Photoconduction absorption edge

First, we consider the associated changes in the photoelectronic properties of the samples. The spectral characteristics of photoconductivity of the samples display a red shift after irradiation. Such behavior of the photoconductivity is not surprising, because it is in full agreement with the shift in the absorption edge. Additionally, the photoconductivity decreases after photodarkening [2]. The decrease may be attributed to the creation of new defect states or altering the existing localized states. 

The threshold of the photocurrent is at the energy of 1 eV. This value is less than the absorption edge enagy, what proves the existence of deep levels inside the forbidden gap. The absence of the structure at the beginning of the interband transitions excludes the possibility that the photocurrent is due to the surface exciton dissociation. Attempts to observe good photoconduction in ris-(CH) were unsuccessful. The photocunent in cis-(CH)n was three orders of magnitude lower than in trans-(CH) . 

Besides photoconductivity, photo-electro-motive (e.m.f.) forces were observed in copper polyacetylenides. The common results characterize the hypso-chromic shift of the photo-emf spectra compared to the photoconductivity ones. The maximum of the photoconductivity spectra coincide with the minima of the photo-emf spectra. The optical activation energies of the photosensitivity defined from the photoconductivity and photo-emf spectra are in close agreement and equal to the energy of the absorption edge. 

The optical properties of this new family of semiconductors are the subject of Volume 21, Part B. Phenomena discussed include the absorption edge, defect states, vibrational spectra, electroreflectance and electroabsorption, Raman scattering, luminescence, photoconductivity, photoemission, relaxation processes, and metastable effects. 

Indeed, the fact that the onset of photoconductivity occurs at a higher energy than the absorption edge in the polydiacetylenes, both in single crystal samples and in thin films cast from solution is a clear indication that the photoexcitations generated below 2.3 eV are neutral bound excitons [200]. 

The observation of photocurrent response at low fields, the onset of photoconductivity at a photon energy that coincides with the absorption edge, and the absence of correlation between Aa(E)/o- and - AIl(E)/1l all suggest... 

For samples pyrolyzed above about 370 C the optical absorption spectra are qualitatively different as shown In Figure 4. Absorption Is strong (>10 cm l) and comparatively flat with no maxima evident below 3.5 eV and no absorption edge above 0.7 eV. The shift of the optical absorption edge to lower energy with Increasing pyrolysis temperature between 220 and 440 C Is consistent with the trend of the photoconductivity and optical absorption data of Hlral and Nakada (17) and of Ohlgashl (19). 

In short, there are some encouraging results, particularly the adjustable optical absorption edge and some discouraging results such as the low photoconductivity. The charge transport mechanism Is still poorly understood. An evaluation of the suitability of PAN as a photovoltaic material requires better data on charge transport, doping and junction formation. 

Fic. 18. Composite optical absorption spectrum for a-Si H determined from optical transmission, pbotoacoustic deflection, and photoconductivity measurements. The linear fits to the data indicate exponential absorption lges with characteristic widths of 48 and 60 meV. [Reprinted with permission from Solid State Communications, C. B. Roxlo, B. Abeles, C. R. Wronski, G. D. Cody, and T. Tiedje, Comment on the optical absorption edge in a-Si H, Copyright 1983, Pergamon Press, Ltd.]... 

Weiser and Stuke (1969) observed a relatively broad band in a-Se near 2.1 eV (that is below the electrical band gap and the photoconduction edge) in electroreflectance. They interpreted it as additional evidence for the excitonic nature of the absorption edge in a-Se (excitons in Seg rings, cf. Section 4.4.3). This is a strong point for this interpretation, because electroreflectance is known to be particularly sensitive to excitonic effects. 

The photoconductivity of PPV prepared by the precursor route has been studied by several groups [142-145]. The polymer has a photoconductivity threshold at 506 nm that coincides well with the absorption edge [145]. Measurements of the transient photocurrent indicate a dispersive type of transport. The current is predominantly carried by holes... 

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