CdCl2 Annealing Research Articles

Effect of annealing time of CdCl2 vapor treatment on CdTe/CdS interface properties

Photoluminescence (PL) spectroscopy was used to study the influence of the CdCl2 annealing time on the CdTe/CdS solar cell structures. The intensity of the PL peaks related to the CdTe and CdS regions drastically changes after chlorine activation. Regardless of the annealing time of the treatment process, the intensity of the CdTe PL-related defect band increases and the CdS-associated PL emission is reduced due to partial consumption of the CdS layer. It was observed that an increase of the annealing time leads to a redshift of the emission around 1.9 eV related to the inclusion of Te into the CdS layer. Additionally, as annealing time increases, a shoulder, which we correlate with intermixing region, appears around 1.55 eV and reaches its maximum intensity for an annealing time of 60 min. The PL measurements were correlated with the electrical characteristic of the solar cells fabricated with this heterostructure. It was observed that after 60 min of annealing time, there was an increase in both the open circuit voltage (VOC) and the short circuit current (JSC), but for higher annealing times these parameters were reduced, decreasing the efficiency of the solar cell.

Effect of CdCl2 annealing treatment on CdS thin films and CdTe/CdS thin film solar cells

AbstractIn order to study the effect of CdCl2 annealing treatment on thin CdS films and CdTe/CdS thin film solar cells, a comparative study was carried out on three types of CdTe/CdS solar cells, which had different kinds of CdS window layer: as‐deposited CdS, air‐annealed CdS without CdCl2 pre‐coating, and CdCl2‐annealed CdS. When annealed in air the CdS film was partially oxidated to CdO and CdSO4. These oxides increased the series resistance of the CdTe solar cell and led to the lowest fill factor. The presence of CdCl2 on the surface of a CdS thin film during heat treatment in air protected it from oxidation and promoted the recrystallization of the CdS film, resulting in large and closely packed grains with a grain size of ∼ 50 ‐150 nm. CdTe/CdS solar cell with such a kind of CdS window layer showed the largest short circuit current and highest conversion efficiency of 12.4%. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ oxygen incorporation and related issues in CdTe∕CdS photovoltaic devices

Cd Te ∕ Cd S ∕ Sn O 2 ∕ ITO : F solar cell devices were investigated using quantitative secondary ion mass spectrometry (SIMS) depth profiling. They were grown on sapphire substrates and potentially active impurity species were analyzed. The SIMS data were calibrated for both CdS window layer (grown by sputtering) and CdTe absorber layer (deposited by close-space sublimation). For comparison, some of the samples were grown with and without oxygen incorporation into the CdTe layer during its deposition, and with and without postgrowth cadmium chloride (CdCl2) annealing in air and chemical etching. These devices were back contacted using Mo∕Sb2Te3 sputtered layers. It was shown that for CdTe and CdS layers there was a correlation between the concentrations of oxygen and chlorine. In situ oxygen incorporation in the CdTe layer yielded a substantial improvement in the device parameters and achieved an efficiency of 14% compared to 11.5% for devices fabricated in the same conditions without oxygen incorporation in CdTe. In light of our previous reports, this study also led to a clear determination of the origin of Na and Si traces found in these devices.

CdTe solar cell in a novel configuration

AbstractPolycrystalline thin‐film CdTe/CdS solar cells have been developed in a configuration in which a transparent conducting layer of indium tin oxide (ITO) has been used for the first time as a back electrical contact on p‐CdTe. Solar cells of 7·9% efficiency were developed on SnOx:F‐coated glass substrates with a low‐temperature (<450°C) high‐vacuum evaporation method. After the CdCl2 annealing treatment of the CdTe/CdS stack, a bromine methanol solution was used for etching the CdTe surface prior to the ITO deposition. The unique features of this solar cell with both front and back contacts being transparent and conducting are that the cell can be illuminated from either or both sides simultaneously like a ‘bi‐facial’ cell, and it can be used in tandem solar cells. The solar cells with transparent conducting oxide back contact show long‐term stable performance under accelerated test conditions. Copyright © 2004 John Wiley & Sons, Ltd.

Flexible CdTe solar cells on polymer films

AbstractLightweight and flexible CdTe/CdS solar cells on polyimide films have been developed in a ‘superstrate configuration’ where the light is absorbed in CdTe after passing through the polyimide substrate. The average optical transmission of the approximately 10‐μm‐thin spin‐coated polyimide substrate layer is more than ∼75% for wavelengths above 550 nm. RF magnetron sputtering was used to grow transparent conducting ZnO:Al layers on polyimide films. CdTe/CdS layers were grown by evaporation of compounds, and a CdCl2 annealing treatment was applied for the recrystallization and junction activation. Solar cells of 8·6% efficiency with Voc = 763 mV, Isc = 20·3 mA/cm2 and FF = 55·7% were obtained. Copyright © 2001 John Wiley & Sons, Ltd.

Effect of CdCl2 Treatment Conditions on the Deep Level Density, Carrier Lifetime and Conversion Efficiency of CdTe Thin Film Solar Cells

ABSTRACTThe effect of CdCl2 annealing conditions of glass/TCO/n/CdS/p-CdTe solar cell structures on the deep level density and carrier lifetime of the p-CdTe layer and correlation with the solar cell conversion efficiency was investigated. CdCl2 treatment was carried out at temperatures ranging from 370 to 460°C for 15 min. A clear correlation between trap density, carrier lifetime, conversion efficiency and the CdCl2 annealing conditions was observed. Un-annealed structures had a conversion efficiency of 5.7%, hole trap energy of EV+0.42eV, hole trap density of 8.71×1014cm−3, and decay lifetime of 0.15μs. The optimum CdCl2 annealing temperature was found to be 415°C for structures grown at a substrate temperature of 595°C, where the conversion efficiency, hole trap energy, hole trap density, decay lifetime were 13.4%, EV+0.44eV, 8.10×1012 cm−3 and 0.40μs, respectively.

Investigation of induced recrystallization and stress in close-spaced sublimated and radio-frequency magnetron sputtered CdTe thin films

We have induced recrystallization of small grain CdTe thin films deposited at low temperatures by close-spaced sublimation (CSS), using a standard CdCl2 annealing treatment. We also studied the changes in the physical properties of CdTe films deposited by radio-frequency magnetron sputtering after the same post-deposition processing. We demonstrated that the effects of CdCl2 on the physical properties of CdTe films are similar, and independent of the deposition method. The recrystallization process is linked directly to the grain size and stress in the films. These studies indicated the feasibility of using lower-temperature processes in fabricating efficient CSS CdTe solar cells. We believe that, after the optimization of the parameters of the chemical treatment, these films can attain a quality similar to CSS films grown using current standard conditions.