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The light intensity dependence of the main photoelectrical parameters of the nonfullerene small-molecule bulk heterojunction (BHJ) solar cells p-DTS(FBTTh2)2:perylene diimide (T1:PDI) shows that the nongeminate recombination losses play an important role in this system. A simple approach for the quantitative analysis of capacitance spectroscopy data of the organic BHJ solar cells, which allows to determine the density of free charge carriers as a function of applied bias under standard working conditions, is demonstrated. Using the proposed capacitance spectroscopic technique, the nongeminate recombination losses in the T1:PDI solar cells are quantitatively characterized in the scope of the bimolecular- and trap-assisted recombination mechanisms. Their contributions are separately analyzed within a wide range of the applied bias.

The field of organic photovoltaics has recently produced highly efficient single-junction cells with power conversion efficiency >10%, yet the open-circuit voltage (VOC) remains relatively low in many high performing systems. An accurate picture of the density of states (DOS) in working solar cells is crucial to understanding the sources of voltage loss, but remains difficult to obtain experimentally. Here, the tail of the DOS is characterized in a number of small molecule bulk heterojunction solar cells from the charge density dependence of VOC, and is directly compared to the disorder present within donor and acceptor components as measured by Kelvin probe. Using these DOS distributions, the total energy loss relative to the charge transfer state energy (ECT)—ranging from ≈0.5 to 0.7 eV—is divided into contributions from energetic disorder and from charge recombination, and the extent to which these factors limit the VOC is assessed.

© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Whereas the role of molecularly mixed domains in organic photovoltaic devices for charge generation is extensively discussed in the literature, the impact on charge recombination and thus fill factor is largely unexplored. Here, a combination of soft X-ray techniques enables the quantification of phases at multiple length scales to reveal their role regarding charge recombination in a highly efficient solution processed small molecule system 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2). A quantitative (linear) relationship between the average composition variations and the device fill-factor is observed. The results establish the complex interrelationship between average phase purity, domain size, and structural order and highlight the requirement of achieving sufficient phase purities to diminish bimolecular and geminate recombination in solution processed small molecule solar cells.