| Inorganic/organic hybrid perovskite, with structures comprised of alternating layers of corner-sharing metal halide octahedra and organic cation layers, represent a family of promising materials, enabling the assembly of organic and inorganic components within a single molecular-scale composite for high-performance semiconductor devices, especially photovoltaic cells. Recently, millimeter-scale crystal grains achieved by a solution-based hot-casting technique has promised intriguing optoelectronic properties, such as excellent crystallinity and very large carrier mobility comparable to that of crystalline silicon. Also with tuning the perovskite chemical composition, the energy level can be readily controlled, that offers the flexibility towards electron/hole selective materials with different band energies for solar cell applications. Thus, solution-processed PVSK based solar cells can rapidly achieve 20.1% power conversion efficiencies, following the initial report in 2009. Now the perovskite technology has been regarded as a serious player in the photovoltaics, which is able to compete with/against crystalline silicon solar cells.In this work, we systematically explored high performance perovskite devices with different architactures. More importantly, photogenerated charge behavior has been carefully probed to illustrate the device working mechanism by using electrochemical impedance spectra, photovoltage/photocurrent decay, Kelvin probe force microscopy(KPFM), and capacitance-voltage(C-V) analysis. Meanwhile, our results show carbon material can work efficiently in perovskite device as counter electrode and we fabricated PSC devices by using low-cost and robust mesoporous TiO2 and NiO layers as electron and hole selective contacts with doctor-blade technique, respectively. In such devices, a mesoporous ZrO2 layer(insulator) was adapted as a space separator between TiO2(n-type semiconductor) and Ni O(p-type semiconductor) layers to realize a p-i-n layer configuration, which has better performance than devices with p-n configuration. And the high device stability may arise from the hydrophobic property of carbon material and a large amount of CH3NH3PbI3(MAPbI3) grains in the porous of carbon counter electrode. And in electron(hole) selective layer free devices a negligible built-in potential was formed across the perovskite/ITO heterojunction, meaning that the built-in potential in this heterojunction is not essential for electron extraction. Moreover, a large capacitance in such device is measured, and the enhanced charge storage capacity should ensure the photovoltage and provide concentration carrier concentration gradient that drives charge transfer from perovskite to ITO. Though p-i-n type device structure still dominates the field, we argue alternative device design without electron/hole selective contact is promising according to our previous success in hole transport layer free devices.And we report an investigation on hysteresis effect in TiO2 based planar heterojunction PVSK photovoltaic cells by considering the contribution from ion motion, charge carrier selective contacts, trap/de-trap processes, and ferroelectric properties. Systematic experiments, including trap energy modification and perovskites with different cations, were adopted for exploring material character on hysteresis, while the influence of charge transportation in devices was estimated by applying different hole transport materials(HTM). Results clearly reveal that J-V hysteresis in PVSK solar cells mainly originates from ion motion of the organic cation component responding to the external electric field. Further, we find that a shallow defect with energy level of almost 80 meV in the bulk perovskite are not the origin for hysteresis. Similarly, ferroelectric effects show negligible impact on hysteresis when CH3NH3PbI3 devices operate at room temperature, being higher than the Curie temperature of CH3NH3PbI3(~180 K).For the stability, our results show two different dynamic in perovskite degradation. For CH3NH3PbI3, it degraded to Pb I2 due to thermal instability with the acceleration of moisture in air while the CH(NH2)2PbI3(FAPbI3) reacted with moisture giving a new material. However, no matter which degradation will cause the decline in power convertion efficiency. |
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