Bulk Heterojunction Solar Cells with Large Open‐Circuit Voltage: Electron Transfer with Small Donor‐Acceptor Energy Offset

Gong, Xiong; Tong, Meiping; Brunetti, Fulvio G.; Seo, Jung Hwa; Sun, Yanming; Moses, D.; Wudl, Fred; Heeger, Alan J.

doi:10.1002/adma.201003768

Cited by 253 publications

(188 citation statements)

References 28 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: 71%

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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: 71%

“…Under the broadband solar illumination, however, the excess energy of the photons incident upon the polymer may be sufficient to dissociate excitons (35)(36)(37)(38). Moreover, impurities or dopants in the bulk of the polymer may provide another channel for efficient exciton dissociation into free carriers.…”

Section: Discussionmentioning

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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“…Fig. 3 shows the energy diagram of the inverted OLED using a-ZSO as ETL, a-C12A7:e as EIL, and CBP:Ir(ppy) 3 as EML. Smooth electron transport from the cathode to the organic EML can be expected from the small injection barriers between these layers and an ohmic contact between the cathode and the ETL.…”

Section: Resultsmentioning

confidence: 99%

“…inverted OLEDs | electron injection | electron transport | amorphous oxide semiconductor | low work function material E lectronic and photonic devices based on organic semiconductors have attracted much attention due to their intrinsic characteristics that are difficult to achieve in inorganic semiconductors, such as flexibility and the capability of precise molecular design of active organic layers (1)(2)(3). However, organic devices suffer from the material properties that organic semiconductors in general have: rather high lowest unoccupied molecular orbital (LUMO) levels and low electron mobilities, such as 10 −6 to 10 −3 cm 2 /(V·s).…”

mentioning

confidence: 99%

Transparent amorphous oxide semiconductors for organic electronics: Application to inverted OLEDs

Hosono

Kim

Yoshitake

et al. 2016

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

115

112

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Efficient electron transfer between a cathode and an active organic layer is one key to realizing high-performance organic devices, which require electron injection/transport materials with very low work functions. We developed two wide-bandgap amorphous (a-) oxide semiconductors, a-calcium aluminate electride (a-C12A7:e) and a-zinc silicate (a-ZSO). A-ZSO exhibits a low work function of 3.5 eV and high electron mobility of 1 cm 2 /(V · s); furthermore, it also forms an ohmic contact with not only conventional cathode materials but also anode materials. A-C12A7:e has an exceptionally low work function of 3.0 eV and is used to enhance the electron injection property from a-ZSO to an emission layer. The inverted electron-only and organic light-emitting diode (OLED) devices fabricated with these two materials exhibit excellent performance compared with the normal type with LiF/Al. This approach provides a solution to the problem of fabricating oxide thin-film transistor-driven OLEDs with both large size and high stability.inverted OLEDs | electron injection | electron transport | amorphous oxide semiconductor | low work function material E lectronic and photonic devices based on organic semiconductors have attracted much attention due to their intrinsic characteristics that are difficult to achieve in inorganic semiconductors, such as flexibility and the capability of precise molecular design of active organic layers (1-3). However, organic devices suffer from the material properties that organic semiconductors in general have: rather high lowest unoccupied molecular orbital (LUMO) levels and low electron mobilities, such as 10 −6 to 10 −3 cm 2 /(V·s). This leads to inefficiency in electron injection/transport between an electrode and an organic active layer and a large energy loss. In other words, the creation of materials suitable for efficient electron injection layers (EILs) and electron transport layers (ETLs), with very low work functions, reasonable chemical stability, and high electron mobility, should lead to organic devices with improved performance.Organic light-emitting diodes (OLEDs) are regarded as nextgeneration flat-panel displays (4). Although small high-resolution OLED displays are used widely for smart phones/tablets, and large televisions are being commercialized, several issues still remain, such as lifetime, image sticking, power consumption, and production cost (5). The recent high-resolution OLED pixels (6) with reduced aperture ratios [areal ratio of thin-film transistor (TFT) to pixel] raise the importance of the top emissiontype structure for practical use. For large OLEDs, it is unrealistic to use low-temperature polycrystalline silicon (LTPS) TFTs for backplanes. Only oxide TFTs, as represented by a-In-Ga-Zn-O (a-IGZO) (7), are practical candidates for the backplane (8, 9). The current small OLEDs use normal-type structures combined with p-channel LTPS TFTs. However, only n-channel TFTs are possible for oxide TFTs in actual applications. Simple substitution of the p-channel LTPS TFTs wi...

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“…The unexpected low V oc of P6-based PSCs was speculated to result from the low solubility and poor film quality. The J sc s and FFs of P4-P7-based devices are lower than those of P3HT:PC 61 BM-based devices (J sc : 9.5 mA cm −2 ; FF: 68 %) [53], which resulted in the lower overall PCEs. Surpassing the PCE of P3HT-based PSCs was achieved by Chen and co-workers [54], who developed a Th 4 /BT-based alternating copolymer, P8.…”

Section: Polythiophene-based Polymers Containing 213-benzothiadiazomentioning

confidence: 98%

Recent Advances in P-Type Conjugated Polymers for High-Performance Solar Cells

Cheng

Wang

et al. 2015

Topics in Applied Physics

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Bulk-heterojunction (BHJ) polymer solar cells have achieved significant progress in the recent years, with the efficiency now over 10 %. The p-type polymer in the BHJ blend played a key role in the amazing technology advance. In this chapter, we will timely update over 80 conjugated polymers leading to highperformance solar cells. The principle of molecular design with structure-properties relationship with respect to device characteristics will also be discussed, as the materials and morphology are tightly interconnected.

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Bulk Heterojunction Solar Cells with Large Open‐Circuit Voltage: Electron Transfer with Small Donor‐Acceptor Energy Offset

Cited by 253 publications

References 28 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

Transparent amorphous oxide semiconductors for organic electronics: Application to inverted OLEDs

Recent Advances in P-Type Conjugated Polymers for High-Performance Solar Cells

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