LiF/Al electrode

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)...

Although LiF/Al electrodes are already widely used for enhancing the efficiency of electron injection electrodes for OLEDs, the underlying mechanisms are worth discussing. Several mechanisms can be suggested ... 

A significant increase in the forward current and in the FF is observed for conjugated polymer/fullerene bulk heterojunction solar cells upon insertion of a thin layer of LiF between the organic layer and the Al electrode (negative electrode of the solar cell), as shown in Fig. 5.37a and b. 

Once again, a box plot diagram is chosen to present the results from current/voltage (I/V) measurements for the FF (Fig. 5.37b) and Voc (Fig. 5.37c). At least 6 different devices were evaluated for each LiF thickness, the latter being varied between 0 A and 15 A. Upon insertion of only 3 A of LiF, the FF already increased by about 20% compared to otherwise identical reference devices with a pristine Al electrode. Together with an Isc of 5.25 mA/cm2 and a Uoc of 825 mV, the white light power conversion efficiency under 800 W/m2 at 50°C is calculated to be 3.3%. (Note that this is a white light efficiency which is not corrected by a spectral mismatch factor M.)... 

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]. 

Based on implemented pattern, the sunbeams entered into the photoactive layer with 4-10 mm active area by passing through a 10 nm of lithium fluoride/aluminum (LiF/Al) layer as semi-transparent cathode outer electrode. 

Figure 1.9.2 (a,b) Cyclic voltammograms in eutectic LiF-LiCImelt at 530°C. Electrode materials are aluminum, copper, tungsten, and aluminum-copper alloy (Al/Cu = 95/5, in mass). Scan rate 200mVs. Starting potentials Al electrode -1.55 V, Cu electrode -0.50 V, W electrode 0 V, and Al-Cu alloy electrode —1.57... 

Hoshino S, Nagamatsu S, Chikamatsu M, Misaki M, Yoshida Y, Tanigaki N, Yase K (2002) Device performance of an n-channel organic thin-film transistor with LiF/Al bilayer source and drain electrodes. Jpn J Appl Phys 41 808-810... 

L.S. Hung, C.W. Tang, and M.G. Mason, Enhanced electron injection in organic electroluminescence devices using an Al/LiF electrode, Appl. Phys. Lett., 70 152-154 (1997). 

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