共查询到20条相似文献，搜索用时 15 毫秒
1.
Bhoj R. Gautam Robert Younts Wentao Li Liang Yan Evgeny Danilov Erik Klump Iordania Constantinou Franky So Wei You Harald Ade Kenan Gundogdu 《Liver Transplantation》2016,6(1)
The role of excess excitation energy on long‐range charge separation in organic donor/acceptor bulk heterojunctions (BHJs) continues to be unclear. While ultrafast spectroscopy results argue for efficient charge separation through high‐energy charge‐transfer (CT) states within the first picosecond (ps) of excitation, charge collection measurements suggest excess photon energy does not increase the current density in BHJ devices. Here, the population dynamics of charge‐separated polarons upon excitation of high‐energy polymer states and low‐energy interfacial CT states in two polymer/fullerene blends from ps to nanosecond time scales are studied. It is observed that the charge‐separation dynamics do not show significant dependence on excitation energy. These results confirm that excess exciton energy is not necessary for the effective generation of charges. 相似文献
2.
Organic Photovoltaics: Charge Photogeneration in Organic Photovoltaics: Role of Hot versus Cold Charge‐Transfer Excitons (Adv. Energy Mater. 1/2016) 
下载免费PDF全文
下载免费PDF全文 Bhoj R. Gautam Robert Younts Wentao Li Liang Yan Evgeny Danilov Erik Klump Iordania Constantinou Franky So Wei You Harald Ade Kenan Gundogdu 《Liver Transplantation》2016,6(1)
3.
Solar Cells: Domain Compositions and Fullerene Aggregation Govern Charge Photogeneration in Polymer/Fullerene Solar Cells (Adv. Energy Mater. 11/2014) 下载免费PDF全文
下载免费PDF全文 Sameer Vajjala Kesava Zhuping Fei Adam D. Rimshaw Cheng Wang Alexander Hexemer John B. Asbury Martin Heeney Enrique D. Gomez 《Liver Transplantation》2014,4(11)
4.
Sameer Vajjala Kesava Zhuping Fei Adam D. Rimshaw Cheng Wang Alexander Hexemer John B. Asbury Martin Heeney Enrique D. Gomez 《Liver Transplantation》2014,4(11)
The complex microstructure of organic semiconductor mixtures continues to obscure the connection between the active layer morphology and photovoltaic device performance. For example, the ubiquitous presence of mixed phases in the active layer of polymer/fullerene solar cells creates multiple morphologically distinct interfaces which are capable of exciton dissociation or charge recombination. Here, it is shown that domain compositions and fullerene aggregation can strongly modulate charge photogeneration at ultrafast timescales through studies of a model system, mixtures of a low band‐gap polymer, poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]germole)‐2,6‐diyl‐alt‐(2,1,3‐benzothia‐diazole)‐4,7‐diyl], and [6,6]‐phenyl‐C71‐butyric acid methyl ester. Structural characterization using energy‐filtered transmission electron microscopy (EFTEM) and resonant soft X‐ray scattering shows similar microstructures even with changes in the overall film composition. Composition maps generated from EFTEM, however, demonstrate that compositions of mixed domains vary significantly with overall film composition. Furthermore, the amount of polymer in the mixed domains is inversely correlated with device performance. Photoinduced absorption studies using ultrafast infrared spectroscopy demonstrate that polaron concentrations are highest when mixed domains contain the least polymer. Grazing‐incidence X‐ray scattering results show that larger fullerene coherence lengths are correlated to higher polaron yields. Thus, the purity of the mixed domains is critical for efficient charge photogeneration because purity modulates fullerene aggregation and electron delocalization. 相似文献
5.
Hilary M. Feier Obadiah G. Reid Natalie A. Pace Jaehong Park Jesse J. Bergkamp Alan Sellinger Devens Gust Garry Rumbles 《Liver Transplantation》2016,6(6)
How free charge is generated at organic donor–acceptor interfaces is an important question, as the binding energy of the lowest energy (localized) charge transfer states should be too high for the electron and hole to escape each other. Recently, it has been proposed that delocalization of the electronic states participating in charge transfer is crucial, and aggregated or otherwise locally ordered structures of the donor or the acceptor are the precondition for this electronic characteristic. The effect of intermolecular aggregation of both the polymer donor and fullerene acceptor on charge separation is studied. In the first case, the dilute electron acceptor triethylsilylhydroxy‐1,4,8,11,15,18,22,25‐octabutoxyphthalocyaninatosilicon(IV) (SiPc) is used to eliminate the influence of acceptor aggregation, and control polymer order through side‐chain regioregularity, comparing charge generation in 96% regioregular (RR‐) poly(3‐hexylthiophene) (P3HT) with its regiorandom (RRa‐) counterpart. In the second case, ordered phases in the polymer are eliminated by using RRa‐P3HT, and phenyl‐C61‐butyric acid methyl ester (PC61BM) is used as the acceptor, varying its concentration to control aggregation. Time‐resolved microwave conductivity, time‐resolved photoluminescence, and transient absorption spectroscopy measurements show that while ultrafast charge transfer occurs in all samples, long‐lived charge carriers are only produced in films with intermolecular aggregates of either RR‐P3HT or PC61BM, and that polymer aggregates are just as effective in this regard as those of fullerenes. 相似文献
6.
Organic Electronics: The Roles of Structural Order and Intermolecular Interactions in Determining Ionization Energies and Charge‐Transfer State Energies in Organic Semiconductors (Adv. Energy Mater. 22/2016) 下载免费PDF全文
下载免费PDF全文 Kenneth R. Graham Guy O. Ngongang Ndjawa Sarah M. Conron Rahim Munir Koen Vandewal John J. Chen Sean Sweetnam Mark E. Thompson Alberto Salleo Michael D. McGehee Aram Amassian 《Liver Transplantation》2016,6(22)
7.
Kenneth R. Graham Guy O. Ngongang Ndjawa Sarah M. Conron Rahim Munir Koen Vandewal John J. Chen Sean Sweetnam Mark E. Thompson Alberto Salleo Michael D. McGehee Aram Amassian 《Liver Transplantation》2016,6(22)
The energy landscape in organic semiconducting materials greatly influences charge and exciton behavior, which are both critical to the operation of organic electronic devices. These energy landscapes can change dramatically depending on the phases of material present, including pure phases of one molecule or polymer and mixed phases exhibiting different degrees of order and composition. In this work, ultraviolet photoelectron spectroscopy measurements of ionization energies (IEs) and external quantum efficiency measurements of charge‐transfer (CT) state energies (ECT) are applied to molecular photovoltaic material systems to characterize energy landscapes. The results show that IEs and ECT values are highly dependent on structural order and phase composition. In the sexithiophene:C60 system both the IEs of sexithiophene and C60 shift by over 0.4 eV while ECT shifts by 0.5 eV depending on molecular composition. By contrast, in the rubrene:C60 system the IE of rubrene and C60 vary by ≤ 0.11 eV and ECT varies by ≤ 0.04 eV as the material composition varies. These results suggest that energy landscapes can exist whereby the binding energies of the CT states are overcome by energy offsets between charges in CT states in mixed regions and free charges in pure phases. 相似文献
8.
Safakath Karuthedath Yuliar Firdaus Ru‐Ze Liang Julien Gorenflot Pierre M. Beaujuge Thomas D. Anthopoulos Frdric Laquai 《Liver Transplantation》2019,9(33)
Ternary organic solar cells (OSCs) are among the best‐performing organic photovoltaic devices to date, largely due to the recent development of nonfullerene acceptors. However, fullerene molecules still play an important role in ternary OSC systems, since, for reasons not well understood, they often improve the device performance, despite their lack of absorption. Here, the photophysics of a prototypical ternary small‐molecule OSC blend composed of the donor DR3, the nonfullerene acceptor ICC6, and the fullerene derivative PC71BM is studied by ultrafast spectroscopy. Surprisingly, it is found that after excitation of PC71BM, ultrafast singlet energy transfer to ICC6 competes efficiently with charge transfer. Subsequently, singlets on ICC6 undergo hole transfer to DR3, resulting in free charge generation. Interestingly, PC71BM improves indirectly the electron mobility of the ternary blend, while electrons reside predominantly in ICC6 domains as indicated by fast spectroscopy. The improved mobility facilitates charge carrier extraction, in turn leading to higher device efficiencies of the ternary compared to binary solar cells. Using the (photo)physical parameters obtained from (transient) spectroscopy and charge transport measurements, the device's current–voltage characteristics are simulated and it is demonstrated that the parameters accurately reproduce the experimentally measured device performance. 相似文献
9.
Significant work has been directed at measuring the exciton diffusion length (LD) in organic semiconductors due to its significance in determining the performance of photovoltaic cells. Several techniques have been developed to measure LD, often probing photoluminescence or charge carrier generation. Interestingly, in this study it is shown that when different techniques are compared, both the diffusive behavior of the exciton and active carrier recombination loss pathways can be decoupled. Here, a planar heterojunction device based on the donor–acceptor pairing of boron subphthalocyanine chloride‐C60 is examined using photoluminescence quenching, photovoltage‐, and photocurrent‐based LD measurement techniques. Photovoltage yields the device relevant LD of both active materials as a function of forward bias subject to geminate recombination losses. These values are used to accurately predict the photocurrent as a function of voltage, suggesting geminate recombination is the dominant mechanism responsible for photocurrent loss. By combining these measurements with photocurrent and photoluminescence quenching, the intrinsic LD, as well as the voltage‐dependent charge transfer state dissociation and charge collection efficiencies are quantitatively determined. The results of this work provide a method to decouple all relevant loss pathways during photoconversion, and establish the factors that can limit the performance of excitonic photovoltaic cells. 相似文献
10.
Dorota Jarzab Fabrizio Cordella Jio Gao Markus Scharber Hans‐Joachim Egelhaaf Maria Antonietta Loi 《Liver Transplantation》2011,1(4):604-609
Photoluminescence studies of the charge transfer exciton emission from a narrow‐bandgap polymer‐based bulk heterojunction are reported. The quantum yield of this emission is as high as 0.03%. Low temperature measurements reveal that while the dynamics of the singlet exciton is slower at low temperature, the dynamics of the charge transfer exciton emission is temperature independent. This behavior rules out any diffusion process of the charge transfer excitons and energy transfer from these interfacial states toward lower lying states. Photoluminescence measurements performed on the device under bias show a reduction (but not the total suppression) of the charge transfer exciton recombination. Finally, based on the low temperature results the role of the charge transfer excitons and the possible pathways to populate them are identified. 相似文献
11.
Marcel Schubert Daniel Dolfen Johannes Frisch Steffen Roland Robert Steyrleuthner Burkhard Stiller Zhihua Chen Ullrich Scherf Norbert Koch Antonio Facchetti Dieter Neher 《Liver Transplantation》2012,2(3):369-380
The authors present efficient all‐polymer solar cells comprising two different low‐bandgap naphthalenediimide (NDI)‐based copolymers as acceptors and regioregular P3HT as the donor. It is shown that these naphthalene copolymers have a strong tendency to preaggregate in specific organic solvents, and that preaggregation can be completely suppressed when using suitable solvents with large and highly polarizable aromatic cores. Organic solar cells prepared from such nonaggregated polymer solutions show dramatically increased power conversion efficiencies of up to 1.4%, which is mainly due to a large increase of the short circuit current. In addition, optimized solar cells show remarkable high fill factors of up to 70%. The analysis of the blend absorbance spectra reveals a surprising anticorrelation between the degree of polymer aggregation in the solid P3HT:NDI copolymer blends and their photovoltaic performance. Scanning near‐field optical microscopy (SNOM) and atomic force microscopy (AFM) measurements reveal important information on the blend morphology. It is shown that films with high degree of aggregation and low photocurrents exhibit large‐scale phase‐separation into rather pure donor and acceptor domains. It is proposed that, by suppressing the aggregation of NDI copolymers at the early stage of film formation, the intermixing of the donor and acceptor component is improved, thereby allowing efficient harvesting of photogenerated excitons at the donor–acceptor heterojunction. 相似文献
12.
Eric T. Hoke Koen Vandewal Jonathan A. Bartelt William R. Mateker Jessica D. Douglas Rodrigo Noriega Kenneth R. Graham Jean M. J. Fréchet Alberto Salleo Michael D. McGehee 《Liver Transplantation》2013,3(2):220-230
Polymer:fullerene solar cells are demonstrated with power conversion efficiencies over 7% with blends of PBDTTPD and PC61BM. These devices achieve open‐circuit voltages (Voc) of 0.945 V and internal quantum efficiencies of 88%, making them an ideal candidate for the large bandgap junction in tandem solar cells. Voc’s above 1.0 V are obtained when the polymer is blended with multiadduct fullerenes; however, the photocurrent and fill factor are greatly reduced. In PBDTTPD blends with multiadduct fullerene ICBA, fullerene emission is observed in the photoluminescence and electroluminescence spectra, indicating that excitons are recombining on ICBA. Voltage‐dependent, steady state and time‐resolved photoluminescence measurements indicate that energy transfer occurs from PBDTTPD to ICBA and that back hole transfer from ICBA to PBDTTPD is inefficient. By analyzing the absorption and emission spectra from fullerene and charge transfer excitons, we estimate a driving free energy of –0.14 ± 0.06 eV is required for efficient hole transfer. These results suggest that the driving force for hole transfer may be too small for efficient current generation in polymer:fullerene solar cells with Voc values above 1.0 V and that non‐fullerene acceptor materials with large optical gaps (>1.7 eV) may be required to achieve both near unity internal quantum efficiencies and values of Voc exceeding 1.0 V. 相似文献
13.
The dynamics of energy and charge transfer in the Photosystem II reaction center complex is an area of great interest today.
These processes occur on a time scale ranging from femtoseconds to tens of picoseconds or longer. Steady-state and ultrafast
spectroscopy techniques have provided a great deal of quantitative and qualitative data that have led to varied interpretations
and phenomenological models. More recently, microscopic models that identify specific charge separated states have been introduced,
and offer more insight into the charge transfer mechanism. The structure and energetics of PS II reaction centers are reviewed,
emphasizing the effects on the dynamics of the initial charge transfer.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
14.
Stefan Väth Kristofer Tvingstedt Andreas Baumann Michael C. Heiber Andreas Sperlich John A. Love Thuc‐Quyen Nguyen Vladimir Dyakonov 《Liver Transplantation》2017,7(7)
Triplet exciton formation in neat 7,7‐(4,4‐bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b′] dithiophene‐2,6‐diyl)bis(6‐fluoro‐4‐(5′‐hexyl‐[2,2′‐bithiophen]‐5‐yl)benzo[c][1,2,5]thiadiazole) (p‐DTS(FBTTh2)2) and blends with [6,6]‐Phenyl C70 butyric acid methyl ester (PC70BM), with and without the selective solvent additive 1,8‐diiodooctane, is investigated by means of spin sensitive photoluminescence measurements. For all three material systems, a significant amount of long living triplet excitons is detected, situated on the p‐DTS(FBTTh2)2 molecules. The characteristic zero‐field splitting parameters for this state are identified to be D = 42 mT (1177 MHz) and E = 5 mT (140 MHz). However, no triplet excitons located on PC70BM are detectable. Using electrically detected spin resonance, the presence of these triplet excitons is confirmed even at room temperature, highlighting that triplet excitons form during solar cell operation and influence the photocurrent and photovoltage. Surprisingly, the superior performing blend is found to have the largest triplet population. It is concluded, that the formation of triplet excitons from charge transfer states via electron back transfer has no crucial impact on device performance in p‐DTS(FBTTh2)2:PC70BM based solar cells. 相似文献
15.
Biniam Zerai Tedlla Feng Zhu Matthijs Cox Jeroen Drijkoningen Jean Manca Bert Koopmans Etienne Goovaerts 《Liver Transplantation》2015,5(2)
Triplet exciton (TE) formation pathways are systematically investigated in prototype bulk heterojunction (BHJ) “super yellow” poly(p‐phenylene vinylene) (SY‐PPV) solar cell devices with varying fullerene compositions using complementary optoelectrical and electrically detected magnetic resonance (EDMR) spectroscopies. It is shown that EDMR spectroscopy allows the unambiguous demonstration of fullerene triplet production in BHJ polymer:fullerene solar cells. EDMR triplet detection under selective photoexcitation of each blend component and of the interfacial charge transfer (CT) state reveals that low lying fullerene TEs are produced by direct intersystem crossing from singlet excitons (SEs). The direct CT‐TE recombination pathway, although energetically feasible, is kinetically suppressed in these devices. However, high energy CT states in the CT manifold can contribute to the population of the fullerene triplet state via a direct CT‐SE conversion. This undesirable energetic alignment could be one of the causes for the severe reduction in photocurrent observed when the open‐circuit voltage of polymer:fullerene solar cells is pushed to 1.0 V or beyond. 相似文献
16.
Organic ternary heterojunction photovoltaic blends are sometimes observed to undergo a gradual evolution in open‐circuit voltage (Voc) with increasing amounts of a second donor or an acceptor. The Voc is strongly correlated with the energy of the charge transfer state in the blend, but this value depends on both local and mesoscopic orders. In this work, the behavior of Voc in the presence of a wide range of interfacial electronic states is investigated. The key charge transfer state interfaces responsible for Voc in several model systems with varying morphology are identified. Systems consisting of one donor with two fullerene molecules and of one acceptor with a donor polymer of varying regio‐regularity are used. The effects from the changing energetic disorder in the material and from the variation due to a law of simple mixtures are quantified. It has been found that populating the higher‐energy charge transfer states is not responsible for the observed change in Voc upon the addition of a third component. Aggregating polymers and miscible fullerenes are compared, and it has been concluded that in both cases charge delocalization, aggregation, and local polarization effects shift the lowest‐energy charge transfer state distribution. 相似文献
17.
Organic solar cells lag behind their inorganic counterparts in efficiency due largely to low open‐circuit voltages (Voc). In this work, a comprehensive framework for understanding and improving the open‐circuit voltage of organic solar cells is developed based on equilibrium between charge transfer (CT) states and free carriers. It is first shown that the ubiquitous reduced Langevin recombination observed in organic solar cells implies equilibrium and then statistical mechanics is used to calculate the CT state population density at each voltage. This general result permits the quantitative assignment of Voc losses to a combination of interfacial energetic disorder, non‐negligible CT state binding energies, large degrees of mixing, and sub‐ns recombination at the donor/acceptor interface. To quantify the impact of energetic disorder, a new temperature‐dependent CT state absorption measurement is developed. By analyzing how the apparent CT energy varies with temperature, the interfacial disorder can be directly extracted. 63–104 meV of disorder is found in five systems, contributing 75–210 mV of Voc loss. This work provides an intuitive explanation for why qVoc is almost always 500–700 meV below the energy of the CT state and shows how the voltage can be improved. 相似文献
18.
Guankui Long Bo Wu Ankur Solanki Xuan Yang Bin Kan Xinfeng Liu Dongchang Wu Zhou Xu Wei‐Ru Wu U‐Ser Jeng Jinyou Lin Miaomiao Li Yunchuang Wang Xiangjian Wan Tze Chien Sum Yongsheng Chen 《Liver Transplantation》2016,6(22)
Morphology plays a vital role on the performance of organic photovoltaics. However, our understanding of the morphology‐performance relationships for organic photovoltaics remains lacking. Specifically, it is still an open question why some bulk‐heterojunction blends exhibit electric field dependent J–V curves, while others do not. Through detailed fs‐μs transient absorption spectroscopy and morphology studies on the representative bulk‐heterojunction type small molecule (SM) donor system, a picture of different J–V behaviors from morphology aspects and excited dynamics is revealed. Our findings reveal that amorphous morphology in the lack of percolated pathways leads to the formation of strongly bound charge transfer states (CTSs), which accounts for about one third of the photoexcited species. Therefore, field‐dependent J–V curves are obtained as these CTSs mainly undergo geminate recombination or function as interfacial traps for nongeminate recombination. On the other hand, the CTSs are totally suppressed after post‐treatment owning to the formation of bicontinuous morphology, which results in very high efficiencies from exciton generation, diffusion, dissociation to charge extraction, thus contributes to field‐independent J–V characteristics. The insights gained in this work provide the effective guidelines to further optimize the performance of bulk‐heterojunction type SM‐organic photovoltaics through judicious morphology control and engineering. 相似文献
19.
Guy O. Ngongang Ndjawa Kenneth R. Graham Sonya Mollinger Di M. Wu David Hanifi Rohit Prasanna Bradley D. Rose Sukumar Dey Liyang Yu Jean‐Luc Brédas Michael D. McGehee Alberto Salleo Aram Amassian 《Liver Transplantation》2017,7(12)
In organic solar cells (OSCs), the energy of the charge‐transfer (CT) complexes at the donor–acceptor interface, E CT, determines the maximum open‐circuit voltage (V OC). The coexistence of phases with different degrees of order in the donor or the acceptor, as in blends of semi‐crystalline donors and fullerenes in bulk heterojunction layers, influences the distribution of CT states and the V OC enormously. Yet, the question of how structural heterogeneities alter CT states and the V OC is seldom addressed systematically. In this work, we combine experimental measurements of vacuum‐deposited rubrene/C60 bilayer OSCs, with varying microstructure and texture, with density functional theory calculations to determine how relative molecular orientations and extents of structural order influence E CT and V OC. We find that varying the microstructure of rubrene gives rise to CT bands with varying energies. The CT band that originates from crystalline rubrene lies up to ≈0.4 eV lower in energy compared to the one that arises from amorphous rubrene. These low‐lying CT states contribute strongly to V OC losses and result mainly from hole delocalization in aggregated rubrene. This work points to the importance of realizing interfacial structural control that prevents the formation of low E CT configurations and maximizes V OC. 相似文献
20.
Julia I. Deitz Pran K. Paul Rouin Farshchi Dmitry Poplavskyy Jeff Bailey Aaron R. Arehart David W. McComb Tyler J. Grassman 《Liver Transplantation》2019,9(35)
A new experimental framework for the characterization of defects in semiconductors is demonstrated. Through the direct, energy‐resolved correlation of three analytical techniques spanning six orders of magnitude in spatial resolution, a critical mid‐bandgap electronic trap level (EV + 0.56 eV) within Ag0.2Cu0.8In1?xGaxSe2 is traced to its nanoscale physical location and chemical source. This is achieved through a stepwise, site‐specific correlated characterization workflow consisting of device‐scale (≈1 mm2) deep level transient spectroscopy (DLTS) to survey the traps present, scanning probe–based DLTS (scanning‐DLTS) for mesoscale‐resolved (hundreds of nanometers) mapping of the target trap state's spatial distribution, and scanning transmission electron microscope based electron energy‐loss spectroscopy (STEM‐EELS) and X‐ray energy‐dispersive spectroscopy for nanoscale energy‐, structure, and chemical‐resolved investigation of the defect source. This first demonstration of the direct observation of sub‐bandgap defect levels via STEM‐EELS, combined with the DLTS methods, provides strong evidence that the long‐suspected CuIn/Ga substitutional defects are indeed the most likely source of the EV + 0.56 eV trap state and serves as a key example of this approach for the fundamental identification of defects within semiconductors, in general. 相似文献