Photo Induced Absorption

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. 

There are few data concerning the photoconductivity of polythiophene [192,193]. The conductivity change under the combined action of N02 and white light is discussed assuming the participation of photo-induced absorption and desorption processes. 

By carefully controlling the temperature we were able to obtain fairly good matching of thermally- and photo-induced absorption bands. Thus, it seems probable that low temperatures, polar solvents and irradiation with ultraviolet light all lead to the same absorbing species. 

Keywords ultrafast processes, photo-induced absorption, photoluminescence, organic ma-... 

Fig. 9.26 Temporal characteristics of photo-induced absorptions associated with charge carriers generated by short laser pulses, (a) IR absorption in MEH-PPV (reprinted from Moses et ah, 2001 copyright 2001, with permission from Elsevier) and (b) optical absorption in a poly(indenofluor-ene) (reprinted from Silva et ah, 2001 copyright 2001, with permission from Elsevier). See text for details.

I. PHOTOEXCITATION PHOTO-INDUCED ABSORPTION, PHOTO-INDUCED BLEACHING AND PHOTOCONDUCTIVITY Photo-excitation studies of conjugated semiconductor polymers were stimulated by the calculations of Su and Schrieffer (IJ. which demonstrated that in trans-fCH) an e-h pair should evolve into a pair... 

In summary, semiconductor polymers such as polyacetylene and polythiophene have experimentally demonstrated nonlinear optical processes (photo-induced absorption, photo-induced bleaching and photo-luminescence) with characteristic time scales in the picosecond range or faster. These phenomena are intrinsic and originate from the instability of these conjugated polymers toward structural distortion. 

At the end of the 80s, the first reports of carriers with a polaronic nature were measured with photo-induced absorption, optical conductivity and infrared reflexivity in LaCu04+x and NdCu04 y [46,47]. The advocate idea was that with the assistance of absorbed light, polarons gain the ability to hop from one localized site to another. The origin of the mid-infra red peak oscillations that register on these experiments was linked to polarons. This picture was supported by related theoretical studies of polarons and bipolarons [48]. It is important to emphasize that these carriers were successfully modeled by Holstein-Hubbard and Holstein t — J... 

Multiexcitons are detected by several spectroscopic measurements. The first one was to monitor the signature of multiexciton decay dynamics using transient absorption (TA) spectroscopy (Schaller and Klimov, 2004 Ellingson et al, 2005 Schaller et al, 2005b and 2006 Murphy et al, 2006). The magnitude of the photo-induced absorption change at the band edge is proportional to the number of electron-... 

We have applied the ultrafast confocal microscope to map excited state dynamics in thin films of poly(9,9-dioctylfluorene) (PFO, see chemical structure in figure 2(a)), blended with polymethylmethacrylate (PMMA, 10% wt. PFO in PMMA). PFO is a blue-emitting polymer, with an absorption maximum at 385 nm (see Fig. 2(a)), while PMMA is transparent at our pump wavelength and it does not interact with PFO [6] so that it is optically inert. Figure 2(b) shows the macroscopic AT/T spectrum of PFO measured at x = 1 ps at 570 nm probe wavelength we observe a photo-induced absorption (PA) due to photo-generated polarons [7],... 

In parallel with the foregoing studies on inorganic semiconductors and biological molecules, ODMR has been expanded to include electroluminescence (EL)-, conductivity (detected magnetic resonance [25,26,28,29,31,48]. These variants are conceptually identical to the conventional PL-detected ODMR. This... 

The half-field PL-detected resonance of a CN-PPV film and EL-detected resonance of a typical ITO/PPV/CN-PPV/Al bilayer LED at various temperatures are shown in Figure 7.49. Two features are noteworthy (a) In contrast to the difference between PPV films and the early ITO/PPV/Ca/Al LEDs (Figure 7.47), the lineshapes of the PL- and EL-detected half-field triplet resonances in the CN-PPV film and associated diodes are very similar. This indicates that the EL-detected resonance is due to the presence of triplets in the CN-PPV layer of the device, (b) The EL-detected resonance is observable at temperatures considerably higher than those achieved in the early ITO/PPV/Ca/Al diodes (Figure 7.48). This behaviour is also entirely consistent with the weaker temperature dependence of the triplet-triplet photo-induced absorption transition of CN-PPV as compared to PPV [80,84], providing further evidence that the effects of the triplets on the EL are occurring in the CN-PPV layer. 

Poplawski et al. [178,179] find in their photo-induced absorption spectrum, two strong bands at 0.80 eV and 1.54 eV and smaller absorption peaks located at 0.96 eV, 1.08 eV, and 1.68 eV. Similar results... 

Most of the linear optical properties and also some of the nonlinear properties like photo-induced absorption were reported in Section 4.1.2 above. We will here concentrate on phenomenological properties like the refractive index and the second and third order hyperpolarizabilities. 

F.L.Pratt, K.S.Wong, W.Hayes and D.Bloor, Infrared photo-induced absorption in polydiacety-... 

D.W. McBranch, M.B. Sinclair, Ultrafast Photo-Induced Absorption in Nondegenerate Ground State Conjugated Polymers Signatures of Excited States, in [9], p. 587. 

Fig.3. Photo-induced absorption spectra of [Pt(en)2][Pt(en)2Cl2](0104)4 single crystal at 77K. The spectra were measured for the light polarized parallel(E//b solid lines) and perpendicular to the chain(Ejl), dashed line). The numbers are the time of photo-excitation (minutes) with the constant irradiation of the laser light(2.41eV, 0.24mW/mm2).

Instead of the single triplet-triplet absorption band centered around 1.35 eV in the photo-induced absorption (PIA) of pristine MEH-PPV, a sharp PI A edge at 1.1 eV and a plateau at 1.6—2.0 eV is observed in the MEH-PPV C6o blends [16]. 

Figure 10.6 also provides a possible explanation of the high energy feature observed at 1.35 eV in the photo-induced absorption spectrum of trans-polyacetylene, which is attributed to neutral states. A vertical excitation from... 

Photo-induced absorption from the l B triplet indicates another triplet, the l Aj state, at approximately 1.4 eV higher in energy, and essentially degenerate with the 2 A+ state. This triplet state is the lowest pseudomomentum branch of the family of n = 2 Mott-Wannier triplet excitons. As... 

From Fig.lOa and Fig.lla, the photo-induced absorption changing spectrum AA(/1) = A (A)-Aq(A) and the photo-induced dichroism spectrum AAd(X.) = Ai(X.)-A//(X.)= lg(T//(X)/T (X)) were obtained, which are shown as solid lines in Fig.lOb and Fig.llb respectively. Assuming that AA and AAd are zero outside of the band 300 800nm, the photo-induced refractive index changing spectrum An(A) = n (A)-iiQ X) and the photo-induced birefringence spectrum Ans(X.) =n X)-n// X) can be calculated according to the Kramers-Kronig relation PI, where jie, tic, ni i and are the refractive indexes of E-form, C-form and of the film... 

According to theoretical considerations and experimental evidence obtained from electron energy-loss spectroscopy by Fink and Leising [79], from photo-induced absorption measurements by Vardeny et al. [80] and from electron-nuclear double resonance (ENDOR) experiments by Grupp et al. [81], the value of U seems to be between 7 and 9 eV in polyacetylene. This case where neither A nor U dominates is very difficult to treat theoretically. 

CW experiments as well as time-resolved measurements down to picosecond resolution have been carried out for both PA and PC. Photo-induced absorption measurements yield information on the number of photo-excited particles (charged and neutral), whilst the photocurrent is due to the product of number of carriers and their mobility, and is sensitive only to charged excitations. Therefore comparison of the results of both types of experiment gives maximum information on photoexcitations in polyacetylene. 

Figure 1.60. Photo-induced absorption spectrum of trans-polyacetylene after illumination with 2.4 eV (at 10 K). (Reprinted with permission from ref 80)...

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