Triplet Exciton Generation in Bulk-Heterojunction Solar Cells based on Endohedral Fullerenes

Organic bulk-heterojunctions (BHJ) and solar cells containing the trimetallic nitride endohedral fullerene 1-[3-(2-ethyl)hexoxy carbonyl]propyl-1-phenyl-Lu3N@C80 (Lu3N@C80-PCBEH) show an open circuit voltage (VOC) 0.3 V higher than similar devices with [6,6]-phenyl-C[61]-butyric acid methyl ester (PC61BM). To fully exploit the potential of this acceptor molecule with respect to the power conversion efficiency (PCE) of solar cells, the short circuit current (JSC) should be improved to become competitive with the state of the art solar cells. Here, we address factors influencing the JSC in blends containing the high voltage absorber Lu3N@C80-PCBEH in view of both photogeneration but also transport and extraction of charge carriers. We apply optical, charge carrier extraction, morphology, and spin-sensitive techniques. In blends containing Lu3N@C80-PCBEH, we found 2 times weaker photoluminescence quenching, remainders of interchain excitons, and, most remarkably, triplet excitons formed on the polymer chain, which were absent in the reference P3HT:PC61BM blends. We show that electron back transfer to the triplet state along with the lower exciton dissociation yield due to intramolecular charge transfer in Lu3N@C80-PCBEH are responsible for the reduced photocurrent

Optimization of morphology of P3HT/PCBM films for organic solar cells: effects of thermal treatments and spin coating solvents

We demonstrate, by means of light-induced EPR and optical spectroscopy, that thermal treatment of P3HT/PCBM films prepared from CB solution results in a decrease of the charge transfer yields. The improved performance of solar cells is therefore attributed to optimization of the morphology, in particular molecular order in the P3HT regions, which could be monitored by optical absorption and resonant Raman scattering (RRS). We further found that spin coating from slowly evaporating solvents leads directly (i.e., without thermal treatment) to optimal morphologies of the P3HT/PCBM films, with similar results for charge transfer, morphology and solar cell efficiencies

Dual crystallization behaviour of polythiophene/fullerene blends

The nanoscale morphology of the active layer in bulk-heterojunction solar cells consisting of regioregular poly(3-hexylthiophene-2, 5-diyl) (P3HT) and methanofullerene([6-6]-phenyl C61 butyric acid methyl ester) (PCBM) was extensively studied using Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and optical microscopy. Different weight ratios of P3HT:PCBM were investigated as a function of annealing temperature and time revealing the occurrence of crystallization of both components. Firstly, the as-prepared films can be described as a semi-crystalline blend. Secondly, it has been demonstrated that for a short annealing time (5 min) at lower annealing temperatures (75–115 °C) an increased crystallization of P3HT occurs. Thirdly, it has been observed that a prolonged annealing at the given temperature range or a short annealing at higher temperatures (≥ 120 °C) leads to the formation of a new ordered crystalline structure of PCBM. These new ordered structures, a few μm up to 100 μm in length, form a network of needle-like and even fan-shaped crystals. Key-parameters to “tune” this new ordered structure of PCBM are blend ratio and annealing conditions. The growth mechanism of these new PCBM-structures is described by means of diffusion

21st European Photovoltaic Solar Energy Conference, 4-8 September 2006, Dresden, Germany ORGANIC SOLAR CELL STRATEGY AND DEVELOPMENTS IN FLANDERS

ABSTRACT: IMEC started up its activity on organic solar cells in 1998 and since 2005 this topic became one of the key technologies of the Flemish Innovation Policy in the field of Photovoltaics. The aim of the paper is to present the overall Flemish approach and strategy to this field as well as to highlight the recent achievements of the Organic Photovoltaics Technology Program. The strategy of this program is built around three focal points: enhancement of the conversion efficiency, improvement of the cell stability and development of a printing technology to realize monolithic modules on a flexible foil. This strategy is being executed mainly within IMEC with its large expertise in the field of solar cell technology development and its associated lab, IMOMEC, and the University of Hasselt which has a strong background in synthesis and characterization of conjugated polymers. In addition, there is considerabl

Relation between Photoactive Layer Thickness, 3D Morphology, and Device Performance in P3HT/PCBM Bulk-Heterojunction Solar Cells

To get an efficient organic solar cell, as much light as possible should be absorbed by the photoactive layer; as a consequence, thick layers should be preferable. However, it is often observed that much thinner photoactive layers result in more efficient devices than the corresponding thicker layers absorbing more light. Besides light absorption, other aspects such as efficient exciton dissociation, charge transportation, and charge collection are of crucial importance, and all of them are strongly influenced by the volume morphology of the photoactive layer. In this study of bulk-heterojunction solar cells based on poly(3- hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM), we show that the resulting P3HT/PCBM morphology is strongly determined by the layer thickness because the kinetics of solvent evaporation and crystallization is different in films of different thickness. For the preparation conditions chosen in this study, an optimum morphological organization of the photoactive layer characterized by high crystallinity of P3HT, viz. numerous crystalline P3HT nanowires forming a genuine three-dimensional network, and enrichment of crystalline P3HT closer to the hole collecting electrode can only be achieved for relatively thin (100 nm) P3HT/PCBM layers. Corresponding devices absorb only a limited fraction of all available photons but have the highest efficiency. © 2009 American Chemical Society