High Current Density and Low Hysteresis Effect of Planar Perovskite

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Surfaces, Interfaces, and Applications

High Current Density and Low Hysteresis Effect of Planar Perovskite Solar Cell via PCBM-doping and Interfacial Improvement He Jiang, Gelei Jiang, Weiwei Xing, Weiming Xiong, Xiaoyue Zhang, Biao Wang, Huiyan Zhang, and Yue Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06020 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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ACS Applied Materials & Interfaces

High Current Density and Low Hysteresis Effect of Planar Perovskite Solar Cell via PCBM doping and Interfacial Improvement He Jiang1,2, Gelei Jiang1,2, Weiwei Xing1,2, Weiming Xiong1,2, Xiaoyue Zhang1,2, Biao Wang1,3 *, Huiyan Zhang1,2 & Yue Zheng1,2 *

1

State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun Yat-sen University, Guangzhou 510275, China.

2

Micro and Nano Physics and Mechanics Research Laboratory, School of Physics, Sun Yat-sen University, Guangzhou 510275, China.

3

Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai, 519082, China.

*Correspondence and requests for materials should be addressed to Y.Z. (email: [email protected]) or B. W. (email: [email protected]).

KEYWORDS: low hysteresis effect, high current density, perovskite solar cell, carrier recombination, PCBM-doped peorvskite 1

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ABSTRACT We propose a doping method by using [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) to fill the grain boundary interstices of the methylammonium lead iodide (CH3NH3PbI3) perovskite for the elimination of pinholes. A sandwiched PCBM layer is also used between the perovskite and TiO2 layers to improve the interfacial contact. By using these two methods, the fabricated perovskite solar cell shows low hysteresis effect (LHE) and high current density (HCD), which result from the improved compactness at the grain boundaries of perovskite surface and the interface between TiO2/perovskite layers. The theoretical and experimental results indicate that PCBM can effectively suppress carrier recombination, regardless of interfacial layer or dopant. We also found that the dark current reduced in the analysis of dark state current-voltage (I-V) characteristics. The slope of the I-V curve for the FTO/PCBM-doped perovskite/Au device reduces monotonically with the PCBM concentration increasing from 0.01 wt% to 0.1 wt%, which suggests the decreasing defects in perovskite layer. Through tuning the PCBM doping and controlling the preparation process, we have successfully fabricated the planar TiO2/PCBM-based PCBM-doped peorvskite photovoltaic device that reaches a high current density of 22.6 mA/cm2 and an outstanding photoelectric conversion efficiency up to 18.3 %. The controllability of the PCBM doping concentration and interfacial preparation shed the light of the further optimization for the photoelectric conversion efficiency of perovskite solar cell.

INTRODUCTION 2


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Of particular interest in current organic-inorganic hybrid perovskite (ABX3, where A represents alkylammonium cations, B is a metal cation such as lead or stannum and C represents halogen element) solar energy research field is how to improve the photovoltaic conversion efficiency to meet the current giant requirement of renewable energy.1,

2

Possessing notable optical-electronic properties such as long carrier

diffusion length, low exciton ionization energy, appropriate band gap, simple preparation technology and excellent light absorption,3-11 perovskite materials have been utilized in photovoltaic and electroluminescence field.12-20 Due to the unique optical-electronic nature, organic-inorganic hybrid perovskite materials have been introduced into dye-sensitized solar cells by Kojima’s group.3 The prototype of perovskite solar cell is typically a sandwich structure similar to emission diode, which consists of an electron transfer layer (ETL), a perovskite active layer and a hole transfer layer (HTL). Many efforts on organic-inorganic hybrid perovskite materials have been devoted to improving the photoelectric conversion efficiency (PCE) of solar cell. The field of perovskite solar cell is burgeoning without signs of decline.21-28 Recently, a great improvement on perovskite solar cells has been achieved. The PCE of perovskite solar cell has increased from original 3.9 % up to more than 20 %, which is comparable with commercial silicon solar cells.26-30 Conventional organolead halide perovskite solar cell in the form of n-i-p architecture consists of a sequence of a transparent conductive oxide, mesoporous/compact TiO2, perovskite absorb layer, Spiro-OMeTAD and metal electrode. For conventional perovskite solar cell without mesoporous scaffold, the hysteresis effect, a phenomenon 3



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of non-overlapped current density-voltage (J-V) curves under different scanning directions, appears in the PCE measurement, which is mostly due to the capture of electrons from ineluctable oxygen vacancy in TiO2 compact layer.31-33 Moreover, morphological and compositional defects in the perovskite layer are inevitable in the fabrication process of solution-processed organolead halide perovskite, especially the methylammonium (CH3NH3+) lead halides (Cl, Br and I). These defects in TiO2 and perovskite layers facilitate the recombination and the trapping of charge carriers, which can induce the current density reduced and the hysteresis effect in the practical application of perovskite solar cell.32, 34-39 To solve these issues, the modification and improvement of compact layer and interface have been performed to eliminate the inherent defects.27, 40-47 However, for perovskite absorb layer, most of researches focus on the element composition rather than the interior or surface defects.

24, 48-50

Optimizations of the perovskite layer by suppressing morphological defects, including grain boundaries and interfacial defects, remain still unknown. 32, 51-54 In our work, taking the improvement of perovskite layer and TiO2/perovskite interface into account, we propose a facile method to fill the grain boundary interstices of

methylammonium

lead

iodide

(CH3NH3PbI3)

perovskite

by

using

[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as dopants. To reduce the interface recombination between TiO2 and perovskite, an additional PCBM layer is deposited on TiO2 layer forming a dual-ETL framework. One-step solution-processing technique is employed throughout the preparation process of perovskite solar cells in this work. By tuning the PCBM doping and controlling the preparation process, the optimal planar 4


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PCBM-doped perovskite solar device possessing high current density (HCD) and low hysteresis effect (LHE) has been obtained in the configuration of FTO/TiO2/PCBM/0.1 wt% PCBM-doped perovskite/Spiro-OMeTAD/Au.

RESULTS AND DISCUSSION Device configurations. Figure 1a and 1b show the energy level diagram and the SEM cross-view image of the n-i-p configurational TiO2/PCBM-based perovskite solar device in our work (other ETL schemes see Figure S1 in Supporting Information). The energy levels of electrodes and transport layers are shown in Figure 1a are obtained from previous literatures. The energy levels of undoped and 0.1 wt% PCBM-doped perovskites are determined by their absorption spectra combining with the Ultraviolet Photoelectron Spectroscopy (UPS) measurement (see Figure S10 in Supporting Information). A PCBM layer is inserted between the TiO2 and perovskite layers to improve the interfacial contact. PCBM molecules are dispersed in CH3NH3PbI3 perovskite by using solution synthetic method. The schematic diagram of PCBM molecules in perovsktite is shown in Figure 1c (see the PCBM doping concentrations in Figure S2 in Supporting Information).

5



Figure 1. Planar heterojunction perovskite solar cell with dual-ETL (TiO2/PCBM) configuration. (a) Schematic energy levels of corresponding layers. (b) SEM cross-view image of TiO2/PCBM-based solar device sample in this work (scale bar: 500nm). (c) Schematic diagram of PCBM molecules in CH3NH3PbI3 perovskite.

Surface topography and optical, electrical properties of perovskite films. The topography of perovskite surface has great impact on the performance of fabricated devices. To study the effect of PCBM doping, the SEM images were first taken for the surface of perovskite film on FTO/PCBM substrate with different PCBM doping concentrations. As shown in Figure 2a, a clear grain boundary is found on the surface of undoped perovskite film, which is similar to the typical surface of hybrid perovskite materials. The grain boundaries attenuate gradually when the PCBM concentrations increase from 0 to 0.1 wt%, as shown in Figure 2a-f, respectively. The well-dispersed 6


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PCBM in the film makes the perovskite surface smoother and fewer grain boundaries, which is attributed to the adsorption effect of PCBM molecules. The perovskite grains are not conspicuous at the PCBM concentration over 0.05 wt%. However, when the doping concentration is beyond 0.1 wt%, the deposition of the PCBM-doped perovskite onto the PCBM layer become difficult, which is probably caused by the electrostatic repulsion between PCBM and PCBM-doped perovskite layers. To investigate the doping concentrations over 0.1 wt%, PCBM-doped perovskite films (0.14, 0.18, 0.22, 0.26 and 0.3 wt%) were prepared directly on the FTO substrate directly without any PCBM layers. Some rounded islands were observed on the surface. The amounts of the island structure increase with the PCBM doping concentration changing from 0.14 to 0.3 wt% (see Figure S3a-e in Supporting Information). The islands are considered as the precipitations of PCBM molecules when the doping is oversaturated. Figure S3f also shows the surface roughness of the film samples. It is found that some granules increase when PCBM doping concentration increases from 0.14 to 0.3 wt%, which is in agreement with the SEM images in Figure S3a-e. Compared with 0.3 wt% PCBM-doped perovskite samples, the 0.1 wt% doped film exhibits a compact and smooth surface with well specular reflection (see Figure S4 in Supporting Information), which directly reflects the quality of perovskite film.

7



Figure 2. SEM top-view of different PCBM doping concentrations of perovskite films established on FTO/PCBM substrate. (a) undoped, (b) 0.01wt%, (c) 0.03 wt%, (d) 0.05 wt%, (e) 0.07 wt% and (f) 0.1 wt%. The scale bars are 5 µm and 1 µm (inset), respectively.

Figure 3 depicts the optical and optical-electronic performances of perovskites with and without PCBM doping. To seek the advantage of PCBM doping, dark state current-voltage (I-V) characteristic of PCBM-doped perovskite films was studied. Figure 3a shows the I-V characteristic of FTO/perovskite/Au devices with different

8


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PCBM doping concentrations (0-0.3 wt%) under the dark condition. The change of I-V curve indicates that the difference in dark current in devices. Figure 3b shows the slopes of the I-V curves, among which the 0.1 wt% PCBM-doped perovskite sample has the lowest slope, indicating the lowest dark current and fewest grain boundaries in this sample. The grain boundaries on the perovskite layer are filled by PCBM molecules, which leads to the reduced current leakage between two electrodes. Meanwhile, the PCBM molecules in perovskite grain boundaries can effectively separate charge carrier and act as the electron trapping sites,15, 20 resulting in the reduction of interface barriers. As shown as the dotted lines in Figure 2a, when the PCBM doping concentration is beyond 0.22 wt%, the linear I-V curve indicate a constant resistance, which suggests that the PCBM-doped perovskite films are no longer semiconductors. On the basis of the above results, only the performance of devices and material characteristics with lower concentrations of PCBM (no larger than 0.1 wt%) is discussed. Benefitting from the compact and smooth perovskite film due to the PCBM doping, the light absorption is enhanced with the increase of doping concentration from 0 to 0.1 wt% (Figure 3c and 3e), which is consistent with the results of surface characterization and I-V characteristic. For each sample, the thickness was measured and averaged from 5 different sites. All perovskite films, except 0.1 wt% PCBM-doped film, have similar thicknesses around 300 nm (see Table S1 in Supporting Information). The 0.1 wt% PCBM-doped perovskite sample is approximately 10 nm thicker than other samples, which may be due to the oversaturation of PCBM molecules on the surface. To study whether the increase of the light absorption is caused by the PCBM doping 9



or the film thickening, we prepared two undoped perovskite samples with the thicknesses of 302 and 316 nm, respectively. The results show that the two undoped samples have similar absorption spectra. Moreover, the light absorption of the 316 nm undoped sample is lower than the 312 nm 0.1 wt% doped one. Therefore, we can conclude that the enhanced absorption is attributed to the PCBM doping which can cause better compactness and less grain boundaries in perovskite films. To further reveal the advantages of PCBM doping, the photoluminescence (PL) characteristics of PCBM-doped perovskite films have been investigated. Figure 3d and 3e show that the PL intensity decreases with the increasing of PCBM doping, which suggests the PCBM doping facilitates charge separation and reduces radiative recombination. The EHL-free devices in the configuration of FTO/PCBM-doped perovskite/Spiro-OMeTAD/Au were fabricated to investigate the condition of the FTO/perovskite interface. The fill factor (FF) obtained from J-V characteristics, under the condition of forward scan, increases from 21.2% to 44.2% as the doping concentration increases from 0 to 0.1 wt% (Figure 3f and Figure S6). The increase of the FF reveals the improved FTO/perovskite interface due to the fewer defects in perovskite layer by PCBM doping. Figure S6 also shows the notable improvement of FF from 0.07 to 0.1 wt% PCBM doping concentration. According to the thickening of the 0.1 wt% doped sample, a thin PCBM interfacial layer may be formed at FTO/perovskite interface as the PCBM doping oversaturates. These optoelectronic performances of PCBM-doped perovskite films offer a solid evidence of the merit of PCBM doping. 10


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Figure 3. Optical and electrical characteristics analysis on undoped and PCBM-doped perovskite films. (a) I-V characteristics analysis on prepared FTO/perovskite/Au devices under dark condition. (b) Dependence of curve slope (at 0.6 V) on the PCBM doping concentrations. (c) Absorption spectra of perovskite samples with different PCBM doping concentrations. (d) Steady-state PL spectra for each sample on FTO substrate. (e) Dependency relationship between PCBM doping concentration and integrated intensity of PL and absorption, respectively, which are calculated from (c) and (d). (f) J-V characteristics of planar 11



undoped

and

0.1

wt

%PCBM-doped

perovskite

solar

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cells

without

ETL

(FTO/perovskite/Spiro-OMeTAD/Au) measured at reverse and forward scans.

Distribution of PCBM molecules in CH3NH3PbI3 perovskite. PCBM molecules commonly have low miscibility with the solvents of perovskite materials, for example N,N-Dimethylformamide (DMF) and Methyl sulfoxide (DMSO). It is necessary to figure out the chemisorption properties between PCBM and essential compositions in perovskite including Lead (II) iodide (PbI2) and Methylammonium iodide (CH3NH3I). X-ray photoelectron spectroscopy (XPS) was employed to study the distribution of PCBM in CH3NH3PbI3 perovskite. Undoped perovskite and 0.1 wt% PCBM-doped perovskite films were coated onto a FTO/PCBM substrate via spin-coating method. The two samples show distinct profiles of the core-level spectra for carbon (C), oxygen (O) and lead (Pb). From the C core-level spectra in Figure 4a, the spectroscopic peaks at 286 eV and 284.8 eV correspond to CH3NH3+ in perovskite and trace pollution, respectively. However, 0.1 wt% PCBM-doped perovskite sample shows the presence of extra carbon at 284.7 eV corresponding to PCBM molecule.32 Meanwhile, the trace of PCBM C=O bond at 533.2 eV (O1s A) in O core-level spectrum reveals the existence of PCBM in the 0.1 wt% doped perovskite, which does not appear in the undoped perovskite (Figure 4b). The PCBM doping also increases the intensity of the O1s peak at 532.5 eV corresponding to C-O bonds in PCBM molecules. As shown in Figure 4c, the peaks at 138 eV and 142.9 eV correspond to Pb(2+) in perovskite in both samples. However, two weak spectral peaks at 136.9 eV (Pb4f) and 141.3 eV (Pb4f A) emerges in the 0.1 wt% PCBM-doped perovskite, corresponding to the elemental Pb(0).

12


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As known, the Pb(0) contributes as an effective electron donor in perovskite55 to the improvement on the current density of the lead halide perovskite solar cell. For other elements as iodine and nitrogen, the two samples show no difference in core-level spectra (see Figure S7). To study the effect of PCBM doping on the crystallinity of perovskite, the X-Ray Diffraction (XRD) pattern has been performed for undoped and 0.1 wt% doped samples. Figure 4d shows that both two samples have the same peaks at 14.2º, 28.52º and 43.26º corresponding to three typical facets of (1 1 0), (2 2 0) and (3 3 0). However, when the PCBM doping concentration increases, the XRD peak around 6.38º become more and more discernible, which is related to the peak of the pure PCBM (Figure 4e). Using the XRD pattern of pure PCBM sample as baseline (Figure 4e gray line), the integrated intensity of XRD pattern in this region increase with the concentration of PCBM doping, as shown in Figure 4f. The results of XRD patterns enable us to conclude that the PCBM molecules are well dispersed in the perovskite layer.

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Figure 4. Distribution of PCBM molecules in CH3NH3PbI3 perovskite. (a,b,c) XPS core level spectra of C, O and Pb elements of undoped and 0.1 wt% PCBM-doped perovskites on FTO/PCBM substrate. (d) XRD characteristic of undoped and 0.1 wt% PCBM-doped pervksites on FTO substrate. (e) XRD pattern under small diffraction angle with different PCBM doping concentrations of perovskite and PCBM samples. (f) Integrated intensity of XRD pattern on small diffraction angle region.

To reveal the mechanism of PCBM doping at the molecular level, we have performed the density of function theory (DFT) calculation to study the density of state (DOS) of the PCBM-doped perovskite. Based on the PCBM distribution in perovskite, we placed PCBM molecules at the defective CH3NH3PbI3 perovskite surface. The PCBM/perovskite adsorption structure is used to model the PCBM doping. In DFT simulation, one or two PCBM molecules are settled nearby the (0 0 1) crystal facet of defective (the Pb-I antisite-defects, i.e. I atom occupies Pb atom position) CH3NH3PbI3 perovskite (modeling and calculating parameters see the Calculation description in Supporting Information), shown in Figure 5a and 5b. Figure 5c and 5d depict the calculation results of total DOS and partial DOS (PDOS) of p-orbital iodide. The DOS results show that two deep trap states (a gray area with arrows in Figure 5c) emerge within the band gap of defective perovskite surface structure, due to the existence of Pb-I-antisite defects. The PDOS shown in Figure 5d reveals that these trap states originate from the iodide of perovskite in p-orbital (gray area in Figure 5d). The trap state of low-energy direction disappears when PCBM anchors onto the perovskite surface. The PDOS diagram suggests that the disappeared trap state should be in valence band (red and blue areas in Figure 5d). On the contrary, the trap state at

14


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high-energy direction moves to the conduction band as the adsorbed PCBM molecules increases, which means the high-energy trap state transfers from deep state to shallow state. The deep trap state in semiconductor plays a role of recombination centers during the photon transition, which can influence carrier lifetime, especially minority carriers. Therefore, we conclude that the PCBM doping may improve the photoelectric conversion performance of perovskite solar cell.

Figure 5. DFT calculation of density of state (DOS) on PCBM/CH3NH3PbI3 adsorption structures. (a,b) Schematic diagrams of adsorption structures. (c,d) Total DOS and partial DOS of different structures, the VB and CB represent valence band and conduction band, respectively.

Performance of planar perovskite solar cells. As is well known, conventional TiO2-based planar perovskite solar cell suffers serious hysteresis under different 15



scanning directions in the PCE measurement. In our work, the prepared solar cell device in the FTO/TiO2/undoped perovskite/Spiro-OMeTAD/Au configuration (aliased as TiO2/undoped perovskite) also presents a severe hysteresis effect (black curves in Figure 6a and Table 1). The hysteresis effect arises from two aspects: the existence of oxygen vacancy in TiO2 layer induces carrier trapping and recombination; defective TiO2 and perovskite layers lead to an inverse current from HTL to FTO, thus resulting in a current loss and ionic migration. Considering these two factors, we deposit PCBM on the TiO2 layer as an additional ETL to improve the interface between TiO2 and perovskite. As expected, the device in the configuration of FTO/TiO2/PCBM/undoped-perovskite/Spiro-OMeTAD/Au (aliased as TiO2/PCBM/ undoped-perovskite) demonstrates further enhancements in FF (75.3 %) and Jsc (19.3 mA/cm2). Meanwhile, the hysteresis effect was slightly suppressed compared with the TiO2-based (without PCBM) device (Figure 6a and Table 1). Figure 6b shows the results of steady-state PL intensity, which demonstrates a phenomenon of repressed radiation recombination owing to an additional PCBM layer. Besides, the PL intensity of FTO/PCBM-based perovskite (curve in blue) is lower than that of FTO/TiO2-based perovskite (curved in red), due to less defects in PCBM layer. Time-resolved PL spectrum of FTO/TiO2/PCBM-based perovskite shows the fastest decay in comparison with the FTO/TiO2-based and FTO/PCBM-based perovskite samples after the excitation by a pulsed laser (Figure 6c). The results of steady-state and time-resolved PL spectra confirm that the PCBM as an additional EHL can effectively promotes carrier transport, which can be also verified by DFT calculation of the DOS in 16


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TiO2/PCBM adsorption structure (see Figure S8 in Supporting Information). PCBM molecules can form an electronic state at forbidden band of TiO2 indicating more electrons can be transited.

17



Figure 6. Performance and characterization for different ETH schemes of undoped perovsite solar devices. (a) J-V characteristics of planar undoped-perovskite solar cells with 18


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TiO2, PCBM and TiO2/PCBM ETL measured at reverse and forward scans. (b,c) Steady-state and time-resolved PL spectra of undoped-perovskite on FTO, FTO/TiO2, FTO/PCBM and FOT/TiO2/PCBM substrates. Table 1. Summary of planar undoped-perovskite solar device parameters with different component of ETL.

Scanning

Jsc

Voc

FF

PCE

directions

(mA/cm2)

(V)

(%)

(%)

Reverse

19.2

1.05

71.4

14.3

Forward

19.2

1.02

60.9

11.8

Reverse

19.1

1.04

74.7

14.8

Forward

19.3

1.01

67.5

13.2

Reverse

19.3

1.07

75.3

15.4

Forward

19.2

1.05

69.4

14

ETH schemes

TiO2

PCBM

TiO2/PCBM

By tuning the PCBM doping concentration and introducing the additional PCBM layer, the device in the configuration of FTO/TiO2/PCBM/0.1 wt% PCBM-doped perovskite/Spiro-OMeTAD/Au (aliased as TiO2/PCBM/0.1 wt% PCBM-doped perovskite) demonstrates the optimal PCE of 18.3 % and 17.5 % at reverse and forward scans, respectively, with the same value of 22.6 mA/cm2 in Jsc, which presents LHE (Figure 7a). The substantial enhancement on Jsc is mainly due to the improved interfacial contact and repressed carrier recombination, which can be attributed to the better match between the energy levels of PCBM and PCBM-doped perovskite induced by the PCBM doping. The UPS measurement shows a subtle difference in work

19



function between undoped and 0.1 wt% PCBM-doped perovskites (Figure S9c). Combining with the calculated band-gap values from the absorption spectra (Figure S9a), we find a small shift in energy levels of PCBM-doped perovskite. As shown in Figure S9b, a better match of energy level between PCBM and perovskite is observed after the PCBM doping. From dark J-V measurement (Figure 7b), the rectification characteristics shows that the TiO2/PCBM/0.1 wt% PCBM-doped perovskite device has the lowest dark current, thus indicating a lower defect states and current loss. In addition, the repressed hysteresis effect in PCBM-perovskite hybrid solar device has been validated by E.H.S. et al. in conductive atomic force microscopy study.32 To verify the robustness of our results, 72 devices have been fabricated and measured. Figure S10 shows a histogram of PCE among 72 devices that presents a centralized efficiency around 17.7 %. In addition, Hall Effect measurement was carried out to study semiconductor characteristics of perovskite films. The results show that the perovskite material presents N-type property regardless of the PCBM doping, which is in good agreement with previous studies.56, 57 Meanwhile, the 0.1 wt% PCBM-doped perovskite sample presents the electronic mobility up to 35.5 cm2/Vs which is higher than the undoped-perovskite sample (16.9 cm2/Vs), indicating a faster electron transfer under an external voltage. The external quantum efficiency (EQE) spectra and integrated Jsc of the corresponding solar devices are shown in Figure 7c. Similarly, the TiO2/PCBM/0.1 wt% PCBM-doped perovskite device exhibits the highest photon-to-electron conversion efficiency, particularly at short wavelength region (300-500 nm). This 20


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enhancement in short wavelength fully indicates a less incident light loss from FTO to perovskite layer, which is induced by the better interface contact in the device.

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Figure 7. The optimal performance of planar PCBM-doped perovskite solar cell and characterizations comparison. (a) J-V characteristics of the FTO/TiO2/PCBM/0.1 wt% PCBM-doped perovskite/Spiro-OMeTAD/Au configurational solar device measured at reverse and forward scans. (b) Logarithmic dark J-V characteristics of corresponding devices. The inset is the curves at voltage from -0.1 V to 0.1 V. (c) EQE spectra and integrated Jsc for the corresponding devices.

The property of charge carrier transport is a critical factor, which directly determines the performance of photoelectric conversion in perovskite solar cells. Minority carrier lifetime measurement was performed to investigate the influence of PCBM doping. Due to the polycrystalline nature of solution-processed perovskite film, 25 points on each film sample (1.5×1.5 cm area) were chosen in the process of measurement. As shown in Figure S11, the 0.1 wt% PCBM-perovskite sample shows an averaged minority carrier lifetime up to 99.211 µs which is higher than the undoped-perovskite sample (53.91 µs). Combining with the calculated results of DOS (Figure 5c and 5d), we conclude that the increase of the minority carrier lifetime could be caused by the PCBM doping, which leading to the deep levels moving towards band edge and thereby the non-radiative recombination reducing. Please note that our minority carrier lifetime values are not consilient with previous report6, which is due to the different test method in our work. However, our results present an observable tendency and comparability. In addition, as shown in Figure S12, the PCE of double-EHL (TiO2/PCBM) devices exhibit an improvement with the increase of PCBM doping concentration. Meanwhile, the PCBM doping enhances the Voc to some extent (see Table S2), implying the reduced charge loss at TiO2/PCBM/perovskite interfaces. Based on the scheme above, 22


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we obtain the highest PCE of 15.5 % in the large-area (0.84 cm2) solar device in the FTO/TiO2/PCBM/0.1

wt%

PCBM-doped

perovskite/Spiro-OMeTAD/Au

configuration (see Figure S13). However, the experimental reproducibility of the lager area device is barely satisfactory. As the area expands, the defects in TiO2 increase and the surface coverage fraction controllability is reduced when the PCBM doping concentration increases to 0.1 wt%.

CONCLUSION In conclusion, we have shown a compact CH3NH3PbI3 perovskite surface with reduced grain boundaries achieved by the PCBM doping approach, with an optimal doping concentration of 0.1 wt%. Combining with the modification of the interface at TiO2/perovskite by inserting additional PCBM layer, a planar PCBM-doped perovskite solar cell with HCD and LHE has been obtained, better than the ones of the referenced device with FTO/TiO2/undoped-perovskite/Spiro-OMeTAD/Au configuration. The steady-state and time-resolved PL spectroscopy reveal that PCBM as dopant or an additional ETL is capable of both promoting carrier extraction and reducing radiative recombination effectively. Furthermore, DFT calculations show that the deep trap states, which are induced by the inherent Pb-I antisite-defects on CH3NH3PbI3 perovskite surface, can be shallowed by the increasing of the adsorptive PCBM molecules. Meanwhile, minority carrier lifetime measurement shows that the lifetime is enhanced as the PCBM doping concentration rises from 0 to 0.1 wt%, which is in accordance with the DOS analysis. The Jsc, Voc, FF and PCE can be further enhanced by the approaches of transport layer improvement and interface engineering. 23



Combining with applying the PCBM as EHT directly and the PCBM doping in perovskite solar cell, this work may provide insights on the low-temperature and stretchable engineering technology aiming to obtain a high efficiency perovskite solar device with HCD and LHE.

METHODS Solution synthesis. For PCBM-doped perovskite precursor solutions, PCBM (Lumtec, >99.5%) powders were dissolved in DMF (Acros, 99.8%) and DMSO (Acros, 99.7+%) mixed solution (2:1 v/v) with the change of concentration between 0.01 and 0.5 wt%, then stirred at 110 Cº for 12h. After cooling down, PbI2 (Alfa, 99.9985%) and CH3NH3I (Lumtec, >99.5%) with mole ratio of 1:1 with concentration of 40 wt% were dissolved in prepared DMF/DMSO (with PCBM doping) solution then stirred at 60 Cº for 8h. For undoped perovskite precursor solution, the process of PCBM doping was omitted. For TiO2 precursor solution, while stirring 3 ml 2-Propanol (SIGMA, ≥99.5%), add 210 µl Titanium(IV) isopropoxide (Alfa, 99.995%) dropwise to form a colloidal sol, then 5 µl hydrochloric acid was doped into solution. For Spiro-OMeTAD precursor solution, refer to the reported in literature for the synthesis method. 58 Planar perovskite solar cells fabrication. A thin TiO2 (about 30nm) compact layer was deposited on cleaned FTO substrate by using spin-coating (6000 rpm 30s) method, followed by annealing at 300 Cº for 20 min and 500 Cº for 30 min in muffle at ambient air (~50 RH%). For TiO2-based device, prepared perovskite precursor solution was coated onto FTO/TiO2 substrate at a procedural speed of 3000 rpm with an acceleration of 2000 rpm/s for the first 25s and 500 rpm for the last 10s. Small amount of 24


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chlorobenzene solution was dropped on the spinning substrate during the last 10s. In order to ensure the doped PCBM not to be washed off from perovskite film, chlorobenzene was slowly dropped onto precursor film within 3-5s. For perovskite film annealing process, a temperature control scheme was employed, which raised the temperature from room-temperature to 100 Cº within 5 min and then maintained for 1h. For TiO2/PCBM based device, PCBM (2 wt% in chlorobenzene solution) was coated (3000 rpm 30s) on TiO2 layer and then annealed at 110 ºC for 30 min before spinning the perovskite layer. For PCBM-based device, PCBM was deposited on FTO substrate using the same scheme. For ETL-free device, perovskite was coated on FTO substrate directly. Spiro-OMeTAD as HTL was deposited by spin-coating (3000 rpm 30s) on perovskite. Finally, 60 nm thick top electrode (Au) was deposited on Spiro-OMeTAD by using thermal evaporation with a metal pattern plate. Photoelectric conversion efficiency measurement. The J-V characteristics of all solar devices were measured by using a Keithley 2400 under the illumination of the solar simulator (ABET TECHNOLOGIES Sun 3000) with AM1.5 G spectrum and light intensity of 100mW/cm2 (1 sun), which was calibrated by a standard crystalline silicon solar cell with a KG5 optical filter and an optical power meter. For small area (0.16 cm2) devices, a circular aperture mask (0.071 cm2) was used during the measurement, with a voltage step of 30 mV from -0.2 V to 1.2 V for reverse scan and 1.2 V to -0.2 V for forward scan. For large area (1.05 cm2) devices, a square mask (0.84 cm2) was used with the same measuring parameters expect for the voltage range (0-1.2 V for forward scan and 1.2 V-0 for reverse scan). 25



Other characterizations. High-resolution SEM images (top-view and corss-view) were obtained by using Quanta 250FEG (FEI, USA) with an acceleration voltage of 10 kV. Absorption spectra were measured by ultraviolet spectrophotometer U-3900 (Hitachi, Japan). Dark state I-V curves of both undoped and PCBM-doped perovskite films were obtained by a Semiconductor Characterization System (Keithly 4200-SCS), which was also used to measure the dark J-V curves of solar devices. PL spectra were obtained by using FLS980 Spectrometer (Edinburgh, Germany) with an excitation source of 350 nm lasing wavelength. XRD patterns were obtained by D-MAX 2200 VPC (RIGAKU, Japan). XPS measurements were carried out by using an ESCALAB 250Xi (Thermo Fisher, USA). The work function of film samples were measured by UPS (He I α: 21.22 eV) with a bias voltage of -5V. EQE measurements were performed by using a quantum efficiency measurement system QEXL (PV Measurements, USA), which was calibrated by a standard single-crystal silicon reference cell. Carrier mobility was determined via RH2035B (PhysTech, Germany) Hall effect measurement system using the van-der-Pauw method. Minority carrier lifetimes were determined by WT-2000 (SEMILAB, Hungary) with a laser wavelength of 904 nm via µ-PCD (Micro-wave photoconductivity decay) method. Thickness of all perovskite films were measured by using a Surfcorder ET150 (Kasada Liaboratory Litd, Japan).

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Supporting Information Other experimental and theoretical results and DFT calculation parameters are available and described in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail (Yue Zheng): [email protected] *E-mail (Biao Wang): [email protected]

ORCID Yue Zheng: 0000-0002-2165-7859 Author contributions Y.Z. initiated and performed this work and manuscript. B.W. conceived and designed the basic idea and structures. H.J. performed the experiments. G.L.J. performed the simulations. H.J., W.M.X., W.W.X., X.Y.Z., H.Y.Z. and Y.Z. analyzed the results of simulations. H.J., G.L.J. and Y.Z. co-wrote the manuscript. All authors contributed to discussion and reviewed the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Key Basic Research Program of China (No. 27



2015CB351905), NSFC (Nos. 11672339, 11474363, 11402312, 11602310). We thank S.X. L. from College of Information Science and Engineering at Huaqiao University for linguistic assistance and valuable discussion. We thank J.Y. L. from School of Engineering at Sun Yat-sen University for linguistic assistance. We thank Z.G. C. from the School of Chemistry at Sun Yat-sen University for providing J-V and thickness of film measurement systems. We thank D.Y. Z. from the School of Physics at Sun Yat-sen University for helping with XPS and UPS spectra measurements.

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High Current Density and Low Hysteresis Effect of Planar Perovskite

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