Despite the constant improvement of their power conversion efficiencies, organic solar cells based on an interpenetrating network of a conjugated polymer as donor and fullerene derivatives as acceptor materials still need to be improved for commercial use. In this context, we present a study on the optimization of solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) by varying a specific cell parameter, namely the concentration of the active layer components in the liquid phase before blend film deposition, in order to improve device performance and to better understand the relation between morphology and device operation. Our study shows a significant increase of the short-circuit current, open-circuit voltage and cell efficiency by properly choosing the formulation of the initial blend before film deposition. We demonstrate that the active layer morphology, which is strongly dependent on the initial material concentrations and the processing conditions, can greatly impact the electronic characteristics of the device, especially regarding charge recombination dynamics at the donor-acceptor interface. Our optimized P3HT:PCBM device exhibits both slow recombination and high photocurrent generation associated with an overall power conversion efficiency of 4.25% under 100 mW cm(-2) illumination (AM1.5G).

Carbon nanotubes (CNTs) are well-known electron acceptor materials for use in optoelectronic devices due to their appropriate electronic properties. In this work, we report on their use as a part of the multi-component active layer in organic solar cells to provide better transport of photogenerated charges. To optimize transport, orientation of CNTs perpendicular to the electrodes of solar cells with an active layer of poly(3-hexylthiophene)-CNT-(6,6-phenyl C 61 butyric acid methyl ester) (P3HT-CNT-PCBM) composite is preferable. In order to achieve this preferred orientation by applying an electric field on the device electrodes, polarization of CNT solutions was studied using dielectric spectroscopy. We find that an improved orientation can be achieved with a high-frequency AC electric field due to the alternating force exerted rapidly on field-induced dipoles of the nanotubes. Following these experiments, the orientation method was applied to organic solar cells with a sandwich structure in which the active layer consists of P3HT-CNT-PCBM. The electric field was applied continuously on the electrodes during annealing at a temperature near the glass transition of the polymer matrix. We demonstrate that a significant increase in cell performance (i.e. open-circuit voltage = 0.61 V, efficiency = 4.53%, fill factor = 0.46) can be achieved, which may signify a better path created by the oriented nanotubes for carrier transport to the electrodes.