Over the last couple of years, photonic materials with tailored -i.e. with deliberately introduced- structural disorder have attracted considerable interest in photovoltaics due to their extended spectral and angular range of effectiveness . Notably, quasi-random nanostructures realized by e-beam lithography (EBL) have been integrated in solar cells as broadband light trapping elements, and have proved to approach the theoretical (Lambertian) limit . Despite recent research efforts aiming at increasing the EBL writing speed , alternative routes based on self-assemblies still possess major advantages for an industrial implementation of disordered structures as they allow to rapidly process them over large areas (>>cm2).
In this communication, we show that the up-scalable polymer blend lithography technique can be used as a versa-tile platform for fabricating 2D planar, disordered nanostructures that can be exploited in both top-down and bottom-up strategies. Tailored disorder is achieved here by adjusting the process parameters (polymer blend composition and deposition conditions), enabling to tune the morphology and the spatial distribution of the nanostructures produced, and in turn their light harvesting properties.
We first use our approach to pattern a resist etching mask, which is employed for transferring disordered nanoholes into a thin hydrogenated amorphous silicon layer by dry etching (top-down route). We report an enhancement of its integrated absorption of +90% under normal incidence, and of up to +200% at large incident angles with respect to an unprocessed absorber . In a second example, we demonstrate that similar structures can serve as a template in a bottom-up configuration, whereby copper indium diselenide nanocrystals are infiltrated into the disordered nano-holes formed in a resist layer. This route, paving the way to wet-processable "photonized" absorbers, is compared to a previous work relying on a serial writing process , and the optical properties of the resulting patterned absorbing layers are analysed.
We finally elaborate on the significance of these findings for the reverse problem, namely for light extraction in broadband light-emitting diodes.
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Thin–film silicon tandem solar cells consist of an amorphous silicon top cell and a microcrystalline silicon bottom cell stacked in series. In order to match the photocurrents of the top cell and the bottom cell, a proper photon management is essential. In this regard, we present the conceptual design and optical simulations of an intermediate reflector consisting of a stack of microcrystalline silicon oxide layers of different, alternating refractive indices. In contrast to 1–layer intermediate reflectors, the spectral and directional selectivity of these intermediate reflectors result in a gain for the top cell current while simultaneously increasing the charge carrier generation in the bottom cell.