Global-warming issues coupled with high oil prices have become a major driving force for the use of advanced solar power technology, where a key component lies in the development of high-efficiency and low-cost photovoltaic cells. Next generation photovoltaics, hence, demand an efficiency-boosting mechanism in order to render solar energy cost competitive with conventional sources of electricity.[1] Fundamentally, the conversion efficiency of a solar cell depends on the photon absorption, carrier separation, and carrier collection. [2,3] Therefore, an effective antireflection (AR) coating, minimized recombination loss, and good Ohmic contacts are particularly important. Metal grids that inevitably block the transmission of solar energy also require optimization in order to reduce the series resistance. The trade-off between the electrode and the AR coating areas is one of the efficiency-limiting factors in a conventional solar cell.The conventional AR coating is usually composed of a quarter wavelength stack of dielectrics with different refractive indices. Broad angular and spectral AR is achievable at the price of multiple layers.[ [4][5][6][7] Over the past few years, versatile subwavelength structures (SWS) have emerged as promising candidates for AR coatings, due to the characteristics of zero-order gratings, or the so-called moth-eye effects. [8][9][10][11][12][13][14] However, the fabrication costs, which involve either electron-beam (e-beam) lithography or various etching processes, can be significant. In addition, the resulting surface-recombination loss due to dry or wet etching could further hinder the applications of SWS in commercial solar cells. Recently, multiple studies have been carried out on indium tin oxide (ITO), titanium dioxide (TiO 2 ), and silicon dioxide (SiO 2 ) nanostructures employing oblique-angle deposition methods, [15][16][17] where the refractive indices of the nanoporous materials can be engineered by adjusting the air volume ratio. Still, the materials require multiple layers to effectively suppress the Fresnel reflection.In this paper, we demonstrate a practical photovoltaic application of ITO nanocolumns serving as a conductive AR layer for GaAs solar cells. As in standard GaAs cells, the use of a nanostructured AR layer could be otherwise limited due to severe front-surface recombination. The characteristic ITO nanocolumns, prepared by glancing-angle deposition with an incident nitrogen flux, offer omnidirectional and broad-band AR properties for both s-and p-polarizations, up to an incidence angle of 708 for the 350-900 nm wavelength range. Calculations based on a rigorous coupled-wave analysis (RCWA) method indicate that the superior AR characteristics arise from the tapered column profiles, which collectively function as a graded-refractive-index layer. The conversion efficiency of the GaAs solar cell with the nanocolumn AR layer increases by 28% compared to a cell without any AR treatment. Moreover, nearly 42% enhancement is achieved for photocurrents generated at wavele...

The donor (D)-π spacer-acceptor (A) framework with electronic push–pull effects provides suitable molecular architectures for molecular design used as efficient light-harvesting sensitizers in dye-sensitized solar cells (DSSCs). Efficiencies of light harvesting and electron injection to the semiconductor of sensitizers play critical roles in DSSC performance. Here, we employed density functional theory to systematically and comparatively investigate the effects of π-spacers of D-π spacer-A types of dyes in solution and adsorbed on a (TiO2)38 anatase cluster on various photophysical properties. The absorption spectra, electron transfer probability, and related photophysical properties of D-π spacer-A types of dyes were investigated as functions of different types (thiophene (Th)- and phenyl (Ph)-based), lengths, and planarity (bridging two neighboring rings; dithieno-thiophene (DTT) and fluorene (FL)-based) of π-spacers, while the D (diphenylamine) and A (cyano-acrylic acid) moieties remained the same. Spacers could significantly influence the λmax values and electron transfer probability. The spacer length has a red-shifted effect in λmax for the Th-, DTT-, and FL-based sensitizers due to their planar conjugated structures; nevertheless, the λmax values are saturated by ring number three. In contrast, the Ph-based spacers induce a blue-shift in λmax with spacer length due to their nonplanar structures. Interestingly, the Th- and DTT-based spacers with lower LUMO energy levels trap more electron density and thus reduce the probability of electron density transfer to TiO2 φET(λmax, TiO2) upon photoexcitation; moreover, the φET(λmax, TiO2) values decrease significantly with ring number. On the other hand, the φET(λmax, TiO2) values for the Ph- and FL-based sensitizers are less sensitive to the spacer length. Interestingly, the orders of theoretical maximum short-circuit current density of four studied families of molecules are correlated with their λmax values. Our study shows the Th–Th motif used as a π-spacer balances the spectral match with solar radiation and φET(λmax, TiO2) suitable for DSSC applications. Our results based on molecular and electronic structures could be used for rational sensitizer design of organic dyes for DSSC applications.

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