Erik C. Garnett

Enhanced thermoelectric performance of rough silicon nanowires

Approximately 90 per cent of the world’s power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30–40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT > 1 (refs 2–4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20–300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.

Light Trapping in Silicon Nanowire Solar Cells

Thin-film structures can reduce the cost of solar power by using inexpensive substrates and a lower quantity and quality of semiconductor material. However, the resulting short optical path length and minority carrier diffusion length necessitates either a high absorption coefficient or excellent light trapping. Semiconducting nanowire arrays have already been shown to have low reflective losses compared to planar semiconductors, but their light-trapping properties have not been measured. Using optical transmission and photocurrent measurements on thin silicon films, we demonstrate that ordered arrays of silicon nanowires increase the path length of incident solar radiation by up to a factor of 73. This extraordinary light-trapping path length enhancement factor is above the randomized scattering (Lambertian) limit (2n(2) approximately 25 without a back reflector) and is superior to other light-trapping methods. By changing the silicon film thickness and nanowire length, we show that there is a competition between improved absorption and increased surface recombination; for nanowire arrays fabricated from 8 mum thick silicon films, the enhanced absorption can dominate over surface recombination, even without any surface passivation. These nanowire devices give efficiencies above 5%, with short-circuit photocurrents higher than planar control samples.

Silicon Nanowire Radial p−n Junction Solar Cells

We have demonstrated a low-temperature wafer-scale etching and thin film deposition method for fabricating silicon n-p core-shell nanowire solar cells. Our devices showed efficiencies up to nearly 0.5%, limited primarily by interfacial recombination and high series resistance. Surface passivation and contact optimization will be critical to improve device performance in the future.

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.

Oligo- and Polythiophene/ZnO Hybrid Nanowire Solar Cells

We demonstrate the basic operation of an organic/inorganic hybrid single nanowire solar cell. End-functionalized oligo- and polythiophenes were grafted onto ZnO nanowires to produce p-n heterojunction nanowires. The hybrid nanostructures were characterized via absorption and electron microscopy to determine the optoelectronic properties and to probe the morphology at the organic/inorganic interface. Individual nanowire solar cell devices exhibited well-resolved characteristics with efficiencies as high as 0.036%, J(sc) = 0.32 mA/cm(2), V(oc) = 0.4 V, and a FF = 0.28 under AM 1.5 illumination with 100 mW/cm(2) light intensity. These individual test structures will enable detailed analysis to be carried out in areas that have been difficult to study in bulk heterojunction devices.

Hybrid Silicon Nanocone−Polymer Solar Cells

Recently, hybrid Si/organic solar cells have been studied for low-cost Si photovoltaic devices because the Schottky junction between the Si and organic material can be formed by solution processes at a low temperature. In this study, we demonstrate a hybrid solar cell composed of Si nanocones and conductive polymer. The optimal nanocone structure with an aspect ratio (height/diameter of a nanocone) less than two allowed for conformal polymer surface coverage via spin-coating while also providing both excellent antireflection and light trapping properties. The uniform heterojunction over the nanocones with enhanced light absorption resulted in a power conversion efficiency above 11%. Based on our simulation study, the optimal nanocone structures for a 10 μm thick Si solar cell can achieve a short-circuit current density, up to 39.1 mA/cm(2), which is very close to the theoretical limit. With very thin material and inexpensive processing, hybrid Si nanocone/polymer solar cells are promising as an economically viable alternative energy solution.

Dopant profiling and surface analysis of silicon nanowires using capacitance-voltage measurements

Silicon nanowires are expected to have applications in transistors, sensors, resonators, solar cells and thermoelectric systems. Understanding the surface properties and dopant distribution will be critical for the fabrication of high-performance devices based on nanowires. At present, determination of the dopant concentration depends on a combination of experimental measurements of the mobility and threshold voltage in a nanowire field-effect transistor, a calculated value for the capacitance, and two assumptions--that the dopant distribution is uniform and that the surface (interface) charge density is known. These assumptions can be tested in planar devices with the capacitance-voltage technique. This technique has also been used to determine the mobility of nanowires, but it has not been used to measure surface properties and dopant distributions, despite their influence on the electronic properties of nanowires. Here, we measure the surface (interface) state density and the radial dopant profile of individual silicon nanowire field-effect transistors with the capacitance-voltage technique.

Passivation Coating on Electrospun Copper Nanofibers for Stable Transparent Electrodes

Copper nanofiber networks, which possess the advantages of low cost, moderate flexibility, small sheet resistance, and high transmittance, are one of the most promising candidates to replace indium tin oxide films as the premier transparent electrode. However, the chemical activity of copper nanofibers causes a substantial increase in the sheet resistance after thermal oxidation or chemical corrosion of the nanofibers. In this work, we utilize atomic layer deposition to coat a passivation layer of aluminum-doped zinc oxide (AZO) and aluminum oxide onto electrospun copper nanofibers and remarkably enhance their durability. Our AZO-copper nanofibers show resistance increase of remarkably only 10% after thermal oxidation at 160 °C in dry air and 80 °C in humid air with 80% relative humidity, whereas bare copper nanofibers quickly become insulating. In addition, the coating and baking of the acidic PEDOT:PSS layer on our fibers increases the sheet resistance of bare copper nanofibers by 6 orders of magnitude, while the AZO-Cu nanofibers show an 18% increase.

Optimization of non-periodic plasmonic light-trapping layers for thin-film solar cells

Non-periodic arrangements of nanoscale light scatterers allow for the realization of extremely effective broadband light-trapping layers for solar cells. However, their optimization is challenging given the massive number of degrees of freedom. Brute-force, full-field electromagnetic simulations are computationally too time intensive to identify high-performance solutions in a vast design space. Here we illustrate how a semi-analytical model can be used to quickly identify promising non-periodic spatial arrangements of nanoscale scatterers. This model only requires basic knowledge of the scattering behaviour of a chosen nanostructure and the waveguiding properties of the semiconductor layer in a cell. Due to its simplicity, it provides new intuition into the ideal amount of disorder in high-performance light-trapping layers. Using simulations and experiments, we demonstrate that arrays of nanometallic stripes featuring a limited amount of disorder, for example, following a quasi-periodic or Fibonacci sequence, can substantially enhance solar absorption over perfectly periodic and random arrays.

Large-area free-standing ultrathin single-crystal silicon as processable materials.

Silicon has been driving the great success of semiconductor industry, and emerging forms of silicon have generated new opportunities in electronics, biotechnology, and energy applications. Here we demonstrate large-area free-standing ultrathin single-crystalline Si at the wafer scale as new Si materials with processability. We fabricated them by KOH etching of the Si wafer and show their uniform thickness from 10 to sub-2 μm. These ultrathin Si exhibits excellent mechanical flexibility and bendability more than those with 20-30 μm thickness in previous study. Unexpectedly, these ultrathin Si materials can be cut with scissors like a piece of paper, and they are robust during various regular fabrication processings including tweezer handling, spin coating, patterning, doping, wet and dry etching, annealing, and metal deposition. We demonstrate the fabrication of planar and double-sided nanocone solar cells and highlight that the processability on both sides of surface together with the interesting property of these free-standing ultrathin Si materials opens up exciting opportunities to generate novel functional devices different from the existing approaches.