• Energy Research

AbstractExamination of results from the pilot production of buried‐contact solar cells (BCSC) allows several new insights into the effects of the substrate resistivity, the differences between upright and inverted pyramid texturing, the reflection after encapsulation and the doping level at which the emitter begins to dominate the overall recombination. A lower substrate resistivity in conjunction with thicker wafers reduces the effects of a high back surface recombination velocity and allows both higher voltages and efficiencies. In BCSCs with low substrate resistivities, the voltage is not limited by the back but by the emitter diffusion and the dislocation formation at the surface. Contrary to previous reports, best results have been realized with upright pyramids rather than inverted pyramids. In addition, the relative performance of the upright pyramids improves after encapsulation owing to the less than optimal unencapsulated reflection of these surfaces in the regions of the pyramid peaks where oxide layers are too thin to gain benefits as an antireflection layer. Recent results also indicate that the contributions to the dark saturation current from both the heavily phosphorus‐diffused region beneath the metal contact and the more lightly diffused top surface emitter are less than indicated previously. Finally, comparison between experimentally obtained voltages and those predicted through modelling with PC‐1D provides an estimate of the bulk material lifetimes in the pilot line cells.

In this paper we present novel light trapping designs applied to multiple junction thin film solar cells. The new designs incorporate one dimensional photonic crystals as band pass filters that reflect short light wavelengths (400 - 867 nm) and transmit longer wavelengths(867 -1800 nm) at the interface between two adjacent cells. In addition, nano structured diffractive gratings that cut into the photonic crystal layers are incorporated to redirect incoming waves and hence increase the optical path length of light within the solar cells. Two designs based on the nano structured gratings that have been realized using the scattering matrix and particle swarm optimization methods are presented. We also show preliminary fabrication results of the proposed devices.

Abstract The double sided buried contact (DSBC) silicon solar cells have consistently shown high open-circuit voltages (Voc) than its single sided buried contact counterpart because of better rear surface passivation. The rear surface passivation which is provided by the rear floating junction is effective only when there is no leakage in the rear floating junction. However, the partial shunting of the rear floating junction can cause a drop in the fill factor of the cell which has been the only parameter limiting the realization of the structure's potentials. In this paper, LBIC (light beam induce current), spectral response, dark I-V and Jsc-Voc measurements for DSBC cells have been carried out to help explain some of the experimentally observed attributes of this structure. The partly shunted rear floating junction has been identified by LBIC measurement as low current regions near the rear metal contacts.

We have investigated the GaP/Si heterojunction interface for application in silicon heterojunction solar cells. We performed X-ray photoelectron spectroscopy (XPS) on thin layers of GaP grown on Si by metal organic chemical vapor deposition and molecular beam epitaxy. The conduction band offset was determined to be 0.9 ± 0.2 eV, which is significantly higher than predicted by Anderson's rule (0.3 eV). XPS also revealed the presence of Ga–Si bonds at the interface that are likely to be the cause of the observed interface dipole. Via cross-sectional Kelvin probe force microscopy ( x -KPFM), we observed a charge transport barrier at the Si/GaP interface which is consistent with the high-conduction band offset determined by XPS and explains the low open-circuit voltage and low fill factor observed in GaP/Si heterojunction solar cells.

Abstract The recombination limiting the voltage of the present buried contact solar cell (BCSC) can be reduced by replacing the present high recombination sintered aluminium back with a floating rear junction for passivation, heavy boron diffusion below the rear contact, and by limiting the rear surface contact area. Analysis of these implementations in the double sided laser grooved (DSLG) structure shows that the floating junction passivation is effective in reducing the recombination component at the rear surface and that the boron diffusion in the rear groove comprises up to half of the total saturation current. Limiting the area of the heavily diffused boron grooves allows open-circuit voltages of 685 mV while maintaining the simplicity of the BCSC processing sequence. An open-circuit voltage of 685 mV represents nearly a 50 mV increase over the conventional BCSC.

The DC-AC system with a small continuous load is analyzed putting emphasis on the inverter efficiency drop at part-load condition. The system robustness and lifecycle cost are calculated in various conditions to investigate the effects of the small continuous load. In addition to the large energy consumption by the small continuous load, inverter efficiency drop observed at part-load condition makes it more important to design a cost-effective system.

Very High Efficiency Solar Cell (VHESC) program is developing integrated optical/photovoltaic modules for portable applications that operate at 50 percent efficiency. Test sub-modules incorporating four p-n junctions and corresponding optics have been realized and are predicted to realize efficiency greater than 40%. Phased implementation requires corresponding measurement to inspect accomplished work and provide improvement direction for the next step. The comparison between the real performance of the four-junction test sub-module and the theoretical prediction of its efficiency is a significant indication of the realizability of the final VHESC module including six junctions which is designed to achieve 50% efficiency. For the sub-module measurement, a test bed was set up for outdoor test. Previous outdoor measurements of the VHESC test sub-modules resulted in a preliminary sub-module efficiency of 36.2% [1]. As solar cells with better performance were fabricated, the measurement methodology was refined and corresponding improvements were made to the initial test bed. Three test sub-modules containing new solar cells were measured with the new test setup for three different concentration levels at University of Delaware (UD). One test sub-module demonstrated efficiency as high as 39.5%, coupled with 44.3% efficient solar cells and 89.1% efficient optics, at 30.48X concentration. The measurements were taken when the direct light intensity was over 860W/m2 and the I sc was not calibrated to 1000W/m2. Another two test sub-modules including solar cells in the same batch as the ones tested at UD were taken to National Renewable Energy Laboratory (NREL). The Isc data of the two test sub-modules were recorded outdoors at NREL when the direct light intensity was over 970 W/m2. In addition, the I sc was calibrated to the standard spectrum condition using ASTM G173 direct data. Comparison of the results shows the difference between the test sub-module efficiency measured at UD and NREL is less than 4%.

Abstract Despite the growing commercial success of the conventional buried contact solar cell (BCSC), significant improvements in cell performance, simplicity of fabrication, and costs are being made. Five strands to this work with five corresponding variations of the cell structure are responsible for the developments. Hybrid solar cells (standard BCSC front surface with photolithographically defined rear metal contact scheme) have demonstrated open-circuit voltages approaching 700 mV while a simpler cell design requiring no photolithography has demonstrated open-circuit voltages as high as 685 mV. Large area, 20 sun BCSC concentrator cells have been developed with very low metal shading losses (below 3%) due to the redesigning of the groove structure to recess the metal to below the top surface. The resulting record efficiency of 21.5% has been independently confirmed (Sandia). The most recent BCSC structure, where the emphasis is on simplicity and low cost, has the number of high temperature processing steps reduced to one, while efficiencies in the vicinity of those achieved by the conventional BCSC are anticipated. The highest efficiencies demonstrated to date with any of the BCSC structures are well above 21% (Sandia) although all five variations of the BCSC structure appear capable of achieving similar performance levels in the future.

AbstractA major hindrance to the development of devices integrating III-V materials on silicon, where it is an active component of the device, is the preservation of its electronic quality. In this contribution, we report on our effort to identify the mechanism behind the severe decrease in the bulk minority-carrier lifetime of silicon after heteroepitaxial growth of gallium phosphide, in our molecular beam epitaxy (MBE) system. We identify that the drop in lifetime occurs at a threshold temperature of 500 °C; we assign the increased recombination rate to extrinsic, fast-diffusing impurities coming from the MBE chamber environment. Impurities can be gettered by phosphorous diffusion, leading to a lifetime recovery. Moreover, we narrow the list of contaminants based on specific experimental observations and compare our hypothesis to modeling of injection-dependent lifetime spectra. Finally we show that coating the silicon wafer with a sacrificial silicon nitride film helps significantly to reduce contamination and provides a path to successful III-V growth on silicon.