Collection asymmetry in a drift-driven p–i–n solar cell J.M. Asensi * , D. Soler, M. Fonrodona, J. Bertomeu, J. Andreu Departament F  ısica Aplicada i  Optica, Universitat de Barcelona, Avinguda Diagonal 647, E-08028 Barcelona, Spain Abstract An analytical expression for the voltage dependence of the internal collection efficiency of amorphous silicon p–i–n solar cells is presented. The influence of arbitrary drift lengths (asymmetrical case) is taken into account and it is shown how this asymmetry affects the spectral dependence of the collection. A new experimental technique is proposed to determine the so-called collection voltage. Comparison between preliminary experimental data and theoretical results reveals the limitations of the analytical description. Ó 2002 Published by Elsevier Science B.V. 1. Introduction In the case of a-Si:H based p–i–n solar cells photocurrent depends on bias voltage (V). It is because in p–i–n structures photocurrent is mainly produced in the i-layer, where the field supports photocarrier collection. The field diminishes under increasing forward voltage. This enhances the re- combination and lets the photocurrent decrease. Various analytical approaches have been pro- posed to describe the voltage dependence of col- lection in terms of the quality of the i-layer [1–3]. These models require strong assumptions in order to solve the transport equations. For instance, it is usual to assume that the electric field through the i-layer is uniform and, in many cases, only condi- tions of weakly absorbed light are considered. Another useful hypothesis is to assume symmetric collection [2], i.e., that the drift lengths for elec- trons and holes are the same. Seemingly, in the case of a-Si:H there is an experimental justification in assuming symmetric collection [4]. However, there is no difficulty about developing the more general case of arbitrary drift lengths (or ls- products) for electrons and holes. In this work we review the concept of collection voltage, which is a very useful parameter to eval- uate the performance of drift-driven solar cells (see [2]). We present analytical expressions for the spec- tral dependence of this parameter in the general case of unequal drift lengths. 2. The collection voltage V c A straightforward experiment that demon- strates the sensitivity of photocurrent to the applied voltage is the variable illumination mea- surement of the short-circuit resistance R sc , i.e., the reciprocal slope of the I ðV Þ curve at the short- circuit point. Over a wide range of illumination levels, R sc is inversely proportional to the short- circuit current I sc . In other words, the R sc I sc -prod- uct is independent of the light intensity. This means that the dependence of R sc on light intensity Journal of Non-Crystalline Solids 299–302 (2002) 1142–1146 www.elsevier.com/locate/jnoncrysol * Corresponding author. E-mail address: jmasensi@fao.ub.es (J.M. Asensi). 0022-3093/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII:S0022-3093(01)01082-1