Perspectives and designs towards solar cell performance limits

The solar cells can be broadly classified into two groups: (i) high performance (but relatively expensive) crystalline cells with efficiencies approaching the thermodynamic limit, and (ii) lower cost thin film alternatives that offer improved cost-efficiency tradeoff. For the first group, the challenge is to develop a meticulous understanding of cell operation so that the remaining efficiency gap can be closed. For the second group, the challenge is to develop a material-specific understanding of cell operation to improve their efficiency and reliability. In the first set of studies, we explore the limits of solar cells spanning both type of above mentioned technologies. The thermodynamic study of photovoltaic (PV) devices explains the performance limit and fundamental loss mechanisms for an idealized ‘PV-perfect’ material. First, based on a 2-energy-level system, we qualitatively discuss and develop intuitive understandings of fundamental physics of PV operation. Next, we revisit the solar cell physics using the well-known ‘detailed particle balance’ formalism. We extend the theory to discuss the detrimental effect of heterojunction (HJ)-limited recombination on solar cell, which defines the performance limits of organic photovoltaics (OPVs). The study also provides insights on critical binding condition for exciton dissociation, and prospects of HJ-free OPVs. The detailed balance theory is also used to explain the thermodynamics of a novel class of solar cells: the bifacial tandem. Such a cell accepts light from both faces (direct sunlight and albedo) which enhances the output and also relaxes the current matching conditions in certain cases. The bifacial tandem may as well be the next generation of high output solar cell with very little added fabrication complexity. In the second set of studies, we apply the insights from the thermodynamic analysis to solve practical design problems—these are sub-divided into three major topics: light management, opto-electric designs, and addressing poor carrier transport (for OPVs). First , among the light management schemes, our nanowire (NW) based photon absorption scheme allows independent control of reflection and absorption. We have also proposed a conceptual planar design with a back ‘meta-mirror’ that can perfectly trap the photons that are coupled into the structure. This approach has the potential to break the geometric light trapping limit, and also can be easily integrated into a planar thin film PV device. Second, we establish a coupled absorption-emission-transport simulation framework necessary to meticulously study high efficiency solar cell designs. We, for the first time, defined the practical constraints for the successful implementation of angle restricted solar cells (APVs). We also use this opto-electric simulation framework to study a perovskite-HIT bifacial tandem cell in a practical scenario. Finally, we propose a ‘collection limited’ theory for HJ-free OPVs corroborated by rigorous numerical and experimental verification. The poor collection in the HJ-free OPVs is then addressed by two possible designs: layer-split-tandem, and projected electrode configuration. The understandings on OPVs may introduce new design paths for better performing HJ-free OPVs