Mauro Pravettoni

Spectral response measurement of double-junction thin-film photovoltaic devices: the impact of shunt resistance and bias voltage

Multijunction photovoltaic (PV) thin-film modules are becoming more and more important on the market, due to their low cost and improved module efficiency now well above 10%. The spectral response (SR) measurement of multijunction thin-film cells presents additional challenges with respect to the SR measurement procedure for single-junction devices. Several works have appeared in the last 15 years in the PV literature, describing certain measurement artefacts that typically appear when measuring the SR of multijunction cells without applying an appropriate voltage bias to the entire cell. In this paper, the authors revise the theoretical description of SR measurements on multijunction devices, show how to detect the possible origin of measurement artefacts from the dark SR and show why bias voltage sometimes is not enough to avoid such artefacts or why it is not even necessary. An experimental confirmation of the theoretical approach is finally given.

Characterization of a Pulsed Solar Simulator for Concentrator Photovoltaic Cell Calibration

The upgrade of a large area xenon lamp pulsed solar simulator to a 1500X pulsed solar simulator is presented in detail. Spectral match, spatial non-uniformity and temporal instability are analyzed according to the existing international standards, resulting in a Class CAA solar simulator: ongoing improvements to meet Class AAA requirements are discussed. The procedure for the calibration of a set of c-Si reference cells equipped with neutral density filters for calibrated measurements at high intensity is also presented. The paper finally highlights the importance of the spectral match to the standard AM1.5d (direct beam) spectrum over a wider range than described in the available standards for solar simulator classification.

Modeling the Effects of Inhomogeneous Irradiation and Temperature Profile on CPV Solar Cell Behavior

Solar cells that integrate concentration photovoltaic systems are usually exposed to inhomogeneous illumination and temperature profiles which influence their performance. Under such conditions, the solar cell behavior is only accurately modeled if the diode and resistive losses are considered to be distributed across the solar cell instead of being gathered, as in the conventional lumped model. This paper presents a distributed diode model and its experimental validation, which was carried out for standard test conditions and a range of temperature and concentration levels going from 25 to 70 °C and 1 to 30 suns, respectively, for both homogeneous and a set of inhomogeneous profiles. Modeled and experimental results showed good agreement, thus validating the model. The results of traditional and distributed model approaches are compared with homogeneous and inhomogeneous profiles of irradiation and temperature. Finally, we analyze the effect of different profiles on cell performance.

Luminescent solar concentrators: Nanorods and raytrace modeling

Nanorods are a novel and promising component for luminescent solar concentrators (LSCs). In particular, their spectra suggest reduced re-absorption losses. We report the incorporation of core-shell nanorods in homogeneous and thin film LSCs. The rods in the solid host appear to retain their spectral features compared to their dissolved state. Short-circuit current measurements with a calibrated solar cell have been compared with the computational simulation of the LSCs. Our raytrace model was applied to fit the fundamental emission spectrum and extract the quantum efficiency (QE) of the nanorods in the concentrators. In the case of the homogeneous LSC, the extracted QE was (67±4)%, which is in good agreement with the quoted value of about 70% for rods in solution. The thin film samples showed noticeably worse performance, which was attributed to possible agglomeration of rods and to macroscopic defects in the film. Finally, the raytrace model was applied to compare the self-absorption between a typical quantum dot concentrator and a nanorod concentrator. The result supported the argument that nanorods exhibit a smaller spectral overlap and consequently less re-absorption losses.

From an existing large area pulsed solar simulator to a high intensity pulsed solar simulator: Characterization, standard classification and first results at ESTI

The development and the characterization of ESTIs new high intensity pulsed solar simulator for concentrator photovoltaic (CPV) cells is presented, as the upgrade of an old large area pulsed solar simulator. The new setup is currently able to reach intensities as high as 1500 suns over a testing area of few centimeter diameter. Classification according to the international standard IEC 60904-9 and the ASTM 927-09 standard is given, resulting in a Class CAA solar simulator. Class C spectrum does not affect the characterization of single-junction CPV devices, thanks to the use of proper reference cells to reduce the spectral mismatch. Nevertheless, a better classification is needed to allow the current-voltage characterization of multi-junction CPV cells. The improvement is ongoing, but a better current balance is already shown when proper spectrally selective filters are used.

Results of the Fifth International Spectroradiometer Comparison for Improved Solar Spectral Irradiance Measurements and Related Impact on Reference Solar Cell Calibration

This paper reports on the results of the fifth spectral irradiance measurement intercomparison and the impact these results have on the spread of spectral mismatch calculations in the outdoor characterization of reference solar cell and photovoltaic (PV) devices. Ten laboratories and commercial partners with their own instruments were involved in the comparison. Solar spectral irradiance in clear sky condition was measured with both fast fixed and slow rotating grating spectroradiometers. This paper describes the intercomparison campaign, describes different statistical analysis used on acquired data, reports on the results, and analyzes the impact these results would have on the primary calibration of a c-Si PV reference cell under natural sunlight.

Characterization of CPV cells on a high intensity solar simulator: A detailed uncertainty analysis

The authors modified a large area pulsed solar simulator to a high intensity pulsed solar simulator, moving the test measurement plane to various distances close to the xenon lamp. Measurement results reported in this work have confirmed the r−2 dependence of the total irradiance on the cell-to-lamp distance, up to the maximum measured intensity at 3000X. Classification of the solar simulator based on the international standard IEC 60904–9 is reported. Though the standard procedure for current-voltage measurements of non-concentrating cells prescribes a calibrated reference cell to detect the total irradiance independently, a “self-reference” method is usually preferred at high intensities by the CPV community. In this work the authors apply a detailed uncertainty analysis to both the procedures. The result highlights the pros and contra of the two methods, giving a useful tool in the pre-normative work for the preparation of norms and standard procedures for terrestrial CPV cells characterization.

Electrical characterization of concentrator PV cells: A comparison between outdoor testing under direct solar radiation and indoor measurements on a high intensity solar simulator

The European Solar Test Installation (ESTI) laboratory and the Department of Electrical and Computer Engineering of the University of Cyprus (UCY) have shared their exploratory research on indoor/outdoor characterization of concentrating photovoltaic (CPV) cells and modules. The activity is performed within the 5-year APOLLON project, a co-financed activity of the VII research framework program of the European Commission. In this work we present results on the indoor characterization of c-Si and III–V group CPV cells by means of the new high intensity pulsed solar simulator developed at ESTI and the outdoor measurement under concentrated irradiance that has been carried out at UCY to corroborate the indoor results.

Standard Characterization of Multi-junction Thin-film Photovoltaic Modules - Spectral Mismatch Correction to Standard Test Conditions and Comparison with Outdoor Measurements

Multi-junction thin-film devices have emerged as very promising PV materials due to reduced cost, manufacturing ease, efficiency and long term performance. The consequent growing interest of the PV community has lead to the development of new methods for the correction of indoor measurements to standard test conditions (STC), as presented in this paper. The experimental setup for spectral response measurement of multi-junction large-area thin-film modules is presented. A method for reliable corrections of indoor current-voltage characterization to STC is presented: results are compared with outdoor measurements where irradiance conditions are close to standard ones, highlighting ongoing challenges in standard characterization of such devices.

Outdoor characterization of Luminescent Solar Concentrators and their possible architectural integration on a historically relevant site in Milan (Italy)

Luminescent Solar Concentrators (LSCs) typically consist of transparent plastic slabs, doped with luminescent species: incident light is partially absorbed and internally re-radiated by luminescence towards the edges of the slab, where photovoltaic cells are placed to convert light into electricity. Interest in LSCs has recently been raised thanks to improvements in module efficiency. A LSC is able to collect both direct and diffuse light and it is therefore very promising as a potentially cheap residential concentrating system. Outdoor characterization of various LSC prototypes has been performed at the outdoor solar field of the European Solar Test Installation in Ispra (VA), Italy. Results enable a first estimate of the annual energy yield of a hypothetical building integration of LSCs, given the annual average irradiance and the geometry of the installation. In this work, we also present a possible architectural integration concept in the historic environment of the Roman Empire archaeological site of Milan.