Photovoltaics Lab IOFFE

Photovoltaic performance measurements of solar cells (including multijunction structures), laser power converters (including multijunction devices) and small-aperture area concentrators such as Fresnel lenses or mirrors have imposed specific requirements on solar simulating equipment and relevant testing methods. For the accurate indoor measurements, the following installations were designed and manufactured at Photovoltaics Laboratory:

The Installation is a measuring system for recording the following spectral-dependent parameters:

• Absolute values of the external and internal quantum efficiency (photoresponse);
• Reflectance coefficients of the mirror-like samples (including structures with Bragg layers);
• Optical transmittance of the samples.

SPRM-Installation (Fig. 1) is equipped with two light sources on the base of Halogen and Xenon lamps. For multijunction devices, additional light bias conditions are produced by a set of semiconductor lasers. Evaluation of the photoresponse absolute units takes place due to comparison of the photocurrent magnitudes from a calibrated reference cell and the solar cell under test at monochromatic illumination in the so-called lock-in technique. The computerized measurements are carried out within wavelength spectral range of λ=200-2200 nm using several calibrated photodetectors: Si (200-1200 nm), Ge (300-1900 nm), GaInAs extended (1000-2200 nm). For multijunction solar cells with pronounced optical coupling, SPRM measurement results are corrected based on a specially developed technique that eliminates the negative influence of optical coupling processes and determines absolute values of the photoresponse. Reflectance coefficients are obtained owing to a comparison of the light fluxes reflected from a calibrated reference mirror and the sample under test.
In addition, all SPRM-measurements can be carried out in a wide temperature range of 80-450K using a nitrogen-cooled cryostat with an optical chamber evacuated by turbo pumping station up to high vacuum.
An electronic unit controlled by a computer is used to treat the analog signals being measured. SPRM-measurements are managed via multifunctional software (Fig. 2)

Figure 1. SPRM-Installation	Figure 2. Software for SPRM-Installation

Flash Tester (Fig. 3) for Characterization of the Solar Cells is a system for I-V curve measurements in solar cells, small-size arrays and concentrator PV submodules under light concentration up to 5000X. It measures both forward and reverse parts of the I-V characteristics under flash illumination and/or in the dark. A flash is produced by single, doubled or multiple xenon lamps and allows generating a light pulse controlled for the shape and intensity. Set of filters provides spectral composition of radiation corresponding to AM0 or AM1.5D conditions. A specialized software (Fig. 4) has been developed for the described tester. This software provides recording, indication and representation of the I-V curves (both graphically and digitally), control of irradiation intensity, voltage and current wave forms as a function of time, calculation of standard I-V curve parameters (Isc, Voc, Iopt, Vopt, Pmax, FF, Eff.).

Figure 3. Flash Tester for Characterization of the Solar Cells.	Figure 4. Software for Flash Tester.

Flash testers build-in on the proposed principles can be modified to serve for the different goals:

• characterization of concentrator modules 0.5x0.5 m² (Fig 5);
• characterization of flat panels up to 2x3 m² (Fig 6).

Solar simulator provides AM0, AM1.5G and AM1.5D spectral compositions of irradiation (Fig. 7). It is equipped with systems for measuring current-voltage characteristics and thermal stabilization of the samples under study. Broadband (250-1700 nm) spectroradiometer is used to monitor the spectral irradiance.

The installation allows forming uniform irradiation on an area with a diameter of up to 100 mm. The irradiance variations are within 3%, which meets the requirements for class B simulators according to the “non-uniformity” criterion. The main part of the installations shown in Fig. 8 and correspond to:
(1) Light source (continuous gas discharge xenon lamp of 150W) with ellipsoid reflector and set of optical filters;
(2) Highly stabilized power supply for the light source and feedback-loop on the base of photoelectric luminous flux sensor;
(3) Integrating sphere that provides radiation divergence of 32 ± 2 ang. min in correspondence with an angular size of natural Sun;
(4) Scanning sensor with 100-500 μm apertures for precision light profiling within focal spot;
(5) Measuring unit which allows signal recording via lock-in technique;
(6) Collimating spherical mirror, which forms a parallel flux (diameter of 190 mm) toward a tested sample.
Spectrum tailored with the help of a set of color-glass and interference filters is in good agreement with the standard AM1.5D sunlight irradiance (Fig. 9).

Figure 8. Installation for measuring optical-power and spectral characteristics of small-size concentrators (Fresnel lenses)	Figure 9. Comparison of the AM1.5D solar spectrum (black) with spectrum formed in the installation by means of an optical system (red).

Installation is intended to determine photo- and electroluminescent spectra of tested samples in the mode of passing current or in the mode of applying voltage (if necessary, the entire I-V characteristic can also be measured). Registration of signals is carried out using a CCD (spectral range 200-1000 nm) or spectrometers with different detectors suitable for UV and NIR ranges. For electroluminescent studies the installation is equipped with a set of laser. Also, the functionality of the installation can be expanded using a closed-loop cryostat to provide temperature studies.

Figure 10. Installation for PL&EL measurements: scheme (left) and photo (right)

Figure 11. Visualization of PL&EL measurement results taken from CCD camera (left) and from spectrometer (right).