Molecular structure simplification of the most common hole transport materials in perovskite solar cells

Abstract

2,2′,7,7′-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) currently is the most common hole transport material (HTM) used in perovskite solar cells (PSCs). Whereas, the hole mobility of amorphous spiro-OMeTAD remains low and may result in severe charge recombination. In addition, the harsh conditions and high cost of spiro-OMeTAD synthesis are also disadvantageous for future large-scale applications. In this paper, we report the synthesis of a new, high-yielding spiro-type HTM, 2,7-di(N,N-di-p-methoxyphenylamine)-2′,7′-di-tert-butyl-9,9′-spirobifluorene (spiro-027). The replacement of two 4,4′-dimeoxyl-diphenylamine at the 2′ and 7′ positions of conventional spiro-OMeTAD with two tert-butyl in spiro-027 results in a greatly simplified molecular structure with unaffected thermal stability or highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). In addition, enhanced hole mobility is observed in spiro-027. Thus, the PSC devices based on spiro-027 demonstrated a high power conversion efficiency (PCE) of 16.60%, which was higher than the 16.12% PCE achieved by those based on spiro-OMeTAD. By combining these results with theoretical calculations, our work confirmed the feasibility of simplifying the molecular structure of spiro-OMeTAD, which contributed to our knowledge of spiro-type optoelectronic functional materials. Moreover, our results offer a novel strategy for the replacement of expensive HTMs and the enhancement of PSCs performance.