CdTe Solar Cells at CEA / LETI
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1210-Q08-31
CdTe Solar Cells at CEA /LETI
Sergio Bernardi, Sebastien Renet and Alain Million Commissariat à l’Energie Atomique CEA – LETI MINATEC, 17 Rue des Martyrs, 38054 Grenoble Cedex, France
ABSTRACT The present communication presents the results obtained at CEA/LETI in Grenoble, France aimed at directly testing the chances of success of this technology. By means of Closed Space Sublimation (CSS), Chemical Bath Deposition (CBD) processes, with appropriate annealing treatments and specific procedures good electric contact on CdTe was obtained. For both structures, deposited on 3 mm thick low-iron containing sodalime glass (SLG) and Borosilicate glass (BSG), using the state-of-art technology with a conversion efficiency of about 15% under standard illumination conditions in “superstrate” cells configuration. EXPERIMENTAL and RESULTS The CdS/CdTe Solar cell technology [1] has been developed by CSS and CBD processes at CEA/LETI in Grenoble (France), where its improvements continue to be part of our R&D programs. The results here presented concern “Thin Films Absorber” Cells, that is, CdTe absorbing layers in the 4-6 µm thickness range [2]. In order to attain the state-of-the-art for superstrate “Thin Films Absorber” Cells, we set up deposition and annealing[3] processes on (7x7) cm2 area substrates, based both on 0.5 mm thick Borosilicate (BSG) and 3 mm thick Sodalime (SLG) glasses. In Figure 1, two of our PV-structures realized on BSG and SLG substrates are shown.
. Figure 1. Two (7x7) cm2 CEA/LETI CdS/CdTe PV-structures realized on 0.5 mm thick BSG (left) and 3 mm thick SLG (right).
For both types of base substrates, ITO (TCO) of thickness of 200 nm were sputtered to form an n-side window electrode and no high-resistivity (HR buffer) layers were interposed between the ITO and CdS film. We developed both CSS and CBD deposition simultaneously for CdS window layer. Their comparative impact on PV-prototype cells performances is a matter of systematic investigations, even though cell efficiencies in the of 13-15% range have been measured for cells made from structures with both types of CdS films on both types of glass substrates (SLM and BSG). Because, CBD CdS films are deposited at low temperature (8085ºC), they require annealing [4] to be used as reliable CdTe cell window layers. This step is generally even more critical than the chemical bath deposition process. CBD CdS films of excellent micro-morphological quality and thickness uniformity in the 50-70 nm range can be systematically obtained and annealed at 450ºC without damaging the films. Due to the problem of increasing native defects in CBD CdS films of thickness over ~70 nm, thick CdS films (>70nm), deposition were carried out in two consecutive CBD steps. On the other hand, CSS CdS-films have high morphological quality even up to 200-300 nm (when deposited on ITO), but their lateral uniformity is more critical than that of CBD films, reflecting the lateral temperature gradients of the deposition system used. Figure 2 compares the visual aspect of the
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