ITIC-based bulk heterojunction perovskite film boosting the power conversion efficiency and stability of the perovskite solar cell

Solar Energy 178 (2019) 90–97

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ITIC-based bulk heterojunction perovskite film boosting the power conversion efficiency and stability of the perovskite solar cell

T

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Ranbir Singha,b, , Vivek Kumar Shuklac a

Department of Energy Materials Science and Engineering, Dongguk University, Seoul 04620, Republic of Korea Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea c Department of Applied Physics, Gautam Buddha University, Greater Noida, G B Nagar, UP, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Bulk heterojunction Perovskite solar cells Photoluminescence Methylammonium lead iodide Transient photovoltage Time-resolved photoluminescence

Herein we report a 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′] dithiophene (ITIC) small molecule additive based bulk heterojunction (BHJ) perovskite solar cell (PSC). The BHJ PSC with 0.2 wt% of ITIC exhibits a power conversion efficiency (PCE) of 17.6% with remarkably enhanced short-circuit current of 23.7 mA cm−2 as compared to reference PSC (without ITIC). The addition of ITIC plays an important role in improving the quality of the perovskite film like enhanced absorption, compact grains with reduced roughness (≈12.3 nm), higher electron mobility (μe = 1.43 × 10-3 cm2 V-1 s−1), better charge extraction and thermal stability of the solar cells. The BHJ PSC device degraded only by ≈18% after 53 days at 80 °C in atmospheric condition. In addition, several important characteristics of solar cell like current-voltage hysteresis, charge transfer, and recombination losses have been studied.

1. Introduction Organometal halide perovskite solar cells (PCSs) have attracted enormous attention due to their ever increasing power conversion efficiencies (PCEs), since the first perovskite solar cell has been reported in 2009 (Kojima et al., 2009). PSCs have achieved great success in short span of time because of facile fabrication procedure, low-cost photovoltaic materials, and promising performance for solar cells (Burschka et al., 2013; Cai et al., 2016; Ding et al., 2016; Tong et al., 2016; Yang et al., 2016). In addition to the strong absorption in the ultraviolet (UV)-visible region, perovskites have exciting intrinsic properties, such as long exciton diffusion length, excellent crystallinity and charge carrier mobility comparable to silicon, bipolar transport and longer charge carrier diffusion length, which enable high-performance of the PSC devices (Burschka et al., 2013; Liu et al., 2013; Yang et al., 2015b). PSCs can be fabricated by solution as well as evaporation methods (Nie et al., 2015; Zhou et al., 2018). The main issue of the planar heterojunction PSCs is uncontrolled complicated morphology of the perovskite film when deposited via solution process. To overcome this issue, various processing approaches, such as substrate annealing, solvent additive, anti-solvents spin-coating and mixed solvents were used to control the crystal growth (Dualeh et al., 2014b; Eperon et al., 2014; Kim et al., 2014; Liang et al., 2014; Mao et al., 2016; Song et al., 2015;

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Sun et al., 2015; Xia et al., 2016). In early stage, Grätzel and Snaith et al. maximized the coverage of perovskite thin film over the substrate by optimizing the annealing temperature and thickness of perovskite solar cell (Dualeh et al., 2014b; Eperon et al., 2014). Also, Song et al. utilized 1-chloronaphthalene as an additive in the CH3NH3PbI3–xClx thin film to facilitate homogeneous nucleation and improve the coverage of perovskite film, and as a result, the PCE of the cells improved by 30% (Song et al., 2015). Besides, Jeon and Kim et al. reported the control of the morphology of the perovskite film (Jeon et al., 2014; Kim et al., 2014). They obtained uniform crystal domains and smooth surface using the mixed solvents (γ-butyrolactone (GBL), N,N-dimethylformamide (DMF) and Dimethyl sulfoxide (DMSO)) for solar cell. In another work, Wu et al. have fabricated a PCBM-perovskite BHJ via two-step-spin coating method to synthesize high-quality perovskite films for PSC and achieved an efficiency of 16% (Chiang and Wu, 2016). Another major issue with perovskite materials is poor long-term stability in ambient conditions, which has been considered as one of the major obstacles in commercialization of the PSCs (Leijtens et al., 2015; Li et al., 2015; You et al., 2015). There are few key factors causing degradation of the perovskite film; moisture, thermal energy, light soaking, photo-induced reaction and ions/molecules migration (Conings et al., 2015; Dualeh et al., 2014a; Leijtens et al., 2013; Liu

Corresponding author at: Department of Energy Materials Science and Engineering, Dongguk University, Seoul 04620, Republic of Korea. E-mail address: [email protected] (R. Singh).

https://doi.org/10.1016/j.solener.2018.12.014 Received 9 September 2018; Received in revised form 18 November 2018; Accepted 3 December 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.

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material was prepared by dissolving 30 mg of po-spiro-MeOTAD, 21.5 µL dilute solution of 4-tert-butylpyridine (1 ml 4-tert-butylpyridine mixed with 1 ml of acetonitrile), 21.5 µL of a stock solution of 170 mg ml−1 lithium bis(trifluoromethylsulphonyl)imide in acetonitrile added in 1 ml anhydrous chlorobenzene. Finally, Au (80 nm) anode was thermally deposited under high vacuum (< 2 × 10-6 Torr) through a shadow mask to create devices area of 0.0555 cm2.

et al., 2014). Among all responsible factors, ions/molecules migration is a serious issue when we look for long-term stability. Tremendous efforts have been already made to suppress ions migration, like introduction of carbon counter electrode, composition engineering, using heavily doped metal oxide, hydrophobic materials, surface passivation, and crystal growth with reduced defects (Abate et al., 2014; Bi et al., 2015; Jeon et al., 2015; Leijtens et al., 2015; McMeekin et al., 2016; Zheng et al., 2014). All these approaches have their own unique morphology control mechanisms, and their merits and demerits. In this work, we have utilized ITIC small molecule to fabricate an efficient and stable BHJ CH3NH3PbI3:ITIC thin film for PSC. ITIC is a well-known electron accepting material and widely used in the BHJ organic solar cells (Wang et al., 2017). Better charge transport properties and favorable energy levels (−3.78 (LUMO) and −5.51 eV (HOMO)) of ITIC make it suitable to form BHJ PSCs (Zhao et al., 2016). The energy level diagram of all the material used in solar cell are shown in supporting information (Fig. S1). The BHJ CH3NH3PbI3:ITIC films were prepared by two-step spin coating solution process, a mixed solution of PbI2 and ITIC in a solvent of DMF:DMSO (6:4 vol%) (Singh et al., 2017a,b) was spin coated on a glass/FTO/TiO2 substrate and then a methylammonium iodide (CH3NH3I) solution is added via spin coating followed by thermal annealing at 100 °C in N2 filled glovebox. Both CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC films were characterized for their optical, morphological, electrical properties and thermal stability. The solar cell fabricated with BHJ CH3NH3PbI3:ITIC film has shown 16% increase in PCE over reference PSC (without ITIC) under AM 1.5 G, 1sun. Fabricated PSCs were further investigated for the differences in photovoltaic properties via analyzing the charge generation, charge carrier mobility, and recombination losses.

2.3. Characterization of perovskite solar cells The electrical characteristics were measured with a Keithley 4200 unit under 1 sun condition in a N2-filled glove box. The light was generated with an Oriel 1-kW solar simulator referenced using a Reference Cell PVM 132 calibrated at the US National Renewable Energy Laboratory. A photomodulation spectroscopic set-up (model Merlin, Oriel) was used to measure the IPCE as a function of light wavelength. The power density of the monochromatic light was calibrated using a Si photodiode certified by the National Institute for Standards and Technology. Photoexcitation intensity dependent current density-voltage (J-V) curves were measured with same solar simulator setup by varying the intensity from 0.1 to 1 sun. 2.4. Morphological characterizations FESEM images were obtained using Hitachi S-4800 Field Emission Scanning Electron Microscope and AFM images were taken using a SPA300HV instrument equipped with a SPI3800 controller (Seiko Instruments). 1D X-ray diffraction (1D-XRD) measurements were performed using X-ray diffractometer (Rigaku D/MAX-2500/PC) with CuKα X-rays.

2. Experimental detail 2.5. SCLC measurement 2.1. Materials The electron and hole mobility data were extracted from the dark J–V characteristics of electron-only devices: ITO/TiO2/CH3NH3PbI3 or BHJ CH3NH3PbI3:ITIC perovskite/Ca/Al, and hole-only devices: ITO/ PEDOT:PSS/CH3NH3PbI3 or BHJ CH3NH3PbI3: ITIC perovskite/Au. The electrical characteristics were measured using Keithley 4200 in a N2filled glove box.

Lead (II) iodide 99% (PbI2), methylammonium iodide (CH3NH3I) and titanium isopropoxide were purchased from Sigma-Aldrich, and used as received. The ITIC small molecule and 2,2′,7,7′-Tetrakis[N-4methoxyphenyl-N'-2-methoxyphenyl)amino]-9,9′-spirobifluorene (pospiro-MeOTAD), a hole transporting materials were purchased from 1materials company.

2.6. Charge carrier dynamics 2.2. Preparation of perovskite solar cells Transient photovoltage (TPV) and transient photocurrent (TPC) were measured using a TDC3054C digital oscilloscope connected to high-speed reamplifiers: SR560 and DHPCA-100. The samples were excited by a 3 ns pulsed laser at 532 nm (OBB, NL4300, and OD401) under AM 1.5G illumination at an intensity of 0.3–1 sun.

The inverted structure of the PSCs was prepared with stack glass/ Fluorine-doped Tin Oxide (FTO)/TiO2 (40 nm)/CH3NH3PbI3 or BHJ CH3NH3PbI3: ITIC/po-spiro-MeOTAD/ (60 nm)/Au (80 nm). First, cleaned FTO-coated glass substrates were treated with UV/ozone for 30 min, a hole-blocking layer of compact TiO2 was deposited by spincoating a mildly acidic solution of titanium isopropoxide (254 µL) in ethanol (2 ml) and HCl (3.4 µL), and then baked at 500 °C for 30 min in the furnace. For CH3NH3PbI3 film deposition, 1.2 M PbI2 solution was prepared in a mixed solvent ratio of DMF:DMSO (6:4 vol%) at 70 °C, and 45 mg ml−1 CH3NH3I solution was prepared in IPA at RT with overnight stirring in N2 filled glove box. The PbI2 film was spin-cast at 3000 rpm for 20 s, dried on 60 °C hot plate for 5 min. Dried PbI2 film was spin-coated at 3000 rpm with prepared CH3NH3I solution for 30 s, and then immediately transferred on the hot plate at 100˚C for 45 min to perovskite conversion. For BHJ perovskite films, separate solutions of PbI2:ITIC were prepared in mixed solvents with different concentration (0.05 wt%, 0.1 wt%, 0.2 wt% and 0.3 wt%) of ITIC small molecule. The BHJ CH3NH3PbI3:ITIC layers were fabricated by spin coating of PbI2:ITIC solution on to the FTO/TiO2 substrates and dried PbI2:ITIC films were then spin-coated with CH3NH3I solution. All the films were immediately transferred on the hot plate in similar conditions as had been used to form CH3NH3PbI3 film. The hole extracting material was then spin-coated at 4000 rpm for 30 s. The solution of hole extracting

3. Results and discussion 3.1. Optical and photovoltaic properties Fig. 1a&b shows UV–visible absorption spectra and J-V characteristics of the CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC films made with different concentrations (0.05, 0.1, 0.2 and 0.3 wt%) of ITIC. All the films displayed absorption onset around 780 nm, a property of the band-gap absorption of CH3NH3PbI3 perovskite film (Xie et al., 2015). The absorption intensities of both CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC films are more prominent in lower wavelength region and maximum absorption intensity recorded for the BHJ perovskite film with 0.2 wt% of ITIC. The slightly improved absorption for the BHJ CH3NH3PbI3:ITIC can be assigned to the improved morphological and crystalline properties of the perovskite films (discussed later in morphological studies section). The photovoltaic properties of fabricated PSCs are summarized in Table1. PSCs were fabricated with device structure of FTO/TiO2/CH3NH3PbI3 or BHJ CH3NH3PbI3:ITIC/po-spiro91

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Fig. 1. (a) UV–vis absorption spectra of perovskite and BHJ perovskite films and (b) J-V characteristics of PSCs fabricated using perovskite and BHJ perovskite films formed with different concentrations (0, 0.05, 0.1, 0.2, and 0.3% wt%) of ITIC. (c) IPCE spectra and integrated JSC and (d) photocurrent density versus effective voltage (Jph − Veff) characteristics for PSCs. For absorption, all the perovskite films were spin-coated on the FTO/TiO2 substrate and solar cells were fabricated with device structure FTO/TiO2/CH3NH3PbI3or CH3NH3PbI3:ITIC/po-Spiro-MeOTAD/Au.

perovskite solar cell like slow transient capacitive current, ion migrations, trapping and detrapping process, and ferroelectric polarization etc (Chen et al., 2016). The better understanding and elimination of the J-V hysteresis is crucial for further advancement of PSCs. Fig. 1c shows the incident photon-to-current conversion efficiency (IPCE) (left vertical scale) and integrated JSC (right vertical scale) for both CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC PSCs. The improved IPCE spectrum by 1–8% in the region 320–788 nm of the solar cell is well followed by absorption spectrum of BHJ perovskite film. Also, the enhanced JSC and IPCE demonstrate that presence of ITIC in BHJ perovskite solar cell influence the exciton dissociation and free charge carrier separation. In Fig. S3, the photoluminescence (PL) spectra for both the perovskite films are presented, where films were excited with 470 nm wavelength. The PL spectrum of BHJ perovskite film is significantly weak, indicating that addition of ITIC small molecule plays an important role in exciton dissociations or in reducing recombination sites. Next, to see how efficiently the excitons are separating into free charge carriers, the relationship between photocurrent (Jph) and effective voltage (Veff = V0 − V) has been studied under AM 1.5G illumination as shown in Fig. 1d. V is the externally applied voltage and V0 is the voltage at which illuminated current is crossed to dark current. At high

MeOTAD/Au. The PSC fabricated using reference CH3NH3PbI3 perovskite film showed short-circuit current density (JSC) = 22.3 mA cm−2, open circuit voltage (VOC) = 0.98 V, fill factor (FF) = 68.2%, and PCE = 15.14%. Out of the BHJ CH3NH3PbI3:ITIC based PSC devices, which were fabricated with different concentrations of ITIC, the device fabricated using 0.2 wt% of ITIC in CH3NH3PbI3:ITIC film showed best photovoltaic properties; JSC = 23.74 mAcm−2, VOC = 1.0 V, FF = 72.8%, and PCE = 17.59%. The improved photovoltaic properties of BHJ CH3NH3PbI3:ITIC with respect to reference CH3NH3PbI3 are investigated for exciton dissociation, morphology, charge carrier transport, and recombination losses. PSCs are well known to have hysteresis in the J-V characteristics, when measured in different scanning directions. Therefore, to check the hysteresis in our devices, both CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC (0.2 wt%) based PSCs were investigated for the reverse and forward scans (supplementary information Fig. S2). BHJ CH3NH3PbI3:ITIC based PSC showed smaller hysteresis compared to CH3NH3PbI3 based PSC, which indicates that addition of ITIC affects the hysteresis favorably. In our work, we suspect that the presence of ITIC at grain boundaries prevent the ions migration in perovskite film. There are several other reasons that can be responsible for the hysteresis in the

Table 1 Photovoltaic parameters of PSCs with different concentrations of ITIC. Average of photovoltaic parameters were taken for six devices. Active material

wt% of ITIC

Voc (V)

Jsc (mA cm−2)

FF (%)

CH3NH3PbI3:ITIC

0% 0.05% 0.1% 0.2% 0.3%

0.98 ± 0.005 1.0 ± 0.003 1.0 ± 0.002 1.0 ± 0.004 1.0 ± 0.002

22.36 23.08 23.41 23.74 23.63

68.2 69.8 70.6 72.8 71.2

92

± ± ± ± ±

0.15 0.15 0.21 0.23 0.21

± ± ± ± ±

PCE (%) 0.83 0.83 0.45 0.57 0.44

14.94 16.12 16.34 17.28 16.34

± ± ± ± ±

PCEmax. (%) 0.18 0.27 0.48 0.31 0.48

15.14% 16.39% 16.82% 17.59% 16.82%

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Fig. 2. (a, b) Field Emission Scanning Electron Microscope (FESEM) images, (d, e) AFM, and (c, f) 1D-XRD of the CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC films. AFM images were scanned for 5 × 5 µm2 area.

(2 2 0) peak at 14.22° and 28.66° with lattice parameters a = 8.7182 Å, c = 6.1908 Å, representing preferential tetragonal CH3NH3PbI3 structure with other peaks at (1 1 2), (2 1 1), (2 0 2),(3 1 0), and (3 1 2) (Singh et al., 2017a,b; Tao et al., 2015). The higher intensity of (1 1 0) peak for BHJ CH3NH3PbI3:ITIC film indicates the higher order of crystallinity (inset of Fig. 2). The full width at half maximum (FWHM) of the (1 1 0) peak was found to be reduced from 0.39° (CH3NH3PbI3) to 0.34° (BHJ CH3NH3PbI3) indicating improvement in the crystallinity of the films. The increase in crystallinity of the film may arise due to increase in degree of structural order (Bi et al., 2014; Cho et al., 2016; Singh et al., 2018). There are some more noticeable differences in CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC films, like: the intensity of the (1 1 0), (2 2 0) and (3 1 0) peaks in BHJ perovskite film becomes stronger, which indicates directional crystal growth in BHJ perovskite film, supporting the better transport properties (measured in next section) in the BHJ PSC device.

reverse biased voltage, Jph is saturated for all the solar cells, suggesting that the photogenerated excitons are dissociating into free charge carriers. Under short-circuit condition, the charge separation probabilities (Pdiss = Jph/Jsat) were calculated as 97.1% for CH3NH3PbI3 and 98.2% for BHJ CH3NH3PbI3 :ITIC (0.2 wt%), respectively. The high values of Pdiss for BHJ perovskite film suggest that excitons are efficiently dissociated into free charge carrier and collected by the electrodes (Cao et al., 2016).

3.2. Morphological study Morphology of the CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC films were investigated with Field Emission Scanning Electron Microscope (FESEM), atomic force microscopy (AFM) and 1-D XRD characterization tools (Fig. 2). All the samples were prepared on FTO/TiO2 substrates in similar fashion as adopted for solar cells fabrication. In case of CH3NH3PbI3 film, clusters of grains with grain size of 0.2–0.8 µm were observed with surface roughness of 24.9 nm, whereas for BHJ CH3NH3PbI3:ITIC film, the grains are appeared as more compact. The grain boundaries of BHJ film are not well defined to clearly distinguish the grains size. It seems that grain boundaries are covered with ITIC small molecules. Also, the average surface roughness of the film was reduced to 12.3 nm. AFM phase images of the CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC perovskite films are shown in Supporting Information Fig. S4. In short, BHJ CH3NH3PbI3:ITIC film show bigger grains with smooth surface, which may be due to the reason that ITIC fills the grain boundaries and appears to have bigger grains (Xu et al., 2015). The stronger absorption in the compact BHJ CH3NH3PbI3:ITIC film as compared to CH3NH3PbI3 film of same thicknesses has been found in agreement with the similar reports in literature where a different small molecule was added in perovskite to make an efficient thin film (Chiang and Wu, 2016). Further, to locate the exact position of ITIC in BHJ perovskite film, energy-dispersive X-ray spectroscopy (EDS) has been performed at different positions like; inside the grains, and grain boundaries (Fig. S5). FESEM images in Fig. S5 present the exact location at which EDS spectrum was recorded. The sulfur (S) atom in ITIC molecule was traced and well pronounced peak at the grain boundaries (position 2 and 3) indicating that the ITIC molecules accumulate at the grain boundaries, not inside the perovskite grains. Fig. 2c&f show1D-XRD patterns of BHJ CH3NH3PbI3:ITIC and CH3NH3PbI3 films. The TiO2 has diffraction peaks at same positions as reported in the literature (Ren et al., 2014) (supporting information Fig. S6). Both the perovskite films have shown diffraction planes (1 1 0) and

3.3. Charge carrier mobility The space charge limited current (SCLC) mobility for both the films was measured on fabricated carrier-controlled devices. The mobility values were calculated from the dark J–V curves by fitting with MottGurney equation (Chiang and Wu, 2016; Singh et al., 2017a,b) (supporting information Fig. S7) and the resulted carrier mobility values are listed in Table S1. The electron mobility of the BHJ CH3NH3PbI3:ITIC film (μe = 1.43 × 10-3 cm2 V-1 s−1) is much higher than the electron mobility of CH3NH3PbI3 film (μe = 6.56 × 10-4 cm2 V-1 s−1), whereas hole mobility (µh = 3.41 × 10-4 cm2 V-1 s−1) of BHJ CH3NH3PbI3:ITIC film is improved by 24.2 times to the hole mobility (µh = 8.32 × 105 cm2 V-1 s−1) of CH3NH3PbI3 film. The balanced and high charge carrier mobility in BHJ CH3NH3PbI3:ITIC film might be one of the reasons for the better FF and PCE of the solar cell. In order to see the effect of traps distribution and their effect on charge carrier transport properties, low temperature-dependent electron mobility measurements were carried out on CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC thin films based controlled devices. Temperaturedependent electron and hole mobility values were determined through their dark J–V characteristics at different temperatures in an identical fashion as for the calculation of electron mobility at room temperature. In Fig. 3, electron and hole mobility values of the devices are plotted as a function of 1/temperature (1/T), where temperature was varied from 140 K to 300 K and the data were fitted with the Arrhenius equation μ(T) = μ0exp(−Ea/kT); here Ea corresponds to the activation energy, 93

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decrease in slope for BHJ CH3NH3PbI3:ITIC indicating that trap states are greatly passivated by ITIC at the grain boundaries that is well supported by the better morphological and charge transport properties of the film. To calculate the recombination rate of charge carriers in CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC PSCs, TPV and TPC measurements were performed. The charge carrier lifetime (τ) was obtained from the TPV data, and the carrier density (n) was estimated from the TPC data using a differential capacitance technique described previously (Jiménez-López et al., 2017; Singh et al., 2017a,b). Fig. 4c represents that at same value of n = 7 × 1018 cm−3, the charge carrier lifetime of BHJ CH3NH3PbI3:ITIC PSC is 3.3 times longer than in the CH3NH3PbI3 PSC, suggesting a much slower recombination rate in the BHJ PSC. Experimental data in Fig. 4c was fitted with equation τ (n) = τ0 n−λ , where τ0 is the intercept at n = 0, and λ is the magnitude of the slope, which is related to the order of the non-geminated recombination (λ + 1) (Foertig et al., 2014). The slope λ extracted from CH3NH3PbI3 PSC was 1.95 and the value extracted from BHJ CH3NH3PbI3:ITIC PSC was 1.52. These results suggest that the CH3NH3PbI3 PSC is strongly limited to monomolecular recombination (trap-assisted recombination) rather than bimolecular recombination as seen in intensity dependent JSC data, whereas the BHJ CH3NH3PbI3:ITIC PSC has shown decrease in trap-assisted recombination. Fig. 4d represents the TRPL decay curves for the CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC films; all these perovskite films were deposited on glass/FTO/TiO2 substrates. The TRPL decay curves were fitted with the bi-exponential decay equation, y = A1e−(x / t1) + A2e−(x / t2) , and the extracted lifetimes (t1 and t2) of the systems were shown in Table 2. For both perovskite films, the dominant fast decay component is associated with charge carrier transfer from perovskite to TiO2 (Huang et al., 2017; Jiménez-López et al., 2017). The faster decay lifetime (t1) of the CH3NH3PbI3 film is 0.93 ns and the t1 decrease with the addition of ITIC to 0.55 ns for CH3NH3PbI3:ITIC film indicating better charge transfer in BHJ films. The slower decay lifetime (t2), which is associated with the recombination losses in the perovskite films, increased from 7.62 ns for CH3NH3PbI3 to 8.93 ns for BHJ CH3NH3PbI3:ITIC films. These results suggest that the photogenerated carriers in the BHJ film are effectively transferred from perovskites to TiO2 and decrease in the recombination rate. The charge carrier recombination is considered to be a combination of trap-assisted recombination, free electron-hole bimolecular recombination, and Auger recombination (Johnston and Herz, 2016; Wen et al., 2016). The efficient charge carrier transfer may be due to the better charge transport properties of BHJ CH3NH3PbI3:ITIC and decrease in the recombination losses arise due to the passivation of the defects sites at the grain boundaries.

Fig. 3. Temperature-dependent charge carrier mobility in electron-only and hole-only devices.

which is closely related to the width of the energetic disorder or the traps distribution (Singh et al., 2017a,b; Zhang et al., 2017). The extracted values of Ea are 84.3 meV for CH3NH3PbI3 and 51.8 meV for BHJ CH3NH3PbI3:ITIC in electron-only devices, and 173 meV for CH3NH3PbI3 and 49.4 meV for BHJ CH3NH3PbI3:ITIC in hole only devices, respectively. The lower value of Ea for CH3NH3PbI3:ITIC indicates a smaller energetic disorder that supports the higher electron and hole mobility of the devices. The decrease in energetic disorder also indicates the decrease in the defected states or charge carrier trapping states in the photoactive layer, which also supports the reduced hysteresis in BHJ CH3NH3PbI3:ITIC PSCs (Hsieh et al., 2018).

3.4. Recombination analysis

3.5. Device thermal stability

To understand the recombination mechanism in BHJ CH3NH3PbI3:ITIC film, the J–V characteristics were measured for same PSCs under different light intensities, and intensity dependent JSC and VOC were plotted in Fig. 4a & b. The experimental data fitting with power law(Bi et al., 2013) (JSC ∝ Iα) yielded α = 0.981 for CH3NH3PbI3, α = 0.979 for BHJ CH3NH3PbI3:ITIC films, respectively. Almost same values of α obtained for both the devices within the error limits, suggesting that mixing of ITIC does not affect the bimolecular recombination in devices. The VOC of the devices shows a linear relationship with I in semi-logarithmic scale plot when fitted with equation VOC ∝ (kBT/q)ln(I) (Li et al., 2016). A trap-free relationship should have a slope of kBT/q. The slope of 1.57kBT/q implies significant Shockley–Read–Hall recombination prevalent in CH3NH3PbI3 PSC, and the slope decreased to 1.32kBT/q for BHJ CH3NH3PbI3:ITIC PSC. The charge carrier traps in the CH3NH3PbI3-based device may be resulted from interfacial defects or traps at grain boundaries, which may also support the lower VOC and large hysteresis for the solar cell. The

To investigate the thermal stability of the PSCs, both CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC based solar cells were kept on hot plate at 80 ˚C in atmospheric conditions for 53 days. The PCE, JSC, VOC and FF decay curves of the devices were recorded as functions of the thermal annealing time and normalized by their original values (Fig. 5a–d). In comparison to the solar cell device fabricated using CH3NH3PbI3, the BHJ CH3NH3PbI3:ITIC based solar cell has been found to have better thermal stability. The PCE of the CH3NH3PbI3 device was decreased by 68% from its initial value, whereas the PCE of the devices fabricated using BHJ CH3NH3PbI3:ITIC was dropped only by 18% from its initial value. It is expected that compact perovskite films have more resistance to degradation and results in better device stability (Yang et al., 2015a; Zhu et al., 2017). Notably, the poor PCE stability of CH3NH3PbI3 based solar cell mainly arises due to the drastic drop of FF parameter by 66%, along with relatively small drop in parameters JSC and VOC. The improved PCE and FF stability for CH3NH3PbI3:ITIC was ascribed to the passivation of grain boundaries (Niu et al.) and the possibility of 94

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Fig. 4. (a, b) The light intensity dependence of Jsc and VOC, (c) carrier lifetime (τ) and carrier density (n) are measured for both PSCs. (d) TRPL traces for the CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC films are recorded on the substrate (FTO/TiO2). Dotted lines in (a) and (b) represent the fitting according to the equation present in literature (Bi et al., 2013; Li et al., 2016). Dotted lines in (c) represent the fitting according to the equation τ (n) = τ0 n−λ and in (d) represent the fitting with bi-exponential equation (Foertig et al., 2014).

Yuan and Huang (2016)) and Niu et al., who found small molecule and perovskite interactions at grain boundaries could suppress the ion movements. Also, the color of CH3NH3PbI3 device has changed from dark brown to yellow color because of degradation of perovskite material due to thermal decomposition, whereas the color of BHJ CH3NH3PbI3:ITIC persistent indicates better thermal stability of the BHJ perovskite material after 53 days (see in the inset of Fig. 5).

Table 2 Parameters extracted from the Fig. 4c and d. Active material

τn = 7 × 1018 (in s)

λ

t1 (ns)

t2 (ns)

CH3NH3PbI3 CH3NH3PbI3 + 0.2 wt% of ITIC

1.28 × 10−5 4.21 × 10−5

1.95 1.52

0.93 0.55

7.62 8.93

improving hydrogen bonding interactions (El-Mellouhi et al., 2016) at the grain boundaries, that might have suppressed the ions migration in perovskite crystals. Similar kind of observations has been reported by

4. Conclusion In summary, we have successfully fabricated an efficient and stable

Fig. 5. Photovoltaic parameters versus time for CH3NH3PbI3 and BHJ CH3NH3PbI3:ITIC thin film based solar cells, where all the PSCs were kept on a hot plate at 80 °C in atmospheric conditions. 95

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R. Singh, V.K. Shukla

BHJ CH3NH3PbI3:ITIC thin film for solar cell applications. The BHJ CH3NH3PbI3:ITIC perovskite film has shown improvement in morphological properties, with reduced grain boundaries and smooth surface. In addition, ITIC was demonstrated as an effective quencher for perovskites PL emission and a candidate for better charge transfer. The BHJ PSCs fabricated with 0.2 wt% of ITIC yielded a PCE of 17.6% with negligible J-V hysteresis. The improved device performance attributed to the advantageous properties of the perovskite photoactive layer, such as its improved film coverage, a high charge carrier mobility, and much longer charge carrier lifetime.

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