial degradation or degradation of bulk perovskite layer. We analysed this by investigating the interfacial and bulk phenomena and conducted detailed structural, micro-structural, optical, electronic, and electrical measurements of individual layers in the PSC devices as elaborated in subsequent sections.

diffraction pattern at 2θ ¼ 12.6� which corresponds to PbI2 phase which usually forms after degradation of perovskite layer3c, 21 and hence, its absence rules out degradation of CH3NH3PbI3 i.e. perovskite film as a function of time, even after 150 h. Hence, this data concludes that bulk phase of perovskite is rather stable under storage conditions within the glove box. Fig. 5 shows the images of the surface morphology of c-TiO2 layer on FTO coated glass substrates i.e. FTO/c-TiO2 (a(i), a(ii), and a (iii) in column I of Fig. 5), of PC60BM layer with underlying structure as FTO/TiO2/PC60BM (b(i), b(ii), and b(iii) in column II of Fig. 5) and of perovskite layer with structure as FTO/TiO2/PC60BM/Perovskite (c(i), c (ii), and c(iii) in column III of Fig. 5) after 0 h, 50 h and 150 h time intervals. While surface roughness evolution could be signature of degradation in devices as reported previously [14]. In our case, the roughness does not change appreciably suggesting other possible rea­ sons behind observed degradation after 50 h as well as difference be­ tween the PC60BM and non-PC60BM containing samples. Similar was observed for FESEM images as well (Fig. S1). The device with PC60BM as an interlayer showed enhancement in

3.2. Structural and optical analysis Fig. 4(a) shows the X-ray diffraction patterns of perovskite films after 0 h, 50 h and 150 h time interval. The characteristic peaks at 2θ ¼ 14.1� , 28.4� , 31.7� , 40.7� , and 43.0� in XRD patterns confirm formation of the tetragonal phase of perovskite [40]. Fig. 4(b) shows the enlarged view of 6

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Fig. 6. (a) Absorbance spectra of triple layer (FTO/c-TiO2/PC60BM/MAPbI3), (b) PL spectra of Perovskite on FTO/TiO2 and FTO/TiO2/PC60BM excited at 470 nm, (c) Schematic of PL blueshift due to PC60BM passivation and (d) PL spectra of perovskite on FTO/TiO2/PC60BM after different time interval of 0 h, 25 h, 50 h, 100 h, 150 h and 200 h (All PL spectra were taken from glass side having excitation wavelength of 470 nm).

efficiency so to further understand the insight, we did absorbance and PL of FTO/c-TiO2/PC60BM/MAPbI3film as absorbance spectra also reveal the bulk characteristics of the perovskite layer [41]. We also looked at the absorption spectra of the perovskite layers as a function of time after being deposited on PC60BM layer and the results are shown in Fig. 6(a). Absorbance spectra of FTO/c-TiO2/PC60BM films were taken at regular interval and not much change in absorbance spectra was observed with the time (Fig. 6(a)). The film shows no degradation even after 150 h. Since XRD and absorbance data did not yield any information on structural changes, we further conducted photoluminescence (PL) spectroscopy on films which is a useful technique to find out whether the defects are at the interfaces or in the bulk [17]. PL measurements were conducted on the perovskite films on FTO/c-TiO2 and FTO/c-­ TiO2/PC60BM and the results, as shown in Fig. 6(b), clearly suggest PL quenching for the case of perovskite film coated on FTO/c-TiO2/PC60BM surface. Excitation wavelength of 470 nm having penetration depth less than 100 nm was used, which is lower than the thickness of perovskite (~300 nm). The light was irradiated from the top side i.e. directly on the perovskite film from the air side to examine the bulk defects while the

spectra from the glass side were collected to examine the interfacial phenomenon at PC60BM/perovskite interface (Fig. S2 (a)). We did not observe any shifting of PL peak when excitation was made from the perovskite side suggesting no degradation or formation of traps in the perovskite layer. However, when PL spectra of perovskite on c-TiO2 with and without PC60BM were taken from the glass side, the data showed PL quenching with slight blue shift (Fig. 6(b)). If we compare it with band edge transition, there will be red shift in PL spectra if spontaneous radiative recombination will occur and blue shift will happen if there is passivation of these traps as shown in Fig. 6(c) [17]. Next we compare the time dependent PL spectra of perovskite on FTO/c-TiO2/PC60BM/glass, collected from the glass side. Fig. 6(d) shows that the PL quenches significantly as we measure from 0 h to 50 h, also marked by an increase in the efficiency accompanied by a blue shift of ca. 3 nm which is attributed to the passivation effect of PC60BM at the c-TiO2/perovskite interface [17,42]. It has been shown previously that annealing of PC60BM causes its diffusion into the perovskite or in its grain boundaries and thus, cause blue shift in PL spectra. As it is known that, there are deep traps and shallow traps in perovskite films. The deep 7

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3.3. Electrical analysis We further investigated the ageing and degradation effects with time using Impedance spectroscopy (IS), conducted in dark with bias voltage of 0.6 V within the range of 100 Hz to 10 kHz. Fig. 7 shows the fitted data for Nyquist plot for 0 h, 50 h and 150 h showing two overlapping semicircles, one at higher frequency and other at lower frequency. These can be modelled using the equivalent circuit using RC elements, as shown in inset of Fig. 7, where CPE depicts a constant phase element. First semicircle denotes the transport resistance at HTL and ETL or charge transfer resistance at perovskite interfaces and has been denoted by Rct, whereas second arc denotes recombination resistance related to perovskite layer (Rrec) [18,44]. The value of Rct is 19.7 kΩ for 0 h which reduces significantly to 9.7 kΩ after 50 h and then to 8.1 kΩ after 150 h suggesting improved charge transfer with time. However, the recombi­ nation resistance (Rrec) increases from 1.01 kΩ at 0 h to 2.45 kΩ after 50 h before decreasing to 0.96 kΩ after 150 h. Therefore, while charge transfer is good in devices till 150 h, it is the recombination of charges that reduces the device performance after 50 h. This could be due to the degradation of perovskite layer at the interface of perovskite/PC60BM. This also manifested in the change in the energy levels of the three phases as studied using KPFM as shown in Fig. 8. KPFM is an atomic force microscopy technique which measures the distributed contact potential difference (CPD) between the tip and the sample, through equation (1), which can be used to assess the surface potential

Fig. 7. Nyquist plots different time intervals of 0 h, 50 h, and 150 h along with fitted data. Inset showing the equivalent circuit diagram.

traps are primarily at the surface of perovskite while shallow traps are present at the grain boundaries [12]. The above results indicate that PC60BM is at the surface of perovskite at 0 h and it started interacting with the traps of grain boundaries (GB) after 25 h and maxima reaching at 50 h. Passivation of the traps by PC60BM thus reduces the interfacial charge recombination and increases the FF and Voc which also can be seen in the device characteristics in Fig. 3 and Table 2. It has been suggested that PC60BM has higher conductivity than perovskite which causes balanced charge collection leading to improved FF [32,43]. After 50 h time interval, there is a little red shift in PL spectra for 100 h, 150 h and 200 h (Fig. 6(d)) samples which is also manifested in the device efficiencies and thus reveals that the interface starts deteriorating due to the formation of traps.

VCPD ¼

ϕtip

ϕsample e

(1)

Here, e is the electronic charge and φtip and φsample are the work functions of the tip and the sample, respectively. The technique has previously been used to measure the energy levels of thin films in de­ vices including organic solar cells as reported in literature [39,45]. The samples were kept in the glove box for KPFM measurement and were taken out at the required intervals. During KPFM measurement, the humidity was below 40% RH and temperature was between maintained

Fig. 8. Energy levels of TiO2 layer, TiO2/PC60BM layer and TiO2/PC60BM/Perovskite layer after different time intervals as determined from the KPFM imaging. 8

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22 � C–24 � C. For calculations, φtip has been taken as 5.0 eV and Fermi level of sample was calculated as φsample ¼ φtip - eVCPD [46]. The results of KPFM measurements as shown in Fig. 8 suggest that while electrons do not face an energy barrier until 50 h while moving towards cathode, the perovskite Fermi level and hence the LUMO level goes down by 90 meV w.r.t PC60BM in sample measured for 150 h. This happens due to intrinsic degradation of perovskite because samples were kept in the nitrogen filled glove box. Hence, formation of deep traps leads to downward shift of Fermi level and hence LUMO level [14,47]. This clearly shows formation of a barrier at the PC60BM/Perovskite interface leading to reduced charge injection from perovskite to PC60BM towards cathode thus leading to degradation in performance and is attributed to degradation of the perovskite layer. Here, one can clearly see that while TiO2 and PC60BM are quite stable until 150 h, degradation of perovskite leads to drop in the performance.

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4. Conclusions In this work, we have studied the aging characteristics of perovskite solar cells by incorporating a thin PC60BM layer between TiO2 and perovskite layer. Our results showed that insertion of PC60BM allows the efficiency to increase up to 50 h which suggests that PC60BM acts as ETL and also causes the passivation of traps and grain boundary defects in the perovskite layer which is also manifested in the reduced series resistance. The device efficiencies increase till 50 h followed by rapid decrease after 150 h. While absorbance and XRD data do not show any material degradation, a combination of photoluminescence spectros­ copy, impedance analysis and Kelvin probe force microscopy suggest the microscopic degradation emanating from the interface in PSC device whereas bulk remains stable. It is found that while charge transfer is good in devices till 150 h, it is the recombination of charges that reduces the device performance after 50 h which is attributed to the degradation of perovskite layer at the interface of perovskite/PC60BM. 5. Associated content Supporting information SEM image and PL spectra from perovskite side for different time intervals. This material is available free of charge. Notes The authors declare no competing financial interest. Author contributions section Rahul Ranjan: Conceptualization, Investigation, Writing- Original draft. Belal Usmani: Resources, Writing- Review & Editing. Sowjanya Pali: Resources, Writing- Review & Editing. Sudhir Ranjan: Investi­ gation, Writing- Review & Editing. Anand Singh: Supervision, WritingReview & Editing. Ashish Garg: Supervision, Writing- Review & Edit­ ing. Raju Kumar Gupta: Supervision, Writing- Review & Editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Authors acknowledge the financial support from Department of Science and Technology (DST), India through Grant No. DST/TMD/ CERI/C140(G) under Clean Energy Research Initiative. This work was also supported by the UKRI Global Challenge Research Fund project, 9

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