MDMO-PPV,

Figure 15-16. Schematic flat band diagram of a MDMO-PPV/Cfcu system (a) and under short circuit conditions (b).

Figure 15-33. Time dependence of specific absorption bands. MDMO-PPV (1506 cm1) (downward full triangles ), 1 3 mixture of MDMO-PPV/C, at the MDMO-PPV band at 1506 cm-1 (downward open triangles).

Figure 15-4 shows the intensity of the photoluminescence as a function of the fullercnc concentration in MDMO-PPV/PCBM composites. The strong quenching... 

Figure 15-9. (a) IJglil-induccd electron spin resonance spectra of MDMO-PPV/PCBM upon successive illumination with 2.41 eV argon ion laser, (b) Integrated LESR intensity [ESR (illuminatcd)-ESR (dark)] of MDMO-PPV/PCBM (reproduced after Ref. 1401). 

Figure 15-34. t/V curves of Al/PVK-MDMO-PPV-PCBM/1TO photocells. The concentration of the conventional polymer PVK in the blend is denoted in the ligurc. Tlie devices were illuminated through the ITO side by 41) iiiWA.nr at 4Kb inn. 

An Austrian team boosts the performance of plastic cells by mixing a conducting polymer, MDMO-PPV (an asymmetrically substituted polyphenylenevinylene),with a molecule made from carbon fullerene. .. 

In accord with the work of Rau [193], the EQEpv and EL data collected over a given range of temperatures and PV illumination intensities for PCBM blends with MDMO-PPV, P3HT, and APF03 donor polymers were then used, for each blend. 

Fig. 16 Parameters for defining the charge-transfer state energy cx in organic solar cells. Charge-transfer state energy for MDMO-PPV PCBM blend device determined by Fourier transform photocurrent spectroscopy and electroluminescence measurements. Reprinted figure with permission from [188]. Copyright 2010 by the American Physical Society...

Thin films of simple Er111 complexes with aza aromatic ligands such as 2,2 -bipyridine (bpy), 1,10-phenanthroline (phen) or 2-dithienyl-2,2/-bipyridazine (dithi, fig. 115) doped into polymethylmethacrylate (PMMA), orpolyl[2-mcthoxy-5-(3, 7 -dimethyl-octylo.xy)]-/ -phenylene vinylene (MDMO-PPV) have been fabricated. The dithi complex is the most promising, with the strong luminescence of the MDMO-PPV matrix quenched by a factor 100 upon doping (Koppe et al., 2001). No other quantitative data are however reported. 

EDOT) coated CdSe nanorods as a function of the quantity of poly [2-methoxy-5-(3, 7 -dimethy locty loxy)-1,4-phenylenevinylene] (MDMO-PPV) added (solvent CHC13).151 (Reprinted with permission from D. Aldakov et al., Eur. Phys. J. Appl. Phys. 2006, 36, 261-265. Copyright the European Physical Journal.)... 

Fig. 1.16. PIA spectrum of MDMO-PPV (a) with and (b) without fullerenes. Spectra are taken at T = 100 K and with a time resolution of 8 ms...

In order to learn about the true quantum efficiency of photogeneration one therefore has to study the photoinduced charge generation mechanism at faster time scales. Pump probe spectroscopy utilising a few optical-cycle laser pulses (5-6 fs) in the visible spectral range with broadband frequency conversion techniques [89] now makes it possible to study extremely fast optically-initiated events with unprecedented time resolution. Such a setup was used to time-resolve the kinetics of the charge transfer process from a polymer chain to a fullerene moiety in thin films of poly[2-methoxy, 5-(3, 7 -dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) and [6,6]-phenyl C6i butyric acid methyl ester (PCBM). Solutions prepared from 1 wt% solutions of toluene on thin quartz substrates were studied. 

The excited state pattern of a conjugated polymer/fullerene composite is shown in Fig. 1.16. First, the dynamics of pure MDMO PPV excited by a sub-10-fs pulse is compared with the dynamics of MDMO PPV/PCBM... 

Fig. 1.18. Spectrally resolved pump-probe spectrum of pristine MDMO-PPV compared to highly fullerene-loaded MDMO-PPV/PCBM composites at various delay times, (a) Absorption spectrum of a pure MDMO-PPV film (solid line) and AT/T spectrum at 200 fs pump-probe delay (dashed line), (b) AT/T spectra of the MDMO-PPV/PCBM blend (1 3 wt. ratio) at various time delays following resonant photoexcitation by a sub-10-fs optical pulse. The CW PA of the blend ( ) was measured at 80 K and 10-5 mbar. Excitation was provided by the 488 nm line of an argon ion laser, chopped at 273 Hz...

By adding PCBM to the polymer matrix, the excited state evolution scenario changes dramatically. Figure 1.18b shows a sequence of A T/T spectra for MDMO-PPV/PCBM composites excited by a sub-10-fs pulse. At early time delays (see the 15 fs and 33 fs data) the spectrum closely resembles that of pure MDMO-PPV, confirming the predominant excitation of this molecule. The SE band from MDMO-PPV rapidly gives way to a photoinduced absorption (PA) feature, the formation of which is completed within... 

Fig. 1.19. Quenching of the coherent vibrational oscillations of MDMO-PPV upon photoinduced charge transfer. The AT/T dynamics for pure MDMO-PPV (continuous line) and for MDMO-PPV/PCBM (1 3 wt. ratio) (dashed line), excited by a sub-10-fs pulse, was recorded at the probe wavelength of 610 nm. The inset shows the Fourier transform of the oscillatory component of the MDMO-PPV signal, the nonresonant Raman spectrum of MDMO-PPV (excitation 1064 nm) and the resonant Raman spectrum of an MDMO-PPV/PCBM sample (excitation 457 nm). For the resonant Raman spectrum of MDMO-PPV, it was necessary to quench the strong background luminescence by adding PCBM...

Experiments carried out on various blends with MDMO-PPV PCBM weight ratios ranging from 1 3 to 1 0.5 all displayed the same ultrafast electron transfer process, with a dynamics which was found to be almost independent of concentration. For much lower PCBM concentrations (weight ratios lower than 1 0.05), the formation time of the PA band increases to a few ps and the formation rate becomes a linear function of PCBM concentration. This indicates that, as previously observed [94], at low acceptor concentrations we enter a new regime in which the charge transfer process is mediated by disorder-induced diffusion of the excitations, which migrate until they reach a site favourable for charge transfer. 

Fig. 1.20. Time resolution of photoinduced charge transfer in MDMO-PPV/PCBM composites. AT/T dynamics for pure MDMO-PPV (continuous line) and MDMO-PPV/PCBM ( ) at probe wavelengths of 580 nm and 700 nm. Dotted lines are single exponential fits to the PA of the composites...

Fig. 1.21. (a) Light-induced ESR intensity as a function of the 3-factor in an MDMO-PPV/PCBM blend, = 9.5 GHz, T = 100 K, Aexc = 488 nm, P = 20 lW, 20 mW, and 200 mW. (b) A doubly integrated LESR signal of the prompt contribution as a function of the excitation power dependence. Squares correspond to the positive polaron signal and circles to Cg0... 

Fig. 1.22. High-field LESR in an MDMO-PPV/PCBM blend, = 95 Hz, T = 100 K, Aexc = 448 nm, = 10 mW...

Fig. 5.3. Formation of a bulk heterojunction and subsequent photoinduced electron transfer inside such a composite formed from the interpenetrating donor/acceptor network, plotted with the device structure for such a junction (a). The diagrams showing energy levels of an MDMO-PPV/PCBM system for flat band conditions (b) and under short-circuit conditions (c) do not take into account possible interfacial layers at the metal/semiconductor interface...

By exchanging one of the electrodes, such a diode can be altered from a unipolar hole device into an ambipolar device. Figure 5.10 shows the I/V characteristics of an ITO/PEDOT/MDMO-PPV/LiF-Al device. Here, the LiF-Al electrode should guarantee electron injection under forward bias. 

Fig. 5.10. Temperature dependent I/V characteristics of a p-type diode (ITO/ PEDOT/MDMO-PPV/LiF-Al), in which the different work functions of the electrodes guarantee ambipolar charge injection (electrons at the LiF-Al electrode, holes at the ITO/PEDOT electrode)...

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