Controlling the Morphology of Nanocrystal–Polymer Composites for Solar Cells

Abstract

We have shown recently that the use of high aspect ratio inorganic nanorods in conjunction with conjugated polymers is a route to obtaining efficient solar cells processed from solution. Here, we demonstrate that the use of binary solvent mixtures in which one of the components is a ligand for the nanocrystals is effective in controlling the dispersion of nanocrystals in a polymer.By varying the concentration of the solvent mixture, phase separation between the nanocrystal and polymer could be tuned from a micron scale down to nanometer scale. In addition, we can achieve nanocrystal surfaces that are free of surfactant through the use of weak binding ligands, which can be removed through heating. Combined, the control of film morphology together with surfactant removal result in nanorod-polymer blend photovoltaic cells with high external quantum efficiency of 59% under 0.1 mW/cm 2 illumination at 450 nm.

Absence of Photoinduced Charge Transfer in Blends of PbSe Quantum Dots and Conjugated Polymers

“…Figure 4 shows TEM images of both PbSe/ MDMO-PPV (Figure 4A,B) and PbSe/P3HT (Figure 4C,D) films spin-coated from the same solutions used to produce the films for the PIA data in Figure 3. Similar to previous reports of CdSe/polymer blends, 5,40,56 the PbSe nanoparticles show some aggregation, but a high loading is achieved, and there is extensive polymer/PbSe interfacial area to support photoinduced charge transfer. To explain the almost complete lack of PL quenching or long-lived polaronic PIA signals in the P3HT blends solely on the basis of morphology, most of the polymer volume would need to be contained in polymer domains that are free of any PbSe and are several tens of nanometers across.…”

“…While some unmixed void areas are evident in Figures 4AϪD, a far larger percentage of the film appears to contain PbSe quantum dots mixed on the 5Ϫ10 nm scale comparable to the reported exciton diffusion lengths for both polymers. Since these TEM micrographs appear similar to those reported for CdSe/polymer blends, 5,40,56 which do show significant PL quenching PIA signals, we conclude that morphology alone cannot be the primary cause of the lack of spectroscopic signatures for charge transfer in these PbSe/polymer blends.…”

Study of the influence of the Cd:Se precursor ratio during the synthesis of CdSe nanocrystals on the performance of CdSe/P3HT hybrid solar cells

“…The roughness peaks up to 30–40 nm at an average active layer thickness of approximately 100 nm. The roughness is very high as compared to purely organic P3HT:PCBM solar cells 33–35, but comparable to results obtained for CdSe/P3HT by other groups 36, 37. The increased surface roughness of blended thin films at high nanoparticle loads is probably caused by the tendency of nc‐CdSe to agglomerate at higher concentrations 38.…”

Photovoltaic Devices Using Blends of Branched CdSe Nanoparticles and Conjugated Polymers

“…Incomplete replacement led to poor solubility of the nanoparticles in the chloroform/pyridine mixture, producing rough surfaces and reduced device efficiencies. The dependence of the efficiency on processing conditions has been studied in detail by Huynh et al for polymer/ nanorod composites; 14 we also find that efficiency is sensitive to the solvent mixture used. However, we find that the tetrapods give consistently higher efficiencies than rods under the same processing conditions.…”