Novel electronic approach for efficient energy harvesting from Cu2S/CdS solar cell

Abstract

Remarkable progress has been made in the development of high efficiency Copper-Sulfide/Cadmium-Sulphide (Cu2S/CdS) solar cells. The stability degradation mechanisms of these devices are of utmost importance. Cu2S/CdS solar cell is fabricated of four main layers and deposited on a glass substrate. CdS film exhibited high degree of transmittance in the infrared region and showed sharp absorption edge at nearly 0.5 μm wavelength. Transmittance curves of Cu2S films diminished transmission in the near infrared region and have maximum transmittance value at 0.7 μm wavelength. The spectral response measurements for the Cu2S/CdS films prove that the light is absorbed mainly by the Cu2S layer. The sample exposed to 100 oC annealing (20 min.) yielded maximum photogenerated current. The annealing improves the open circuit voltage (Voc) of the samples, and Voc increases considerably with time while the degradation of the sample under illumination becomes smaller. The load characteristics of the cell washed for five minutes and heated at 200oC is improved by enhancement of the exchange reaction and formation of Cu2S/CdS layer. Another achievement of the cell efficiency is to deposit of a very thin layer of copper followed by additional heat treatment at 200oC for 5 minutes to ensure the formation of chalcocite phase. Improvements have been made by varying the thickness of Cu2S layer. More than 80% efficiency has been detected at 5 μm thicknesses with the highest efficiency in the gradual transition between 0.5 and 0.8 μm. This is the reason behind the choice of a few microns (2–5μm) thickness of the Cu2S layers. Creating a highly doped p++ layer at the surface of Cu2S can reduce the effects of the back contact potential barrier produces tunneling or thermally assisted tunneling carrier transport mechanism. The tunnel diode is formed by high doping thin layer of Cu2S to produce p++, then depositing another highly doped CdS to produce n++. The tunnel diode is designed to be able to have a peak current that may exceed 50mA/cm2. Measured I-V characteristics of a Cu2S/CdS solar cell with Cu2S (p++) CdS (n++) tunnel diode at the short circuit current of the cell to be lower than the maximum tunneling current push the short circuit current up as long as the photogenerated current is increased, this consequently means that the maximum power output of solar cell is increased.