Enhancing the efficiency of low-temperature planar perovskite solar cells by modifying the interface between perovskite and hole transport layer with polymers

In this work, planar perovskite solar cells (PSCs) based on CH3NH3PbI3 perovskite layer and low-temperature processed TiO2 have been fabricated. Polymers including poly(methylmethacrylate) (PMMA), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-pheny- lenevinylene] (MEH-PPV) and polyethylene glycol (PEG) in chlorobenzene solution have been selected to modify the interface between perovskite and hole transport layer (HTL), respectively. The concentrations of the three polymer solutions have been optimized. The effect of interfacial modification by different polymer solutions on the photoelectric properties of perovskite layer and the performance of PSCs has been systematically investigated. The microstructure and photoelectric properties of the modified perovskite films has been systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), conducting force microscopy (CFM) and Kelvin probe force microscopy (KPFM). The results reveal that the modified perovskite films with tetrahedral perovskite structure have lager grain size, lower roughness and better photoelectric properties compared with the reference sample. The electron trap state density (Dtrap), charge extraction, carrier transfer and recombination process in the PSCs have been investigated by current-voltage (I-V) characteristic curves, steady-state photoluminescence (PL), photo-voltage decay and electrochemical impedance spectroscopy (EIS). The results indicate that the polymeric interface modification at the optimum concentration can reduce the Dtrap, promote the charge transfer and suppress carrier recombination, resulting in the improved performance of PSCs. All of the modified PSCs at an optimum concentration exhibit the improved fill factor (FF) and open circuit voltage (Voc), thus the power conversion efficiency (PCE) is enhanced to over 17% from 15.49%.