Indolo[3,2-a]carbazole-Based Hole-Selective Layers for Silicon Heterojunction Solar Cells

Abstract

Organic–inorganic hybrid solar cells represent a promising avenue for realizing cost-effective and scalable energy solutions. Here, we introduce a hybrid heterocyclic organic small molecule, indolo[3,2-a]carbazole (C₁₄IC), as a prospective material for the preparation of a hole-selective layer (HSL) in silicon solar cells. The cost-effective material, which was synthesized using a single-step acid-catalyzed condensation method, is expected to be attractive from a commercial perspective compared to other expensive HSL materials like PEDOT:PSS and Spiro-OMeTAD. C₁₄IC possesses a HOMO energy of −5.39 eV and a bandgap of 3.26 eV, which provides favorable band alignment with the valence band (VB) of silicon (−5.17 eV). This enables it to function as a good HSL in silicon heterojunction solar cells. A proof-of-concept device was fabricated on an n-type textured silicon wafer using the C₁₄IC molecule for the HSL and the LiF molecule for the electron-selective layer (ESL). The optoelectronic properties of the fabricated devices were evaluated through illuminated and dark J-V measurements, along with external quantum efficiency (EQE) analysis. The devices exhibited a promising open-circuit voltage (V_oc) of approximately 500 mV and an average EQE of 70%. An ideality factor of 1.83 and a reverse saturation current density (J₀) of 7.08 × 10^–8 A/cm² confirmed efficient diode behavior, highlighting the potential of indolo[3,2-a]carbazole as a material for effective hole transfer.