A Study On The Passivation Layer In α-Si/C-Si Heterojunction Solar Cells

Amorphous silicon/crystalline silicon heterojunction solar cells have a greatpotential in development of energy conversion efficiency. They have also advantages ofgood response to weak light and lower temperature coefficient. They are now regardedas an important orientation for future development of wafer-based solar cells. Thishas been confirmed by commercial production of this type of high efficiency solar cellsby Panasonic of Japan. However, the detailed technologies for fabrication of this typeof solar cells have so far been tightly blockaded. This makes the R&D for amorphoussilicon/crystalline silicon heterojunction solar cells not only a great challenge forphotovoltaic academics and industry in the world, but also of a great potential value todevelopment of PV industry in China. Passivation of the hetero-interface betweenamorphous silicon films and crystalline wafer substrates has been identified as one ofthe key factors for amorphous silicon/crystalline silicon heterojunction solar cells. Thebasic known passivation technology is to deposit a high quality intrinsic amorphoussilicon thin film on the crystalline silicon substrate as a passivation layer. The presentthesis reports our studies on the passivation layers. The main stream passivation layerbased on hydrogenated intrinsic amorphous silicon (α-Si:H) thin film is adopted. Adomestic plasma enhanced chemical vapor deposition (PECVD) system is employed.The major studies and results are listed as following:（1）Effect of rounding the pyramid structures of textured monocrystallinesilicon wafers on passivation by α-Si:H thin films (Chapter2). HF/HNO3mixedacidic polishing is used for rounding textured pyramidal monocrystalline siliconstructures, and varying dipping time of wafers in mixed acid should control roundinglevel of pyramidal structure. Relative decrease of roughness Rz is used toquantitatively characterize the degree of rounding, DR. Improvement of thepassivation effect of α-Si:H films deposited by PECVD from the rounding of thetextured wafers has been experimentally studied. The results indicate a basicallylinear increase of the minor carrier lifetimes with increase of DR value, except theinitial extraordinary phenomenon. The relative improvement with thinner films isfound to be more sensitive to the DR value. Dependence of light reflectivity on the DR value is measured, and a linear relationship is identified. A typical result is:260%relative increase of minor carrier lifetime can be achieved by a rounding with DR of6%, when the passivation film is7nm thick. A3%absolute increment in lightreflectivity of the naked rounded surface is also generated.（2）PECVD processing of a-Si:H thin films on monocrystalline silicon and itsoptimization for passivation of the monocrystalline silicon wafers (Chapter3). Withthe deposition rate decreasing and the substrate temperature increasing, thepassivation effect improves, but to a certain extent, will deteriorate further. Lowerdeposition rate and higher substrate temperature lead to more dense and orderlystructure of deposited films, until transiting to crystallization. These finding reveal theshort-range order and high density of α-Si:H film structure are benefit to passivationeffects, until passivation effects deteriorate once the film start to epitaxy on wafersubstrate. Trace air leak of PECVD system was also found experimentally passivationgain, the reason may be due to trace amounts of oxygen. The best minority carrierlifetime value of2951microseconds obtained on a commercial Cz-Si wafer samplepassivated with α-Si:H films represents a surface recombination rate less than2.4cm/s, which shows that domestic equipment can achieve excellent passivation effect.（3） PECVD processing of hydrogenated amorphous silicon suboxide(α-SiOx:H) thin films and their passivation effect (Chapter4). Incorporation of CO2on the base of silane and hydrogen mixed sources gases to form α-Si:H films duringPECVD process, the α-SiOx:H films are successfully prepared. The passivation effectof α-SiOx:H films is similar to α-Si:H films, and the formers deposition pressureprocess window is notably widened than the latter. This is very meaningful forα-Si:H/c-Si heterojunction solar cell industrialization.（4）Passivation effect of stacked p-type doped α-Si:H thin films deposited onintrinsic α-Si:H thin films (Chapter5). To form emitter layers of solar cells, B-dopedp-type α-Si:H films are deposited on5nm thick intrinsic α-Si:H films or α-SiOx:Hfilms, respectively. With the doped layer stacked on passivation layer, the passivationeffect further improves, then enhances significantly after annealing samples.