Efficient and Stable Perovskite Quantum Dot Photovoltaics enabled by Using Dopant-Free Hole Transport Materials with Rigid Segments

CsPbI₃ perovskite quantum dots (PQDs) with ideal optoelectronic properties offer high thermal stability for photovoltaics. In device fabrication, a ligand exchange process is required to enhance electrical coupling within PQDs. However, this dynamic ligand exchange is imperfectly performed, resulting in surface traps and inevitable exposure of PQDs to external conditions containing H₂O or O₂. So far, most PQD photovoltaics (PQDPVs) have adopted Spiro-OMeTAD as hole transport material (HTM), which requires deliquescent dopants to enhance its hole mobility, resulting in moisture that penetrates the perovskite crystal and greatly accelerates the decomposition of PQDs. In this point of view, hydrophobic HTMs on the PQD surface are crucial for achieving high device stability. Conjugated polymeric HTMs basically have strong molecular packing for efficient charge transport, but excessive intermolecular interactions degrade film-forming properties in perovskite, which causes interfacial nonradiative recombination. In this respect, side chain engineering (e.g., extended-, polar-, and asymmetric-side chains) is developed for optimizing the morphology. Nevertheless, these strategies reduce the charge hopping efficiency. Therefore, in this study, we propose a novel D-A conjugated polymeric HTM design employing unsubstituted rigid segments and fabricate three D-A conjugated polymeric HTMs: Asy-PDTS, Asy-PSDTS, and Asy-PSeDTS. Particularly, Asy-PSeDTS with a rigid segment locally induces strong intermolecular interactions, enhancing the charge hopping efficiency while exhibiting appropriate intermolecular interaction to maintain excellent film-forming properties. Owing to this synergy, Asy-PSeDTS-based PQDPVs achieved 15.2% power conversion efficiency (PCE) and maintained 80% of the initial PCE after 40 days, which is the highest PCE and stability among dopant-free HTM-based PQDPVs so far.