MDMO characteristics

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)...

Fig. 5.11. Comparison of the unipolar and ambipolar transport characteristics of a p-type semiconductor (MDMO-PPV) based diode at different temperatures...

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)...

FET mobility measurements thus constitute a sound method for investigating changes in the mobility of an organic semiconductor due to morphology variations. On the basis of the FET characteristics of MDMO-PPV films spin-cast from different solvents, we will discuss the influence of interchain polymer aggregates on the hole field-effect mobility and further consequences for the short-circuiting of solar cells. 

Figure 5.25 shows the FET characteristics of a device in p-channel mode with MDMO-PPV as the channel material. Figures 5.25a and b show the situation for MDMO-PPV spin-cast from a toluene solution. The hole current... 

Fig. 5.25. (a) Ids versus Kis characteristics of a toluene-based MDMO-PPV FET with Au contacts and L = 3 pm. (c) The same for chlorobenzene-based MDMO-PPV. (b) Right hand axis, circles Ids plotted as a function of Vgs for Vis = —90 V on a logarithmic scale. Left hand axis, triangles I 2 plotted as a function of Vrgs. From the slope at high negative VgS, the field-effect mobility of toluene-based MDMO-PPV for holes is calculated to be pfe = 5 x 10 6 cm2/V s. (d) The same for chlorobenzene-based MDMO-PPV with pfe = 3 x 10-5 cm2/V s... 

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... 

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. 5.21. 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.

Fig. 5.22. Experimental plots of I-V characteristics of a typical MDMO-PPV/PCBM bulk heterojunction solar cell with a LiF/Au electrode [124],...

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. 

FIG. 6.21. (a) Output characteristics of the MDMO-PPV PCBM (1 4) based photOFET fabricated on top of a PVA gate-insulator with LiF/Al as top source and drain electrodes in the dark, (b) transfer characteristics of the device in the dark (filled square symbol curves), under AM1.5 (1 mW/cm2) illumination (open square symbol curves) and in the dark after illumination (filled triangular symbol curves) measured at Vds = +80 V. The arrows show the sweep directions [152]. 

Fig. 22 Optical transmission spectra of 100-nm-thick MDMO-PPV PCBM (1 4 by wt.) films spin cast onto glass substrates from either toluene (dashed line) or chlorobenzene (solid line) solutions (a). Incident photon to collected electron (IPCE) spectra (b) and current-voltage characteristics (c) for photovoltaic devices using these films as the active layer. (Reprinted with permission from [34], 2001, American Institute of Physics)...

Fig. 30 Photovoltaic properties of an MDMO-PPV based polymer-fullerene solar cell with an active area of 0.1 cm. a External quantum efficiency (EQE) of [70]PCBM MDMO-PPV cells, spin-coated from chlorobenzene (triangles) and ODCB (squares) and of [60]PCBM MDMO-PPV devices spin-coated from chlorobenzene (open circles) b current-voltage characteristics of [70]PCBM MDMO-PPV devices, spin-coated from ODCB in the dark (open circles) and under illumination (AM 1.5, 1000 W/m squares). The inset shows the I-V characteristics in a semilogarithmic plot. (Reproduced with permission from [170], 2003, Wiley-VCH)...

CdSe nanocrystal based solar cells were substantially improved by Sun et al a twofold increase in the EQE was achieved for MDMO-PPV based blends by application of CdSe nanotetrapods instead of nanorods [254]. The tetrapods, due to their shape, induced better directed electron transport normal to the film plane, yielding overall power conversion efficiencies of 1.8%. The current-voltage characteristics of this device are displayed in Pig. 57. 

Figure 10.10. The onset of the absorption of PTPTB is around 780 nm, which corresponds to an optical band gap of 1.6 eV. When mixed with the electron acceptor PCBM, the characteristic spectroscopic signatures of photoinduced charges are observed [74]. Spectrally resolved photocurrent measurements of a device based on PTPTB/PCBM show clearly that the spectral sensitivity is extended to longer wavelength by almost 200 nm as compared to the MDMO-PPV-PCBM mixture. The power conversion efficiency of 1% is limited by the low FF.

TABLE 1 Photoelectrical characteristics of photovoltaic fibers having different having different photoactive layers (P3HT PCBM and MDMO-PPV PCBM). 

Figure 2.18 (top) Typical l-V curves as measured for each point of the l-V spectroscopy scan (128 x 128 pixels) demonstrating the heterogeneous l-V characteristics of the MDMO-PPV matrix (bottom) for four biases the corresponding... 

Figure 71 Effect of LiF interlayer on the I-V characteristics of MDMO-PPV/[60]PCBM soiar ceiis (a), (b), and (c) are box piots with the statistics of the FF and the Voc from six separate solar cells. (Reproduced from Ref. 205. American Institute of Physics, 2009.)...

Relaxation-assisted charge separation was later shown to explain the J-V characteristics of MDMO-PPV PCBM OPVs very well using experimentally... 

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