Scott Burroughs

Printing-based assembly of quadruple-junction four-terminal microscale solar cells and their use in high-efficiency modules

Expenses associated with shipping, installation, land, regulatory compliance and on-going maintenance and operations of utility-scale photovoltaics can be significantly reduced by increasing the power conversion efficiency of solar modules through improved materials, device designs and strategies for light management. Single-junction cells have performance constraints defined by their Shockley-Queisser limits. Multi-junction cells can achieve higher efficiencies, but epitaxial and current matching requirements between the single junctions in the devices hinder progress. Mechanical stacking of independent multi-junction cells circumvents these disadvantages. Here we present a fabrication approach for the realization of mechanically assembled multi-junction cells using materials and techniques compatible with large-scale manufacturing. The strategy involves printing-based stacking of microscale solar cells, sol-gel processes for interlayers with advanced optical, electrical and thermal properties, together with unusual packaging techniques, electrical matching networks, and compact ultrahigh-concentration optics. We demonstrate quadruple-junction, four-terminal solar cells with measured efficiencies of 43.9% at concentrations exceeding 1,000 suns, and modules with efficiencies of 36.5%.

A high concentration photovoltaic module utilizing micro-transfer printing and surface mount technology

We describe a high concentration photovoltaic (CPV) module utilizing micro-transfer printed (µ-TP) dual-junction GaInP/GaAs solar cells and an ELO (Epitaxial Lift-Off) process used to fabricate very small cells (<0.5 mm2) using 1st use and reused GaAs substrates. The benefits of this technology include high efficiency, simple distributed heat transfer at high concentration ratios, and short optical paths. This approach enables the use of low cost, high reliability surface mount assembly of large backplanes for integration into CPV modules. To minimize compound semiconductor use and maximize cell efficiency, we combine plano-convex primary and spherical secondary optics to concentrate sunlight 1000X over a +/−0.8 degree angle of acceptance. Receiver efficiencies of ELO dual-junction GaInP/GaAs cells of >30% at 1,000 sun concentration are reported. Coupled with a >80% efficient optical train, module efficiencies greater than 24% have been achieved with dual-junction µ-TP solar cells.

A New Approach For A Low Cost CPV Module Design Utilizing Micro‐Transfer Printing Technology

Semprius is applying a novel massively parallel, automated production process to address CPV’s reliability, performance, cost, and scalability requirements. The new design approach utilizing patented micro‐transfer printing technology enables the use of many very small cells (0.36 mm2) with benefits including high efficiency, simple distributed heat transfer, high concentration ratio, and small thin concentrating optical elements. We briefly describe the design approach and provide detailed supporting on‐sun measurements.

Concentrator photovoltaic module architectures with capabilities for capture and conversion of full global solar radiation

Significance Concentrator photovoltaic (CPV) systems, wherein light focuses onto multijunction solar cells, offer the highest efficiencies in converting sunlight to electricity. The performance is intrinsically limited, however, by an inability to capture diffuse illumination, due to narrow acceptance angles of the concentrator optics. Here we demonstrate concepts where flat-plate solar cells mount onto the backplanes of the most sophisticated CPV modules to yield an additive contribution to the overall output. Outdoor testing results with two different hybrid module designs demonstrate absolute gains in average daily efficiencies of between 1.02% and 8.45% depending on weather conditions. The findings suggest pathways to significant improvements in the efficiencies, with economics that could potentially expand their deployment to a wide range of geographic locations. Emerging classes of concentrator photovoltaic (CPV) modules reach efficiencies that are far greater than those of even the highest performance flat-plate PV technologies, with architectures that have the potential to provide the lowest cost of energy in locations with high direct normal irradiance (DNI). A disadvantage is their inability to effectively use diffuse sunlight, thereby constraining widespread geographic deployment and limiting performance even under the most favorable DNI conditions. This study introduces a module design that integrates capabilities in flat-plate PV directly with the most sophisticated CPV technologies, for capture of both direct and diffuse sunlight, thereby achieving efficiency in PV conversion of the global solar radiation. Specific examples of this scheme exploit commodity silicon (Si) cells integrated with two different CPV module designs, where they capture light that is not efficiently directed by the concentrator optics onto large-scale arrays of miniature multijunction (MJ) solar cells that use advanced III–V semiconductor technologies. In this CPV+ scheme (“+” denotes the addition of diffuse collector), the Si and MJ cells operate independently on indirect and direct solar radiation, respectively. On-sun experimental studies of CPV+ modules at latitudes of 35.9886° N (Durham, NC), 40.1125° N (Bondville, IL), and 38.9072° N (Washington, DC) show improvements in absolute module efficiencies of between 1.02% and 8.45% over values obtained using otherwise similar CPV modules, depending on weather conditions. These concepts have the potential to expand the geographic reach and improve the cost-effectiveness of the highest efficiency forms of PV power generation.

Transfer-printed microscale integrated circuits

Transfer-printing is an emerging technology that enables massively parallel assembly of microscale semiconductor devices onto virtually any target substrate, including glass, plastics, metals or other semiconductors. Transfer-printing is accomplished using a microstructured elastomeric stamp to selectively pick-up devices from a source wafer and then print the devices onto a target substrate. The process is massively parallel as the stamps are designed to transfer hundreds to thousands of discrete devices in a single pick-up and print operation. Previous studies using bare silicon chips [1] demonstrated transfer-print yields in excess of 99% and chip placement accuracy better than ± 5 µm. For the first time, foundry-produced CMOS integrated circuits have been designed and transfer-printed. The ICs were designed and built using a commercially available silicon-on-oxide (SOI) CMOS process. The buried oxide (BOx) underneath the device layer is used as a sacrificial layer to “release” the ICs from the handle wafer. Microfabricated silicon bridges, or tethers, are used to fasten the ICs to the handle wafer following the sacrificial etch. A process was developed to remove the sacrificial BOx while protecting the interlayer dielectric (ILD) and Aluminum wiring levels present in the ICs. The microscale ICs have been transfer-printed with yields in excess of 99.5% and with placement accuracies better than ± 5 µm. Surface topography present on the ICs did not negatively impact the transfer-printing process. Initial studies show that transfer-printing has negligible impact on the I-V characteristics of transistors.

Performance of a micro-cell based transfer printed HCPV system in the South Eastern US

Printed micro-cells are the basis of a more cost effective and highly efficient HCPV module design. Module efficiencies of 33.9% at STC have been demonstrated for modules designed for commercial use. An array of these modules were deployed in a 3.5 kW system in Huntsville, AL to validate the design, demonstrate the reliability of the system and collect on-sun data to understand system performance. The first six months of performance data is presented along with results from soiling experiments.

Development of InGaAs solar cells for >44% efficient transfer-printed multi-junctions

Transfer-printing is a key enabling technology for the realization of ultra-high-efficiency, mechanically stacked II-IV solar cells with low cost. In this work, we describe the development of InGaAs solar cells, designed to harvest long wavelength photons when stacked in tandem with a high efficiency InGaP/GaAs/InGaAsNSb triple junction solar cell. High performance InGaAs solar cells, grown on InP by MOCVD, were achieved through a combination of detailed modeling, material development and device characterization. The transfer printing apparatus of Semprius Inc. was used to create a four-terminal device with an uncertified conversion efficiency of 44.1% at 690 suns.

Optical cell temperature measurements of multiple CPV technologies in outdoor conditions

It is well known that photovoltaic performance is dependent on cell temperature. Although various methods have been explored to determine outdoor concentrating photovoltaic (CPV) cell temperature, no method has proven to work across all module technologies and result in desirable uncertainties. Menard (2012) has recently published results claiming accurate measurements of cell temperature using the wavelength shift of light emitted from the sub-cells of a Semprius CPV module. This work focuses on efforts to verify Menards results using additional CPV technologies that are on-sun at NREL. Baseline electro-luminescence emission is recorded for modules under a low level forward bias and under isothermal conditions using thermal chambers. The same modules or sister modules are then placed on NRELs high accuracy two-axis tracker for outdoor measurements. Photo-luminescence emission peaks are measured for multiple modules at stable wind and irradiance conditions. Emission results from the sub-cells are compared to what is documented in the literature for the given semiconductor material. The signal to background ratio is analyzed and the possible broad applicability of this procedure is discussed.

On‐Sun Performance of a Novel Microcell Based HCPV System Located in the Southwest US

Semprius has developed a novel microcell based, highly scalable HCPV module that addresses performance, cost and reliability requirements for utility scale solar installations. Semprius has fabricated dual junction cell based engineering prototype modules with 1000X concentration based on this technology. A 1 kW HCPV system using these modules was installed in Tucson to validate the technology and acquire on‐sun data. Eight months of on‐sun results from this system are presented.

Ultrahigh Efficiency HCPV Modules and Systems

Semprius manufactures high-concentration photovoltaic (HCPV) modules and systems. Module characterization and quality control methods are described in this paper. Module efficiency distribution for thousands of modules is presented, currently showing an increase in average efficiency from 33% in 2013 to 35%. Data from modules and systems in the field are presented showing consistent performance over all seasons and no apparent degradation after 3 years. Finally, the effect of ambient temperature and wind speed on performance was studied.