Vipin P. Gupta

Optimal cell connections for improved shading, reliability, and spectral performance of microsystem enabled photovoltaic (MEPV) modules

Microsystems enabled photovoltaics (MEPV) is a recently developed concept that promises benefits in efficiency, functionality, and cost compared to traditional PV approaches. MEPV modules consist of heterogeneously integrated arrays of ultra-thin (∼2 to 20 µm), small (∼100 µm to a few millimeters laterally) cells with either one-sun or micro-optics concentration configurations, flexible electrical configurations of individual cells, and potential integration with electronic circuits. Cells may be heterogeneously stacked and separated by dielectric layers to realize multi-junction designs without the constraints of lattice matching or series connections between different cell types. With cell lateral dimensions of a few millimeters or less, a module has tens to hundreds of thousands of cells, in contrast to todays PV modules with less than 100. Hence, MEPV modules can operate at high voltages without module DC to DC converters, reducing resistive losses, improving shading performance, and improving robustness to individual cell failures.

Microscale c-Si (c)PV cells for low-cost power

We are exploring fabrication and assembly concepts developed for Microsystems/MEMS technology to reduce the cost of solar PV power. These methods have the potential to reduce many system level costs of current PV systems including, among others, silicon material costs, module assembly costs, and installation costs. We have demonstrated a direct c-Si material reduction of approximately 20X (including wire-saw kerf loss and polishing loss). The cells have achieved efficiencies of almost 9% and Jsc of 30 mA/cm2. We are currently using integrated-circuit (IC) fabrication tools that will lead to higher efficiencies and improved yield. These advantages and the material reduction are expected to reduce the current module manufacturing costs.

Leveraging scale effects to create next-generation photovoltaic systems through micro- and nanotechnologies

Current solar power systems using crystalline silicon wafers, thin film semiconductors (i.e., CdTe, amorphous Si, CIGS, etc.), or concentrated photovoltaics have yet to achieve the cost reductions needed to make solar power competitive with current grid power costs. To overcome this cost challenge, we are pursuing a new approach to solar power that utilizes micro-scale solar cells (5 to 20 μm thick and 100 to 500 μm across). These micro-scale PV cells allow beneficial scaling effects that are manifested at the cell, module, and system level. Examples of these benefits include improved cell performance, better thermal management, new module form-factors, improved robustness to partial shading, and many others. To create micro-scale PV cells we are using technologies from the MEMS, IC, LED, and other micro and nanosystem industries. To date, we have demonstrated fully back-contacted crystalline silicon (c-Si), GaAs, and InGaP microscale solar cells. We have demonstrated these cells individually (c-Si, GaAs), in dual junction arrangements (GaAs, InGaP), and in a triple junction cell (c-Si, GaAs, InGaP) using 3D integration techniques. We anticipate two key systems resulting from this work. The first system is a high-efficiency, flexible PV module that can achieve greater than 20% conversion efficiency and bend radii of a few millimeters (both parameters greatly exceeding what currently available flexible PV can achieve). The second system is a utility/commercial scale PV system that cost models indicate should be able to achieve energy costs of less than

Ultrathin Flexible Crystalline Silicon: Microsystems-Enabled Photovoltaics

0.10/kWh in most locations.

Defect formation dynamics during CdTe overlayer growth

We present an approach to create ultrathin (<;20 μm) and highly flexible crystalline silicon sheets on inexpensive substrates. We have demonstrated silicon sheets capable of bending at a radius of curvature as small as 2 mm without damaging the silicon structure. Using microsystem tools, we created a suspended submillimeter honeycomb-segmented silicon structure anchored to the wafer only by small tethers. This structure is created in a standard thickness wafer enabling compatibility with common processing tools. The procedure enables all the high-temperature steps necessary to create a solar cell to be completed while the cells are on the wafer. In the transfer process, the cells attach to an adhesive flexible substrate which, when pulled away from the wafer, breaks the tethers and releases the honeycomb structure. We have previously demonstrated that submillimeter and ultrathin silicon segments can be converted into highly efficient solar cells, achieving efficiencies up to 14.9% at a thickness of 14 μm. With this technology, achieving high efficiency (>;15%) and highly flexible photovoltaic (PV) modules should be possible.

Microfabrication of microsystem-enabled photovoltaic (MEPV) cells

The presence of atomic-scale defects at multilayer interfaces significantly degrades performance in CdTe-based photovoltaic technologies. The ability to accurately predict and understand defect formation mechanisms during overlayer growth is, therefore, a rational approach for improving the efficiencies of CdTe materials. In this work, we utilize a recently developed CdTe bond-order potential (BOP) to enable accurate molecular dynamics (MD) simulations for predicting defect formation during multilayer growth. A detailed comparison of our MD simulations to high-resolution transmission electron microscopy experiments verifies the accuracy and predictive power of our approach. Our simulations further indicate that island growth can reduce the lattice mismatch induced defects. These results highlight the use of predictive MD simulations to gain new insight into defect reduction in CdTe overlayers, which directly addresses efforts to improve these materials.

Investigating the allegations of Indian nuclear test preparations in the Rajasthan desert

Microsystem-Enabled Photovoltaic (MEPV) cells allow solar PV systems to take advantage of scaling benefits that occur as solar cells are reduced in size. We have developed MEPV cells that are 5 to 20 microns thick and down to 250 microns across. We have developed and demonstrated crystalline silicon (c-Si) cells with solar conversion efficiencies of 14.9%, and gallium arsenide (GaAs) cells with a conversion efficiency of 11.36%. In pursuing this work, we have identified over twenty scaling benefits that reduce PV system cost, improve performance, or allow new functionality. To create these cells, we have combined microfabrication techniques from various microsystem technologies. We have focused our development efforts on creating a process flow that uses standard equipment and standard wafer thicknesses, allows all high-temperature processing to be performed prior to release, and allows the remaining post-release wafer to be reprocessed and reused. The c-Si cell junctions are created using a backside point-contact PV cell process. The GaAs cells have an epitaxially grown junction. Despite the horizontal junction, these cells also are backside contacted. We provide recent developments and details for all steps of the process including junction creation, surface passivation, metallization, and release.

Cost analysis of flat-plate concentrators employing microscale photovoltaic cells for high energy per unit area applications

This paper describes how commercial satellite imagery was used along with news reports and published scientific articles to investigate the December 1995 allegations of Indian nuclear test preparations in the Rajasthan Desert. Taking the allegations against India as an example of a future CTB compliance dispute, the investigation was conducted to test the utility of commercial satellite imagery for CTB verification. The technical inquiry produced a series of findings on India`s nuclear testing history and on the recent nuclear test allegations. These findings included the exact location of the subsidence crater created by the May 18, 1974 nuclear test, the discovery of an adjacent military range near Khetolai village, and the observation of recent large-scale, unusual activity at this military range in the immediate vicinity of the 1974 test site. The image-derived information was used to sift fact from fiction in the conflicting media reports. It was then integrated into the collection of credible evidence and analyzed to determine whether the observed activity at the Khetolai military range was conventional, missile testing, nuclear, or innocuous. 95 refs., 1 tab.

Back-contacted and small form factor GaAs solar cell

Microsystems Enabled Photovoltaics (MEPV) is a relatively new field that uses microsystems tools and manufacturing techniques familiar to the semiconductor industry to produce microscale photovoltaic cells. The miniaturization of these PV cells creates new possibilities in system designs that can be used to reduce costs, enhance functionality, improve reliability, or some combination of all three. In this article, we introduce analytical tools and techniques to estimate the costs associated with a hybrid concentrating photovoltaic system that uses multi-junction microscale photovoltaic cells and miniaturized concentrating optics for harnessing direct sunlight, and an active c-Si substrate for collecting diffuse sunlight. The overall model comprises components representing costs and profit margin associated with the PV cells, concentrating optics, balance of systems, installation, and operation. This article concludes with an analysis of the component costs with particular emphasis on the microscale PV cell costs and the associated tradeoffs between cost and performance for the hybrid CPV design.

Cost analysis for flat-plate concentrators employing microscale photovoltaic cells

We present a newly developed microsystem enabled, back-contacted, shade-free GaAs solar cell. Using microsystem tools, we created sturdy 3 µm thick devices with lateral dimensions of 250 µm, 500 µm, 1 mm, and 2 mm. The fabrication procedure and the results of characterization tests are discussed below. The highest efficiency cell had a lateral size of 500 µm and a conversion efficiency of 10%, open circuit voltage of 0.9 V and a current density of 14.9 mA/cm2 under one-sun illumination.