M. Ryyan Khan

Bifacial Si heterojunction-perovskite organic-inorganic tandem to produce highly efficient ( ηT* ∼ 33%) solar cell

As single junction photovoltaic (PV) technologies, both Si heterojunction (HIT) and perovskite based solar cells promise high efficiencies at low cost. Intuitively, a traditional tandem cell design with these cells connected in series is expected to improve the efficiency further. Using a self-consistent numerical modeling of optical and transport characteristics, however, we find that a traditional series connected tandem design suffers from low JSC due to band-gap mismatch and current matching constraints. Specifically, a traditional tandem cell with state-of-the-art HIT ( η=24%) and perovskite ( η=20%) sub-cells provides only a modest tandem efficiency of ηT∼ 25%. Instead, we demonstrate that a bifacial HIT/perovskite tandem design decouples the optoelectronic constraints and provides an innovative path for extraordinary efficiencies. In the bifacial configuration, the same state-of-the-art sub-cells achieve a normalized output of ηT* = 33%, exceeding the bifacial HIT performance at practical albedo re...

Enhanced light trapping in solar cells with a meta-mirror following generalized Snell’s law

As the performance of photovoltaic cells approaches the Shockley-Queisser limit, appropriate schemes are needed to minimize the losses without compromising the current performance. In this paper we propose a planar absorber-mirror light trapping structure where a conventional mirror is replaced by a meta-mirror with asymmetric light scattering properties. The meta-mirror is tailored to have reflection in asymmetric modes that stay outside the escape cone of the dielectric, hence trapping light with unit probability. Ideally, the meta-mirror can be designed to have such light trapping for any angle of incidence onto the absorber-mirror structure. We illustrate the concept by using a simple gap-plasmon meta-mirror. Even though the response of the mirror is non-ideal with the unwanted scattering modes reducing the light absorption, we observe an order of magnitude enhancement compared to single pass absorption in the absorber. The bandwidth of the enhancement can be matched with the range of wavelengths close to the solar cell absorber band-edge where improved light absorption is required.

Fundamentals of PV efficiency interpreted by a two-level model

We consider the physics of photovoltaic (PV) energy conversion in a two-level, atomic PV and explain the conditions for which the Carnot efficiency is reached and how it can be exceeded. The loss mechanisms—thermalization, angle entropy, and below-bandgap transmission—explain the gap between Carnot efficiency and the Shockley-Queisser limit. Techniques developed to reduce these losses (e.g., solar concentrators, tandem cells, etc.) are reinterpreted using a simple two-level model. Remarkably, this simple model captures the essence of PV operation and reproduces the key results and important insights that that have been previously obtained using more complicated derivations.

Wavelength-dependent absorption in structurally tailored randomly branched vertical arrays of InSb nanowires.

Arrays of semiconductor nanowires are of potential interest for applications including photovoltaic devices and IR detectors/imagers. While nominally uniform arrays have typically been studied, arrays containing nanowires with multiple diameters and/or random distributions of diameters could allow tailoring of the photonic properties of the arrays. In this Letter, we demonstrate the growth and optical properties of randomly branched InSb nanowire arrays. The structure mentioned can be approximated as three vertically stacked regions, with average diameters of 20, 100, and 150 nm within the respective layers. Reflectance and transmittance measurements on structures with different average nanowire lengths have been performed over the wavelength range of 300-2000 nm, and absorbance has been calculated from these measurements. The structures show low reflectance over the visible and IR regions and wavelength-dependent absorbance in the IR region. A model considering the diameter-dependent photonic coupling (at a given wavelength) and random distribution of nanowire diameters within the regions has been developed. The diameter-dependent photonic coupling results in a roll-off in the absorbance spectra at wavelengths well below the bulk cutoff of ∼7 μm, and randomness is observed to broaden the absorbance response. Varying the average diameters would allow tailoring of the wavelength dependent absorption within various layers, which could be employed in photovoltaic devices or wavelength-dependent IR imagers.

Thermodynamic limit of bifacial double-junction tandem solar cells

A traditional single-junction solar panel cannot harness ground-scattered light (albedo reflectance, RA), and also suffers from the fundamental sub-band-gap and the thermalization losses. In this paper, we explain how a “bifacial tandem” panel would dramatically reduce these losses, with corresponding improvement in thermodynamic performance. Our study predicts (i) the optimum combination of the band-gaps, empirically given by Eg(t)opt≈Eg(b)opt(2+RA)/3+(1−RA) and the (ii) corresponding optimum normalized output power given by ηT(opt)*≈RA (2ηSJ(opt))+(1−RA)ηDJ(opt). Empirically, ηT(opt)* interpolates between the thermodynamic efficiency limit of classical double-junction tandem cell ( ηDJ) and twice that of a single-junction cell ( ηSJ). We conclude by explaining how the fundamental loss mechanisms evolve with RA in a bifacial tandem cell.

Directing solar photons to sustainably meet food, energy, and water needs

As we approach a “Full Earth” of over ten billion people within the next century, unprecedented demands will be placed on food, energy and water (FEW) supplies. The grand challenge before us is to sustainably meet humanity’s FEW needs using scarcer resources. To overcome this challenge, we propose the utilization of the entire solar spectrum by redirecting solar photons to maximize FEW production from a given land area. We present novel solar spectrum unbundling FEW systems (SUFEWS), which can meet FEW needs locally while reducing the overall environmental impact of meeting these needs. The ability to meet FEW needs locally is critical, as significant population growth is expected in less-developed areas of the world. The proposed system presents a solution to harness the same amount of solar products (crops, electricity, and purified water) that could otherwise require ~60% more land if SUFEWS were not used—a major step for Full Earth preparedness.

Vertical bifacial solar farms: Physics, design, and global optimization

There have been sustained interest in bifacial solar cell technology since 1980s, with prospects of 30–50% increase in the output power from a stand-alone panel. Moreover, a vertical bifacial panel reduces dust accumulation and provides two output peaks during the day, with the second peak aligned to the peak electricity demand. Recent commercialization and anticipated growth of bifacial panel market have encouraged a closer scrutiny of the integrated power-output and economic viability of bifacial solar farms, where mutual shading will erode some of the anticipated energy gain associated with an isolated, single panel. Towards that goal, in this paper we focus on geography-specific optimization of ground-mounted vertical bifacial solar farms for the entire world. For local irradiance, we combine the measured meteorological data with the clear-sky model. In addition, we consider the effects of direct, diffuse, and albedo light. We assume the panel is configured into sub-strings with bypass-diodes. Based on calculated light collection and panel output, we analyze the optimum farm design for maximum yearly output at any given location in the world. Our results predict that, regardless of the geographical location, a vertical bifacial farm will yield 10–20% more energy than a traditional monofacial farm for a practical row-spacing of 2m (corresponding to 1.2m high panels). With the prospect of additional 5–20% energy gain from reduced soiling and tilt optimization, bifacial solar farm do offer a viable technology option for large-scale solar energy generation.

Self-assembly of single dielectric nanoparticle layers and integration in polymer-based solar cells

A single, self-assembled layer of highly uniform dielectric alumina nanoparticles improves the photovoltaic performance of organic semiconductor bulk heterojunction solar cells. The block copolymer based self-assembly approach is readily amenable to the large areas required for solar cell fabrication. A fraction of the performance gain results from incident light scattering which increases active layer absorption and photocurrent output, consistent with device simulations. The nanoparticle layer also roughens the device electrode surface, increasing contact area and improving device fill factor through more efficient charge collection.

Thermodynamic efficiency limits of classical and bifacial multi-junction tandem solar cells: An analytical approach

Bifacial tandem cells promise to reduce three fundamental losses (i.e., above-bandgap, below bandgap, and the uncollected light between panels) inherent in classical single junction photovoltaic (PV) systems. The successive filtering of light through the bandgap cascade and the requirement of current continuity make optimization of tandem cells difficult and accessible only to numerical solution through computer modeling. The challenge is even more complicated for bifacial design. In this paper, we use an elegantly simple analytical approach to show that the essential physics of optimization is intuitively obvious, and deeply insightful results can be obtained with a few lines of algebra. This powerful approach reproduces, as special cases, all of the known results of conventional and bifacial tandem cells and highlights the asymptotic efficiency gain of these technologies.

Nonideal Effects Limit the Efficiency Gain for Angle-Restricted Solar Cells

Performance of a conventional solar cell at 1 sun is limited to ~31%, defined by the Shockley-Queisser (SQ) limit. The thermodynamic limit can be improved to ~41% by reducing the angle mismatch between incident light and emitted rays. This can be achieved either by increasing the range of incident angles (as in a concentrator cell; CPV) or by reducing the emission angle from the device (angle-restricted photovoltaics; APVs). The equivalence is exact in the radiative limit. However, the gains of angle-restricted cells may erode rapidly once nonradiative recombination and other optical losses are accounted for, suggesting the need for quantitative/careful design. Here, we report a detailed self-consistent absorption-emission-transport study to 1) define the limiting efficiency of APV scheme and 2) explain the efficiency loss due to practical constraints, such as Shockley-Read-Hall (SRH) and Auger recombination, and mirror reflectivity. Our analysis reflects a growing consensus in the field that any imperfection of the mirror quickly diminishes the advantages of angle restriction. We conclude that a 50-nm angle-restricted cell with moderate material quality, but with nearly perfect mirror and Lambertian trapping, may be ~36-37% efficient.