Showing papers by "Miro Zeman published in 2008"

Optical and electrical modeling of thin-film silicon solar cells

Abstract: This article focuses on the modeling and simulation of thin-film silicon solar cells to obtain increased efficiency. Computer simulations were used to study the performance limits of tandem and triple-junction, silicon-based solar cells. For the analysis, the optical simulator SunShine, which was developed at Ljubljana University, and the optoelectrical simulator ASA, which was developed at Delft University of Technology, were used. After calibration with realistic optical and electrical parameters, we used these simulators to study the scattering properties required, the absorption in nonactive layers, antireflective coatings, and the crucial role of the wavelength-selective intermediate reflector on the performance of the solar cells. Careful current matching was carried out to explore whether a high photocurrent [i.e., more than 15 mA/cm2 for a tandem hydrogenated amorphous silicon (a-Si:H)/hydrogenated microcrystalline silicon (μc-Si:H) solar cell and 11 mA/cm2 for a triple-junction a-Si:H/amorphous silicon germanium (a-SiGe:H)/μc-Si:H solar cell] could be obtained. In simulations, the extraction of the charge carriers, the open-circuit voltage, and the fill factor of these solar cells were improved by optimizing the electrical properties of the layers and the interfaces: a p-doped, a-SiC layer with a larger band gap (EG > 2 eV) and buffer layers at p/i interfaces were used. Simulations demonstrated that a-Si:H/μc-Si:H solar cells could be obtained with a conversion efficiency of 15% or higher, and triple-junction a-Si:H/a-SiGe:H/μc-Si:H solar cells with an efficiency of 17%.

Simulation studies on the effect of a buffer layer on the external parameters of hydrogenated amorphous silicon p-i-n solar cells

Abstract: Device modeling of p-i-n junction amorphous silicon solar cells has been carried out using the amorphous semiconductor analysis (ASA) simulation programme. The aim of the study was to explain the role of a buffer layer in between the p-and i-layers of the p-i-n solar cell on the external parameters such as dark current density and open circuit voltage. Investigations based on the simulation of dark I–V characteristics revealed that as the buffer layer thickness increases the dark current for a given voltage decreases.

Periodic Structures for Improved Light Management in Thin-film Silicon Solar Cells

Abstract: Advanced light management in thin-film solar cells is important in order to improve the photo-current and, thus, to raise up the conversion efficiencies of the solar cells. In this article two types of periodic structures iV one-dimensional diffraction gratings and photonic crystals,are analyzed in the direction of showing their potential for improved light trapping in thin-film silicon solar cells. The anti-reflective effects and enhanced scattering at the gratings with the triangular and rectangular features are studied by means of two-dimensional optical simulations. Simulations of the complete microcrystalline solar cell incorporating the gratings at all interfaces are presented. Critical optical issues to be overcome for achieving the performances of the cells with the optimized randomly textured interfaces are pointed out. Reflectance measurements for the designed 12 layer photonic crystal stack consisting of amorphous silicon nitride and amorphous silicon layers are presented and compared with the simulations. High reflectance (up to 99 %) of the stack is measured for a broad wavelength spectrum. By means of optical simulations the potential for using a simple photonic crystal structure as a back reflector in an amorphous silicon solar cell is demonstrated.

Modelling the light induced metastable effects in amorphous silicon

Abstract: We present results of computer simulations of the light induced degradation of amorphous silicon solar cells. It is now well established that when amorphous silicon is illuminated the density of dangling bond states increases. Dangling bond states produce amphoteric electronic mid-gap states which act as efficient charge trapping and recombination centres. The increase in dangling bond states causes a decrease in the performance of amorphous silicon solar cells. To show this effect, a modelling approach has been developed which uses the density of localised states with exponentially increasing band-tails and dangling bond defect states distribution chosen according to the defect pool model. The calculation of the evolution of dangling bond state density during illumination has been achieved through a dynamic scaling relation derived from a defect creation model. The approach considers the amphoteric nature of the dangling bond state and thus accounts for the contributions of the different charge states of the dangling bond during the degradation process. This paper attempts to describe the simulation approach which calculates the defect density as a function of energy, position in the solar cell and illumination time. In excellent agreement with other workers, our simulation results show that the increase in the density of neutral dangling bond states during illumination is higher than of the charged states.

Structural and Opto-Electronic Properties of a-Si:H/a-SiNx:H Superlattices

Abstract: A series of multilayer structures consisting of alternating layers of hydrogenated amorphous silicon (a-Si:H) and amorphous silicon nitride (a-SiNx:H) was fabricated using plasma enhanced chemical vapor deposition. The overall thickness and a-Si:H-to-a-SiNx:H ratio was kept constant for the different multilayer samples fabricated. A blue shift of the optical bandgap was observed with decreasing a-Si:H well layer thickness. High-Resolution Transmission Electron Microscopy was used to estimate the abruptness of the layer-to-layer transitions. The thickness of the interface mixing layer for transitions from a-Si:H to a-SiNx:H was estimated to be 0.5 − 1 nm, while for the reverse transition a thickness of 2-2.5 nm was found. Results from Fourier Transform Infrared Spectroscopy support the found thickness range.