Development of thin film amorphous silicon oxide/microcrystalline silicon double-junction solar cells and their temperature dependence

We have developed thin film silicon double-junction solar cells by using micromorph structure. Wide bandgap hydrogenated amorphous silicon oxide (a-SiO:H) film was used as an absorber layer of top cell in order to obtain solar cells with high open circuit voltage (V_oc), which are attractive for the use in high temperature environment. All p, i and n layers were deposited on transparent conductive oxide (TCO) coated glass substrate by a 60 MHz-very-high-frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique. The p-i-n-p-i-n double-junction solar cells were fabricated by varying the CO₂ and H₂ flow rate of i top layer in order to obtain the wide bandgap with good quality material, which deposited near the phase boundary between a-SiO:H and hydrogenated microcrystalline silicon oxide (μc-SiO:H), where the high V_oc can be expected. The typical a-SiO:H/μc-Si:H solar cell showed the highest initial cell efficiency of 10.5%. The temperature coefficient (TC) of solar cells indicated that the values of TC for conversion efficiency (η) of the double-junction solar cells were inversely proportional to the initial V_oc, which corresponds to the bandgap of the top cells. The TC for η of typical a-SiO:H/μc-Si:H was −0.32%/ °C, lower than the value of conventional a-Si:H/μc-Si:H solar cell. Both the a-SiO:H/μc-Si:H solar cell and the conventional solar cell showed the same light induced degradation ratio of about 20%. We concluded that the solar cells using wide bandgap a-SiO:H film in the top cells are promising for the use in high temperature regions.