Micro-morphology Controlling And Performance Optimizing For P3HT/ZnO Hybrid Bulk Heterojunction By Introducing Diketopyrrolopyrrole(DPP)-based Narrow Bandgap Liquid Crystalline

During the last decades, conjugated polymer/inorganic semiconducting nanocrystals hybrid bulk heterojunction solar cells consisting of an electron donor-conducting conjugated polymer and an electron acceptor-inorganic semiconducting nanocrystals have attracted considerable attention, mainly due to the potential of combining the advantages of the characteristic properties of the two parts, such as low fabrication costs, excellent processability, high optical absorbance and flexible substrate manufacturing of the conjugated polymer and high electron mobility, tunable optical bandgap, and perfect physical and chemical stability of the inorganic semiconducting nanocrystals, and it is widely recognized as one of the most promising approaches for producing high-efficent and low-cost solar cells. Limited by the diffusion length of excitons in conjugated polymers and to obtain efficient charge transport, achievement of ordered interpenetrating networks on nanoscale is the key to fabricate high-efficient and stable organic/inorganic hybrid bulk heterojunction solar cells. Considering liquid crystalline molecules can self-assemble to ordered and orientational microstructures on nanoscale, incorporation of liquid crystalline molecules into the photoactive materials in hybrid solar cells will provide an effective approach to control the micromorphology of hybrid system.Herein, we have synthesized a kind of narrow bandgap liquid crystalline molecules, 3,6-bis(5-(4-hexyl-phenyl)-2-thienyl)-2,5-bis(2-ethylhexyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione(DPP-TP6), and added it into P3HT/ZnO hybrid system as tertiary component to control the micro-morphology and improve the orderness of P3HT/ZnO bulk heterojunction via the self-assembly and ordered orientation of liquid-crystalline molecules, and hence improve the power conversion efficiency(PCE) of P3HT/ZnO hybrid solar cells. The results show that the incorporation of DPP-TP6 can effectively improve the aggregation of ZnO nanocrystals and achieve better dispersion of ZnO in organic phase. Interestingly, upon thermal annealing treatment at 140°C(mesomorphase temperature), the enhancement in crystallinity and ordering of the P3 HT polymer chain is induced by self-assembly behavior of DPP-TP6 liquid crystalline molecules, improving the orderness of P3HT/ZnO bulk-heterojunction and offering a more efficient path for carrier transport. Meanwhile, induced by the liquid crystals ordering, the micromophology of P3HT/ZnO hybrid system is also improved and nanoscale microphase separation is obtained. On this basis, the PCE of P3HT/ZnO hybrid solar cells is improved mainly due to the increase of short circuit current density(Jsc) and fill factor(FF). Furthermore, absorption enhancement caused by narrow bandgap liquid crystal DPP-TP6 can also contribute to the increase of Jsc. Overall, introducing narrow bandgap liquid crystal into P3HT/ZnO is a efficient approach to optimize micromophology of hybrid thin film and improve solar cell device performance.