Spectral Response Data

Photoemitters are characterized by rather definite upper and lower energy response limits, as stressed throughout this chapter. Spectral sensitivity is commonly plotted in units of amps of cathode emission per watt of illumination. When plotting tube sensitivity versus wavelength, this scale contorts constant quantum efficiency contours into angled lines sensitivities can be particularly ambiguous near zero wavelength. Data for NEA devices are more frequently found plotted directly in units of quantum yield. 

Excellent summaries of the various response curves available with commercial photomultipliers can be found in commercial sales publications and review articles [5.11] and we will not repeat such data in detail here. One response summary for classical photoemitters is given in Fig. 5.20 [5.11]. Note that the maximum yield is in the order of 0.3 electron per incident photon, with the higher efficiencies and broader response generally obtained with more complex materials. Note also that the IR-sensitive S-1 surface has by far the poorest quantum efficiency over the visible spectrum. Summaries of the general advantages and applications of each generic surface are given in [5.1, 11]. 

Example curves for one of the more important generic surfaces, the S-20 (NaKCsSb) and its extended red modifications, are illustrated in Fig. 5.21 [5.167]. Each variant is slightly better adapted to, for example, the detection of radiation from one or more laser devices. Similar optimized families of variants exist for several other common generic photoemissive surfaces. 

The spectral response of most NE A emitters with thresholds above 1 pm are shaped similarly to one another regardless of the bandgap of the material. A typical curve is that for (cooled) NEA InGaAsP in Fig. 5.18, where a sharp threshold is followed by a flat quantum efficiency plateau. For longer wavelength threshold devices and in particular for InGaAs, the response curves show inflection points which do vary with bandgap (see Fig. 5.16). At present 

NEA tubes are about a factor of five more expensive than their classical counterparts, but GaAs devices are dropping in price and amorphous GaAsP (comparable to S-20 in response) is less expensive and suffers from no lifetime problems [5.18, 125]. With increasing production quantities, both limitations (price and lifetime) will tend to disappear, although the very critical NEA surface required for 1 pm devices may make increasing operational life more problematical. 

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More recently, Faughnan and Hanak (1983) have used spectral response data to determine that the concentration of acceptors is —1019 cm-3 for p-type a-Si H layers containing — 1021 boron atoms cm -3 (as determined by SIMS) for a doping efficiency of — 1 %. Dresner (1983) has estimated that the doping efficiency of boron in a-Si H is — 0.1 % for films containing between 1019 and 1021 boron atoms cm-3. Thus, more recent estimates of the doping efficiency are in the range 0.1-1.0%. Apparently, many of the dopant atoms do not go into electronically active substitutional sites. 

A numerical simulation of this cell based on a one-dimensional model has been carried out by Ernst (2001), Grasso et al. (2002) and by Burgelman and Grasso (2004). In the work of Ernst and Grasso et al., the spectral response data could be simulated with reasonable accuracy using only a few adjustable parameters. These simulations confirm the electron diffusion length in the p-type CdTe films to be approximately 150 nm. The recombination centre density was found to be lO cm . These data indicate that the nanocrystalline CdTe films are of inferior quality than the material used in the conventional, planar CdTe solar cells, where diffusion lengths of 2 //m and defect densities of lO cm are typical. 

The spectral- and frequency-response characteristics of representative detectors are shown, respectively, in Figure 5.10a,b, where the spectral-response data for the frequency of maximum response and the frequency-response data for the wavenumber of maximum spectral response are shown, respectively. The quantity used for the ordinates of these figures is called specific detectivity, which is denoted by D A,f) and defined as... 

We will deal with the integral of (2.15) by either an in-band approach, or by using relative spectral response data to determine an irradiance or exitance that is effective at a specific wavelength. The first method allows us to calculate the average in-band responsivity of the detector the second yields the responsivity at a specific wavelength. 

Spectral response data can be presented per watt or per photon. Spectral response measurements yield normalized or relative values we are given the responsivity at each wavelength as a fraction of the responsivity at some reference wavelength, but we are not given the actual responsivity. Misunderstandings can result because there are no units associated with the reported values to tell us whether the data are per watt or per photon. We describe the determination of the absolute responsivity from the relative responsivity in Chapter 10 (Testing). 

Calibration. In general, standards used for instrument calibration are physical devices (standard lamps, flow meters, etc.) or pure chemical compounds in solution (solid or liquid), although some combined forms could be used (e.g., Tb + Eu in glass for wavelength calibration). Calibrated lnstr iment parameters include wavelength accuracy, detection-system spectral responsivity (to determine corrected excitation and emission spectra), and stability, among others. Fluorescence data such as corrected excitation and emission spectra, quantum yields, decay times, and polarization that are to be compared among laboratories are dependent on these calibrations. The Instrument and fluorescence parameters and various standards, reviewed recently (1,2,11), are discussed briefly below. 

Fluorescence data could be used to quantify oxygen demand values (chemical and biochemical) and total organic carbon values. Furthermore, the fluorescence spectral response can be apportioned to biodegradable (BOD) and non-biodegradable (COD-BOD) dissolved organics [71]. Other studies outline the advantages and drawbacks of the use of fluorescence techniques for waste-water quality monitoring [72,73]. 

The use of a linear detector array in the image plane of a polychromator in place of the fluorescence monochromator in Figure 12.1 enables the parallel data accumulation of complete fluorescence spectra. Silicon photodiode arrays, operated in a CCD mode(34) are the most widely used detector elements. The spectral response of the diodes enables fluorescence to be detected from the near-UV up to ca. 1100 nm with a peak response in the near-IR. Up to 8192 elements are now available commercially in a single linear array at low cost. However, the small length of each element (ca. 10 [im) presently limits sensitivity and hence cylindrical lens demagnification is often necessary. 

As for all absorbance-based spectral measurements, the intensity data represented in a raw (single-beam) chemical image are a combination of the spectral response of both the instrument and the sample. In order to remove the instrument response component, it is necessary to ratio the data to a background reference. For reflectance measurements, the background is a separate data cube typically acquired from a uniform. 

For a statistical analysis to adequately characterize the distribution of a component, the data should first be processed to obtain the optimum selectivity for that component. In this application the PLS model produces a score image that effectively separates the spectral response of the API from the excipients. Even though there is very little observable contrast in the images of the well blended samples, the results from the poorly blended samples convey conhdence that the method is effective at tracking the API distribution. 

For this example, much of the spectral variance has been explained after two factors (99-51%) vvith only 52.1% of the concentration variance explained. The fact that only a small amount of the concentration variance is accounted for may indicate a problem with the data, such as significant error in the concentration reference values or the lack of a Beer s law relationsliip between concentrations and spectral responses. 

We have shown that the radiant flux spectrum, as recorded by the spectrometer, is given by the convolution of the true radiant flux spectrum (as it would be recorded by a perfect instrument) with the spectrometer response function. In absorption spectroscopy, absorption lines typically appear superimposed upon a spectral background that is determined by the emission spectrum of the source, the spectral response of the detector, and other effects. Because we are interested in the properties of the absorbing molecules, it is necessary to correct for this background, or baseline as it is sometimes called. Furthermore, we shall see that the valuable physical-realizability constraints presented in Chapter 4 are easiest to apply when the data have this form. 

Figure 8 Spectral responses (IPCE) of N3- and black dye-sensitized 2 solar cells. IPCE is plotted as a function of wavelength. (Data from Ref. 20.)...

Figure 15. (top,) Spectral response curves for a reduced SrTiOs electrode before and after aging in F solution, (bottom) 1-V data for the same aging conditions. The I-V and spectral response curves were obtained in 0.1 M NaOH, and the electrode was aged in 1M KF for 2 days at +5 V vs. SCE. 

Note These data were obtained using Westinghouse HCLs and a single experimental setup. No correction has been made for the spectral response of the monochromator/photomultiplier tube system. These data were taken from Reference 2. 

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