Theoretical investigation of the core-related electronic and charge transport properties of 2D expanded small molecule hole-transporting materials

Abstract

Generally, thiophene/benzene-fused π-conjugated systems have been the go-to core units for enhancing the electrical conduction and charge transport characteristics of 2D hole-transporting materials (HTMs). However, while thiophene-fused units have been extensively studied, the investigation into benzene-fused π-conjugated cores has been relatively limited. In order to explore the untapped potential of benzene-fused π-conjugated systems in HTMs, we have designed three new molecules utilizing polycyclic triphenylene, pyrene and perylene as core units. These molecules were then simulated using density functional theory in conjunction with the Marcus theory of electron transfer. Our findings reveal that the highest occupied molecular orbital (HOMO) levels of these newly developed HTMs are 0.14–0.29 eV lower compared to Spiro-OMeTAD, a commonly used HTM, potentially leading to an increase in the open circuit voltage of solar cell devices. Additionally, the enhanced electronic interaction between neighboring molecules and lowered reorganization energies contribute to high hole mobilities, measured at 5.08 × 10⁻³ cm² V⁻¹ s⁻¹, 7.38 × 10⁻³ cm² V⁻¹ s⁻¹ and 9.31 × 10⁻¹ cm² V⁻¹ s⁻¹, respectively. Furthermore, these new molecules exhibit weak visible light absorption and significant Stokes shifts, both of which are advantageous for HTM performance enhancement. Overall, our calculations suggest that these benzene-fused π-conjugated core molecules are promising as HTM candidates, paving the way for more effective solar cells.