Too Many Junctions?: A Case Study of Multijunction Thin-Film Silicon Solar Cells

The benefit of two-terminal multijunction solar cells in regard to the numberof junctions (subcells) is critically evaluated. The optical and electrical lossesinherent in the construction of multijunction cells are analyzed using informationfrom thin-film silicon photovoltaics as a representative case. Althoughthe multijunction approach generally reduces the thermalization and nonabsorption losses, several types of losses rise with the number of subcells.Optical reflection and parasitic absorption are slightly increased by addingsupporting layers and interfaces. The output voltages decline because of thetunnel recombination junctions, and more importantly of the illuminationfiltered and reduced by the top subcell(s). The loss mechanisms consumethe potential gains in efficiency of multijunction cells. For thin-film silicon,the triple-junction is confirmed to be the best performing structure. Moregenerally, only when each component subcell shows a high ratio between theoutput voltage and the bandgap of the absorber material, a multijunction cellwith a large number of subcells can be beneficial. Finally, the high voltageand low current density of multijunction cells with a large number of subcellsmake them difficult to optimize and manufacture, vulnerable to any changesin the solar spectrum, and thus less practical for the ordinary terrestrialapplications.Photovoltaic Materials and DevicesElectrical Sustainable Energ