Vasilios Palekis

High efficiency CSS CdTe solar cells

Cadmium telluride (CdTe) has long been recognized as a strong candidate for thin film solar cell applications. It has a bandgap of 1.45 eV, which is nearly ideal for photovoltaic energy conversion. Due to its high optical absorption coefficient essentially all incident radiation with energy above its band-gap is absorbed within 1‐2 mm from the surface. Thin film CdTe solar cells are typically heterojunctions, with cadmium sulfide (CdS) being the n-type junction partner. Small area efficiencies have reached the 16.0% level and considerable efforts are underway to commercialize this technology. This paper will present work carried out at the University South Florida sponsored by the National Renewable Energy Laboratory of the United States Department of Energy, on CdTe/CdS solar cells fabricated using the close spaced sublimation (CSS) process. The CSS technology has attractive features for large area applications such as high deposition rates and efficient material utilization. The structural and optical properties of CSS CdTe and CdS films and junctions will be presented and the influence of some important CSS process parameters will be discussed. q 2000 Elsevier Science S.A. All rights reserved.

Vapor chloride treatment studies of CdTe/CdS solar cells

The most commonly used processing step for the fabrication of CdTe thin film solar cells is a heat treatment in the presence of CdCI/sub 2/. This paper discusses results on the effect of a vapor CdCI/sub 2/ heat treatment on the characteristics of thin film CdTe/CdS solar cells. The heat treatment was carried out in three different ambient environments, He, O/sub 2/, and H/sub 2/, and over a wide range of temperatures. Best solar cell performance to-date was achieved for cells heat-treated in the presence of O/sub 2/. Solar cells heat-treated in He and H/sub 2/ exhibited lower ffs and Vocs respectively. The process was optimized for high throughput and demonstrated state of the art Vocs and ffs for short annealing times.

Pulsed laser induced ohmic back contact in CdTe solar cells

Creating an ohmic back contact has long been a problem for making efficient CdTe solar cells. Current devices utilize some combination of preferential chemical etching, buffer layer, and Cu doping with additional cost, time, and complexity added for each step. In this Letter, these processes are eschewed and replaced with a nanosecond pulsed ultraviolet laser treatment. It is shown that this treatment can eliminate the rollover effect seen in photovoltaic current-voltage (J-V) curves that is indicative of a non-ohmic back contact. Transfer length measurements show that a single UV laser pulse can reduce the specific contact resistivity by a factor of 24 versus untreated samples. X-Ray photoemission spectroscopy shows evidence of increased conductivity and of elemental Te created at the surface by laser pulses. Finally, finite element modeling is used to model the laser-sample interaction, which predicts both the temperature and the amounts of Cd and Te lost during a laser pulse.

Pulsed UV laser annealing of polycrystalline CdTe

Presented here are the results of a three dimensional, finite element simulation that models pulsed, ultraviolet (UV) laser annealing of polycrystalline CdTe. The model considers heat generated by the absorption of a 25 ns, 248 nm laser pulse normally incident to a 5 μm thick CdTe thin film deposited on a polycrystalline alumina substrate. In particular, focus is on the spatial and temporal distribution of temperature from laser fluences that achieve a sub-melting condition. The model shows that there are very large temperature gradients both in depth and in-plane directions. These predictions, as well as the onset of melting, are confirmed with cross sectional scanning electron microscopy. Additionally, the model predicts that the heat generated dissipates rapidly after the pulse has ended. This has implications if pulse trains are to be used experimentally.

Effect of Cu and Cl on EVT-CdTe solar cells

The stoichiometry of CdTe plays an important role in the formation of point defects and can therefore impact solar cell performance. In this study, the effect of Cu and Cl on CdTe films is investigated as a function of stoichiometry. The stoichiometry of CdTe films was altered by varying the gas phase composition (amount of Cd and Te vapors) during the Elemental Vapor Transport (EVT) deposition process. Superstrate CdTe/CdS solar cells were fabricated using EVT-CdTe. Controlled amounts of Cu were incorporated into the CdTe cells during the back contact formation process. Solar cell performance improved for cells fabricated with EVT-CdTe deposited under Te-rich conditions. The dual benefits of Cu viz. improving doping concentration and eliminating the Schottky barrier at the back contact were observed. Capacitance-voltage (CV) measurements revealed an increase in net doping with a decrease in the Cd/Te vapor ratio, which can be attributed to an increase in the native acceptor defects, cadmium vacancies (VCds). Open circuit voltages up to 850 mV with FF up to 70 % were measured for EVT-CdTe solar cells with Cd/Te vapor ratio 0.7. 1-photon Time Resolved Photoluminescence (TRPL) measurements revealed higher lifetimes for solar cells fabricated with Cd/Te ratio 0.7. The incorporation of Cu reduced lifetimes.

Surface stoichiometry of pulsed ultraviolet laser treated polycrystalline CdTe

The effects of nanosecond pulsed ultraviolet laser annealing on the surface stoichiometry of close-space sublimated polycrystalline thin films are investigated using angle-resolved x-ray photoemission spectroscopy (XPS). The raw data suggest the formation of a Cd-rich surface layer, but this is counter to the expectation based on Cd and Te vapor pressures above CdTe that predicts a Te-rich layer and to direct observation of elemental Te at the surface. In order to explain this apparent discrepancy, we analyze our XPS data in the context of prior reports of lateral segregation of Cd and Te at the surface after pulsed laser treatments with a simple model of angular dependent XPS in the presence of surface roughness. This analysis reveals that a uniform Te layer cannot explain our results. Instead, our analysis suggests that Te enrichment occurs near grain boundaries and that a sub-monolayer Cd layer exists elsewhere. These complex yet repeatable results underscore the challenges in measuring surface stoichi...

CdTe films by Elemental Vapor Transport

The electro-optical properties of CdTe films deposited by Elemental Vapor Transport (EVT) are being investigated. The EVT process, unlike most processes currently used for the deposition of CdTe thin films, allows for the creation of excess Cd or excess Te conditions during the deposition, which can be used to influence the formation of defects and improve doping in CdTe. Using resistivity measurements, it has been demonstrated that the Cd/Te ratio used during the deposition process influences the incorporation of Cu in CdTe. Photoluminescence measurements have shown that the Cd/Te ratio also influences the formation of defect complexes in CdTe. Junctions formed with CdS suggest that the conductivity of CdTe can be adjusted p- or n-type by creating Te or Cd-rich conditions respectively. Structurally, the EVT-CdTe films have been found to be densely packed and highly oriented.

Cu effects on CdS/CdTe thin film solar cells prepared on flexible substrates

Thin film CdS/CdTe solar cells of the substrate configuration (foil/back contact (BC)/CdTe/CdS/TCO) have been prepared on the molybdenum (Mo)-coated stainless steel substrates. Copper (Cu) is introduced in the cell by briefly immersing the device in a CuCl solution at different stages of the fabrication process in order to study the effect of Cu on cell performance. The structural and morphological properties of the CdTe and CdS films have been investigated. CdS/CdTe solar cells with and without Cu incorporation are characterized. The results suggest that cell performance can be improved by incorporating Cu under certain conditions. Solar cells with efficiencies above 7% have been prepared in this study.

Stoichiometric effects in polycrystalline CdTe

The effect of the vapor phase Cadmium (Cd) to Tellurium (Te) ratio on the electronic properties of CdTe films is being studied. The stoichiometry of CdTe films is being altered by varying the gas phase Cd/Te ratio during the Elemental Vapor Transport (EVT) deposition process. Resistivity-temperature measurements and solar cells made with polycrystalline EVT-CdTe suggest changes in the native cadmium vacancy (VCd) concentration and carrier lifetime. The findings are in good agreement with the recently updated defect levels using the HSE approximation. Photoluminescence (PL) analysis has also demonstrated variation in the formation of defect complexes with the Cd/Te ratio. 2-Photon TRPL lifetime measurements showed improved minority-carrier lifetime for CdCl2 treated samples deposited at low Cd/Te ratios.

Tellurides as back contacts for substrate CdTe thin film solar cells on flexible foil substrates

The formation of a low resistance ohmic contact on p-type CdTe is a significant challenge for the superstrate solar cell configuration, where surface preparation techniques including wet etches are often used in order to modify the surface of CdTe prior to applying the contact material(s). It is even a greater challenge for the less commonly used substrate configuration, since the p-contact/CdTe interface will be formed first and will be exposed to the remaining solar cell fabrication steps. This paper presents results on the properties of two tellurides and their performance as back contacts in substrate type CdTe solar cells fabricated on flexible foil substrates. Solar cell efficiency is limited by the properties of the back contact and is currently in the 6.0–6.5% range.