KEYWORDS: Interfaces, Solar cells, Fourier transforms, Computer simulations, Absorption, Thin film solar cells, Atomic force microscopy, Silicon solar cells, Thin films, External quantum efficiency
Analysis of randomly textured interface in solar cells is an ambitious effort considering the numerous structures and variation in the texture. We use the Fourier transform based method, which takes randomly textured interface as initial input and manipulates its frequency spectrum to synthesize interface texture with the desired texture behavior. Afterward, the synthesized interfaces are applied to simulate flexible thin-film silicon tandem (aSi/μcSi) solar cells. Using the simulation, we have shown how interface between the back and front areas of the solar cell evolves, and the effect of the back contact; the front area roughness is analyzed and interference fringes observed on experiments are discussed. For the simulations, a finite integration technique with time-harmonic inverse iterative method is used. All simulations are performed on high-performance computers, which allows simulation of a big simulation domain (>5 μm×5 μm) with fine mesh size (<10 nm). The analysis performed shows that the interface roughness at the front contact remains similar to the initial back contact roughness. Furthermore, a solar cell with flat back contact can be as efficient as a solar cell with rough back contact when the front area roughness is well optimized.
The efficiency of thin film solar cells can be improved by various known and extensively studied light trapping techniques such as textured interfaces, nano-particles, and so on. But all these methods are not expected to improve the efficiency of the solar cells with by same amount. In this paper, textured interfaces and nano-particles are simulated and their respective effect on the absorption of the active layer, aSi : H in this case, is observed. The nano-particles are silver nano-particles and for the interface simulations, different AFM scans are used. For the nano-particles simulations different position in the layer stack is simulated. To achieve this, Maxwell's equations are solved with Finite Difference Method(FDM) and Finite Integration Technique(FIT) and special scheme that accounts for the negative permittivity of silver. To accurately simulate the silver nano-particles the simulation is done with a fine discretization and run on high performance machines with a highly parallel iterative scheme.
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