Photoconductive solar cells

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

Madan et al. [515] have presented the effect of modulation on the properties of the material (dark conductivity and photoconductivity) and of solar cells. They also observe an increase in deposition rate as a function of modulation frequency (up to 100 kHz) at an excitation frequency of 13.56 MHz, in their PECVD system [159]. The optimum modulation frequency was 68 kHz, which they attribute to constraints in the matching networks. Increasing the deposition rate in cw operation of the plasma by increasing the RF power leads to worse material. Modulation with a frequency larger than 60 kHz results in improved material quality, for material deposited with equal deposition rates. This is also seen in the solar cell properties. The intrinsic a-Si H produced by RF modulation was included in standard p-i-n solar cells, without buffer or graded interface layers. For comparison, solar cells employing layers that were deposited under cw conditions were also made. At a low deposition rate of about 0.2 nm/s, the cw solar cell parameters... 

Photoconductivity forms the basis for photocopying, and photovoltaic effects form the basis for solar cells being developed to harvest light energy. 

Grey selenium exhibits both photovoltaic and photoconductive properties, which make it useful in the production of photocells and solar cells. Moreover,... 

In Chapter 3, Swartz presents a model for the p-i-n solar cell consisting of two transitions (like two portions of a p-n junction) separated by a photoconductive i layer. Various physical properties can be incorporated... 

Fig. 3. Equivalent circuit of amorphous silicon solar cell showing voltage source, photoconductive resistance, and fixed series resistance.

Optical measurements showed that a small concentration of 2% is effective in quenching the PL of MDMO but a high concentration of more than 67% is required to increase the photoconductivity or the short circuit current of a BHJSC. The solar cells efficiency measurements show that the optimum PCBM concentration is 80%. See next section. 

Illumination creates excess electrons and holes which populate the extended and localized states at the band edges and give rise to photoconductivity. The ability to sustain a large excess mobile carrier concentration is crucial for efficient solar cells and light sensors and depends on the carriers having a long recombination lifetime. The carrier lifetime is a sensitive function of the density and distribution of localized gap states, so that the study of recombination in a-Si H gives much information about the nature of the gap states as well as about the recombination mechanisms. 

The importance of these amorphous layers derives from their electronic structure. There are no longer sharp bands characterized by a definite band gap, but quasi-continuous changes in the density of states are observed leading to differences between the optical gap and the mobility gap . Thus, interesting optoelectronic properties and applications are reported, e.g., photoconductivity and solar cells [204, 205], optical vnndows for solar cells [206, 207], electroluminescence and light emitting diodes (LED) [207, 208], or thermistors for IR sensors [209]. 

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