High-Performance Bulk-Heterojunction Polymer Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 171

Fig. 6 a Schematic representation of the fabrication of solar cells featuring a self-organized
bilayer (P3HT:PCBM/PEG) structure. b SEM images of the surfaces of active layers prepared at
PEG concentrations of i 0 ii 5, and iii 10 % [ 28 ]
PEG spontaneously migrated to the surface of the active layer. As a result, a
nanoscale functional interlayer was created. Scanning electron microscopy (SEM)
revealed morphological evidence supporting the formation of vertically-phase-
separated PEG molecules (Fig. 6 b). The corresponding thin film containing no
PEG exhibited a smooth surface morphology. After blending with 5 or 10 % PEG,
we clearly observed additional ''dot-like'' phases on the surfaces of the films. We
assigned this new phase to assemblies of PEG molecules. In terms of device
performance, the addition of 5 wt% PEG into the active layer improved the PCE
from 2.21 to 3.97 %. Furthermore, OPVs fabricated using this method also
exhibited superior device stability under illumination, presumably because a PEG
interface might have lower sensitivity toward moisture and oxygen from the
atmosphere. In addition, because the PEG molecules preferred to segregate to the
top of the polymer blend, interdiffusion through the contact was inhibited, forming
a thermodynamically stable interface [ 28 ].
4 Light Trapping Technologies
As we stated in the introduction to this chapter, it is difficult to improve the
absorption efficiency and charge collection efficiency simultaneously. Therefore,
many light trapping strategies have been proposed to increase the absorption
efficiency without affecting the charge collection efficiency [ 30 - 43 ]. For example,