Nano- and microlenses as concepts for enhanced performance of solar cells

Both metallic nanoparticles exhibiting plasmonic effects and dielectric nanoparticles coupling the light into resonant modes have shown successful applications to photovoltaics. On a larger scale, microconcentrator optics promise to enhance solar cell efficiency and to reduce material consumption. Here, we want to create a link between the concentrators on the nano- and on the microscale. From metallic nanospheres, we turn to dielectric ones and then look at increasing radii to approach the microscale. The lenses are investigated with respect to their interaction with light using three- dimensional simulations with the finite-element method. Resulting maps of local electric field distributions reveal the focusing behavior of the dielectric spheres. For larger lens sizes, ray tracing calculations, which give ray distributions in agreement with electric field intensities, can be applied. Calculations of back focal lengths in geometrical optics coincide with ray tracing results and allow insight into how the focal length can be tuned as a function of particle size, substrate refractive index, and the shape of the microlens. Despite the similarities we find for the nano- and the microlenses, integration into solar cells needs to be carefully adjusted, depending on the goals of material saving, concentration level, focal distance, and lens size

Influence of substrate and its temperature on the optical constants of CuIn1−xGaxSe2 thin films

We investigate the influence of substrate and its temperature on the optical constants of CuIn1−xGaxSe2 (CIGSe) thin films using the transfer-matrix method. The optical constants of a CIGSe layer on top of a transparent conducting oxide (TCO) layer were calculated considering the realistic optical constants of the TCO layer after CIGSe deposition. It was found that TCO substrates could influence the optical constants of CIGSe layers and that the ITO (Sn doped In2O3) substrate had a greater impact than IMO (Mo doped In2O3) for the CIGSe (x = 0.4) film when compared to a reference on bare glass substrate. Additionally, the varied substrate temperatures did not impact the optical constants of CGSe (x = 1). For CIGSe (x = 0.4), the refractive index n stayed relatively independent although at low temperature the grain size was reduced and the Ga/(Ga+In) profile was altered compared to that at high temperature (610 °C). In contrast, the extinction coefficient k at low temperature showed higher absorption at longer wavelengths because of a lower minimum bandgap (Eg,min) originating from reduced inter-diffusion of Ga–Se at a low substrate temperature

An Efficient Method for Calculating the Absorption Enhancement in Solar Cells with Integrated Plasmonic and Photonic Nanoparticles

We present a method for calculating the plasmonic and photonic enhancement of the absorption in solar cells. The method involves coupling between a transfer matrix method to describe light propagation in the layered stack and Mie theory for calculating the absorption and angular scattered field distribution from the nanoparticles. We also compare the method to rigorous simulations

Enhanced absorption in tandem solar cells by applying hydrogenated In2O3 as electrode

To realize the high efficiency potential of perovskite/chalcopyrite tandem solar cells in modules, hydrogenated In2O3 (IO:H) as electrode is investigated. IO:H with an electron mobility of 100 cm2 V−1 s−1 is demonstrated. Compared to the conventional Sn doped In2O3 (ITO), IO:H exhibits a decreased electron concentration and leads to almost no sub-bandgap absorption up to the wavelength of 1200 nm. Without a trade-off between transparency and lateral resistance in the IO:H electrode, the tandem cell keeps increasing in efficiency as the IO:H thickness increases and efficiencies above 22% are calculated. In contrast, the cells with ITO as electrode perform much worse due to the severe parasitic absorption in ITO. This indicates that IO:H has the potential to lead to high efficiencies, which is otherwise constrained by the parasitic absorption in conventional transparent conductive oxide electrode for tandem solar cells in modules

Light Extraction from Plasmonic Particles with Dielectric Shells and Overcoatings

We rigorously simulate light scattering via the FEM from core-shell plasmonic particles and plasmonic particles with an isolating overcoat, in order to recommend design principles for maximising plasmonic scattering gains

Nano-optical concept design for light management

Efficient light management in optoelectronic devices requires nanosystems where high optical qualities coincide with suitable device integration. The requirement of chemical and electrical passivation for integrating nanostrutures in e.g. thin film solar cells points towards the use of insulating and stable dielectric material, which however has to provide high scattering and near-fields as well. We investigate metal@dielectric core-shell nanoparticles and dielectric nanorods. Whereas core-shell nanoparticles can be simulated using Mie theory, nanorods of finite length are studied with the finite element method. We reveal that a metallic core within a thin dielectric shell can help to enhance scattering and near-field cross sections compared to a bare dielectric nanoparticle of the same radius. A dielectric nanorod has the benefit over a dielectric nanosphere in that it can generate much higher scattering cross sections and also give rise to a high near-field enhancement along its whole length. Electrical benefits of e.g. Ag@oxide nanoparticles in thin-film solar cells and ZnO nanorods in hybrid devices lie in reduction of recombination centers or close contact of the nanorod material with the surrounding organics, respectively. The optical benefit of dielectric shell material and elongated dielectric nanostructures is highlighted in this paper

Concentrating light in Cu(In,Ga)Se2 solar cells

Light concentration has proven beneficial for solar cells, most notably for highly efficient but expensive absorber materials using high concentrations and large scale optics. Here, we investigate the light concentration for cost- efficient thin-film solar cells that show nano- or microtextured absorbers. Our absorber material of choice is Cu(In,Ga)Se2 (CIGSe), which has a proven stabilized record efficiency of 22.6% and which—despite being a polycrystalline thin-film material—is very tolerant to environmental influences. Taking a nanoscale approach, we concentrate light in the CIGSe absorber layer by integrating photonic nanostructures made from dielectric materials. The dielectric nanostructures give rise to resonant modes and field localization in their vicinity. Thus, when inserted inside or adjacent to the absorber layer, absorption and efficiency enhancement are observed. In contrast to this internal absorption enhancement, external enhancement is exploited in the microscaled approach: mm-sized lenses can be used to concentrate light onto CIGSe solar cells with lateral dimensions reduced down to the micrometer range. These micro solar cells come with the benefit of improved heat dissipation compared with the large scale concentrators and promise compact high-efficiency devices. Both approaches of light concentration allow for reduction in material consumption by restricting the absorber dimension either vertically (ultrathin absorbers for dielectric nanostructures) or horizontally (microabsorbers for concentrating lenses) and have significant potential for efficiency enhancement