EXCITON DIFFUSION LENGTH IN MEH-PPV AND CdS NANOPARTICLES SIZE DISTRIBUTION BY SURFACE PHOTOVOLTAGE METHOD

摘要

The exciton diffusion length is a crucial parameter characterizing polymer materials for solar cells.Method of surface photovoltage (SPV) used here for its evaluation is simple and quick. It utilizes an electric field atsurface of the sample for dissociation of photogenerated excitons, which can then lead to generation of the photocurrentin an external circuit. In our case, such field is formed in the space charge region (SCR) at the ITO/polymer interface.The advantage of our SPV model is possibility to apply it to samples with arbitrary ratio of thickness of the SCR and ofthe bulk. The samples under study were MEH-PPV layers prepared by spin-coating technique using CHCl_3 or THFsolvent. Ellipsometric measurement yielded data on thickness of the samples, absorption coefficients needed forevaluation of SPV spectra were obtained from transmittance and reflectance measurements. Fitting of theory toexperiment yielded the exciton diffusion lengths (13-15 nm). CdS nanoparticles were synthesized by thermaldecomposition of cadmiumethylxanthate in triethanolamine or from cadmium chloride and ammonium thiosulfatesolutions. Sigma-Aldrich CdS nanocrystals were also studied. TEM and SPV techniques were applied to characterize thesize of the nanoparticles.