Photocurrent absorption

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. 

It is difficult to measure metal/polymer Schottky energy barriers smaller than about 0.5 eV using internal pholoemission. Small Schotiky energy barriers lead to thermal emission currents produced by the absorption of light in the metal which are difficult to separate from true photocurrents 134]. If the structure is cooled to try to improve this contrast, it is often found that the significant decrease in the electrical transport properties of the polymer [27 [ makes it difficult to measure the internal photoemission current. To overcome this limitation, internal photoemission and built-in potential measurements are combined to measure small Schottky energy barriers, as described below. 

The spectral dependence of the photoresponse of these bilayer heterojunction devices, illuminated from the 1TO side, is displayed in Figure 15-22. The onset of photocurrent at hv— 1.7 cV follows the absorption of the fullerene, indicating a symmetric hole transfer from the excited fullerene to the MEH-PPV. The minimum in the photocurrent at /iv=2.5 eV corresponds to the photon energy of maximum absorption of MEH-PPV. The MEH-PPV layer, therefore, acts as a filter, which reduces the number of photons reaching the MEH-PPV/C()0 interlace. Thus, the thickness of the MEH-PPV layer determines the anlibatic spectral be-... 

Erley G, Gorer S, Penner RM (1998) Transient photocurrent spectroscopy Electrical detection of optical absorption for supported semiconductor nanocrystals in a simple device geometry. Appl Phys Lett 72 2301-2303... 

Fig. 5.17 CdS-ZnO coupled semiconductor system (a) interaction between two colloidal particles showing the principle of the charge injection process and (b) light absorption and electron transfer on an electrode surface leading to the generation of photocurrent. (Reproduced from [330])...

Photoirradiation of the modified electrode with nanoclusters of Cj qN alone or the mixture of C oN and MePH afforded anodic photocurrents. The photocurrent action spectrum was in fair agreement with the absorption spectrum of the THF-H2O (2 1) mixed solution containing nanoclusters of the mixture of CfioN and MePH or C oN alone. These results strongly indicate that the photocurrents can be ascribed to photoexcitation of the nanoclusters of C qN. ... 

The dependence of steady photocurrents on excitation wavelength corresponded well with the absorption spectra of AMP in LB films as shown in Figure 7 independent of the molar ratio or the surface pressure of deposition. Much higher photocurrents were observed at the same bias voltage in LB films deposited at 50 mN nr1 (film A) than those at 20 mN m1 (film B). Figure 21 shows a bias voltage dependence of the ratio of photocurrents at 560 nm for film A and film B, I50/I2O/ deposited from 1 5 and 1 10 mixtures. The ratio was... 

Quite differently, Pleux et al. tested a series of three different organic dyads comprising a perylene monoimide (PMI) dye linked to a naphthalene diimide (NDI) or C60 for application in NiO-based DSSCs (Fig. 18.7) [117]. They corroborated a cascade electron flow from the valance band of NiO to PMI and, finally, to C60. Transient absorption measurements in the nanosecond time regime revealed that the presence of C60 extends the charge-separated state lifetime compared to just PMI. This fact enhanced the device efficiencies up to values of 0.04 and 0.06% when CoII/m and P/Ij electrolytes were utilized, respectively. More striking than the efficiencies is the remarkable incident photon-to-current efficiency spectrum, which features values of around 57% associated to photocurrent densities of 1.88 mA/cm2. 

---