Correlation of morphology, structure, and function in bulk heterojunctions solar cells with controlled solvent vapor exposure

Organic photovoltaics (OPVs) have attracted considerable interest recently as a lightweight, low-cost and easily-fabricated replacement for inorganic photovoltaics. The conversion of solar energy to electricity in OPVs occurs in three consecutive steps: the generation of excitons, the dissociation of excitons into free charge carriers, and the transportation of electrons and holes to the electrodes. Research shows that the initial structure of the active layer in OPVs upon casting, its evolution to equilibrium, and final equilibrium structure upon annealing dramatically impact photovoltaic function. In this presentation, recent work in our group that examines the change in morphology and depth profile of P3HT:PCBM bulk heterojunctions with controlled exposure to CS ₂ vapor will be discussed. A meter-long column with a solvent vapor pressure gradient is used to control the extent of solvent vapor exposure. These studies show that the crystallinity and crystal size of the P3HT increase initially with solvent vapor pressure and annealing time, but longer exposure to solvent decreases P3HT crystallinity and photovoltaic efficiency. Neutron reflectivity indicates that the PCBM segregates to the Si substrate in the as-cast thin film, but migrates to the surface with solvent annealing. The structural variation with solvent exposure is correlated to photovoltaic function, which shows that the solvent annealing provides a window of optimum efficiency. Moreover, the control of depth profile and structure should be generally applicable to a broad range of polymer-nanoparticle mixtures and thus these results provide fundamental information that enables control of the depth profile, morphology and function of thin film nanocomposites.