Beyond efficiency: phenothiazine, a new commercially viable substituent for hole transport materials in perovskite solar cells

Abstract

Two triphenylbenzene (TPB) derivatives, 1,3,5-tris(2′-((N,N-di(4-methoxyphenyl)amino)phenyl))benzene (TPB(2-MeOTAD)) and 1,3,5-tris(2′-(N-phenothiazylo)phenyl)benzene (TPB(2-TPTZ)), have been synthesized via two cost-efficient two step processes, and fully characterized by ¹H/¹³C NMR spectroscopy and mass spectrometry. For the first time in perovskite solar cells, phenothiazine has been introduced, as a low cost substituent to replace commonly used dimethoxydiphenylamine, which constitutes almost 90% of the final cost of hole transporting materials (HTMs). The use of a more flexible central core than state of the art spirobifluorene (SBF) lowers the highest occupied molecular orbital (HOMO) energy level, increases solubility and decreases the glass transition temperature. The derivatives were employed as hole-transport materials for the fabrication of mesoporous ZnO–Mg–EA(NH₃⁺)/CH₃NH₃PbI₃/HTM/Au solar cells. The best cells obtained have an optimized PCE of 12.14% and 4.32% for cells based on 4,4′-dimethoxydiphenylamine and phenothiazine substituents, respectively. Due to the extremely low cost of TPB(2-TPTZ) which is equal to 3.43 $ g⁻¹, in solar cells it delivers the lowest cost per peak Watt of 0.014 $ per W_p, which is 15 times lower than spiro-MeOTAD. This shows that the approach is commercially viable with potential to deliver HTMs with a cost contribution to the final module of as little as 1%.