Absorption conjugated polymers

In low-dimensional systems, such as quantum-confined. semiconductors and conjugated polymers, the first step of optical absorption is the creation of bound electron-hole pairs, known as excitons [34). Charge photogcncration (CPG) occurs when excitons break into positive and negative carriers. This process is of essential importance both for the understanding of the fundamental physics of these materials and for applications in photovoltaic devices and photodctcctors. Since exciton dissociation can be affected by an external electric field, field-induced spectroscopy is a powerful tool for studying CPG. 

An alternative explanation for the photoinduced absorption in the bulk polymer has been discussed by the Cambridge group [28]. It was shown that the amount of stimulated emission depends critically on the degree of photooxidation of the conjugated polymer. Figure 10-5 compares the stimulated emission of pristine PPV (see Fig. 10-5 a) and its heavily photooxidized counterpart (sec Fig. 10-5b). 

The excellent agreement between the TSC and P1A results has two implications. First, since the TSC method probes the product of mobility and carrier density, while the P1A probes only the carrier density, there seems to be no dominant influence of temperature on the carrier mobility. This was also found in other conjugated polymers like /ra/ry-polyacetylene [19, 36]. Second, photoconductivity (observed via the thermal release of photoexcited and trapped earners) and photo-induced absorption probe the same charged entity [36, 37J. 

Lincar 1R absorption studies proved that there is no interaction between the conjugated- polymer backbone and the CWi molecule in the ground slate, as al-... 

EL), conjugated polymers are also of interest as materials for optically or electrically pumped stimulated emission. For effects of this type, the ratio of stimulated emission to photoinduced absorption (PA) is of particular interest for conjugated polymers. In this context, the orign of the PA is controversial the PA can be a result of the formation of either charge-separated polaron pair -states or excimers. Initial experiments support the conjecture that LPPP 26 is significantly superior [49], as the stimulated emission of LPPP 26 is markedly more intense than that of PPV under comparable conditions. 

Figure 9 shows the electronic absorption spectrum of a PTTB film which has undergone extensive but incomplete reaction with bromine in a non-in-situ experiment. The absorption spectrum is that expected for a one-dimensional conjugated polymer. The sharpest absorption edge is at about 1490 nm (o.83 eV) and the absorption maximum is located at 1240 nm (1.0 eV). Thus, this material has a bandgap of about 0.83 eV. Note that two small... 

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