MDMO-PPV/PCBM solar cell

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. 5.21. Top figure The experimental plots of dark and illuminated J—V characteristics of a typical MDMO-PPV/PCBM solar cell for different concentrations of PCBM. The currents are plotted on the log scale. Curves 1 are for dark currents and curves 2 are for illuminated currents. The open circles show the dark currents needed to make the output current zero at the open circuit voltage. Bottom figure The current densities are plotted on the linear scale for two PCBM concentrations. Curves 1 are for dark currents and curves 2 are for illuminated currents.

Hoppe et al. studied MDMO-PPV PCBM solar cells for decoding the different nanophases within these MDMO-PPV PCBM blends cast from the two solvents (toluene and chlorobenzene) [55]. A large difference in the scale of phase separation could be identified as a major difference between toluene and chlorobenzene cast blends (see Fig. 23), but this could not directly explain the observed differences in photocurrent generation [55]. 

FIGURE 10.13 The effect of on the photo-CELIV transients recorded for an MDMO-PPV/PCBM solar cell. 

A further improvement of MDMO-PPV based bulk heterojunctions was achieved by the application of a new C70 fullerene derivative, which was substituted with the same side chains as PCBM and is therefore called [70]PCBM [170]. Due to the reduced symmetry of C70 as compared to the football sphere (icosahedral symmetry) of Ceo. more optical transitions are allowed and thus the visible hght absorption is considerably increased for [70]PCBM. This led to an improved external quantum efficiency (EQE) of MDMO-PPV based solar cells reaching up to 66% (Fig. 30). As a result the power conversion efficiency was boosted to 3% under AM 1.5 solar simulation at 1000 W/m [170]. 

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...

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. 5.39. 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...

Brabec and coworkers have made extensive measurements of the dark and illuminated currents of MDMO-PPV/PCBM BHSCs [124,141], The experimental results for the BH-SCs from their paper [124] are shown in Fig. 5.22. There are points of inflexion in the dark current of the organic solar cell confirming that the dark current is the space charge limited currents. These features, particularly the points of inflexion, are absent in the illuminated characteristics. At larger voltages the forward dark current intersects the illuminated current and the illuminated current becomes larger than the dark current. 

Martens T., d Haen J., Munters T., Beelen Z., Goris L., Manca J., d Olieslaeger M., Vanderzande D., de Schepper L. and Andriessen R. (2003), Disclosure of the nanostructure of MDMO-PPV PCBM bulk heterojunction organic solar cells by a combination of SPM and TEM , Synthetic Metals 138, 243-247. 

Fig. 28 Solar cell parameters for MDMO-PPV PCBM polymer solar cells under slightly elevated temperatures, as expected for realistic operation conditions. Interestingly, the short circuit photocurrent increases with temperature, while the open circuit voltage drops. As a result the power conversion efficiency is maximized for temperatures of 50 (Reprinted with permission from [122], 2001, American Institute of Physics)...

Synthesis via the sulfinyl route led to a reduced number of defects on the MDMO-PPV donor polymer and showed some improved performances in MDMO-PPV PCBM bulk heterojunctions [ 167,168]. The lower defect density resulted in a more regioregular (head-to-taU) order within the MDMO-PPV, leading to charge carrier mobihty improvements and ultimately to an improved efficiency of 2.65% for MDMO-PPV PCBM based bulk heterojimc-tions [ 169]. This was accompanied by a fill factor of 71% [169], which to date has not been exceeded by any other polymer solar cell device. 

Hoppe H, Glatzel T, Niggemann M, Schwinger W, Schaeffler F, Hinsch A, Lux-Steiner MC, Sariciftci NS (2006) Efficiency limiting morphological factors of MDMO-PPV PCBM plastic solar cells. Thin Solid Films 511-512 587... 

Martens T, Beelen Z, D Haen J, Munters T, Goris L, Manca J, D Olieslaeger M, Vanderzande D, Schepper LD, Andriessen R (2003) Morphology of MDMO-PPV PCBM bulk hetero-junction organic solar cells studied by AFM, KFM and TEM. In Kafafi Z H, Fichou D (eds) Organic photovoltaics III. SPIE Proc 4801 40... 

Munters T, Martens T, Goris L, Vrindts V, Manca J, Lutsen L, Ceunick WD, Vanderzande D, Schepper LD, Gelan J, Sariciftci NS, Brabec CJ (2002) A comparison between state-of-the-art gilch and sulphinyl synthesised MDMO-PPV/PCBM bulk hetero-junction solar cells. Thin Solid Films 403-404 247... 

Significant amount of research has been dedicated to two material systems MDMO-PPV PCBM (MDMO-PPV poly(2-methoxy-5-(3, 7 -dimethyl-octyloxy)-l,4-phenylenevinylene)) and RR-P3HT PCBM, which represent the state-of-the-art polymer solar cell technology. Because there are excellent review papers on the MDMO-PPV PCBM system in the literature, in this section we will only focus on the recent improvements in RR-P3HT PCBM system, especially the approaches to optimize the active blend layer for enhancing device efficiency. 

M.T. Rispens, et al, Influence of the solvent on the crystal structure of PCBM and the efficiency of MDMO-PPV PCBM plastic solar cells. Chemical Communications, 2003 p. 2116-2118. 

Manca, J.V., et al. 2003. State-of-the-art MDMO-PPV PCBM bulk heterojunction organic solar cell Materials, nanomorphology, and electro-optical properties. Proc SPIE 4801 15. 

FIGURE 10.14 Photo-CEIIV transients recorded in MDMO-PPV/PCBM (1 4) bulk heterojunction solar cells at various delay times at room temperature. 

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