Designing novel spiro compounds as favorable hole transport materials for quantum dot sensitized photovoltaics

Abstract

Some hole transport materials (HTMs) containing 9,9´-spiro-bifluorene central moiety (core) were developed to achieve high performance quantum dot sensitized solar cells (QDSSCs). The properties of all HTMs were explored via density functional theory (DFT) calculations along with Marcus and Einstein theories. It was exhibited that the HOMO and LUMO energies of all HTMs were positioned upper than the valence and conduction bands of CdS and CdSe QDs, respectively. Thus, they were appropriately aligned which could result in efficient hole injection from the CdS and CdSe QDs towards the HTMs. The negative solvation energies of all HTMs approved that they were highly stable and soluble in CH2Cl2 solution. All spiro-based HTMs displayed relatively good light harvesting efficiency (LHE) for light absorption and the materials with X = CH3 and F substituents exhibited the greatest LHE values of 0.3485 and 0.3317. Hirshfeld analysis illustrated that the H∙∙∙H contacts were the most important interactions that covered the greatest areas (41.1–71.2%) in all HTMs except for the X = CF3 which indicated F…H interaction as the most significant interaction with 42.2% surface area. The highest hole mobility (μh = 1.65 × 10-2 cm2V-1s−1) was measured for the HTM including the X = H substituent that was greater than that of the famous Spiro-OMeTAD. As a result, it was suggested as the most favorable HTM for QDSSCs as it could afford higher performances than those of Spiro-OMeTAD and other HTMs designed in this work.