Device modelling and characterization are indispensable tools in the design of photovoltaic devices. In the contribution we present two challenging issues related to accurate modelling and efficient characterization of light scattering at nanotextured interfaces or other nanophotonic structures used in solar cell technologies. The model based on finite element method, which is upgraded with the Huygens’ expansion theorem is presented. It enables to calculate the angular distribution function of scattered light in the near and far field. It accounts also for the antireflection effects originating from nanoroughnesses. To characterize scattered light efficiently a camera based angular resolved spectroscopy system is presented. It captures the spatial angular distribution function in broad angular range at one shot.
We studied the optical properties of polymer layers filled with phosphor particles in two aspects. First, we used two different polymer binders with refractive indices n = 1.46 and n = 1.61 (λ = 600 nm) to decrease Δn with the phosphor particles (n = 1.81). Second, we prepared two particle size distributions D50 = 12 μm and D50 = 19 μm. The particles were dispersed in both polymer binders in several volume concentrations and coated onto glass with thicknesses of 150 - 600 μm. We present further a newly developed optical model for simulation and optimization of such luminescent down-shifting (LDS) layers. The model is developed within the ray tracing framework of the existing optical simulator CROWM (Combined Ray Optics / Wave Optics Model), which enables simulation of standalone LDS layers as well as complete solar cells (including thick and thin layers) enhanced by the LDS layers for an improved solar spectrum harvesting. Experimental results and numerical simulations show that the layers of the higher refractive index binder with larger particles result in the highest optical transmittance in the visible light spectrum. Finally we proved that scattering of the phosphor particles in the LDS layers may increase the overall light harvesting in the solar cell. We used numerical simulations to determine optimal layer composition for application in realistic thin-film photovoltaic devices. Surprisingly LDS layers with lower measured optical transmittance are more efficient when applied onto the solar cells due to graded refractive index and efficient light scattering. Therefore, our phosphor-filled LDS layers could possibly complement other light-coupling techniques in photovoltaics.
Efficient transparent light converters have received lately a growing interest from optical device industries (LEDs, PV,
etc.). While organic luminescent dyes were tested in PV light-converting application, such restrictions as small Stokes
shifts, short lifetimes, and relatively high costs must yet be overcome. Alternatively, use of phosphors in transparent
matrix materials would mean a major breakthrough for this technology, as phosphors exhibit long-term stability and are
widely available. For the fabrication of phosphor-filled layers tailored specifically for the desired application, it is of
great importance to gain deep understanding of light propagation through the layers, including the detailed optical
interplay between the phosphor particles and the matrix material. Our measurements show that absorption and
luminescent behavior of the phosphors and especially the scattering of light by the phosphor particles play an important
role. In this contribution we have investigated refractive index difference between transparent binder and phosphors.
Commercially available highly luminescent UV and near-UV absorbing μm-sized powder is chosen for the fabrication of
phosphor-filled layers with varied refractive index of transparent polymer matrix, and well-defined particle size
distributions. Solution-processed thick layers on glass substrates are optically analyzed and compared with simulation
results acquired from CROWM, a combined wave optics/ray optics home-built software. The results demonstrate the
inter-dependence of the layer parameters, prove the importance of careful optimization steps required for fabrication of
efficient light converting layers, and, thus, show a path into the future of this promising approach.
Individual shunts and "weak diodes" can have a significant effect, one much larger than implied by their physical area,
on the performance of laboratory-sized (~ 1cm2) solar cells. For larger areas typical of thin-film modules, the sheet
resistance of the transparent contact minimizes the impact of a single, small-area non-uniformity. If there are significant
numbers of shunts or weak diodes throughout a module, however, its performance may also be reduced. In this case, the
number, the magnitude, the nature, and the distribution of the non-uniformities combine to affect the degree of reduction.
In particular, a concentration of most shunts or weak diodes in a small number of module cells will be less destructive
than if they are distributed among a greater number of cells. In the case of non-uniform illumination, however, module
performance is less degraded if the shadowing is spread relatively uniformly over all or most of the cells.
The potential of three advanced optical designs in tandem micromorph silicon solar cells are analysed by means of optical simulations: enhanced light scattering, intermediate reflector (interlayer) and antireflective coating (ARC) on glass. The effects on quantum efficiency, QE, and short circuit current density, JSC, of the top and bottom cell are investigated. In case of enhanced light scattering, the role of haze parameter and angular distribution function of scattered light is analysed separately. High haze parameter improves light trapping in top and bottom cell. However, the improvement in QE and JSC of the bottom cell is limited at higher haze parameters due to increased absorption in top cell and increased optical losses in realistic textured ZnO/Ag back contact. Broad ADF plays an important role for improving the performances of both, top and bottom cell. The role of refractive index of an interlayer between top and bottom cell is analysed. Significant increases in QE and JSC of the top cell are revealed for small refractive indexes of the interlayer (n < 2.0). At the same time noticeable decrease in the performance of the bottom cell is observed. Optimisation of thickness and refractive index of a single-layer ARC on glass is carried out in order to obtain maximal JSC either in top or in bottom cell. Moderate increases in JSC and QE are obtained for optimised ARC parameters. Among the three optical designs, the greatest potential, considering the improvements in both cells, is revealed for enhanced light scattering.