Nowadays fossil fuels remain the most common source of energy. Limited reserves, adverse effect on human health and the environment are demanding more intense research on sustainable and renewable alternatives. Solar energy has the largest potential among all renewable energy sources and it can be transformed into usable electricity by photovoltaic conversion in solar cells [1].
Silicon solar cells have been the dominant driving force in photovoltaic technology for the past several decades due to their well-known mechanism of action, the longevity and reliability during the operation. In addition, silicon is the abundant element in earth's crust and its nature is environmentally friendly. The efficiency of current thin-film silicon solar cells is about 26%, but this requires the very pure silicon and complex technological processes [2]. For these reasons, the silicon-based solar cell production is still expensive.
Perovskite solar cells (PSCs) can be one of the alternatives to silicon solar cells. PCSs have recently attracted a lot of attention due to their high power conversion efficiency (PCE), inexpensive starting materials, and ease of production. In less than a decade, these photovoltaics have made tremendous progress in effectiveness, rising PCE from 3.8% to 25.5% [3]. Perovskites possess intrinsic properties like wide absorption spectrum, fast charge separation, long transport distance of electrons and holes, long carrier separation lifetime. The PSC consists of an active perovskite layer located between the electron transporting layer (ETL) and the hole transporting layer (HTL). The HTL plays an important role in transporting holes, blocking electrons and protecting the perovskite from external factors, including moisture heat, and oxygen [4]. The main disadvantage of HTL is its conductivity, which can be increased by the addition of dopants. The use of dopants, however, impairs the stability of the perovskite solar cells, so intensive research is conducted to abandon them.
One way to avoid the use of dopants in HTL is to use self-assembling (SAM) positive charge-carrying compounds. Triarylamine-type molecules are considered as very efficient functional units that are widely used as photoconductors and charge carriers [5]. Combining triphenylamine with amide moieties can allow molecules to self-assemble into helical aggregates when exposed to visible light. Meanwhile, presence of the diacetylene functional groups allows formed structure to be fixated with covalent bonds into stable aggregate during the photopolymerization induced by ultraviolet light.
In this work several triphenylamine-based organic semiconductors with amide and reactive functional groups were synthesized as dopant-free SAM HTMs. Their optical, photophysical and thermal properties were investigated.