Absorbed photon flux

The transient current, derivable from equation 1, is given in equations 2 and 3 where T is the transit time and I is the absorbed photon flux. The parameter a can be further derived as equation 4 (4), where Tis the absolute temperature and is the distribution width (in units of kT) of a series of exponential traps. In this context, the carrier mobdity is governed by trapping and detrapping processes at these sites. 

In practical cases the absorption edge is not sharp and absorption coefficients of semiconductors decrease as the wavelength approaches the bandgap wavelength. The absorbed photon flux can then be written as [109]... 

Pig. 7. Relative carrier yield versus excitation wavelength. All data are normalized to the same absorbed photon flux. The electric field was t300 V/cm oriented approximately in the polymer chain direction in a standard surface cell configuration. The peak photocurrent-to-dark current ratio was 300 at 300 °K and >600 at 120 °K. Also shown is the absorption spectrum of PTS. The solid portion of the curve refers to the polymer chain direction. The dashed portion is an extrapolation derived from absorption measurements for a dispersion of PTS in a KBr pellet24 ... 

Accurate measurement of photon flux is essential in order to calculate quantum efficiency (whether based on absorbed photons flux or the flux of photons impinging on the surface). The response of commercial sensors is wavelength dependence, hence, the readout, which is in energy flux units (W cm ), is usually calibrated according to either the 365 nm line of mercury or the 254 nm line, depending on the sensor. For more details on radiation sources see de Lasa et al. (2005). A different method for measuring photon flux is actinometry. This method was very popular in the past, however, very few researchers still use it due to its complexity and the time it consumes. 

Fig. 10.3 illustrates the current-voltage characteristics of a p-i-n sensor in the dark and in light. The photocurrent rapidly saturates in reverse bias when there is full collection of the incident absorbed photon flux. The photovoltaic properties in slight forward bias are characterized by the short circuit current J, the open circuit voltage, Vog, and the fill factor, F. The maximum power delivered by the device is the product of the three terms... 

The photoinduced change in conductivity is iarge (111. For an absorbed photon flux of 10 5 cm 2 per pulse, the photocurrent (at -50 ps) is lp = 4 X 10 4 A (i.e., current density - i03 A/cm2) with good reproducibility from sample to sample. This corresponds to a conductivity of about 0.3 S/cm with a possible error of a factor of two due to uncertainty in ad in the top-iilumination measurement geometry. The increase in gph over the dark room temperature vaiue is five to six orders of maanitudel... 

Under illumination, a photocurrent is observed under depletion conditions if the photogenerated electron and hole are spatially separated before recombination can occur. The hole reacts at the surface (Eq. 1) and the electron is collected at the counter electrode. For the simplest case in which no recombination occurs at the surface, Gartner [29] derived an expression for the photocurrent density y pH taking into account the absorbed photon flux , the absorption coefficient a, and the minority carrier diffusion length Lmin... 

Fig. 3.18 - Current voltage plots for semiconductor/electrolyte junctions in the dark and under illumination (a) n-type, (b) p-type. The photocurrent is linearly proportional to the absorbed photon flux in the saturation region.

Absorption within an ionization chamber is controllable by selection of fill-gas composition and pressure. For reliable absorption measurements, ionization chambers must be carefully constructed and operated at sufficiently high bias voltage that they are linear, i.e. the output current is linearly related to the absorbed photon flux. Under these conditions the chamber is said to be in the plateau region of the flux versus bias voltage curve. 

Fig. 4.1 Relation between irradiation time requited to decompose one mole of reactant and absorbed photon flux, qip ab, for three values of the reaction quantum yield,...

The absorbed photon flux, p,ab, is a fraction of the flux of photons incident in the reaction cell, p,in, generally time independent. If the exciting radiation is absorbed only by R, the relation between p ab ad is the Lambert-Beer law... 

The previous integrated equations are valid only for irradiation with monochromatic light or, approximately, with light transmitted by an interference filter sufficiently narrow to assume that the reactant has a constant molar absorption coefficient in the entire band. Otherwise, in the case of uneven absorbance of the reactant and constant emission of the lamp in the spectral range of the filter, the relation between the overall incident photon flux and the overall absorbed photon flux can be evaluated by considering a continuous sequence of very narrow (e.g., 1 nm) wavelength intervals (Fig. 4.5) in this way, the relation can be numerically calculated by means of the equation... 

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