Flexible, high-efficiency, low-cost solar cells can enable applications that take advantage of high specific power, flexible form factors, lower installation and transportation costs. Here, we report a certified record efficiency of 16.4% for a flexible CdTe solar cell that is a marked improvement over the previous standard (14.05%). The improvement was achieved by replacing chemical-bath-deposited CdS with sputtered CdS:O and also replacing the high-temperature sputtered ZnTe:Cu back contact layer with co-evaporated and rapidly annealed ZnTe:Cu. We use quantum efficiency and capacitance-voltage measurements combined with device simulations to identify the reasons for the increase in efficiency. Both device simulations and experimental results show that higher carrier density can quantitatively account for the increased open circuit voltage (VOC) and Fill Factor (FF), and likewise, the increase in short circuit current density (JSC) can be attributed to the more transparent CdS:O.
High-performance transparent conductive indium-tin-oxide (ITO) films on flexible glass have been flextested to 25–50k bend cycles without breakage, and with ∼0.1% change in sheet resistance. In contrast, commercial ITO/PET samples undergo ∼50–100% increase in sheet resistance in the same test, indicating that such coatings/substrates may not be acceptable for use in some products or fabrication procedures. The flexible glass substrate enables high-temperature processing, which facilitates the high performance of the coatings. Measurements of the volume resistivity and water vapor transmission rate (WVTR) indicate that Corning ® Willow ® Glass is suitable as a PV substrate material without need for barrier coatings or glass lamination.
Theoretical predictions of thin-film CdS/CdTe photovoltaic (PV) devices have suggested performance may be improved by reducing recombination due to Te-vacancy (VTe) or Te-interstitial (Tei) defects. Although formation of these intrinsic defects is likely influenced by CdTe deposition parameters, it also may be coupled to formation of beneficial cadmium vacancy (VCd) defects. If this is true, reducing potential effects of VTe or Tei may be difficult without also reducing the density of VCd. In contrast, post-deposition processes can sometimes afford a greater degree of defect control. Here we explore a post-deposition process that appears to influence the Te-related defects in polycrystalline CdTe. Specifically, we have exposed the CdTe surface to Te prior to ZnTe:Cu/Ti contact-interface formation with the goal of reducing VTe but without significantly reducing VCd. Initial results show that when this modified contact is used on a CdCl2-treated CdS/CdTe device, significantly poorer device performance results. This suggests two things: First, the amount of free-Te available during contact formation (either from chemical etching or CuTe or ZnTe deposition) may be a more important parameter to device performance than previously appreciated. Second, if processes have been used to reduce the effect of VTe (e.g., oxygen and chlorine additions to the CdTe), more » adding even a small amount of Te may produce detrimental defects. « less
A back contact containing a sputtered ZnTe:Cu interface layer can produce high-performing thin-film CdS/CdTe photovoltaic devices. We have found that varying the ZnTe:Cu sputtering target fabrication processes and deposition temperature can affect material properties of the ZnTe:Cu films and the resulting device performance. Two different target "recipes" with various copper contents were used to study changes in the compositional, structural, optical, and electrical properties of ZnTe:Cu films. Substrate temperature during deposition was also varied to investigate the temperature dependence of the films. It was found that the target recipe, Cu concentration in the target, and deposition temperature affect the composition of the ZnTe:Cu films, which impacts their structural, optical, and electrical properties.
A back contact containing a sputtered ZnTe:Cu interface layer can produce high-performing thin-film CdS/CdTe photovoltaic devices. We have found that small changes in ZnTe:Cu sputtering target fabrication processes affect the properties of the ZnTe:Cu films, which affect the performance of the resulting devices. Different target manufacturing techniques were investigated to study changes in ZnTe:Cu film properties and how they impact device performance. Compositional, optical, and electrical properties of films made from different target recipes were studied. It was found that the amount of oxygen in the targets and films is strongly linked to changes in material properties, especially in band tailing and optical bandgap.
We conducted T= 6 K cathodoluminescence (CL) spectrum imaging with a nanoscale electron beam on beveled surfaces of CdTe thin films at the critical stages of standard CdTe solar cell fabrication. We find that the through-thickness CL total intensity profiles are consistent with a reduction in grain-boundary recombination due to the CdCl2 treatment. The color-coded CL maps of the near-band-edge transitions indicate significant variations in the defect recombination activity at the micron and sub-micron scales within grains, from grain to grain, throughout the film depth, and between films with different processing histories. We estimated the grain-interior sulfur-alloying fraction in the interdiffused CdTe/CdS region of the CdCl2-treated films from a sample of 35 grains and found that it is not strongly correlated with CL intensity. A kinetic rate-equation model was used to simulate grain-boundary (GB) and grain-interior CL spectra. Simulations indicate that the large reduction in the exciton band intensity and relatively small decrease in the lower-energy band intensity at CdTe GBs or dislocations can be explained by an enhanced electron-hole non-radiative recombination rate at the deep GB or dislocation defects. Simulations also show that higher GB concentrations of donors and/or acceptors can increase the lower-energy band intensity, while slightly decreasing the exciton band intensity.
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