On the identification of deeper defect levels in organic photovoltaic devices

Carr, Jack; Chaudhary, Sumit

doi:10.1063/1.4818324

Cited by 45 publications

(30 citation statements)

References 47 publications

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“…In practice, the impedance spectroscopy data (see Fig. S1) indicate the presence of high density of defect states acting as p-type dopants in the semiconductor (27)(28)(29)(30). Under solar illumination, these defects are filled by photoinduced electrons, and they form a space-charge region (with corresponding nonlinear bending of the energy band diagram) near the organic low work function metal (27) (the solid lines of Fig.…”

Section: Interpretation Of the Key Experimentsmentioning

confidence: 99%

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: Interpretation Of the Key Experimentsmentioning

confidence: 99%

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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“…Since the relation between morphology and doping phenomena in semiconductor polymers is not totally clear, we evaluated the SEM images of the films to gain further insight into this problem (see Fig. ).…”

Section: Resultsmentioning

confidence: 99%

Effects of FeCl₃ as oxidizing agent on the conduction mechanisms in polypyrrole (PPy)/pc–ZnO hybrid heterojunctions grown by oxidative chemical vapor deposition

Castro-Carranza

Nolasco

Bley

et al. 2016

J Polym Sci B Polym Phys

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P-type polypyrrole (PPy) films are deposited on glass and on n-type polycrystalline ZnO (pc-ZnO) substrates by oxidative chemical vapor deposition under three different amounts of FeCl 3 used as oxidizing agents to form hybrid heterojunctions. Their microstructure, morphology, and electrical characteristics are studied. Particularly, current-voltage characteristics of the PPy/pc-ZnO heterojunctions are analyzed by defining an electrical equivalent circuit. The extracted parameters, together with the estimated heterojunction barrier height and the HOMO energy level of the PPy, indicate that a thermionic emission of holes at the heterojunction determines the saturation current of the diode at low voltage. For larger FeCl 3 amounts, the diode ideality factor increases indicating an increment of recombination by tunneling of charge carriers occurring at the heterojunction. This is attributed to a narrowing of the space charge region due to an increment of the number of charge carriers with a growing amount of FeCl 3 . At high voltages, the PPy thickness influences the ohmic and spacecharge limited current mechanisms at the bulk region. V C 2016

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“…electrodes into the bulk material. References [24]- [26] show that both shallow and deep trap states exist in the bulk and at the interfaces of OBHJ material systems. As shown in Step 2 of Fig.…”

Section: Light-modulated Contact Injectionmentioning

confidence: 99%

Photoconductivity and Photoconductive Gain in Organic Bulk Heterojunction Materials

Liang

Danielson

Ooi

et al. 2015

IEEE Trans. Electron Devices

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A comprehensive study on photoconductive gain in organic bulk heterojunction (OBHJ) materials is reported. High photoconductive gains in OBHJs occur in operating regions where the device characteristics are dominated by photomodulated contact injection effects. The photomodulated injection current contributes to the total device current by augmenting the photocurrent resulting from photoabsorption in the device. An experimentally confirmed simulation of bulk photoconduction in lateral P3HT:PC 71 BM bulk heterojunction blend devices is presented, and it clarifies the origins and operating regimes in these devices. High gains occur at lower light intensities, where the total device current is dominated by photomodulated contact injection rather than bulk photoconduction effects. At higher light intensities, the total device current is dominated by bulk photoconduction effects, where the photocurrent results from light absorption in the bulk material. In agreement with previous research, gains also increase with higher applied biases. The inclusion of field-dependent mobilities plays a critical role in accurately modeling this relationship between applied voltage bias and gain.

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On the identification of deeper defect levels in organic photovoltaic devices

Cited by 45 publications

References 47 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

Effects of FeCl₃ as oxidizing agent on the conduction mechanisms in polypyrrole (PPy)/pc–ZnO hybrid heterojunctions grown by oxidative chemical vapor deposition

Photoconductivity and Photoconductive Gain in Organic Bulk Heterojunction Materials

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On the identification of deeper defect levels in organic photovoltaic devices

Cited by 45 publications

References 47 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

Effects of FeCl3 as oxidizing agent on the conduction mechanisms in polypyrrole (PPy)/pc–ZnO hybrid heterojunctions grown by oxidative chemical vapor deposition

Photoconductivity and Photoconductive Gain in Organic Bulk Heterojunction Materials

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Effects of FeCl₃ as oxidizing agent on the conduction mechanisms in polypyrrole (PPy)/pc–ZnO hybrid heterojunctions grown by oxidative chemical vapor deposition