Response, spectral

The spectral response of any solar cell is important since it dictates how much of the solar spectrum may be utilised in the generation of photocurrent. 

1) In the long wavelength region of the spectrijm the absorption coefficient of silicon falls off rapidly until at wavelengths of v 1100 nm and above there is no appreciable absorption within the silicon. There is therefore no cell response beyond this wavelength. 

2) In the short wavelength region of the spectrum the absorption coefficient of silicon is extremely high and photons are absorbed within a very small distance of the semiconductor surface. For p-n junction cells where the surface layer is heavily doped and minority carrier lifetimes are short, recombination is such that very few minority carriers reach the junction. Response in this part of the spectrum is small. For SBSCs there is no such layer and a greater response within this region is to be expected. 

However, Li et have shown that the short wavelength response 

if is the rms signal voltage at the output of a detector of area A measured at frequency / in response to incident radiation of rms power P and irradiance H modulated at frequency / from a black body of temperature T the black body responsivity is 

Although responsivity is also a valid figure of merit for detectors operating in the visible spectrum, the units are sometimes different. Since the lumen is a standard unit of visible radiant power, it is common practice to measure the responsivity of detectors such as photomultipliers in units of amps/lumen. Again, either a spectral or a black body reference can be employed. A useful black body reference temperature for visible detectors is 2870 K, the temperature at which the peak emission is at 1 pm. 

Because the performance of infrared detectors is limited by noise, it is important to be able to specify a signal-to-noise ratio in response to incident radiant power. An area-independent figure of merit is D ( dee-star ) defined as the rms signal-to-noise ratio in a 1 Hz bandwidth per unit rms incident radiant power per square root of detector area. D can be defined in response to a monochromatic radiation source or in response to a black body source. In the former case it is known as the spectral D, symbolized by D (A,/1) where A is the source wavelength, / is the modulation frequency, and 1 represents the 1 Hz bandwidth. Similarly, the black body D is symbolized by D (T,/, 1), where T is the temperature of the reference black body, usually 500 K. Unless otherwise stated, it is assumed that the detector field of view is hemispherical (2n ster). The units of D are cm Hz /watt. The relationship between Df measured at the wavelength of peak response and Z) (500 K) for an ideal photon detector is illustrated in Fig. 2.14. For an ideal thermal detector, Df D T) at all wavelengths and temperatures. 

Jones [2.157] has defined D ( dee-double-star ), a figure of merit appropriate to background limited detectors, which removes the need to specify the field of view when listing D. As discussed in Section 2.4, the idealized dependence of D upon angular subtense of the emitting background is given by 

PMTs are available in a wide variety of types. Tfrey can be classified in various ways, such as according to the design of the dynode chain, size and sh, spectral response, or tempond response. 

Note tfiat the quantum efficioicy is not constant over any reasonable range of wavelmgths. Hiis fact, coujded wdth the wavelength-d ndent efficiencies of monochromators, is the origin of the nonideal wavelengdi response of spectrofluorometers. 

Requirements for such polymers (either p- or n-type) are manifold  

The structure of the low bandgap polymeric semiconductor and the dopant dye is plotted in Fig. 5.19. The average thickness of the active layers, determined by AFM measurements, is between 80 and 110 nm. In order to obtain a better understanding of the transport behavior of polymer blends, low temperature studies of cells with pristine MDMO-PPV and MDMO-PPV/PTPTB 1 1 (wt. %) with Au electrodes were carried out. Au has a high work function and should therefore be a good hole injection contact and provide a high barrier for electron injection. The device will therefore be a hole-only device, as described earlier in this chapter [14]. 

In order to understand the performance of the tandem device, low temperature transport studies are a valuable tool. Diodes made from pristine MDMO-PPV and in composites with PTPTB are compared. ITO/PEDOT and Au electrodes are chosen to guarantee hole-only devices. This special choice of the electrodes is a successful technique for improving our understanding of transport failures. The proper choice of contacts allows us to produce p-type or n-type diodes from the same semiconductor, depending on the selectivity of the contact. For instance, Au is a hole-injection contact for most of the polymeric semiconductors, while Ca is an electron-injection 

Polymeric solar cells with a bandgap 2 eV are spectrally so badly mismatched to the solar spectrum that their efficiency is severely restricted. It is essential to develop polymeric semiconductors with lower bandgap. Low bandgap polymeric semiconductors behave similarly in conjunction with fullerenes as n-type semiconductors (acceptors). 

Recently systematic studies on the dependence of the dc and ac photocurrent of MEH-PPV photodiodes on the film thickness as well as applied voltage have been presented [76]. Generally the same features as for PPV diodes were observed in the case of MEH-PPV. 

There is experimental consensus on the most important parameters of singlelayer polymer photovoltaic devices, the short circuit current 4, the open circuit voltage Vof, and the filling factor FF. From these parameters the efficiencies of PPV based devices were typically calculated to be around 0.1% under monocliro-matic low light intensities. Efforts to extend the classical semiconductor picture 

15 Conjugated Polymer Based Plastic Solar Cells 

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Light sources can either be broadband, such as a Globar, a Nemst glower, an incandescent wire or mercury arc lamp or they can be tunable, such as a laser or optical parametric oscillator (OPO). In the fomier case, a monocln-omator is needed to achieve spectral resolution. In the case of a tunable light source, the spectral resolution is detemiined by the linewidth of the source itself In either case, the spectral coverage of the light source imposes limits on the vibrational frequencies that can be measured. Of course, limitations on the dispersing element and detector also affect the overall spectral response of the spectrometer. 

There are important figures of merit (5) that describe the performance of a photodetector. These are responsivity, noise, noise equivalent power, detectivity, and response time (2,6). However, there are several related parameters of measurement, eg, temperature of operation, bias power, spectral response, background photon flux, noise spectra, impedance, and linearity. Operational concerns include detector-element size, uniformity of response, array density, reflabiUty, cooling time, radiation tolerance, vibration and shock resistance, shelf life, availabiUty of arrays, and cost. 

When considering light of a certain spectral energy distribution falling on an object with a given spectral reflectance and perceived by an eye with its own spectral response, to obtain the perceived color stimulus it is necessary to multiply these factors together as ia Eigure 6. Standards are clearly required for both the observer and the illuminant. 

Fig. 6. The stimulus perceived as color is made up of the spectral power (or, as here, energy) curve of a source times the spectral reflectance (or transmittance) curve of an object times the appropriate spectral response curves (one shown here) of the eye (3).

Slightly less sophisticated are spectrocolorimeters that determine spectral response curves for further computation but from which the spectral curve itself is not available. An example is the X-Rite 948. 

For each EA spectrum, the transmission T was measured with the mechanical chopper in place and the electric field off. The differential transmission AT was subsequently measured without the chopper, with the electric field on, and with the lock-in amplifier set to detect signals at twice the electric-field modulation frequency. The 2/ dependency of the EA signal is due to the quadratic nature of EA in materials with definite parity. AT was then normalized to AT/T, which was free of the spectral response function. To a good approximation [18], the EA signal is related to the imaginary part of the optical third-order susceptibility ... 

The PL spectrum and onset of the absorption spectrum of poly(2,5-dioctyloxy-para-phenylene vinylene) (DOO-PPV) are shown in Figure 7-8b. The PL spectrum exhibits several phonon replica at 1.8, 1.98, and 2.15 eV. The PL spectrum is not corrected for the system spectral response or self-absorption. These corrections would affect the relative intensities of the peaks, but not their positions. The highest energy peak is taken as the zero-phonon (0-0) transition and the two lower peaks correspond to one- and two-phonon transitions (1-0 and 2-0, respectively). The 2-0 transition is significantly broader than the 0-0 transition. This could be explained by the existence of several unresolved phonon modes which couple to electronic transitions. In this section we concentrate on films and dilute solutions of DOO-PPV, though similar measurements have been carried out on MEH-PPV [23]. Fresh DOO-PPV thin films were cast from chloroform solutions of 5% molar concentration onto quartz substrates the films were kept under constant vacuum. 

Figure 15-8. Spectral response of the steady state photoconductivity of MEH-PPV alone and MEH-PPV/Cjo for several concentrations at 300 K and a biasing field of 104 V/cm (reproduced by permission of Elsevier Science from Ref. (18)].

Comparison of the spectral response and of the power efficiency of these first conjugated polymer/fullerene bilayer devices with single layer pure conjugated polymer devices showed that the large potential of the photoinduced charge transfer of a donor-acceptor system was not fully exploited in the bilayers. The devices still suffer from antibatic behavior as well as from a low power conversion efficiency. However, the diode behavior, i.e. the rectification of these devices, was excellent. 

Figure 15-24. Spectral response or devices nude wilh different PEOPT polymer thicknesses Al/C ) (35 nm)/PEOPT (30 nin)/PEDOT-PSS (110 unU/lTO (120 ninj/glass (solid circles) and AI/Cm (35 nm)/PEOPT (40 iini)/Pl DOT-PS.S (110 mn)/lTO (120 ninj/glass (open circles). The absorption spectrum of the PEOPT polymer is plollcd for comparison (solid line) (reproduced by permission of Wiley-VCH from Ref. (92]).

The conversion of radiated power (P in watts) to luminous flux (F in lumens) is achieved by considering the variation with wavelength of the human eye s photopie response. Then the spectral power from the source (PA in, lor example, W/nnt) is convoluted with the relative spectral response of the eye (V tabulated by the CIE) according to ... 

Kigurc 15-22. Spectral response of die photocurrcnl in ITO/MEH-PFV/Qj/Au photodiode al (reverse) -1 V bias (reproduced by pennission of llie American Insti-lule of Physics from Rel. [89J). 

Silicon is not as prominent a material in optoelectronics as it is in purely electronic applications, since its optical properties are limited. Yet it finds use as a photodetector with a response time in the nanosecond range and a spectral response band from 0.4 to 1.1 im, which matches the 0.905 im photoemission line of gallium arsenide. Silicon is transparent beyond 1.1 im and experiments have shown that a red light can be produced by shining an unfocused green laser beam on a specially prepared ultrathin crystal-silicon slice.CVD may prove useful in preparing such a material. 

Definition and Uses of Standards. In the context of this paper, the term "standard" denotes a well-characterized material for which a physical parameter or concentration of chemical constituent has been determined with a known precision and accuracy. These standards can be used to check or determine (a) instrumental parameters such as wavelength accuracy, detection-system spectral responsivity, and stability (b) the instrument response to specific fluorescent species and (c) the accuracy of measurements made by specific Instruments or measurement procedures (assess whether the analytical measurement process is in statistical control and whether it exhibits bias). Once the luminescence instrumentation has been calibrated, it can be used to measure the luminescence characteristics of chemical systems, including corrected excitation and emission spectra, quantum yields, decay times, emission anisotropies, energy transfer, and, with appropriate standards, the concentrations of chemical constituents in complex S2unples. 

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. 

Spectral Responsivity Standards (for Corrected Spectra). Depending on the conditions, many different organic and inorganic compounds in various solvents have been used as standards for determining the spectral responsivity of instruments. Several measurement proce-... 

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