Efficiency Enhancement of Perovskite Solar Cells by Pumping Away the Solvent of Precursor Film Before Annealing
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A new approach to improve the quality of MAPbI3 − x Cl x perovskite film was demonstrated. It involves annealing the precursor film after pumping away the solvent, which can decrease the influence of solvent evaporation rate for the growth of the MAPbI3 − x Cl x perovskite film. The resulting film showed improved morphology, stronger absorption, fewer crystal defects, and smaller charge transfer resistance. The corresponding device demonstrated enhanced performance when compared with a reference device. The averaged value of power conversion efficiency increased from 10.61 to 12.56 %, and a champion efficiency of 14.0 % was achieved. This work paves a new way to improve the efficiency of perovskite solar cells.
KeywordsPerovskite solar cells Pump away the solvent High-quality MAPbI3 − xClx layer
Organo-metal halide perovskite solar cell, as a rising star in the field of thin-film photovoltaic cells, has drawn much attention, not only due to the superior optical and electrical properties of perovskite materials, such as broad light absorption range , low exciton binding energy [2, 3], longer carrier diffusion length [4, 5, 6], and higher charge carrier mobilities , but also due to its low cost and easy fabrication process. The first work employing perovskite as a light-harvesting material in solar cells was reported by Miyasaka and co-workers in 2009 with an efficiency of only 3.8 % . Almost double of this efficiency (6.5 %) was reported by Park’s research group in 2011 . Due to the dissolubility of perovskite in liquid electrolyte, devices in both works showed poor stability. When a solid-state hole conductor of 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) was introduced to replace the liquid electrolyte, both stability and performance were improved dramatically . Amazing progress has been made in recent times [5, 10, 11, 12, 13, 14, 15, 16, 17, 18] as a champion efficiency over 20 % has been reported for perovskite-based solar cell , making it an ideal candidate for next-generation photovoltaic cells. The development of perovskite solar cells came with the challenge of controlling the morphology of the perovskite layer. Such morphology is strongly influenced by several parameters such as fabrication method [5, 12, 13, 15, 20, 21, 22], additives [15, 23, 24, 25, 26], and annealing process [16, 27, 28, 29, 30, 31]. Hence, efforts have been made to improve the morphology of the perovskite layer. For example, Snaith’s group introduced a dual-source thermal evaporation technique . They obtained a much compact and uniform MAPbI3 − x Cl x perovskite layer compared with the traditional one-step spin-coating method. Liang et al. added a small amount of 1,8-diiodooctane (DIO) to the MAPbI3 − x Cl x precursor solution and found that a high-quality MAPbI3 − x Cl x perovskite film was formed with improved coverage and absorption. As a result, the power conversion efficiency (PCE) was increased by 20 % . Yang Yang et al. investigated the influence of annealing conditions on the perovskite layer. They annealed the precursor film in a humid environment (~30 %) which greatly improved the film quality, grain size, carrier mobility, and lifetime. This method produced planar devices with a pretty high PCE approaching 17.1 % . Rira et al. found that a solvent evaporation rate can strongly influence the growth of the MAPbI3 − x Cl x perovskite film. By controlling the solvent evaporation rate, they obtained a MAPbI3 − x Cl x perovskite layer with improved surface coverage. Thus, improved performance was achieved .
In this letter, we report a new approach to obtain a high-quality MAPbI3 − x Cl x perovskite layer, by pumping away the solvent of precursor film before annealing to decrease the influence of solvent evaporation rate on the growth of MAPbI3 − x Cl x perovskite film. This approach was proven to be effective as a compact and uniform perovskite film with stronger absorption, fewer crystal defects, and smaller charge transfer resistance. Devices based on this high-quality perovskite film showed enhanced performance compared with the reference device. The averaged efficiency increased from 10.61 to 12.56 % and a champion PCE of 14.0 % was achieved.
Methylammonium iodide (MAI) was synthesized by reacting 10 ml of hydroiodic acid (57 wt.% in water, Alfa Aesar) with 24 ml of methylamine (33 wt.% in ethanol, Sigma-Aldrich) in ice bath under nitrogen atmosphere with constant stirring. After reacting for 2 h, the resulting white powder of MAI was collected by rotary evaporator at 50 °C. The MAI was dissolved into ethanol and evaporated for further purification. This step was repeated two times, and the MAI powder was finally collected and dried in a vacuum oven at 60 °C for 30 h. Poly(3,4-ethylenedioxythiophene):poly(p-styrene sulfonate) (PEDOT:PSS, Clevios AI 4083) and [6,6]-phenyl-C60-butyric acid methylester (PC60BM) were bought from Heraeus (Germany) and Nichem Fine Technology Co. Ltd. (Taiwan), respectively. To prepare MAPbI3 − x Cl x (30 wt.%) precursor solution, MAI and PbCl2 (Sigma-Aldrich) were dissolved into N,N-dimethylformamide (DMF) solvent with a molar ratio of 1:1 under constant stirring. The concentration of PC60BM solution was 20 mg/ml in chlorobenzene.
Measurements and Characterization
Current density-voltage (J-V) characteristics of perovskite solar cells were measured in air using a programmable Keithley 2400 source meter under AM1.5G solar irradiation at 100 mW/cm2 (Newport, Class AAA solar simulator, 94023A-U). The light intensity was calibrated by a certified Oriel Reference Cell (91150 V) and verified with an NREL-calibrated Hamamatsu S1787-04 diode. The external quantum efficiency (EQE) was measured by a certified IPCE instrument (Zolix Instruments, Inc., Solar Cell Scan 100). We utilized a field emission scanning electron microscope (FEI Quanta 200) to investigate the morphology of the perovskite layer. The absorption spectra were measured with a UV/vis spectrophotometer (PerkinElmer Lambda 750). The steady-state photoluminescence spectra were measured by utilizing Horiba Jobin-Yvon LabRAM HR800. Impedance spectroscopy (IS) measurements were performed using a Wayne Kerr 6550B precision impedance analyzer with a 50-mV perturbation oscillation signal in a frequency range from 20 Hz to 20 MHz.
Results and Discussion
Photovoltaic parameters of both the reference and modified MAPbI3 − x Cl x -based perovskite solar cells
J sc (mA/cm2)
V oc (V)
R s (Ω cm2)
R sh (Ω cm2)
We also utilized IS to investigate the series resistance (R s) of both devices. The R s consists of sheet resistance (R sheet) of the electrodes and charge transfer resistance (R CT). The following three parts contribute to the R CT: the interfaces between electrodes and charge extraction layer, the interfaces between charge extraction layer, and perovskite layer as well as the bulk of the perovskite layer . Figure 5b shows the Nyquist plots of both devices tested under applied voltage conditions approaching the V oc of perovskite solar cells. R s values of 4.9 and 2.8 kΩ were obtained for the reference and modified devices, respectively. Noticeably, the modified device showed much smaller R s. Since the main difference is located at the perovskite layer in this study, it indicates that the MAPbI3 − x Cl x perovskite layer prepared by annealing the precursor film after pumping away the solvent exhibits much smaller R CT. Enhanced performance was therefore obtained for the modified device.
A new approach which involves the annealing of a precursor film after pumping away its solvent component was introduced to obtain a high-quality MAPbI3 − x Cl x perovskite film. The device based on such high-quality film showed enhanced performance compared with the reference device. The averaged efficiency increased from 10.61 to 12.56 %, and a champion efficiency of 14.0 % was achieved. SEM, UV-vis absorption, steady-state photoluminescence spectra, and impedance spectroscopy results indicated that the improvement in device efficiency is mainly attributed to the improved morphology, stronger absorption, and fewer crystal defects as well as smaller charge transfer resistance of the modified MAPbI3 − x Cl x perovskite film. This work paves a new way to improve the efficiency of perovskite-based solar cells.
We acknowledge financial support from the Youth 973 Program (Grant No. 2015CB932700), the National Natural Science Foundation of China (Grant No. 51290273, 91433107, and 61177016), and the Natural Science Foundation of Jiangsu Province (Grant No. BK20130288). This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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