Photocurrent excitation spectroscopy

Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy. Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap. The other methods are related to the absorption process or its derivative (resonant Raman scattering). Most of these methods require cryogenic temperatures. 

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. 

The direct proof that H is present in certain centers in Ge came from the substitution of D for H, resulting in an isotopic energy shift in the optical transition lines. The main technique for unraveling the nature of these defects, which are so few in number, is high-resolution photothermal ionization spectroscopy, where IR photons from an FTIR spectrometer excite carriers from the ls-like ground state to bound excited states. Phonons are used to complete the transitions from the excited states to the nearest band edge. The transitions are then detected as a photocurrent. 

Unfortunately, the redox potential of the Pt4 + /3+ couple is not known in literature. Although some stable Ptm compounds have been isolated and characterized (37), the oxidation state III is reached usually only in unstable intermediates of photoaquation reactions (38-40) and on titania surfaces as detected by time resolved diffuse reflectance spectroscopy (41). To estimate the potential of the reductive surface center one has to recall that the injection of an electron into the conduction band of titania (TH) occurs at pH = 7, as confirmed by photocurrent measurements. Therefore, the redox potential of the surface Pt4 + /3+ couple should be equal or more negative than —0.28 V, i.e., the flatband potential of 4.0% H2[PtClal/ TH at pH = 7. From these results a potential energy diagram can be constructed as summarized in Scheme 2 for 4.0% H2[PtCl6]/TH at pH = 7. It includes the experimentally obtained positions of valence and conduction band edges, estimated redox potentials of the excited state of the surface platinum complex and other relevant potentials taken from literature. An important remark which should be made here is concerned with the error of the estimated potentials. Usually they are measured in simplified systems - for instance in the absence of titania - while adsorption at the surface, presence of various redox couples and other parameters can influence their values. Therefore the presented data may be connected with a rather large error. 

The illumination of semiconductor electrodes can give rise to a photocurrent due to the interband excitation of electrons. Although semiconductor photo-electrochemistry lies outside the scope of this chapter (an excellent review has been published by Morrison [57]), photocurrent spectroscopy has found more general application as an in-situ technique for the characterisation of surface films formed on metal electrodes such as Fe [58] and Pb [59] during corrosion. Quantitative analysis of photocurrent spectra can be used to identify semiconductor surface phases and to characterise their thickness and electronic properties. 

In part II, the photoresponses for the heterogeneous quenching of ZnTPPC by ferrocene derivatives were studied by intensity-modulated photocurrent spectroscopy (IMPS). The different contributions, that is, the electron injection, the recombination-product separation competition, and the attenuation due to the uncompensated resistance and interfacial capacitance (RC) time constant of the cell were deconvoluted in the frequency domain. The flux of electron injection was described as a competition between the relaxation of the porphyrin-excited state and the electron-transfer step. Experimental results confirmed that the electron-transfer rate increases with the Galvani potential difference (Butler-Volmer behavior), but the ZnTPPC coverage was potential-dependent. 

Whereas CM in bulk materials is usually determined in photocurrent device measurements, that is, by collecting the carriers, CM in QDs is studied by (optical) spectroscopic measurements, in which the orbital occupation of the QDs is probed on ultrafast (picosecond) timescales. Hence, the commonly used experimental procedures to determine CM in QDs (ultrafast spectroscopy) and in bulk (device measurements) are rather different. While time-resolved optical and IR spectroscopies are ideally suited to probe carrier populations in colloidal QDs, " light of terahertz (THz) frequencies interacts strongly with free carriers in the bulk material and allows the direct characterization of carrier density and mobility. From THz-time domain spectroscopy (TDS) experiments, one can quantitatively assess the number of photogenerated carriers in bulk semiconductors picoseconds after the light is absorbed. Additionally, as a result of the contact-free nature of the THz probe, it is possible to determine the CM factor in isolated samples of bulk semiconductors without the need to apply contacts, which is necessary in the device measurements. For these reasons, THz-TDS experiments have been employed to quantify CM in bulk PbSe and PbS on ultrafast timescales " in order to make a bulk-QD comparison in the context of the CM controversy. The CM factor in bulk PbS and PbSe was determined for excitation with various photon energies from the UV to the IR. 

---