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

Solution-processed metal halide perovskite semiconductors, such as CH3NH3PbI3, have exhibited remarkable performance in solar cells, despite having non-negligible density of defect states. A likely candidate is halide vacancies within the perovskite crystals, or the presence of metallic lead, both generated due to the imbalanced I/Pb stoichiometry which could evolve during crystallization. Herein, we show that the addition of hypophosphorous acid (HPA) in the precursor solution can significantly improve the film quality, both electronically and topologically, and enhance the photoluminescence intensity, which leads to more efficient and reproducible photovoltaic devices. We demonstrate that the HPA can reduce the oxidized I2 back into I−, and our results indicate that this facilitates an improved stoichiometry in the perovskite crystal and a reduced density of metallic lead.

Lead-halide perovskites are promising materials for opto-electronic applications. Recent reports indicated that their mechanical and electronic properties are strongly affected by the lattice vibrations. Herein we report far-infrared spectroscopy measurements of CH3NH3Pb(I/Br/Cl)3 thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra. We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites, which indicates dynamic disorder of the lead-halide cage at room temperature. We determine the frequencies of the transversal and longitudinal optical phonons, and use them to calculate the static dielectric constants, polaron masses, electron-phonon coupling constants, and upper limits for the phonon-scattering limited charge carrier mobilities. Our findings place an upper limit in the range of 200 cm 2 V −1 s −1 for the room temperature charge carrier mobility in MAPbI3 single crystals, and are important for the basic understanding of charge transport processes and mechanical properties in metal halide perovskites.