Photocurrents excitation spectra

Before photocurrent excitation spectra can be normalised to allow for the wavelength dependence of the illumination intensity, it is essential to establish the relation between the photocurrent response and the incident photon flux. This can be done using calibrated neutral density fdters. The relationship is generally not linear for photoconductive systems or for systems in which processes such as surface recombination or photocurrent multiplication occur (Peter, 1990). The incident photon flux can be measured using a UV-enhanced silicon photodiode with known sensitivity. 

Fig. 14 Photocurrent excitation spectrum of an undoped anthracene crystal whose traps were filled at 83 K current, 3 x 10-I3A full scale. (After Rohrbacher and Karl, 1975)...

Photoeffects at copper electrodes coated with copper phenylacetylide (CuPA) films were discovered during an in-situ laser Raman investigation [35], where substantial cathodic photocurrents were observed. A subsequent detailed study by photocurrent spectroscopy showed that the photocurrent conversion efficiency was of the order of a few percent, even for thicker films which absorbed a substantial fraction of the incident light. Figure 18 contrasts the photocurrent excitation spectrum with the absorption spectrum measured on an OTE coated with the CuPA polymer. The coincidence in the vibrational structure in the two spectra is striking, suggesting that the absorption gives rise to a state with considerable molecular character. 

A typical photocurrent action spectrum is illustrated in Figure 10 together with the excitation spectrum of the ZnTOAPP monolayer. The good correspondence between the two curves indicates that the dye is, in fact, the source of the photocurrents observed. 

A plot of the photocurrent quantum yield versus excitation wavelength is termed the photoaction or photocurrent action spectrum. These spectra are obtained at short-circuit in a two-electrode arrangement or with an external bias in a three-electrode configuration. The photocurrent quantum yield is defined as the number of electrons measured in the external circuit divided by the number of absorbed photons. It is experimentally difficult to calculate the number of absorbed photons and corrections for scattered or transmitted light often appear to be fudge factors that increase the uncertainty of the absolute photocurrent yield. Therefore, the incident photocurrent yield is often reported which represents a lower limit of the true photocurrent quantum yield. 

Light excitation of a Chla membrane deposited on Pt resulted in a weak photocurrent with the Pt/Chla behaving as a cathode. The photocurrent action spectrum resembled the Chla absorption spectrum which ruled out a direct photoemission process. The direction of the photocurrent was rationalized by invoking a p-type conductivity in the Chla films. Bolton and co-workers explored related monolayer assemblies with spatially organized quinones to increase efficiencies ]41]. In a more promising system, Chla monolayers deposited on A1 contacts with uboquinone ac-... 

The long effective pathlength and high surface area afforded by these colloidal semiconductor materials allow spectroscopic characterization of interfacial electron transfer in molecular detail that was not previously possible. It is likely that within the next decade photoinduced interfacial electron transfer will be understood in the same detail now found only in homogeneous fluid solution. In many cases the sensitization mechanisms and theory developed for planar electrodes" are not applicable to the sensitized nanocrystalline films. Therefore, new models are necessary to describe the fascinating optical and electronic behavior of these materials. One such behavior is the recent identification of ultra-fast hot injection from molecular excited states. Furthermore, with these sensitized electrodes it is possible to probe ultra-fast processes using simple steady-state photocurrent action spectrum. 

Deeper traps can in some cases be emptied by irradiation with light. Figure 8.42 shows as an example the excitation spectrum of the photocurrent in an anthracene crystal which was doped with 10 tetracene molecules and whose hole traps were previously filled at a lower temperature, so that the anthracene crystal contained tetracene radical-cations before the excitation. The excitation spectrum shows the 0,0 transition and the vibronic series of the energetically lowest doublet-doublet transition Di Dq in the tetracene radical-cation. The combined evaluation of the thermally and the optically-stimulated currents yielded in this special case a value = 0.42 eV for the depth of the hole traps represented by tetracene in an... 

Fig. 8.43 The excitation spectrum of the photocurrent in a high-purity naphthalene crystal after its traps had been filled at T=77 K. The spectrum consists of the 0,0 transition and the vibronic series of the strongly forbidden T] Sq transition. In this way, primarily triplet excitons T] are excited, which empty the charge-carrier traps by annihilation and thus contribute to the current. After [44].

Similar results were also obtained with p-type semiconductors, such as p-GaP (Figure 10.12a) [20]. In this case, a cathodic sensitization process was observed which corresponds to a reduction of the excited dye via the valence band (see also excitation spectrum in Figure 10.8). Since the Uf of p-GaP occurs at a very positive electrode potential, the cathodic photocurrent onset also occurs in a fairly positive potential range and remains constant far into the cathodic region. Here, the energy bands are bent downward so that the holes injected into the valence band are pushed toward the bulk by the electric field across the space charge layer... 

The resemblance of the photocurrent to the optical adsorption spectrum has suggested the involvement of molecular excited states in the creation of charge carriers. While this resemblance is by no means universally observed, the concept of carrier creation via exciton interactions at or very near the illuminated electrode has become increasingly favored. Many of the data leading to these conclusions have been obtained by the use of pulsed light techniques (6, 7,3). These methods are virtually independent of electrode effects and the subsequent analysis of the transient current has led to considerable advances in the theory of charge transfer in molecular crystals. 

When the interfacial supramolecular triad is irradiated in the presence of I- under solar cells conditions, appreciable photocurrents are obtained. The profile of the photoaction spectrum shows clearly that photoinjection into TiC>2 takes place upon excitation of the ruthenium center. However, the IPCE values obtained are lower than those observed for the model compound, thus suggesting that injection is less efficient in the heterotriad. Of major interest is the mechanism for charge injection. Two different pathways can be envisaged. First, the charge injection may be a two-step process and takes place via the rhodium center as shown in the following equations ... 

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 ... 

Electrons are optically excited from the metal Fermi energy over the Schottky barrier by the internal photoemission mechanism. The carriers are collected at the opposite contact and detected as a photocurrent when the junction is held in reverse bias. The energy required to excite carriers is less than the band gap energy, so that the internal photoemission spectrum is distinguished from bulk band-to-band carrier generation. Models of the effect predict that the photoresponse spectrum is... 

Sanda et al. (1988) measured thermally stimulated currents in DEH doped PC. A peak in the spectrum was observed at 0.50 eV. The peak was attributed to the thermal excitation of holes from DEH donor states. The activation energy is in good agreement with values later derived from conventional photocurrent transient measurements by Schein and Mack (1988), Mack et al. (1989), and Kitamura and Yokoyama (1991). A secondary peak was observed at higher energies and attributed to trap states. 

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