Avi Braun

Hybrid photovoltaic-thermoelectric system for concentrated solar energy conversion: Experimental realization and modeling

An experimental demonstration of the combined photovoltaic (PV) and thermoelectric conversion of concentrated sunlight (with concentration factor, X, up to ∼300) into electricity is presented. The hybrid system is based on a multi-junction PV cell and a thermoelectric generator (TEG). The latter increases the electric power of the system and dissipates some of the excessive heat. For X ≤ 200, the systems maximal efficiency, ∼32%, was mostly due to the contribution from the PV cell. With increasing X and system temperature, the PV cells efficiency decreased while that of the TEG increased. Accordingly, the direct electrical contribution of the TEG started to dominate in the total system power, reaching ∼20% at X ≈ 290. Using a simple steady state finite element modeling, the cooling effect of the TEG on the hybrid systems efficiency was proved to be even more significant than its direct electrical contribution for high solar concentrations. As a result, the total efficiency contribution of the TEG reach...

Photovoltaic performance enhancement by external recycling of photon emission

Experimental evidence of enhancing the performance of ultra-efficient solar cells by external recycling of photon emission is presented – predicated on the strategy of increasing cell open-circuit voltage by reducing radiative recombination. It is equivalent to restricting the angular range of photon emission, and can only be effective in photovoltaics with high external luminescent efficiency. This has precluded the voltage enhancement from being observable in todays photovoltaic technologies. As shown here, however, it is attainable with the latest generation of champion single-junction one sun thin-film GaAs cells. The measurements are understandable in terms of basic photovoltaic thermodynamics.

Current-limiting behavior in multijunction solar cells

Experimental measurements on tandem GaInAsP/InGaAs concentrator solar cells are presented that demonstrate how the short-circuit current can shift from that of the higher current subcell to that of the lower current subcell as irradiance increases. Theoretical modeling illustrates how this can occur when the current-limiting subcell has a noticeably nonzero slope in its current-voltage curve near short-circuit, and should be general to all series-connected multijunction cells of this nature.

Multiple-bandgap vertical-junction architectures for ultra-efficient concentrator solar cells

A novel multiple-bandgap multi-terminal vertical-junction (side-illumination) architecture for concentrator solar cells is proposed and simulated, wherein each stacked sub-cell operates at its own maximum power point, without the need for tunnel diodes, current matching, lattice matching, metallization grids or spectrum splitting. Practical implementation is limited to indirect bandgap semiconductors (including Si and Ge), with a dramatic reduction in series resistance such that ultra-efficient operation at irradiance levels of thousands of suns appears tenable – rivalling in principle todays best multi-junction III–V cells.

Analytic solution for quasi-Lambertian radiation transfer

An analytic solution is derived for radiation transfer between flat quasi-Lambertian surfaces of arbitrary orientation, i.e., surfaces that radiate in a Lambertian fashion but within a numerical aperture smaller than unity. These formulas obviate the need for ray trace simulations and provide exact, physically transparent results. Illustrative examples that capture the salient features of the flux maps and the efficiency of flux transfer are presented for a few configurations of practical interest. There is also a fundamental reciprocity relation for quasi-Lambertian exchange, akin to the reciprocity theorem for fully Lambertian surfaces. Applications include optical fiber coupling, fiber-optic biomedical procedures, and solar concentrators.

Fundamentally new aspects of tunnel diode transitions in multi-junction photovoltaics

Tunnel diodes constitute an essential part of multi-junction concentrator photovoltaics. These tunnel junctions exhibit a transition from low-resistance tunneling to high-resistance thermal diffusion, commonly at current densities of the order of 102-103 mA/mm2. Experimental evidence of a fundamentally new effect is reported and confirmed in distinct cell architectures: the dependence of the threshold current density on the extent of localized irradiation. It is also shown that photovoltaic cells with a non-uniform metal grid can possess an additional spatial dependence to the threshold current density. These new phenomena should be observable in all solar cell tunnel diodes subjected to inhomogeneous illumination, and are posited to stem from the lateral spreading of excess majority carriers (similar to current spreading in LEDs). The implications for concentrator solar cells are also addressed.

Angular restriction of photon emission for ultra-efficient photovoltaics

We present experimental evidence for improving the open-circuit voltage – and thereby efficiency - of photovoltaics via the external recycling of photon emission. This strategy is equivalent to limiting the angular extent of photon emission - effective only in photovoltaics with high external luminescent efficiency. This is why the effect has not been observed in current solar cell technologies. It is attainable with the latest generation of ultra-efficient single-junction non-concentrator thin-film GaAs cells. The findings are explained in terms of basic photovoltaic thermodynamics.

Irradiance-dependent current-limiting behavior of multijunction solar cells

Measurements of tandem GaInAsP/InGaAs concentrator solar cells are presented and demonstrate how the short-circuit current of multijunction cells can shift from the higher-current subcell to the lower-current subcell as irradiance increases. Basic modeling shows how this can occur for cells that exhibit a noticeably non-zero slope in the low-voltage regime of the current-voltage curve of the current-limiting subcell. This observation should be general to all series-connected multijunction cells of this nature.

Temperature coefficients of concentrator solar cells up to ultra-high irradiance

A flash-like, real-sun optical system has been used to experimentally measure the effect of irradiance (from 11 up to 8,600 suns) on the temperature coefficients of the key performance parameters of multijunction concentrator solar cells. Particular attention is paid to the time scales and magnitudes of junction heating, hence the degree to which the cell can be deemed isothermal, with implications for corresponding measurements from solar simulators with pulsed artificial light, and for the performance evaluation of concentrator photovoltaics. The theoretical bound for the magnitude of the temperature coefficient of Voc (as function of irradiance) has also been calculated for current commercial triple-junction structures.

Quasi-lambertian radiative transfer: exact analytic solutions

The evaluation of radiative exchange among quasi-lambertian surfaces has been deemed the domain of raytrace simulations (quasi-lambertian referring to flux distributions that are uniform spatially but within a numerical aperture NA less than unity). The familiar view factors for fully-lambertian surfaces are not valid in this domain. Analytic, physically-transparent solutions are derived for radiation transfer between quasi-lambertian surfaces - solutions that also obviate the need for case-specific time-intensive simulation methods. A generalized reciprocity relation is shown to follow in analogy to the reciprocity theorem for fully-lambertian flux exchange. Representative results of flux maps and flux transfer efficiency for several geometries of practical interest are presented to illustrate the principal features.