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Chlorine passivation treatment of cadmium telluride (CdTe) solar cells improves device performance by assisting electron-hole carrier separation at CdTe grain boundaries. Further improvement in device efficiency is observed after alloying the CdTe absorber layer with selenium. High-resolution secondary ion mass spectroscopy (NanoSIMS) imaging has been used to determine the distribution of selenium and chlorine at the CdTe grain boundaries in a selenium-graded CdTe device. Atomistic modeling based on density functional theory (DFT-1/2) further reveals that the presence of selenium and chlorine at an exemplar (110)/(100) CdTe grain boundary passivates critical acceptor defects and leads to n-type inversion at the grain boundary. The defect state analysis provides an explanation for the band-bending effects observed in the energy band alignment results, thereby elucidating mechanisms for high efficiencies observed in Se-alloyed and Cl-passivated CdTe solar cells.

The effects of the CdCl2passivation treatment on thin-film CdTe photovoltaic films and devices have been extensively studied. Recently, with an addition of CdSeTe layer at the front of the absorber layer, device conversion efficiencies in excess of 19% have been demonstrated. The effects of the CdCl2passivation treatment for devices using CdSeTe has not been studied previously. This is the first reported study of the effect of the treatment on the microstructure of the CdSeTe /CdTe absorber. The device efficiency is < 1% for the as-deposited device but this is dramatically increased by the CdCl2treatment. Using Scanning Transmission Electron Microscopy (STEM), we show that the CdCl2passivation of CdSeTe/CdTe films results in the removal of high densities of stacking faults, increase in grain size and reorientation of grains. The CdCl2treatment leads to grading of the absorber CdSeTe/CdTe films by diffusion of Se between the CdSeTe and CdTe regions. Chlorine decorates the CdSeTe and CdTe grain boundaries leading to their passivation. Direct evidence for these effects is presented using STEM and Energy Dispersive X-ray Analysis (EDX) on device cross-sections prepared using focused ion beam etching. The grading of the Se in the device is quantified using EDX line scans. The comparison of CdSeTe/CdTe device microstructure and composition before and after the CdCl2treatment provides insights into the important effects of the process and points the way to further improvements that can be made.

Photovoltaics is an important energy technology for large scale energy generation. In the past few years cost of photovoltaic module manufacturing and installation as well as electricity generation has substantially decreased while the production volume has seen a steep increase. These changes can be attributed to improvement in solar cell efficiencies as well as better manufacturing practices. There are several photovoltaic technologies available in the market but the two primary technologies commercially manufactured for large scale installations are polycrystalline thin-film CdTe and crystalline silicon. Crystalline Si is the oldest and the most widely installed technology while thin-film CdTe is the technology that has demonstrated the largest growth and lowest LCOE (levelized cost of energy). In this study, commercial modules from both these technologies are installed side by side for an accurate comparison of their performance. The modules for comparison are installed with the same approximate nameplate capacity in three different configurations viz. Roof-top, floating on water and ground. Their performance is monitored and analyzed over a 3 month period. Thin-film CdTe demonstrated substantial advantage under all three conditions over crystalline Si in Thailand's tropical climate which is characterized by high temperatures and humidity throughout the year. Advantage demonstrated by thin-film CdTe is further supported by greater economic, environmental, reliability and life-cycle advantages that are summarized in the later part of the study.

Thin film CdTe (cadmium telluride) is an important technology in the development of sustainable and affordable electricity generation. More than 10 GW of installations have been carried out using this technology around the globe. It has been demonstrated as a sustainable, green, renewable, affordable and abundant source of electricity. An advanced sublimation tool has been developed that allows highly controlled deposition of CdTe films onto commercial soda lime glass substrates. All deposition and treatment steps can be performed without breaking the vacuum within a single chamber in an inline process that can be conveniently scaled to a commercial process. In addition, an advanced cosublimation source has been developed to allow the deposition of ternary alloys such as CdxMg1−xTe to form an electron reflector layer which is expected to address the voltage deficits in current CdTe devices and to achieve very high efficiency. Extensive materials characterization, including but not limited to scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, high resolution transmission electron microscopy and electron back-scatter diffraction, has been performed to get a better understanding of the effects of processing conditions on CdTe thin film photovoltaics. This combined with computer modeling such as density function theory modeling gives a new insight into the mechanism of CdTe photovoltaic function. With all these efforts, CdTe photovoltaics has seen great progress in the last few years. Currently, it has been recorded as the cheapest source of electricity in the USA on a commercial scale, and further improvements are predicted to further reduce the cost while increasing its utilization. Here, we give an overview of the advantages of thin film CdTe photovoltaics as well as a brief review of the challenges that need to be addressed. Some fundamental studies of processing conditions for thin film CdTe are also presented along with fabrication conditions using the closed-space sublimation method.

Cadmium Sulphide is the conventional n-type buffer layer used in thin film Cadmium Telluride solar cells. It is well known that Cadmium Sulphide causes optical losses and sulphur diffuses into the absorber during high temperature activation. Sputter-deposited Mg-doped ZnO (MZO) has been shown to be an attractive buffer layer for Cadmium Telluride solar cells due to its transparency and tuneable band gap. It is also stable to high temperature processing and avoids diffusion of elements into the cadmium telluride absorber during the cadmium chloride activation treatment. However, degradation is observed in solar cells incorporating MZO buffer layers. Analysis of the MZO film surface potential has revealed significant fluctuations in the thin film work function once the layer is exposed to the atmosphere following deposition. These fluctuations are attributed to the high reactivity to water vapour of the MgO contained in the MZO films. This has been analysed using X-ray Photoelectron Spectroscopy to determine corresponding changes in the surface chemistry. The Zinc Oxide component is relatively stable, but the analysis shows that MgO forms a Mg(OH)2 layer on the MZO surface which forms a secondary barrier at the MZO/CdTe interface and/or at the interface between MZO and the Fluorine-doped SnO2. This affects the Fill Factor and as a consequence it degrades the conversion efficiency.