At first glance, the semiconducting polymers used in organic light-emitting diodes and photovoltaics may seem to require similar properties, and yet materials which work well for one application are often unsuitable for the other. Highly photoluminescent polymers such as polyfluorenes have long exciton lifetimes which should aid charge separation in an organic solar cell, but instead tend to give poor efficiencies. Recent work by Y.W. Soon and J.R. Durrant of Imperial College London and their co-workers suggests that one cause of this discrepancy may be a fast energy transfer mechanism which competes with electron transfer to the acceptor material.
The group’s article in the online edition of Chemical Science (DOI: 10.1039/c0sc00606h) compares the photophysics of photovoltaic devices made from blends of the electron acceptor [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and either of two indenofluorene-based polymers. One of the polymers (an indeno-fluorene analogue of F8BT) shows no photovoltaic activity, while the other achieves reasonable power conversion efficiencies of up to 1.9%. The photo-luminescence spectrum of the ineffective blend shows emission from a PCBM singlet exciton which is not present for the latter superior polymer and suggests that energy transfer to the acceptor material is taking place. Given the strong photoluminescence from this polymer and its larger degree of overlap with the absorption spectrum of PCBM, there should indeed be fast Förster resonant energy transfer between the two materials.
For a photocurrent to be generated in the blend, an electron excited in the polymer has to be transferred to the PCBM and from there to an electrode. In the poorly performing poly-indenofluorene this process is slow compared to a transfer of the electron’s energy to create a PCBM exciton which, instead of generating a current, either relaxes to the ground state or crosses to a triplet state. A photocurrent could nonetheless be generated from this process if, in reverse fashion, the hole generated in the highest occupied molecular orbital (HOMO) of the PCBM were to hop to the HOMO of the polymer. For this polymer it seems that there is not enough energy remaining in the PCBM exciton to drive this process, but it is likely that hole transfer is responsible for photocurrents generated in other polymers which show strong photoluminescence toward the blue end of the spectrum
The research goes a long way toward explaining the difficulties of using intensely photoluminescent polymers in organic solar cells and provides an important clue for designing photovoltaics that absorb blue light. Polymers for this purpose would benefit from having weakly emissive excitons that do not favor the wasteful energy transfer.
An energy level diagram showing the fast energy transfer to PCBM which follows photoexcitation of IF8BT (poly[2,8-(6,6,12,12-tetraoctylindenofluorene)-co-4,7-(2,1,3-benzothiodiazole]) and competes with slower charge transfer (CT). Reproduced with permission from Chem. Sci. (2011) DOI: 10.1039/c0sc00606h. © 2011 The Royal Society of Chemistry.
