MDMO-PPV/PCBM

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. 

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...

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...

Fig. 5.12. Temperature dependent I/V characteristics of a bulk heterojunction device (ITO/PEDOT/MDMO-PPV PCBM/LiF-Al) in the dark (top) and under illumination (bottom)...

Figure 5.13 (top) displays the frequency spectra of the measured capacitance for temperatures ranging from 20 K to 300 K for a standard cell (ITO/PEDOT/MDMO-PPV PCBM/Al). The arrow indicates increasing temperatures. One clearly observes a step which is shifted to higher frequencies as the temperature increases. In order to evaluate the position of the steps, it is better to plot wdC/dw versus w, rather than C(u>) versus w. Figure 5.13 (bottom) shows the normalised deviated frequency spectrum of the capacitance. The steps now appear as maxima within the individual curves, and the corresponding critical frequency wq can be derived more ac-... 

For bulk heteroj unction solar cells made from MDMO-PPV/PCBM, measured under metal halogenide lamps, a mismatch factor of M 0.76 has been determined, while for xenon high-pressure lamps, M rs 0.9 is used. 

Fig. 5.20. AFM images (acquired in the tapping mode) showing the surface morphology of MDMO-PPV PCBM blend films (1 4 by wt.) with a thickness of approximately 100 nm and the corresponding cross-sections, (a) Film spin-coated from a toluene solution, (b) Film spin-coated from a chlorobenzene solution. The images show the first derivative of the actual surface heights. The cross-sections of the true surface heights for the films were taken horizontally along the dashed lines...

To compare the impact of these different morphologies on photovoltaic performance, devices are fabricated in an identical manner except for the choice of solvent (either toluene or chlorobenzene) used for spin-coating the active layer (MDMO PPV PCBM, 1 4 by wt.). Characterization of the devices is performed under illumination by a solar simulator. The AM 1.5 power... 

Fig. 5.23. (a) Optical absorption spectra of 100 nm thick MDMO PPV PCBM films (1 4 by wt.) spin-coated onto glass substrates from either toluene (dash-dotted line) or chlorobenzene (solid line) solutions, (b) IPCE spectra for photovoltaic devices using these films as the active layer... 

Fig. 5.33. I/V curves for MDMO-PPV/PCBM photovoltaic devices with different metal electrodes. The inset shows the //V curves on a linear scale...

The influence of the work function of the negative electrode on the value of Voc for MDMO-PPV/PCBM solar cells is shown in Fig. 5.34b. It is important to note that the x-axis now covers more than 2 eV. Once again, a linear model is fitted to the experimental data and a slope of S2 0.1 is calculated as the best fit. This result shows that the work function of the metal has a considerably weaker effect on the Voc values than the reduction potential. 

Fig. 5.37. (a) I/V characteristics of typical MDMO-PPV/PCBM solar cells with a LiF/A1 electrode of various LiF thicknesses ( 3 A, 6 A, 12 A) compared to the performance of a MDMO-PPV/PCBM solar cell with a pristine A1 electrode ( ). (b) and (c) are box plots with the statistics of the FF and Voc from 6 separate solar cells. LiF or SiOx were thermally deposited at a rate of 1-2 A/min from a tungsten boat in a vacuum system with a base pressure of 10-4 Pa. We emphasize that, for thickness values of the order of 1 nm, LiF/SiOx does not form a continuous, fully covering layer, but instead consists of island clusters on the surface of the photoactive layer. Slow evaporation conditions are essential for more homogenous distribution of the LiF on the organic surface. The nominal thickness values given here represent an average value across the surface of the substrate. The metal electrode (either aluminum or gold) was thermally deposited with a thickness of 80 nm... 

Fig. 5.39. I/V plot of a typical MDMO PPV/PCBM bulk heterojunction solar cell with a Au electrode (continuous line) and an LiF/Au electrode (dotted line), respectively, in the dark and under illumination...

Table 5.3. Solar cell characteristics (PF and Voc) of MDMO-PPV/PCBM bulk heterojunction devices for various interfacial layers (LiF, SiO ) with different thicknesses compared to a solar cell with a pristine A1 electrode, and also calculated diode characteristics Rs and Rp found using (5.39) for the various interfacial layers...

Fig. 5.40. Photon density and integrated photon density of the AM 1.5 spectrum compared to the absorption spectrum (arbitrary units) of a MDMO-PPV/PCBM bulk heterojunction composite...

Fig. 5.42. I/V curves under AM 1.5 conditions top) and in the dark (bottom) for a device with PTPTB/PCBM 1/3, PTPTB/PCBM 1/3 +10% Nile red, and PTPTB/MDMO-PPV/PCBM 0.5/0.5/4, as indicated...

Fig. 5.56. (a) Isc of various PS/MDMO PPV/PCBM devices under different excitation intensities vs. PS percentage. The inset shows the dependence of Isc on the electroactive component concentrations (100% PS wt. %) in a log-log plot. Lines are power law fits according to 7SC [wt. %] . Best fits are obtained with a 3. (b) Power efficiency r/efr of various PS/MDMO-PPV/PCBM cells vs. excitation intensity. Lines are drawn as a guide to the eye. Excitation is provided by Ar+ laser at 488 nm... 

FIG. 5.17. TEM images of MDMO-PPV/PCBM blends with different weight percentages of PCBM as indicated on the upper right corner [136]. 

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