Mukul Agrawal

Design of wide-angle solar-selective absorbers using aperiodic metal-dielectric stacks

Spectral control of the emissivity of surfaces is essential in applications such as solar thermal and thermophotovoltaic energy conversion in order to achieve the highest conversion efficiencies possible. We investigated the spectral performance of planar aperiodic metal-dielectric multilayer coatings for these applications. The response of the coatings was optimized for a target operational temperature using needle-optimization based on a transfer matrix approach. Excellent spectral selectivity was achieved over a wide angular range. These aperiodic metal-dielectric stacks have the potential to significantly increase the efficiency of thermophotovoltaic and solar thermal conversion systems. Optimal coatings for concentrated solar thermal conversion were modeled to have a thermal emissivity <7% at 720K while absorbing >94% of the incident light. In addition, optimized coatings for solar thermophotovoltaic applications were modeled to have thermal emissivity <16% at 1750K while absorbing >85% of the concentrated solar radiation.

High performance solar-selective absorbers using coated sub-wavelength gratings

Spectral control of the emissivity of surfaces is essential for efficient conversion of solar radiation into heat. We investigated surfaces consisting of sub-wavelength V-groove gratings coated with aperiodic metal-dielectric stacks. The spectral behavior of the coated gratings was modeled using rigorous coupled-wave analysis (RCWA). The proposed absorber coatings combine impedance matching using tapered metallic features with the excellent spectral selectivity of aperiodic metal-dielectric stacks. The aspect ratio of the V-groove can be tailored in order to obtain the desired spectral selectivity over a wide angular range. Coated V-groove gratings with optimal aspect ratio are predicted to have thermal emissivity below 6% at 720K while absorbing >94% of the incident light. These sub-wavelength gratings would have the potential to significantly increase the efficiency of concentrated solar thermal systems.

2-D numerical simulation and modeling of monocrystalline selective emitter solar cells

This paper presents a detailed analysis of the dependence of the performance of crystalline silicon (c-Si) selective emitter solar cells on geometrical parameters and doping profiles. Based on two dimensional drift-diffusion TCAD simulations, we report the effects of the front contact pitch and doping profiles on the most important output parameters of solar cells. Simulations show that a significant gain in terms of output power of the cell may arise compared to the homogeneous emitter solar cell. We also present an analysis of the main loss mechanisms and trade-offs for this solar cell.

Numerical simulation and modeling of rear point contact solar cells

High efficiency silicon monocrystalline solar cells commonly adopt point contacted rear surfaces to reduce the recombination losses in the rear side of the device. However, the reduction of the rear contact surface leads to an increase of series resistance losses. Modeling and analysis of rear point contact solar cells is strategic to optimize the device design by taking into account several competing physical mechanisms. Owing to their complicated geometries, the analysis of these devices requires three-dimensional (3-D) numerical simulation. In this work we analyze the influence of the most important geometrical and electrical parameters on the conversion efficiency of rear point contact solar cells.

Comprehensive experimental and numerical optimization of surface morphology of transparent conductive oxide films for tandem thin film photovoltaic cells

Typical thin-film photovoltaic cells incorporate a textured transparent conductive oxide to efficiently harvest solar energy. What should be the ideally desired morphology of this textured surface for best cell performance - is highly debated but remains an unsolved mystery. We present a comprehensive methodology to 1) accurately model, 2) extract macroscopically sufficient statistical finger-prints and 3) predict best desired values for these statistical finger-prints of such a randomly textured surface for the best performance of the cell.

Development of high efficiency mono-crystalline silicon solar cells: Optimization of rear local contacts formation on dielectrically passivated surfaces

An integration process was developed for the fabrication of rear passivated point contact solar cells achieving 19.36% conversion efficiency by using 156×156mm, pseudo square, p-type single crystalline silicon wafers. This is a significant improvement when compared to unpassivated, full area aluminum back surface field solar cells, which exhibit only 18.64% conversion efficiency on the same wafer type. The rear surface was passivated with a Al2O3 layer and a SiNX capping layer. The thicknesses of individual films were optimized to obtain maximum minority carrier lifetimes. The rear surface contact pattern was created by laser ablation and the contact geometry was optimized to obtain voids free contact filling resulting in a uniform back surface field. Internal quantum efficiency and reflectance measurement show significant improvement in rear passivated cells in the infrared wavelength region in comparison to reference cells. The rear surface internal reflectivity for the passivated cell was 93% while that for the reference cell was only about 73%. The rear surface recombination velocity for the rear passivated cell was about 52 cm/s while that for the reference cell was about 300 cm/s. The efficiency gain in rear passivated cells over the reference cells is mainly due to improved short circuit current and open circuit voltage. However, rear passivated solar cells show lower fill factors due to increased series resistance.

Optimal light trapping in ultra-thin photonic crystal crystalline silicon solar cells

Using thin films of crystalline silicon to make solar cells reduces the cost by reducing the amount of material needed and allowing poorer quality material with shorter carrier diffusion lengths to be used. However, the indirect band gap of silicon requires that a light trapping approach be used to maximize optical absorption. Here, a photonic crystal (PC) based approach is used to maximize solar light harvesting in a 400 nm-thick silicon layer by tuning the coupling strength of incident radiation to quasiguided modes over a broad spectral range. The structure consists of a double layer PC. We show an enhancement of maximum achievable photocurrent density from 7.1 mA/cm2 for an unstructured film to 21.8 mA/cm2 for a structured film for normal incidence. This photocurrent density value approaches the limit of 26.5 mA/cm2, obtained using the Yablonovitch light trapping limit for the same volume of active material.

Progress in production-worthy point contact solar cells process

This work reports on the integration process of rear point contact solar cells with reduced recombination and better light trapping than the conventional cells. Al₂O₃/SiN_x passivation stacks were used to ensure the backside passivation and the effective lifetime of minority carrier is found to be >;100 μs (estimated surface recombination velocity ~50 cm/s) on solar p-type Cz wafers. The estimated fixed charge associated with the Al₂O₃ is in the range of -5e12 C/cm². Laser ablation of Al₂O₃/SiN_x stack and aluminum alloying are studied in detail to understand the process window for clean ablation and back surface field. It is found that laser energy plays an important role cleanly ablating the Al₂O₃/SiN_x passivation stack. The solar cell is fabricated using standard processes based on screen-printed aluminum paste onto laser ablated passivation layer consisting of Al₂O₃/SiN_x. Aluminum alloying is dependent on the firing profile and amount of Al melted into Si. Back point contact cells show improvements in J_sc by 1 mA/cm² and V_oc by 10 mV due to better response in infrared spectrum. The best conversion efficiency of back point contact solar cells fabricated with standard industrial emitter and backside passivation is 19.35%.

Design and optimization of next generation high aspect ratio periodic thin film photovoltaic cells

The typical thin-film photovoltaic cells incorporate a textured transparent conductive oxide film to efficiently harvest solar energy [1]. It is well known that the light trapping performance provided by the random texturing has largely fallen short as compared to the initial expectations based on the performance achieved from similar features in thick wafer based cells [1–3]. Several reasons have been accounted for this including interference and coherent effects, smaller photonic density of states in thin film layers, and inability to construct Lambertian scattering surfaces with small textures [1–3]. Coherent periodic structures that can better confine the incident solar energy in the volume occupied by the efficient photoactive materials have been proposed [2,3]. Several research groups have concluded that the high aspect ratio structures such as a periodic array of very thin but tall pillars or cones will be needed for high performance cells. Similarly, high aspect ration holes have also been proposed [2,3]. In this work we performed rigorous opto-electronic analysis of several different device configurations for the next generation high aspect ratio periodic thin film photovoltaic cells. Each device configuration is exhaustively optimized for all geometrical variables to achieve maximum short circuit current under 1SUN AM1.5g radiation. This enables us to objectively compare the potential of different proposed cell designs. We show results from our studies of high aspect ratio pillars versus high aspect ratio perforations (holes); constant cross-section cylindrical pillars versus tapered cross-section conical pillars; square cross-section versus circular cross-section pillars and holes and square periodic lattice versus hexagonal periodic lattice. We also studied differences between the cases when a-Si p-i-n stack is conformally deposited on the high aspect ratio periodic features and when a thick layer of TCO is used to planarize the features before a p-i-n stack is grown. Results will be shown for both the substrate and superstrate configuration of devices. We developed a fully rigorous and coherent opto-electronic simulation platform that is used for this work. In the past we developed a Fourier domain based rigorous coherent optical simulator REMS that was used for studying the random textured devices. We showed that this simulator predicted both the optical scattering properties such as spectral haze and reflectivity of textured TCO surfaces as well as the QE of single junction and tandem junction devices grown on these surfaces with very good accuracy [1–3]. In the present work we extend this simulation capability to analyze and optimize next generation patterned periodic solar cells. In addition to the optical light trapping, these high aspect ratio structures may greatly alter the charge carriers transport pattern and collection efficiency. In a p-i-n stack conformally deposited on a columnar structure, the charge carriers are expected to be collected in the direction perpendicular to the axis of the columns while photons are expected to be absorbed while waveguiding in the direction parallel to the axis. Hence, it is very important to rigorously study the charge carrier dynamics coupled with a full 3D Maxwell solver. Here, we integrated the very fast REMS 3D Maxwell solver with the established device simulator Sentaurus™ from SynpsysR Inc. REMS is Fourier domain based method and is very fast compared to the FDTD or FEM based methods. This allows us to exhaustively optimize the several geometrical features of the device configuration in reasonable amount of time [1–3].