Date Published: May 02, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Junjie Ma, Guang Yang, Minchao Qin, Xiaolu Zheng, Hongwei Lei, Cong Chen, Zhiliang Chen, Yaxiong Guo, Hongwei Han, Xingzhong Zhao, Guojia Fang.
Reducing the energy loss and retarding the carrier recombination at the interface are crucial to improve the performance of the perovskite solar cell (PSCs). However, little is known about the recombination mechanism at the interface of anode and SnO2 electron transfer layer (ETL). In this work, an ultrathin wide bandgap dielectric MgO nanolayer is incorporated between SnO2:F (FTO) electrode and SnO2 ETL of planar PSCs, realizing enhanced electron transporting and hole blocking properties. With the use of this electrode modifier, a power conversion efficiency of 18.23% is demonstrated, an 11% increment compared with that without MgO modifier. These improvements are attributed to the better properties of MgO‐modified FTO/SnO2 as compared to FTO/SnO2, such as smoother surface, less FTO surface defects due to MgO passivation, and suppressed electron–hole recombinations. Also, MgO nanolayer with lower valance band minimum level played a better role in hole blocking. When FTO is replaced with Sn‐doped In2O3 (ITO), a higher power conversion efficiency of 18.82% is demonstrated. As a result, the device with the MgO hole‐blocking layer exhibits a remarkable improvement of all J–V parameters. This work presents a new direction to improve the performance of the PSCs based on SnO2 ETL by transparent conductive electrode surface modification.
Perovskite solar cells (PSCs) have attracted considerable research interest because of their excellent photovoltaic performance and simple fabrication process.1, 2 Perovskite materials have many advantages such as excellent charge‐carrier mobility, effective ambipolar charge transfer, and high optical absorption coefficient.[[qv: 1g,3]] Perovskite materials can be coated on the compact electron transport layer (ETL) immediately to form a planar heterojunction structure.[[qv: 3b,f,4]] Many semiconductors, such as TiO2 and ZnO, are proved to be good ETLs.5 But for the TiO2 ETL, the electron recombination rates are very high due to the low electron mobility and the ZnO ETL suffers from the issue of chemical instability. Recently, tin oxide (SnO2) has emerged as a promising candidate of ETL, which shows much higher electron mobility, good antireflection, low‐temperature process, and no ultraviolet (UV) photocatalysis effect in PSCs.6 Further efforts are made to improve the performance of PSCs with planar structure based on SnO2 ETLs.
Figure1a presents the scheme of PSCs with regular structure in this study: an FTO or ITO‐coated glass as the anode, an MgO nanolayer as the HBL (MgO was discontinuously distributed on the surface of the anode), an SnO2 thin film as the ETL, a perovskite absorber layer (CH3NH3PbI3), a 2,2,7,7‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9‐spirobifluorene (spiro‐OMeTAD) as the hole transport layer (HTL), and Au as the back electrode. The energy band diagram is shown in Figure 1b. As MgO has a large energy bandgap of ≈7.8 eV,[[qv: 17b]] it ensured efficient blocking charge recombination in the process of charge transport from perovskite absorber layer to FTO negative electrode. The lower valence band position can enhance the ability of blocking the holes, and the electrons can easily tunnel through the MgO film because the MgO is a good tunneling material and the film is very thin.20
By utilizing an ultrathin MgO insulating nanolayer at the interface of anode and SnO2 ETL, improved device performance for planar perovskite solar cells was demonstrated. The enhancement mechanism was studied systematically by using AFM, PL, EIS, OCVD measurements, and diode model calculations. It was found that thin layer of MgO can make the FTO smoother and enhance the transmission of FTO. Most importantly, MgO layer can facilitate the hole blocking and suppress the electron–hole recombination at the FTO/SnO2 ETL interface owing to its wide bandgap and insulating properties. Finally, devices with MgO HBL layer yield a PCE of over 18%, which was greatly improved than device without MgO HBL. Our work points out the role of MgO in charge carrier transport regulation and reveals the effect of recombination at anode/SnO2 interface with a n‐i‐p junction working mechanism. Besides, we demonstrate the universal application of MgO on different anodes and provide a novel interface engineering strategy on the road to improving the performance of the PSCs based on SnO2 ETL.
Materials: FTO glass with a sheet resistance of 14 Ω sq−1 and ITO glass with a sheet resistance of 10 Ω sq−1 were purchased from Asahi Glass (Japan). Magnesium acetate (Aladdin, 99.99%) was dissolved in deionized water to prepare aqueous magnesium acetate. SnCl2·2H2O (Alfa, 99.9985%) was used to prepare precursor solutions. Methylammonium iodide (MAI) was prepared according to a literature.18 Lead iodide (PbI2) was purchased from Aladdin reagent. MAI and PbI2 (1:1, mol mol−1) were dissolved in the mixed solution of dimethyl sulfoxide (DMSO) and N,N‐dimethylformamide (DMF) with a concentration of 1.38 m. The solution was stirred at 60 °C for 12 h inside an argon glovebox. The hole transport material was made up of 68 × 10−3m spiro‐OMeTAD (Shenzhen Feiming Science and Technology Co., Ltd., 99.0%), 55 × 10−3m TBP (Aladdin reagent), and 26 mg Li‐TFSI (Aladdin reagent) in acetonitrile and chlorobenzene (1:10 in volume ratio). The purity of gold wire is 99.99%.
The authors declare no conflict of interest.