Relating the morphology of a poly(p-phenylene vinylene)/methanofullerene blend to bulk heterojunction solar cell performance

Abstract

Few techniques are available where the morphology of blends can be determined in all three dimensions on a nanometer scale. Here we describe the use of a combination of techniques to resolve the morphology both laterally and vertically in spin cast films of a poly(p-phenylene vinylene)/methanofullerene blend. These blends are used as photoactive films in plastic bulk heterojunction solar cells where the performance strongly depends on the molecular morphology. In addition to depth profiling by dynamic time-of-flight secondary ion mass spectrometry (TOF-SIMS), tapping-mode atomic force microscopy (TM-AFM), transmission electron microscopy (TEM), absorption and emission spectroscopy and time-correlated single photon counting fluorescence measurements were used to investigate the morphology of ~100 nm thick spin cast composites with varying composition of 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) and poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV). Combining all results showed clustering of PCBM into rather pure separate domains in a matrix of ~50 wt% MDMO-PPV for composites with PCBM concentrations 80-100 wt%, and no spontaneous stratification. Electrical characterization, under illumination and in the dark, of the composite devices revealed the percolation threshold for electron transport and showed a reduction in recombination-limitation for the photocurrent of these solar cells as a result of an improved charge transport and collection efficiency with increasing fullerene concentration.

Tuning organic magnetoresistance in polymer-fullerene blends by controlling spin reaction pathways

“…3d. In both figures, we observe very pronounced trends, which can be correlated with different morphology regimes of the polymer-PCBM blend 27 , schematically represented in Fig. 3c.…”

“…When increasing x beyond 70%, regime (3), phase separation occurs, creating separate regions dominated by singlecarrier electron and hole currents. The exact concentration at which this happens for our blends is known from the literature 27 , but has also been verified by means of atomic force microscopy (see Supplementary Fig. S3).…”

“…At low concentration, the fullerene effectively quenches excitons into weakly bound, spatially separated charge-transfer (CT) states (CTSs), thereby reducing the exciton densities 25,26 . At higher concentration, phase separation additionally leads to separate electron and hole current pathways through the device 25,27,28 . Following earlier suggestions by Wang et al 8 on MFEs in polymer-fullerene blends, we succeeded in fully correlating pronounced changes in the MFEs to the complementary (spin) physics in the different concentration regimes.…”

Investigation of PCBM Concentration on the Performance of Small Organic Solar Cell

“…The variation of V OC in Figure 6 maybe does not attribute to variation of PCBM concentration in the mixture of BHJ layer. Studies on MDMO-PPV:PCBM devices showed that the V OC was composition independent for PCBM contents above about 10% by weight [21]. For organic BHJ materials, the general perception is that V OC is determined by the energy difference between the HOMO of the donor and the LUMO of the acceptor [15].…”

“…A BHJ is usually obtained by relying on the phase separation (on the nanometer scale) of donor and acceptor materials in a mixed layer, where insufficient phase separation often compromises the device performance owing to carrier recombination and poor charge transport [21]. In general, and as a result of phase separation on the nanometer scale occurring in the active layer blend, the donor/acceptor interfaces (photoactive p-n heterojunction sites) spread within the entire volume of the BHJ composite layer.…”