Amorphous and microcrystalline silicon thin films grown by photon assisted electron cyclotron resonance chemical vapor deposition for heterojunction solar cells and thin film transistors.

摘要

The role of light illumination (5--10 W/cm2) onto substrates (photon assist (PA)) during the hydrogenated amorphous silicon (a-Si:H) growth by electron cyclotron resonance chemical vapor deposition (ECR-CVD) has been investigated in terms of chemical bonding, light absorption, carrier mobility, carrier lifetime, dark and photoconductivity, stability and photovoltaic properties. Compared to the film grown without PA, the film deposited by PA has an improved hydrogen-related chemical structure resulting from the enhanced decomposition of weak Si-H bonds and superior surface diffusion of atomic species during the film formation. The PA process gave less dihydride content, higher photoconductivity, improved carrier lifetime, higher light absorption in the solar spectrum, and 30% and 70% increases in field effect mobility (mufe) and undoped a-Si:H/p-type crystalline silicon (c-Si) solar cell efficiency, respectively.; Carrier transport mechanisms in undoped a-Si:H/ p-type c-Si heterojunctions with and without a microcrystalline silicon (muc-Si) buffer layer was studied. The recombination process involving the interface states on the a-Si:H/mu c-Si side dominates at low forward bias (V 0.3 V), whereas multi-step tunneling capture emission (MTCE) dominates in the higher bias region (0.3 V 0.55 V) until the conduction becomes space charge limited (V> 0.55 V). Despite the domination of MTCE in the indicated voltage range, the reduced band offset at the interface increases current levels by the enhanced diffusion and/or emission processes.; Low-cost thin film silicon/c-Si heterojunction solar cells and thin film transistors (TFTs) were developed and investigated. As an emitter in the cell, a-Si:H thin films, single or multi-layer mu c-Si thin films and their combinations were examined and compared in terms of cell structure, thin film silicon growth conditions, interface and throughput. Three different designs including a-Si:H (850 A)/c-Si, multi-layer muc-Si (700 A)/c-Si, and a-Si:H (700 A)/mu c-Si (200 A)/c-Si provided the efficiency of 9--10%, without an anti-reflective coating.; Finally, the dependence of TFT performance on the device structure and silicon thin film material properties was studied. Among various designs, the TFT with an a-Si:H active layer generally showed superior properties (mufe ∼ 0.63 cm2/V-s, Ion/Ioff ∼ 106 ), possibly due to a lower defect density in the a-Si:H film. TFTs with a muc-Si active layer showed comparable or degraded mufe values, smaller I on/Ioff ratios, and threshold voltage (Vt) values compared with the a-Si:H based TFT. The decreased Ion/Ioff ratios and Vt values could be explained by the larger conductivity and more n-type nature in the muc-Si film.