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

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We demonstrate the use of a simple charge extraction measurement to determine the charge carrier densities n in annealed poly(3-hexylthiophene):methanofullerene solar cells under operating conditions. By applying charge extraction to the device under forward bias in the dark (Jdark), we find Jdark∝n2.6. This dependence on charge density is the same as that we find for bimolecular recombination losses observed in such devices under irradiation at open circuit, suggesting that the dark current originates from bimolecular recombination at the polymer/fullerene interface.

Quantum dot photovoltaics (QDPV) offer the potential for low-cost solar cells. To develop strategies for continued improvement in QDPVs, a better understanding of the factors that limit their performance is essential. Here, we study carrier recombination processes that limit the power conversion efficiency of PbS QDPVs. We demonstrate the presence of radiative sub-bandgap states and sub-bandgap state filling in operating devices by using photoluminescence (PL) and electroluminescence (EL) spectroscopy. These sub-bandgap states are most likely the origin of the high open-circuit-voltage (VOC) deficit and relatively limited carrier collection that have thus far been observed in QDPVs. Combining these results with our perspectives on recent progress in QDPV, we conclude that eliminating sub-bandgap states in PbS QD films has the potential to show a greater gain than may be attainable by optimization of interfaces between QDs and other materials. We suggest possible future directions that could guide the design of high-performance QDPVs.

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.