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This work presents the results of the first interlaboratory comparison of linearity measurements of short-circuit current versus irradiance that includes a wide variety of photovoltaic (PV) device types, from reference cells to full-size modules. The aim of this inter-comparison was to compare the methods employed and to collect new inputs useful for the revision of the standard IEC 60904-10, which deals with linearity measurements for PV devices. The procedures and facilities employed by the partners include the differential spectral responsivity, the white light response, the solar simulator method and the two-lamp method. The facilities are generically described and compared and their main sources of uncertainty are discussed. Comparison results show good agreement within declared uncertainties between all partners. A few minor exceptions under low-light conditions raise questions of possible uncertainty underestimation for these specific conditions. The overall outcome of the comparison also highlights the importance of considering correlations in the uncertainty budget, which can potentially improve the overall stated uncertainty. A critical review is made of the data analysis adopted in the standard IEC 60904-10 to calculate the linearity degree of the short-circuit current towards irradiance. The analysis review suggests a way to make results based on different methods more comparable and less prone to erroneous linearity assessment. more...

Within the EURAMET ENG55 “PhotoClass” project, several characteristics of photovoltaic (PV) devices beyond their performance at Standard Test Conditions were investigated, including measurements at varying irradiance and temperature. Four groups of PV devices of different size and technology were prepared and corresponding round-robins were run between partner laboratories with substantially different facilities and methods – namely based on spectral or integral measurements. This paper presents the outcome of the four inter-laboratory comparisons dealing with temperature coefficient measurements of the short-circuit current of PV devices, from reference-cell size to full-size commercial modules of mainly several c-Si technologies, but also with some examples of CIGS and GaAs devices. The measurement results are compared via En number assessment, hence including measurement uncertainties. The main outcome of this measurement exercise is a very good agreement of all the laboratories although completely different approaches were applied. In some cases, laboratory measurement uncertainties are even considered rather conservative and could therefore be revised. Furthermore, a comparison between bare cells and commercial modules of the same technology is made, which may represent useful information for PV manufacturers. more...

We measure and discuss the complex refractive index of conventional ethylene vinyl acetate (EVA) and an EVA with enhanced UV-transmission based on spectroscopic ellipsometry, transmission and reflection measurements over the wavelength range from 300-1200 nm. Ray tracing of entire solar cell modules using this optical data predicts a 1.3% increase in short circuit current density (Jsc) at standard test conditions for EVA with enhanced UV transmission. This is in good agreement with laboratory experiments of test modules that result in a 1.4% increase in Jsc by using a UV transparent instead of a conventional EVA. Further, ray tracing simulations with realistic irradiation conditions with respect to angular and spectral distribution reveal an even larger Jsc increase of 1.9% in the yearly average. This increase is largest in the summer months with an increase of up to 2.3%. German Federal Ministry for Economic Affairs and Energy/0325641 more...

In a previous contribution it was found by evaluating 2-dimensional device simulation-based photoluminescence (PL) images of solar cells that the previous methods of PL image evaluation give good results for imaging the local series resistance, but lead to an erroneous distribution of an inhomogeneous saturation current density J01. This is caused by the model of independent diodes used in these evaluation methods, which does not take into account the distributed character of the series resistance and horizontal balancing currents occurring in inhomogeneous solar cells. In this contribution it is checked whether two alternative PL evaluation methods, which were already published but did not become popular yet, may overcome this problem. These are the differential luminescence imaging technique proposed by Rau et al. and the Laplacian-based method proposed by Glatthaar et al. By simulating PL images of a model device based on a 2-dimensional equivalent circuit with a known distribution of J01 and applying these PL evaluation methods, retrieved images of J01 are obtained and compared with the input J01 distribution. It is found that the differential luminescence imaging technique, at least in its presently used form, leads to wrong results, as the traditional PL evaluation techniques do. The Laplacian-based method, on the other hand, indeed has the potential to image J01 correctly. However, until now this method lacks in sensitivity. In this contribution some improvements are proposed, which may enhance the sensitivity of the Laplacian-based method. One important improvement is to enable a scaling measurement at a high instead of a low current resp. illumination intensity. Thereby the signal-to-noise ratio of the scaling measurement, which until now limited the accuracy of the whole PL evaluation, may be improved. Another proposal is that the actual PL measurement is performed under open circuit condition and not under current extraction. First experimental results show the effectiveness of this new method, but limitations still remain. more...

The conventional quantitative evaluation of dark lock-in thermography (DLIT), electroluminescence (EL), and photoluminescence (PL) images of solar cells is based on the model of independent diodes, where each image pixel is assumed to be connected with the terminals by an independent series resistance. In reality, however, the solar cell represents a 2-dimensional resistance-diode network. In this work solar cells containing well-defined spatial distributions of the saturation current density J01 and also containing J02-type and ohmic shunts are modeled for various externally applied biases and illumination conditions realistically as a 2-dimensional resistance-diode network. The resulting local diode voltage distributions are converted into DLIT, EL and PL images, which are further processed by conventional evaluation methods, which rely on the simple model of independent diodes. These are the so-called “Local-IV” method for the DLIT analysis, which may be supported by EL results to obtain series resistance images, and “C-DCR” for the PL analysis. This leads to calculated images of the local effective series resistance Rs and of J01. Regarding the resulting Rs images, PL shows the expected series resistance distribution and is not affected by the shunt regions. The DLIT–EL Rs images instead yield expected values only in the homogeneous regions, which are not affected by the assumed shunts. DLIT–EL determines higher values of Rs in local shunt regions and lower values around these regions and in spatially extended shunt regions. Regarding the J01 images both methods again give the expected results if J01 is distributed homogeneously. However, in the shunted regions, PL suffers from balancing currents within the emitter and DLIT from optical blurring. By comparing local and extended regions of increased J01 we find that DLIT approximates the expected J01 value better than PL, which clearly underestimates even extended local maxima of J01. For a local current analysis of silicon solar cells we recommend the use of DLIT for the determination of J01 images and PL for the determination of Rs images. more...