Brent A. Wacaser

High-Concentration Photovoltaics—Effect of Inhomogeneous Spectral Irradiation

At high solar concentration, subtle optical and electrical effects in combination can have a substantial impact on photovoltaic power (PV) generation. We have identified such an effect through its clear signature: a “ripple” in the output current with respect to the pointing angle of the concentrated PV (CPV) system to sun direction. At small angular misalignment, this effect can lower cell current by as much as 15% at 1600x concentration in full sun. At medium concentration between 500 and 1000x, while not as clearly visible in single cells, the effect also reduces output by a smaller amount. The disappearance of the “ripple” signature at low concentrations below 300x indicates that the effect is not a linear effect, such as a light loss. We attribute the pronounced angular sensitivity of power output at high concentrations to a combination of inhomogeneous spectral irradiation incident on the multijunction solar cell and of the impact of the finite lateral resistance of the cell.

Exploring the limits of concentration for UHCPV

Practical multi receiver ultra high (1000+ Suns) concentration photovoltaic (UHCPV) systems experience large radiation, thermal and electrical loads in addition to large power density transients under routine operation. This report is a summary of the issues involved in determining the practical limits to concentration. How high is too high? Explorations into UHCPV have both theoretical and experimental aspects. Understanding the theoretical device physics and circuit limitations is often essential to determining which experiments to do and in interpreting results. On the experimental side the work can be divided into two fields depending on the type of light source. The first is artificial or simulated sources and the second is working in the field with direct solar irradiation. Both fields have advantages and disadvantages. Direct solar radiation was selected for the current experiments due to the low cost and ability to produce ultra high concentrations (4000+) over relatively large areas (25+ mm2). Several experimental examples from these direct solar measurements shed light on some of the basic theories of how concentrated light affects the performance of multi junction photovoltaic cells. Out of these examples and theoretical foundations we conclude that for practical devices the first order constraint to optimum efficiency at ultra high concentrations is the series resistance. We also present a simple model based on published data and our results that can be used to predict the total system series resistance needed to optimize a system for a particular concentration.

Multi receiver concentrator photovoltaic testing at extreme concentrations

Practical multi receiver concentrator photovoltaic systems operating at high solar concentration levels up to 2000 suns experience large radiation, thermal and electrical loads in addition to large power density transients under routine operation. These systems require efficient cooling to manage the associated incident power densities between 100 to 200 W/cm2. Photovoltaic cells and thermal interface materials experience considerable stress under these load conditions. Their assembly is sensitive to contamination and process optimization. Efficient optical coupling of light at high concentration requires precise component alignment and tracking. We will discuss high power testing of single and multi receiver, high concentration systems comprising commercial triple junction cells, Fresnel optics, electric actuators, and cooled through a metal thermal interface using active and passive cooling methods.

Nanoscale chemical templating of Si nanowires seeded with Al

We describe a new approach for achieving controlled spatial placement of VLS-grown nanowires that uses an oxygen-reactive seed material and an oxygen-containing mask. Oxygen-reactive seed materials are of great interest for electronic applications, yet they cannot be patterned using the approaches developed for noble metal seed materials such as Au. This new process, nanoscale chemical templating, takes advantage of the reactivity of the blanket seed layer by depositing it over a patterned oxide that reacts with the seed material to prevent nanowire growth in undesired locations. Here we demonstrate this technique using Al as the seed material and SiO2 as the mask, and we propose that this methodology will be applicable to other reactive metals that are of interest for nanowire growth. The method has other advantages over conventional patterning approaches for certain applications including reducing patterning steps, flexibility in lithographic techniques, and high growth yields. We demonstrate its application with standard and microsphere lithography. We show a high growth yield and fidelity, with no NWs between openings and a majority of openings occupied by a single vertical nanowire, and discuss the dependence of yield on parameters.

Optimizing defocus to increase efficiency in concentrator photovoltaic modules

We describe a process for increasing power efficiency of concentrator photovoltaic systems by optimizing the lens-to-cell spacing. We find that there is an optimum defocus position with improved power output and reduced sensitivity to pointing errors, which in combination can result in a more than 10% enhancement. The improvement can be realized by minor changes to module cases which should not require changes to other manufacturing, installation, or component costs. In fact optimizing the defocus position allows for lower costs per unit power due to increased power and relaxed system tolerances. The paper focuses on detailed data illuminating the behavior of ultra high concentration photovoltaic modules. While one can look forward to optimizing defocus through sufficiently detailed simulation, at present, we find that an empirical determination of optimum defocus is necessary. The data reveals that even without design parameters changing, supply chain changes can have a significant impact on the optimum defocus - data from five different module configurations with components from different manufacturing lots are presented. These different configurations serve to illustrate the consequences of component changes and the importance of verifying the optimum defocus. A discussion of the effects that are important to determining the optimum defocus and which underlie these differences is included.

Ultra-high CPV system development and deployment in Saudi Arabia

This paper discusses the development and deployment of an ultra-high concentrating PV module that utilizes concentration above 1400X on multijunction solar cells. The development process included the selection of cell assemblies, primary and secondary optics, and focal distance. The systems were deployed in Saudi Arabia inside the Solar Village near Riyadh and in Khafji near the border of Saudi and Kuwait, following the deployment of first prototype in Yorktown, NY. Data from operation in those areas are shown here, and next steps of optimizing the module performance are discussed.

Technical advantages and challenges for core-shell micro/ nanowire large area PV devices

A promising field for future low cost, medium efficiency solar cell devices is the use of vapor-liquid-solid (VLS) grown nanowires or micropillars (NWs referring to both) as the active region of large scale (greater than 1 mm2 area) photovoltaic devices. There are several advantages of using NWs. The NWs can be doped as grown, helping with formation of a PV structure. NW-based PV structures require shorter carrier diffusion distances than are needed for a similarly thick planar absorber layer. At the same time, due to scattering and other optical phenomena the NW structure is able to trap more light and improve the overall light absorption. This, combined with the ability to grow nanowires on cheap substrates or reuse the growth substrate multiple times, makes NWs promising for future generation PV devices. In order for NWs to perform to their full potential several technical challenges need to be overcome. In this paper we will discuss these technical challenges in conjunction with the advantages of using NWs in large scale PV devices. We will also outline the progress that we and others have made in overcoming these challenges on the way to making nanowires a viable PV technology.

Materials challenges for III-V/Si co-integrated CMOS

This review focuses on material challenges associated with III-V co-integration with Si for future CMOS. There is a huge volume of literature on this topic as implementation of III-V monolithic integration with Si has been the holy grail for last four decades; targeting a wide range of applications including RF devices, LEDs, lasers, photo-detectors and the like. The key drivers have been the cost reduction, scalability with Si wafer diameter, and accessibility to highly scaled integrated circuits next to III-V devices. With the current focus on CMOS the pace of progress on monolithic integration has accelerated by leaps and bounds partly because of its vast impact on CMOS scaling, and partly due to the aggressive CMOS roadmap requirements. The discussion below concentrates on In0.53Ga0.47As channel which is the dominant III-V material being pursued for future technology. Despite the narrow focus, fundamental and engineering challenges posed by this material encompass a broad range of material topics including epitaxial growth, crystallographic defects and their dynamics during growth and subsequent processing, clever device architecture to alleviate adverse impact of defects on device leakage, and innovative engineering for material improvement.

Templating silicon nanowires seeded with oxygen reactive materials

The nanopatterning of semiconductors and other surfaces in a controlled manor is of a great interest for industrial application. The current technique is a new method of controlling the spatial placement of the growth of nanowires (NWs) seeded with oxygen reactive materials such as aluminum, which is a standard metal in silicon process line. The technique is based about patterning a semiconductor substrate or other like substrate which is capable of forming a semiconductor alloy with an oxygen reactive element during a subsequent annealing step. Moreover, it does not require removal of the patterned compound oxide layer.