Papers by Keyword: Solar Cell

Abstract: Local defects, as micro-fractures, precipitates and other material inhomogeneities in solar cell structure, evidently modify electrical and photoelectrical behavior of the latter. To improve the efficiency and lifetime of existing solar cells, it is important to localize these defects which influence the p-n properties, and assign them corresponding electrical characteristics. Although the electric breakdown can be evident in current-voltage plot, the localization of local defects in the sample, that generate this breakdown, is not so easy task. It has to be done by microscopic investigations and measurement of light emission from defects under electrical bias conditions. Thus to contribute to this end, the structure of defects is microscopically investigated and consequently, the defects can be removed by focused ion beam milling. The experimental results obtained from samples before and after milling are also discussed.

Abstract: In the present report, the synthesis process of CuIn_xGa_1-xSe₂ nanoparticles as an absorption layer in tetraethylene glycol using metallic chloride and Se powder for the purpose of solar cell application. Whole processes were performed under glovebox condition. Nanoparticles sizes were achieved via manipulation of reaction temperature and various precursor concentrations. CuIn_xGa_1-xSe₂ or CIGS nanoparticles with diameters in the range of about 20-50 nm were prepared via polyol route and purified through centrifugation and precipitation processes. Then nanoparticles were dispersed to obtain stable inks that could be directly used for thin-film deposition via spin coating. Then, CIGS nanoparticles were coated on soda lime glass for fabrication of inorganic thin film solar cell via spin coating as a film. In those devices, the prepared films yielded relatively dense CuInGaSe₂ films with some void spaces. For elimination of the void spaces, the nanocrystals were exposed to selenium vapor atmosphere. Filling the voids with selenium can lead to the fabrication of CIGS absorptive layers having good dense structures and high efficiency. CIGS thin films were characterized by various analytical tools, such as XRD, UV-Visible spectroscopy and SEM imaging.

Abstract: Electronic properties along with the absorption coefficients and dielectric tensors of MoS₂ thin films in (0001) direction and the bulk state have been computed using density functional theory within full potential linearized augmented plane wave method. Surface energies and work functions are also deduced for different number of layers to check the quantum size effects and thereby the stability of thin films. The dielectric tensors and absorption coefficients (optical properties) of these materials are discussed to explore the utility of MoS₂ in photovoltaic applications.

Abstract: There is no national standard and international standard about nominal module operating temperature (NMOT) test methods for Building Integrated Photovoltaic (BIPV) modules. An in-lab testing method for measuring NMOT of BIPV modules is proposed. Based on the real operation condition of solar cell, the effect of ambient temperature, wind speed and optimal load resistor to operating temperature were analyzed. The surface temperature, interior temperature and back temperature of BIPV module were measured and the NMOT was determined.

Abstract: Isopropyl alcohol (IPA) is widely used as an additive to enhance the alkaline texturing process of mono-crystalline silicon solar cells currently. However, due to its low boiling point and high volatilization, some negative effects are brought into large scale production especially in stability, cost and environment. In this paper, an IPA-free texturing process was studied by using other additive instead of IPA. The influences of concentration of KOH and additive on etching rate, surface morphology and weighted reflectance were investigated. It is found that the additive has an opposite effect on etching rate and pyramid size compared to KOH. The etching rate and average pyramid size decrease with the concentration of additive increased. The best weight reflectance of 10.8% and lowest average pyramid size of 1.1 um were obtained on mono-crystalline silicon surface by an optimized solution of 1.5 wt% KOH and 1.5 wt% additive at 80oC for 20 minutes. Finally, the effects of KOH and IPA-free additive on the texturing process were also discussed in detail.

Abstract: This paper proposes a mathematical programming based approach for optimal estimation of photovoltaic cell model parameters. In this study, solar cell models are used to represent the current-voltage characteristics of the solar cell. The model is represented as a non-linear function that relates the cell current and voltage with some parameters to be estimated. No direct general analytical solution exists for such function. Given the input-output characteristic data of the solar cell, a mathematical programming technique is used to solve a set of transcendental equations to optimally estimate the solar cell parameters.

Abstract: This study proposed a light trapping module to improve the light path in a solar cell in order to increase its light absorption efficiency. The microlens on a transparent substrate concentrates incident light into several light beams, which it leads into the optical channel on the back side. The optical channel is designed by coating highly reflective metals on the same transparent substrate, then an optical channel opening is made at the light beam focus so the light beams can pass through the optical channel and irradiate the solar cell. The light reflected by the solar cell is reflected again by the metal surface to the upper film of the solar cell, thus, increasing the absorption efficiency of the solar cell and reducing the film thickness of the solar cell to obtain better electrical properties. In this simulation the refractive index of the microlens was set as 1.43, the optical channel was 25 μm and the spacing was 0.27 mm, thus, the simulated absorption efficiency reached over 80%. The feasibility of this study was thus proved.

Abstract: This paper describes results of our study aimed at understanding mechanism (s) of dislocation generation and propagation in multi-crystalline silicon (mc-Si) ingots, and evaluating their influence on the solar cell performance. This work was done in two parts: (i) Measurement of dislocation distributions along various bricks, selected from strategic locations within several ingots; and (ii) Theoretical modeling of the cell performance corresponding to the measured dislocation distributions. Solar cells were fabricated on wafers of known dislocation distribution, and the results were compared with the theory. These results show that cell performance can be accurately predicted from the dislocation distribution, and the changes in the dislocation distribution are the primary cause for variations in the cell-to-cell performance. The dislocation generation and propagation mechanisms, suggested by our results, are described in this paper.

Abstract: To investigate transition metal precipitates in Si, synchrotron based measurements, like micro x-ray fluorescence (μXRF) or detailed transmission electron microscopy (TEM) studies, are usually necessary. Transition metals are among the most detrimental defects in multi-crystalline (mc) silicon material for solar cell applications, due to their impact on minority charge carrier lifetime and possible shunt formation. We present another possibility to investigate transition metal precipitates by 3-dimensional focused ion beam (3D-FIB) cutting using a combined scanning electron microscope (SEM) SEM-FIB-system. This method is able to detect transition metal precipitates down to 5 nm in radius and provides additional information about the 3D shape, size and spatial distribution of the precipitates.

Abstract: The removal of dissolved iron from the wafer bulk is important for the performance of p-type multicrystalline silicon solar cells. In this paper we review some recent progress in understanding both external and internal gettering of iron. Internal gettering at grain boundaries and dislocations occurs naturally during ingot cooling, and can also be driven further during cell processing, especially by moderate temperature anneals (usually below 700 °C). Internal gettering at intra-grain defects plays key a role during such precipitation annealing. External gettering to phosphorus diffused regions is crucial in reducing the dissolved iron concentration during cell processing, although its effectiveness depends strongly on the diffusion temperature and profile. Gettering of Fe by boron and aluminum diffusions is also found to be very effective under certain conditions.