Optimization of the Antireflection Coating of Thin Epitaxial Crystalline Silicon Solar Cells

AbstractIn this work we use an effective weighting function to include the internal quantum efficiency (IQE) and the effective thickness, Te, of the active cell layer in the optical modeling of the antireflection coating (ARC) of very thin crystalline silicon solar cells. The spectrum transmitted through the ARC is hence optimized for efficient use in the given cell structure and the solar cell performance can be improved. For a 2-μm thick crystalline silicon heterojunction solar cell the optimal thickness of the Indium Tin Oxide (ITO) ARC is reduced by ∼8nm when IQE data and effective thickness are taken into account compared to the standard ARC optimization, using the AM1.5 spectrum only. The reduced ARC thickness will shift the reflectance minima towards shorter wavelengths and hence better match the absorption of very thin cells, where the short wavelength range of the spectrum is relatively more important than the long, weakly absorbed wavelengths. For this cell, we find that the optimal thickness of the ITO starts at 63nm for very thin (1μm) active Si layer and then increase with increasing Te until it saturates at 71nm for Te > 30μm

Carrier Selective, Passivated Contacts for High Efficiency Silicon Solar Cells based on Transparent Conducting Oxides

AbstractWe describe the design, fabrication and results of passivated contacts to n-type silicon utilizing thin SiO2 and transparent conducting oxide layers. High temperature silicon dioxide is grown on both surfaces of an n-type wafer to a thickness <50Å, followed by deposition of tin-doped indium oxide (ITO) and a patterned metal contacting layer. As deposited, the thin-film stack has a very high J0,contact, and a non-ohmic, high contact resistance. However, after a forming gas anneal, the passivation quality and the contact resistivity improve significantly. The contacts are characterized by measuring the recombination parameter of the contact (J0,contact) and the specific contact resistivity (ρcontact) using a TLM pattern. The best ITO/SiO2 passivated contact in this study has J0,contact = 92.5 fA/cm2 and ρcontact = 11.5 mOhm-cm2. These values are placed in context with other passivating contacts using an analysis that determines the ultimate efficiency and the optimal area fraction for contacts for a given set of (J0,contact, ρcontact) values. The ITO/SiO2 contacts are found to have a higher J0,contact, but a similar ρcontact compared to the best reported passivated contacts

Diodes for Optical Rectennas

Two types of ultra-fast diode are fabricated, characterized, and simulated for use in optical rectennas. A rectenna consists of an antenna connected to a diode in which the electromagnetic radiation received by the antenna is rectified in the diode. I have investigated metal/insulator/metal (MIM) tunnel diodes and a new, geometric diode for use in rectenna-based infrared detectors and solar cells. Factors influencing the performance of a rectenna are analyzed. These include DC and optical-frequency diode-characteristics, circuit parameters, signal amplitude, and coherence of incoming radiation. To understand and increase the rectification response of MIM-based rectennas, I carry out an in-depth, simulation-based analysis of MIM diodes and design improved multi-insulator tunnel barriers. MIM diodes are fundamentally fast. However, from a small-signal circuit model the operating frequency of a rectenna is found to be limited by the diode\u27s RC time constant. To overcome this limitation, I have designed and simulated a distributed rectifier that uses the MIM diode in a traveling-wave configuration. High-frequency characteristics of MIM diodes are obtained from a semiclassical theory for photon-assisted tunneling. Using this theory, the dependence of rectenna efficiency on diode characteristics and signal amplitude is evaluated along with the maximum achievable efficiency. A correspondence is established between the first-order semiclassical theory and the small-signal circuit model. The RC time constant of MIM diodes is too large for efficient operation at near-infrared-to-visible frequencies. To this end, a new, planar rectifier that consists of an asymmetrically-patterned thin-film, is developed. The diode behavior in this device is attributed to the geometric asymmetry of the conductor. Geometric diodes are fabricated using graphene and measured for response to infrared illumination. To model the I(V) curve of geometric diodes, I have implemented a quantum mechanical simulation based on the tight-binding Hamiltonian. The simulated and the measured current-voltage characteristics are consistent with each other. I have also derived a semiclassical theory, analogous to the one for MIM diodes, for analyzing the optical response of geometric diodes

OPTICAL RECTENNA SOLAR CELLS USING GRAPHENE GEOMETRIC DIODES

ABSTRACT A solar cell using micro-antennas to convert radiation to alternating current and ultrahigh-speed diodes to rectify the AC can in principle provide extremely high conversion efficiencies. Currently investigated rectennas using metal/insulator/metal (MIM) diodes are limited in their RC response time and have poor impedance matching to the antenna. We have investigated a new rectifier, referred to as a geometric diode, which can overcome these limitations. The geometric diode consists of a conducting thin-film, such as graphene, patterned in a geometry that leads to diode behavior. We have experimentally demonstrated geometric diodes made from graphene and simulated their characteristics using the Drude model for charge transport. Here we compare the characteristics of rectennas using MIM diodes with those based on geometric diodes and show the improved performance of the latter

A Search for New Back Contacts for CdTe Solar Cells

There is widespread interest in reaching the practical efficiency of cadmium telluride (CdTe) thin-film solar cells, which suffer from significant open-circuit voltage loss due to high surface recombination velocity and Schottky barrier at the back contact. Here, we focus on back contacts in the superstrate configuration with the goal of finding new materials, that can provide improved passivation, electron reflection and hole transport properties compared to the commonly used material, ZnTe. We performed a computational search among 229 binary and ternary tetrahedrally-bonded structures using first-principles methods and transport models to evaluate critical materials design criteria, including phase stability, electronic structure, hole transport, band alignments, and p-type dopability. Through this search, we have identified several candidate materials and their alloys (AlAs, AgAlTe2, ZnGeP2, ZnSiAs2, CuAlTe2) that exhibit promising properties for back contacts. We hope these new material recommendations and associated guidelines will inspire new directions in hole transport layer design for CdTe solar cells

{CdTe}-based thin film photovoltaics: Recent advances, current challenges and future prospects

Cadmium telluride (CdTe)-based cells have emerged as the leading commercialized thin film photovoltaic technology and has intrinsically better temperature co-efficients, energy yield, and degradation rates than Si technologies. More than 30 GW peak (GWp) of CdTe-based modules are installed worldwide, multiple com-panies are in production, modules are shipping at up to 18.6% efficiency, and lab cell efficiency is above 22%. We review developments in the science and technology that have occurred over approximately the past decade. These achievements were enabled by manufacturing innovations and scaling module production, as well as maximizing photocurrent through window layer optimization and alloyed CdSexTe1-x (CST) absorbers. Improved chlorine passivation processes, film microstructure, and serendipitous Se defect passivation significantly increased minority carrier lifetime. Efficiencies &gt;22% have been realized for both Cu and As doped CST-based cells. The path to further efficiency gains hinges primarily on increasing open circuit voltage (Voc) and fill factor (FF) through innovations in materials, fabrication methods, and device stacks. Replacing the longstanding Cu doping with As doping is resulting in better module stability and is being translated to large-scale production. To realize 25% efficiency and &gt;1 V Voc, research and development is needed to increase the minority carrier lifetime beyond 100 ns, reduce grain boundary and interface recombination, and tailor band diagrams at the front and back interfaces. Many of these goals have been realized separately however combining them together using scalable manufacturing approaches has been elusive to date. We review these achievements and outstanding opportunities for this remarkable photovoltaic technology

CdTe-based thin film photovoltaics: recent advances, current challenges and future prospects

Cadmium telluride (CdTe)-based cells have emerged as the leading commercialized thin film photovoltaic technology and has intrinsically better temperature coefficients, energy yield, and degradation rates than Si technologies. More than 30 GW peak (GWp) of CdTe-based modules are installed worldwide, multiple companies are in production, modules are shipping at up to 18.6% efficiency, and lab cell efficiency is above 22%. We review developments in the science and technology that have occurred over approximately the past decade. These achievements were enabled by manufacturing innovations and scaling module production, as well as maximizing photocurrent through window layer optimization and alloyed CdSexTe1-x (CST) absorbers. Improved chlorine passivation processes, film microstructure, and serendipitous Se defect passivation significantly increased minority carrier lifetime. Efficiencies >22% have been realized for both Cu and As doped CST-based cells. The path to further efficiency gains hinges primarily on increasing open circuit voltage (Voc) and fill factor (FF) through innovations in materials, fabrication methods, and device stacks. Replacing the longstanding Cu doping with As doping is resulting in better module stability and is being translated to large-scale production. To realize 25% efficiency and >1 V Voc, research and development is needed to increase the minority carrier lifetime beyond 100 ns, reduce grain boundary and interface recombination, and tailor band diagrams at the front and back interfaces. Many of these goals have been realized separately however combining them together using scalable manufacturing approaches has been elusive to date. We review these achievements and outstanding opportunities for this remarkable photovoltaic technology.</p