The Research Of Photo-generated Carriers Transport Mechanism In SIS Structure Devices

Semiconductor-insulator-semiconductor(SIS) heterojunction devices attracted lots of attention for the advantages in simple structure, low cost, low temperature manufacture, stable & considerable photon-electron conversion efficiency, and so on. In this paper, the working principles, carriers transport mechanism,photovoltaic effect, temperature- & time-dependent conversion efficiency, and “S” shape current-voltage characteristics of SIS devices were studied, based on the semiconductor physics & device principle of solar cells, and semiconductor manufacture & analysis technology. Furthermore, we developed a novel integrated system using on-wafer bypass diode and high efficiency Si-based solar cell through incorporating SIS device into HIT solar cell. The research methods and major results are summarized as follows:(1) A simplified heterojunction of ITO/SiOx/n-Si solar cells were successfully fabricated by radio-frequency magnetron sputtering, possessing the best photovoltaic parameters that concluding conversion efficiency of 11.5%, open circuit voltage of 0.55 V, short circuit current density of 34.22 mA/cm2, and fill factor of 74.13%, respectively. The current-voltage characteristics, chemical states and microstructure of devices were investigated. The results indicated that the build-in-potential was successfully established in SIS devices, derived from the difference of work function between ITO and n-Si. In the meantime, the upward bending of energy band near the surface region of n-Si substrate, induced by space charge of n-type dopants, is acting as an inversion layer and emitter. And the inversion layer together with n-Si base would make up a quasi p-n junction in SIS devices. Furthermore, it was also found that the interfacial ultra-thin SiOx(0 < x <2) layer, fabricated by shallow ion implantation intermixing, was able to enhance the hole-like carriers tunneling. The obtained results would play an important role in improving conversion efficiency of SIS structure solar cells.(2) Temperature dependent photovoltaic properties of ITO/SiOx/n-Si devices were investigated in low temperature control system under illumination of 405 nm laser, halogen tungsten lamp and standard one sun spectra simulator, respectively. The results showed that the low temperature properties of Si material, temperature dependent property of bandgap, shift of Fermi energy level and phonon involved absorption for indirect bandgap, resulted in open circuit voltage decreasing and short circuit current increasing with temperature. On the other side, the high-temperature and time- dependent current- voltage characteristics of solar cells were investigated, which performed a good temperature coefficient of-0.34%/℃ and photovoltaic stability over past 355 days. The results in these investigations indicated that ITO/SiOx/n-Si structure solar cells would be potential for the application in generating electricity.(3) The phenomenon of “S” shape current-voltage characteristics in both AZO/SiOx/n-Si and ITO/SiOx/n-Si devices were analyzed based on the working principle and carriers transport mechanism of SIS structure devices. The results show that photo-generated hole carrier and electron carrier dominate the forward current and reverse current, respectively. And, because of the thicker interfacial layer blocking the transport of both hole and electron carriers, carrier transports could be equivalent to two opposite polarity diodes in parallel connection and connected with PV diode in series. Based on the equivalent circuit of device, the equation of “S” shape current-voltage characteristics was reasonably deduced, which could successfully describe the abnormal curve of SIS devices. The theoretical explanation of “S” shape curve in SIS devices would be an important reference in developing similar structures, especially new types of heterojunction PV devices.(4) An integrated system through bypass diode- high efficiency solar cell was fabricated through incorporating SIS structure of outstanding rectification characteristics into HIT solar cell of high conversion efficiency. The results show that the integrated system possesses definite PV characteristics from HIT structure at forward voltage and high leakage current from SIS structure at reverse voltage. The novel integrated structure of bypass diode- solar cell would provide new ideas and methods in research and development on photovoltaic device.