ABSTRACT
Chapter 10), which opens up the possibility of high-efficiency multijunction cells lattice-matched to silicon.
13.1.2 Collection in Solar-Cell Junctions 13.1.2.1 Constraints on solar-cell length scales
A solar-cell junction works by absorbing photons, converting them into electron-hole pairs, and then collecting the minority carriers as current at the junction. Photons that are not absorbed, or minority carriers that recombine at traps rather than being converted to current at the junction, represent losses in cell efficiency. There are three important length scales of the junction:
1. The thickness d of the absorbing region 2. The absorption length 1/a(?) at the photon wavelength k of interest 3. The collection length 1 from the junction in which minority carriers
are collected without significant recombination loss
The need for efficient photovoltaic conversion sets constraints on these length scales as design rules for an efficient solar cell. For most of the incident photons to be absorbed, it is necessary that d > 1/oc(k). Furthermore, for complete collection of the minority carriers generated by the absorbed photons, it is necessary that 1> d. For GaAs cells, these constraints call for a thickness d = 3 gm and a collection length 1> d. Indeed, solar cells, with their critical dependence on minority-carrier collection, can be used as test structures for the minority-carrier properties of their component semiconductors.
