Effect of Affinity Discontinuity on Heterojunction p-i-n Solar Cell Performance

Effect of the band alignment on the performance of heterojunction p-i-n solar cell is investigated in the framework of one-dimensional transport model. An affinity-based approach is introduced in order to identify a set of material parameters and doping levels which would result in lower series resistance R_s, higher open-circuit voltage V_oc, and higher fill factor FF. The search reveals key ingredients of the nongraded heterojunction optimized design, whose efficiency can be as high as 27%. On one hand, certain band alignment determined by affinity-related quantities has to be observed to ensure the case of staggered gaps, once the dimensional requirements in the absorber and in the charge collectors are met. On the other hand, an affinity-based constraint is shown to control the potential profile which mitigates the adverse effect of barriers and accumulation regions at the junction interfaces. The constraint is shown to result in lower R_s and, thus, higher FF. Analytical approach reveals a whole region in the affinity space where the reverse saturation current J₀ is at minimum, which also strongly overlaps with the constraints pinning down R_s. It is concluded that the performance figures are featured with shallow minima as functionals in roughly same region of the affinity space, where those numbers are relatively stable versus changes in the band alignment. The findings aim to make material screening more feasible and widens significantly the options in the design of new p-i-n cells.