Colloidal quantum dots (CQDs) can be used to extend the response of solar cells, enabling the utilization of solar power that lies to the red of the bandgap of c‐Si and perovskites. To achieve largely complete absorption of infrared (IR) photons in CQD solids requires thicknesses on the micrometer range; however, this exceeds the typical diffusion lengths (≈300 nm) of photoexcited charges in these materials. Nanostructured metal back electrodes that grant the cell efficient IR light trapping in thin active layers with no deterioration of the electrical properties are demonstrated. Specifically, a new hole‐transport layer (HTL) is developed and directly nanostructured. Firstly, a material set to replace conventional rigid HTLs in CQD devices is developed with a moldable HTL that combines the mechanical and chemical requisites for nanoimprint lithography with the optoelectronic properties necessary to retain efficient charge extraction through an optically thick layer. The new HTL is nanostructured in a 2D lattice and conformally coated with MoO3/Ag. The photonic structure in the back electrode provides a record photoelectric conversion efficiency of 86%, beyond the Si bandgap, and a 22% higher IR power conversion efficiency compared to the best previous reports.

Ordered arrays of metal nanoparticles offer new opportunities to engineer light–matter interactions through the hybridization of Rayleigh anomalies and localized surface plasmons. The generated surface lattice resonances exhibit much higher quality factors compared to those observed in isolated metal nanostructures. Template‐induced colloidal self‐assembly has already shown a great potential for the scalable fabrication of 2D plasmonic meta‐molecule arrays, but the experimental challenge of controlling optical losses within the repeating units has so far prevented this approach to compete with more standard fabrication methods in the production of high‐quality factor resonances. In this manuscript, the optical properties of plasmonic arrays are investigated by varying the lattice parameter (between 200 and 600 nm) as well as the diameter of the gold colloidal building‐blocks (between 11 ± 1 and 98 ± 6 nm). It is systematically studied how the internal architecture of the repeating gold‐nanoparticle meta‐molecules influences the optical response of the plasmonic supercrystals. Combining both experimental measurements and simulations, it is demonstrated how, reducing the size of the gold nanoparticles it is possible to switch from strong near‐field plasmonic architectures to high‐quality factors (>60) for lattice plasmon resonances located in the visible spectral range.

Noble metal decoration of wideband gap semiconductors enables the excitation of surface plasmons in the visible range that upon relaxation generate hot carriers used for catalysis. However, this strategy leads to photocatalytic conversion efficiencies that are still low. Here, a light‐trapping scheme is used to amplify the light‐harvesting efficiency of the TiO2 semiconductor beyond the UV region by coupling a 2D‐photonic crystal to Au decorated titania. This approach is easily scalable using soft nanoimprinting lithography to prepare Au/TiO2 2D‐photonic photocatalysts. In a first process, gold nanoparticles (Au NPs) are in situ infiltrated in the superficial 50 nm of a mesoporous titania (mTiO2) scaffold patterned with the photonic structure, while in a second one 2D‐photonic crystals with a homogeneous volume distribution of the Au colloids are achieved. The dependence of the optical properties of the photonic crystals on the lattice parameter, geometry, and metal loading is presented through extinction measurements and analyzed through simulations. The improved photocatalytic performance of the substrates is tested for hydrogen production where a maximum of 8.5 mmol gcat−1 h−1 of H2 is recorded and attributed to photonic–plasmonic effects. These results may open new avenues in solar harvesting for hydrogen production using photonic crystals as photocatalysts.

in balancing out optimum thicknesses for charge extraction and efficient light absorption. [6] The ultimate goal of a lighttrapping strategy is to achieve complete absorption of light in a broad spectral range while using the minimum amount of photoactive material. Traditional waferbased technologies have extensively exploited light-trapping strategies up to a point where the classical limit of light trapping [7,8] is almost reached. [9] There is, however, an increasing interest in further decreasing the thickness of the active layer below the micron, where new properties appear for classic materials (such as flexibility for silicon [10] ) and new materials like plastics [11] or nanocrystals [12,13] can be used at lower processing costs. Capturing light in such ultrathin devices requires redesigning the strategy, moving away from traditional ray optics approximations into the wave optics regime. [6] Many wave optics based designs are being currently investigated; photonic crystals (PhCs), [14,15] plasmonics, [16,17] and microresonators [18][19][20] provide new and exciting means of confining light in subwavelength thin films. Nevertheless, strong absorption enhancements are not restricted exclusively to the above-mentioned photonic architectures. Kats et al. [21] demonstrated flat nanometric films (up to 25 nm) of amorphous germanium (a-Ge) with 90% absorption in the visible range when deposited on optically thick gold films. Simply put, nanometric semiconductor coatings on metallic films act as Gires-Tournois interferometers [22,23] (a Fabry-Perot resonator where one of the interfaces is metallic). Such absorption comes from a strong interference effect originated by both the nonideal behavior of the metal film at visible frequencies (allowing field penetration in the metal) and the high values of the complex refractive index of the semiconductor. In-depth analysis of these results revealed that this system sustains Fabry-Perot resonances (FPRs) coupled with Brewster modes, [24] which are incident wave modes with no scattered wave that can be accessed from air. The excitation of this Brewster mode is possible at normal incidence and endows the system with omnidirectional strong absorption [25] (Figures S1 and S2, Supporting Information). The thickness of the semiconductor layer determines the position of the minimum in the reflectivity (maximum in absorption) of these resonances with the particularity that, because of the penetration depth in the metal, the resonant condition is below the classical λ/4 condition. SuchThe design of ultrathin semiconducting materials that achieve broadband absorption is a long-sought-after goal of crucial importance for optoelectronic applications. To date, attempts to tackle this problem consisted either of the use of strong-but narrowband-or broader-but moderate-light-trapping mechanisms. Here, a strategy that achieves broadband optimal absorption in arbitrarily thin semiconductor materials for all energies above their bandgap is presented. This stems from the strong interplay...