Characteristics and performance of silicon solar cells between low-and high-level injection—Part II: Results of the study

Abstract

Applying the extension of the transport velocity transformation method described in Part I of this paper [1], the I-V characteristics of several types of solar-cell structures have been studied in the range between the validity of the low-level or of the high-level injection assumptions. The structures investigated were a more heavily doped "wide-base diode," a lowly doped "narrow-base diode," and a structure with a high-low junction in its base. Various effects have been seen to dominate in different ranges of the I-V characteristic, causing observable changes in its slope. Depending on the particular structure, the major effects are: changes in the majority-carrier distribution, ohmic effects and changes in conductivity modulation, changes in minority-carrier lifetime, and reduction of the high-low junction barrier height, both of the latter resulting from increasing carrier concentrations. In addition, the influence of the impurity concentration in the more lowly doped layer of the base in solar cells with a high-low junction, on the conversion efficiency and on its dependence on the light intensity was investigated for optical concentration ratios up to 1000. In these studies, the doping range of 5E15-5E17/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> was seen to yield a broad efficiency maximum, with the higher doping more favorable except for its limitation by the onset of the heavy doping effects. It has also been seen that the collection efficiency is increased or decreased by some of the effects investigated. In consequence, the collection efficiency can no longer be rigorously considered as independent of the light intensity. In some cases, this dependency occurs even when the low-level injection condition is still fulfilled.