Colton R. Bukowsky

Silicon heterojunction solar cells with effectively transparent front contacts

We demonstrate silicon heterojunction solar cells with microscale effectively transparent front contacts (ETCs) that redirect incoming light to the active area of the solar cell. Replacing standard contact electrodes by ETCs leads to an enhancement in short circuit current density of 2.2 mA cm−2 through mitigation of 6% shading losses and improved antireflection layers. ETCs enable low loss lateral carrier transport, with cells achieving an 80.7% fill factor. Furthermore, dense spacing of the contact lines allows for a reduced indium tin oxide thickness and use of non-conductive, optically optimized antireflection coatings such as silicon nitride. We investigated the performance of ETCs under varying light incidence angles, and for angles parallel to the ETC lines find that there is no difference in photocurrent density with respect to bare indium tin oxide layers. For angles perpendicular to the ETC lines, we find that the external quantum efficiency (EQE) always outperforms cells with flat contact grids.

Photon and carrier management design for nonplanar thin-film copper indium gallium diselenide photovoltaics

Nonplanar structured photovoltaic absorber design has potential to achieve high solar cell efficiency with significantly reduced material use. We report optoelectronic simulations that highlight photon and generated carrier management opportunities for improvement of thin film Cu(InxGa1−x)Se2 (CIGS) device performance. Structures realized via either self-assembly or patterning via nanoimprint lithography, and also a combination of both are predicted to exhibit significant increases in short circuit current density and open circuit voltage simultaneously. The structures investigated include: 1) self-assembled nonplanar structures that strongly scatter incident light and enhance carrier generation near regions of high electric potential, 2) lithographicallypatterned embedded periodic dielectric structures, 3) planar dielectric layers that separate the CIGS absorber from the molybdenum back-contact via reduced-area contacts that minimize optical and electronic losses, 4) a combination of these for combined effects. We find that the self-assembled nonplanar CIGS cells with 700 nm planar equivalent thickness, combined with dielectric separation layers yield increases in short circuit current density and open circuit voltage up to 3.4 mA cm and 29 mV, respectively. The absolute efficiency increases from 15.4% to 18.1%, compared to the predicted efficiency for planar CIGS thin film cells of equivalent thickness. The addition of a single layer MgF2 anti-reflection coating brings the maximum predicted efficiency up to 19.7% for randomly textured devices.

Effectively transparent contacts (ETCs) for solar cells

We have developed effectively transparent contacts (ETCs) that allow for increased current in heterojunction solar cells. Micro-meter scaled triangular cross-section grid fingers with micro-meter scaled distance redirect light efficiently to the active area of the solar cell and hence, omit losses through reflection at the front finger grid. Furthermore, the grid fingers are placed close together such that only a very thin layer of transparent conductive oxides (TCO) is necessary which avoids parasitic absorption and can decrease material costs. In this paper we experimentally show current enhancement of ~2 mA/cm2 in silicon heterojunction solar cells using ETCs. 1 mA/cm2 is gained through less parasitic absorption and 1 mA/cm2 is gained by efficient redirection of light and therefore, absent shadowing losses.

Design Criteria for Micro-Optical Tandem Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) harness light generated by luminophores embedded in a light-trapping waveguide to concentrate onto smaller cells. LSCs can absorb both direct and diffuse sunlight, and thus can operate as flat plate receivers at a fixed tilt and with a conventional module form factor. However, current LSCs experience significant power loss through parasitic luminophore absorption and incomplete light trapping by the optical waveguide. Here, we introduce a tandem LSC device architecture that overcomes both of these limitations, consisting of a poly(lauryl methacrylate) polymer layer with embedded cadmium selenide core, cadmium sulfide shell (CdSe/CdS) quantum dot (QD) luminophores and an InGaP microcell array, which serves as high bandgap absorbers on the top of a conventional Si photovoltaic. We investigate the design space for a tandem LSC, using experimentally measured performance parameters for key components, including the InGaP microcell array, CdSe/CdS QDs, and spectrally selective waveguide filters. Using a Monte Carlo ray-tracing model, we compute the power conversion efficiency for a tandem LSC module with these components to be 29.4% under partially diffuse illumination conditions. These results indicate that a tandem LSC-on-Si architecture could significantly improve upon the efficiency of a conventional Si photovoltaic cell.

Absorption enhancing and passivating non-planar thin-film device architectures for copper indium gallium selenide photovoltaics

The sub-micrometer absorber regime is currently being explored to reduce materials usage and deposition time while simultaneously increasing device voltages due to increased generated carrier concentration. In order to realize these benefits, the absorption of photons must be maintained or even increased while avoiding detrimental recombination. Reported here are optoelectronic simulations that highlight photon and generated carrier management opportunities for improvement of thin film Cu(InxGa1-x)Se2 (CIGSe) device performance. Structures that could be created via either self-assembly, patterning by nanoimprint lithography, or a combination of both are predicted to significantly increase short circuit current density and open circuit voltage simultaneously.

Waveguide modes in sparse III-V nanowire arrays for ultra-broadband tunable perfect absorbers(Conference Presentation)

Design of perfect absorbers and emitters has been a primary focus of the metamaterials community owing to their potential to enhance device efficiency and sensitivity in energy harvesting and sensing applications, specifically photovoltaics, thermal emission control, bolometers and photodetectors, to name a few. While reports of perfect absorbers/emitters for a specific frequency, wavevector, and polarization are ubiquitous, a broadband and polarization- and angle-insensitive perfect absorber remains a particular challenge. In this work, we report on directed optical design and fabrication of sparse III-V nanowire arrays as broadband, polarization- and angle-insensitive perfect absorbers and emitters. Specifically, we target response in the UV-Vis-NIR and NIR-SWIR-MWIR via two material systems, InP (Eg=1.34 eV) and InSb (Eg=0.17 eV), respectively. Herein, we present results on InP and InSb nanowire array broadband absorbers, supported by experiment, simulation and analytic theory. Electromagnetic simulations indicate that, with directed optical design, tapered nanowire arrays and multi-radii nanowire arrays with 5% fill fraction can achieve greater than 95% broadband absorption (λInP=400-900nm, λInSb=1.5-5.5µm), due to efficient excitation and interband transition-mediated attenuation of the HE11 waveguide mode. Experimentally-fabricated InP nanowire arrays embedded in PDMS achieved broadband, polarization- and angle-insensitive 90-95% absorption, limited primarily by reflection off the PDMS interface. Addition of a thin, planar VO2 layer above a sparse InSb nanowire array enables active thermal tunability in the infrared, effecting a 50% modulation, from 87% (insulating VO2) to 43% (metallic VO2) average absorption. These concepts and results along with photovoltaic and other optical and optoelectronic device applications will be discussed.

Scalable, epitaxy-free fabrication of super-absorbing sparse III-V nanowire arrays for photovoltaic applications(Conference Presentation)

III-V compound semiconductor nanowire arrays are promising candidates for photovoltaics applications due to their high volumetric absorption. Uniform nanowire arrays exhibit high absorption at certain wavelengths due to strong coupling into lossy waveguide modes. Previously, simulations predicted near-unity, broadband absorption in sparse semiconductor nanowire arrays (<5% fill fraction) with multi-radii and tapered nanowire array designs [1]. Herein, we experimentally demonstrate near-unity broadband absorption in InP nanowire arrays via a scalable, epitaxy-free fabrication method, using nanoimprint lithography and ICP-RIE to define nanowire arrays in bulk InP wafers. In addition to mask pattern design (wire radius and spacing) and etch chemistry (wire taper), appropriate selection of a hard mask for the InP etch is critical to precise dimension control and reproducibility. Polymer-embedded wires are removed from the bulk InP substrate by a mechanical method that facilitates extensive reuse of a single bulk InP wafer to synthesize many polymer-embedded nanowire array thin films. Arrays containing multiple nanowire radii and tapered nanowires were successfully fabricated. For both designs, the polymer-embedded arrays achieved ~90% broadband absorption (λ=400-900 nm) in less than 100 nm planar equivalence of InP. The addition of a silver back reflector increased this broadband absorption to ~95%. The repeatable process of imprinting, etching and peeling to obtain many nanowire arrays from one single wafer represents an economical manufacturing route for high efficiency III-V photovoltaics. [1] K.T. Fountaine, C.G. Kendall, Harry A. Atwater, “Near-unity broadband absorption designs for semiconducting nanowire arrays via localized radial mode excitation,” Opt. Exp. (2014).