Sub-bandgap

The photogeneration of hydrogen from a water-methanol mixture using Nb-doped and Mg-doped FCjOj powder suspensions has been examined by Somorjai and coworkers The reaction was driven mainly by bandgap (2.2 eV) radiation. However, small amounts of were also formed upon sub-bandgap illumination. 

Marin PI, Hamstra MA, Vanmaekelbergh D (1996) Greatly enhanced sub-bandgap pbotocurrent in porous GaP pbotoanodes. J Electrochem Soc 143 1137-1142... 

Thermally assisted solar electrolysis consists of (i) light harvesting, (ii) spectral resolution of thermal (sub bandgap) and electronic (super-bandgap) radiation, the latter of which (iiia) drives photovoltaic or photoelectrochemical charge transfer V(iH2o) while the former (iiib) elevates water to temperature T, and pressure, p ... 

When captured at a thermal efficiency of reheat, the sub bandgap insolation power is r heatoc heat Psun. Other available system heating sources include absorbed super-bandgap photons which do not effectuate charge separation, Pecomb, and non-insolation sources, Pm, such as heat available from the ambient environment heat... 

Fig. 85. Quantum yield for the sub-bandgap photocurrent of (100) n-Si in 1MHC1 at a bias potential of + 1.0V/SCE. The sharp increase of tj near 0.95 eV is related to the onset of the band-to-band transition.

In this section, we consider some techniques, other than d.c. photocurrent measurements, that use illumination as a probe. Most directly related to the previous section are techniques employing sub-bandgap radiation and we shall consider these first. We shall then treat those techniques involving the emission of radiation and then photovoltage and a.c. photocurrent measurements. Finally, we shall treat electroreflectance and thermoreflectance measurements. 

Experimentally, it is found that the observed photocurrent-wavelength variation does not show a sharp onset at the bandgap threshold, but frequently a rather lengthy tail into the sub-bandgap region. An example is shown for n-Si/1 MKC1 in Fig. 85 [161]. The sub-bandgap photocurrent was interpreted here in terms of an expression of the form Ec... 

This technique has been elaborated as an electrochemical tool by Tench and co-workers [164-166]. Its main purpose is to explore the deep-lying bulk and surface levels and the principle of the technique is that the main role of sub-bandgap irradiation in a semiconductor will be to cause optical excitation to or from a bulk or surface state this, in turn, will cause an alteration in the potential distribution from that existing in the dark this alteration will manifest itself in the behaviour of the interfacial capacitance. 

It is clear, from the foregoing, that luminescence studies will be immeasurably aided by a simultaneous study of the sub-bandgap photocurrent and of the photocapacitance behaviour. 

The n-type character of these zinc sulfides is indicated by the observation of an anodic photocurrent upon illumination of a powder-coated electrode. The action spectrum of the photocurrent parallels the wavelength dependence of the apparent quantum yield of the photoreduction of water by sodium formate, except that hydrogen evolution starts already in the sub-bandgap region at 370 nm, a wavelength where no photocurrent was observable [98]. 

Changes in space-charge capacity can be used to observe the effect of charging and discharging of electronic states in a semiconductor subject to sub-bandgap illumination. Show that the change in observed capacity caused by illumination can be referenced to the unilluminated case by... 

Trupke T., Green M. A. and Wiirfel P. (2002b), Improving solar cell efficiencies by up-conversion of sub-bandgap light , J. Appl. Phys. 92,4117-4122. 

Fig. 3. Mechanisms that generate free electrons and holes (and photocurrent) by absorption of sub-bandgap photons. Continuous vertical arrows represent optical transitions and broken vertical arrows thermal transitions. Interfacial electron transfer is indicated by the horizontal arrow.

---