Evidence for space-charge-limited conduction in organic photovoltaic cells at open-circuit conditions

Nuzzo, Daniele Di; Reenen, S Stephan van; Janssen, Raj René; Kemerink, Martijn; Meskers, Stefan C. J.

doi:10.1103/physrevb.87.085207

Cited by 17 publications

(12 citation statements)

References 40 publications

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“…Based on the inversion of the polarity of the F2 signal in Figure 7, the Fermi level E F of the polymer can be estimated as −4.7 eV, slightly above the E F of ITO (−4.8 eV). The depleted semiconductor model is supported by impedance spectroscopy, 69,70 capacitance measurements, 28,71 and time-of-flight electron-collection. 72 Furthermore, XPS and UPS studies also confirm an electrochemical equilibrium between P3HT:PCBM and substrate.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

P3HT:PCBM Bulk-Heterojunctions: Observing Interfacial and Charge Transfer States with Surface Photovoltage Spectroscopy

et al. 2014

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Surface photovoltage (SPV) spectra are reported for separate films of (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) and for regioregular and regiorandom poly(3-hexylthiophene) (P3HT):PCBM bulk heterojunctions, as a function of wavelength, film thickness, thermal annealing, and substrate. In PCBM films, two photovoltage features are observed at 1.1–1.4 eV (F1) and 1.4–2.3 eV (F2), which are assigned to excitation of charge transfer states at the interface (F1) and in the bulk (F2) of the film. In BHJ films, five different photovoltage features are observed at 0.75–0.9 eV (F1), 0.9–1.3 eV (F2), 1.3–1.8 eV (F3), 1.8–2.0 eV (F4), and 2.0–2.4 eV (F5). This data can be analyzed on the basis of optical absorbance and fluorescence spectra of the films, and using SPV spectra for PCBM and P3HT only films, and for a BHJ film containing P3HT nanofibers for comparison. SPV features are assigned to states at the polymer–substrate interface (F1 and F2), the P3HT:PCBM charge transfer state (F3), the self-ionized (CT) state of P3HT (F4), and the band gap transition of P3HT (F5). This interpretation is also consistent with molecular orbital energy diagrams and electron microscopy-derived topological maps of the films. Photovoltage sign and substrate dependence can be understood with the depleted semiconductor model. Features F1–4 are caused by polarization of electrostatically bound charge pairs by the built-in electric field at the substrate–BHJ interface, whereas F5 is due to transport of free charge carriers through the film and through the substrate film interface. This work will promote the understanding of photochemical charge generation and transport in organic photovoltaic films.

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Section: ■ Results and Discussionmentioning

confidence: 99%

P3HT:PCBM Bulk-Heterojunctions: Observing Interfacial and Charge Transfer States with Surface Photovoltage Spectroscopy

et al. 2014

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“…The space charge limit (SCL) effect is a universal phenomenon in semiconductor devices involving light emitting diodes, solar cells, and photodetectors 1 2 3 4 5 6 7 8 9 . It also sets a fundamental electrostatic limit in electrical properties of organic semiconductor devices with unbalanced photocarriers (electrons and holes) mobility and high exciton generation efficiency 10 11 12 13 14 .…”

mentioning

confidence: 99%

“…In other words, positive space charges are accumulated due to the unbalanced photocarriers mobility and a long transport path of holes. As a result, the short-circuit current and fill factor of OSCs will drop significantly due to both the bulk recombination and space charge formation 4 7 9 23 24 . In this work, we will demonstrate the SCL breaking in the OSCs incorporating metallic (Ag or Au) nanostructures, which offers a novel route to eliminate the SCL effect in semiconductor devices.…”

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confidence: 99%

Breaking the Space Charge Limit in Organic Solar Cells by a Novel Plasmonic-Electrical Concept

Sha

Choy

2014

Sci Rep

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As a fundamental electrostatic limit, space charge limit (SCL) for photocurrent is a universal phenomenon and of paramount importance for organic semiconductors with unbalanced photocarriers mobility and high exciton generation. Here we proposed a new plasmonic-electrical concept to manipulate electrical properties of organic devices including photocarriers recombination, transport and collection. As a proof-of-concept, organic solar cells (OSCs) comprising metallic planar and grating electrodes are systematically investigated with normal and inverted device structures. Interestingly, although strong plasmonic resonances induce abnormally dense photocarriers around a grating anode, the grating-inverted OSC is exempt from space charge accumulation (limit) and degradation of electrical properties in contrast to the planar-inverted and planar-normal ones. The particular reason is that plasmonically induced photocarriers redistribution shortens the transport path of low-mobility holes, which are collected by the grating anode. The work demonstrated and explained the SCL breaking with the plasmonic-electrical effect. Most importantly, the plasmonic-electrical concept will open up a new way to manipulate both optical and electrical properties of semiconductor devices simultaneously.

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“…In terms of specific analysis, examination of the temporal decay of photocurrent transients has been used to measure the chargetransport properties of organic semiconductors [26][27][28], while the integral of these transients has been taken to quantify initial amounts of photogenerated charge [29][30][31]. Additionally, charge-extraction transients are routinely used to probe semiconductor recombination kinetics, average doping densities, and carrier mobilities [32][33][34][35][36][37].…”

Section: Introductionmentioning

confidence: 99%

Theory of Current Transients in Planar Semiconductor Devices: Insights and Applications to Organic Solar Cells

2015

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Time-domain current measurements are widely used to characterize semiconductor material properties, such as carrier mobility, doping concentration, carrier lifetime, and the static dielectric constant. It is therefore critical that these measurements be theoretically understood if they are to be successfully applied to assess the properties of materials and devices. In this paper, we derive generalized relations for describing current-density transients in planar semiconductor devices at uniform temperature. By spatially averaging the charge densities inside the semiconductor, we are able to provide a rigorous, straightforward, and experimentally relevant way to interpret these measurements. The formalism details several subtle aspects of current transients, including how the electrode charge relates to applied bias and internal space charge, how the displacement current can alter the apparent free-carrier current, and how to understand the integral of a charge-extraction transient. We also demonstrate how the formalism can be employed to derive the current transients arising from simple physical models, like those used to describe charge extraction by linearly increasing voltage (CELIV) and time-of-flight experiments. In doing so, we find that there is a nonintuitive factor-of-2 reduction in the apparent free-carrier concentration that can be easily missed, for example, in the application of charge-extraction models. Finally, to validate our theory and better understand the different current contributions, we perform a full time-domain drift-diffusion simulation of a CELIV trace and compare the results to our formalism. As expected, our analytic equations match precisely with the numerical solutions to the drift-diffusion, Poisson, and continuity equations. Thus, overall, our formalism provides a straightforward and general way to think about how the internal space-charge distribution, the electrode charge, and the externally applied bias translate into a measured current transient in a planar semiconductor device.

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Evidence for space-charge-limited conduction in organic photovoltaic cells at open-circuit conditions

Abstract: Evidence for spacecharge-limited conduction in organic photovoltaic cells at open-circuit conditions. Physical Review B, 87(8), 085207-1/11. [085207].

Cited by 17 publications

References 40 publications

P3HT:PCBM Bulk-Heterojunctions: Observing Interfacial and Charge Transfer States with Surface Photovoltage Spectroscopy

P3HT:PCBM Bulk-Heterojunctions: Observing Interfacial and Charge Transfer States with Surface Photovoltage Spectroscopy

Breaking the Space Charge Limit in Organic Solar Cells by a Novel Plasmonic-Electrical Concept

Theory of Current Transients in Planar Semiconductor Devices: Insights and Applications to Organic Solar Cells

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