Christine Jandl

Simulation of tandem thin-film silicon solar cells

A sophisticated light-management is indispensable for silicon thin-film silicon solar cells based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon. The optical properties of thin-film solar cells have a significant influence on the conversion efficiency. The topology of the nano-textured interfaces affects the optical path and absorption. A rough transparent conductive oxide (TCO) film leads to a high quantum efficiency and shortcircuit current density. Simulations of various geometries indicate the optimal texture. Therefore, we simulate 3-dimensional tandem thin-film solar cells with different interfaces. The roughness can be identified by atomic force microscope (AFM) scans. In order to accurately analyze all aspects of the light propagation in solar cells, numerical simulations of Maxwells equations are needed. By standard simulation programs for solving Maxwells equations, it is difficult to simulate realistic textures of the solar cell layers. Therefore, a simulation tool based on the finite difference time domain (FDTD) method and the finite integration technique (FIT) is developed, which is able to integrate AFM scan data. To incorporate the nanostructure of a relevant section in the AFM scans, high computational domains are needed. This leads to a large number of grid points in the resulting discretization. Parallel computations on high performance computers are needed to meet the large computational requirements. The simulations show that the light propagation in the investigated thin-film device is a complex phenomenon depending on the wavelength and phase of the incident light.

Simulation of Silicon Thin-Film Solar Cells for Oblique Incident Waves

To optimize the quantum efficiency (QE) and short-circuit current density (JSC) of silicon thin-film solar cells, one has to study the behavior of sunlight in these solar cells. Simulations are an adequate and economic method to analyze the optical properties of light caused by absorption and reflection. To this end a simulation tool is developed to take several demands into account. These include the analysis of perpendicular and oblique incident waves under E-, H- and circularly polarized light. Furthermore, the topology of the nanotextured interfaces influences the efficiency and therefore also the short-circuit current density. It is well known that a rough transparent conductive oxide (TCO) layer increases the efficiency of solar cells. Therefore, it is indispensable that various roughness profiles at the interfaces of the solar cell layers can be modeled in such a way that atomic force microscope (AFM) scan data can be integrated. Numerical calculations of Maxwells equations based on the finite integration technique (FIT) and Finite Difference Time Domain (FDTD) method are necessary to incorporate all these requirements. The simulations are performed in parallel on high performance computers (HPC) to meet the large computational requirements.

Simulation of light-trapping in thin film solar cells by high performance computing

A sophisticated light management is important to construct thin film solar cells with optimal efficiency. This is based on suitable nanostructures of different layers and materials with optimized optical properties. To design thin film solar cells with high efficiency, simulation of light-trapping is a very helpful tool. Such a simulation has to take into account the underlying physical properties like plasmonic effects of silver or interferences. To this end, it is important to solve Maxwells equations on a discretization grid. To obtain an accurate simulation, the roughness of the top transparent conductive oxide (TCO) layer is described by AFM-scan data. To meet the high computational amount in solving Maxwells equations on a finite difference discretization grid, high performance computers (HPC) are used.

High Performance Computing for the Simulation of Thin-Film Solar Cells

To optimize the optical efficiency of silicon thin-film solar cells, the absorption and reflection of sunlight in these solar cells has to be simulated. Since the thickness of the layers of thin-film solar cells is of the size of the wavelength, a rigorous simulation by solving Maxwell’s equations is important. However, large geometries of the cells described by atomic force microscope (AFM) data lead to a large computational domain and a large number of grid points in the resulting discretization. To meet the computational amount of such simulations, high performance computing (HPC) is needed. In this paper, we compare different high performance implementations of a software for solving Maxwell’s equations on different HPC machines. Simulation results for calculating the optical efficiency of thin-film solar cells are presented.

High-performance computing simulates light trapping in solar cells

Sophisticated light management is crucial for optimal thin-film solar cell efficiency.1 One successful approach, based on nanostructured composite layers and illustrated in Figure 1, incorporates materials with highly optimized optical properties. As might be expected, the design, development, and testing of these new solar cell prototypes is a time-consuming process. For this reason, suitable models and simulation techniques are required for the analysis of optical properties within thin-film solar cells.2 These can be widely applied to nanostructure designs and may include ray tracing, optical admittance analysis, a transfer matrix method, or finite difference discretization of Maxwell’s equations. The latter leads to the most accurate simulations because it includes optical effects such as interference, optical near-field properties, and plasmon effects. Suitable discretization methods are finite edge elements (FE), the finite integration technique (FIT), and finite difference time domain (FDTD). However, these approaches are all computationally intensive owing to the difficulty of achieving the randomness of rough interfaces between composite layers formed on rough metallic surfaces. We have developed a simulation tool for calculating quantum efficiency and short-circuit current density of thin-film solar cells. We use the FIT method since it can be used for curvilinear interfaces and is less computationally intensive compared with FE. We optimized the software using the standard messagepassing interface (MPI) to harness the power of several thousand processors. The topography of interfaces between different layers used to build our simulation uses data from atomic force microscopy (AFM) scans of real surfaces. For the simulation itself, suitable boundary conditions must be defined to handle the non-periodic structure of the AFM scan data. We modeled Figure 1. Structure of a thin-film solar cell. Onto a rough silver (Ag) surface are deposited, in order, transparent conductive oxide (TCO), microcrystalline and amorphous silicon ( c-Si:H and a-Si:H, respectively), a second layer of TCO, and glass.