Fundamentals and Design Considerations for Tandem Solar Cells with 2-, 3- and 4-Terminals

Tandem solar cells with two or more subcells can outperform single-junction solar cells, but their additional complexity brings with it additional design considerations. One high-level decision which must be made is the number of external electrical connections to be used. Here we will discuss design decisions associated with two-junction tandem solar cells with two, three, or four terminals. The discussion will be framed in terms of 2-junction tandem solar cells (comprised of a top subcell and a bottom subcell), but the concepts are extensible to tandem solar cells with more junctions.<br/> <br/>In a 4-terminal tandem solar cell, each subcell (top and bottom) has two contacts such that power from each is collected independently. In a 2-terminal tandem, the subcells are series-connected such that the same current passes through both subcells (I_top = I_bottom). In a 3-terminal tandem, the third terminal collects any excess current from between the subcells, thereby eliminating any current-matching constraints. Instead, a 3-terminal tandem is typically constrained by module circuitry to operate in a voltage-matched configuration with V_top/V_bottom = m/n, where m and n are integers.<br/> <br/>In principle, a 4-terminal tandem should maximize the energy collected as the operating conditions and incident solar spectra vary. In practice, these advantages must be balanced against the parasitic losses associated with the additional current-collection layers required for a 4-terminal device. At the other extreme, a 2-terminal tandem will be most sensitive to changes in incident spectra, but its simpler architecture typically has fewer inherent losses, which can give it a higher baseline efficiency under any given fixed spectrum. A 3-terminal tandem is a compromise between these two extremes, with less spectral sensitivity than a 2-terminal tandem, but fewer parasitic losses than a 4-terminal tandem (because it has one less current-extracting layer). However, 3-terminal tandems will typically be connected in an interlaced voltage-matched configuration, which creates some additional complexity.<br/> <br/>The fundamentals and design/material considerations for each of these configurations (2-, 3- and 4-terminal) will be discussed and compared.