Film thickness and chemical processing effects on the stability of cadmium telluride solar cells

Accelerated stress testing and diagnostic analysis of degradation in CdTe solar cells

“…The CBD solution consisted of 550 ml DI water, 8 ml of 0.033 M cadmium acetate, 15 ml of reagent-grade ammonium hydroxide (28-30%), and 4.7 ml of 1.0 M ammonium acetate heated to 92 °C where it was then titrated (2 ml at 10 m intervals) with 8 ml of 0.067M thiourea solution for 37 m. The CSS process was subsequently performed at a substrate growth temperature of 620 °C and CdTe source temperature of 660 °C in an ambient of 1 torr O 2 and 15 torr He for 5 m. This processing resulted in CdS and CdTe layers of approximately 80 nm and 9-10 microns respectively. Resulting glass/SnO 2 /CdS/CdTe layers were then annealed at 400 °C suspended in a CSS configuration over a pelletized, anhydrous CdCl 2 source in an ambient of 100 torr O 2 and 400 torr helium for 8 m. Prior to contact application, a 1:88:35 volume mixture of HNO 3 :H 2 PO 4 :H 2 O acid etch was used to remove surface oxides and produce a Te-rich layer to improve initial performance [10] and stability [9]. Contacts consisted of a Cu-doped, graphite paste mixture brushed onto the etched CdTe surface and subsequently annealed in He at 280 °C for 25 m followed by the final application of a Ag-paste (Acheson) conductor.…”

“…Though most use fixed stress temperatures, others have considered temperature itself as a variable [6,7]. Recently, it has been reported that processing can impart strong differences in stability, and thus potentially different degradation mechanisms [8,9]. Viable stress protocols require acceleration temperatures that only invoke mechanisms expected under actual use conditions.…”

CdTe thin film solar cells: Interrelation of nucleation, structure, and performance

“…As a consequence there is a strong tendency to form recombination pathways along grain boundaries and pinholes existing in the films which lead to rather low parallel resistances and thus to a strong loss in performance of most CdTe solar cells and modules (see e. g. a comparison of CdTe solar cells to related GaAs cells given by Sites [10]). Recombination losses across the absorber layer are of even stronger influence after chemical etching using etching in nitric-phosphoric acid (NP etch) [11] or if thinner films are approached to consider the physically defined needs for improved solar cells or tandem cells. For such improved cells absorber films are needed with maximized lateral dimensions and vertical dimensions close to the absorption length of about 1 µm.…”

The use of 2^nd and 3^rd level correlation analysis for studying degradation in polycrystalline thin-film solar cells

“…The effects of back contact and processes on cell durability have been discussed for example in [3][4][5]. More recently, the detrimental effects of localized shunts [6], the general effects of film micrononuniformities [7], and the effect of cell-fabrication processing through formal design-of-experiments methodologies [8] have been reported.…”