Photovoltaics are expected to play an important role in the future energy infrastructure. However, achieving simultaneously high efficiency in both light absorption and carrier collection remains a challenging tradeoff. Photon management, which refers to the engineering of materials and device structures to control the spatial distribution of optical energy, offers a number of promising routes to optimizing this tradeoff. Progress in fabrication of nanostructured materials combined with advances in the understanding of nanophotonic devices has enabled new strategies for photon management in a range of photovoltaic devices. Prominent among these are structures with pronounced surface topography or graded refractive-index profiles that reduce surface reflectivity; materials processing that increases optical absorption in materials such as silicon; incorporation of semiconductor nanostructures that enables simultaneous improvements in optical absorption and photogenerated carrier collection; and coherent light trapping in optical waveguide modes via plasmonic or optical scattering effects. The articles in this issue review some of these emerging directions.

BaZrO3–BaTiO3 ceramics exhibit a shift in transformation temperatures as revealed by dielectric and viscoelastic spectroscopy; a phase diagram has been established. Sigmoid anomalies in Poisson’s ratio and bulk modulus during the ferroelastic transitions were observed in doped materials, which are not predicted by standard theories for phase transformations. “Hashin–Shtrikman” composite model with negative stiffness heterogeneity can well explain this phenomenon. Negative stiffness heterogeneity is considered to be caused by the strained BaTiO3 unit cells in the vicinity of BaZrO3-rich zones under the perturbation of lattice reconstruction.

Thin-film photovoltaic technologies have an enormous potential to reduce the cost of solar electricity. However, because thin photoactive layers are used, optical absorption is incomplete unless light-trapping strategies are employed. Since conventional light-trapping approaches based on geometric scattering are less effective in thin-film cells, coherent light-trapping approaches that exploit the wave nature of light are being explored to enhance optical absorption. In this article, we look at the various strategies for coherent light trapping in thin-film solar cells, including photonic crystals, metal nanostructures, and multilayer stacks. The suitability of a particular strategy depends on factors such as configuration of the solar cell, process compatibility, cost, desired angular response, and materials usage. We also discuss the physical limits of light trapping in thin films.

(Pb0.87−0.07xBa0.10+0.07x)La0.02(Zr0.7Sn0.15Ti0.15)O3 ceramics with 0 ≤ x ≤1 were prepared by conventional solid-state reaction process, and their dielectric and electric field–induced pyroelectric properties were systemically investigated. Compared with conventional pyroelectric materials, (Pb0.87−0.07xBa0.10+0.07x)La0.02(Zr0.7Sn0.15Ti0.15)O3 ceramics exhibited higher pyroelectric coefficient and figure of merit, which are beneficial for the development of pyroelectric devices. The specimens with x = 0.65 showed good pyroelectric properties for practical applications. When a 500 V/mm dc bias field was applied, they showed the maximum pyroelectric coefficient of 12,200 μC/m2K and the figure of merit of 106 × 10−5 Pa−0.5 at 45 °C, which are larger than those observed from conventional pyroelectric materials. Improvement of pyroelectric property is beneficial for the development of infrared detectors.

Self-separated Pb(Zr0.52Ti0.48)O3 (PZT) films were processed by a hydrothermal deposition and a rapid thermal separation method, followed by a sol–gel filling and sintering process. The films possess excellent piezoelectric and electromechanical properties close to those of bulk material. The maximum remnant polarization is over 30 μC/cm2 and the electromechanical coupling factor (kt) reaches as high as 0.52. The unique microstructure characteristics of the PZT films, such as their highly dense structure, columnar grains, well-connected grain boundaries, and well-dispersed nanopores, could all contribute to the enhanced piezoelectric and electromechanical properties.

In recent years, there has been rapid development in the field of nanoscale light trapping for solar cells. This has been driven by the decrease in thickness of solar cells in order to reduce materials costs, as well as advances in fabrication technology and computer power for simulating nanoscale structures. Nanoscale light trapping offers the possibility of enhancing absorption beyond the limits achievable with geometrical optics for certain structures. It also allows the optical design to be separated from the electrical design, as for example in plasmonic solar cells. Most importantly, thin-film cell designs will need to incorporate nanophotonic light trapping in order to reach their ultimate efficiency limits. In this article, we review the major types of nanophotonic light trapping, including plasmonic, diffraction gratings, and random scattering surfaces and describe the major advantages and disadvantages of each. In addition, we describe the most important related fabrication and characterization technologies and provide an outlook on future directions in this field.

To harness the full spectrum of solar energy, optical reflections at the surface of a solar photovoltaic cell must be reduced as much as possible over the relevant solar spectral range and over a wide range of incident angles. The development of antireflection coatings embodying omni-directionality over a wide range of wavelengths is challenging. Recently, nanoporous films, fabricated by oblique-angle deposition and having tailored- and very low-refractive index properties, have been demonstrated. Tailorability of the refractive index and the ability to realize films with a very low-refractive index are properties critical in the performance of broadband, omnidirectional antireflection coatings. As such, nanoporous materials are ideally suited for developing near-perfect antireflection coatings. Here, we discuss multilayer antireflection coatings with near-perfect transmittance over the spectral range of 400−2000 nm and over widely varying angles of acceptance, 0−90°. The calculated solar optical-to-electrical efficiency enhancement that can be attained with nanoporous multilayer coatings over single-layer quarter-wave films is 18%, making these coatings highly attractive for solar cell applications.