Interface Stability of Polymer and Small-Molecule Organic Photovoltaics - Organic Solar Cells: Materials and Device Physics - page 151

Fig. 6.6 a Characteristic capacitance response (100 Hz) of a device (ITO/PEDOT:PSS/
P3HT:PCBM/Al), as a function of bias voltage. Vertical lines separate voltage regions for
different capacitances. b Mott-Schottky curve (100 Hz) which exhibits a straight line from which
the values V fb = 0.49 V and N A = 3.8 9 10 15 cm -3 are determined, assuming e = 3 for
P3HT:PCBM [ 55 ]. (Reproduced by permission of the PCCP Owner Societies)
Fig. 6.7 (left) IS in the dark of the nontreated device, after the first post-treatment (30 s at 110
C and 1 V forward bias) and after the second post-treatment (450 s at 110 C and 2 V forward
bias); (right) IS after the two consecutive post-treatment steps at different bias voltages [ 58 ].
(Copyright John Wiley and Sons. Reproduced with permission)
contacts. The device performance is also likely enhanced by the large space charge
region and a positive doping gradient toward the hole contact due to the post-
treatment, where charge separation is improved and electron recombination is
reduced at the hole contact side due to possible electron transport in PEDOT:PSS
layer.
Furthermore, Glatthaar et al. demonstrated that electrical IS can be a useful tool
to identify two limiting factors in OPV efficiency by providing information about
the conductivity of different regions within the device [ 59 ]. One is that the exis-
tence of p-type impurity doping in organic semiconductor results in a Schottky
type contact at the Al interface, decreasing the charge collection efficiency and
photocurrent, as there is no electrical field in the bulk region, which facilitates the
charge separation. The other one is that a poor interface is permeating charge
carriers due to the Al corrosion, leading to low FF. The authors explained that