A Simple and Cost-Effective Thiophene-Benzene-Thiophene Based Hole Transporting Material for Stable Perovskite Solar Cells

Abstract

The efficiency and stability of perovskite solar cells (PSCs) are often constrained by the limitations of the conventional hole transporting material (HTM), Spiro-OMeTAD, which exhibits low intrinsic conductivity, poor thermal endurance, dopant-induced instability and high synthetic cost. In this work, two low-cost HTMs, NS-1 and NS-2, were designed and synthesized via a facile process. Both materials share a common thiophene-benzene-thiophene (TBT) conjugated core with triarylamine terminal groups but differ in their side chains: NS-1 contains methoxy (-OCH3) substituents, whereas NS-2 incorporates polar ethylene glycol moieties on the TBT unit. These HTMs were synthesized through a cost-effective route, with material costs of $35/g and $37/g for NS-1 and NS-2, respectively, lower than that of the commercially available Spiro-OMeTAD (~$333/g). The influence of side-chain engineering on their optical, electrochemical, photophysical, charge-transport, and photovoltaic properties was systematically investigated. Among the two, NS-2 exhibited superior molecular planarity, enhanced hole mobility and better energy-level alignment, leading to an impressive power conversion efficiency (PCE) of 17.50%, compared to 4.96% for NS-1, which suffered from poor solubility and non-uniform film formation. Moreover, NS-2 based devices retained 84% of their initial efficiency after 1200 hours of operation, surpassing Spiro-OMeTAD in stability. The combination of high efficiency, long-term stability and low-cost synthesis establishes NS-2 as a promising HTM for scalable PSCs.