Graphene-Based Hybrid Materials For Solar Cells

Hybrid polymer solar cells （HPSCs） based on conjugated polymers as electron donor and inorganic semiconductor nanocrystals as electron acceptor are attractive for low-cost solar cells, and they combine the advantages of both inorganic （e.g., tunable absorption, high charge mobility, good chemical stability） and organic （e.g., light-weight, good flexibility, easy film-formation） materials. In this dissertation, we study the formation mechanism of graphene-based hybrid materials and the performance in their HPSCs. The main contents and results are summarized as follows:（1） The hybrid featuring the aggregrates of CuInS₂ quantum dots （CuInS₂-QDs） on reduced graphene oxide （rGO） sheets （i.e., rGO/CuInS₂-QDs） is synthesized by a one-pot solvothermal approach, and the growth mechanism of the hybrid is elucidated. It is found that adsorption of Cu²⁺ cations mainly takes place at epoxy/hydroxyl groups on graphene oxide （GO） sheets at room temperature and the adsorbed Cu⁺ cations resulting from the Cu²⁺ reduction at solvothermal temperature act as the nucleation for the surface growth of CuInS₂-QDs into three-dimensional aggregates on GO sheets that are simultaneously reduced into rGO. The experiments results show that the existence of the CuInS₂-QDs aggregates also promoted the reduction of the GO.（2） We use the blend of rGO/CuInS₂-QDs hybrids and polymer MEH-PPV to prepare the photoactive layer for HPSCs, and study the feature of charge generation and transportation in them. Result show that rGO/CuInS₂-QDs hybrid is an effective hybrid electron acceptor, and the devices exhibit a power conversion efficiency of 1.5%; Compared to the counterpart HPSCs based on rGO, rGO/CuInS₂-QDs provide not only a complementary absorption but also an higher efficiency of polymer exciton dissociation, the rGO-CuInS₂ devices exhibit significantly increased electron transit time and electron lifetime, with an open-circuit voltage predominatly determined by the energy difference between the highest occupied molecular orbital level of MEH-PPV and the work-function of rGO but significantly related to the electron concentration in rGO.（3） The hybrids featuring Mn₃O₄ nanoparticles on rGO sheets （i.e., rGO/Mn₃O₄） is synthesized by one-pot solvothermal approach, and the HPSCs with a power conversion efficiency of 1.04%are prepared by blending them with polymer MEH-PPV. Result show that, with increasing the Mn₃O₄ content, the morphology of Mn₃O₄ nanoparticles on rGO sheets changes from a dispersing state into three-dimensional aggregates, but the device performance becomes worse for which the reason is mainly attributed to the increasing interfacial charge recombinantion and the reduced electron lifetime as a results of the poor conductivity in Mn₃O₄ nanoparticle aggregates.