Mohammad Ryyan Khan

Design of GaAs Solar Cells Operating Close to the Shockley–Queisser Limit

With recent advances in device design, single-junction GaAs solar cells are approaching their theoretical efficiency limits. Accurate numerical simulation may offer insights that can help close the remaining gap between the practical and theoretical limits. Significant care must be taken, however, to ensure that the simulation is self-consistent and properly comprehends thermodynamic limits. In this paper, we use rigorous photon recycling simulation coupled with carrier transport simulation to identify the dominant loss mechanisms that limit the performance of thin-film GaAs solar cells.

Performance-limiting factors for GaAs-based single nanowire photovoltaics

GaAs nanowires (NWs) offer the possibility of decoupling light absorption from charge transport for high-performance photovoltaic (PV) devices. However, it is still an open question as to whether these devices can exceed the Shockley-Queisser efficiency limit for single-junction PV. In this work, single standing GaAs-based nanowire solar cells in both radial and vertical junction configurations is analyzed and compared to a planar thin-film design. By using a self-consistent, electrical-optically coupled 3D simulator, we show the design principles for nanowire and planar solar cells are significantly different; nanowire solar cells are vulnerable to surface and contact recombination, while planar solar cells suffer significant losses due to imperfect backside mirror reflection. Overall, the ultimate efficiency of the GaAs nanowire solar cell with radial and vertical junction is not expected to exceed that of the thin-film design, with both staying below the Shockley-Queisser limit.GaAs nanowires (NWs) offer the possibility of decoupling light absorption from charge transport for high-performance photovoltaic (PV) devices. However, it is still an open question as to whether these devices can exceed the Shockley-Queisser efficiency limit for single-junction PV. In this work, single standing GaAs-based nanowire solar cells in both radial and vertical junction configurations is analyzed and compared to a planar thin-film design. By using a self-consistent, electrical-optically coupled 3D simulator, we show the design principles for nanowire and planar solar cells are significantly different; nanowire solar cells are vulnerable to surface and contact recombination, while planar solar cells suffer significant losses due to imperfect backside mirror reflection. Overall, the ultimate efficiency of the GaAs nanowire solar cell with radial and vertical junction is not expected to exceed that of the thin-film design, with both staying below the Shockley-Queisser limit.

Nanostructured Electrodes for Organic Solar Cells: Analysis and Design Fundamentals

Nanostructured electrodes (NEs) improve optical absorption and charge collection in photovoltaic (PV) devices. Traditionally, the electrodes have been designed exclusively for higher optical absorption. Such an optical design of the electrodes does not necessarily ensure better charge collection. Since the efficiency of organic PV (OPV) devices is hindered by the low carrier mobility of the organic semiconductors, the charge collection property of the NEs provides an interesting design alternative. The goal of this paper is the formulation of the essential design rules for NEs to improve charge collection in the low-mobility organic materials. We use detailed optoelectronic device simulation to explore the physics of NEs embedded in the organic semiconductors and quantify its effect on the performance gain of organic solar cells. Our analysis suggests that an optimum codesign of electrodes and morphology is essential for significant performance improvement (mainly through fill factor) in OPV cells.

Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells

Significance We demonstrate that, instead of the charge generation mechanism, charge collection can readily explain the bottleneck toward higher efficiency single organic semiconductor based OPVs (SS-OPVs). This change in archetype has the potential to transform the design rules for materials used in OPV devices and would inspire searches for a completely different set of polymers for OPV cells. Furthermore, we believe that our findings will broaden the understanding of the physics of charge transport and the impact of charged defect states in organic electronic devices. Hence, this work should have deep and immediate impact on the chemists, materials scientists, and device physicists in the field and would be of broad interest to the organic electronics community. The bulk heterojunction (BHJ) organic photovoltaic (OPV) architecture has dominated the literature due to its ability to be implemented in devices with relatively high efficiency values. However, a simpler device architecture based on a single organic semiconductor (SS-OPV) offers several advantages: it obviates the need to control the highly system-dependent nanoscale BHJ morphology, and therefore, would allow the use of broader range of organic semiconductors. Unfortunately, the photocurrent in standard SS-OPV devices is typically very low, which generally is attributed to inefficient charge separation of the photogenerated excitons. Here we show that the short-circuit current density from SS-OPV devices can be enhanced significantly (∼100-fold) through the use of inverted device configurations, relative to a standard OPV device architecture. This result suggests that charge generation may not be the performance bottleneck in OPV device operation. Instead, poor charge collection, caused by defect-induced electric field screening, is most likely the primary performance bottleneck in regular-geometry SS-OPV cells. We justify this hypothesis by: (i) detailed numerical simulations, (ii) electrical characterization experiments of functional SS-OPV devices using multiple polymers as active layer materials, and (iii) impedance spectroscopy measurements. Furthermore, we show that the collection-limited photocurrent theory consistently interprets typical characteristics of regular SS-OPV devices. These insights should encourage the design and OPV implementation of high-purity, high-mobility polymers, and other soft materials that have shown promise in organic field-effect transistor applications, but have not performed well in BHJ OPV devices, wherein they adopt less-than-ideal nanostructures when blended with electron-accepting materials.

Optics-Based Approach to Thermal Management of Photovoltaics: Selective-Spectral and Radiative Cooling

For commercial one-sun solar modules, up to 80% of the incoming sunlight may be dissipated as heat, potentially raising the temperature 20–30 °C higher than the ambient. In the long term, extreme self-heating erodes efficiency and shortens lifetime, thereby dramatically reducing the total energy output. Therefore, it is critically important to develop effective and practical (and preferably passive) cooling methods to reduce operating temperature of photovoltaic (PV) modules. In this paper, we explore two fundamental (but often overlooked) origins of PV self-heating, namely, sub-bandgap absorption and imperfect thermal radiation. The analysis suggests that we redesign the optical properties of the solar module to eliminate parasitic absorption (selective-spectral cooling) and enhance thermal emission (radiative cooling). Comprehensive opto-electro-thermal simulation shows that the proposed techniques would cool one-sun terrestrial solar modules up to 10 °C. This self-cooling would substantially extend the lifetime for solar modules, with corresponding increase in energy yields and reduced levelized cost of electricity.

Electrodeposition of InSb branched nanowires: Controlled growth with structurally tailored properties

In this article, electrodeposition method is used to demonstrate growth of InSb nanowire (NW) arrays with hierarchical branched structures and complex morphology at room temperature using an all-solution, catalyst-free technique. A gold coated, porous anodic alumina membrane provided the template for the branched NWs. The NWs have a hierarchical branched structure, with three nominal regions: a “trunk” (average diameter of 150 nm), large branches (average diameter of 100 nm), and small branches (average diameter of sub-10 nm to sub-20 nm). The structural properties of the branched NWs were studied using scanning transmission electron microscopy, transmission electron microscopy, scanning electron microscopy, x-ray diffraction, energy dispersive x-ray spectroscopy, and Raman spectroscopy. In the as-grown state, the small branches of InSb NWs were crystalline, but the trunk regions were mostly nanocrystalline with an amorphous boundary. Post-annealing of NWs at 420 °C in argon produced single crystalline st...

Optimization and performance of bifacial solar modules: A global perspective

With the rapidly growing interest in bifacial photovoltaics (PV), a worldwide map of their potential performance can help assess and accelerate the global deployment of this emerging technology. However, the existing literature only highlights optimized bifacial PV for a few geographic locations or develops worldwide performance maps for very specific configurations, such as the vertical installation. It is still difficult to translate these location- and configuration-specific conclusions to a general optimized performance of this technology. In this paper, we present a global study and optimization of bifacial solar modules using a rigorous and comprehensive modeling framework. Our results demonstrate that with a low albedo of 0.25, the bifacial gain of ground-mounted bifacial modules is less than 10% worldwide. However, increasing the albedo to 0.5 and elevating modules 1 m above the ground can boost the bifacial gain to 30%. Moreover, we derive a set of empirical design rules, which optimize bifacial solar modules across the world and provide the groundwork for rapid assessment of the location-specific performance. We find that ground-mounted, vertical, east-west-facing bifacial modules will outperform their south-north-facing, optimally tilted counterparts by up to 15% below the latitude of 30°, for an albedo of 0.5. The relative energy output is reversed in latitudes above 30°. A detailed and systematic comparison with data from Asia, Africa, Europe, and North America validates the model presented in this paper.

Approaching the Shockley-Queisser limit in GaAs solar cells

With recent advances in device design, single junction GaAs solar cells are approaching their theoretical efficiency limits. Accurate numerical simulation may offer insights that can lead to further improvement. Significant care must be taken, however, to ensure that the simulation properly comprehends thermodynamic limits. In this paper, we use rigorous photon recycling simulation coupled with carrier transport simulation to identify the dominant loss mechanisms that limit the performance of thin film GaAs solar cell.

A novel approach to thermal design of solar modules: Selective-spectral and radiative cooling

For commercial solar modules, up to 80% of the incoming sunlight may be dissipated as heat, potentially raising the temperature 20-30°C higher than the ambient. In the long run, extreme self-heating may erode efficiency and shorten lifetime, thereby, dramatically reducing the total energy output by almost ~10% Therefore, it is critically important to develop effective and practical cooling methods to combat PV self-heating. In this paper, we explore two fundamental sources of PV self-heating, namely, sub-bandgap absorption and imperfect thermal radiation. The analysis suggests that we redesign the optical and thermal properties of the solar module to eliminate the parasitic absorption (selective-spectral cooling) and enhance the thermal emission to the cold cosmos (radiative cooling). The proposed technique should cool the module by ~10°C, to be reflected in significant long-term energy gain (~ 3% to 8% over 25 years) for PV systems under different climatic conditions.

Untangling the essence of bulk heterostructure organic solar cells: Why complex need not be complicated

The A new class of Macroelectronic devices appropriate for large area flexible electronics, supercapacitors, batteries, and solar cells rely on the biological dictum that ‘form defines function’ and use structural geometry to compensate for the poor intrinsic transport in materials processed at low temperature [1]. Bulk heterostructure solar cells with two intermixed polymers acting as the acceptor/donor layers provide a striking example of such biomimetic design. The optimization of this class of devices has long been stymied by a lack of theoretical tools and transport models that can treat the geometry of structure at par with transport characteristics and that does not resort to classical transport theories originally developed for spatially homogenous media. In this presentation, I will use theories of spinodal decomposition, geometric transform, and percolation models to demonstrate how simple ideas can untangle the complex (transport) geometry of BH solar cells [2–5]. Indeed, the essence of the most important aspects of organic solar cells like short circuit current [2–4], open circuit voltage [2], fill-factor, reliability [5], etc. can be described by no more than a few lines of algebra. Our approach allows an intuitive understanding of the essential features of performance and reliability of BH solar cells and establish the fundamental principle of optimization and trade-off that must dictate the design of such devices.