Effect of number and position of methoxy substituents on fine-tuning the electronic structures and photophysical properties of designed carbazole-based hole-transporting materials for perovskite solar cells: DFT calculations

Abstract

In perovskite solar cells (PSCs), the state-of-art Spiro-OMeTAD, which used as a hole-transporting material (HTM), suffered from complicated multistep synthesis and difficult purification that make this material cost ineffective, in addition to it being UV-unstable. Thus, new, cost-effective and easy to synthesize small organic molecules is still required. As reported, a carbazole-based compound (R01) was synthesized using a simple two steps method from low-cost commercially available compounds and used as an HTM. R01 exhibited higher conductivity and hole-mobility compared to that of the Spiro-OMeTAD. PSCs fabricated with R01 produced a power conversion efficiency of 12.03%, equivalent to that obtained in devices where Spiro-OMeTAD was the HTM. These findings highlighted R01 as a highly promising HTM with high performance, facile synthesis, and low cost. From a structural perspective, methoxy groups (–OCH3) in the HTM structure are controlling the HOMO level of the compound, apart from the critical role they play in anchoring the material onto the core perovskite layer. In this paper, we report a systematic study of the electronic structures and photophysical properties of twelve designed derivatives of R01. R01 was modified by substituting some hydrogen in the carbazole rings by two, four and six methoxy groups at different positions. The ground and excited state geometries are optimized by applying density functional theory (DFT) and its time-dependent functional (TDDFT), respectively. Detailed investigation of two factors: (i) the number and (ii) position of methoxy groups on the frontier molecular orbitals (FMOs), absorption and emission wavelengths, ionization potential, electron affinity, reorganization energies and charge mobility are examined and discussed. The electro-optical and nonlinear optical (NLO) properties are finely tuned in the R01 derivatives. By incorporating methoxy substituents into this carbazole-based compound, systematic design of potential materials for PSCs can be feasible.