• Energy Research

Abstract In this work we present a modified version of the commonly used semiconductor simulation program PC1D, named cmd-PC1D 6.0, which applies Fermi–Dirac (F–D) statistics, thus improving on the existing Boltzmann approximation that is employed in the original program. Several up-to-date models for crystalline Si have been implemented in the new program, including injection dependent band gap narrowing, carrier mobility and Auger recombination. The results obtained using cmd-PC1D 6.0 are verified against well-accepted and modern simulations tools for a range of both phosphorus- and boron-doped emitters with varying surface concentration. We find a good correspondence between the simulation results obtained with the different programs, with a deviation of less than 2 fA/cm2 for the calculated emitter saturation current density using F–D statistics and less than 0.8 % relative deviation in all the main solar cell parameters under 1 sun illumination. By using the correct carrier statistics and the appropriate set of models it is now possible to avoid the commonly used effective models and adapted values, thus taking a large step in bringing PC1D further towards a more general, consistent and physically meaningful simulation tool for Si solar cells. The new program can be run from a command line or within a recently developed, powerful graphical user interface. Both cmd-PC1D 6.0 and the graphical user interface are freely available for download.

AbstractSolar cells featuring a dielectrically coated rear side combine excellent electrical passivation, yielding high open circuit voltages, with high light trapping capabilities, yielding high short circuit currents. The roughness of the rear side is a crucial parameter for the optical properties of these devices. We therefore analyze the influence of the rear surface roughness on the charge carrier generation and the spectral reflection of the devices using predictive, three dimensional modeling of the rear surface based on the transfer-matrix formalism. After the verification of this so-called tilted-mirrors-model, we compare it to the widely used Phong model. When fitting the spectral reflectance of the Phong model using its two empirical parameters to the spectral reflectance obtained with the predictive model, we calculate 0.2 - 0.4mA/cm2 lower photogenerated current densities. We further use the predictive model to demonstrate the optical impact of variations of the thickness of the passivation layers.

The optimization of the metallization pattern and the emitter doping profiles and geometry for selective emitter solar cells require reliable and fast simulation models. The computational effort of one-dimensional models is usually much lower than that of two-dimensional models, which in turn allow for more realistic calculations. We review the literature on one-dimensional and two-dimensional models for the simulation of selective emitter solar cells. We compare the approaches for various emitter profiles and widths of the highly doped areas. We show that the one-dimensional and the two-dimensional approaches show similar trends and only small deviations concerning the short-circuit current density and the open-circuit voltage. Concerning the fill factor and the efficiency, the agreement is still reasonable for the investigated selective emitter structures. However, the one-dimensional approach leads to a more profound understanding and a more realistic simulation of the fill factor.

AbstractAfter completion of the solar cell manufacturing process the current–density versus voltage curves (J(U) curves) are measured to determine the solar cell's efficiency and the mechanisms limiting the efficiency. An accurate and robust analysis of the measured curves is essential. In this work it is shown that fitting the two‐diode model is inappropriate to quantify recombination in the space charge region and ohmic losses due to series resistance. Three fill factors, namely the fill factor of the illuminated J(U) curve, the pseudo fill factor of the sunsVoc curve and the ideal fill factor of the single diode model, are the base of a quick loss analysis that is evaluated in the present paper. It is shown that for an accurate analysis the distributed character of the series resistance and the network character of the solar cell cannot be neglected. An advanced current–voltage curve analysis including fill factors and fit is presented. Copyright © 2010 John Wiley & Sons, Ltd.

Abstract Current losses at surfaces play a crucial role in the optimization of high-efficiency silicon solar cells. We present a new approach to characterize the surface recombination activity of interdigitated-back-contact (IBC) silicon solar cells by comparing experimental and simulated photoluminescence images (PL). The different recombination properties of the p- and n-doped regions of IBC cells in combination with the operating condition lead to contrast profiles in the PL image that vary with bias voltage. We achieve a good matching of experimental and simulated data for the investigated cells enabling the analysis of how sensitive the simulated contrast patterns are to changes in the surface recombination at the emitter, back- and front-surface-field and if a better matching of the experimental PL images is possible. Using these PL images in combination with simulations around the open circuit voltage Voc the determination of surface recombination velocities S and emitter saturation currents J0 on finished cells is made possible for the first time. This approach also opens a new path towards loss analysis of finished PERL, PERC, MWT and other silicon solar cells.

This work introduces a novel industrial feasible back-contact back-junction crystalline silicon solar cell technology with exclusively highly doped surfaces. The near-surface phosphorus doping is realized in one single tube furnace diffusion step, while the alloying of screen-printed aluminum forms the p+-doping. A first experimental verification leads to a conversion efficiency of 20.1%. Further possible improvements are proposed and verified with numerical simulation. We perform comprehensive step-by-step optimization led by insights from free energy loss analysis, resulting in a conversion efficiency potential of 22.4% under realistic assumptions for design optimization.

AbstractDouble side collecting silicon solar cells such as the emitter wrap through and the both sides contacted and collecting concept are one possibility to maintain high efficiencies even on low lifetime material. To investigate the intrinsic advantages and limitations of this concept we use the solution of the continuity equation under low level injection conditions for a double side collecting (n+pn+) and a single side collecting (n+pp+) solar cell. In general, compared to the single side collecting structure, the double side collecting structure yields the highest advantages concerning the roduct of the short circuit current density and the open circuit voltage Jsc·Voc on highly doped material with diffusion lengths in the range of half the cell thickness. A realistic calculation of the fill factor of such devices is done using two dimensional numerical simulations. It is shown that a major reduction of the fill factor is caused by laterally varying voltage induced non-generation losses and ohmic losses, both caused by the majority carrier flow in the base. Electro- and photoluminescence images verify and illustrate the lateral voltage distribution under different illumination levels and terminal voltages.

AbstractPhotoluminescence (PL) imaging is a promising measuring technique to rate the quality of multi-crystalline silicon wafers. Defect structures which form during crystallization can be observed in the PL-images. In recent approaches, image features are measured and correlated to global solar cell parameters. So far, an underlying physical effect has not been connected to the large variety of observed PL-image patterns. Next to global solar cell parameters, also spatially resolved solar cell parameters like the dark saturation current density can be measured to rate wafer quality. It is shown that these spatially resolved measurements strongly correlate to the global open circuit voltage. Based on the observation that special patterns within the as-cut PL-images correlate with regions of high and low dark saturation current, a method for the modelling of physically relevant image features in PL-images is introduced in this work. The modelling framework is based on the transfer of spatially resolved quality data of the dark saturation current onto prototypical patterns within the PL-images of as-cut wafers by means of pattern recognition techniques. We evaluate our approach by predicting the images of the local dark-saturation current of the finished cells based on PL-images of as-cut wafers.