COMMUNICATIONshadow loss, and the sheet resistance and absorption losses associated with planar layers that facilitate lateral carrier transport to the grid fi ngers. [ 22,23 ] For high effi ciency silicon heterojunction (HIT) solar cells, contact design requires a trade-off between grid fi nger resistance and the sheet resistance and transmission losses of the transparent conducting oxide (TCO)/ amorphous silicon structures coating the cell front surface. [ 24 ] In this paper, we describe a new front contact design principle that overcomes both shadowing losses and parasitic absorption without reducing the conductivity. By redirecting the scattered light incident on the front contact to the solar cell active absorber layer surface, micrometer-scale triangular crosssection grid fi ngers can perform as effectively transparent and highly conductive front contacts. Previously, researchers have designed light harvesting strings that serve to obliquely refl ect light, which is then redirected into the cell by total internal refl ection from the encapsulation layers. [ 16 ] By contrast our front contact design does not require total internal refl ection at the encapsulation layer. Furthermore in our design, the contact fi ngers are micrometer sized and can be placed very close together such that a TCO with reduced thickness can be used-and in some cases the TCO layer might possibly be omitted completely. We demonstrate with simulations and experimental results that designs utilizing effectively transparent triangular cross-section grid fi ngers rather than conventional front contacts have the potential to provide 99.86% optical transparency while ensuring effi cient lateral transport corresponding to a sheet resistance of 4.8 Ω sq −1 due to their close spacing of only 40 µm. Thus effectively transparent contacts have potential as replacements for both the front grid and TCO layer used, e.g., in HIT solar cells. While related schemes for contacts were envisioned early in the development of photovoltaics technology, [ 25 ] they have not found application in current photovoltaic technology, which is increasingly dominated by high effi ciency silicon photovoltaics. Moreover, the effectively transparent front contact design is conceptually quite general and applicable to almost any other front-contacted solar cell or optoelectronic device. For example, we obtained similar experimental results when applying our structures to InGaP-based solar cells.Figure 1 a,b shows the steady-state electric fi eld magnitude distribution of a freestanding triangular contact and a fl at contact, respectively, with 550 nm monochromatic plane wave illumination normally incident at the top of the simulation cell. For planar grid fi ngers, part of the incident light is refl ected back toward the incidence direction, as is apparent from the high electric fi eld density above the contact plane. By contrast, the triangular cross-section grid fi nger does not exhibit a similar back refl ection, as indicated by the lack of an increased electric

We report ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely low angular (0°to 50°) and spectrally averaged reflectivity of <1% of the incident 400−1100 nm illumination. After isolating the microwires from the substrate with a polymer infill and peel off process, the arrays were found to absorb 89.1% of angular averaged incident illumination (0°to 50°) in the equivalent volume of a 20 μm thick Si planar slab, reaching 99.5% of the classical light trapping limit between 400 and 1100 nm. We explain the broadband absorption by enhancement in coupling to waveguide modes due to the tapered microstructure of the arrays. Time-resolved microwave photoconductivity decay measurements yielded charge-carrier lifetimes of 0.75 μs (more than an order of magnitude higher than vapor−liquid−solid-grown Si microwires) in the tapered microwires, resulting in an implied V oc of 0.655 V. The high absorption and high aspect ratio in these ordered microwire arrays make them an attractive platform for high-efficiency thin-film crystalline Si solar cells and as well as for the photoelectrochemical production of fuels from sunlight.

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.

A transparent, flexible contact is developed using Ni nanoparticles and Ag nanowires and demonstrated on free-standing, polymer embedded, Si microwire solar cells. Contact yields of over 99% and a series resistance of 14 Ω cm² are demonstrated.

We have investigated the GaP/Si heterojunction interface for application in silicon heterojunction solar cells. We performed X-ray photoelectron spectroscopy (XPS) on thin layers of GaP grown on Si by metal organic chemical vapor deposition and molecular beam epitaxy. The conduction band offset was determined to be 0.9 ± 0.2 eV, which is significantly higher than predicted by Anderson's rule (0.3 eV). XPS also revealed the presence of Ga-Si bonds at the interface that are likely to be the cause of the observed interface dipole. Via cross-sectional Kelvin probe force microscopy (x-KPFM), we observed a charge transport barrier at the Si/GaP interface which is consistent with the high-conduction band offset determined by XPS and explains the low open-circuit voltage and low fill factor observed in GaP/Si heterojunction solar cells.