R. G. Dhere

Fabrication procedures and process sensitivities for CdS/CdTe solar cells

This paper details the laboratory processes used to fabricate CdS/CdTe solar cells at the National Renewable Energy Laboratory. The basic fabrication technique includes low-pressure chemical vapor deposited SnO2 , chemical-bath deposited CdS, close-spaced sublimated CdTe, solution-CdCl2 treatment, and an acid-contact etch, followed by application of a doped-graphite paste. This paper also describes the results of a reproducibility study in which cells were produced by multiple operators with an average AM1·5 efficiency of 12·6%. And finally, this paper discusses process sensitivities and alternative cell fabrication procedures and reports the fabrication of a cell with an AM1·5 efficiency of 15·4%. Copyright

Minority Carrier Lifetime Analysis in the Bulk of Thin-Film Absorbers Using Subbandgap (Two-Photon) Excitation

We describe a new time-resolved photoluminescence (TRPL) analysis method for the determination of minority carrier lifetime τB. This analysis is based on subbandgap excitation (two-photon excitation, or 2PE) and allows selective lifetime determination at the surface or in the bulk of semiconductor absorbers. We show that for single-crystal CdTe, τB could be determined even if surface recombination velocity is >105 cm s-1. Two-photon excitation TRPL measurements indicate that radiative lifetime in undoped CdTe is >>66 ns. We also compare one-photon excitation (1PE) and 2PE TRPL data for polycrystalline CdS/CdTe thin films.

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.

Deposition and properties of CBD and CSS CdS thin films for solar cell application

Abstract We deposited cadmium sulfide (CdS) thin films using the chemical-bath deposition (CBD) and close-spaced sublimation (CSS) techniques. The films were then treated in CdCl 2 vapor at 400 °C for 5 min. The CSS CdS films had hexagonal structure, and good crystallinity. The CdCl 2 treatment did not produce major changes, but there was a decrease in the density of planar defects. The untreated CBD CdS films had cubic structure and poorer crystallinity than the CSS films. After the CdCl 2 treatment, these films recrystallized to the hexagonal phase, resulting in better crystallinity and a lower density of planar defects. The conformal coverage and the presence of bulk oxygen are the key issues in making the CBD films more suitable for photovoltaic applications.

Electrical and optical properties of ion beam sputtered ZnO:Al films as a function of film thickness

Electrical and optical properties of as‐deposited, ion beam sputtered, Al‐doped ZnO films have been studied as a function of film thickness and carrier concentration. Hall effect measurements reveal that the bulk electrical resistivity of the film generally decreases with increasing film thickness. Additionally, it is observed that the rate of decreasing resistivity depends on the particular film thickness regime. For thinner films (100–200 nm), the resistivity decreases rapidly with increasing film thickness and is due to increases in both carrier concentration and Hall mobility. However, for thicker films, the resistivity decreases more slowly with increasing film thickness and approaches a nearly constant value at a thickness of 1100 nm. In this thickness regime, the slight decrease in resistivity with increasing film thickness is found to be due to an increase in carrier concentration alone. The above observations suggest the presence of at least two scattering mechanisms. It is speculated that grain ...

High-efficiency polycrystalline CdTe thin-film solar cells with an oxygenated amorphous cds (a-CdS:O) window layer

In the conventional CdS/CdTe device structure, the poly-CdS window layer has a bandgap of /spl sim/2.4 eV, which causes absorption in the short-wavelength region. Higher short-circuit current densities (J/sub sc/) can be achieved by reducing the CdS thickness, but this can adversely impact device open-circuit voltage (V/sub oc/) and fill factor (FF). Also, poly-CdS film has about 10% lattice mismatch related to the CdTe film, which limits the improvement of device V/sub oc/ and FF. In this paper, we report a novel window material: oxygenated amorphous CdS film (a-CdS:O) prepared at room temperature by rf sputtering. The a-CdS:O film has a higher optical bandgap (2.5-3.1 eV) than the poly-CdS film and an amorphous structure. The preliminary device results have demonstrated that J/sub sc/ of the CdTe device can be greatly improved while maintaining higher V/sub oc/ and FF. We have fabricated a CdTe cell demonstrating an NREL-confirmed J/sub sc/ of 25.85 mA/cm/sup 2/ and a total-area efficiency of 15.4%.

Intermixing at the CdS/CdTe Interface and its Effect on Device Performance

A study of the CdS/CdTe interface was performed on glass/SnO 2 /CdS/CdTe device structures. CdS layers were deposited by chemical solution growth to a thickness of 80–100 nm, and CdTe was deposited by close-spaced sublimation at substrate temperatures of 500°, 550°, and 600°C. Post-deposition CdCl 2 heat treatment was performed at 400°C. Samples were analyzed by optical spectroscopy, secondary ion mass spectrometry (SIMS), spectral response, and current-voltage measurements. SIMS analysis shows that the intermixing of CdS and CdTe is a function of substrate temperature and post-deposition CdCl 2 heat treatment. The degree of intermixing increases with increases in substrate temperature and the intensity of CdCl 2 heat treatment. Optical analysis and X-Ray diffraction data show that the phases of CdS x Te 1-x are also a function of the same parameters. Formation of a Te-rich CdS x Te 1-x alloy is favored for films deposited at higher substrate temperatures. Spectral response of the devices is affected by the degree of alloying at the interface. The degree of alloying is indicated by simultaneous changes in long wavelength response (due to the formation of lower bandgap intermixed CdS x Te 1-x ) and the short wavelength response (due to the change in CdS thickness). Device performance is heavily influenced by alloying at the interface. With optimized intermixing, improvements in V oc , and diode quality factors are observed in the resulting devices.

Cathodoluminescence of Cu diffusion in CdTe thin films for CdTe/CdS solar cells

We investigate the distribution of Cu acceptor states in CdTe thin films used in high-efficiency solar cells. These states are CuCd and Cui+–VCd− complexes, which are relatively deep and shallow acceptors, respectively. Cathodoluminescence spectroscopy and imaging show that, first, CuCd and Cui+–VCd− do not coexist and, second, the primary diffuser is represented by Cui+–VCd− complexes. Our results are used to discuss the effect of grain boundaries on Cu diffusion.

Investigation of potential and electric field profiles in cross sections of CdTe/CdS solar cells using scanning Kelvin probe microscopy

We investigated cross sections of working CdTe/CdS solar cells using scanning Kelvin probe microscopy (SKPM). The cross sections were prepared by polishing to avoid steps between the glass substrate and film that generally make the analysis difficult. However, this process resulted in strong pinning of the Fermi level. During the measurements, the cells were biased under different conditions, revealing the distribution of the electrical potential inside the device. We were able to identify different regions inside the device: in the region away from the CdTe/CdS junction, there was only a small variation in the potential; closer to the junction, the potential increased, due to the increase in the depletion regions with the reverse bias; at the junction, there was a sudden increase in the potential, which was attributed to interdiffusion between CdTe and CdS. By taking the first derivative of the potential, we were able to calculate the electric field inside the device. The maximum of the electric field, w...

Determination and observation of electronic defect levels in CuInSe2 crystals and thin films

The effects of heat and surface treatments used in the fabrication of CuInSe2 (CIS) single‐crystal and thin‐film solar cells are investigated, and the associated dominant defect states are identified using high‐resolution photoluminescence (PL) and deep‐level transient spectroscopy (DLTS) measurements. The results are correlated with junction electrical characteristics, and comparisons between thin films and single crystals are established. For the first time, direct evidence for several major defect types (Cu vacancies, Cu at In sites, and In at Cu sites) responsible for the majority‐carrier type of the CIS is presented based upon spectroscopic atomic imaging of the n‐ and p‐type semiconductor surfaces. Radiative recombination levels of extrinsic origin resulting from the surface processing (including polishing, etching, and annealing) were detected in the near surface region of the CIS single crystals, but not in the thin‐film material having the same composition. The energy locations and depth of these...