Thin silicon solar cells need light trapping to compensate for their short optical path, and structuring the back-reflector as a photonic crystal (PC) — a structure whose refractive index varies periodically on the scale of the optical wavelength — was proposed as a route that combines mirror-like stop-band reflection with coupling of light into resonant modes that propagate along the absorber. A 2008 paper by O'Brien and co-workers (Adv. Mater. 20, 1577) showed the optical mechanisms but explicitly left the reduce-to-practice hurdles open: how the enhancement generalizes to non-normal and diffuse illumination, how rear PCs combine with engineered front surfaces, how conducting rear contacts can be integrated without destroying the optical benefit, and how realistic material absorption and fabrication non-idealities affect the predicted gains. The later literature (2009–2019) partially resolves each hurdle at laboratory scale — conducting selectively-transparent PCs have been used as working rear contacts and intermediate reflectors, periodic back reflectors have been fabricated in real cells with measured gains, and the angular response of periodic light-trapping structures has been measured — but no work reports the complete quantitative device-level assessment (PC back-reflector plus conducting contact plus engineered front surface under realistic angular/diffuse illumination, with quantified losses and a benchmark against standard reflectors) needed to validate the concept for production-capable thin silicon photovoltaics. That combined assessment, with its electrical-parity question against standard ZnO/Ag back contacts, is the surviving open core.

Background

Silicon is a weak absorber near its band edge, so photovoltaic cells made much thinner than the conventional 200–300 µm wafer — thin-film hydrogenated amorphous silicon (a-Si:H), nanocrystalline silicon, or sub-10 µm crystalline films — lose a large fraction of the incident near-bandgap photons unless light is trapped inside the absorber. Classical light trapping uses roughened surfaces to randomize ray directions; the ray-optics (Lambertian) limit gives a maximum path-length enhancement of about 4n24n^2 for a medium of refractive index nn, and 4n2/sin⁡2θ4n^2/\sin^2\theta when the angular escape cone has half-angle θ\theta. A photonic crystal (PC) is a dielectric or metallic structure whose refractive index varies periodically on the scale of the optical wavelength; over certain frequency bands (stop bands, part of the photonic band gap) a PC cannot support propagating waves and reflects incident light almost completely, which is the same principle used by a one-dimensional PC known as a distributed Bragg reflector (a stack of alternating-index quarter-wave layers). When such a PC is placed directly behind a thin semiconductor film, two effects were shown to enhance absorption: the PC acts as a nearly perfect mirror over its stop band, and the PC–film interface diffracts light into resonant modes that travel along the plane of the film, lengthening the optical dwell time (Bermel, Luo, Zeng, Kimerling and Joannopoulos, Opt. Express 15, 16986 (2007); O'Brien, Kherani, Chutinan, Ozin, John and Zukotynski, Adv. Mater. 20, 1577 (2008)). A practical back reflector, however, must do more than reflect: it must also extract photogenerated carriers, so a transparent conducting oxide (TCO, e.g. ZnO) and a metal contact occupy the region where the PC would need to sit optically close to the absorber. The 2008 paper framed the outstanding issues as a reduce-to-practice agenda: its electromagnetic calculations treated only normally incident plane waves, whereas real solar illumination and any textured or anti-reflective front surface deliver a spread of incident angles; the interplay between front-surface engineering and the rear PC was unexplored; conducting, transparent, index-contrast-rich and manufacturable PC materials and geometries had not been established; and the effects of realistic absorption, scattering and fabrication tolerances on the predicted enhancements had not been quantified. Later theory (Yu, Raman and Fan, PNAS 107, 17491 (2010)) generalized the light-trapping limit to nanophotonic resonant-mode systems and made the role of the angular escape cone explicit, and later experiments (Curtin, Biswas and Dalal, Appl. Phys. Lett. 95, 231102 (2009); Biswas et al., Sol. Energy Mater. Sol. Cells 94, 2337 (2010); Chen et al., J. Appl. Phys. 116, 064508 (2014)) fabricated periodic back reflectors under real thin-film silicon cells and measured external quantum efficiency (EQE, the ratio of collected carriers to incident photons) and short-circuit current gains. The question that remains is whether these ingredients combine into a validated device-level strategy.

Problem Statement

Determine whether a photonic-crystal back-reflector can deliver its predicted absorption enhancement in a realistic thin silicon photovoltaic cell — a thin-film a-Si:H/nanocrystalline-Si cell or a sub-10 µm crystalline-Si cell — once the full device stack is included, using electromagnetic simulation, experimental device realization, or a combination of the two. Specifically, quantify (i) how both enhancement mechanisms (stop-band mirror reflection and coupling into film-resonant modes) generalize from normally incident plane waves to the spread of incidence angles and in-plane wavevectors produced by real solar and diffuse illumination and by any front-surface texturing; (ii) the combined optical gain, and any adverse interaction, when an engineered front surface (textured transparent conducting oxide or antireflective coating) is co-designed with the rear photonic-crystal reflector; (iii) whether a conducting, optically transparent, high-index-contrast periodic back-contact structure can extract carriers while staying optically close to the absorber — within a distance small compared with the optical wavelength, so that near-field coupling is retained — and match the standard ZnO/Ag back contact in the three quantities that define electrical parity, namely series resistance, fill factor and short-circuit current, each measured under the same conditions; and (iv) how realistic material absorption and scattering in the PC and contact layers, fabrication tolerances, surface roughness, imperfect periodicity and finite lateral extent degrade the predicted enhancements. Resolving the task requires quantifying, for a single named platform of the solver's choice among those above, the device-level external quantum efficiency, short-circuit current and efficiency relative to state-of-the-art back reflectors and the Lambertian light-trapping reference, and determining from them whether photonic-crystal back-reflectors are viable for production-capable thin silicon photovoltaic devices. All four parts (i)–(iv) are jointly required; addressing a subset alone does not resolve the task.

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