Benjamin Lipovšek

Modeling and optimization of white paint back reflectors for thin-film silicon solar cells

Diffusive dielectric materials such as white paint have been demonstrated as effective back reflectors in the photovoltaic technology. In this work, a one-dimensional (1D) optical modeling approach for simulation of white paint films is developed and implemented in a 1D optical simulator for thin-film solar cells. The parameters of white paint, such as the paint film thickness, the pigment volume concentration (PVC), and the pigment/binder refractive index ratio (RIR), are examined and optimized to achieve the required optical properties for back reflector application. The simulation trends indicate that white paint back reflectors with sufficient film thickness and higher PVC and RIR values exhibit improved reflectivity characteristics which results in an increased long-wavelength quantum efficiency of thin-film silicon solar cells. The simulation results based on the 1D model agree very well with the experimental data obtained from reflectance measurements of various white paint compositions and quantum efficiency measurements of amorphous silicon solar cells with white paint back reflectors.

Highly transmissive luminescent down-shifting layers filled with phosphor particles for photovoltaics

We study 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 study the influence of 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 with thicknesses of 150-600 µm onto glass substrates. 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 used numerical simulations to determine optimal layer composition for application in realistic photovoltaic devices.

Optical model for simulation and optimization of luminescent down-shifting layers filled with phosphor particles for photovoltaics.

We developed an optical model for simulation and optimization of luminescent down-shifting (LDS) layers for photovoltaics. These layers consist of micron-sized phosphor particles embedded in a polymer binder. The model is based on ray tracing and employs an effective approach to scattering and photoluminescence modelling. Experimental verification of the model shows that the model accurately takes all the structural parameters and material properties of the LDS layers into account, including the layer thickness, phosphor particle volume concentration, and phosphor particle size distribution. Finally, using the verified model, complete organic solar cells on glass substrate covered with the LDS layers are simulated. Simulations reveal that an optimized LDS layer can result in more than 6% larger short-circuit current of the solar cell.

Optimization of Microtextured Light-Management Films for Enhanced Light Trapping in Organic Solar Cells Under Perpendicular and Oblique Illumination Conditions

To improve light absorption in organic solar cells, microscale surface-textured light-management (LM) films are applied on top of the front glass substrate. In this study, numerical simulations are employed to determine the optimal texture of the LM films that would result in the highest short-circuit current density of the solar cells in perpendicular, as well as oblique, illumination conditions. Different types of 2-D periodic surface textures are analyzed (pyramidal, parabolic, sinusoidal), and the effects of the period and groove height sizes are investigated. Numerical simulations are based on a model that combines geometric optics and wave optics and, thus, enables simulation of light propagation through the thick microtextured LM film and glass, as well as thin layers of the device, respectively. Results show that parabolic textures are the most advantageous for the solar cells to achieve high performance operating in changing illumination conditions. When properly optimized, they enable over 14% boost of the short-circuit current density in a broad range of illumination incident angles, with the maximum of 22% for perpendicular incidence, with respect to that of the nontextured cell.

Photonic Crystal Back Reflector in Thin-film Silicon Solar Cells

One-dimensional photonic crystals having desired broad region of high reflectance ( R ) were fabricated by alternating the deposition of amorphous silicon and amorphous silicon nitride layers. The effect of the deposition temperature and angle of incidence on the optical properties of photonic crystals deposited on glass substrate was determined and an excellent matching was found with the simulated results. The broad region of high R of photonic crystals deposited on flat and textured ZnO:Al substrates decreases when compared to the R of photonic crystals de-posited on glass. The performance of amorphous silicon solar cells with 1-D photonic crystals integrated as the back reflector was evaluated. The external quantum efficiency measurement demonstrated that the solar cells with the photonic crystals back reflector had an enhanced re-sponse in the long wavelength region (above 550 nm) compared to the cells with the Ag reflector.

Design of periodic nano- and macro-scale textures for high-performance thin-film multi-junction solar cells

Surface textures in thin-film silicon multi-junction solar cells play an important role in gaining the photocurrent of the devices. In this paper, a design of the textures is carried out for the case of amorphous silicon/micro-crystalline silicon (a-Si:H/mu c-Si:H) solar cells, employing advanced modelling to determine the textures for defect-free silicon layer growth and to increase the photocurrent. A model of non-conformal layer growth and a hybrid optical modelling approach are used to perform realistic 3D simulations of the structures. The hybrid optical modelling includes rigorous modelling based on the finite element method and geometrical optics models. This enables us to examine the surface texture scaling from nano- to macro-sized (several tens or hundreds of micrometers) texturisation features. First, selected random and periodic nanotextures are examined with respect to critical positions of defect-region formation in Si layers. We show that despite careful selection of a well-suited semi-ellipsoidal periodic texture for defect-free layer growth, defective regions in Si layers of a-Si: H/mu c-Si: H cell cannot be avoided if the lateral and vertical dimensions of the nano features are optimised only for high gain in photocurrent. Macro features are favourable for defect-free layer growth, but do not render the photocurrent gains as achieved with light-scattering properties of the optimised nanotextures. Simulation results show that from the optical point of view the semi-ellipsoidal periodic nanotextures with lateral features smaller than 0.4 mu m and vertical peak-to-peak heights around or above 0.3 mu m are required to achieve a gain in short-circuit current of the top cell with respect to the state-of-the-art random texture (>16% increase), whereas lateral dimensions around 0.8 mu m and heights around 0.6 mu m lead to a > 6% gain in short-circuit current of the bottom cell.

Determination of the Complex Refractive Index of Powder Phosphors

We demonstrate a novel 2-step method to precisely determine both n and k of phosphors, luminescent inorganic particles, in the visible spectrum. To measure n we modified the Becke Line immersion method and verified its applicability in the absorption/ emission regions of phosphor particles (step 1). Particles were then embedded into a transparent binder and coated in thick layers (100-500 µm) on glass. Absorptance of the layers was measured with a novel approach: spectral angular resolved measurements. This method delivers accurate results by avoiding any errors from intense scattering inside the layers. A computational model was employed to extract k of particles from the measured absorptance data taking into account luminescence, scattering and re-absorption (step 2). The entire method was verified on reference materials. Finally, based on the proposed method, we determined in a broad wavelength range the n and k parameters for a variety of commonly used phosphors with few or no earlier reports on their n and k values (the complete set of numerical data is fully disclosed in the supplementary materials).

Detailed optical modelling and light-management of thin-film organic solar cells with consideration of small-area effects

We present detailed numerical and experimental investigation of thin-film organic solar cells with a micro-textured light management foil applied on top of the front glass substrate. We first demonstrate that measurements of small-area laboratory solar cells are susceptible to a significant amount of optical losses that could lead to false interpretation of the measurement results. Using the combined optical model CROWM calibrated with realistic optical properties of organic films and other layers, we identify the origins of these losses and quantify the extent of their influence. Further on, we identify the most important light management mechanisms of the micro-textured foil, among which the prevention of light escaping at the front side of the cell is revealed as the dominant one. Detailed three-dimensional simulations show that the light-management foil applied on top of a large-area organic solar cell can reduce the total reflection losses by nearly 60% and improve the short-circuit current density by almost 20%. Finally, by assuming realistic open-circuit voltage and especially the realistic fill factor that deteriorates as the absorber layer thickness is increased, we determine the optimal absorber layer thickness that would result in the highest power conversion efficiency of the investigated organic solar cells.

Light Management in Thin-Film Solar Cell

Employment of advanced light management techniques presents an important aspect in the design of thin-film solar cells. In this chapter, we highlight a number of light management approaches leading towards higher cell performances. Efficient light scattering within the cell, which can boost the photocurrent generation, can be achieved by optimised surface textures. Random textures and periodic textures for efficient light scattering are addressed. Besides surface texturing, the role of metal nanoparticles in thin-film solar cell structures is investigated in the scope of improved light trapping. To minimise optical losses, antireflective coatings and advanced back reflectors are employed. As examples, photonic crystal structures and diffusive dielectric materials are presented. And finally, better utilisation of the solar spectrum can be achieved by multi-bandgap multi-junction cells. For efficient spectrum harvesting, a concept of wavelength-selective intermediate reflectors is investigated. Further on, a concept of spectrum splitting and dislocated cells connected in a multi-terminal configuration is presented.

Micro-scale textures for enhanced performance of organic solar cells

To enhance light harvesting in organic solar cells, micro-scale surface-textured plastic foils laminated to the front side of the cells present an efficient low-cost solution. In this work, numerical modelling based on a combined geometric-optics/wave-optics model is used to investigate the potential of different surface texture profiles of light-management foils and to determine the optimal texture parameters. Results indicate that properly optimized parabolic textures are most advantageous for this purpose and enable a more than 15% boost of the short-circuit current density in a broad range of illumination incident angles, with the peak of 22% at perpendicular incidence, compared to that of the nontextured solar cell.