Janez Krč

Effect of surface roughness of ZnO:Al films on light scattering in hydrogenated amorphous silicon solar cells

Abstract Experimental investigation combined with computer modeling is used for analysis of light scattering process in hydrogenated amorphous silicon (a-Si:H) solar cells deposited on textured glass/ZnO:Al substrates. Descriptive scattering parameters—haze and angular distribution functions (ADFs)—for the textured ZnO:Al films with different surface roughness are determined. The haze parameters of all internal interfaces in the a-Si:H solar cells are calculated using equations of scalar scattering theory calibrated on the measurements of the substrates. The ADFs determined for the substrates are modified and applied to the internal interfaces. The scattering parameters are incorporated in our optical model and used to simulate the effect of the ZnO:Al surface roughness on the quantum efficiency (QE) of the solar cells. The simulations reproduce the measured QE of all solar cells with different roughness of the substrate very well.

CH 3 NH 3 PbI 3 perovskite / silicon tandem solar cells: characterization based optical simulations

In this study we analyze and discuss the optical properties of various tandem architectures: mechanically stacked (four-terminal) and monolithically integrated (two-terminal) tandem devices, consisting of a methyl ammonium lead triiodide (CH(3)NH(3)PbI(3)) perovskite top solar cell and a crystalline silicon bottom solar cell. We provide layer thickness optimization guidelines and give estimates of the maximum tandem efficiencies based on state-of-the-art sub cells. We use experimental complex refractive index spectra for all involved materials as input data for an in-house developed optical simulator CROWM. Our characterization based simulations forecast that with optimized layer thicknesses the four-terminal configuration enables efficiencies over 30%, well above the current single-junction crystalline silicon cell record of 25.6%. Efficiencies over 30% can also be achieved with a two-terminal monolithic integration of the sub-cells, combined with proper selection of layer thicknesses.

Modulated surface textures for enhanced light trapping in thin-film silicon solar cells

Substrates with a modulated surface texture were prepared by combining different interface morphologies. The spatial frequency surface representation method is used to evaluate the surface modulation. When combining morphologies with appropriate geometrical features, substrates exhibit an increased scattering level in a broad wavelength region. We demonstrate that the improved scattering properties result from a superposition of different light scattering mechanisms caused by the different geometrical features integrated in a modulated surface texture.

Optical modeling of a-Si:H solar cells deposited on textured glass/SnO2 substrates

In this article we determine descriptive scattering parameters—haze and angular distribution functions—of scattered light for textured glass/SnO2 Asahi U-type substrates. These scattering parameters are input parameters of our optical model that enables us to analyze multilayer optical systems with rough interfaces. The scalar scattering theory is used to calculate the haze parameters of all internal rough interfaces in the a-Si:H solar cells deposited on the glass/SnO2 substrates. In the equations of the scalar scattering theory the correction functions are introduced in order to match the calculations with the measurements of the haze parameters of the substrates. The angular distribution functions of the substrates are applied to the rough internal interfaces. Using these scattering parameters we investigate the optical behavior of a-Si:H solar cells with different intrinsic layer thicknesses deposited on the textured glass/SnO2 substrates with different roughnesses.

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.

Modeling plasmonic scattering combined with thin-film optics

Plasmonic scattering from metal nanostructures presents a promising concept for improving the conversion efficiency of solar cells. The determination of optimal nanostructures and their position within the solar cell is crucial to boost the efficiency. Therefore we established a one-dimensional optical model combining plasmonic scattering and thin-film optics to simulate optical properties of thin-film solar cells including metal nanoparticles. Scattering models based on dipole oscillations and Mie theory are presented and their integration in thin-film semi-coherent optical descriptions is explained. A plasmonic layer is introduced in the thin-film structure to simulate scattering properties as well as parasitic absorption in the metal nanoparticles. A proof of modeling concept is given for the case of metal-island grown silver nanoparticles on glass and ZnO:Al/glass substrates. Using simulations a promising application of the nanoparticle integration is shown for the case of CuGaSe(2) solar cells.

Modulated photonic-crystal structures as broadband back reflectors in thin-film solar cells

A concept of a modulated one-dimensional photonic-crystal (PC) structure is introduced as a back reflector for thin-film solar cells. The structure comprises two PC parts, each consisting of layers of different thicknesses. Using layers of amorphous silicon and amorphous silicon nitride a reflectance close to 100% is achieved over a broad wavelength region (700–1300 nm). Based on this concept, a back reflector was designed for thin-film microcrystalline silicon solar cells, using n-doped amorphous silicon and ZnO:Al. Simulations show that the short-circuit current of the cell with a modulated PC back reflector closely resembles that of a cell with an ideal reflector.

Advanced Numerical Simulation Tool for Solar Cells - ASA5

Current amorphous and microcrystalline silicon thin-film solar cells use textured substrates for enhancing the light absorption and buffer and graded layers in order to improve the overall performance of the cells. The resulting solar cell structures are very complex, thus, for a detailed understanding and further optimization computer modeling has become an essential tool. In this article a new version of the Advanced Semiconductor Analysis computer program (ASA5) is introduced, which is particularly suitable for the modeling of these complex solar cell structures. The new features of the ASA program include: a new optical model Genpro3 for calculating the absorption profiles in solar cells with rough interfaces, a simulation mode for metal/insulator/semiconductor (MIS) structures, and a model for Charge Deep-Level Transient Spectroscopy (Q-DLTS) [1]. The Genpro3 model takes both coherent (specular) and incoherent (scattered) light propagation into account. The capabilities of the ASA5 program are demonstrated with simulations of a micromorph silicon tandem solar cell and the simulation of the Q-DLTS signal of an amorphous silicon MIS structure. The illuminated J-V characteristics and the external quantum efficiency of an optimized micromorph silicon solar cell approaching 15% efficiency are presented. Using simulations of the Q-DLTS signal a spatial and energy profile of the defect density of states in amorphous silicon is extracted

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.

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.