Rebecca Saive

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.

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.

The Influence of Spectral Albedo on Bifacial Solar Cells: A Theoretical and Experimental Study

We have investigated the influence of the spectral albedo on the power output of bifacial solar cells. We adapted the Shockley–Queisser radiative flux balance framework to account for a variation of the spectrum and intensity of the incoming light. We find that the ideal band gap and the maximum efficiency depend on the spectral albedo of the surroundings and that optimal cell performance cannot be assessed when only accounting for a spectrally independent albedo. With a spectral albedo model, we predict that the power output for a bifacial silicon solar cell surrounded by green grass is 3.1% higher than for a wavelength-independent albedo, and even 5.2% higher for white sand. We experimentally verify this trend for silicon heterojunction solar cells and we derive the ideal spectral albedo.

Fabrication of Single Crystal Gallium Phosphide Thin Films on Glass

Due to its high refractive index and low absorption coefficient, gallium phosphide is an ideal material for photonic structures targeted at the visible wavelengths. However, these properties are only realized with high quality epitaxial growth, which limits substrate choice and thus possible photonic applications. In this work, we report the fabrication of single crystal gallium phosphide thin films on transparent glass substrates via transfer bonding. GaP thin films on Si (001) and (112) grown by MOCVD are bonded to glass, and then the growth substrate is removed with a XeF2 vapor etch. The resulting GaP films have surface roughnesses below 1 nm RMS and exhibit room temperature band edge photoluminescence. Magnesium doping yielded p-type films with a carrier density of 1.6 × 1017 cm−3 that exhibited mobilities as high as 16 cm2V−1s−1. Due to their unique optical properties, these films hold much promise for use in advanced optical devices.

GaP/Si heterojunction solar cells

The world record efficiency and open circuit voltage for crystalline silicon solar cells are held by a-Si/Si heterojunction devices. While a-Si provides excellent surface passivation, these heterojunction devices are limited by non-ideal optical and electronic properties. Gallium phosphide is a candidate material for replacing a-Si in a heterojunction device, promising lower parasitic absorption and better carrier mobilities. In this work, we present our results in growing high quality GaP thin films directly on Si using a two-step nucleation and growth scheme with metalorganic chemical vapor deposition, characterization, and x-ray photoelectron spectroscopy band offset measurements toward realizing a GaP/Si heterojunction device.