Charge Carriers Motion in P3HT:Capped ZnO Nanoparticles Blend Films; Impact of Capping Agents

Blend of conjugated polymer poly(3-hexylthiophene) or P3HT and Zinc Oxide nanoparticles (ZnO-NP) has been intensively used as active material for high performance hybrid solar cells. However, agglomeration of ZnO-NP hinders efficient charge carrier s transfer both from P3HT to ZnO-NP and their transport within ZnO-NP which cause to low performance of solar cells. Capping of ZnO-NP is currently applied to avoid this agglomeration effect. In this study, we used three different capping agents to cap ZnO-NP, namely small semiconducting molecules squaraine, 2-naphthalene and insulating polymer polyvinylpyrrolidone. We conducted temperature dependence of photoinduced infrared absorption spectroscopy to study charge carriers motion in the P3HT:capped ZnO nanoparticles blend films. The measurement was carried out with light irradiation of 532 nm and temperature ranging from 78 to 300 K.  The spectra were analyzed by a bimolecular carrier recombination method with Arrhenius activation energy. Two parallel charge carrier recombination processes are observed, namely polarons recombination along polymer chain (intra-chain) and inter-chain polarons recombination in the P3HT-chains. At low temperatures, polarons recombine along polymer chains (intra-chain) with activation energy between 19-23 eV for all samples. However, the inter-chain polaron motion is influenced by capping agents as shown by a variation in its activation energy ranging from 28 to 58 eV.