Effects of Self-Assembled Polymer-Based Hole Transport Monolayer on Organic Photovoltaics

In this study, the first attempt is made to implement conjugated polymer-based self-assembled monolayer (SAM), poly[3-(6-carboxyhexyl) thiophene-2,5-diyl] (P3HT-COOH), is implemented as the hole transport layer (HTL) in fabricatiing organic photovoltaics (OPVs). The scanning tunneling microscopy (STM) results show that those P3HT-COOH molecules with periodic carboxylic acid anchoring groups pack periodically on the indium tin oxide (ITO) surface and form a monolayer. Further, this monolayer is smooth and dense with a polar feature that minimizes defects, forms an excellent interface with the photoactive layer, and tunes the work function of ITO beneficial for hole extraction. OPVs with this P3HT-COOH polar monolayer as HTL exhibit an improved exciton dissociation rate, enhanced polymer crystallinity of the photoactive layer with increased hole mobility for more balanced charge transport, reduced trap state density, and weaker bimolecular recombination with larger recombination resistance. The improved charge transport properties lead to a ≈9% increment in power conversion efficiency (PCE) of OPVs relative to those using well-known PEDOT:PSS as HTLs. Additionally, the hydrophobic feature of P3HT-COOH SAM stabilizes the OPVs with residual PCE of ≈80% even after 5.3 months. The proposed approach is very useful and timely and provides a key step for developing optoelectronic devices with high-efficiency, high stability, and cost-effective production.