An Optimal Driving Force for Converting Excitons into Free Carriers in Excitonic Solar Cells

Coffey, David C.; Larson, Bryon W.; Hains, Alexander W.; Whitaker, James B.; Kopidakis, Nikos; Boltalina, Olga V.; Strauss, Steven H.; Rumbles, Garry

doi:10.1021/jp302275z

Cited by 156 publications

(213 citation statements)

References 44 publications

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“…Furthermore, most of the D/A OPV systems are strongly dependent on the fullerene ratio. Their electronic structure calculations on the large clusters of functionalized fullerenes is well consistent with the transient pump-push-photocurrent data, which predicts several features of charge generation process, outside the framework of conventional theories, i.e., Onsager theory [71][72][73] and Marcus theory [74,75]. In each case, the DOS accessible at the donor−acceptor heterojunction is depicted.…”

Section: Charge Separated Statessupporting

confidence: 72%

Towards High Performance Organic Photovoltaic Cells: A Review of Recent Development in Organic Photovoltaics

2014

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Organic photovoltaic cells (OPVs) have been a hot topic for research during the last decade due to their promising application in relieving energy pressure and environmental problems caused by the increasing combustion of fossil fuels. Much effort has been made toward understanding the photovoltaic mechanism, including evolving chemical structural motifs and designing device structures, leading to a remarkable enhancement of the power conversion efficiency of OPVs from 3% to over 15%. In this brief review, the advanced progress and the state-of-the-art performance of OPVs in very recent years are summarized. Based on several of the latest developed approaches to accurately detect the separation of electron-hole pairs in the femtosecond regime, the theoretical interpretation to exploit the comprehensive mechanistic picture of energy harvesting and charge carrier generation are discussed, especially for OPVs with bulk and multiple heterojunctions. Subsequently, the novel structural designs of the device architecture of OPVs embracing external geometry modification and intrinsic structure decoration are presented. Additionally, some approaches to further increase the efficiency of OPVs are described, including thermotics and dynamics modification methods. Finally, this review highlights the challenges and prospects with the aim of providing a better understanding towards highly efficient OPVs.

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Section: Charge Separated Statessupporting

confidence: 72%

Towards High Performance Organic Photovoltaic Cells: A Review of Recent Development in Organic Photovoltaics

2014

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“…Regarding the modest structural evolution on the polymer, we note that Ö sterbacka et al 46 concluded that an increase in polymer chain stiffness is related to a smaller polaron relaxation energy. In a different vein, Coffey et al 47 have suggested that a key route for improving future excitonic solar cells might be to reduce the reorganization energy inherent to photoinduced electron transfer. Indeed, we propose that correlating reorganization dynamics with macroscopic optoelectronic parameters will enable a fundamental understanding of the critical factors that lead to efficient photovoltaic systems at a molecular level.…”

Section: Discussionmentioning

confidence: 99%

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions

et al. 2014

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In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This results in distinctive signatures in the vibrational modes of the polymer. Here, we probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its timeresolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 300 fs. Surprisingly, further structural evolution on t50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that near equilibrium. We interpret this as suggestive that charges are free from their mutual Coulomb potential because we would expect rich vibrational dynamics associated with charge-pair relaxation. We address current debates on the photocarrier generation mechanism at molecular heterojunctions, and our work is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.

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“…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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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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An Optimal Driving Force for Converting Excitons into Free Carriers in Excitonic Solar Cells

Cited by 156 publications

References 44 publications

Towards High Performance Organic Photovoltaic Cells: A Review of Recent Development in Organic Photovoltaics

Towards High Performance Organic Photovoltaic Cells: A Review of Recent Development in Organic Photovoltaics

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions

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

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