Effects of precursor solution composition on the performance and I-V hysteresis of perovskite solar cells based on CH3NH3PbI3-xClx
© The Author(s). 2017
Received: 17 December 2016
Accepted: 28 January 2017
Published: 3 February 2017
Precursor solution of CH3NH3PbI3-xClx for perovskite solar cells was conventionally prepared by mixing PbCl2 and CH3NH3I with a mole ratio of 1:3 (PbCl2:CH3NH3I). While in the present study, CH3NH3PbI3-xClx-based solar cells were fabricated using the precursor solutions containing PbCl2 and CH3NH3I with the mole ratios of 1:3, 1.05:3, 1.1:3, and 1.15:3, respectively. The results display that the solar cells with the mole ratio of 1.1:3 present higher power conversion efficiency and less I-V hysteresis than those with the mole ratio of 1:3. Based on some investigations, it is concluded that the higher efficiency could be due to the smooth and pinhole free film formation, high optical absorption, suitable energy band gap, and the large electron transfer efficiency, and the less I-V hysteresis may be attributed to the small low frequency capacitance of the device.
Organometal halide perovskite solar cells (PSCs) have attracted much attention over the last several years due to their outstanding properties, such as large absorption coefficient, high electron-hole diffusion length, and high charge carrier mobility [1–6]. The power conversion efficiency (PCE) has increased from 3.8 to 22% . The typical architectures of PSCs mainly contain electron transporting layer (ETL)/perovskite/hole transporting layer (HTL) (n-i-p) and HTL/perovskite/ETL (p-i-n) structures . In the CH3NH3PbX3 (X = I, Br, Cl) family, a mixed halide perovskite CH3NH3PbI3-xClx (MAPbI3-xClx) has been proved a large diffusion length (~1 μm), which could be applied for planar heterojunction solar cells with improved device performance [9, 10]. Some groups have reported the results of the MAPbI3-xClx-based solar cells [11–13], in which the highest PCE is 19.3% .
The precursor solution of MAPbI3-xClx is conventionally prepared by mixing PbCl2 and CH3NH3I with a mole ratio of 1:3 (PbCl2:CH3NH3I). While there was no or only trace amount of Cl to be detected [15, 16]. Some studies have been performed to investigate the role of Cl in the MAPbI3-xClx film formation [17, 18]. A widely accepted opinion is that Cl ion in organometal halide perovskite can boost the mobility of excitons and the charge carrier transport [19–21]. A few groups have fabricated MAPbI3-xClx solar cells using the precursor solutions containing excess PbCl2 to investigate its effect on the performance of solar cells based on the I-V measurement with single scan direction [18, 21–23]. It has been reported that hysteretic effects were observed during the I-V measurement of the perovskite solar cells . I-V hysteresis could lead to an over- or underestimation of the PCE if it is not considered. Up to now, there are few reports to investigate the effects of excess PbCl2 on the PCE and I-V hysteresis of MAPbI3-xClx solar cells by considering the hysteretic effect.
Therefore in the present study, MAPbI3-xClx-based solar cells were fabricated using the precursor solutions containing different mole ratios of PbCl2, and CH3NH3I. I-V measurements were carried out with reverse scan (RS) and forward scan (FS). The photovoltaic parameters were obtained from the I-V curves averaged with RS and FS. Based on the measurements, the effects of excess PbCl2 on the PCE and the I-V hysteresis of the solar cells were investigated. One of the novelties of this work is that the photovoltaic parameters were obtained by an average of RS and FS to improve the accuracy of data. The other is the observation and investigation of the effect of excess PbCl2 on I-V hysteresis.
Methylammonium iodide (CH3NH3I) was synthesized with a method reported in the literature . The perovskite precursor solutions (40 wt%) were obtained by mixing PbCl2 and CH3NH3I (MAI) in anhydrous N,N-Dimethylformamide (DMF) at 60 °C with the mole ratios of 1:3, 1.05:3, 1.1:3, and 1.15:3 (PbCl2 to MAI), respectively.
Solar cell fabrication
Perovskite solar cells with a structure of n-i-p were fabricated. FTO-coated glass substrate (~15 ohm/sq, NPG, Japan) was patterned and cleaned with detergent, acetone, 2-propanol, and ethanol for 15 min by sonication. Then the substrate was treated by oxygen plasma for 20 min. A hole-blocking layer of compact TiO2 was deposited by spin-coating, a mildly acidic solution of titanium isopropoxide (Aladdin reagent) in ethanol (350 μl in 5 ml ethanol with 0.013 M HCl) at 2000 rpm for 30 s and annealed at 500 °C for 30 min. A mesoporous TiO2 layer composed of commercial TiO2 paste (Dyesol 18NRT, Dyesol) diluted in ethanol (1:3.5, weight ratio) was then deposited on the top of compact layer by spin-coating at 5000 rpm for 30 s. After drying at 125 °C, the TiO2 films were annealed at 500 °C for 30 min. The perovskite precursor solution was spin-coated on the mesoporous TiO2 film at 2000 rpm for 45 s in an argon-filled glove box. The sample was dried on a hotplate for 60 min at 110 °C. The hole-transporter layer was formed by spincoating a spiro-OMeTAD solution at 2000 rpm for 45 s. Finally, a gold layer with the thickness of 80 nm was deposited on top of the device by thermal evaporation in air.
X-ray diffraction (XRD) patterns were carried out on a DX-2700 diffractometer. UV-vis absorption spectra were performed on a UV–vis spectrophotometer (Varian Cary 5000). Morphologies and microstructures were obtained by a scanning electron microscope (SEM, JEM-7001 F, JEOL). Photocurrent-voltage (I-V) curves were carried out with a Keithley 2440 Sourcemeter under AM 1.5 G illumination with 100-mW/cm2 intensity from a Newport Oriel Solar Simulator. The active area of the device was 0.1 cm2 determined with a mask. Steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra were collected using a fluorometer (FLS 980E, Edinburgh Photonics). Capacitance-frequency measurements were performed under a forward bias of 0.6 V under 1 sun illumination conditions using an electrochemical workstation (RST5200, Zhengzhou Shiruisi Instrument Co., Ltd.) with the frequency range from 0.1 to 1000 Hz. The electrochemical impedance (IS) measurements were carried out with an electrochemical workstation (CHI660e, Shanghai CHI Co., Ltd.) in the frequency range from 0.1 to 100 kHz, in which an alternative signal with 5 mV magnitude was applied.
Results and discussion
Photovoltaic parameters of perovskite solar cells as a function of different mole ratios of PbCl2 and MAI
V oc (V)
J sc (mA/cm2)
0.76 ± 0.01
18.1 ± 0.2
61.9 ± 1.6
8.8 ± 0.1
0.82 ± 0.02
19.3 ± 0.3
64.0 ± 1.5
9.3 ± 0.2
0.88 ± 0.01
19.7 ± 0.1
65.0 ± 0.5
11.3 ± 0.2
0.85 ± 0.01
18.5 ± 0.2
61.5 ± 1.5
9.3 ± 0.3
Fitting parameters for EIS data
The solar cells based on MAPbI3-xClx were fabricated using the precursor solutions containing the mole ratio of 1:3, 1.05:3, 1.1:3, and 1.15:3. I-V curves were obtained by both reverse scan and forward scan, from which the photovoltaic parameters were calculated by taking the average of them. The results displayed that the solar cells with the mole ratio of 1.1:3 present higher PCE and less I-V hysteresis. To get an insight into the results, some investigations were performed. The higher PCE could be due to the smooth and pinhole-free film formation, high optical absorption, suitable energy band gap, and the large electron transfer efficiency. The less I-V hysteresis may be attributed to the small low frequency capacitance of the device.
This work is supported by the NSFC-Henan Province Joint Fund (U1604144), Science Fund of Henan Province (162300410020), National Science Research Project of Education Department of Henan Province (No.17A140005), Science and Technology Development Project of Henan Province (No.142102210389), and Program for Innovative Research Team (in Science and Technology) in University of Henan Province (No. 13IRTSTHN017).
Z-LZ, B-QM, and Y-LM carried out the main part of experiment and drafted the manuscript. Other authors provided assistance with experimental measurements, data analysis, and the manuscript writing. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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