Improving the performance of low-temperature planar perovskite solar cells by adding functional fullerene end-capped polyethylene glycol derivatives

Improving the performance of low-temperature planar perovskite solar cells by adding functional fullerene end-capped polyethylene glycol derivatives

Journal of Power Sources 396 (2018) 49–56 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/loca...

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Journal of Power Sources 396 (2018) 49–56

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Improving the performance of low-temperature planar perovskite solar cells by adding functional fullerene end-capped polyethylene glycol derivatives

T

Qiqi Qina, Zongbao Zhanga, Yangyang Caia, Yang Zhoua, Hui Liua, Xubing Lua, Xingsen Gaoa, Lingling Shuib, Sujuan Wua,∗, Junming Liuc a

Institute for Advanced Materials, Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, 510006, China b Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, 510006, China c Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China

H I GH L IG H T S

end-capped polyethylene glycol (PCBPEG) derivatives were synthesized. • Fullerene perovskite solar cells (PSCs) with PCBPEG as additives were fabricated. • Planar the grain size and decreased trap state in the modified perovskite films. • Improved • Promoted charge transport and reduced recombination rates in the modified PSCs.

A R T I C LE I N FO

A B S T R A C T

Keywords: Functional fullerenes Planar perovskite solar cells Photoelectric properties

Functional fullerene derivatives play an important role in improving the performance of perovskite solar cells (PSCs) by promoting charge transfer and passivating trap states in perovskite film. In this work, the planar PSCs with the structure of FTO/TiO2/modified CH3NH3PbI3/Spiro-OMeTAD/Ag are fabricated by one-step method. Fullerene end-capped polyethylene glycol derivative (PCBPEG) is synthesized by a simple process and added into the perovskite precursor solution to improve microstructure and photoelectric properties of PSCs. At the optimum concentrations of PCBPEG additives and the annealing time of perovskite film, PSCs with PCBPEG additives yield the average efficiency of over 17.3%, being much higher than 15.28% of the reference PSC. Moreover, the unencapsulated PSCs with PCBPEG additives prepared at the optimal process demonstrate the enhanced stability. Compared to the reference perovskite film, the modified perovskite films demonstrate larger grain size, improved electric properties at nanoscale level and reduced electron trap state density, which will contribute to the favorable photovoltaic performance. The improved performance of the modified PSCs is primarily attributed to the promoted carrier transfer and suppressed charge recombination. These results provide a facile and feasible method to fabricate functional fullerene for high performance PSCs.

1. Introduction Due to high absorption coefficient, long carrier diffusion length and ambipolar charge transport characteristics, organic-inorganic hybrid halide perovskites are considered to be promising material for photoelectric device [1–5]. The power conversion efficiency (PCE) of the halide perovskite solar cells (PSCs) rapidly increases from 3.8% to 22.1% [6–15] in a few years. This was attributed to the regulated composition of perovskite layer and the optimized interface and device's structure [1,2,16–24]. However, lots of efficient PSCs are based on the meso-superstructure with a layer of mesoporous metal oxide ∗

Corresponding author. E-mail address: [email protected] (S. Wu).

https://doi.org/10.1016/j.jpowsour.2018.05.091 Received 6 February 2018; Received in revised form 27 April 2018; Accepted 29 May 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.

materials such as TiO2 and Al2O3, which need to be sintered at high temperature (> 450 °C). On the other hand, planar PSCs have been widely studied due to their unique advantages such as low cost, easy processing and low-temperature process [25–27]. In planar PSCs, each layer has an important influence on the device's performance [28–32]. Especial significantly, the perovskite layer is important to obtain efficient PSCs. However, the perovskite film prepared by solution method has suffered from some problems such as poor coverage and inferior morphology [12,33], which may lead to undesirable photovoltaic performance. In order to improve the microstructure of PSCs, incorporating additives into the perovskite solutions

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aqueous solution for 50 min at 70 °C. Then the TiO2 film was washed with deionized water and alcohol, followed by annealing at 200 °C for 30 min. The methylammonium iodide (MAI) was synthesized according to the previous reported [27]. 1 M MAI and 1 M PbI2 (99.9985%) were dissolved in dimethylsulfoxide (DMSO) (≥99.9%) and gamma-butyrolactone (GBL) (≥99.9%) at a 3:7 vol ratio to obtain a 40 wt% perovskite precursor solution, and then stirred at 60 °C for 12 h. The PCBPEG-4k and PCBPEG-20 k were synthesized by the reported method [42]. For the modified sample, PCBPEG-4k and PCBPEG-20 k with different concentrations were added to the perovskite precursor solutions, respectively. The perovskite films were prepared by spin-coating the CH3NH3PbI3 precursor solution at a speed 4000 rpm for 40 s in the glove box, and the chlorobenzene solution was dropped on the as-spun perovskite films during the program at 20 s. Then the perovskite films were annealed at 120 °C for 20 min. Subsequently, a thin layer of 2, 2′, 7, 7′- Tetrakis (N,N-di-p-methoxyphenylamine) - 9, 9′-spirobifluorene (Spiro-OMeTAD) was deposited on the perovskite film by spin-coating a chlorobenzene solution containing 80 mM Spiro-OMeTAD, 64 mM tertbutylpyridine (TBP) and 24 mM Li-bis(trifluoromethanesulfonyl)-imide (Li-TFSI) (520 mg/ml in acetonitrile) at 5000 rpm for 30 s. These samples were left in dry air overnight in the dark. Finally, silver electrode with a thickness of 80 nm was evaporated onto the top of the device through a shadow mask. The active area of the device was 0.045 cm2.

have been widely used due to its feasibility and facility. Functional fullerenes have made many impressive achievements during the past ten years in organic solar cells [34,35]. On the one hand, functional fullerenes can maintain the properties of the original fullerenes such as excellent electronic transmission and high electron affinity. On the other hand, the physical, chemical or thermodynamic properties of fullerene can be adjusted by introducing appropriate functional molecules into the fullerene. Moreover, fullerene has been successfully introduced into the perovskite layer to improve the performance of PSCs [36,37]. Shao et al. have proved that the phenyl-C61-butyric acid methyl ester (PCBM) can penetrate into the grain boundary of the perovskite during annealing, which can provide channel for carrier transport [13]. Furthermore, it has been reported that PCBM can effectively passivate the traps in the perovskite films [13]. Researchers have found that adding functional fullerene into perovskite precursor solution can result in better morphology and higher crystallinity of perovskite films [4,32,38,39]. Among them, perovskite films with long-chain polymer additives can form more compact films with network structure. Chang et al. have reported that the morphology and the coverage of perovskite film can be controlled by adding poly (ethylene glycol) (PEG) into the perovskite film [40]. It has been proved that the PEG additive can delay the growth of the perovskite crystal and reduce the pores between the perovskite grains during the phase transition. Zhao et al. have used PEG as a three-dimensional skeleton structure to support the growth of perovskite crystal, resulting in the slowed down crystallization of perovskite [41]. However, the insulating property of PEG will impede the charge transport and increase the device's serial resistance, leading to the relatively low photocurrent and fill factor. Therefore, we try to use PEG and PCBM to synthesize the fullerene end-capped polyethylene glycol (PCBPEG) to solve the problem. Although the PCBPEG has been used as an additive in organic solar cell to control the microstructure of active layers and improve the devices' performance [42], it has not been used in the PSCs. Moreover, the effects of PCBPEG additives on the microstructure and photoelectric of perovskite layer and PSCs have been systemically investigated in this work. Based on these considerations, the fullerene derivative PCBPEG has been synthesized by one-step transesterification from PEG and PCBM. The synthesized PCBPEG was used as additive into the perovskite precursor solution to regulate the microstructure and photoelectric properties of perovskite layer. The effects of concentrations of PCBPEG additives and molecular weight of PEG on the microstructure and photoelectric properties of perovskite layer and PSCs have been systemically investigated. At the optimized process, the best PCE of planar PSC increases from 15.35% to 17.72%. The results indicate that the PCBPEG modified PSCs can promote the grain growth of perovskite film, resulting in the promoted carrier transfer and collection, and suppressed charge recombination in PSCs. For convenience of presentation, these structures are abbreviated as “the reference films/ PSCs” in which perovskite film without any modification is included, “PCBPEG-4k or PCBPEG-20 k films/PSCs” in which the PCBPEG-4k or PCBPEG-20 k was added into the perovskite precursor solution, respectively. The PCBPEG-4k and PCBPEG-20 k were synthesized by PCBM and PEG with the molecular weight of 4000 and 20,000, respectively. This work provides a simple method to fabricate functional fullerene derivatives as effective additives for perovskite layer to improve the performance of PSCs.

2.2. Characterizations Fourier Transform Infrared (FTIR) spectra were measured by using an IR spectrometer (FT/IR-660 plus, Jasco). The photovoltaic performance of these PSCs was recorded on a Keithley 2400 source meter under an illumination of 100 mW/cm2 (Newport 91160 equipped with an AM 1.5 G filter). The light intensity was calibrated by a standard Si solar cell. All the unencapsulated devices were measured in the air. The X-ray diffraction (XRD) (X'Pert PRO, Cu Ká radiation) was used to investigate the crystallinity of perovskite films. The surface morphology of the perovskite films was obtained by the scanning electron microscopy (SEM) (ZEISS ULTRA 55) and the grain size of the films was calculated by the Nano Measurer. Atomic force microscope (AFM) (MFP-3D-Stand Alone, Asylum Research Cypher) was used to measure the films surface roughness, photo-current and contact potential using a white light-emitting diode (LED) with an irradiance of 10 mW/cm2. The external quantum efficiency (EQE) was recorded by EQE measurement system (Newport 66902). The UV-vis absorption spectra of the perovskite films were measured by spectrophotometer (SHIMADZU UV2550). The electrochemical impedance spectroscopy (EIS) measurements were carried out by the Zahner Zennium electrochemical workstation under the 100 mW/cm2 white light. For the EIS measurements, a 50 mV ac-sinusoidal signal source was employed over the constant bias with the frequency tuned from 1 Hz to 1 MHz. The steady-state photoluminescence (PL) was measured by a fluorescence spectrometer (HITACHI F-5000) exited at 515 nm in wavelength. 3. Results and discussion As illustrated in Fig. 1a, the PCBPEG-4k and PCBPEG-20 k were synthesized according to the reported method [42]. As shown in Fig. 1b, the FTIR spectroscopy was carried out to investigate the molecular structure difference between PCBPEG-4k and the mixtures of PEG-4k and PCBM. It can be seen that the FTIR spectrum of the mixture of PCBM and PEG are simply the superposition of the two substance peaks. However, the patterns in PCBPEG-4k are quite different. The peak at 1730 cm−1 is wider and notably the relative intensities of three peaks at 841, 955 and 1280 cm−1 are reduced, suggesting that there is a decreased crystallinity of PEG. The reduced crystallinity is primarily attributed to the break of old chains in original PEG when fullerene is combined with it. This is consistent with the previous report [42]. Fig. 2a displays the schematic diagram of the fullerene derivative

2. Experimental section 2.1. Material and methods Fluorine-doped tin oxide conductive glass (FTO, 15 Ohm/square) was patterned properly with zinc powder and dilute hydrochloric acid, then sequentially washed with detergent, deionized water, acetone, isopropanol and alcohol. The TiO2 compact layer was deposited on the cleaned FTO substrate which was immersed in a 0.04 M TiCl4 (98%) 50

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Fig. 1. (A) Synthesis route of PCBPEG; (b) the FTIR spectra of PEG-4k, PCBM, mixture of PCBM and PEG-4k, and PCBPEG-4k.

respectively. Fig. S1 (Supporting Information) presents the J-V curves of PSCs with different additive concentrations of PCBPEG-4k or PCBPEG-20 k and the PCE values as a function of concentrations, respectively. The reference PSC exhibits a champion PCE of 15.35% with an open circuit voltage (Voc) of 1.038 V, short current density (Jsc) of 20.20 mA/cm2 and fill factor (FF) of 73.21%. For the PSCs with PCBPEG-4k or PCBPEG-20 k additive, the concentrations vary from 0.03 to 0.11 wt%. The PCBPEG-4k PSC with the concentration of 0.07 wt% PCBPEG-4k additive achieves the highest PCE of 17.72%, yielding a Jsc of 21.28 mA/cm2, a Voc of 1.073 V and a FF of 77.62%. The PCBPEG-20 k PSC with a concentration of 0.07 wt% PCBPEG-20 k additive also indicates a best PCE of 17.36% with a Jsc of 21.21 mA/ cm2, a Voc of 1.074 V and a FF of 76.17%. For all of the modified PSCs, a higher concentration than the optimum value will lead to a dramatic decrease of Jsc, Voc and FF, thus a lower PCE. It is speculated that this is due to the insulation of PEG [40]. Fig. S2 shows the J-V curves of PSCs with the PCBPEG-4k or PCBPEG-20 k additive annealed for different time at 120 °C and the PCE values as a function of annealing time. It can be seen that the optimum annealing time for PSCs with PCBPEG-4k or PCBPEG-20 k additive is 20 min. Therefore, we fix the concentration of PCBPEG-4k and PCBPEG-20 k additives at 0.07 wt% and the annealing time of perovskite films at 20 min. The measured results are shown in Figs. 4–8 and Fig. S3.

modified PSC. The XRD patterns are measured to investigate the effect of PCBPEG-4k or PCBPEG-20 k additive on the crystal structure of perovskite films. Fig. 2b shows the XRD spectra of the reference, PCBPEG-4k and PCBPEG-20 k films, respectively. The peaks at 23.39, 26.45 and 37.74° are attributed to the FTO/TiO2 substrate. The peaks at 14.06, 19.94, 28.34 and 31.86° correspond to the (100), (112), (220) and (310) crystal planes of CH3NH3PbI3 with tetrahedral perovskite structure [43], respectively. It is noted that the reference, PCBPEG-4k and PCBPEG-20 k films show the comparative XRD intensity and no other phases appears. This indicates that the PCBPEG additives do not change the crystalline structure of the perovskite films. The cross-sectional SEM images of the reference, PCBPEG-4k and PCBPEG-20 k PSCs are shown in Fig. 2c–e. It can be seen that the PSCs without and with PCPBEG additives demonstrate the similar thickness. The PCBPEG-4k and PCBPEG-20 k PSCs show a slightly larger grain size in the perovskite films. This is consistent with the results of the surface SEM images of the perovskite films without and with PCBPEG additives, as shown in Fig. 4. In order to optimize the process, the effects of the concentrations of PCBPEG-4k and PCBPEG-20 k, and the annealing time of perovskite film on the performance of PSCs have been investigated. The PCBPEG4k and PCBPEG-20 k with the concentrations of 0.03, 0.05, 0.07, 0.09 and 0.11wt% are added into the perovskite precursor solutions,

Fig. 2. (A) Schematic diagram of PSC with the PCBPEG additive; (b) XRD patterns of the reference, PCBPEG-4k and PCBPEG-20 k perovskite films; Cross-sectional SEM images of the reference, PCBPEG-4k and PCBPEG-20 k PSCs: (c) Reference; (d) PCBPEG-4k; (e) PCBPEG-20 k. 51

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Fig. 3. (A) J-V curves of reference, PCBPEG-4k and PCBPEG-20 k PSCs; (b) Efficiency statistics histograms of reference, PCBPEG-4k and PCBPEG-20 k PSCs; (c), (d) Voc, Jsc, FF and Rsh/Rs distribution of reference, PCBPEG-4k and PCBPEG-20 k PSCs from 25 devices.

Fig. 4. SEM images and the histogram of calculated grain size and Gauss fitting curves of the statistical data from SEM images of the reference, PCBPEG-4k and PCBPEG-20 k films: (a), (d) Reference; (b), (e) PCBPEG-4k; (c), (f) PCBPEG-20 k.

PSCs. The PCBPEG-4k PSC shows a slight higher PCE. The PCE of PCBPEG-4k PSC increases to 17.72% from 15.35% of the reference PSC. The reason of the improved performance after the PCBPEG additives will be discussed below. In order to better compare the performance of

Fig. 3a presents the J-V curves for the reference and modified PSC with PCBPEG-4k or PCBPEG-20 k additive. The corresponding photovoltaic parameters have been listed in Table 1. It can be noted that the PCBPEG-4k or PCBPEG-20 k additive all improve the performance of

Fig. 5. (A) EQE and integrated Jsc curves; (b) Absorption spectra of the reference, PCBPEG-4k and PCBPEG-20 k PSCs, respectively. 52

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Fig. 6. CAFM images for the reference, PCBPEG-4k and PCBPEG-20 k films: (a) Reference; (b) PCBPEG4k; (c) PCBPEG-20 k. KPFM images for the reference, PCBPEG-4k and PCBPEG-20 k films: (d) Reference; (e) PCBPEG-4k; (f) PCBPEG-20 k. Histograms of average photo-current (g) and CPD (h) obtained from the CAFM and KPFM measurement, respectively.

reference PSCs. Obviously, the modified PSCs demonstrate the higher FF than that of the reference PSC. The value of FF is related to the ratio of shunt resistance (Rsh) to series resistance (Rs), Rsh/Rs. The higher FF in the modified PSCs is ascribed to the larger Rsh/Rs [21,44]. Fig. 3c and d shows the detailed photovoltaic parameter variation including Jsc, Voc, FF and Rsh/Rs of the reference and modified PSCs. It can be seen

the reference and modified PSCs, a statistical histogram of the PCE for 25 individual devices is shown in Fig. 3b. The average PCE for reference, PCBPEG-4k and PCBPEG-20 k PSCs are 15.28, 17.31 and 17.30%, respectively. It is clear to see that the dispersions of PCE in the modified PSCs are narrower than that of the reference PSCs, which implys better reproducibility of the modified PSCs compared with the

Fig. 7. Dark I-V curves of the electron-only devices displaying the VTFL kink point behavior: (a) Reference; (b) PCBPEG-4k; (c) PCBPEG-20 k. (d) Histograms of the calculated Dtrap. 53

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Fig. 8. (A) Nyquist plots of the reference, PCBPEG-4k and PCBPEG-20 k PSCs measured under light illumination. The open symbol represents the experimental data and the solid line is the fitting results. Insert: the equivalent circuit diagram which is used to fit the data of Nyquist plots; (b) Recombination resistance obtained from the fitting results of Nyquist plots; (c) PL spectra of the reference, PCBPEG-4k and PCBPEG20 k films deposited on the FTO/TiO2 substrate, (d) PCE evolution as a function of time for the reference, PCBPEG-4k and PCBPEG-20 k PSCs.

benefit to promote charge transport, resulting in the enhanced PCE. Fig. 5 shows the EQE, the integrated Jsc curves and the UV-vis absorbance spectra of the reference and the modified PSCs, respectively. Compared with the reference PSC, the modified PSCs demonstrate the higher EQE in the range of 370–750 nm. The integral Jsc from the EQE for the reference, PCBPEG-4k and PCBPEG-20 k PSCs are 18.71, 19.82 and 19.69 mA/cm2, respectively. They agree very well with the measured Jsc shown in Table 1. As shown in Fig. 5b, the light absorption of the modified PSCs is similar to that of the reference PSCs. Therefore, it is speculated that the increased EQE may be attributed to the promoted carrier transfer and suppressed charge recombination in the modified PSCs [48]. They will be further discussed in the following. To investigate the effect of PCBPEG-4k or PCBPEG-20 k additive on the local electrical properties of perovskite layer at nanoscale level, the average photo-current and contact potential difference (CPD) were studied by conductive atomic force microscopy (CAFM) and Kelvin probe force microscopy (KPFM), respectively [49]. Fig. 6a–c demonstrate the photo-current images of the reference film, PCBPEG-4k and PCBPEG-20 k films, respectively. The corresponding average photocurrent values for the three samples are shown in Fig. 6g, respectively. The average photo-current values increase from 195 pA of reference film to 276 pA of PCBPEG-4k film, and 238 pA of PCBPEG-20 k film. It is found that the obtained average photo-current from CAFM is consistent with the current of bulk devices, which is well correlated with the device performance [50]. The photo-current variation is consistent with the enhanced Jsc in the modified PSCs. This can display the reason that the modified PSCs demonstrate the higher Jsc than that of reference PSC [51]. Fig. 6d-f shows the KPFM images of the reference and modified perovskite films, respectively. The average CPD obtained from the KPFM images are shown in Fig. 6h. It can be seen that there is a slight increase in the average CPD from 261 mV of reference film to 296 mV of PCBPEG-4k film and 280 mV of PCBPEG-20 k film. It was reported that the average CPD at nanometer scale was consistent with the bulk devices [52]. Thus the higher Voc is associated with the larger CPD in the modified PSCs, which corresponds to the effective photo-generated charge separation [52]. In order to explore the reason for the enhanced PCE in the modified PSCs, the trap state density (Dtrap) of perovskite films without and with PCBPEG-4k or PCBPEG-20 k additive was investigated. A series of

Table 1 Photovoltaic parameters of the reference, PCBPEG-4k and PCBPEG-20 k PSCs at the optimum process. Device

Voc (V)

Jsc (mA/ cm2)c

Jsc (mA/ cm2)d

FF (%)

PCE(%)

Reference PCBPEG-4k PCBPEG-20 k

1.038 1.073 1.074

20.20 21.28 21.21

18.71 19.82 19.69

73.21 77.62 76.17

15.35a(15.28)b 17.72a (17.31)b 17.36a (17.30)b

a b c d

Best PCE. Average PCE of 25 devices. Measured Jsc by the solar simulator. Integrated Jsc from the EQE curves.

that all the modified PSCs exhibit the increased Jsc, Voc and FF compared with the reference PSC, thus the increased PCE. In order to explore the reason of the improved performance in the modified PSCs with the PCBPEG additive, the morphology of the modified perovskite films is investigated. Fig. 4 displays the SEM images of the reference and modified perovskite films. The corresponding Gaussian fitting of the grain sizes statistical data are shown in Fig. 4d–f. It can be seen that the grain size in the reference film is about 170 nm. For the PCBPEG-4k and PCBPEG-20 k films, the grains increase to about 243 and 238 nm, respectively. The increased grain size reduces the density of grain boundaries, resulting in the improved PCE of the modified PSCs [45,46]. In addition, the surface characteristics of perovskite films also has an important impact on the optoelectronic properties of PSCs [46]. The surface morphology and roughness of the reference, PCBPEG-4k and PCBPEG-20 k films are investigated by AFM, as shown in Fig. S3. The roughness root mean square (RMS) values are 10.043, 7.898 and 8.933 nm for the reference, PCBPEG-4k and PCBPEG-20 k films, respectively. It is noted that the PCBPEG-4k or PCBPEG-20 k additive decreases the RMS of perovskite film, resulting in the smoother perovskite films. The reduced RMS value in the PCBPEG4k and PCBPEG-20 k films is beneficial to improve the interface contact with the hole transport layer, and promote the charge separation and transport in the modified PSCs [47]. The results of SEM and AFM indicate that the PCBPEG-4k or PCBPEG-20 k additive can regulate the microstructure of perovskite film. The improved micrograph will 54

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4. Conclusions

devices with the structure of FTO/TiO2/CH3NH3PbI3/PCBM/Ag were prepared. The dark current-voltage (I-V) curves of the three devices are shown in Fig. 7a–c, respectively. The Dtrap value can be obtained from the I-V analysis for electron-only devices [53]. In the I-V at low bias voltage, the linear relation indicates the ohmic response of the electrononly device. When the bias voltage is over the kink point, the current will increases nonlinearly, suggesting that the trap states are completely filled. The Dtrap value can be calculated by the trap filled limit voltage (VTFL) using the flowing equation (1) [54]:

VTFL =

In this work, functional fullerene end-capped polyethylene glycol (PCBPEG-4k and PCBPEG-20 k) are synthesized by a simple method. The effect of PCBPEG-4k or PCBPEG-20 k additive on the photoelectric properties of perovskite films and PSCs has been systematically investigated. The annealing time of perovskite films and concentrations of PCBPEG-4k and PCBPEG-20 k additives have been optimized. At the optimum process, the PCBPEG-4k and PCBPEG-20 k PSCs demonstrate the champion efficiency of 17.72% and 17.36% respectively, much higher than the efficiency of 15.35% for the reference PSC. This improvement is majorly attributed to the much improved microstructure with large grain size and low defect density, promoted charge transport and reduced recombination rates. This work provides a simple method to synthesize functional fullerene derivatives to regulate the microstructure and photoelectric properties of PSCs.

eDtrap L2 2εε 0

(1)

where e is the elementary charge of the electron (e = 1.6 × 10−19 C), L is the thickness of perovskite film (≈200 nm), as presented in Fig. S4. ε is the relative dielectric constant of CH3NH3PbI3 (ε = 28.8), ε0 is the vacuum permittivity (ε0 = 8.854 × 10−12 F/m), and Dtrap is the trap state density of CH3NH3PbI3 film [54]. The obtained VTFL values of reference, PCBPEG-4k and PCBPEG-20 k films are respectively listed in Fig. 7a–c. They are 0.196, 0.108 and 0.123 V, respectively. The calculated Dtrap values of reference, PCBPEG-4k and PCBPEG-20 k films are 1.56 × 1016, 0.86 × 1016 and 0.98 × 1016 cm−3, respectively. They are illustrated in Fig. 7d. It is noted that the Dtrap values decrease after adding the PCBPEG-4k or PCBPEG-20 k additive. This indicates that the PCBPEG-4k or PCBPEG-20 k additive can passivate the electron trap [54]. The PCBPEG-4k film has the lowest Dtrap value, thus the best PCE [55,56]. The result is consistent with the J-V result. To further investigate the effects of PCBPEG-4k or PCBPEG-20 k additive on the carrier transfer and charge recombination, the electrochemical impendence spectroscopy (EIS) and photoluminescence (PL) spectra were measured. Fig. 8a shows the Nyquist plots of the three PSCs measured under light illumination, where the solid lines are the fitting results by using the inserted model (Inserted in Fig. 8a). It can be seen that the Nyquist plots can be fitted well with the equivalent circuit model. It was reported that the size of the arc is related to the charge recombination process at the TiO2/MAPbI3/Spiro-OMeTAD interfaces [57]. It is clear to see that the size of arc in the PCBPEG-4k and PCBPEG-20 k PSC are larger than that of the reference PSC. The bigger arc in the PCBPEG-4k and PCBPEG-20 k PSCs indicates that the recombination resistance (Rrec) values in the modified PSCs are higher than that of the reference PSC [58,59]. The value of Rrec can be gotten from the fitting results. Fig. 8b shows the value of Rrec for reference, PCBPEG-4k and PCBPEG-20 k PSCs. It is clear to see that the modified PSCs all show higher Rrec than that of the reference PSC, suggesting the reduced charge recombination rates in the modified PSCs [60]. Moreover, the PCBPEG-4k PSC shows the largest Rrec, thus the lowest charge recombination rates in this PSC, which is consistent with the J-V result. In addition, the steady PL spectra were carried out to investigate the charge extraction and transport [61,62]. Fig. 8c shows the PL spectra for reference, PCBPEG-4k and PCBPEG-20 k films deposited on the FTO/TiO2 substrate, respectively. It is noted that the intensity of the emission peak drastically decreases in the PCBPEG-4k and PCBPEG-20 k films compared to the reference sample. This indicates that the charge recombination rates are drastically reduced in the modified perovskite films, suggesting the enhanced charge extraction ability in the modified sample [63]. The result is in keeping with the EIS and J-V results. In addition, the stability of PSCs without and with PCBPEG additives was investigated. The unencapsulated PSCs are stored in air with a relative humidity of about 32% at room temperature. As shown in Fig. 8d, when the PCBPEG-4k and PCBPEG-20 k PSCs are exposed to such an environment for 360 h, the PCE values can still be maintained at 76.1% and 73.3% of the initial value, respectively. However, the reference PSC can only retain 44.2% of the initial value. The result shows that the PCBPEG-4k or PCBPEG-20 k additive can improve the stability of the PSCs. This is consistent with the reported result [64].

Acknowledgements We acknowledge the financial support of the National Key R & D Program of China (2016YFB0401502, 2016YFA0201002), the National Natural Science Foundation of China (Grant Nos. 51431006, 51472093, 61574065), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016), the Characteristic Innovation Project of Guangdong Provincial Department of Education (Science 2016, 22), the Natural Science Foundation of Guangdong Province (No. 2016A030313421, 2016A030308019), the Program for Changjiang Scholars and Innovative Research Team in University (IRT_17R70), the Guangdong Innovative Research Team Program (No. 2011D039) and the MOE International Laboratory for Optical Information Technologies,the Science and Technology Planning Project of Guangdong Province (Grant No. 2016B090906004, 2015B090927006). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jpowsour.2018.05.091. References [1] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Science 338 (2012) 643–647. [2] J. Burschka, N. Pellet, S.J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin, M. Gratzel, Nature 499 (2013) 316–319. [3] M. Liu, M.B. Johnston, H.J. Snaith, Nature 501 (2013) 395–398. [4] H. Zhou, Q. Chen, G. Li, S. Luo, T. Song, H.S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Science 345 (2014) 542–546. [5] N.J. Jeon, J.H. Noh, W.S. Yang, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Nature 517 (2015) 476–480. [6] S.S. Shin, E.J. Yeom, W.S. Yang, S. Hur, M.G. Kim, J. Im, J. Seo, J.H. Noh, S. Seok, Science 356 (2017) 167–171. [7] H. Tan, A. Jain, O. Voznyy, X. Lan, T.P.G. Arquer, J.Z. Fan, Q.B. Rafael, M. Yuan, B. Zhang, Y. Zhao, F. Fan, P. Li, L.N. Quan, Y. Zhao, Z.H. Lu, Z. Yang, S. Hoogland, E.H. Sargent, Science 355 (2017) 722–726. [8] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050–6051. [9] M. Saliba, T. Matsui, J.Y. Seo, K. Domanski, J.P. Correa-Baena, M.K. Nazeeruddin, S.M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Gratzel, Energy Environ. Sci. 9 (2016) 1989–1997. [10] C. Wehrenfennig, G.E. Eperon, M.B. Johnston, H.J. Snaith, L.M. Herz, Adv. Mater. 26 (2014) 1584–1589. [11] L.Q. Zhang, X.W. Zhang, Z.G. Yin, Q. Jiang, X. Liu, J.H. Meng, Y.J. Zhao, H.L. Wang, J. Mater. Chem. 3 (2015) 12133–12138. [12] P. Docampo, J.M. Ball, M. Darwich, G.E. Eperon, H.J. Snaith, Nat. Commun. 4 (2013) 2761. [13] Y. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Nat. Commun. 5 (2014) 5784. [14] N.G. Park, M. Grätzel, T. Miyasaka, K. Zhu, K. Emery, Nature Energy 1 (2016) 16152. [15] W.S. Yang, B.W. Park, E.H. Jung, N.J. Jeon, Y.C. Kim, D.U. Lee, S.S. Shin, J. Seo, E.K. Kim, J.H. Noh, S.I. Seok, Science 356 (2017) 1376–1379. [16] N. Pellet, P. Gao, G. Gregori, T.Y. Yang, M.K. Nazeeruddin, J. Maier, M. Gratzel, Angew. Chem. Int. Ed. 53 (2014) 3151–3157.

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