Novel ferroelectric-semiconductor photovoltaics

Abstract

Solar cells have been traditionally developed for optimizing three key steps for charge carriers: generation, separation, and transport. Conventional solar cells are essentially PN junction based, and utilize the internal electric field near the junction interface for realizing charge carrier separation. However, this kind of structure limits material choices and device fabrication to form a working junction due to issues such as lattice mismatch, doping, and band alignment. Ferroelectric photovoltaic devices with typical capacitor structure have been developed to overcome the junction caused disadvantage but suffer from the poor charge transport issue. In this work, novel ferroelectric-semiconductor photovoltaic devices were developed and investigated in detail with experimental results and theoretical simulation. This type of solar cell is fundamentally different with traditional PN junction based solar cells, utilizing ferroelectric polarization for charge separation in semiconductor layer. Systematical works have been conducted on: (1) device working principle and mechanism study; (2) effect of electrode; (3) influence of device key dimension parameters. The new cells showed the rectifying behavior and effective photovoltaic effect after specific asymmetric polarization. Furthermore, the device performance has been improved through adjusting electrode design and semiconductor layer thickness, which is mainly due to the optimized electric field strength and distribution resulting from polarization.

As low cost commercial semiconductor, the multicrystalline silicon (mc-Si) has great potential application in the novel ferroelectric-semiconductor photovoltaic devices. However, the grain boundaries with high density of defects limit the material electric properties. In order to improve the multicrystalline silicon transport property, a polar molecules system was developed to play the role in grain boundaries passivation. The small polar molecule composition and solution passivation process were carried out to optimize the passivation effect. The result showed the developed ZK series solutions reduced the Rsheet across large-angle grain boundaries by up to more than one order to be close to the bulk Rsheet. Also, the correlation between the grain misorientation and passivation effectiveness was built up.