| dc.description.abstract | With the rapid development of science and technology, the sustainable energies get more and more attention, and the photovoltaic without pollutions is regard as the most important sustainable energy in the future. In the many kinds of solar cells, Si based solar cell has advantages of low cost and mature technology, and the Si hetero-junction (SHJ) solar cell can achieve the highest efficiency of 25.6 % in 2015. In this dissertation, we use homemade electron cyclotron resonance chemical vapor deposition (ECR-CVD) method with high plasma density characteristic to develop several films such as: amorphous hydrogenated silicon carbide (a-Si1-xCx:H), boron doped hydrogenated nanocrystalline silicon (B-dopd nc-Si:H), and epitaxial germanium (epi-Ge) for SHJ solar cell application and novel III-V/Si tandem solar cell development. At first, we complete Si:H phased diagram to demonstrate the high plasma density of ECR-CVD, which is a significant result to further adjust growth parameters to develop new thin films. Then we grow and investigate structural, optical, electrical, and passivation qualities of a-Si1-xCx:H and B-dopd nc-Si:H. We provide another choice of passivation and emitter layers for a SHJ solar cell. Furthermore, the Ge films development opens the newly created application for III-V/Si solar cell development. Epitaxial 40 nm thick Ge on Si with smooth surface can be grown at an ultra-low temperature of 180 ˚C. Such a thin Ge on Si is suitable being the buffer layer for III-V/Si tandem solar cell with theoretical conversion efficiency above 30 %.
Si:H film phase diagram has been completed by ECR-CVD at a low temperature of 180 ˚C. The amorphous to microcrystalline phase transition occurs at a lower hydrogen dilution ratio (H2/SiH4= 1) due to the high plasma density of ECR-CVD, compare with conventional PECVD process with higher dilution ratio (H2/SiH4~ 10). Then we introduce the CH4 and B2H6 gases to further deposit a-Si1-xCx:H and B-doped nc-Si:H for SHJ solar cell, respectively. The typical structure of SHJ solar cell is used a-Si:H as passivation and emitter layers grown at a low temperature (< 200 ˚C) by PECVD. However, the bandgap of a-Si:H is about 1.85 eV which will reduce photo generation at the short wavelength. The a-Si1-xCx:H film has a higher optical bandgap is a good solution, but it usually needs a high temperature above 300 ˚C to achieve a high quality by PECVD. We can use low temperature of 180 ˚C, satisfied SHJ solar cell requirement, to obtain an a-Si1-xCx:H film for a passivation layer. The film, having an excellent passivation quality (Lifetime= 680 sec, surface recombination velocity= 21 cm/sec) and lower absorption coefficient than a-Si:H, can keep the Voc performance and reduce the optical loss in a passivation layer meanwhile. In the other hand, the a-Si:H has a poor conductive property so that the SHJ solar cell needs a highly conductive transparent conductive oxide layer to reduce resistance loss. For the B-doped nc-Si:H deposited by ECR-CVD, the formation of nanocrystal results a high conductivity for resistance reduction, and the a-Si:H matrix passivates the p/i interface for Voc preservation. We also apply them to fabricate planar Ag grid/ITO/nc-Si:H(p)/a-Si1-xCx:H (i)/c-Si(n)/Ag SHJ solar cell without back surface field and passivation layer at rear side, and the conversion efficiency is 13.66 % (Voc= 518 mV, Jsc= 37.95 mA/cm2, FF= 69.5 %).
Ge on Si is attracted attention recently because of several applications like: infrared photodetector, Ge laser, solar cell, and monolithic integration of III-V semiconductor devices on c-Si. Usually, the growth of epi-Ge on Si needs high growth temperature (> 600 ˚C) and thick thickness (> 200 nm) by conventional epitaxy process like ultra-high vacuum chemical vapor deposition (UHV-CVD) or molecular beam epitaxy (MBE). However, the high temperature process limits the materials or devices integration, and enhances the thermal budget. ECR-CVD method with high plasma density can solve these problems. We first grow highly conductive B-doped Ge:H films to realize the growth mechanism of it, and then successfully grow a 40 nm thick Ge on Si at an ultra-low temperature of 180 ˚C. The film, having a good crystal quality (XRD FWHM= 683 arcsec) and smooth surface (RMS roughness= 0.342 nm), is suitable being a buffer layer for III-V/Si tandem solar cell to solve lattice mismatched between III-V and Si materials. More importantly, the thin thickness of 40 nm can reduce the optical loss in a buffer layer, that the Ge has a lower bandgap 0.66 eV than Si. Different to the conventional thick Ge on Si application, the thin Ge on Si developed by ECR-CVD with high plasma density starts a new concept of III-V/Si tandem solar cell. By theoretically calculation, the efficiency can achieve above 30 % (Voc= 2.19 V, Jsc= 14.78 mA/cm2, FF= 93 %). | en_US |