The spectral performance analysis conducted in Section 7.3 demonstrated that the currently available supercontinuum radiation allows forISC mea-surements of silicon solar cells despite the lack of UV radiation.
Thus, in very first measurements with the new experimental approach (presented in Section 7.2), the ISCs of two 4 cm2 silicon solar cells have been measured.7 The results are shown in Table 7.1 and demonstrate an excellent agreement ofISCs obtained from the new differential super-continuum approach (SC) to those from the DSR-method, that exhibits lowest uncertainties but is orders of magnitude slower (compare discus-sion in Section 7.1). Naturally, the ISC measurement with the currently available supercontinuum still requires a spectral mismatch correction due to its lack of UV radiation. Consequently, the spectral responsivity of the device under test (sTC) has to be measured as well to achieve MM ac-cording to Eq. (7.2). However, due to the twofold appearance ofsTC in Eq. (7.2), a relative measurement ofsTC suffices, thereby significantly re-ducing the spectral measurement effort. Furthermore, by application of an appropriate PCF, generating a supercontinuum that covers the entire spectral range, any mismatch correction becomes redundant and sTC is not required anymore forISC measurements with this new approach.
The cells applied for the measurements presented in Table 7.1 are con-sidered to be linear. Thus, the level of bias irradiation does not vary the ISC response of the test cells. However, it has to be mentioned that the presented measurement approach readily allows for fast and accurateISC
measurements of nonlinear cells as well. In that case severalISC measure-ments at different bias irradiation levels are required to achieve the ISC
under standard test conditions (STC) as described in Section 2.2.2.2.
Furthermore and in analogy to the white-light-response (WLR) method proposed [115], this approach yields that level of bias irradiation that rep-resents STC and that needs to be applied for absolute spectral responsivity
7For these measurements the same optical setup as introduced in Fig. 6.7 has been applied. As discussed there, this currently limits the irradiated area to 5x5 cm2. How-ever, it has to be mentioned that the generated supercontinuum power is sufficiently high to measureISCs of large area solar cells (up to 18 cm2edge length).
measurements. Thus, the differential supercontinuum approach might be readily implemented in the spectral responsivity measurement routine of the new setup (presented in Chapter 6) for evaluating test cell linearity and appropriate bias irradiation level.
7.5 Conclusions
In this chapter a new measurement approach was presented that takes advantage of the spectral shaping capabilities of supercontinuum radia-tion. Using the ultrashort laser pulses from the laser system applied in Chapter 6, a supercontinuum was generated by highly nonlinear optical effects in a PCF. Afterwards the supercontinuum was spectrally shaped to resemble a desired standard solar spectrum by real-time amplitude variations using a GLV and spatial amplitude masks.
In order to overcome the general problem of low output power from supercontinuum sources, a differential approach was presented for fast and accurate ISC measurements. Illuminating a device under test by steady and high powered bias irradiation in addition to chopped supercontinuum radiation, allows for retrieving the differential current response of the test cell to the virtually perfect supercontinuum spectrum. First experimental results demonstrated the excellent accuracy of this new approach.
The general capabilities of supercontinuum radiation were demon-strated by a detailed analysis of spectral mismatch and its uncertainty.
Cell MM uMM/% ISC/mA
Dev./%
SC DSR
Si A 1.021 0.16 143.98 144.06 0.05 Si B 1.004 0.12 129.17 129.28 0.08
Table 7.1: ISCs measured with differential supercontinuum approach (SC) and compared to theISCs from the differential spectral respon-sivity (DSR) method demonstrating excellent agreement. Moreover, the spectral mismatches (MM) and associated uncertainties (uMM) are given.
The shaped supercontinuum is capable of outperforming any state-of-the-art solar simulator regarding spectral properties. The analysis also revealed two major conclusions for the further development of super-continuum solar simulators: although a virtually perfect replication of a standard solar spectrum can be achieved, firstly, a less detail-rich ver-sion of a standard spectrum is favorable and, secondly, spectrally well matching reference cells are still required for achieving lowest possible measurement uncertainties.
In future development steps of this approach, a new PCF should be ap-plied that generates a supercontinuum covering the entire relevant spec-tral range, thereby making any specspec-tral mismatch corrections redundant.
In the meantime, applying an additional UV-LED covering parts of the currently missing UV spectrum appears to be most promising regarding spectral mismatch and its uncertainty.
EQE-Measurement of CPV Modules
In this chapter a novel experimental approach for a direct mea-surement of the external quantum efficiency (EQE) of concen-trator photovoltaic (CPV) modules will be presented that has been developed in the course of this work.1 Firstly, the state-of-the-art in EQE determination of CPV modules and challenges involved in a direct measurement of this EQE will be outlined.
Afterwards it will be discussed, which special requirements an experimental setup for such a direct measurement needs to meet and how these are achieved in the presented approach.
Finally, a validation of this novel experimental approach will be given by comparing measured short circuit current density and optical efficiency of a CPV mono module with indepen-dent indoor and outdoor measurements as well as simulations.
Parts of the results of this chapter were published in [128].
1The work presented in this chapter has been accomplished in collaboration with Christoph Rapp and Thomas Mißbach from the CPV-calibration group at Fraunhofer ISE.
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8.1 Challenges and State-Of-The-Art
The external quantum efficiency (EQE) is an important measurand for characterization and calibration of photovoltaic (PV) devices (see also section Section 2.2.2.2). Firstly, it gives spectrally resolved insight into the electro-optical performance of a device and, secondly, it enables spectral mismatch corrections as well as highly precise short circuit current density determination for any spectral irradianceEλ via
JSC= Z
EQE (λ)Eλ(λ) qλ
hc0dλ. (8.1)
Especially for multi-junction cells a precise JSC determination of the individual sub-cells is of tremendous importance, since imperfections in spectral mismatch correction might result in current-limiting artifacts that bias the electrical characterization of multi-junction devices. As these cells are typically applied in highly concentrating PV (CPV) modules,2 that focus the sun’s irradiation onto a smaller cell area, knowledge of the individual sub-cellJSC and, thus, of the CPV module EQE (EQEmod) is essential.
However, the various approaches for generation of quasi-monochromatic radiation vital for EQE-measurements are limited to EQE-measurements of PV cells3or non-concentrator (flat) PV modules [17, 53, 101, 105, 107, 110]. When it comes to CPV modules, above measurement approaches cannot be applied without any further angular radiation shaping. This results from the fact that the concentrator optics in CPV modules are specifically designed for the angular properties of the solar radiation.
Consequently, any deviations in the angular properties of the radiation from EQE-measurement facilities to the sun’s angular properties result in erroneous concentrator and, thus, current generation performance.
Therefore, any EQE-measurement approach for CPV modules requires a
2Throughout this chapter module denotes the combination of a single solar cell and any optical component (e.g. glasses, lenses or mirrors) and it does not necessarily include the consideration of interconnection of multiple cells inside a module.
3Naturally, the termcellsincludes non-concentrator and concentrator cells.
tight control of the angular radiation characteristics (in addition to its spectral and spatial properties).
As there is no measurement facility available yet for a direct EQE-measurement of CPV modules, EQEmodis typically computed from mea-sured cell-EQEs and simulated optical efficienciesηoptof the concentrator optics according to
EQEcompmod (λ) = EQEmeascell (λ)ηopt(λ). (8.2) The spectral optical efficiencyηopt(λ) of the concentrator optics gen-erally depends on various parameters like e.g. geometry, absorption and refractive index, that are partially temperature-dependent. For silicone-on-glass Fresnel lens concentrators Hornung et al. developed a FEM-based numerical model for calculation of ηopt(λ) that they successfully validated by experimental results [129]. These results were combined with solar irradiation spectra and ambient temperature-conditions to esti-mate the temperature-impact on energy generation in CPV modules [130].
Steiner et al. [131] applied both, ηopt(λ) and standard spectral irradi-ance Eλ(λ) ˆ=AM1.5d (according to ASTM G173-03 [132]), to compute individual sub-cellJSCs of multi-junction cells according to
JSC= Z
EQEmeascell (λ)ηopt(λ)EAM1.5d(λ) qλ hc0
dλ. (8.3)
With these sub-cellJSCs they simulated the current-voltage character-istics of a CPV module taking into account the cell interconnections and predicted the energy yield over a period of one year with a maximum deviation of 3%.
Although the above outlined approach has been applied successfully, direct experimental access to EQEmod might significantly reduce the ef-fort as an individual consideration of ηopt(λ) would become redundant.
Therefore, an experimental approach for direct measurement of the EQE of a CPV module has been developed in the course of this work [128], that takes advantage of the new laser-based measurement facility presented in Chapter 6. In the subsequent sections this approach will be detailed and
results will be presented that demonstrate its robustness, reproducibility and precision.