Electricity produced by cadmium telluride (CdTe) photovoltaic modules is the lowest cost in the solar industry, and now undercuts fossil fuel-based sources in many regions of the world. This is due to recent efficiency gains brought about by alloying selenium into the CdTe absorber, which has taken cell efficiency from 19.5% to its current record of 22.1%. While the addition of selenium is known to reduce the bandgap of the absorber material and hence increase cell short-circuit current, this effect alone does not explain the performance improvement. Here, by means of cathodoluminescence (CL) and secondary ion mass spectrometry (SIMS), we show that selenium enables higher luminescence efficiency and longer diffusion lengths in the alloyed material, indicating that selenium passivates critical defects in the bulk of the absorber layer. This passivation effect explains the recordbreaking performance of selenium-alloyed CdTe devices, and provides a route for further efficiency improvement that can result in even lower costs for solar generated electricity.

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Sublimated thin film CdTe photovoltaic devices with conversion efficiencies over 18% and a fillfactor greater than 79% have been repeatedly obtained using high-rate fabrication processes on commercial soda-lime glass substrates used in CdTe modules. Four major improvements to the device have enabled an increase in efficiency from a baseline of approximately 12% to 18.7%: 1) A sputtered multilayer metal-oxide anti-reflection layer; 2) total replacement of the CdS window layer with a higher bandgap sputtered MgxZn1-xO (MZO) window layer; 3) deposition of the CdTe layer at a higher thickness and substrate temperature; and 4) an evaporated tellurium back-contact. This work describes the effect of these changes on the device performance and film microstructural characteristics using various methods. Multiple devices with comparable high efficiency have been fabricated and demonstrated using methods described in this study, yielding some of the highest efficiencies for CdTe polycrystalline thin-film photovoltaics. Thin film CdTe photovoltaics have consistently demonstrated the lowest cost solar electricity generation, particularly for utility scale applications. CdTe is a p-type absorber that has a bandgap of 1.5 eV which is nearly optimal for photovoltaic conversion. Approximately 2 µm is sufficient to absorb most of the visible solar spectrum. 1,2 CdTe films are typically deposited on glass substrates using low-cost hardware and high-rate deposition processes 3,4,5 reducing production costs. Typical crystalline silicon photovoltaics require wafers that are 150-200 microns thick and use a more complex and capital-intensive fabrication process. 3 The low-cost manufacturing of thin-film CdTe PV has enabled agreement for a record low cost power purchase agreement of ¢3.8/kWh for a 100 MW field, 6 which is significantly lower than the average cost of electricity in the U.S. of ¢11/kWh. 7 With recent improvements, research-scale small devices have record efficiencies of 22.1%, 8 while modules with up to 18.6% 9 efficiency have been produced. The leading CdTe PV manufacturer, First Solar Inc., has increased average production module efficiency from 13.5% in the first quarter of 2014 10 to 16.7% in the first quarter of 2017. 11 Further improving the efficiency without substantial increase in production cost will reduce the levelized cost of energy for CdTe photovoltaics. 12,13 Maintaining the dual requirement of high efficiency and low cost requires the use of film deposition techniques suitable for mass production of millions of solar modules per year. The vapor deposition methods used for this study, including sublimation, evaporation, and sputter deposition, have been used in large scale manufacturing for solar and other industries. Commercially available 3.2-mm soda-lime glass with a fluorine-doped tin-oxide (FTO) transparent conducting layer is a standard substrate for thin-film PV manufacturing, including for CdTe, due to its sufficient strength, reliability, and low cost. Using processes suitable for large scale manuf...

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The effects of the CdCl 2 passivation 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 CdCl 2 passivation 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 CdCl 2 treatment. Using Scanning Transmission Electron Microscopy (STEM), we show that the CdCl 2 passivation of CdSeTe/CdTe films results in the removal of high densities of stacking faults and increase and reorientation of grains. The CdCl 2 treatment 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 CdCl 2 treatment provides insights into the important effects of the process and points the way to further improvements that can be made.

CdTe-based solar cell efficiency has rapidly improved over the last few years. Some of the reasons have been a change to the absorber composition including the incorporation of selenium, and better front contact and emitter materials in CdTe photovoltaic devices. In addition to the increase in short-circuit current by reducing the bandgap, Se plays other important roles in passivation of defects thus improving the conversion efficiency of CdSeTe/CdTe graded absorber photovoltaic devices. Here, we combine structural and optical characterizations with first principles calculations to investigate the role of Se and Cl segregation in CdSeTe devices. We find that in the presence of Se and Cl, the minority carrier lifetime improves due to a reduction of midgap defect states. We also correlate this effect with defect passivation in CdSeTe devices and suggest innovative ways to further improve CdTe-based photovoltaic efficiency.

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