Fundamentals of PV Efficiency Interpreted by a Two-Level Model

Elementary physics of photovoltaic energy conversion in a two-level atomic PV is considered. We explain the conditions for which the Carnot efficiency is reached and how it can be exceeded! The loss mechanisms - thermalization, angle entropy, and below-bandgap transmission - explain the gap between Carnot efficiency and the Shockley-Queisser limit. Wide varieties of techniques developed to reduce these losses (e.g., solar concentrators, solar-thermal, tandem cells, etc.) are reinterpreted by using a two level model. Remarkably, the simple model appears to capture the essence of PV operation and reproduce the key results and important insights that are known to the experts through complex derivations.Comment: 7 pages, 6 figure

Thermodynamic efficiency limits of classical and bifacial multi-junction tandem solar cells: An analytical approach

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 bandgapcascade and the requirement of current continuity make optimization of tandem cellsdifficult 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. 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

Bifacial Si heterojunction-perovskite organic-inorganic tandem to produce highly efficient (η T * ~ 33%) solar cell

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 JSC due to band-gap mismatch and current matching constraints. Specifically 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 ηT~ 25%. Instead we 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