Charge carrier collection

Electrons and holes generated in the i-layer by incident light are driven to the n- and p-layer by the internal electric field, respectively. The material quality of the intrinsic layer and the strength and distribution of the electric field are responsible for the charge carrier collection and mainly determine the electrical solar cell performance. Defects affect the charge carrier collection in two different ways On the one hand they act as recombination centers, and on the other hand their charge state modifies the electric field distribution in the i-layer. 

The quantum efficiency of a device is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the cell surface. Two types of quantum efficiencies are often quoted the internal quantum efficiency (IQE) that includes the effects of losses through light transmission and reflection and the external quantum efficiency (EQE) that excludes these effects and instead considers only the fraction of photons that were actually absorbed. 

Besides the considerations on the parameters of the semiconductor and the redox electrolyte, performance of the counterelectrode also matters for designing an efficient PEC cell. The counter-electrode in a PEC cell is a charge carrier collecting material, and catalyzes the electron transfer, but is not chemically involved in the electrochemical reaction. 

It is defined as ratio of number of charge carriers collected xmder short circuit condition to the number of photons falling on the solar cell. 

CHARGE CARRIERS Collectively electrons and holes created by the interaction of the gamma-ray with the detector that comprise the detector signal. 

Fig. 3 Band diagrams of a 300 nm thick active layer (a) with doping (]Va = lO cm ) and (b) without doping. In case of doping, a small space charge region and a larger field free region are created. It can be shown that in the field free region, charge carrier collection is reduced...

FIGURE 5.4 Cross-sectional view for the comparison of photogenerated charge-carrier collection at planar and high-aspect-ratio semiconductor photoelectrodes. (From Price, M.J. and Maldonado, S., J. Phys. Chem C, 113, 11988,2009.)... 

An attractive method to improve the light absorption and charge-carrier collection involves high aspect ratio cylindrical absorbers, such as ID nanowires [40], To fabricate such solar cells, the following methods are required (1) preparation of large area arrays that are vertically aligned, (2) fabrication of electrical junctions to such wire arrays, and (3) fabrication of electrical contacts to the backsides of the devices. 

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