• Energy Research

The remarkable development in photovoltaic (PV) technologies over the past 5 years calls for a renewed assessment of their performance and potential for future progress. Here, we analyse the progress in cells and modules based on single-crystalline GaAs, Si, GaInP and InP, multicrystalline Si as well as thin films of polycrystalline CdTe and CuInxGa1−xSe2. In addition, we analyse the PV developments of the more recently emerged lead halide perovskites together with notable improvements in sustainable chalcogenides, organic PVs and quantum dots technologies. In addition to power conversion efficiencies, we consider many of the factors that affect power output for each cell type and note improvements in control over the optoelectronic quality of PV-relevant materials and interfaces and the discovery of new material properties. By comparing PV cell parameters across technologies, we appraise how far each technology may progress in the near future. Although accurate or revolutionary developments cannot be predicted, cross-fertilization between technologies often occurs, making achievements in one cell type an indicator of evolutionary developments in others. This knowledge transfer is timely, as the development of metal halide perovskites is helping to unite previously disparate, technology-focused strands of PV research. Nearly all types of solar photovoltaic cells and technologies have developed dramatically, especially in the past 5 years. Here, we critically compare the different types of photovoltaic technologies, analyse the performance of the different cells and appraise possibilities for future technological progress.

Proteins are attractive as functional components in molecular junctions. However, control-ling the electronic charge transport via proteins, held between two electrodes, requires in-formation on their frontier orbital energy level alignment relative to the electrodes’ Fermi level (EF), which normally requires studies of UV Photoemission Spectroscopy (UPS) with HeI excitation. Such excitation is problematic for proteins, which can denature under stand-ard measuring conditions. Here we use high-sensitivity soft UV photoemission spectroscopy (HS-UPS) combined with Constant Final State Yield Spectroscopy (CFS-YS) to get this in-formation for electrode/protein contacts. Monolayers of the redox protein Azurin, (Az) and its Apo-form on Au substrates, have HOMO onset energies, obtained from CFS-YS, differ by ~ 0.2 eV, showing crucial role of the Cu redox centre in the electron transport process. We find that combined HS-UPS / CFS-YS measurements agree with the Photoelectron Yield Spectroscopy (PYS), showing potential of the HS-UPS + CFS-YS as a powerful tool to char-acterize and map the energetics of a protein-electrode interfaces, which will aid optimizing design of devices with targeted electronic properties, as well as for novel applications.

In their Article, Cui et al.1 describe the fabrication and characterization of planar p–n junction solar cells based on lead-halide perovskites. The formation of a p–n junction is noteworthy given the doping densities, measured using the Hall effect, which were reported to vary from ND = 1 × 1012 cm−3 to 8 × 1012 cm−3 for the solution-processed n-type layer and to equal NA = 8 × 109 cm−3 for the evaporated p-type layer. Although these devices outperform their counterparts that are supposedly undoped, the results raise three important questions. Are the reported doping densities high enough to change the electrostatic potential distribution in the device from that of undoped ones? Are the doping densities high enough for the p–n junction to remain intact under typical photovoltaic operation conditions? Is a p–n junction beneficial for photovoltaic performance given the typical properties of lead-halide perovskites.

2D PVSCs with “favorable” ion accumulation are realized via thermal–light post-treatment, which increases the built-in potential and device performance.