U. Jayamaha

Radio-frequency-magnetron-sputtered CdS/CdTe solar cells on soda-lime glass

We report the fabrication of an 11.6% efficient, polycrystalline thin‐film CdS/CdTe solar cell in which both semiconductor layers were deposited by planar‐magnetron‐radio‐frequency sputtering at 380 °C on commercially available soda‐lime float‐glass substrates coated with SnO2:F. We show that the magnetron magnetic field is critical to obtaining high cell efficiency. Much stronger photoluminescence and higher electrical conductivity are found in films and cells grown with unbalanced‐field magnetrons. The magnetic field dependence is interpreted as arising from the enhanced electron and ion bombardment of the film growth interface when unbalanced magnetrons are used.

Photoluminescence of Cu-doped CdTe and related stability issues in CdS/CdTe solar cells

We explore Cu electronic states in CdTe using photoluminescence as the main investigative method. Our results are consistent with some Cu atoms occupying substitutional positions on the Cd sublattice and with others forming Frenkel pairs of the type Cui+–VCd− involving an interstitial Cu and a Cd vacancy. In addition, we find that Cu-doped CdTe samples exhibit a significant “aging” behavior, attributable to the instability of Cu acceptor states as verified by our Hall measurements. The aging appears to be reversible by a 150–200 °C anneal. Our results are used to explain efficiency degradation of some CdTe solar-cell devices which use Cu for the formation of a backcontact.

Optimization of laser scribing for thin-film photovoltaics

In this paper, the authors report on studies of four different lasers for the scribing of thin-film photovoltaic materials including CdTe, CIS, SnO/sub 2/, ZnO, and Mo. They have used cw-lamp-pumped and flashlamp-pumped Nd:YAG lasers (/spl lambda/=532/1064 nm), a copper-vapor laser (511/578 nm), and an XeCl-excimer laser (308 nm), with pulse durations ranging from 180 nsec down to 8 nsec. The purpose of this work is to identify the effects of laser wavelength and laser pulse duration on the most effective energy utilization and on the quality of the scribe line. Ablation spots and scribe lines were examined by optical microscopy and stylus profilometry. The ablation threshold has been identified for the combinations of four lasers and five materials. The energy density for optimum removal of material has been identified for two of the laser systems (three wavelengths).

Pulse duration and wavelength effects in laser scribing of thin-film polycrystalline PV materials

This project is focussed on a study of wavelength-dependent effects and pulse-duration effects on laser scribing of polycrystalline thin-film PV materials. The materials studied here are CdTe, CI(G)S, SnO2, ZnO, molybdenum and gold. This paper provides a summary of thresholds and optimum scribing energy densities for two types of Nd:YAG lasers, a 308 nm excimer laser, and a copper vapor laser. A comparison is presented of glass-side vs. film-side scribing. Discussion is also given of scribing of multilayer films such as ZnO/CIS/moly and gold/CdTe/SnO2.

Admittance spectroscopy of CdTe-based solar cells

Frequency-dependent complex admittance measurements have been extensively used for the analysis of semiconductor p-n junctions. Properties such as carrier concentration, density and spatial distribution of traps can, in principle, be extracted from this type of measurement. In this study, the authors present results of high-precision complex admittance measurements performed on thin-film CdTe solar-cell devices at frequencies ranging between 1 Hz to 100 KHz, temperatures varying between -15/spl deg/C and 60/spl deg/C with and without Cu-doping. The extended range of frequencies allows the study of long-lived traps, which are important in understanding the physical processes in CdTe devices and the relation between the formation of deep traps and possible degradation mechanisms. The data are interpreted using a model of continuous, energy dependent density of states in the forbidden gap of the device.

Interdiffusion of CdS/CdTe in laser-deposited and RF sputtered alloys, bilayers and solar cells

We have studied interdiffusion between CdS and CdTe in solar cells, bilayers, and in alloy films by Rutherford backscattering (RBS), Raman scattering, photoluminescence (PL), optical absorption and X-ray diffraction. The cells and films were fabricated by laser physical vapor deposition (LPVD) or by RF sputtering (RFS). We also have prepared films of the ternary alloy material (CdS/sub x/Te/sub 1-x/) by LPVD, related film composition to target composition by WDS, and obtained values for band bowing, lattice constant, phonon frequencies, and low temperature PL characteristics. The RBS spectra, obtained from thin bilayers of CdTe/CdS on fused silica, provide information on interdiffusion with /spl sim/200 /spl Aring/ depth resolution.

Raman and RBS studies of interdiffusion in RF-sputtered CdS/CdTe solar cells

The performance of CdS/CdTe photovoltaic devices is strongly determined by the properties of the CdS/CdTe interface region which forms during the heat treatment of the solar cell. Due to interdiffusion of sulfur and tellurium across the original CdS/CdTe junction and the formation of CdSxTe1−x at the interface, material properties such as the bandgap and the absorption coefficient of the newly formed material will be changed. In order to improve our understanding of the interface and to be able to control it, near resonant Raman scattering on a series of single-phase CdSxTe1−x alloys was performed and the Stokes shifts of the longitudinal optical (LO) phonons were measured over the entire composition x. The data have been fitted according to the modified random element isodisplacement (MREI) model. The results gained from the investigation of the alloys have then been applied to study the CdS/CdTe interface region of sputter-deposited solar cells. The formation of a two-phase CdSxTe1−x alloy region at the...

Capacitance - frequency analysis of CdTe photovoltaics

We present data on frequency dependent capacitance for fresh and degraded CdS/CdTe devices in a broad range of frequency from 0.1 to 10/sup 5/ Hz and for various temperatures. The data are consistent with the model where defects have continuous, energy dependent density of states (DOS) in the forbidden gap. The DOS is found to change in the course of device degradation. We introduce the concept of a single defect capacitance and pay special attention to interpreting the temperature dependence of our data. To ensure the interpretation where DOS is temperature independent we take into account the multiphonon character of the trapping-detrapping electron transitions.

Effects of Magnetic Field Configuration on Rf Sputtering for CdS/CdTe Solar Cells

We report studies of solar cells prepared by rf planar magnetron sputtering in which the films were deposited using magnetic field structures ranging from approximately balanced to strongly unbalanced in the “type II” configuration. For films grown with the unbalanced configurations, we find much stronger photoluminescence and much better cell performance than for the balanced configuration. The CdTe films show differences in electrical performance depending on magnetic field as well. These effects are interpreted as arising from the enhanced electron and ion bombardment of the film growth interface for the unbalanced magnetrons. Using two unbalanced magnetrons we have fabricated an all-rf-sputtered cell with NREL-verified efficiency of 11.66% at air mass 1.5 illumination.

Performance vs. microstructure in RF sputtered CdS/CdTe solar cells

In RF sputtering the film microstructure can be affected by many factors including substrate temperature, sputter gas pressure, substrate bias, and the magnetic field of the magnetron. In this paper we examine principally the effects of pressure, substrate bias, and the magnetic field of the planar magnetron sputter source on the microstructure and relate this to the performance of CdS/CdTe solar cells in which both semiconductor layers were grown by RF sputtering. With careful choice of gas pressure and magnetic field configuration, we have demonstrated an all-RF-sputtered CdS/CdTe solar cell with AM 1.5 efficiency of 11.6% as confirmed by NREL.