Spectral, mismatch

The match between a simulator spectrum E s(A) and the reference AM 1.5G spectrum r(A) is never perfect, even for the best solar simulators. Furthermore, a spectral mismatch is introduced because the spectral responses St (A) of the device under test and Sr.(A) of the reference cell are not identical. In order to correct for this, a spectral mismatch factor M can be computed from... 

Fig. 5.22. Characteristics for devices with an active layer that is spin-coated from a toluene solution (open squares) Jsc = 2.33 mA/cm2, V, . = 0.82 V, FF = 0.50, 77AM 1.5 = 0.9%, and from a chlorobenzene solution (open circles) Jsc = 5.25 mA/cm2, Voc = 0.82 V, FF = 0.61, t am 1.5 = 2.5%. Data are for devices illuminated with an intensity of 80 mW/cm2 and an AM 1.5 spectral mismatch factor of 0.753. The temperature of the samples during measurement was 50°C...

Once again, a box plot diagram is chosen to present the results from current/voltage (I/V) measurements for the FF (Fig. 5.37b) and Voc (Fig. 5.37c). At least 6 different devices were evaluated for each LiF thickness, the latter being varied between 0 A and 15 A. Upon insertion of only 3 A of LiF, the FF already increased by about 20% compared to otherwise identical reference devices with a pristine Al electrode. Together with an Isc of 5.25 mA/cm2 and a Uoc of 825 mV, the white light power conversion efficiency under 800 W/m2 at 50°C is calculated to be 3.3%. (Note that this is a white light efficiency which is not corrected by a spectral mismatch factor M.)... 

One of the concerns raised with the direct UV absorbance approach is that without HPLC separation the presence ofimpurities may cause erroneous solubility values to be reported. This is also a concern for turbidimetric methods. However, with strict purity criteria for registration of compounds into the company collection, this has helped to lessen this concern. In addition, for this assay an algorithm has been written that checks the UV spectrum of the sample against that of the calibration. Any significant impurities or decomposition occurring during the 24 h agitation period are readily picked out as a spectral mismatch [23]. 

Waldauf C (2007) Device ID RMS NREL certified, personal communication Corrected for spectral mismatch of the solar simulator... 

The spectral sensitivity of filter radiometers is not constant over the sensitive wavelength region. Two typical spectral sensitivities of UV radiometers are shown in Figure 5.13. The response of such radiometers depends on the measured spectral power distribution. If the calibration distribution differs from the measured distribution, a spectral mismatch must be taken into account. If the measured spectral distribution is known, the mismatch can be calculated. 

Spectral Mismatch Factors Calculated with Respect to the AM 1.5 G Reference Spectrum for Various Test Cell/Reference Cell Combinations... 

H. Mullejans, A. loannides, R. Kenny, W. Zaaiman, H.A. Ossenbrink, E.D. Dunlop, Spectral mismatch in calibration of photovoltaic reference devices by global sunlight method. Meas. Sci. Technol. 16, 1250-1254 (2005)... 

Different classes of solar simulators are available the classification is dependent on the spectral mismatch of the true AM 1.5 G spectrum. These devices are... 

Standard for determining PV efficiencies in the US [44], A reference cell is a solid-state PV device that has had its calibrated under the AM 1.5 G spectrum. Short-circuit current is the parameter that is most sensitive to the spectral distribution of the light source [44]. It is important that the band gap of the reference cell be as close as possible to the material being tested, because the calibration cell can only correct for spectral mismatch if it absorbs the same portion of the spectrum as the test electrode. Otherwise, a reference cell can be fitted with short-pass filters, to mimic wider band gap devices, and calibrated. Calibration of reference cells can be performed by any of several recognized institutions (NREL, Fraunhofer, AIST, and others). Integration of the spectral response over the AM 1.5 G spectrum is another method that can be used to obtain jsc and yield a reference cell that does not require outside testing. 

Two important issues for the measurement of materials in solution that must be taken into consideration are temperature and solvent-solute interactions. The impact of temperature was implied previously. One of the main reasons for considering thermal control when measuring solutions is that it is common practice to subtract (in absorbance) or ratio out the spectral components of the solvent from the solution spectrum. This exercise will be successful only if the temperatures of both the solution and the solvent are identical, or very similar, when the spectra are recorded. Wide differences in the temperatures will result in significant artifacts in the solvent-subtracted spectra due to spectral mismatch of the solvent components. 

For accurate PV measurements one uses a solar simulator with a light spectrum similar to that of AM 1.5G and a cahbrated reference cell to set the intensity. The SC measurement is divided into two steps (a) the determination of the solar simulator spectral mismatch factor, M and (b) measurement of the I-Vcurve of the solar cell referenced to standard test conditions (STC). The match between the simulator and the AM1.5G reference spectra [Es( ) and Er(> ), respectively] is not perfect. Furthermore, while for crystalline Si SCs, M = 1.00 0.02 (stable calibrated reference cells are available), for OSCs suitable and stable reference cells do not exist - in this case M = 0.9 was reported. 

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