Spiro[fluorene-9,9′-xanthene] core comprising green imidazole-sulfonylurea moieties devised as excellent hole transport materials for perovskite solar cells

Fifteen green and benign imidazole-sulfonylurea derivatives of Spiro[fluorene-9,9′-xanthene] (SFX) were devised as hole transporting materials (HTMs) for perovskite solar cells (PSCs) by density functional theory (DFT) method to evaluate their capability of hole extraction from perovskite and transportation within photovoltaic devices. It was indicated that in all HTMs, the highest occupied molecular orbital (HOMO) levels were situated between energy levels of CsPbCl₃ perovskite's valence band and Ag electrode's potential, verifying such SFX-based materials could efficiently extract and transfer holes from CsPbCl₃ towards Ag contact. The absorption spectra showed that the absorption peaks of all molecules advantageously occurred in the UV (ultraviolet) region (a similar behavior like Spiro-OMeTAD), which certified they did not compete with the CsPbCl₃ perovskite in absorbing solar visible light. Also, smaller hole reorganization energy values were obtained for all samples than their corresponding electron reorganization energy values. Notably, for all SFX-based materials, hole mobilities (μ_h) changed from 0.469 to 13.748 cm²V⁻¹s⁻¹, which were very much larger (by about 10³-10⁵ times) than μ_h values of Spiro-OMeTAD. Moreover, all SFX-based HTMs revealed almost equal fill factors (FFs) of 0.929 and very comparable high open-circuit voltage (V_OC) values (1.858–1.898 V). Such surprising results substantiated that these green and biocompatible imidazole-sulfonylurea derivatives of SFX could be applied as excellent HTM alternatives for the famous and commercial Spiro-OMeTAD to fabricate high-performance PSC photovoltaics.