Organic Salt-Doped Polymer Alloy: A New Prototype Hole Transporter for High-Photovoltaic-Performance Perovskite Solar Cells

Abstract

Hole-transporting layer (HTL) is one of the key components in a regular perovskite solar cell (r-PSC), which has the function of extracting the photon-excited holes from the absorber and then transporting them to the electrode. The most commonly used HTL in r-PSC is LiTFSI and tBP-doped spiro-OMeTAD. The inevitable instability induced by a deliquescent inorganic salt (LiTFSI), the migration of small lithium ions, and the necessary oxidation process in air hinder the commercialization of this technology. In this paper, a new undoped D–A copolymer (P15) is used as a hole-transporting material (HTM) for r-PSC but with moderate photovoltaic performance. Therefore, an organic salt, DPI-TPFB, having a big organic cation and a hydrophobic anion, was used as a dopant to increase the conductivity/hole mobility of P15 while avoiding the instability caused by lithium salt and moisture. Furthermore, an amphiphilic polymer, PDTON (with hole- transporting and perovskite-passivation ability), was added to P15 to form a polymer alloy, (P15 + PDTON), to further enhance the crystallinity and, therefore, the conductivity/hole mobility of P15 via space-confined interaction. As a result, r-PSCs based on DPI-TPFB-doped (P15 + PDTON) HTLs exhibit the highest power conversion efficiency (PCE) of 18.8%, which is higher than those of the cells based on DPI-TPFB-doped P15 (15.08%), DPI-TPFB-doped PDTON (7.37%), and undoped (P15 + PDTON) (15.66%) HTLs. Cells based on DPI-TPFB-doped (P15 + PDTON) HTL also have much better long-term stability than those using LiTFSI and tBP-doped spiro-OMeTAD as an HTL. The studies show that a polymer-compatible organic salt, DPI-TPFB, can be used as a stable dopant to increase the hole mobility of polymeric HTL without sacrificing the stability of the resulting cells, and mixing two ordinary photovoltaic performance polymeric HTLs (such as P15 and PDTON) can form a high- photovoltaic-performance polymer alloy (P15 + PDTON) HTL. Therefore, organic salt-doped polymer alloy can be regarded as a new prototype hole transporter for high-photovoltaic- performance PSCs.