Characerization of a-Si:H based HIT solar cell fabricated by PECVD method

a-Si:H/c-Si heterojunction solar cells are a type of crystalline based solar cells, developing since 1990s, are under great interest of researches due to their easy production and high energy conversion efficieny values. Silicon heterojunction solar cells are obtained by the growth of very thin intrinsic amorphous silicon and doped amorphous silicon layers on crystalline silicon, thus they are called HIT (Heterojunction wth intrinsic thin layer) solar cells. For HIT solar cell which are already in the photovoltaic market, the problems such as the crystaline silicon surface passivation and the charge transport through the heterojuntion but the research and development of HIT solar cells are still in progress. In this thesis, a-Si:H/c-Si heterojunction solar cells whicch have high energy conversion efficiency are analyzed and the current transport mechanism are determined. The influence of the oxide content of the buffer layer of HIT solar cells on electrical properties are studied. The energy-band diagrams of the analysed solar cells, the transport paths, band offsets are determined by current-voltage, capacitance-voltage measurements. The solar cell parameters and the temperature dependent parameters of the solar cells investigated in this thesis are performed. Temperature dependent current-voltage measurements performed at dark ambient are used to understand the current mechanisms. The consistency of the activation energy values calculated from I-V and C-V measurements helps us to achieve the correct current mechanism. Capaticance measurements and IPE measurements are used to determine the bandoffsets at the amorphous/crystalline heterojunction. Shortly, the reasons behind the high efficiency values are aimed to be extracted. On the other hand, the factors that limit the higher efficiency values are also studied. The optimization of the growth conditions of the amorphous siliscon layer and the passivation of the crystalline silicon wafer are the main factors of higher energy conversion efficiencies. To increase the efficiency of HIT solar cells, the diode occurs between the doped a-Si:H/ and ITO layer which acts as transparent conductive oxide layer should be understood clearly. The inverse diode appears due to the difference of the work functions of doped amorphous silicon and ITO materials. The work function of the ITO layer can be aligned with the oxygen content of the layer, and thus the obstacle for the charges can be overcame. With this purpose the oxide content of the ITO layer is varied in order to determine and tune the work function of ITO layer. This study is focused on the determination of the barrier height values of ITO/(n)c-Si and ITO/(i)a-Si:H/(n)c-Si structures through I-V and C-V measurements. Additionally, one of the main reasons of the high efficiencies achieved by the researchers is the reduced interface defect density of amorphous silicon/crystalline silicon interface. A numerical model based on MOS analysis is developed and applied for a-Si:H/c-Si structure and interface defect density is extracted. This value is also confirmed by AFORS-HET simulation studies.