n-Doping of Electron Transporting Layer for High Performance Perovskite Solar Cells with Planar Heterojunction Architecture

Abstract

Organic-inorganic hybrid perovskite has intensively been studied as light absorber for efficient photovoltaic cells because of its unique and superior intrinsic properties such as long charge carrier diffusion length, excellent charge carrier transport, low exciton binding energy, and high absorption coefficient. Furthermore, thin films of organic-inorganic perovskites can be easily fabricated from abundant and inexpensive precursor materials. In spite of their short history, the photovoltaic performances of perovskite solar cells have advanced dramatically with surpassing a power conversion efficiency (PCE) of 20%. Particularly, inverted structure of planar heterojunction perovskite solar cell with a device configuration of ITO/PEDOT:PSS/perovskite(CH3NH3PbI3) /electron transporting material/Al has attracted much attention due to an advantage of low-temperature and solution processibility over the normal structure. For inverted perovskite solar cells, fullerene derivatives such as C60, PCBM, PC71BM, and ICBA have commonly been used as electron transporting layer (ETL) material because of their room temperature solubility and orthogonal solvent processibility on the perovskite layer. In organic photovoltaics, non-fullerene electron acceptors based on polymers and small molecules have recently been developed because of their unique advantages over fullerene derivatives such as high absorption in visible range, tunable energy level, and low production cost. However, since inverted perovskite solar cells require sufficiently thick ETL to prevent direct contact between perovskite layer and metal cathode, the non-fullerene acceptors as ETL material of perovskite solar cells must have sufficiently high electrical conductivity and high electron mobility. 1,3-Dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole (DMBI) has been reported as an effective n-type dopant for enhancing the electrical properties of n-type materials such as fullerene or naphthalene diimide derivatives. Since DMBI has good solubility in common organic solvents, the doping is easily achieved by simply mixing with n-type material in organic solvents. DMBI is expected to effectively dope the electron transporting materials and thus to enhance the solar cell performance of planar heterojunction perovskite solar cells. In this study, a simple n-doping method to enhance the performance of n-type materials as electron transporting layer of perovskite solar cells with planar heterojunction architecture is investigated. n-Type materials used in this study are PCBM and perylene diimide-based small molecules. When a small amount of DMBI is added to ETL materials, it reveals that DMBI effectively dopes the ETL as clearly evidenced by up-shift of the Fermi level. Further, the device with DMBI-doped ETL exhibits high PCE with remarkably enhanced short-circuit current density as compared to the device with undoped ETL, mainly due to the greatly increased electrical conductivity of ETL materials. When the charge transfer characteristics are identified by photoluminescence measurements, it reveals that the electron accepting ability of diPDI is comparable to that of PCBM.