Carissa N. Eisler

Multijunction Solar Cell Efficiencies: Effect of Spectral Window, Optical Environment and Radiative Coupling

Solar cell efficiency is maximized through multijunction architectures that minimize carrier thermalization and increase absorption. Previous proposals suggest that the maximum efficiency for a finite number of subcells is achieved for designs that optimize for light trapping over radiative coupling. We instead show that structures with radiative coupling and back reflectors for light trapping, e.g. spectrum-splitting cells, can achieve higher conversion efficiencies. We model a compatible geometry, the polyhedral specular reflector. We analyze and experimentally verify the effects of spectral window and radiative coupling on voltage and power. Our results indicate that radiative coupling with back reflectors leads to higher efficiencies than previously studied architectures for practical multijunction architectures (i.e., ≤20 subcells).

Spectrum splitting photovoltaics: Polyhedral specular reflector design for ultra-high efficiency modules

A design for ultra-high efficiency solar modules (>50%) using spectrum splitting is proposed. In the polyhedral specular reflector design, seven subcells are arranged around a solid parallelepiped. Incident light enters the parallelepiped and is directed via specular reflection onto each subcell in order from highest to lowest bandgap. We analyze optical losses due to external concentration and parasitic absorption and optimize the design for >50% module efficiency. We find that moderate concentration designs (90-170x) with a high index parallelepiped and perfect shortpass filters meet target efficiencies and demonstrate an initial design.

Spectrum splitting photovoltaics: Materials and device parameters to achieve ultrahigh system efficiency

We describe an approach for spectrum splitting solar module design using 8 independently connected subcells with optimized band gaps. Modified detailed balance calculations using parameters to account for nonideal absorption and recombination behavior are used to identify the efficiency of spectrum splitting modules with 2 to 20 subcells. Potential optical designs for solar spectrum splitting among subcells are briefly described. Spectrum splitting improves use of the power in the solar spectrum by decreasing thermalization losses. A multijunction design utilizing independently connected subcells employed in a concentrator photovoltaic receiver allows flexibility in subcell selection, fabrication and operating power point optimization when a large number of subcells are employed.

Polyhedral specular reflector design for ultra high spectrum splitting solar module efficiencies (>50%)

One pathway to achieving ultra-high solar efficiencies (<50%) is employing a spectrum splitting optical element with at least 6 subcells and significant concentration (100-500 suns). We propose a design to meet these criteria, employing specular reflection to split and divide the light onto appropriate subcells. The polyhedral specular reflector incorporates a high index parallelepiped with seven subcells. The subcells are placed around the parallelepiped such that light entering at normal incidence encounters the subcells in order from highest to lowest bandgap, with the ray path reflecting at a 90° angle until the light is fully absorbed. Previous studies of the design have shown that concentration and filters are necessary to achieve high efficiencies and thus the current iteration of the design employs shortpass filters and two stages of concentration. Ray tracing of the current iteration shows exceeding 50% efficiency is possible for current subcell qualities with perfect shortpass filters while 50% module efficiencies are only possible for very high quality (<6% ERE) subcells with commercially available shortpass filters. However, even with commercially available filters and achievable subcell quality, ray tracing results show very high (<43%) module efficiency.

Enhanced performance of small GaAs solar cells via edge and surface passivation with trioctylphosphine sulfide

We have extended our previous work on trioctylphosphine sulfide (TOP:S) to further elucidate the mechanisms of this chemical passivation for small GaAs solar cells. Photoluminescence (PL) measurements indicate monolayers of TOP:S on GaAs significantly increases the electronic quality of both n- and p-doped wafers. TOP:S was also applied to an “ultra small” GaAs solar cell (0.31 mm2) to test its ability to passivate devices with the relevant dimensions for microconcentrator schemes. After the cells were briefly soaked in TOP:S, the efficiency of the cell was boosted by 1% (absolute), even after a rinse in toluene to remove all but a few monolayers of TOP:S, confirming sidewall passivation.

Electrically independent subcircuits for a seven-junction spectrum splitting photovoltaic module

An electrical system that facilitates independent electrical connection of subcells with different bandgaps is required in order to optimally extract two-terminal power generated by spectrum-splitting photovoltaics. We have designed such a system for a seven subcell spectrum-splitting module using commercially available electrical components. Modified detailed balance modeling of the subcells coupled with HSPICE circuit simulation results in 87.5% power-weighted average subcell-to-grid electrical efficiency. Additional analysis indicates electrical efficiencies in excess of 95% are possible with custom power conditioning components and appropriately designed contacts.

Design improvements for the polyhedral specular reflector spectrum-splitting module for ultra-high efficiency (>50%)

A spectrum-splitting module design, the polyhedral specular reflector (PSR), is proposed for ultra-high photovoltaic efficiency (>50%). Incident light is mildly concentrated (≤16 suns) and subsequently split seven ways by a series of multilayer dielectric filters. The split spectrum is directed into compound parabolic concentrators (CPCs) and each concentrates a given slice of the spectrum onto one of seven subcells for conversion. We have recently made significant improvements to the design, such as vertically stacking each submodule and rearranging the subcell order to increase the optical efficiency of the design. We optimize the concentration and composition of the parallelepiped prism (hollow vs. solid) and model designs with >50% module efficiencies including optical and cell nonidealities.

Nanophotonic design principles for ultrahigh efficiency photovoltaics

To date, solar-cell efficiencies have remained well below the thermodynamic limits. However new nanophotonic and microphotonic approaches to light management that systematically minimize thermodynamic losses can enable ultrahigh efficiencies previously considered to be out of reach.

Positional irradiance measurement: characterization of spectrum- splitting and concentrating optics for photovoltaics

Multijunction photovoltaics enable significantly improved efficiency over their single junction analogues by mitigating unabsorbed sub-bandgap photons and voltage loss to carrier thermalization. Lateral spectrum-splitting configurations promise further increased efficiency through relaxation of the lattice- and current-matching requirements of monolithic stacks, albeit at the cost of increased optical and electrical complexity. Consequently, in order to achieve an effective spectrum-splitting photovoltaic configuration it is essential that all optical losses and photon misallocation be characterized and subsequently minimized. We have developed a characterization system that enables us to map the spatial, spectral, and angular distribution of illumination incident on the subcell reception plane or emerging from any subset of the concentrating and splitting optics. This positional irradiance measurement system (PIMS) comprises four motorized stages assembled in an X-Z-RY configuration with three linear degrees of freedom and one rotational degree of freedom, on which we mount an optical fiber connected to a set of spectrometers covering the solar spectrum from 280-1700 nm. In combination with a xenon arc lamp solar simulator with a divergence half angle of 1.3 degrees, we are able to characterize our optics across the full spectrum of our photovoltaic subcells with close agreement to outdoor conditions. We have used this tool to spectrally characterize holographic diffraction efficiency versus diffraction angle; multilayer dielectric filter transmission and reflection efficiency versus filter incidence angle; and aspheric lens chromatic aberration versus optic-to-receiver separation distance. These examples illustrate the versatility of the PIMS in characterizing optical performance relevant to both spectrum-splitting and traditional multijunction photovoltaics.

Designing and prototyping the polyhedral specular reflector, a spectrum-splitting module with projected >50% efficiency

We investigate a spectrum-splitting design, the polyhedral specular reflector, for an ultra-high efficiency module (>50%). The design employs a series of multilayer dielectric stack filters to divide the incident spectrum onto seven independently connected subcells. We optimized the geometry and components of the design through coupled wave-optics, device physics, electrical circuit, and ray tracing models. We show a wide design space where >50% module efficiencies are possible and have chosen a design with a projected 50.8% module efficiency to prototype. Initial efforts show excellent matching of the fabricated optical splitting prism (90.1% splitting efficiency) to the theoretical design (93% splitting efficiency). Additionally, integrating fabricated concentrators yields an optical structure consistent with a 30% efficiency module.