Abstract
Blends of semiconducting nanocrystals and conjugated polymers continue to attract major research interest because of their potential applications in optoelectronic devices, such as solar cells, photodetectors and light-emitting diodes. In this study we investigate the surface structure, morphological and optoelectronic properties of multilayer films constructed from ZnO nanocrystals (NCs) and poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV). The effects of layer number and ZnO concentration (CZnO) used on the multilayer film properties are investigated. An optimised solvent blend enabled well-controlled layers to be sequentially spin coated and the construction of multilayer films containing six ZnO NC (Z) and MDMO-PPV (M) layers (denoted as (ZM)6). Contact angle data showed a strong dependence on CZnO and indicated distinct differences in the coverage of MDMO-PPV by the ZnO NCs. UV-visible spectroscopy showed that the MDMO-PPV absorption increased linearly with the number of layers in the films and demonstrates highly tuneable light absorption. Photoluminescence spectra showed reversible quenching as well as a surprising red-shift of the MDMO-PPV emission peak. Solar cells were constructed to probe vertical photo-generated charge transport. The measurements showed that (ZM)6 devices prepared using CZnO = 14.0 mg mL-1 had a remarkably high open circuit voltage of ∼800 mV. The device power conversion efficiency was similar to that of a control bilayer device prepared using a much thicker MDMO-PPV layer. The results of this study provide insight into the structure-optoelectronic property relationships of new semiconducting multilayer films which should also apply to other semiconducting NC/polymer combinations.
Highlights
Blends of semiconducting nanocrystals and conjugated polymers continue to attract major research interest because of their potential applications in optoelectronic devices, such as solar cells, photodetectors and light-emitting diodes
We employed solar cell measurements to study the effect of ZnO NC layer thickness on vertical photo-generated charge transport via the short-circuit current density
TEM images obtained for the ZnO NCs (see Fig. S1(a), Electronic supplementary information (ESI)†) gave a number-average diameter of 4.1 Æ 0.4 nm
Summary
Blends of semiconducting nanocrystals and conjugated polymers continue to attract major research interest because of their potential applications in optoelectronic devices, such as solar cells, photodetectors and light-emitting diodes. MDMOPPV was used at the donor in this study because earlier studies showed the photoluminescence behaviour for ZnO NC/MDMOPPV films to be well controlled by the proportion of each species.[29] Our multilayer films consisted of one type of polymer donor and NC acceptor with a repeating bilayer (heterojunction) structure This design simplified the study and enabled a clearer understanding of the structure–optoelectronic properties to be discerned. Solar cell data are considered and the effect of ZnO film layer thickness studied The latter data provide evidence for vertical charge transport pathways through the multilayer films. The sequential spin coating method established here for ZnO NC/MDMO-PPV multilayer films provides optoelectronic properties that are tuneable using the number of layers and NC concentration. Our approach and the results obtained should be applicable to a range of other semiconducting NC/conjugated polymer films or devices
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