4 Search Results

Bifacial tandem cells promise to reduce three fundamental losses (i.e., above-bandgap, below bandgap, and the uncollected light between panels) inherent in classical single junction photovoltaic (PV) systems. The successive filtering of light through the bandgap cascade and the requirement of current continuity make optimization of tandem cells difficult and accessible only to numerical solution through computer modeling. The challenge is even more complicated for bifacial design. In this paper, we use an elegantly simple analytical approach to show that the essential physics of optimization is intuitively obvious, and deeply insightful results can be obtained with a few lines of algebra.more » This powerful approach reproduces, as special cases, all of the known results of conventional and bifacial tandem cells and highlights the asymptotic efficiency gain of these technologies.« less

Here, a traditional single-junction solar panel cannot harness ground-scattered light (albedo reflectance, R_A), and also suffers from the fundamental sub-band-gap and the thermalization losses. In this paper, we explain how a “bifacial tandem” panel would dramatically reduce these losses, with corresponding improvement in thermodynamic performance. Our study predicts (i) the optimum combination of the band-gaps, empirically given by E^opt_g(t) ≈ E^opt_g(b) (2+R_A)/3+(1–R_A) and the (ii) corresponding optimum normalized output power given by η*_T(opt) ≈ R_A (2η_SJ(opt)) + (1–R_A)η_DJ(opt). Empirically, η*_T(opt) interpolates between the thermodynamic efficiency limit of classical double-junction tandem cell (η_DJ) and twice that of a single-junction cell (η_SJ).more » We conclude by explaining how the fundamental loss mechanisms evolve with R_A in a bifacial tandem cell.« less

As single junction photovoltaic (PV) technologies, both Si heterojunction (HIT) and perovskite based solar cells promise high efficiencies at low cost. Intuitively, a traditional tandem cell design with these cells connected in series is expected to improve the efficiency further. Using a self-consistent numerical modeling of optical and transport characteristics, however, we find that a traditional series connected tandem design suffers from low J_SC due to band-gap mismatch and current matching constraints. Furthermore, a traditional tandem cell with state-of-the-art HIT (η = 24%) and perovskite (η = 20%) sub-cells provides only a modest tandem efficiency of η_T~ 25%. Instead, wemore » demonstrate that a bifacial HIT/perovskite tandem design decouples the optoelectronic constraints and provides an innovative path for extraordinary efficiencies. In the bifacial configuration, the same state-of-the-art sub-cells achieve a normalized output of η_T* = 33%, exceeding the bifacial HIT performance at practical albedo reflections. Unlike the traditional design, this bifacial design is relatively insensitive to perovskite thickness variations, which may translate to simpler manufacture and higher yield.« less