Study Of Pin-type Ultrathin Amorphous Silicon Thin Film Solar Cells Deposited On MOCVD-ZnO:B Front Contact

pin-type silicon based multijunction thin film solar cells has already shown high productive potential because of its excellent device properties, such as high conversion efficiency, low production cost, high stability. Recently, the research topic has been focused on fully utilizing the wide solar-spectrum. Generally, pin-type amorphous silicon thin film solar cells directly deposited on front contact is used as top cell. Along the direction of incident light, this top cell is located at the front of optical path. Its output characteristic has a huge influence on the performance of multijunction solar cells. In this thesis, high-haze boron doped zinc oxide deposited by metal organic chemical vapor deposition was used as front contact in amorphous silicon thin film solar cell with an absorber layer thickness only around150nm. The corresponding study can be divided into three parts as follows:First of all, the relationship between the electrical properties of amorphous silicon solar cell(Open circuit voltage; Fill factor) and the as-grown sharp edges of the pyramids at the rough texture surface of ZnO:B was carefully investigated. The result demonstrates that, the V-type surface morphology can cause large defect regions in amorphous silicon material, increase the device current leakage. Argon plasma post-treatment method was introduced to tailor the surface morphology from V-type to U-type, improve the open circuit voltage and fill factor. Unfortunately, this etching process can reduce the light trapping capability. In this thesis, we developed a new dry etching method to modify the surface of ZnO:B thin film:hydrogen and methane mixed gas (H2/CH4) plasma post treatment method. With this etching process, The light trapping capability of ZnO:B thin film in wide solar-spectrum improved simultaneously. The ultrathin amorphous silicon thin film solar cells fabricated on this substrate demonstrated excellent electrical properties. In order to obtain more information about this etching method, the plasma parameters were adjusted to investigate the change of etching process.Secondly, the influence of device interface properties on the open circuit voltage and fill factor was also focused on. Different optimized methods has been introduced to overcome the TCO/p contact potential and the comparison of those methods was also listed in the fourth chapter. The result presented that both the tunneling current and the decrease of potential barrier can both improve the open circuit and fill factor. The other part of fourth chapter was to study the p/i contact properties. The potentail barriers at the bottom of conduction band and the top of valence band were compared both from simulation and experiment. It was found that the former p/i potential barrier showed larger influence on the solar cell electrical performance. With the purpose of improving the p/i contact properties, large bandgap intrinsic layer material a-SiC:H was inserted into p/i interface and the mechanism behind this interface layer has been deeply discussed.Finally, the doped microcrystalline silicon oxide material (p-layer; n-layer) was introduced in ultrathin amorphous silicon thin film solar cells. The distinctive properties of those doped materials can be summarized into two points:firstly, the in-plane conductivity was smaller by five orders of magnitude than the transverse conductivity. As comparison, the in-plane conductivity of doped amorphous silicon is almost same as the transverse conductivity. Secondly, the doped microcrystalline silicon oxide material consisted of two different crystalline phases:microcrystalline silicon and amorphous silicon oxide. Those two phases distributed separately in the whole material region. On the other hand, we also focused on the performance of solar cells when those doped materials were used. It was presented that, the electrical properties of solar cells deposited on very high-haze front contact successfully avoided the deterioration of open circuit voltage and fill factor. With the optimization of those doped materials, the conversion efficiency of single ultrathin amorphous silicon solar cells can reach up to7.76%for an absorber layer thickness only around140nm(Voc:911mV;Jsc:12mA/cm2; FF:71%).