Fabrication Of Back Surface Field Buffer Layer And Cdte Thin Film Solar Cells

Cdmium telluride(CdTe)is an important ?-? compound semiconductor with a direct band gap of 1.45 eV,corresponding to the best solar spectrum.CdTe has an absorption coefficient as high as 105 cm-1,100 times higher than Si.1-?m CdTe film can absorb 99%of photos with energy greater than the CdTe band gap.So,CdTe is suitable for thin film solar cells.CdS/CdTe polycrystalline solar cell has great market potential due to its high efficiency and low material cost.In recent years,CdTe solar cell develops rapidly and now the world highest conversion efficiency is 22.1%,higher than that of polystalline Si solar cell,breaking the market dominance of Si solar cells.However,there are also many problems to fabricate stable and high efficient CdTe solar cell,due to some CdTe's material weak points.Firstly,due to the high work function of CdTe(?5.7 eV),and low p-type carrier density on the order of 1013-1014cm-3 in CdTe,it's a challenge to fabricate stable and low-resistance back contact CdTe solar cell.Second,back contact structure using Cu is effective to make quasi-ohmic contact and improve the efficiency of CdTe solar cell.But copper is well known to diffuse easily from the contact region into CdTe and accumulate at the CdTe/CdS junction,resulting in device performance degradation over time.Thirdly,due to the large carriers recombination in the CdTe bulk and at the interface,the highest open-circuit voltage for polycrystalline CdTe cell is about 75%of ideal for its band gap,far lower than Si and CIGS solar cells.This thesis maimly focuses on the back contact buffer layer of high efficiecny CdTe solar cells.In Chapter ?,we reviewed the background and history of photovoltaics,and introduced the structure,principle and current-voltage characteristics of solar cell.Finally,we summarized the development,structure,material properties and preparation process of CdTe solar cell.In Chapter ?,we discussed the CdTe solar cells fabricated with NiO as a back contact buffer layer.XPS technique was carried out to quantitatively characterize the band alignment at the CdTe/NiO interface.The band alignment obtained from XPS measurement shows that the band alignment of NiO and CdTe are perfectly matched.It can be seen from the band alignment that the conduction band offset(?ECBO)exists at the interface.The ECBO indicated that NiO can act as a back surface field layer(BSF)to dramatically reduce carrier recombination at the contact region of a CdTe cell,leading to an improved Voc and efficiency.To optimize the thickness of the NiO thin film,we fabricated some CdTe solar cells with different NiO thickness and 20nm-NiO was best.We used impedance spectroscopy test to prove the BSF effect of NiO buffer layer.We fabricated a cell with contact structure of 3-nm-Cu/20-nm-NiO/Au and the cell had Voc of 796 mV,Jsc of 24.2 mA/cm2,FF of 70.2%and an efficiency of 13.5%and demonstrated that the stability of 3-nm-Cu/20-nm-NiO/Au cell was better than that of the standard Cu/Au cell.In Chaper III,we discussed the CdTe solar cells with CuI as back contact buffer layer.We used thermal evaporation to prepare CuI films and studied the effects of heat treatment on CuI films through XRD,UV and SEM.The results indicated that heat treatment at 200? is necessary for CuI to cover the substrate.We fabricated CdTe solar cells with and without CuI buffer layer.From the J-V curve,we can see the CuI buffer layer can reduce the Schottky barrier at the CdTe back surface and improve the Voc,FF and efficiency of CdTe solar cells.Finally,XPS was used to test the band alignment of CdTe/CuI interface,and to show the mechanism of carrier transport.