Experimental Studies On Internal Photovoltaic Processes In Polymer/Perovskite Solar Cells By Using In-Situ Measurements

Polymer solar cells and perovskite solar cells have become an attractive component in photovoltaic devices based on possible fabrication, processibility facile tuning of structure, low-cost and mechanically flexible properties. Recently, the research and application of polymer solar cells and perovskite solar cells have made great progress. The recent studies and developments indicate that we need to essentially reveal the internal photovoltaic processes under device working condition in polymer solar cells and perovskite solar cells. This will provide an in-depth understanding on critical parameters in coltrolling optical absorption, charge dissociation, charge transport and collection, and further develop the photovoltaic advancements in polymer solar cells and perovskite solar cells. In this thesis, we use in-situ experiment tools:light-induced dielectric response, transient photocurrent, electric field-induced photoluminescence quenching, and scanning kelvin probe microscopy to investigate the dynamic processes of excited states dissociation, charge transport and charge collection under device working condition. This discussion can reveal and elucidate useful and non-useful processes in coltrolling the optical absorption, short-circuit current and open-circuit voltage. More details are listed as followed:The internal electrical polarizations are studied by using optical absorption and capacitance-voltage measurements upon thermal annealing in the P3HT:PCBM solar cell. It is found that thermal annealing can increase absorption intensity of P3HT:PCBM film and the crystallinity of P3HT and aggregations of PCBM molecules, thus improve the donor and acceptor interpenetrating networks, lead to an enhancement on device performance. Furthermore, the enhancement of absorption coefficient indicates stronger electrical polarizations in the P3HT:PCBM film, which can lead to an increase on the generation of charge carriers at donor:acceptor interfaces, and the transport of generated charge carriers to respective electrode interfaces in the P3HT:PCBM device. Our experimental findings suggest that local electrical polarizations play an important role in the development of photovoltaic processes in organic solar cells.The charge transport layer effects on charge accumulation and collection in polymer solar cells are studied by using light intensity-dependent current-voltage characteristics and light-induced dielectric response measurements. It is found that the fill factor of low-conductivity transport layer device continuously decreases with increasing light intensity. This means that the low-conductivity transport layer device experiences serious charge recombination. The high-conductivity charge transport layer can decrease the surface accumulation at electrode interfaces with the consequence of increasing charge transport and interfacial charge collection, sequentially improve the device performance.The mutually connected bulk and interface parameters involved in light soaking and hysteresis effects are studied. It is found that the perovskite solar cells can exhibit hysteresis and reversible light soaking phenomena, which can decrease the charge accumulation at the electrode interfaces, lead to an enhancement of open-circuit voltage. Light soaking can reduce the density of positively charged bulk defects within the perovskite layer through neutralization by photogenerated electrons upon light illumination. In particular, light soaking can decrease bulk-electrical polarization within the perovskite film. Decreasing the bulk electrical polarization can reduce charge dissociation and increase the charge recombination, consequently generate a decrease on short-circuit current. Clearly, the interfacial polarization and bulk polarization are internally coupled through built-in electric field to control the light soaking and hysteresis effects in perovskite solar cells.The light soaking and hysteresis effects can be suppressed by incorporating an inferfacial layer of PEIE. The electro-photoluminescence and capacitance-voltage measurements provides the direct experimental evidence that the PEIE interlayer forms interfacial dipoles at the PCBM/Al interface. The interfacial dipoles can enhance the effective electric field within the device, which can increase the charge dissociation, transport and collection, lead to a power conversion efficiency of 12.62% in perovskite solar cells. Furthermore, interfacial dipoles can suppress the light soaking and hysteresis effects by decreasing charge accumulation at electrode interface and ion migration within the perovskite film.The additive effects on charge recombination in perovskite solar cells are studied by using transient photocurrent and light intensity-dependent current-voltage measurements. The optical absorption and surface potential of perovskite film can be enhanced with the additive DIO. It indicates that the presence of DIO can increase the crystallinity and local electrical polarizations of perovskite film, thus improve the charge transport and collection, lead to an enhancement on device performance. Furthermore, DIO can significantly decrease the charge extraction time and increase the charge extraction efficiency, consequently suppress the trap-assisted recombination.