Interface Modification Before Organic-inorganic Hybrid Perovskite Layer And The Devices’ Photovoltaic Performance

Organic-inorganic hybrid perovskite solar cells（PSCs） developed very fast recently. With the advantages of high conversion efficiency, low cost and solution-fabrication process, this kind of solar cell attracted people’s great attention. Interface modification was an effective approach to further improve the power conversion efficiency and stability of PSCs. Here, with the aim to solve key problems such as electron’s forward injection, back recombination and energy level matching at the interface before perovskite layer, we carried out related interface modification work. Cesium Carbonate（Cs₂CO₃）, Polyoxyethylene（PEO） and amino-acids were used to modify the TiO2 mesoporous layer and TiOx compact layer. The interface modification effects of them on the devices’ photovoltaic performance were researched carefully. The main results and conclusions were as follows:（1） The effects of Cs₂CO₃ as surface modification material for mesoporous TiO2 film were investigated. XPS results showed that Cs₂CO₃ was physically coated on the surface of TiO2. With optimized conditions, Cs₂CO₃ modified devices showed both enhanced Voc and Jsc. The solar cell’s power conversion efficiency increased from 11.9% to 14.2% after Cs₂CO₃ interface modification. Dark current and EIS tests indicated that Cs₂CO₃ formed a blocking layer during the interface. As a result, the back recombination inside the solar cell was largely retarded, leading to enhanced photovoltaic performance.（2） Thin layer of PEO was introduced into perovskite solar cells to modify the TiOx compact layer. XPS results indicated that there was electrons’ transfer or sharing at the TiOx/PEO interface. That would benefit the electrons’ injection from PEO layer into TiOx layer. UPS results showed that the work function of the TiOx decreased 0.9eV, resulting from the dipole formed at the interface. Devices based on the modified TiOx gave an obviously enhanced Jsc, averagely increasing from 17.1 mA/cm2 to 19.0 mA/cm2. Dark current and EIS tests showed that the improvement of Jsc should mainly be attributed to the retarded back recombination, increased recombination resistance and enhanced electron collection efficiency of the devices with PEO interface modification.（3） Self-assembly of amino-acids was carried out to modify the TiOx compact layer. Among the several water-soluble amino-acids, β-alanine worked best. UPS results showed that with the dipole formed at the interface, the work function of TiOx compact layer decreaed nearly 0.2 eV, being better aligned with the energy levels of perovskite layer. Steady-state photoluminescence spectra and time-resolution photoluminescence spectra indicated that the electrons’ forward injection was enhanced with the alanine self-assembled monolayer. Amino group on the alanine molecule had important influence on the modification effect. TiOx compact layer modified with β-alanine had a work function more close to the LUMO of perovskite than modified with α-alanine. Besides, amino group might interact with the perovskite layer’s interface to promote the electrons’ forward injection.