Albert Polman

Photovoltaic materials: Present efficiencies and future challenges

Surveying the solar cell landscape The rate of development and deployment of large-scale photovoltaic systems over recent years has been unprecedented. Because the cost of photovoltaic systems is only partly determined by the cost of the solar cells, efficiency is a key driver to reduce the cost of solar energy. There are several materials systems being explored to achieve high efficiency at low cost. Polman et al. comprehensively and systematically review the leading candidate materials, present the limitations of each system, and analyze how these limitations can be overcome and overall cell performance improved. Science, this issue p. 10.1126/science.aad4424 BACKGROUND Photovoltaics, which directly convert solar energy into electricity, offer a practical and sustainable solution to the challenge of meeting the increasing global energy demand. According to the Shockley-Queisser (S-Q) detailed-balance model, the limiting photovoltaic energy conversion efficiency for a single-junction solar cell is 33.7%, for an optimum semiconductor band gap of 1.34 eV. Parallel to the development of wafer-based Si solar cells, for which the record efficiency has continually increased during recent decades, a large range of thin-film materials have been developed with the aim to approach the S-Q limit. These materials can potentially be deposited at low cost, in flexible geometries, and using relatively small material quantities. ADVANCES We review the electrical characteristics of record-efficiency cells made from 16 widely studied photovoltaic material geometries and illuminated under the standard AM1.5 solar spectrum, and compare these to the fundamental limits based on the S-Q model. Cells that show a short-circuit current (Jsc) lower than the S-Q limit suffer from incomplete light absorption or incomplete collection of generated carriers, whereas a reduced open-circuit voltage (Voc) or fill factor (FF) reflects unwanted bulk or interfacial carrier recombination, parasitic resistance, or other electrical nonidealities. The figure shows the experimental values for Jsc and the Voc × FF product relative to the S-Q limiting values for the different materials. This graph enables a direct identification of each material in terms of unoptimized light management and carrier collection (Jsc/JSQ < 1) or carrier management (Voc × FF/VSQ × FFSQ < 1). Monocrystalline Si cells (record efficiency 25.6%) have reached near-complete light trapping and carrier collection and are mostly limited by remaining carrier recombination losses. In contrast, thin-film single-crystalline GaAs cells (28.8%) show only minimal recombination losses but can be improved by better light management. Polycrystalline CdTe thin-film cells (21.5%) offer excellent light absorption but have relatively high recombination losses; perovskite cells (21.0%) and Cu(In,Ga)(Se,S)2 (CIGS) cells (21.7%) have poorer light management, although CIGS displays higher electrical quality. Aside from these five materials (Si, GaAs, CdTe, CIGS, perovskite) with efficiencies of >20%, a broad range of other thin-film materials have been developed with efficiencies of 10 to 12%: micro/nanocrystalline and amorphous Si, Cu(Zn,Sn)(Se,S)2 (CZTS), dye-sensitized TiO2, organic polymer materials, and quantum dot solids. So far, cell designs based on these materials all suffer from both light management and carrier management problems. Organic and quantum dot solar cells have shown substantial efficiency improvements in recent years. OUTLOOK The record-efficiency single-crystalline materials (Si, GaAs) have room for efficiency improvements by a few absolute percent. The future will tell whether the high-efficiency polycrystalline thin films (CdTe, CIGS, perovskite) can rival the efficiencies of Si and GaAs. Because the cost of photovoltaic systems is only partly determined by the cost of the solar cells, efficiency is a key driver to reduce the cost of solar energy, and therefore large-area photovoltaic systems require high-efficiency (>20%), low-cost solar cells. The lower-efficiency (flexible) materials can find applications in building-integrated PV systems, flexible electronics, flexible power generation systems, and many other (sometimes niche) markets. High-efficiency (>20%) materials find applications in large-area photovoltaic power generation for the utility grid as well as in small and medium-sized systems for the built environment. They will enable very large-scale penetration into our energy system, starting now and growing as the cost per kilowatt-hour is reduced further by a factor of 2 to 3. This can be achieved by nanophotonic cell designs, in which optically resonant and nonresonant structures are integrated with the solar cell architecture to enhance light coupling and trapping, in combination with continued materials engineering to further optimize cell voltage. Making big steps forward in these areas will require a coordinated international materials science and engineering effort. Limiting processes in photovoltaic materials. An efficient solar cell captures and traps all incident light (“light management”) and converts it to electrical carriers that are efficiently collected (“carrier management”). The plot shows the short-circuit current and product of open-circuit voltage and fill factor relative to the maximum achievable values, based on the Shockley-Queisser detailed-balance limit, for the most efficient solar cell made with each photovoltaic material. The data indicate whether a particular material requires better light management, carrier management, or both. Colors correspond to cells achieving <50% of their S-Q efficiency limit ηSQ (red), 50 to 75% (green), or >75% (blue). Recent developments in photovoltaic materials have led to continual improvements in their efficiency. We review the electrical characteristics of 16 widely studied geometries of photovoltaic materials with efficiencies of 10 to 29%. Comparison of these characteristics to the fundamental limits based on the Shockley-Queisser detailed-balance model provides a basis for identifying the key limiting factors, related to efficient light management and charge carrier collection, for these materials. Prospects for practical application and large-area fabrication are discussed for each material.

Nanophotonics: Shrinking light-based technology

The study of light at the nanoscale has become a vibrant field of research, as researchers now master the flow of light at length scales far below the optical wavelength, largely surpassing the classical limits imposed by diffraction. Using metallic and dielectric nanostructures precisely sculpted into two-dimensional (2D) and 3D nanoarchitectures, light can be scattered, refracted, confined, filtered, and processed in fascinating new ways that are impossible to achieve with natural materials and in conventional geometries. This control over light at the nanoscale has not only unveiled a plethora of new phenomena but has also led to a variety of relevant applications, including new venues for integrated circuitry, optical computing, solar, and medical technologies, setting high expectations for many novel discoveries in the years to come.

Loss mechanisms of surface plasmon polaritons on gold probed by cathodoluminescence imaging spectroscopy

We use cathodoluminescence imaging spectroscopy to excite surface plasmon polaritons and measure their decay length on single crystal and polycrystalline gold surfaces. The surface plasmon polaritons are excited on the gold surface by a nanoscale focused electron beam and are coupled into free space radiation by gratings fabricated into the surface. By scanning the electron beam on a line perpendicular to the gratings, the propagation length is determined. Data for single-crystal gold are in agreement with calculations based on dielectric constants. For polycrystalline films, grain boundary scattering is identified as additional loss mechanism, with a scattering coefficient SG=0.2%.

Directional emission from a single plasmonic scatterer

Directing light emission is key for many applications in photonics and biology. Optical antennas made from nanostructured plasmonic metals are suitable candidates for this purpose but designing antennas with good directional characteristics can be challenging, especially when they consist of multiple elements. Here we show that strongly directional emission can also be obtained from a simple individual gold nanodisk, utilizing the far-field interference of resonant electric and magnetic modes. Using angle-resolved cathodoluminescence spectroscopy, we find that the spectral and angular response strongly depends on excitation position. For excitation at the nanodisk edge, interference between in-plane and out-of-plane dipole components leads to strong beaming of light. For large nanodisks, higher-order multipole components contribute significantly to the scattered field, leading to enhanced directionality. Using a combination of full-wave simulations and analytical point scattering theory we are able to decompose the calculated and measured scattered fields into dipolar and quadrupolar contributions.

Photonic crystals of shape-anisotropic colloidal particles

Spherical silica (SiO2), zinc sulfide (ZnS), and core-shell particles of these materials undergo substantial anisotropic plastic deformation under high-energy ion irradiation. Individual particles can be turned into oblate or prolate ellipsoids with exact control over the aspect ratio. In this letter, we report on the fabrication and optical characterization of thin three-dimensional photonic crystals of spherical particles, which have been anisotropically deformed into spheroidal oblates by means of ion irradiation. As a result of the collective deformation process, both the unit cell symmetry and the particle form factor have been changed leading to appreciable tunability in the optical properties of the photonic crystal.

Deep-subwavelength imaging of the modal dispersion of light.

Numerous optical technologies and quantum optical devices rely on the controlled coupling of a local emitter to its photonic environment, which is governed by the local density of optical states (LDOS). Although precise knowledge of the LDOS is crucial, classical optical techniques fail to measure it in all of its frequency and spatial components. Here, we use a scanning electron beam as a point source to probe the LDOS. Through angular and spectral detection of the electron-induced light emission, we spatially and spectrally resolve the light wave vector and determine the LDOS of Bloch modes in a photonic crystal membrane at an unprecedented deep-subwavelength resolution (30-40 nm) over a large spectral range. We present a first look inside photonic crystal cavities revealing subwavelength details of the resonant modes. Our results provide direct guidelines for the optimum location of emitters to control their emission, and key fundamental insights into light-matter coupling at the nanoscale.

Nanoscale optical tomography with cathodoluminescence spectroscopy

Tomography has enabled the characterization of the Earths interior, visualization of the inner workings of the human brain, and three-dimensional reconstruction of matter at the atomic scale. However, tomographic techniques that rely on optical excitation or detection are generally limited in their resolution by diffraction. Here, we introduce a tomographic technique--cathodoluminescence spectroscopic tomography--to probe optical properties in three dimensions with nanometre-scale spatial and spectral resolution. We first obtain two-dimensional cathodoluminescence maps of a three-dimensional nanostructure at various orientations. We then use the method of filtered back-projection to reconstruct the cathodoluminescence intensity at each wavelength. The resulting tomograms allow us to locate regions of efficient cathodoluminescence in three dimensions across visible and near-infrared wavelengths, with contributions from material luminescence and radiative decay of electromagnetic eigenmodes. The experimental signal can be further correlated with the radiative local density of optical states in particular regions of the reconstruction. We demonstrate how cathodoluminescence tomography can be used to achieve nanoscale three-dimensional visualization of light-matter interactions by reconstructing a three-dimensional metal-dielectric nanoresonator.

Angle-resolved cathodoluminescence spectroscopy

We present a cathodoluminescence spectroscopy technique which combines deep subwavelength excitation resolution with angle-resolved detection capabilities. The cathodoluminescence emission is collected by a paraboloid mirror (effective NA = 0.96) and is projected onto a 2D CCD array. The azimuthal and polar emission pattern is directly deduced from the image. As proof of principle, we use the technique to measure the angular distribution of transition radiation from a single crystalline gold surface under 30 keV electron irradiation. We find that the experiment matches very well with theory, illustrating the potential of this technique for the characterization of photonic structures with deep subwavelength dimensions.

Anisotropic deformation of colloidal particles under MeV ion irradiation

Abstract Spherical silica colloids with a diameter of 1.0 μm , made by wet chemical synthesis, were irradiated with 2–16 MeV Au ions at fluences ranging from 2×10 14 to 11 ×10 14 cm −2 . The irradiation induces an anisotropic plastic deformation turning the spherical colloids into ellipsoidal oblates. After 16 MeV Au irradiation to a fluence of 11×10 14 cm −2 a size-aspect ratio of 4.7 is achieved. The size polydispersity (∼3%) remains unaffected by the irradiation. The transverse diameter increases with the electronic energy loss above a threshold value of ∼0.6 keV/nm. Non-ellipsoidal colloids are observed in the case that the projected ion range is smaller than the colloid diameter. The deformation effect is also observed for micro-crystalline ZnS and amorphous TiO2 colloids, as well as ZnS/SiO2 core/shell particles. No deformation is observed for crystalline Al2O3 and Ag particles. The data provide strong support for the thermal spike model of anisotropic deformation.

Colloidal assemblies modified by ion irradiation

Abstract Spherical SiO2 and ZnS colloidal particles show a dramatic anisotropic plastic deformation under 4 MeV Xe ion irradiation, that changes their shape into oblate ellipsoidal, with an aspect ratio that can be precisely controlled by the ion fluence. The 290 nm and 1.1 μm diameter colloids were deposited on a Si substrate and irradiated at 90 K, using fluences in the range 3×10 13 –8×10 14 cm −2 . The transverse particle diameter shows a linear increase with ion fluence, while the longitudinal diameter shrinks; the particle volume remains constant. Size aspect ratios up to 3.1 are achieved. Applications of the ion beam deformation technique are shown in studies of liquid crystalline colloidal ordering, self-assembled two-dimensional colloidal lithographic masks for thin-film deposition, and in tuning the optical properties of three-dimensional colloidal crystals.