On the pseudo-symmetric current–voltage response of bulk heterojunction solar cells

Ooi, Zi En; Jin, Rui; Huang, Jinsong; Loo, Yueh‐Lin; Sellinger, Alan; deMello, John C.

doi:10.1039/b718563d

Cited by 51 publications

(75 citation statements)

References 20 publications

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“…It is a widely used tool to analyze device parameters. [54][55][56][57] We use a pulsed, almost simultaneous measurement of light and dark currents [ 55 ] through which temperature-and time-dependent effects are avoided and the photocurrent J ph = J light -J dark can be measured much more accurately than by subtracting separately measured current-voltage curves. J ph is then corrected for a photoshunt as explained in the supplementary information and is normalized to its saturation value at high reverse biases, the saturation current J sat .…”

Section: Studies Of Charge Collection Effi Ciency Of the Blend Filmsmentioning

confidence: 99%

Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

Faist

Shoaee

Tuladhar

et al. 2013

Advanced Energy Materials

134

140

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The use of fullerenes with two or more adducts as acceptors has been recently shown to enhance the performance of bulk‐heterojunction solar cells using poly(3‐hexylthiophene) (P3HT) as the donor. The enhancement is caused by a substantial increase in the open‐circuit voltage due to a rise in the fullerene lowest unoccupied molecular orbital (LUMO) level when going from monoadducts to multiadducts. While the increase in the open‐circuit voltage is obtained with many different polymers, most polymers other than P3HT show a substantially reduced photocurrent when blended with fullerene multiadducts like bis‐PCBM (bis adduct of Phenyl‐C61‐butyric acid methyl ester) or the indene C60 bis‐adduct ICBA. Here we investigate the reasons for this decrease in photocurrent. We find that it can be attributed partly to a loss in charge generation efficiency that may be related to the LUMO‐LUMO and HOMO‐HOMO (highest occupied molecular orbital) offsets at the donor‐acceptor heterojunction, and partly to reduced charge carrier collection efficiencies. We show that the P3HT exhibits efficient collection due to high hole and electron mobilities with mono‐ and multiadduct fullerenes. In contrast the less crystalline polymer Poly[[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl (PCDTBT) shows inefficient charge carrier collection, assigned to low hole mobility in the polymer and low electron mobility when blended with multiadduct fullerenes.

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Section: Studies Of Charge Collection Effi Ciency Of the Blend Filmsmentioning

confidence: 99%

Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

Faist

Shoaee

Tuladhar

et al. 2013

Advanced Energy Materials

134

140

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“…The relevant processes during which the losses can occur are geminate recombination during polaron pair dissociation 3,4 , nongeminate recombination during transport of the already separated polarons 5,6 , and charge extraction from the device. 7,8 A detailed analysis considering the interplay of these mechanisms has still to be done, as already the separate processes are not completely described yet. Concerning nongeminate recombination, classically the bimolecular Langevin formalism 9,10 has been used for low mobility materials.…”

Section: Introductionmentioning

confidence: 99%

Origin of reduced polaron recombination in organic semiconductor devices

2009

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We propose a model to explain the reduced bimolecular recombination rate found in state-of-the-art bulk heterojunction solar cells. When compared to the Langevin recombination, the experimentally observed rate is one to four orders of magnitude lower, but gets closer to the Langevin case for low temperatures. Our model considers the organic solar cell as device with carrier concentration gradients, which form due to the electrode/blend/electrode device configuration. The resulting electron concentration under working conditions of a solar cell is higher at the cathode than at the anode, and vice versa for holes. Therefore, the spatially dependent bimolecular recombination rate, proportional to the local product of electron and hole concentration, is much lower as compared to the calculation of the recombination rate based on the extracted and thus averaged charge carrier concentrations. We consider also the temperature dependence of the recombination rate, which can for the first time be described with our model.

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“…Ooi et al demonstrated that the photocurrent is easily overestimated under illumination since the sample heats up such that the dark and illumination currents are measured at different temperatures. 5 An elegant solution to suppress the effect of heating the substrate under illumination is to use a pulsed white light-emitting diode (LED). 5 By keeping the on-off cycles of the LED shorter than the cooling and heating time of the sample, the dark and illumination currents are measured at the same temperature.…”

Section: Introductionmentioning

confidence: 99%

“…5 An elegant solution to suppress the effect of heating the substrate under illumination is to use a pulsed white light-emitting diode (LED). 5 By keeping the on-off cycles of the LED shorter than the cooling and heating time of the sample, the dark and illumination currents are measured at the same temperature.…”

Section: Introductionmentioning

confidence: 99%

Influence of injected charge carriers on photocurrents in polymer solar cells

et al. 2012

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We determine and analyze the photocurrent J ph in polymer solar cells under conditions where, no, one, or two different charge carriers can be injected by choosing appropriate electrodes and compare the experimental results to simulations based on a drift-diffusion device model that accounts for photogeneration and Langevin recombination of electrons and holes. We demonstrate that accounting for the series resistance of the device is essential to determine J ph . Without such correction, the results are, even qualitatively, incorrect. We show that in solar cells with forward bias applied J ph is reduced by recombination of photogenerated charge carriers with injected charge carriers. Self-selective contacts or band bending are not necessary to explain the effects. Without injecting contacts J ph is symmetric around the compensation voltage. A simple analytical model shows that under high forward bias J ph scales inversely with 1 + ξγ pre , in which γ pre represents the extent of Langevin recombination and ξ is a positive constant. Under conditions where Langevin recombination is very low or when electron and hole mobility are a very different, photogenerated charge carriers can affect the space-charge field and modify the injection of charge carriers. We show by simulations and experimentally that under such conditions the photocurrent can exceed that charge generation such that, effectively, photocurrent multiplication occurs.

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On the pseudo-symmetric current–voltage response of bulk heterojunction solar cells

Cited by 51 publications

References 20 publications

Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

Origin of reduced polaron recombination in organic semiconductor devices

Influence of injected charge carriers on photocurrents in polymer solar cells

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