- Nano Express
- Open Access
Enhanced performance of CH3NH3PbI3−x Cl x perovskite solar cells by CH3NH3I modification of TiO2-perovskite layer interface
© The Author(s). 2016
- Received: 29 March 2016
- Accepted: 24 June 2016
- Published: 29 June 2016
In this work, perovskite solar cells (PSCs) with CH3NH3PbI3-x Cl x as active layer and spiro-OMeTAD as hole-transport media have been fabricated by one-step method. The methylammonium iodide (CH3NH3I) solution with different concentrations is used to modify the interface between mesoporous TiO2 (meso-TiO2) film and CH3NH3PbI3−x Cl x perovskite layer. Several techniques including X-ray diffraction, scanning electron microscopy, optical absorption, electrochemical impedance spectroscopy (EIS) and photoluminescence are used to investigate the effect of the interfacial modification. It is found that the interfacial modification by CH3NH3I enhance the crystallinity and increase the grain size of CH3NH3PbI3−x Cl x layer, and improve the surface wetting properties of perovskite precursor on meso-TiO2 film. The sunlight absorption and external quantum efficiency of PSCs in the visible region with wavelength less than 600 nm have been improved. The Nyquist plots obtained from the EIS suggest that the CH3NH3I modification can reduce the charge recombination rates. The photoluminescence measurement shows that the exciton dissociation in the modified devices is more effective than that in the control samples. The photovoltaic performance of the modified devices can be significantly improved with respect to the reference (control) devices. The CH3NH3I modified devices at the optimized concentration demonstrate the average power conversion efficiency of 12.27 % in comparison with the average efficiency of 9.68 % for the reference devices.
- Interfacial modification
- CH3NH3PbI3−x Cl x perovskite solar cells
- Photoelectronic properties
Recently, solar cells based on composites of organometallic halide perovskite have attracted much attention due to their super high absorption coefficients, relatively high carrier mobility and easy fabrication by solution process [1–3]. The efficiency of perovskite (CH3NH3PbX3, X = Cl, Br, I)-based photovoltaic devices has greatly increased from 3.8 % to more than 20 % in just a few years [4–6]. It is well known that the microstructure and crystallinity of perovskite layer have important influence on the performance of perovskite solar cells (PSCs) . The morphology of the perovskite films influences on exciton separation, charge transfer, and recombination . The low crystallinity of the perovskite films will result in a strong leakage path and has a negative effect on the charge dynamics of PSCs [5, 9]. However, a precise control of the morphology and crystallinity of perovskite layer remains a critical challenge due to the complex crystal growth mechanism of the perovskite materials. Substantial effort has been done to improve the microstructure of PSCs by adjusting the perovskite crystallization kinetics, such as additives modification , composition optimization , solvent extraction , and controlling the temperature, annealing time, or atmosphere [13–15]. However, a control of the crystalline property and microstructure just by optimizing the fabrication processing seems to be insufficient.
It is known that surface modification has been widely used to improve the performance of organic solar cells and dye-sensitized solar cells [16–19]. Interfacial engineering has been also used as a new strategy to control the morphology of perovskite layer and improve the efficiency of PSCs. It is found that interfacial modification can significantly promote the charge transfer and reduce the recombination rate for those PSCs with metal oxides as electron transport materials [20–22]. It was reported that a modification of the interface between ZnO and perovskite layer using self-assembled monolayer can optimize the morphology of perovskite layer and improve the performance of PSCs [23, 24]. It was also demonstrated that modifying the TiO2/CH3NH3PbI3 heterojunction interface by glycine can enhance the photovoltaic performance of two-step solution-processed PSCs .
In addition, a modification of the perovskite/TiO2 interface with a nanoscale layer of Al2O3 can reduce the charge losses of the PSCs . Excess CH3NH3 + or methylammonium iodide (CH3NH3I) is very important for the improvement in the optoelectronic properties of perovskite layer. Better coverage, uniform and pinhole-free perovskite films by adding excess CH3NH3 + to the reactants of perovskite layer can be obtained . During the preparation of perovskite layer by sequential deposition method, a proper addition of CH3NH3I to PbI2 solution not only enhances the absorption but also reduces the recombination rate, resulting in the improvement of efficiency in PSCs . These results suggest that it is promise to introduce CH3NH3I to modify the interface of PSCs.
Based on these considerations, in this work, the PSCs with the glass/FTO/compact TiO2/meso-TiO2/CH3NH3PbI3−x Cl x /spiro-OMeTAD/Ag structure are fabricated by the one-step solution method. Here, we choose CH3NH3I to modify the interface between meso-TiO2 and CH3NH3PbI3−x Cl x perovskite layer and investigate the effect of CH3NH3I concentration on the microstructure of CH3NH3PbI3−x Cl x layer and photo-electronic properties of the PSCs. The related mechanism is addressed too. The results show that the CH3NH3I modification at the optimal concentration can improve the sunlight absorption and external quantum efficiency (EQE) in the visible region at the wavelengths less than 600 nm, reduce the charge recombination rate, and promote the charge transfer, resulting in the enhanced performance. The average power conversion efficiency (PCE) of the PSCs can be enhanced from 9.68 to 12.27 %, respectively.
CH3NH3I was synthesized using the reported method . For the CH3NH3I modification, the CH3NH3I of different concentration dissolved in isopropanol was spin-coated on the meso-TiO2 films at 4000 rpm. The untreated samples were chosen as the references. After the modification, these samples together with the reference samples were annealed at 60 °C for 30 min. CH3NH3I and PbCl2 (Aladdin, 99.5 %) were dissolved in N,N-dimethylformamide (Aladdin, 99.9 %) to obtain a 40 wt % precursor solution with a CH3NH3I:PbCl2 molar ratio of 3:1. The solution was filtered with a 0.45-μm pore size filters before spin-coating. To fabricate the PSCs from the above samples, a CH3NH3PbI3−x Cl x layer was deposited onto the meso-TiO2 film by spin-coating a solution of CH3NH3PbI3−x Cl x (40 wt % dissolved in DMF) at 2000 rpm for 30 s in the glove box. Then, these samples were annealed in nitrogen (N2) ambient at 100 °C for 45 min. Subsequently, 0.08 M spiro-OMeTAD in chlorobenzene solution was spin-coated onto the perovskite film. These samples were left in dry air overnight in the dark. Finally, Ag electrodes with thickness of ~100 nm were evaporated on the sample surface through a shadow mask under a vacuum of 1 × 10−4 Pa. All the as-prepared PSCs were fabricated with the standard in-plane size of 3 mm × 4 mm.
The morphology and crystallinity of the perovskite layer were investigated using scanning electron microscopy (SEM, ZEISS ULTRA 55) and the X-ray diffraction (XRD) (X’Pert PRO, Cu Ká radiation). The photovoltaic performance of these PSCs was characterized using a Keithley 2400 source meter under an illumination of 100 mW/cm2 (Newport 91160, 150 W solar simulator equipped with an AM 1.5 G filter). The radiation intensity was calibrated by a standard silicon solar cell (certified by NREL) as the reference. The EQE and the UV-vis absorption spectra were measured using a standard EQE system (Newport 66902). The electrochemical impedance spectroscopy (EIS) measurements were performed on the Zahner Zennium electrochemical workstation in the dark. A 20-mV ac-sinusoidal signal source was employed over the constant bias with the frequency ranging from 1 Hz to 4 MHz. The photoluminescence spectra (PL) were measured by a fluorescence spectrophotometer (HITACHI F-5000) exited at 405 nm. The PL spectra have been normalized to the absorbance and measured in the same conditions.
The photovoltaic parameters of the PSCs modified by CH3NH3I with different concentrations
V oc (mV)
J sc (mA/cm2)
In summary, a series of PSCs based on the structure of glass/FTO/compact TiO2/meso-TiO2/CH3NH3PbI3−x Cl x /spiro-OMeTAD/Ag have been fabricated. CH3NH3I are used to modify the interface between meso-TiO2 and CH3NH3PbI3−x Cl x . It has been revealed that modifying the interface by CH3NH3I with appropriate concentration can significantly improve the performance of PSCs. After the CH3NH3I modification, the PCE of PSCs increases to 12.27 from 9.68 % of the references device. It is suggested that the better performance for CH3NH3I modified device is mainly attributed to the improved crystalline property, increased sunlight absorption in the visible range and reduced charge recombination rate.
We acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51431006, 61271127, 51472093, 21303060,61574065), Guangdong Natural Science Foundation (2016A030313421), Guangdong Engineering Technology Center of Optofluidics Materials and Devices (2015B090903079), International Science and Technology Cooperation Platform Program of Guangzhou (No. 2014 J4500016), the State Key Program for Basic Researches of China (Grant No. 2015CB921202), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2014), Science and Technology Planning Project of Guangdong Province (2015B090927006), and Program for Changjiang Scholars and Innovative Research Team in University (IRT13064).
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