December 2020
H. Baumgartner, B. Oksanen, P. Kärhä and E. Ikonen
IEEE Journal of Photovoltaics 9, 1631 - 1636 (2019)

Abstract: A recently developed method to characterize the band gap energies of III-V optosemiconductors was utilized to determine temperature-invariant band gap features of multijunction solar cells. The method is based on measuring electroluminescent spectra of the solar cells at different temperatures. The normalized spectra reveal temperature-invariant energy values of the different junctions which are further converted to band gap energies. The method utilization requires a calibrated spectroradiometer and a temperature controlled mounting base for the solar cell under test, however, no knowledge about the absolute temperature of the cell under measurement. The method was tested on GaAs and GaInP solar cells that consist of single and dual junctions. The band gap energies were also derived from spectral response measurements. The differences of the determined band gap energies from the literature values were smaller than 1.1%. Compared with other band gap determination methods, the developed method yields temperature-invariant band gap characteristics; with a known uncertainty, that separated the different junctions in a multijunction device without individual biasing for the different junctions. In addition, a temperature-independent characterization parameter ensures that the operating conditions of the device under test do not affect the results.

Dittmann S.
Solar Energy 179, 424-436 (2019)

Abstract: 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.

February 2019
J. Lopez-Garcia, T. Sample and A. Casado
Solar Energy 177, 471-482
ISSN: 0038-092X

Abstract: Bifacial photovoltaic (PV) modules can increase the performance with respect to traditional PV modules because both sides of the cells, front and rear, absorb solar radiation. To assess their performance and quality, PV modules are characterized using international standards. However, currently only a draft IEC technical specification exists for bifacial PV modules and research needs to be done in order to study the indoor performance testing conditions. One of the issues that need to be addressed is how to measure bifacial PV modules correctly and analyse the different testing approaches proposed. This work outlines the indoor performance testing of c-Si bifacial modules under different module mounting setups including open rack, a structure with baffles and 3 modules. For each mounting method a white reflective rear panel of several dimensions was placed at various distances behind the module as a potential approach for a double-sided illumination characterization method. Electrical performance is also studied with a single-side illumination method with a black rear panel. The rear irradiance measurements and non-uniformity are also studied and the performance measurements are validated with a single-side illumination method. Additional rear irradiance allows Pmax increment up to 20% under certain conditions. However, the rear irradiance non-uniformity needs to be improved in order to fulfil the current requirements of the draft technical specification.

R.P. Kenny, J. Lopez-Garcia, B. Haile and E. Garcia Menendez
AIP Conference Proceedings 1999, 020014 (2018) (2018)

Abstract: The crystalline Si bifacial PV module market has grown significantly in recent years due to the opportunity for increased performance in comparison to traditional PV modules because both sides of the cell can absorb solar radiation. Draft technical specification IEC TS 60904-1-2 (Measurement of current-voltage characteristics of bifacial PV devices) sets limits on permissible deviations of operating conditions, e.g. irradiance non-uniformity. One purpose of this work is to ensure correct evaluation of the real outdoor conditions experienced and to understand what is achievable in practice. A second scope is to ensure correct measurement of module operating conditions in long-term outdoor testing where conditions will be highly variable, but nevertheless must be accounted for in energy yield or rating calculations. Two bifacial modules of each of two different types (Type 1: glass-glass and Type 2: glass-foil) are studied. Rear-side albedo is studied for white ground sheets. The specific conditions studied are module temperature, rear-side irradiance non-uniformity and spectral irradiance.

K. Bothe, D. Hinken, B. Min and C. Schinke
IEEE Journal of Photovoltaics 8, 611 - 620 (2018)

Abstract: The determination of the spectral responsivity is an essential part of solar cell characterization. Since solar simulators only approximate the reference spectrum, a spectral mismatch correction has to be performed. This correction procedure requires spectral responsivity data. Apart from the complete differential spectral responsivity procedure, the IEC 60904-8 standard defines four simplifications. In this paper, we provide information on the variations in the spectral responsivity curves for these simplifications. We show that for nonlinear front junction cells, deviations predominantly occur at wavelengths above 700 nm and become largest around 1000 nm. While we found a maximum deviation of 30% for the simplification with lowest requirements in bias irradiance, all other simplifications yield deviations below 10%. For a nonlinear cell measured relative to a world photovoltaic scale reference cell using a class A solar simulator, this transfers to a deviation below 0.01% in the spectral mismatch factor. If one depends on the use of a simplification, we recommend using the multicolor approach. Even though the singlecolor approach might yield lower deviations, this approach requires knowledge about the maximum in the spectral responsivity, which is not generally known in advance of the measurement. Accepting a slightly higher deviation, the white bias approach is a recommendable alternative.

J. Lopez-Garcia, D. Shaw, R.P. Kenny, L. Pinero-Prieto and E. Ozkalay
35th European Photovoltaic Solar Energy Conference and Exhibition
Brussels, BE, 24-09-2018 to 27-09-2018

Abstract: Bifacial crystalline Si photovoltaic modules have attracted considerable interest in the last years, since they can enhance the performance in comparison to traditional PV modules because both sides of the cell can absorb solar radiation. To assess their performance and quality, PV modules are measured and characterized under standard test conditions (STC) as defined by the International Electrotechnical Commission (IEC). Since there is currently no standard for bifacial PV modules, one of the main issues that need to be addressed is the standardisation of bifacial measurements for PV devices. The IEC TC82 is currently working on the development of a draft technical specification. Three different approaches are considered in the draft for the electrical performance of bifacial PV modules, that is, natural sunlight, a solar simulator with adjustable irradiance level for single-side illumination and a solar simulator with double-sided illumination. In order to reproduce the real conditions with both front and rear illumination, the latter approach has been used to characterise bifacial solar cells. However, at a module scale, this approach represents a technical and logistical challenge for many due to the associated cost of using two controlled light sources instead of one and it is considered that the current equipment is not suitable for this setup and requires significant and potentially expensive modifications. However, recent advances in high-power Light Emitting Diode (LED) technology have resulted in a number of LED-based sun simulators coming to market. The aim of this work is the design, development and validation of a double-sided solar simulator based on an existing large-area single long pulse flash solar simulator and a low-cost LED-based rear bias light that can fulfil the standard requirements of rear illumination (up to 200 W/m2) and non-uniformity.

D. Pavanello, A. Casado, J. Lopez-Garcia and T. Sample
33rd European Photovoltaic Solar Energy Conference and Exhibition
Amsterdam, the Netherlands, 25-09-2017 to 29-09-2017

Abstract: The interest on the n-type crystalline silicon bifacial PV modules has increased in the last years since they can increase the performance in comparison to traditional PV modules. As the PV modules are generally sold according to the nominal power output and efficiency under standard test conditions STC, the lack of standards for bifacial modules leads each manufacturer to claim different amounts for the "added value" of the bifaciality that makes it difficult for customers to directly compare between bifacial manufacturers. In this work, we have compared the electrical performance of framed and frameless c-Si bifacial modules from 5 different manufacturers utilising rear covers of different reflectivity and different measurement methods such as a single-sweep flash simulator, the so-called multi-flash (MF) method, a largearea steady state sun simulator and under natural sunlight for the accurate measurement of the module parameters. It was observed that depending on the measuring method, some modules presented STC front power out of the tolerance range given by the manufacturer. The design of the module also plays an important role on the rear power contribution.

Stefan Riechelmann
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