Cu-related recombination in CdS/CdTe solar cells (original) (raw)
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Solar Energy Materials and Solar Cells, 2005
Device performance of thin film CdTe/CdS solar cells having different methods for fabricating the primary back contact are presented. Wet and dry methods for forming the primary contact (Cu 2 Te) were evaluated with Cu layers from 0 to 15 nm. Extensive analysis of J-V curves is presented, including effects of temperature, intensity and accelerated stress. A procedure for recontacting stress-degraded cells allowed separation of contact and junction degradation modes. The junction recombination is shown to be a Shockley-Read-Hall mechanism. Stress increases the recombination current density J 0 by 2-3 orders of magnitude, resulting in a loss in V oc of 100-200 mV which is not restored with recontacting. Rollover is eliminated by recontacting the device while fill factor is partially restored with recontacting. For devices with a Cu layer, no significant differences in illuminated solar cell performance between the wet and dry process were observed before or after stress, but there were large differences in the dark J-V related to a blocking contact. To first order, unstressed devices without Cu contact layers behave similar to stressed devices with Cu; lower V oc , higher resistance, and appearance of a blocking contact. r
IEEE Journal of Photovoltaics, 2013
We study the effects of Cu composition on the CdTe/ZnTe:Cu back contact and the bulk CdTe. For the back contact, its potential barrier (φ bc) decreases with Cu concentration while its saturation current density (J 0bc) increases. For the bulk CdTe, the hole density (N) increases with Cu concentration. We identify a Cu-related deep level at ~0.55 eV whose concentration is significant when the Cu concentration is high. The device performance, which initially increases with Cu concentration then decreases, reflects the interplay between the positive influences (reducing φ bc while increasing J 0bc and N) and negative influences (increasing deep levels in CdTe) of Cu.
Analysis of the ZnTe:Cu Contact on CdS/CdTe Solar Cells
MRS Proceedings
We report on the recent use of cathodoluminescence (CL) to probe the depth-dependent changes in radiative recombination that occur in CdTe devices during ZnTe:Cu contacting procedures. These types of CL measurements may be useful to assist in linking impurity diffusion (e.g., Cu) from the contact with depth-dependent variation in electrical activation within the CdTe layer. Variable-energy CL suggests that diffusion from the ZnTe:Cu contact interface may assist in reducing effects of shallow donors in the CdTe bulk, and yield higher acceptor levels in the region near the contact. CL analysis near abrupt metal discontinuities provides estimates of diffusion lengths for carriers associated with both excitonic and donor-to-acceptor pair recombination. Finally, CL measurements at increasing excitation levels (i.e., increasing electron-beam current) provides estimates of the defect state density, as well as providing evidence that discrete multiple defect bands may exist in CdTe prior to...
Carrier recombination in Cu doped CdS thin films: Photocurrent and optical studies
Applied Surface Science, 2012
Quenching of photocurrent in Cu doped CdS prepared by Spray pyrolysis technique is reported. Anomalous changes in surface morphology are seen at 2% of Cu in CdS. Surface morphology of pure CdS film shows rod like structure. Aspect ratio of such rods has a maximum around 2% Cu substitution. This in turn produces anomalous changes in photoconductivity, which is further supported by marked changes seen in mean crystallite size, strain and grain size, roughness, transmittance, optical band gap, activation energy and finally in the photocurrent. Pronounced effects are seen in transmittance as a broad profile centered on 590 nm. The observed effects are explained in terms of carrier recombination mechanisms.
A comprehensive picture of Cu doping in CdTe solar cells
Journal of Applied Physics, 2013
The importance of Cu for CdTe solar cell absorber doping has been increasingly recognized in recent years. Currently different models are being discussed how to understand the case of Cu Cd substitutional doping in polycrystalline CdTe solar cells. In this work, an understanding is developed, which is based on a low concentration deep acceptor doped CdTe layer (N a $ 5 Â 10 14 cm À3 ,E a $ 300 meV above the valence band). Despite their non-shallow nature, Cu Cd acceptors are fully or at least heavily (>30%) ionized. The low hole concentration in CdTe ($1 Â 10 14 cm À3) originates directly from low Cu solubility in CdTe bulk material and is not caused by partial ionization or compensation as proposed by earlier models. The three to four orders of magnitude difference between bulk acceptor concentration and average Cu concentration in polycrystalline CdTe is attributed to grain boundary segregation of Cu. Our model is derived from substrate and superstrate CdTe solar cell measurements, controlled CdTe doping and quenching, Hall Effect measurements of CdTe films, numerical and analytical calculations, and a broad literature survey. Based on these results, routes to improve the conversion efficiency of CdTe solar cells are discussed. V
Photoluminescence study of CdTe/CdS solar cells grown from a source with Cu residual impurity
Semiconductor Science and Technology, 2005
Three photoluminescence (PL) bands centred at 1.30, 1.35 and 1.45 eV have been observed in the PL spectrum of CdTe/CdS thin film solar cells grown by close space sublimation (CSS) techniques from a source with Cu residual impurity. The bands at 1.30 and 1.45 eV were found to be independent of the technological conditions of the CSS process, while the intensity of the band at 1.35 eV proved to increase with the increase of the source temperature and the decrease of the substrate temperature. This PL band is suggested to correspond to donor-Cu Cd defects related to the incorporation in the CdTe film of the impurity from the source. The other two bands are associated with defects whose formation does not depend upon the technological processes applied, the band at 1.45 eV being attributed to a V Cd-Cl Te defect.
Journal of vacuum science and technology, 2018
A study is reported comparing the electrical and optical properties of CdTe solar cells, prepared using CdS and CdSe buffer layers, to investigate defects in the bulk and interface, carrier transport, and recombination. Temperature dependent capacitance-voltage measurement and admittance spectroscopy were used to extract carrier concentration, resistivity, charge carrier mobility, and their temperature dependence. We identify the presence of two defect signatures corresponding to carrier freeze-out and the formation of a Schottky back-contact barrier. The back-contact barrier height (≈ 300 meV) extracted from the temperature dependent current density-voltage (JVT) experiment was confirmed by conventional admittance spectroscopy. The activation energies of mobility (resistivity) are 101.2 ± 2.5 meV (92.6 ± 2.3 meV) and 84.7 ± 2.7 meV (77.6 ± 4.5 meV) for CdS and CdSe buffer layers, respectively. Intensity dependent photoluminescence analysis demonstrates that the CdSe/CdTe device exhibits lower radiative efficiency than the CdS/CdTe device. This confirms the presence of higher defects in CdSe/CdTe device corroborated by temperature-dependent VOC analysis. The comparative electrical and optical analysis provides insight to improving the performance of CdTe solar cell device by selenization.
Spectroscopic cathodoluminescence studies of the ZnTe:Cu contact process for CdS/CdTe solar cells
Conference Record of the Twenty-Ninth IEEE Photovoltaic Specialists Conference, 2002., 2002
Spectroscopic cathodoluminescence (CL), electronbeam induced current (EBIC), and capacitance-Voltage (C-V) measurements are used to study the formation of CdS/CdTe devices processed using ion-beam milling and a ZnTe:Cu/Ti contact. Results show heating in vacuum at 360°C and ion-beam milling lead to observable changes in the CL emission from the CdCl2-treated CdTe surface. Changes in the CL spectrum are also observed as ZnTe:Cu layer thickness increases. These changes are correlated to published studies of defect levels and shown to be due, possibly, to an n-type region existing between the ZnTe:Cu contact interface and the p-CdTe layers. This n-type region is eliminated once a sufficiently thick ZnTe:Cu layer is produced.