Direct determination of the electronic structure of the poly(3-hexylthiophene):phenyl-[6,6]-C61 butyric acid methyl ester blend

Guan, Zelei; Kim, Jong-Bok; Wang, He; Jaye, Cherno; Fischer, Daniel A.; Loo, Yueh Lin; Kahn, Antoine

doi:10.1016/j.orgel.2010.07.023

Cited by 212 publications

(209 citation statements)

References 23 publications

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“…Second, our EQE measurements indicate the transport gap and optical gap for the polymers considered differ by <0.1 eV, explaining facile dissociation of excitons into free carriers. This result is consistent with previous transient photoconductivity results from Heeger and co-workers (37,39), but differs substantially from those obtained from photophysical experiments, such as ultraviolet photoelectron spectroscopy (UPS)/inverse photoemission spectroscopy (IPES) (40). Therefore, the relevance of these photophysical experiments in interpreting device performance needs to be reconciled in future work on this topic.…”

Section: Discussionsupporting

confidence: 90%

Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells

Ray

Baradwaj

Khan

et al. 2015

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

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The bulk heterojunction (BHJ) organic photovoltaic (OPV) architecture has dominated the literature due to its ability to be implemented in devices with relatively high efficiency values. However, a simpler device architecture based on a single organic semiconductor (SS-OPV) offers several advantages: it obviates the need to control the highly system-dependent nanoscale BHJ morphology, and therefore, would allow the use of broader range of organic semiconductors. Unfortunately, the photocurrent in standard SS-OPV devices is typically very low, which generally is attributed to inefficient charge separation of the photogenerated excitons. Here we show that the short-circuit current density from SS-OPV devices can be enhanced significantly (∼100-fold) through the use of inverted device configurations, relative to a standard OPV device architecture. This result suggests that charge generation may not be the performance bottleneck in OPV device operation. Instead, poor charge collection, caused by defect-induced electric field screening, is most likely the primary performance bottleneck in regular-geometry SS-OPV cells. We justify this hypothesis by: (i) detailed numerical simulations, (ii) electrical characterization experiments of functional SS-OPV devices using multiple polymers as active layer materials, and (iii) impedance spectroscopy measurements. Furthermore, we show that the collection-limited photocurrent theory consistently interprets typical characteristics of regular SS-OPV devices. These insights should encourage the design and OPV implementation of high-purity, high-mobility polymers, and other soft materials that have shown promise in organic field-effect transistor applications, but have not performed well in BHJ OPV devices, wherein they adopt less-than-ideal nanostructures when blended with electron-accepting materials.T he photovoltaic properties of semiconducting polymers arranged in a simple metal-polymer-metal sandwich structure were first demonstrated in 1993 by several groups (1-3). The efficiency values of these early photovoltaic (PV) cells were minuscule mainly because of the poor short-circuit current densities (J sc ∼10 μA cm −2 ) under standard 1 sun illumination. By definition,where J ph ðV Þ is the voltage-dependent photocurrent, η ex is the exciton diffusion-dissociation (or free-charge generation) efficiency, η C is the charge (free-carrier) collection efficiency, and J max is the maximum current density obtained by integrating absorption spectra (J max ∼ 10 − 20 mA cm −2 for typical semi-conducting polymers). The difference between J sc and J max was attributed to an inefficient η ex (<0.1%) arising from ultrashort exciton diffusion lengths (∼10-15 nm) in semiconducting polymers and was eventually interpreted by Onsager's theory of geminate pair recombination (4-6). This classical interpretation assumes, on the other hand, that η C ∼ 100%. To improve η ex , Heeger and co-workers (7) and Holmes and coworkers (8) introduced the concept of bulk heterojunction (BHJ) devices in 1995. Her...

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Section: Discussionsupporting

confidence: 90%

Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells

Ray

Baradwaj

Khan

et al. 2015

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

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“…[9,17,18] 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) with its LUMO located at ~ 5.24 eV [19] should then be a strong p-type dopant upon mixing with the host polymers [9,17] (crystalline) regioregular poly(3-hexylthiophene) (rr-P3HT, HOMO ~ 4.65 eV) and (amorphous) poly [2,3- Several studies find that energy level alignment at organic/electrode and organic/organic interfaces plays a critical role in multilayer stack device, nearly dominating the performance. [20][21][22][23][24] Typically, the energy level alignment of such interfaces follows the trends predicted by the so-called integer charge transfer (ICT) model, i.e. three distinct regimes where the Fermi level is either pinned to negative/positive interface polarons or vacuum level alignment holds.…”

Section: Introductionmentioning

confidence: 99%

Energetics at Doped Conjugated Polymer/Electrode Interfaces

Bao

Liu

Gao

et al. 2014

Adv Materials Inter

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Abstract:The energy level alignment at molecule-doped conjugated polymer/electrode interface is investigated. We find regular plots with a constant thickness-independent displacement away from the original energy level alignment behavior of the pristine polymer at low to intermediate level for molecule-doped conjugated polymer/conducting substrate interfaces.We proposed that two combined processes control the energy level alignment: (i) equilibration of the Fermi level due to oxidation (or reduction) of polymer sites at the interface as per the integer charge transfer model and (ii) a double dipole step induced by image charge from the dopant-polymer charge transfer complex that cause a shift of the work function. Such behavior is expected to hold for all low to intermediate level doped organic semiconductor systems.

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“…15,[30][31][32] Even if the determination of energy levels is done with one consistent method in pristine materials, the results may not be transferable to blend films and devices, because effects such as aggregation, crystallization and interface dipoles can shift energy levels by up to 0.5 eV. [33][34][35][36][37] These uncertainties in energy levels are relevant for material design rules and for the estimation of efficiency potentials as done by Scharber et al 15 The present paper uses electroluminescence (EL) and photoluminescence (PL) measurements of polymer:fullerene devices and films to study the relationship between interfacial energetics and charge generation and recombination. EL spectroscopy probes the emissive states directly in working devices and allows a quantitative determination of the energy of the CT state by measuring its emission.…”

Section: Introductionmentioning

confidence: 99%

Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells

Faist¹,

Kirchartz²,

et al. 2011

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We study the appearance and energy of the charge transfer (CT) state using measurements of Electroluminescence (EL) and Photoluminescence (PL) in blend films of high-performance polymers with fullerene acceptors. EL spectroscopy provides a direct probe of the energy of the interfacial states without the need to rely on the LUMO and HOMO energies as estimated in pristine materials. For each polymer, we use different fullerenes with varying LUMO levels as electron acceptors, in order to vary the energy of the CT state relative to the blend with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). As the energy of the CT state emission approaches the absorption onset of the blend component with the smaller optical bandgap, , we observe a transition in the EL spectrum from CT emission to singlet emission from the component with the smaller bandgap. The appearance of component singlet emission coincides with reduced photocurrent and fill factor. We conclude that the open circuit voltage is limited by the smaller bandgap of the two blend components. From the losses of the studied materials, we derive an empirical limit for the open circuit voltage: 2

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Direct determination of the electronic structure of the poly(3-hexylthiophene):phenyl-[6,6]-C61 butyric acid methyl ester blend

Cited by 212 publications

References 23 publications

Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells

Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells

Energetics at Doped Conjugated Polymer/Electrode Interfaces

Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells

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