Hole transport properties of some spiro-based materials for quantum dot sensitized solar devices

Abstract

Hole transport characteristics of seven compounds including 9,9´-spiro-bifluorene core bound onto two S(O)2(4-C6H4-X) groups with X = CF3, CH3, CH2CH3, OCH3, OCH2CH3, OH and H substituents were examined using density functional theory (DFT) calculations. These materials were designed for employment in quantum dot sensitized solar cells (QDSSCs) to achieve great performances. All HTMs exhibited negative solvation energies in CH2Cl2 solution confirming they had extraordinary stability and solubility. As the HOMO and LUMO energies of all HTMs were situated at higher levels compared to the valence and conduction bands of both CdS and CdSe QDs, respectively, an appropriate alignment was observed which led to effective hole injection from the CdS and CdSe QDs to the HTMs while inhibited the electron backward towards the electrolyte. Rather good light harvesting efficiency (LHE) values were measured for the spiro-based samples so that the HTM composed of X = OCH3 substituent demonstrated the greatest LHE = 0.571. The HTM including the X = H substituent presented the highest hole mobility (μh = 1.19 × 10-1 cm2V-1s−1) that was very larger than that of the eminent Spiro-OMeTAD. Notably, the HTM with the X = OH group depicted a hole mobility of 2.29 × 10-2 cm2V-1s−1 which was superior to that of Spiro-OMeTAD. As well, the HTMs composed of X = CH3, OCH3 and CH2CH3 exhibited slightly smaller hole mobilities compared to that of Spiro-OMeTAD demonstrating they could display nearly similar hole mobility behaviors. Consequently, the sample including the X = H substituent was recommended as the most advantageous HTM for QDSSCs due to its capacity to cause the highest performance among all HTMs investigated in this effort plus Spiro-OMeTAD.