Perovskite solar cells approaching 25% PCE using side chain terminated hole transport materials with low concentration in a non-halogenated solvent process

Synchronously achieving high power conversion efficiency (PCE) at low cost is a great challenge for the star hole transport material (HTM) spiro-OMeTAD-based perovskite solar cells (PSCs) because of their expense. Three side-chain modified HTMs, spiro-mF-V, -A, and -P, are designed and synthesized. The incorporation of side chains, such as vinyloxy, allyloxy, and propargyloxy, effectively improves the hole mobility and matches the energy levels with the perovskite layer. All three HTMs show a high PCE of 23.7, 23.80, and 24.85% for spiro-mF-V, -A, and -P using a non-halogenated solvent, such as toluene for eco-friendly PSC fabrication. Furthermore, spiro-mF-V, -A, and -P exhibit good solubility and homogeneous film with smooth surface morphologies in a non-halogenated solvent system. In particular, the obtained PCE values exceed 23.7% at a low concentration in a non-halogenated system, which is one-third that of the spiro-OMeTAD-based PSCs. To the best of the authors' knowledge, it is the highest reported PCE for HTMs using non-halogenated solvents in PSCs. These findings show that the side chain engineering technique can produce novel HTMs for low-cost, environmentally friendly solvent processing.