Transient absorption spectroscopy of charge photogeneration yields and lifetimes in a low bandgap polymer/fullerenefilm

Clarke, Tracey M.; Ballantyne, Amy M.; Jamieson, Fiona C.; Brabec, Christoph J.; Nelson, Jenny; Durrant, James R.

doi:10.1039/b813815j

Cited by 91 publications

(162 citation statements)

References 11 publications

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“…It is also apparent that the charge density in the dark follows a greatly reduced dependence on cell voltage when compared to a Boltzmann-like form of n dark ðV Þ, which is a further indication of strong disorder. This behavior is consistent with our previous observations that both the bimolecular rate coefficient, k, and polaron mobility measured in P3HT∶PCBM devices are strongly dependent upon charge density (15,16,18,(22)(23)(24) and that these dependencies can be understood in terms of trapping/detrapping dynamics in the presence of energetic disorder (25).…”

Section: Resultssupporting

confidence: 79%

Charge-density-based analysis of the current–voltage response of polythiophene/fullerene photovoltaic devices

Shuttle

Hamilton

O’Regan

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

267

274

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A key challenge for organic electronics research is to develop device models that correctly account for the structural and energetic disorder typically present in such materials. In this paper we report an approach to analyze the electrical performance of an organic electronic device based upon charge extraction measurements of charge densities and transient optoelectronic measurements of charge carrier dynamics. This approach is applied to a poly(3-hexyl thiophene) (P3HT)/6,6 phenyl C61 butyric acid methyl ester (PCBM) blend photovoltaic device. These measurements are employed to determine the empirical rate law for bimolecular recombination losses, with the energetic disorder present in the materials being accounted for by a charge-density-dependent recombination coefficient. This rate law is then employed to simulate the current/ voltage curve. This simulation assumes the only mechanism for the loss of photogenerated charges is bimolecular recombination and employs no fitting parameters. Remarkably the simulation is in good agreement with the experimental current/voltage data over a wide range of operating conditions of the solar cell. We thus demonstrate that the primary determinant of both the open-circuit voltage and fill factor of P3HT∶PCBM devices is bimolecular recombination. We go on to discuss the applicability of this analysis approach to other materials systems, and particularly to emphasize the effectiveness of this approach where the presence of disorder complicates the implementation of more conventional, voltagebased analyses such as the Shockley diode equation.O rganic semiconductor materials are attracting huge interest on account of their application to low-cost microelectronic and optoelectronic devices. However, the detailed physical understanding of organic semiconductor devices still lags behind their application, on account of fundamental differences in the optoelectronic properties of these materials compared to conventional semiconductors, and this lack of understanding limits the scope of material and device design. Among those features that distinguish organic semiconductor devices from inorganic semiconductor based devices are (i) the fact that charges and excited states are localized on individual molecules or molecular segments, with the result that charge and energy transport processes are relatively slow; (ii) the dielectric permittivity is low, leading to stronger space charge effects; (iii) the organic semiconductor materials are electronically disordered, dispersing the rates of charge transfer and transport processes; (iv) the active layers are often heterogeneous, either as multicomponent films or because of nonuniform molecular ordering; and (v) the organic semiconductor is usually not doped, thus precluding the conditions that allow charge dynamics to be linearized in the description of device physics (1).The need for appropriate device physics for organic semiconductor materials is of particular relevance to the organic bulk heterojunction solar cell (Fig. 1). In these d...

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Section: Resultssupporting

confidence: 79%

Charge-density-based analysis of the current–voltage response of polythiophene/fullerene photovoltaic devices

Shuttle

Hamilton

O’Regan

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

267

274

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“…The push-pull copolymer SiPCPDTBT shows a high generation yield at the lowest offset, in agreement with previous reports that show low energetic losses [ 17 , 49 ] and high charge generation yields [ 50 ] at low energetic offsets in push-pull (donor-acceptor) copolymers containing benzothiadiazole (BT) or diketopyrrolopyrrole (DPP). One reason for this could be the strength of the acceptor (pull) unit and the resulting charge-transfer nature of the polymer exciton.…”

Section: Full Papersupporting

confidence: 80%

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

Faist

Shoaee

Tuladhar

et al. 2013

Advanced Energy Materials

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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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“…[28] We note that the calculated errors are around 10%, although it is recognised there is a distinction to be made between the uncertainty in the measurement of the mean value of τ, and the dispersion of τ. The scope of this paper is to determine mean lifetimes in the μs-ms range, which is the most critical timescale for charge relaxation processes in photovoltaic blends.…”

Section: Resultsmentioning

confidence: 99%

Photophysical studies of poly-isocyanopeptide based photovoltaic blends

Finlayson¹,

Whitney²

2010

J. Phys. D: Appl. Phys.

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We report a photophysical study of photovoltaic blends containing perylene-substituted polyisocyanide (pery-PIC) materials, using time-correlated single photon counting (TCSPC) and photo-induced absorption spectroscopy, and compare the key characteristics to analogous perylene-diimide (PDI) monomer blends with polythiophene-and polyfluorene-based conjugated polymers. Pery-PIC consists of semiconducting perylene units, which self-stack in a regular fashion around a rigid helical poly-isocyanopeptide backbone. In particular, the charged state lifetimes in pery-PIC blends are found to be of order 10s of μs; this being typically less than half those of the perylene-anion in the corresponding PDI blends. We consider the influence of photophysical factors on the superior photovoltaic device performance of the pery-PIC blends, relative to the corresponding PDI-based devices, in addition to the morphological effects described in earlier studies.

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Transient absorption spectroscopy of charge photogeneration yields and lifetimes in a low bandgap polymer/fullerenefilm

Abstract: 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.

Cited by 91 publications

References 11 publications

Charge-density-based analysis of the current–voltage response of polythiophene/fullerene photovoltaic devices

Charge-density-based analysis of the current–voltage response of polythiophene/fullerene photovoltaic devices

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

Photophysical studies of poly-isocyanopeptide based photovoltaic blends

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