Transient Photocurrent Characterization Of Organic Solar Cells And Its Application To The Study Of Defect States

Organic solar cells have been attracting more and more attention, due to its light-weight, flexibility and potential for large scale, low-cost fabrication. Nowadays, the power conversion efficiency of organic solar cells is still low compared to traditional silicon based inorganic solar cells. One of the main reasons is that the defect states distributed within organic solar cells could affect charge carrier dynamics and therefore affect the power conversion efficiency. Therefore, scientifically and practically, it is valuable to study the defect states distribution and its effects in organic solar cells.In this dissertation, nanosecond time scale transient photocurrent(ns-TPC) characterization method is used in combination with AC impedance characterization method to study the defect states distribution and its effects on charge carrier dynamics in typical organic solar cells, namely, planar-heterojunction small molecule organic solar cells and bulk-heterojunction polymer solar cells. The following are the two works of the dissertation:(1) A simple method to experimentally determine the accurate RC-constant in ns-TPC measurements on typical organic solar cells. In this work, typical planar-heterojunction small molecule organic solar cells and typical bulk-heterojunction polymer solar cells with the structure of ITO/CuPc/C60/Alq3/Al and ITO/PEIE/P3HT: PC61BM/MoO3/Ag are fabricated. A waveform generator is used to apply a square voltage pulse of 20 mV pulse amplitude and 10 μs pulse width to the ns-TPC test circuit, and the RC-constant of the ns-TPC test circuit is determined by simply analyzing the transient response of the test circuit after the square voltage pulse excitation. In order to verify the correctness of the experimentally determined RC-constant, three verification methods, which are also experimental but only valid under specific conditions, are selectively adopted to verify whether the experimentally determined RC-constant is reliable. And the result is positive.(2) Study of defect states distribution and its effects on charge carrier dynamics in typical bulk-heterojunction polymer solar cells with the structure of ITO/PEIE/P3HT: PC61BM/MoO3/Ag by ns-TPC characterization method. Temperature variation ns-TPC signals of the solar cell from 80 K to 300 K are simulated according to the transfer matrix method and the multiple trapping and release model. The defect states distribution and its effects on charge carrier dynamics are simultaneously determined. In order to make the analysis more reliable, complementary AC impedance characterization method is also used to study the defect states distribution within the solar cell.