We use transient photovoltage and differential charging experiments, complemented by transient absorption data, to determine charge carrier lifetimes and densities in a poly(3-hexylthiophene): methanofullerene solar cell at Voc as a function of white light-bias intensity. For a typical device, the charge carrier decay dynamics are observed to exhibit an approximately third order dependence on charge density (dn∕dt∝n3).

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Solution processed organic solar cells based on blends of semiconducting polymers and soluble fullerene derivatives are showing impressive advances in photovoltaic power conversion efficiency, with recent reports of efficiencies in excess of 6%.[1]One of the key remaining factors limiting the performance of such blend or 'bulk heterojunction' solar cells is that they generally exhibit relatively modest voltage outputs, with the energy corresponding to the open circuit voltage, V OC , typically being less than half the optical gap. This V OC has been shown to be correlated to the energy levels of the donor and acceptor materials of the bulk heterojunction (BHJ). [2] In this paper, we compare the V OC of BHJ fabricated from four

The function of organic solar cells is based upon charge photogeneration at donor/acceptor heterojunctions. In this paper, the origin of the improvement in short circuit current of poly(3‐hexylthiophene)/6,6‐phenyl C61‐butyric acid methyl ester (P3HT/PCBM) solar cells with thermal annealing is examined. Transient absorption spectroscopy is employed to demonstrate that thermal annealing results in an approximate two‐fold increase in the yield of dissociated charges. The enhanced charge generation is correlated with a decrease in P3HT's ionization potential upon thermal annealing. These observations are in excellent quantitative agreement with a model in which efficient dissociation of the bound radical pair into free charges is dependent upon the bound radical state being thermally hot when initially generated, enabling it to overcome its coulombic binding energy. These observations provide strong evidence that the lowest unoccupied molecular orbital (LUMO) level offset of annealed P3HT/PCBM blends may be only just sufficient to drive efficient charge generation in polythiophene‐based solar cells. This has important implications for current strategies to optimize organic photovoltaic device performance based upon the development of smaller optical bandgap polymers.

The time‐of‐flight method has been used to study the effect of P3HT molecular weight (Mn = 13–121 kDa) on charge mobility in pristine and PCBM blend films using highly regioregular P3HT. Hole mobility was observed to remain constant at 10−4 cm2V−1s−1 as molecular weight was increased from 13–18 kDa, but then decreased by one order of magnitude as molecular weight was further increased from 34–121 kDa. The decrease in charge mobility observed in blend films is accompanied by a change in surface morphology, and leads to a decrease in the performance of photovoltaic devices made from these blend films.

In this paper, we employ transient photovoltage, transient photocurrent, charge extraction, and transient absorption measurements to analyze the current/voltage response of bulk heterojunction solar cells employing a poly(3hexylselenophene) (P3HS)/[6,6]-phenyl C 61 butyric acid methyl ester (PC 61 BM) blend photoactive layer. These techniques are employed to determine the charge carrier densities and lifetimes observed in devices held at open circuit as a function of light intensity. Excellent agreement is obtained between charge densities and lifetimes determined by the different techniques, supporting the validity of these analyses. These analyses are employed to calculate the nongeminate recombination flux at open circuit as a function of light intensity, and therefore open circuit voltage. This nongeminate recombination flux is found to be approximately equal and opposite to the short circuit current density measured at the same light intensity, indicating that the dominating charge carrier loss pathway determining the device open circuit voltage is nongeminate recombination. This analysis is extended across the device current/voltage curve by using charge extraction to determine the average charge density in the device as a function of applied light intensity and bias voltage. Using this analysis, and assuming that the nongeminate recombination flux depends only upon this average charge density, we demonstrate that we are able to obtain a reasonable reproduction of the device current/voltage behavior both in the dark and for light intensities up to ∼1 sun without the use of any fitting parameters. We thus conclude that a simple device model based upon a light intensity dependent charge photogeneration term and a charge density dependent nongeminate recombination flux is capable of describing the dominating factors determining the fill factor and open circuit voltage of these devices.

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Charge photogeneration yields and energetics are determined for a low band gap co-polymer, PCPDTBT blended with PC(70)BM; the increase in charge photogeneration with dithiol is correlated with an increase in the free energy of charge separation.