Amy M. Ballantyne

Recombination Dynamics as a Key Determinant of Open Circuit Voltage in Organic Bulk Heterojunction Solar Cells: A Comparison of Four Different Donor Polymers

[*] Prof. J. R. Durrant, Mr. A. Maurano, Dr. R. Hamilton, Dr. C. G. Shuttle, Dr. B. O’Regan, Dr. W. Zhang, Prof. I. McCulloch Departments of Chemistry, Imperial College London South Kensington SW7 2AZ (United Kingdom) E-mail: j.durrant@imperial.ac.uk Dr. A. M. Ballantyne, Prof. J. Nelson Departments of Physics, Imperial College London South Kensington SW7 2AZ (United Kingdom) Dr. H. Azimi, Dr. M. Morana, Prof. C. J. Brabec Konarka Austria, Altenbergerstrasse 69 A-4040 Linz (Austria) Dr. H. Azimi Christian Doppler Laboratory for Surface Optics Johannes Kepler University Linz (Austria)

Analysis of charge photogeneration as a key determinant of photocurrent density in polymer: fullerene solar cells.

Signifi cant progress has been made in relating the voltage output of organic solar cells to materials’ properties, specifi cally to the energy difference between the donor ionisation potential and acceptor electron affi nity. [ 1–3 ] However, progress in predicting device photocurrent densities on the basis of materials or fi lm properties has proved much more problematic. Signifi cant attention has focused upon enhancing light-harvesting effi ciency by reducing the optical bandgap of the photoactive layer, as discussed in recent reviews. [ 4–6 ] Most models of device effi ciency have typically assumed a unity yield for exciton dissociation into separated charges, requiring only that the donor/acceptor LUMO level offset is greater than 0.3 eV (corresponding to the assumed exciton binding energy). In practice, these models have proved rather poor in predicting the photocurrent densities of real devices, even after processing optimization. [ 7 ] Whilst some materials (e.g. P3HT:PCBM) have indeed achieved photocurrent densities consistent with near unity internal quantum effi ciencies for photocurrent generation, most new materials (with some notable exceptions) evaluated for their performance in organic photovoltaic devices have yielded much lower photocurrent densities, and consequently poor device performance. [ 6 , 7 ] In this paper, we consider the extent to which such variations in photocurrent density can be largely understood in terms of the effi ciency of charge photogeneration. The key processes involved in charge photogeneration in organic bulk heterojunciton solar cells are illustrated in Figure 1 . By ‘charge photogeneration’ we refer to the overall process by which photon absorption leads to the generation of dissociated

The effect of applied magnetic field on photocurrent generation in poly-3-hexylthiophene:[6,6]-phenyl C61-butyric acid methyl ester photovoltaic devices

The effect of a magnetic field on the photocurrent generated by a bulk heterojunction solar cell made from poly-3-hexylthiophene (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is investigated. At the operating voltage, increases in photocurrent of ~9% can be obtained at magnetic fields of less than 100 mT. This increase in photocurrent is attributed to an increase in the rate of intersystem crossing, between the singlet and triplet states, leading to a higher net efficiency of exciton dissociation. Close to the open-circuit voltage, an increase of more than two orders of magnitude in the photocurrent could be obtained under applied magnetic field.

TOF mobility measurements in pristine films of P3HT: control of hole injection and influence of film thickness

Time-of-flight (TOF) photocurrent measurements have been used to study charge transport in films of regioregular poly(3-hexylthiophene) (P3HT). Devices in which the P3HT film had been deposited directly onto an indium tin oxide (ITO) electrode produced high dark currents as a result of hole injection into P3HT from ITO. Photocurrent transients in such devices were disperse. It was found however, that these dark currents could be significantly reduced by inserting a dense TiO2 layer between the ITO and the polymer film. The resulting devices gave non-dispersive transients with hole and electron mobilities in the range of 1 - 2 10-4 cm2 V-1 s-1 at room temperature. The mobility values were observed to be almost independent of film thickness over the range of 350 nm to 4.3 μm. Temperature dependence studies showed a weak dependence on temperature with a low energetic disorder parameter according to analysis using the Gaussian Disorder Model (GDM) of 71 meV.