Polymer And Hybrid Polymer/Amorphous Silicon Thin Film Solar Cells

Polymer solar cells (PSC) hold tremendous attention due to their environmental friendly, scalable and low cost manufacturing and their mechanical flexibility. However, although the power conversion efficiency (PCE) of the polymer thin film solar cells has been improved gradually, there are still lots of issues remained. Current work focused on a number of key physical and technical issues in state-of-the-art PSCs and made progresses as follows:First of all, the crystallization and phase separation of poly(3-hexylthiophene) were investigated by a combination of techniques such as differential scanning calorimetry (DSC), wide angle X-ray diffraction and small angle X-ray scattering. It was discovered that high temperature or low temperature crystallites were developed during the isothermal crystallization in P3HT matrices at different conditions. Both high and low crystallites are of same crystalline structure but different order and size, which related to the crystallization temperature. The population of high temperature crystallites can be manipulated by variation of isothermal crystallization conditions. The small angle X-ray scattering revealed a progressive phase separation at appropriate crystallization temperature and/or prolonged crystallization times. Furthermore, phase behaviors of P3HT doping with PC61BM were also investigated and compared. It was demonstrated a much similar crystallization behavior during the isothermal crystallization. The P3HT:PC6iBM mixtures also formed high and low temperature crystallites with similar crystal structure. The understanding of the crystallization phase behavior of the polymer materials may offer a guidance to engineers in device fabrications.Secondly, hybrid polymer-amorphous silicon thin film solar cells combined low cost in fabrication and potentially flexible of the both along with a relatively high charge carrier mobility and high stability of amorphous silicon. It forms a novel but a challenge solar cell technology at its very preliminary developing stage.In current work, it is the first time to fabricate the hybrid P3HT/amorphous silicon heterojunction thin film solar cells on flexible substrates. A number of hybrid P3HTAsr-Si:H thin film solar cells have been fabricated in combination of P3HT with intrinsic amorphous silicon (i-Si), n-doped amorphous silicon (n-Si) and p-doped amorphous silicon (p-Si) thin film(s). It was found that the polymer layer contributed to the external quantum efficiency (EQE) of the hybrid solar cells was dependent on the order of the polymer and amorphous silicon layers fabricated. However, if using P3HT:PC6iBM to replace the pure P3HT in the hybrid solar cells, it was discovered that the polymer layer contributed the most strongly in EQE spectra of the hybrid solar cells regardless of the order of the active layers.The third, a breakthrough in hybrid polymer/a-Si thin film solar cells was achieved which incorporated poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), a polymer with a relatively high carrier mobility with the aim to match the charge carrier mobilities between the polymer and amorphous silicon. In current work, the best hybrid solar cell performance was achieved with a device structure of PEDOT:PSS/i-Si/n-Si as the active layer. The short current density (Jsc) reached 14.08 mA/cm2 and the PCE,4.78%. This is the highest PCE for the polymer/amorphous silicon hybrid solar cells of the similar structure so far reported. It demonstrated that a good matching of charge carrier mobilities of polymer and amorphous silicon can be the critical issue in the hybrid polymer/amorphous silicon thin film solar cells.The fourth, the thin metal oxides films of In2O3, ZnO, ZnO:Al, ZnO:Ag were fabricated using solution processes and annealed at a relatively low temperature, which were used as electron transport layers (ETL) in inverted solar cells. Bilayer of In2O3-ZnO, In2O3-ZnO:Al and In2O3-ZnO:Ag were also investigated for the first time. Bandgap and work function of the films were investigated using absorption spectroscopy and Kelvin probe station. The charge carrier transport mobilities of the In2O3-ZnO, In2O3-ZnO:Al and In2O3-ZnO:Ag layers were measured via the bottom-gate top-contact thin film transistors. These metal oxides films were evaluated to use as ETL in inverted solar cells and relatively high PCE was achieved.Finally, the CuI(I) has been investigated as the hole transport layer (HTL) to replace PEDOT:PSS in polymer bulk-heteroj unction solar cells successfully. The EQE of the devices and the energy level of the thin films were measured as a powerful tool to reveal the functionalities. In a conventional structured polymer solar cells, the PCE with CuI(I) used as HTL was comparable with that using PEDOT:PSS. It can be applied also in inverted polymer thin film solar cells in combination with the low temperature solution-processed metal oxides thin film In2O3-ZnO as ETL and achieve high performance.