Introducing thin, light-weight and high efficiency photovoltaics will make solar cells more suitable to be integrated in urban landscapes or even small gadgets and would largely contribute to solving the global warming threat that we are facing today. Stacking of solar cells with different characteristic bandgaps is the most common strategy to surpass the Shockley-Queisser efficiency limit, but such tandem devices are typically heavy weight, rigid and costly. Thinning down of absorber materials is a good strategy to overcome these restrictions. However, nano- and micro-meter thicknesses come down to the expense of light absorption. An effective approach to tackle the absorption problem in thin materials is nanopatterning the absorbing layer.
In this work we introduce hyperuniform designs as an effective way to control scattered light into particular range of angles (revealed as a ring in k-space of the reflected/transmitted light), with the aim to efficiently trap light in μm-thick Silicon (Si) cells. We first consider the –theoretical and experimental- case of a single Si solar cell, and thanks to an optimization algorithm, we show the highest light absorption in 1 μm-thick Si film to date. We also compare different designs for best anti-reflection effect on top of light trapping and characterize the increased absorption in photoelectrochemical devices. Second, we incorporate a similar light trapping strategy in a tandem solar cell, by using a periodic GaAs nanowire array as a top cell. We introduce two waveguiding effects in GaAs NW-Si thin film architectures to explain the 4-fold light absorption in the Si ultrathin bottom cell for tailored geometries of the NW array. These results represent significant light trapping scheme that is obtained “for free” when using a nanostructured top cell.
Recently, a systematic effort has been undertaken in developing efficient energy harvesting devices on thin Si films. Two main mechanisms have been identified for the efficient light harvesting. One is related to minimizing reflection losses, while the other is related to coupling to quasi-guided modes supported by the silicon film. However, the effects associated with the homogeneity and isotropy of the structures have not attracted much attention. Here, we employ hyperuniform disordered structures(HUDS) to achieve very efficient light harvesting in the wavelength range from 400 to 1000 nm. We show that the surface patterning has a dramatic impact on the number modes that are involved in the absorption process and that the structure needs to be optimized such that the scattering promotes minimization of the energy directed in radiative channels, i.e. inside the light cone of the surrounding air. To provide a through comparison, we also fully optimize a periodic structure taking into account the patterning of the AR layer refractive index. We then examine various HUDS architectures and demonstrate not only the importance of the scattering components, but also the dramatic impact of the structure homogeneity and isotropy on the devices performance. Using this design strategy, we report a broadband solar energy absorption of 84% in a one micron-thick Si membrane, which is, to the best of our knowledge, the best value achieved in such ultra-thin Si membranes.
Phononic crystals, artificial materials with periodically arranged scattering centers, were introduced more than two decades ago as the elastic waves analogue of photonic crystals. These materials, either in two or three dimensions, can exhibit large frequency regions of prohibited propagation of elastic waves, the so-called phononic band gaps (PBGs). On the other hand, typical elastic wave propagation in random structures is associated with diffusion, or in extreme situation with localization, and random structures do not exhibit band gaps. Here, we introduce a new class of structurally disordered phononic structures, hyperuniform disordered phononic structures (HDPS) that exhibit large elastic band gaps. These structures are created from initially arbitrary point patterns by imposing hyperuniform correlations among the points and finally decorating them with a specific scatterers, so that the structure factor becomes isotropic and vanishes for all k-vectors within a specific radius. The disorder can smoothly be tuned to produce structures ranging from totally random to fully periodic by adjusting a single parameter. Such amorphous structures exhibit large band gaps, comparable to the ones found in the periodic counterparts, ballistic and diffusive propagation depending on the modes frequency and a large number of localized modes near the band edges. We discuss the formation of high-Q cavity modes and waveguides with 100% transmission in these disordered structures in the GHz regime. Such phononic-circuit architectures are expected to have a direct impact on integrated micro-electro-mechanical filters/modulators for wireless communications and acoustic-optical sensing devices.