Photoinduced Injection Enhancement in Fullerene-Based Organic Solar Cell Originates from Exciton–Electron Interaction

Li, Wenbin; Yu, Haomiao; Zhang, Jiawei; Yao, Yao; Wu, Chenming; Hou, X. Y.

doi:10.1021/jp501078f

Cited by 10 publications

(8 citation statements)

References 40 publications

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“…radiation damage, charging effects, errors in determining the binding energy of the holetransporting species) and ways to tackle these effects can be found in [11] and [12]. The current state of research on organic heterojunctions and their implications for the development of organic solar cells has recently been addressed by several authors based on experimental results [13][14][15][16] and theoretical approaches [17][18][19]. In particular, Dutton and Robey [20] and Jailaubekov et al [21] have investigated the dynamics of hot chargetransfer excitons in C 60 /phthalocyanine heterojunctions by time-resolved two-photon-photoemission (TR-2PPE).…”

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confidence: 99%

Site-specific probing of charge transfer dynamics in organic photovoltaics

Arion

Neppl

Roth

et al. 2015

Applied Physics Letters

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We report the site-specific probing of charge-transfer dynamics in a prototype system for organic photovoltaics (OPV) by picosecond time-resolved X-ray photoelectron spectroscopy. A layered system consisting of approximately two monolayers of C60 deposited on top of a thin film of CopperPhthalocyanine (CuPC) is excited by an optical pump pulse and the induced electronic dynamics are probed with 590 eV X-ray pulses. Charge transfer from the electron donor (CuPC) to the acceptor (C60) and subsequent charge carrier dynamics are monitored by recording the time-dependent C 1s core level photoemission spectrum of the system. The arrival of electrons in the C60 layer is readily observed as a completely reversible, transient shift of the C60 associated C 1s core level, while the C 1s level of the CuPC remains unchanged. The capability to probe charge transfer and recombination dynamics in OPV assemblies directly in the time domain and from the perspective of well-defined domains is expected to open additional pathways to better understand and optimize the performance of this emerging technology.PACS numbers: 88.40.jr,79.60.-i,78.47.da, 78.20.-e Research on materials for organic photovoltaic (OPV) applications is driven mostly by two figures of merit: the light-to-current conversion efficiency and the cost of production, which would make the technology suitable for widespread implementation. So far, OPVs meet the second criterion, as they can easily be prepared by deposition from solutions, or by low-cost printing techniques, rather than using more demanding vacuum deposition methods. However, the relatively low conversion efficiency of OPVs remains a significant challenge. It is generally accepted that recombination of the photo-induced electron-hole pair or excitonic state in the photoreceptive material is one of the major loss mechanisms. Consequently, many research efforts focus on the identification of methods to quench the recombination process. Empirically, the admixture of C 60 into organic photoreceptor films has led to significant progress in this respect [1][2][3][4][5][6][7][8]. Based on observations of intrinsic fluorescence quenching [1, 2] as well as from photoemission studies of the band alignment in thin film systems [9], it has been concluded that after the initial electron-hole production in the chromophore, the electron is captured by the C 60 molecule, thus spatially separating the electron from the hole in the chromophore molecule. This spatial separation prevents direct electron-hole recombination. Studies by photoelectron spectroscopy (PES) have shown that the charge transfer process is energetically possible, both in layered systems [4,5] as well as in heterogeneous mixtures [10]. Here, we concentrate on the material combination of the organic photoreceptor copper phthalocyanine (CuPC), which is a ubiquitous component of photochemical and light harvesting applications [1][2][3], with C 60 molecules as electron acceptors.A recent report on CuPC-C 60 interfaces indicates that, when CuPC is depos...

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confidence: 99%

Site-specific probing of charge transfer dynamics in organic photovoltaics

Arion

Neppl

Roth

et al. 2015

Applied Physics Letters

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“…[36] In other words, saturated photocurrent is the biggest photocurrent when the applied bias voltage is high enough and the total exciton generation rate can be directly extracted from the saturated photocurrent. Here, the saturated photocurrent of device with different H/J values was compared.…”

Section: The Influence Of H-/j-aggregation On Photophysical Processmentioning

confidence: 99%

Balancing the H- and J-aggregation in DTS(PTTh₂)₂/PC₇₀BM to yield a high photovoltaic efficiency

et al. 2015

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The content and ratio of two stacking styles, namely H-aggregation and J-aggregation, of 5,5'-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)- [1,2,5]thiadiazolo [3,4-c]pyridine}-3 ,3'-di-2-ethylhexylsilylene-2,2'-bithiophene, DTS(PTTh 2 ) 2 had a profound influence on performance of solar cells based on DTS(PTTh 2 ) 2 /[6,6]-phenyl-C71-butyric acid methyl ester (PC 70 BM) (7/3, w/w) blend films. It was found that the H/J ratio could be tuned from 0.30 to 1.40 by controlling boiling point of main solvent, content of additive and the dissolving selectivity of additives (such as 1,8-diiodooctane (DIO), 1,6-diiodohexane (DIH) and 1,4-diiodobutane (DIB)) for DTS(PTTh 2 ) 2 side chain.The power conversion efficiency (PCE) of DTS(PTTh 2 ) 2 /PC70BM was improved firstly, then decreased with increasing of the H/J ratio. The best PCE of 6.51% was achieved by adding 0.20 vol% 1,6-diiodohexane (DIO) into solutions in thiophene (Th), improved 203% compared to the reference without additives, when the H/J ratio is 1.01. A balance between the two stacking styles is needed for device to obtain best performance. On one hand, J-aggregation was favorable to form more excitons because a lower energy was needed to excite the J-aggregation. On the other hand, the H-aggregation favored exciton dissociation for two reasons, providing excitons with longer lifetime and stronger driving force for exciton dissociation. And also, the complementary light absorption of the two stacking styles is advantageous for maximal light absorption. 4 molecules. [15][16][17] The adjacent molecules stack face-to-face to form H-aggregation, and head-to-tail to form J-aggregation. [18] As to the relationship between absorption peaks and H-/J-aggregation, there are two viewpoints. One was that the three peaks (0-0, 0-1, 0-2) taken together represent a vibronic progression, due to a symmetric vinyl stretching and/or ring breathing mode (or modes) in the region from 1200-1500cm -1 . The ratio of the 0-0 peak to the 0-1 peak is very sensitive to the exciton bandwidth. In particular, the peak ratio decreases with bandwidth for H-aggregation and increases with bandwidth for J-aggregation. [19][20][21] The other was that red-shift absorption peak (compared to absorption peak of monomer) represented J-aggregation and the blue-shift one represented H-aggregation. [16,[22][23][24] Each of the two viewpoints has its own merits.Here, we adopted the latter one due to that it was more direct and allowed us to compare H-and J-aggregation intuitively.The H-/J-aggregation state has multiple influences on many important factors concerning about the performance of solar cells. Firstly, the absorption peaks of H-aggregation and J-aggregation are blue-shifted and red-shifted compared to those in isolated state, respectively. [16,25] If the two stacking styles coexist, the absorption range could be enlarged. Secondly, H-aggregation has a higher lowest excited level [19] which provides stronger driving force for exciton dissociation, and excitons may have longer lifetime in H-aggrega...

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“…Although fullerenes have been used widely in OPV devices, their (photo)excited state properties have received relatively little attention up to now . However, the role of fullerenes as light absorber contributing to the photocurrent generation has gained increasing recognition as several recent reports demonstrate . In fact, these studies suggest that fullerenes not only play a role as electron acceptor, but also can contribute significantly to light harvesting in the photoactive layer .…”

Section: Introductionmentioning

confidence: 99%

Charge and Triplet Exciton Generation in Neat PC₇₀BM Films and Hybrid CuSCN:PC₇₀BM Solar Cells

Karuthedath

Gorenflot

Firdaus

et al. 2018

Advanced Energy Materials

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Organic solar cells that use only fullerenes as the photoactive material exhibit poor exciton‐to‐charge conversion efficiencies, resulting in low internal quantum efficiencies (IQE). However, the IQE can be greatly improved, when copper(I) thiocyanate (CuSCN) is used as a carrier‐selective interlayer between the phenyl‐C70‐butyric acid methyl ester (PC70BM) layer and the anode. Efficiencies of ≈5.4% have recently been reported for optimized CuSCN:PC70BM (1:3)‐mesostructured heterojunctions, yet the reasons causing the efficiency boost remain unclear. Here, transient absorption (TA) spectroscopy is used to demonstrate that CuSCN does not only act as a carrier‐selective electrode layer, but also facilitates fullerene exciton dissociation and hole transfer at the interface with PC70BM. While intrinsic charge generation in neat PC70BM films proceeds with low yield, hybrid films exhibit much improved exciton dissociation due to the presence of abundant interfaces. Triplet generation with a rate proportional to the product of singlet and charge concentrations is observed in neat PC70BM films, implying a charge–singlet spin exchange mechanism, while in hybrid films, this mechanism is absent and triplet formation is a consequence of nongeminate recombination of free charges. At low carrier concentrations, the fraction of charges outweighs the population of triplets, leading to respectable device efficiencies under one sun illumination.

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Photoinduced Injection Enhancement in Fullerene-Based Organic Solar Cell Originates from Exciton–Electron Interaction

Cited by 10 publications

References 40 publications

Site-specific probing of charge transfer dynamics in organic photovoltaics

Site-specific probing of charge transfer dynamics in organic photovoltaics

Balancing the H- and J-aggregation in DTS(PTTh₂)₂/PC₇₀BM to yield a high photovoltaic efficiency

Charge and Triplet Exciton Generation in Neat PC₇₀BM Films and Hybrid CuSCN:PC₇₀BM Solar Cells

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Photoinduced Injection Enhancement in Fullerene-Based Organic Solar Cell Originates from Exciton–Electron Interaction

Cited by 10 publications

References 40 publications

Site-specific probing of charge transfer dynamics in organic photovoltaics

Site-specific probing of charge transfer dynamics in organic photovoltaics

Balancing the H- and J-aggregation in DTS(PTTh2)2/PC70BM to yield a high photovoltaic efficiency

Charge and Triplet Exciton Generation in Neat PC70BM Films and Hybrid CuSCN:PC70BM Solar Cells

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Product

Resources

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Balancing the H- and J-aggregation in DTS(PTTh₂)₂/PC₇₀BM to yield a high photovoltaic efficiency

Charge and Triplet Exciton Generation in Neat PC₇₀BM Films and Hybrid CuSCN:PC₇₀BM Solar Cells