Conjugated polymer thin films sandwiched between two metal electrode are usually described using a metal-insulator-metal (MIM) picture (Parker, 1994). The different operating regimes the MIM device due to externally applied voltages is shown in Figure 3. As illustrated in Figure 3(a), the vacuum levels (Evac) of the stacked materials shall align themselves (Shottky-Mott model).
Figure 3(a) indicates the energy diagram of a bulk heterojunction solar cell in open circuit condition. The Evac of the different materials are aligned as explained above, and no electrical field is present within the device. Figure 3 (b) represents the short circuit condition. The Fermi levels of the two electrodes align themselves and a built-in field appears in the bulk, resulting in a constant slope for the HOMO and LUMO levels of the donor and acceptor (respectively, HD, LD, HA, and LA) and for the Evac.
Fig. 3. MIM picture for a polymer diode under different operating modes. (a) open circuit condition, (b) short circuit condition, (c) forward bias, (d) reverse bias.
When polarized in the forward direction (high work function electrode (ITO) connected to (+) and low work function electrode (Al) connected to (-)) as in Figure 3 (c), electrons can be injected from the Al electrode to ITO electrode and holes from ITO electrode to Al electrode. The effective field in the device will ensure the drift of electrons from Al electrode to ITO electrode and hole from ITO electrode to Al electrode. Finally, when the device is polarized in the reverse direction (ITO connected to (-) and Al connected to (+)) (Figure 3 (d), charge injection is hindered by the field present in the device (Dennler & Sariciftci, 2005).
Solar cells are operated between open circuit and short circuit condition (fourth quadrant in the current-voltage characteristics), as shown in Figure 4. In the dark, there is almost no current flowing, until the contacts start to inject heavily at forward bias for voltages larger than the open circuit voltage. Under illumination, the current flows in the opposite direction than the injected currents. The overall efficiency of a solar cell can be expressed by the following formula:
where Voc is the open circuit voltage, I sc is the short circuit current, and Pin is the incident light power. The fill factor ( FF ) is given by
Ff = mpp mpp (2)
~ V I y
where Impp and Vmpp represent the current and voltage at the maximum power point (Pmlx) in the fourth quadrant, respectively (Figure 4).