Interface Stability of Polymer and Small-Molecule Organic Photovoltaics - Organic Solar Cells: Materials and Device Physics - page 152

Fig. 6.8 (left) Electrical impedance of an inverted layer sequence device, where a TiO 2 layer
was added between the electron contact of Ti and the absorber. In the modulus-plot, a semicircle
can be observed at high bias, where built-in potential should be compensated and thus, the
depletion region being vanished; (right) Modeled I-V curve of a device with a low extraction/
injection rate for both charge carrier types at the electron contact [ 59 ]. (Reprinted from [ 59 ], with
permission from Elsevier) ( http://www.sciencedirect.com/science/journal/09270248 )
the kink of I-V curve under illumination is caused by a slow charge transfer at the
Al/organic interface, evidenced by the large capacitance for low frequencies as
shown in Fig. 6.8 (left) due to the accumulated charges injected from the good
contact at the blocking contact. Additionally, a one-dimensional model for such
I-V curves, as shown in Fig. 6.8 (right) was developed by selecting small
extraction velocities for the charge transfer at one interface (10 -4 cm/s instead of
1000 cm/s). Both experiments and simulations are in good agreement.
6.4 Improvement Approaches to Interface Stability
6.4.1 Interface Engineering
In order to improve the interface stability, one effective way is to introduce a stable
interfacial layer in between electrodes and organic photoactive layer. On the one
hand, an air-stable n-type metal oxide can be inserted between active layer and Al
cathode to replace the commonly used Ca or other LWF metals. On the other hand,
at the anode side, an alternative to current PEDOT:PSS is p-type-like or transition
metal oxides as the interfacial layer.
6.4.1.1 Cathode Buffer Layer
Wang et al. presented an extended lifetime of P3HT:PCBM-based BHJ OPVs by
inserting a thin layer of CuO x in between active layer and Al cathode, which was
fabricated by thermal evaporation [ 60 ]. Figure 6.9 shows the comparison of
various parameters as a function of storage time for the OPVs with the interfacial