P-type CdS Research Articles

P-type CdS thin films formed by in situ Cu doping in the chemical bath

The formation of p-type CdS thin films, doped with Cu, in a chemical bath, is reported for the first time. The Cu doped films showed amorphous nature and exhibited high electrical conductivity as compared to undoped films, which were near stoichiometric with n-type conductivity and crystalline in nature. Cu doping influenced the photocurrent response, structural, electrical, and optical properties of the films. The optical band gap of the CdS film varied from 2.35 to 2 eV after Cu doping. The potential application of these films as an absorber layer in thin film solar cells is being investigated.

Preparation and properties of graded band gap CdS xTe 1−x thin film solar cells

A novel polycrystalline thin film solar cell based on alloys of CdS and CdTe is proposed and investigated. The structure consists of an n-type CdS xTer 1−x (0.5< x < 1) surface layer with a graded composition and hence a band gap which varies between 2.4 eV and 1.5 eV. The base layer consists of homogeneous p-type CdTe or p-type CdS 0.5Te 0.5 of uniform band gap. An apparatus suitable for the vacuum deposition of the proposed structure is described which incorporates a novel technique to control the composition of the graded and mixed CdS xTer 1−x layers. The conductivity of both layers is controlled by simultaneous co-evaporation of suitable dopant materials. Preliminary results are presented, showing that in agreement with theoretical expectations the device exhibits a very wide spectral response. Copper electrodes used for contracting the p-type base layers were found, however, to be unsuitable because of the migration of copper atoms along the grain boundaries. Because of a lack of suitable contacting materials a solar cell with a sufficiently low series resistance has not as yet been produced.

Indirect doping as a possibility for Obtaining p-type conductivity CdS

Indirect doping as a possibility for Obtaining p-type conductivity CdS

Photoconductivity of CdS: Li Single Crystals

CdS single crystals activated with lithium impurity of 0.001 and 0.01 weight percents were prepared by a sublimation recrystallization method, and their photoconductive spectra, light intensity vs. photocurrent and temperature vs. photocurrent characteristics and thermally stimulated current were measured. These results showed the existence of electron traps with high density at 0.87 eV under the conduction band and hole traps at about 0.65 eV above the valence band. These electron and hole traps depend directly and/or indirectly on lithium impurity. We propose that interesting features of photoconductive properties of CdS: Li crystals predominantly occure due to these electron trap and hole trap centers. However, even with an addition of 0.01 weight percent of lithium impurity, p-type CdS has not been obtained.

Photo-Hall effect of CdS : Li single crystals

The photo-Hall effect in lithium-doped CdS single crystals has been studied. Samples were prepared by a sublimation recrystallization method. The carrier concentration n was proportional to the excitation intensity, and the Hall mobility obtained was a constant value of about 200 cm2/V sec, at room temperature over the whole excitation region of 100–105 lx. The temperature dependence of the photo-Hall effect was also measured. The results showed unmeasureably low values of the Hall mobility in the temperature region 350–400 K. However, with increasing temperature the Hall mobility and carrier concentration increase again, but the sign of the Hall effect indicates electrons. The p-type CdS has not been obtained, even with an addition of 0.01 wt% of lithium impurity.

Some Optical Properties of Cadmium Sulfide Containing Phosphorus and Arsenic

Ion implantation and sputtering have generated renewed interest in the possibility of obtaining p-type CdS. However, little is known concerning the properties of acceptors. In an attempt to learn something about acceptors in CdS, phosphorus- and arsenic-doped samples have been prepared by diffusion in a saturated Cd atmosphere at 800°C for 24 h. The electrical and optical properties of the doped samples are compared with undoped test samples (fired in saturated Cd atmosphere but without added dopant). The electrical properties indicate only a slight compensation due to the presence of P and As. The optical studies (photoluminescence, photoconductivity, and optical absorption) yield a level about 1-eV above the valence band, which is presumably the binding energy of an electron to a compensated P or As acceptor.

HIGH CONDUCTIVITY p-TYPE CdS

High conductivity p-type CdS was formed by ion implanting Bi into n-type single crystals. Hall measurements and lifetime studies were performed on the implanted layers. p-n junctions were formed in high conductivity n-type material and light emission observed in the forward direction. By making two independent contacts to the p-type layer and measuring their I-V characteristics, it was determined that they were not rectifying.

Progress in Cadmium Sulfide

Considerable progress has been made during the last two years in both the understanding and application of cadmium sulfide, which is an outstanding photoconductor material. Improved crystal growing techniques have resulted in large single crystals with more perfect crystal structure and higher purity. Further improvements in purity are sought by zone refining of cadmium and sulfur. Ultrasonic cutting methods have been applied successfully to shaping the crystals. High-temperature, high-pressure equipment has been used to grow crystals from the melt. P-type CdS crystals have been produced by heavy copper doping. Thin film techniques have been advanced. Surface volume studies of photoconductors have been made, which clarify certain surface deteriorations. Methods of producing ohmic and barrier electrodes have been improved. All of these fundamental studies have contributed to an improvement in the efficiency, stability, and uniformity of elements used in devices such as photocells, gamma detectors, solar generators, and photorectifiers.