Andrej Campa

Analysis and optimisation of microcrystalline silicon solar cells with periodic sinusoidal textured interfaces by two-dimensional optical simulations

Two-dimensional optical model for simulation of thin-film solar cells with periodical textured interfaces is presented. The model is based on finite element method and uses triangular discrete elements for the structure description. The advantages of the model in comparison to other existing models are highlighted. After validation and verification of the developed simulator, simulations of a microcrystalline silicon solar cell with a sinusoidal grating texture applied to the interfaces are carried out. The analysis and optimization of the two grating parameters—period and height of the grooves—are performed with respect to the maximal short-circuit current density of the cell. Up to 45% increase in the current density is identified for the optimized structure, compared to that of the cell with flat interfaces. Optical losses in the periodically textured silver back reflector are determined.

Two Approaches for Incoherent Propagation of Light in Rigorous Numerical Simulations

In multidimensional numerical simulations of optoelec- tronic devices the rigorous Maxwell equations are solved in difierent ways. However, numerically e-cient incoherent propagation of light inside the layers has not been resolved yet. In this paper we present two time- and resource-e-cient approaches for optical simulations of incoherent layers embedded in multilayer structures: (a) phase match- ing and (b) phase elimination approach. The approaches for simulat- ing the incoherent propagation of light in thick layers are derived from Maxwell equations. Both approaches can be applied to any layer in the structure regardless of the position inside the structure and the number of incoherent layers. In rigorous simulations, for low absorbing thick layers scaling down the thickness and increasing extinction coe-cient of the layer proportionally is implemented to shorten computational time. The simulation results are verifled with the experiment on two types of structures: a bare glass incoherent layer and an amorphous silicon solar cell.

Novel approaches of light management in thin-film silicon solar cells

In order to improve light trapping in thin-film silicon solar cells two novel approaches are investigated in this article: angle-selective management of light scattering inside the solar cell and wavelength-selective manipulation of high reflectance or transmittance of light. Diffraction gratings are analyzed as a representative of the first approach. Haze and angular distribution function of scattered (diffracted) light in reflection are measured for aluminum-based rectangular periodic gratings with different period and height of the rectangles. High haze values in specific wavelength region and scattering angles of the investigated gratings measured in air and water agree very well with the theoretical predictions. Considering the actual optical situation in microcrystalline silicon solar cells, optimal period and height of the rectangular gratings applied as a back reflector are calculated for obtaining the total reflection at the front interfaces. In the frame of the second approach, photonic-crystal-like structures are introduced. By means of optical simulations photonic-crystal-like structures are investigated for two possible applications: an intermediate reflector in a micromorph silicon solar cell with wavelength-selective reflectivity and a dielectric back reflector with a high reflectance in the long-wavelength region. The photonic crystal structure consisting of sequences of n-doped amorphous silicon and ZnO layers is designed for the efficient intermediate reflector. For the back reflector with a high reflectance the structures with intrinsic amorphous silicon, SiO2, MgF2 and TiO2 are proposed.

Optical and Electrical Analysis of Tandem Micromorph Silicon Solar Cell to Achieve Record-High Efficiency

Optical and electrical analysis of a micromorph silicon solar cell is carried out using numerical simulations. Electrical analysis reveals that generated charge carriers in p-doped amorphous silicon contribute to short-wavelength quantum efficiency of the top cell. In the bottom cell, non-ideal extraction of charge carriers from a few micrometer thick microcrystalline absorber is indicated. Improvements of optical and electrical properties are investigated in order to achieve high conversion efficiency of the solar cell. Potential for short-circuit current above 16 mA/cm2 is indicated already for relative thin absorbers. Introduction of a (hypothetical) wavelength-selective intermediate reflector (interlayer) results in further improvements in the short-circuit current of the top and bottom cells in comparison to the optimised single interlayer. The interlayer is very important for high absorption especially in the top cell, while for the absorption in the bottom cell a broad angular distribution function of scattered light is of great importance. By optimising electrical parameters of the layers in the top cell (larger energy band-gap, buffer layers), open-circuit voltage of the solar cell is increased. The efficiency over 15 % is obtained in simulations for an optically and electrically optimised micromorph solar cell

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.

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.

Thin film solar cell performance limits and potential

Limitations in performance of single-junction thin film solar cells are reviewed. Conversion efficiency in single junction solar cells is systematically analyzed in terms of energy conversion efficiency, electrical efficiency and optical efficiency. The analysis reveals a strong dependence of limitations in single junction solar cells on the band-gap of the absorber. In the case of CIGS solar cells, the band gap can be varied from 1.04 ev to 1.7 eV, which allows considerable opportunity to optimize for both band gap and the associated temperature coefficient.

Optimization of advanced surface-textures for thin-film silicon solar cells

Optimization and development of (i) periodic U-like single-texture and (ii) periodic+random double-texture are carried out for micromorph silicon solar cell. Based on modeling supported by experiments more than 10% improvement in conversion efficiency is indicated.

Potential of optical design in tandem micromorph silicon solar 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.

Design challenges for light harvesting in photovoltaic devices

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.