MDMO-PPV diode

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)...

Figure 5.11 summarizes the temperature dependent transport behavior of unipolar and ambipolar diodes based on MDMO-PPV. Below 190 K, the hole-controlled device (ITO/PEDOT and Au contact) and the ambipolar device (ITO/PEDOT and LiF-Al contact) behave identically. Trap-free SCLC transport is observed and the mobility at this temperature is estimated to be around 10-8 cm2/Vs. For the ambipolar device, a diode-like turn-on is... 

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

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...

In order to understand the performance of the tandem device, low temperature transport studies are a valuable tool. Diodes made from pristine MDMO-PPV and in composites with PTPTB are compared. ITO/PEDOT and Au electrodes are chosen to guarantee hole-only devices. This special choice of the electrodes is a successful technique for improving our understanding of transport failures. The proper choice of contacts allows us to produce p-type or n-type diodes from the same semiconductor, depending on the selectivity of the contact. For instance, Au is a hole-injection contact for most of the polymeric semiconductors, while Ca is an electron-injection... 

Fig. 27 Compositional dependence of electron and hole mobilities in MDMO-PPV PCBM blend films as obtained by space charge limited diode currents. Clearly the mobility for holes is increased upon addition of the fullerene. (Reproduced from [144] with permission, 2005, Wiley-VCH)...

Figure 19.9 Electroluminescence spectra of ITO/PEDOT/active layer/LiE/Ca devices with 93 (PTPTB) and MDMO-PPV (PPV) as an active layer. Reprinted with permission from C. J. Brabec, C. Winder, N. S. Sariciftci, J. C. Hummelen, A. Dhanabalan, P. A. van Hal, R. A. J. Janssen, A low-bandgap semiconducting polymer for photovoltaic devices and infrared emitting diodes, Adv. Eunct. Mater., 12, 709-712 (2002). Copyright 2002 Wiley-VCH Verlag GmbH Co. KCaA.

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