Fabrication Of High Effcient CdTe Thin Film Solar Cells With Different Back Contacts

Cadmium telluride (CdTe) is considered as the most economical and promising material for solar cells in the photovoltaic industry. CdTe has a band gap of 1.45 eV which suits well with the solar spectrum for photovoltaic energy conversion. The theoretical limit of the energy conversion efficiency is as high as ～29% for a single-junction CdTe solar cell. CdTe is a direct band gap semiconductor having an absorption coefficient around 105 cm-1, which allows absorption of 99% of the photons with energy greater than the band gap within a 2-μm CdTe film. Small-area CdTe cell with an energy conversion efficiency of 22.1% and large-size commercial-scale module with an efficiency of 18.6% have been achieved recently. However, due to the material limitations and lack of knowledge in device physics, some key issues remain to be investigated and understood in a more consistant way to obtain high-efficiency and stable CdTe thin-film solar cells. The key challenges are low carrier concentration in CdTe thin film, the high work function of P-type CdTe, and the fabrication of high-quality p-n junction. Moreover, the photocurrent loss in cadmium sulfide (window layer) and the impact of doping elements on the stability of CdTe thin film solar cell also need to be studied. This thesis focuses on the fabrication of highly efficient CdTe thin-film solar cells and some of the fundamental problems and issues related to the devices.In Chapter I, we have reviewed the background and history of solar cells and introduced the construction, principles and output characteristics of solar cells based on p-n junction.Furthermore, we summarized the fabrication process and the chemical bonding of CdS and CdTe materials as well as the physical properties of these materials.In Chapter II, we have briefly discussed the fabrication steps of highly efficient CdTe thin-film solar cells. CdTe thin-film solar cells with the assembly of Glass/SnO2:F/N-CdS/P-CdTe/Cu-Au were fabricated. Cadmium sulfide (CdS) films were deposited by chemical bath deposition (CBD) and CdTe films were prepared by close spaced sublimation (CSS) technique.In order to obtain high-quality p-n junction, the CdS films were heat treated in a controlled atmosphere, the crystalline quality of the CdS thin films was enhanced and the unnecessary oxidation of the CdS film surface was avoided. This annealing procedure in addition to a proper interdiffusion at the CdS/CdTe interface guaranteed a high-quality p-n junction. Finally, We studied the optimization of the back contact and briefly explained the band alignment in CdTe thin film solar cells.In Chapter Ⅲ, we have discussed the CdTe thin-film solar cells fabricated with copper oxide (CuO) as a buffer layer in the back contact. The choice of back contact plays a key role in the fabrication of CdTe thin film solar cells. Material with low electrical resistivity, relatively high work function, and thermal stability is needed to form a low barrier contact with CdTe absorber film.The introduction of the buffer layer reduced the work function mismatch between CdTe and the back electrode. In this work, thin CuO layer was fabricated as a buffer layer between p-type CdTe and the metal electrode. Quantitative band alignment measurement demonstrated that a relatively low energy barrier (～0.44 eV) was formed at the CuO/CdTe interface. A CdTe solar cell with conversion efficiency of 12.2% was achieved. CdTe solar cell stability was significantly enhanced when CuO buffer layer and less Cu were employed in the back contact.In Chapter IV, we studied the CdTe solar cells fabricated with carbon nanofibers (CNFs) as a buffer layer in the back contact. CNF have admirable electrical and photoelectric properties. In this chapter, CNF film is fabricated as a buffer layer in the back contact of CdTe solar cells to replace the conventional device structure. Quantitative band alignment measurement demonstrated that a relatively low energy barrier at the CNF/CdTe interface was formed.Valance band offset and conduction band offset between CNF and CdTe were calculated 0.47 and 0.80 eV respectively. CdTe solar cell stability was significantly enhanced when a CNF buffer layer and less Cu were employed in the back contact. The device had more than 11.3%conversion efficiency. From the stability tests we conclude that CNF as a buffer layer in CdTe solar cells produced a reliable back contact.In Chapter V, we explained the fabrication process of ultra-thin CdTe solar cell. We kept the thickness of ultra-thin CdTe film from 0.5 to 1.0μm. We fabricated devices having structure of CdS/CdTe(0.5 and 1.0 μm)/Metal electrodes. Furthermore, we studied the impact of different transition metal oxides (TMOs) such as V2O5, NiO, and CuO as a buffer layer in ultra-thin CdTe solar cells. The experimental results showed that 1.0μm thick CdTe is thick enough to produce a reliable conversion effciecny. CuO was used as buffer layer in ultra-thin solar cell, and this produced 6.84% efficiency. We found that CdCl2 heat treatment had a strong impact for ultra-thin CdTe solar cell, especially the treatment time and the amount of CdCl2 powder used are very important to fabricate a uniform and better quality of ultra-thin CdTe film. The experimental results proved that ultra-thin CdTe solar cell used different TMOs as a buffer layer in back contacts could produce reasonable effciecny. It can enhance the device thermal stability and decrease the production cost.In chapter Ⅵ, we summarized the thesis results and briefly explained the concluion and inovation related to the CdTe thin film and ultra-thin film solar cells.