R. Sudharsanan

Growth and process optimization of CdTe and CdZnTe polycrystalline films for high efficiency solar cells

Abstract Polycrystalline CdTe solar cells with efficiencies of approximately 10% were achieved by metal organic chemical vapor deposition growth of CdTe on glass/SnO 2 /CdS substrates. An in situ pre-heat treatment of the CdS substrate at 450 °C in an H 2 ambient was found to be essential for high performance devices because it removes oxygen-related defects on the CdS surface. This heat treatment also results in a cadmium-deficient CdS surface which may, in part, limit the CdTe cell efficiency to 10% owing to cadmium vacancy related interface defects. The CdCl 2 treatment used in CdTe cell processing was found to promote grain growth, reduce series resistance and interface state density, and change to dominant current transport mechanism from thermally assisted tunneling and recombination via interface states to recombination in the depletion region. These beneficial effects resulted in an increase in the CdTe/CdS cell efficiency from around 2% to approximately 9%. In addition to the CdTe cells, polycrystalline 1.7 eV CdZnTe films were grown by molecular beam epitaxy for tandem cell applications. CdZnTe/CdS cells processed using the standard CdTe cell fabrication procedure resulted in 4.4% efficiency, high series resistance, and a band gap shift to 1.55 eV. Formation of ZnO at and near the CdZnTe surface was found to be the source of high contact resistance. A saturated dichromate etch instead of the Br 2 :CH 3 OH etch prior to contact deposition was found to solve the contact resistance problem. The CdCl 2 treatment was identified to be the cause of the observed band gap shift owing to the preferred formation of ZnCl 2 . A model for the band gap shift along with a possible solution using an overpressure of ZnCl 2 in the annealing ambient is proposed. Development of a sintering aid which promotes grain growth and preserves the optimum 1.7 eV band gap is shown to be the key successful wide gap CdZnTe cells.

Investigation of polycrystalline CdZnTe, CdMnTe, and CdTe films for photovoltaic applications

Polycrystalline thin films of CdZnTe and CdMnTe have been grown by molecular beam epitaxy and metal-organic chemical vapor deposition, respectively, on CdS/SnO2/glass substrates, with bandgaps of 1.65 – 1.75 eV for the top of a two-cell tandem design. P-i-n cells were fabricated and tested using Ni/p+-ZnTe as a back contact to the ternary films. CdTe cells were also fabricated using both growth techniques, which resulted in 9 – 10% efficiency and provided a baseline for ternary cell development. It was found that standard CdTe processing (400°C air annealing) reduces the ternary bandgaps from about 1.7 to about 1.55 eV, resulting in significantly reduced subgap transmission with cell efficiencies of 3 – 4%. Optimum air-annealing conditions were determined to retain the 1.7 eV bandgaps; however, the cell performance was still limited by both poor CdZnTe/CdS interface quality and high series resistance. The junction interface was found to improve by annealing in the presence of hydrogen, which resulted in Voc values from 0.500 V to as high as 0.65 V, but the cell performance became increasingly limited by series resistance. The effects of cell processing on the properties of the CdZnTe/CdS interface, the bulk CdZnTe film, and the back-contact region have been investigated to provide guidelines for achieving high efficiency in widegap ternary cells.

A study of polycrystalline Cd(Zn,Mn)Te/CdS films and interfaces

Polycrystalline films of Cd1-xZnxTe (x = 0–0.4) and Cd1-xMnxTe (x = 0–0.25) were grown by MBE and MOCVD, respectively, on CdS/SnO2/glass substrates to investigate their feasibility for solar cell applications. The compositional uniformity and interface quality of the films were analyzed by x-ray diffraction, surface photovoltage, and Auger depth profile measurements to establish a correlation between growth conditions and lattice constant, atomic concentration, and bandgap of the ternary films. MBE-grown polycrystalline Cd1-xZnxTe films showed a linear dependence between Zn/(Cd + Zn) beam flux ratio, Zn concentration in the film, and the bandgap. Polycrystalline Cd1-xZnxTe films grown at 300° C showed good compositional uniformity in contrast to compositionally non-uniform Cd1-xMnxTe films grown by MOCVD in the temperature range of 420–450° C. The MBE-grown Cd1-xZnxTe interface also showed significantly less interdiffusion compared to the MOCVD-grown Cd1-xMnxTe/CdS interface, where preferential exchange between Cd from the CdS layer and Mn from the Cd1-xMnxTe film was observed. The compositional uniformity of MOCVD-grown polycrystalline Cd1-xMnxTe films grown on CdS/SnO2/glass substrates was found to be a strong function of the growth conditions as well as the Mn source.

Effects of annealing and surface preparation on the properties of polycrystalline CdZnTe films grown by molecular beam epitaxy

The effects of annealing and chemical etching on the chemical, compositional, and electrical properties of polycrystalline molecular‐beam epitaxy (MBE)‐grown Cd1−xZnxTe thin films were investigated in detail for the first time to identify and eliminate some of the undesirable process‐induced side effects resulting from the CdZnTe/CdS solar cell fabrication procedure. Depth‐resolved x‐ray photoelectron and Auger electron spectroscopies on processed surfaces along with current‐voltage and capacitance‐voltage measurements on In/Cd1−xZnxTe Schottky diodes were used to characterize the treated Cd1−xZnxTe surface. Films were annealed in air at 200–400 °C and etched in Br:CH3OH and K2Cr2O7:H2SO4 solutions, similar to processes used to achieve high efficiency CdTe/CdS solar cells. Air annealing was found to enhance the uniformity of the p‐type conductivity within the film, yielding a doping value of ∼3×1015 cm−3. However, the air anneal resulted in a highly Zn‐rich surface compared to hydrogen and argon anneals. ...

Wide bandgap thin film solar cells from CdTe alloys

The growing of ternary films of CdZnTe and CdMnTe by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD), respectively, on glass/SnO/sub 2//CdS substrates with a target bandgap of 1.7 to 1.8 eV for solar cell applications is presented. X-ray diffraction, surface photovoltage spectroscopy, and Auger electron spectroscopy measurements were performed to estimate the bandgap, compositional uniformity, and interface quality of the films. Front-wall CdTe cell (glass/SnO/sub 2//CdS/CdTe/ZnTe/metal) efficiencies were found to be approximately 9%, while CdZnTe and CdMnTe efficiencies were approximately 3.6% and 6%, respectively. The n-i-p cell efficiencies were consistently higher than those of the n-p cells. The optimum cell processing temperature for CdZnTe films was found to be less than 400 degrees C. Higher processing temperatures caused a shift in bandgap coupled with film quality degradation.<<ETX>>

An improved understanding of efficiency limiting defects in polycrystalline CdTe/CdS solar cells

Efficiency-limiting mechanisms associated with CdS substrates, CdCl/sub 2/ treated CdTe films, and Cu/Au contacts were investigated. It was found that thermal treatment prior to CdTe deposition removed the oxygen-related defects from the CdS films. However, it also created nonuniform and Cd deficient CdS surface, resulting in an optimum temperature of 450 degrees C. Growth of CdTe films on the CdS under Te-rich conditions enhanced the interdiffusion between CdTe and CdS and reduced the CdTe bandgap to 1.47 eV, whereas growth of CdTe films under Cd-rich conditions retained the CdTe bandgap at 1.5 eV by reducing the interdiffusion. It was found that use of thinner CdS (1000 AA instead of 2000 AA) films increased the cell efficiency from 9.7% to 10.3% by increasing the J/sub SC/ from 22.4 to 24.19 mA/cm/sup 2/.<<ETX>>

Investigation of metalorganic chemical vapor deposition grown CdTe/CdS solar cells

Abstract Polycrystalline CdTe films were grown on CdS/SnO 2 /glass substrates by metalorganic chemical vapor deposition (MOCVD) for solar cell applications. Cells fabricated on these films showed efficiency of 9.7% which is the highest efficiency reported so far for MOCVD grown CdTe solar cells. The bias-dependent spectral response of the 9.7% efficient cell showed an external quantum efficiency greater than 0.85 at zero bias but a significant wavelength-independent reduction in spectral response at higher voltages. The interface recombination model was used to calculate the interface collection function term to quantify the open-circuit voltage ( V oc ) and fill factor losses in the high efficiency cell. It was found that the interface recombination reduces the V oc and fill factor by 60 mV and 0.1 respectively. It was estimated that efficiency as high as 13.5% can be achieved by improving CdTe/CdS interface quality.

Effects of pre-heat treatment of CdS on MOCVD CdTe/CdS solar cell performance

Preheat treatment of CdS/SnO/sub 2//glass substrates prior to CdTe growth was investigated to improve the CdTe/CdS cell performance. The substrates were annealed in an H/sub 2/ atmosphere range of 300 to 450 degrees C. N-i-p cells fabricated on these substrates showed a reduction of interface states with increasing annealing temperature. This may be associated with a reduction in oxygen concentration at the surface of CdS. Cells fabricated on substrates without any heat treatment had poor device performance with V/sub oc/=0.55 V, fill factor (ff)=0.49, and an efficiency of 5.6%. Cell parameters improved with increasing annealing temperature, and the best values, V/sub oc/=0.68 V, ff=0.59, and efficiency = approximately 8%, were obtained at an annealing temperature of 450 degrees C. In spite of the improved performance due to reduced interface states, thermal treatment also results in some detrimental effects, like a nonuniform Cd/Te ratio on the surface of CdS and a Cd-deficient CdS layer near the junction, which tend to limit the cell performance.<<ETX>>

Characterization of MOCVD-grown CdMnTe films by infrared spectroscopy

Single crystal Cd1−xMnxTe (x=0.10–0.30) films have been grown by metalorganic vapor deposition (MOCVD) on (111) GaAs substrates with and without CdTe buffer layers, at substrate temperatures of 380° to 450° C. Infrared phonon spectra reveal that the films grown at 420° C substrate temperature have reasonable Mn concentration (>10%) and are of good quality in agreement with Raman measurements. Spectral analysis also gives values for Mn concentration that agrees with photoluminescence measurements, and determines film thickness.

Metalorganic chemical vapor deposition of PbTe films on GaAs substrates

PbTe films on GaAs substrates were grown by metalorganic chemical vapor deposition for the first time, at substrate temperatures as low as 300 °C. Films grown directly on GaAs below a substrate temperature of 400 °C showed (111) orientation on (100) GaAs and (100) orientation on (111) GaAs. However, above 400 °C on GaAs and on CdTe buffer layers, the films followed the orientation of GaAs and CdTe, respectively. Hall measurements showed n‐type conductivity in PbTe films grown directly on GaAs regardless of the Te/Pb input gas ratio and substrate temperature. The conductivity type of PbTe films grown on CdTe buffer layers depended on the input gas ratio of Te/Pb. The resistivity measured by Hall measurement was higher than that determined from infrared data, suggesting microcracks in the films.