Determination of energy level alignment at interfaces of hybrid and organic solar cells under ambient environment

Davis, R. J.; Lloyd, Matthew T.; Ferreira, Summer Rhodes; Bruzek, Matthew J.; Watkins, Scott E.; Lindell, Linda; Sehati, Parisa; Fahlman, Mats; Anthony, John E.; Hsu, Julia W. P.

doi:10.1039/c0jm02349c

Cited by 148 publications

(138 citation statements)

References 44 publications

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“…22 The onset of the film absorbance spectra were used to estimate the HOMO-LUMO gaps of the three polymers, BFR2, BFS3, and BFS4. Photoelectron spectroscopy in air (PESA) was applied to estimate the highest occupied molecular orbital (HOMO) of these conjugated polymers in thin films ( Figure S6) 23 The experimentally determined HOMO values and the calculated LUMO levels are summarized in Table 1. It is clear that the variations to the polymers described here do not have a significant effect on the energy levels of the materials.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Tailored Donor–Acceptor Polymers with an A–D1–A–D2 Structure: Controlling Intermolecular Interactions to Enable Enhanced Polymer Photovoltaic Devices

et al. 2014

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Extensive efforts have been made to develop novel conjugated polymers that give improved performance in organic photovoltaic devices. The use of polymers based on alternating electron-donating and electron-accepting units not only allows the frontier molecular orbitals to be tuned to maximize the open-circuit voltage of the devices but also controls the optical band gap to increase the number of photons absorbed and thus modifies the other critical device parameterthe short circuit current. In fact, varying the nonchromophoric components of a polymer is often secondary to the efforts to adjust the intermolecular aggregates and improve the charge-carrier mobility. Here, we introduce an approach to polymer synthesis that facilitates simultaneous control over both the structural and electronic properties of the polymers. Through the use of a tailored multicomponent acceptor−donor−acceptor (A−D−A) intermediate, polymers with the unique structure A−D1−A−D2 can be prepared. This approach enables variations in the donor fragment substituents such that control over both the polymer regiochemistry and solubility is possible. This control results in improved intermolecular π-stacking interactions and therefore enhanced charge-carrier mobility. Solar cells using the A−D1−A−D2 structural polymer show shortcircuit current densities that are twice that of the simple, random analogue while still maintaining an identical open-circuit voltage. The key finding of this work is that polymers with an A−D1−A−D2 structure offer significant performance benefits over both regioregular and random A−D polymers. The chemical synthesis approach that enables the preparation of A−D1−A−D2 polymers therefore represents a promising new route to materials for high-efficiency organic photovoltaic devices.

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Section: ■ Results and Discussionmentioning

confidence: 99%

Tailored Donor–Acceptor Polymers with an A–D1–A–D2 Structure: Controlling Intermolecular Interactions to Enable Enhanced Polymer Photovoltaic Devices

et al. 2014

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“…Shifts of the frontier orbitals in P3HT/ PCBM blends have been previously observed with photoelectron spectroscopy and Kelvin probe microscopy. 60,61 As mentioned above, we attribute feature 2 at 0.9−1.3 eV to excitation of interfacial states at the P3HT−substrate interface. In our earlier SPV measurements on pure P3HT films, these states occur at higher energy (1.2−1.7 eV).…”

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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“…E FL;PEN ≈ −4.0 eV and E EX;PEN ¼ 0.5 eV have already been reported in literature. 28,30 These values yield E DP;PEN∕K12 ¼ −4.0 eV − ð−4.6 eVÞ ¼ 0.6 eV as the maximum dipole between PEN and K12. In summary, a dipole shift between P3HT and K12 was estimated to be as large as 0.2 eV.…”

Section: Resultsmentioning

confidence: 99%

Pentacene/K12 solar cells formed by organic vapor phase deposition

et al. 2014

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Abstract. This study reports on an organic solar cell structure utilizing K12 as a new low-temperature processable small-molecule acceptor material. Pentacene (PEN) and K12 were deposited onto indium tin oxide by means of organic vapor phase deposition (OVPD) as bilayer solar cells. The resulting solar cell was characterized electrically by current density-voltage (J − V) measurements and optically by photocurrent and reflectivity measurements. The J − V characteristic under AM 1.5 illumination indicates a short-circuit current of 0.45 mA∕cm 2 (J sc ), a fill factor of 38% (FF), and an open-circuit (V oc ) voltage of 0.71 V. Current generation is found to predominantly occur in the K12 layer, although strong light absorption in the PEN layer is detected. We suggest that either a dipole shift between the layers or the fission of singlet excitons in the PEN layer leads to this observation. Although the efficiency of the device is low in combination with PEN, our experiment successfully demonstrates the use of K12 as a lowtemperature acceptor material in OVPD processes.

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Determination of energy level alignment at interfaces of hybrid and organic solar cells under ambient environment

Cited by 148 publications

References 44 publications

Tailored Donor–Acceptor Polymers with an A–D1–A–D2 Structure: Controlling Intermolecular Interactions to Enable Enhanced Polymer Photovoltaic Devices

Tailored Donor–Acceptor Polymers with an A–D1–A–D2 Structure: Controlling Intermolecular Interactions to Enable Enhanced Polymer Photovoltaic Devices

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

Pentacene/K12 solar cells formed by organic vapor phase deposition

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