The remarkable development in photovoltaic (PV) technologies, including materials, cells and modules, over the past five years call for renewed assessments with an eye towards their future progress. We do not restrict such assessments to solar to electrical power conversion efficiencies (PCEs), but also consider many of the factors that affect power output for each cell type. Where appropriate, we note improvements in control over materials and interfaces, and discovery of new properties in materials. The PCE of "champion cells" for all types of PV technology has improved over the past half decade. We analyse and discuss the remarkable progress in cells and modules, based on single crystal -Si, GaInP and InP, and on thin (polycrystalline) films of , esp. CdTe and Cu(In,Ga)Se 2 (=CIGS). In addition, we analyse PV developments of the more recently emerged lead halide perovskites, together with notable improvements in sustainable chalcogenides, organics and quantum dots. By comparing PV cell parameters across technologies, we can appraise how far each technology may progress in the near future, because, even though accurate or revolutionary developments cannot be predicted, often cross-fertilization occurs, making achievements in one cell type an indicator of evolutionary developments in others. This is extremely relevant in the present time, since the common theme of metal halide perovskites has helped to unite previously disparate, technology-focussed strands of PV research.
Introduction:Undoubtedly, sunlight is the most abundant, safe and clean energy source for sustainable economic growth. One of the efficient and practical ways to use the sunlight as an energy source is to convert it to electricity using solar cells. An upper limit for light to electrical power conversion efficiency, PCE, by a single junction solar cell (i.e., solar photon energy electrical energy) is given by the Shockley-Queisser (S-Q) model and formalism 1 . In this formalism there are assumptions, which postulate that all photons with energies above the bandgap create free electrons and holes, with perfectly chargeselective contacts, thus yielding one electron per absorbed photon to the electrical current flow. The S-Q model also stipulates that all electron-hole recombination events, which occur when the solar cell is generating power, are the inverse process to light absorption and therefore radiative -i.e., they result in the re-emission of light. The S-Q limit is based purely on thermodynamic considerations and takes the optical absorption edge (E G ), the solar spectrum and the operating temperature of the solar cell as the only inputs for the PCE calculation. The efficiency of real-world single junction solar cells will always be below the S-Q limit as real material properties come into play, e.g., the absorption edge is not a step function, as assumed by the S-Q model, and real materials have defects, which will lead