共查询到20条相似文献,搜索用时 15 毫秒
1.
Open‐Circuit Voltage in Organic Solar Cells: The Impacts of Donor Semicrystallinity and Coexistence of Multiple Interfacial Charge‐Transfer Bands 下载免费PDF全文
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
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Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies 下载免费PDF全文
Vasileios C. Nikolis Johannes Benduhn Felix Holzmueller Fortunato Piersimoni Matthias Lau Olaf Zeika Dieter Neher Christian Koerner Donato Spoltore Koen Vandewal 《Liver Transplantation》2017,7(21)
High photon energy losses limit the open‐circuit voltage (VOC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the VOC by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha‐sexithiophene (α‐6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the VOC of an α‐6T/SubNc/SubPc fullerene‐free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D–A interface. By accurately measuring the optical gap (Eopt) and the energy of the charge‐transfer state (ECT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. Eopt – qVOC losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the VOC‐optimized devices, the low‐energy (700 nm) external quantum efficiency (EQE) peak remains high at 79%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low‐voltage losses can be combined with a high EQE in organic photovoltaic devices. 相似文献
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Beyond Langevin Recombination: How Equilibrium Between Free Carriers and Charge Transfer States Determines the Open‐Circuit Voltage of Organic Solar Cells 下载免费PDF全文
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
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Robin E. M. Willems Christ H. L. Weijtens Xander de Vries Reinder Coehoorn Ren A. J. Janssen 《Liver Transplantation》2019,9(10)
For 19 diketopyrrolopyrrole polymers, the highest occupied molecular orbital (HOMO) energies are determined from i) the oxidation potential with square‐wave voltammetry (SWV), ii) the ionization potential using ultraviolet photoelectron spectroscopy (UPS), and iii) density functional theory (DFT) calculations. The SWV HOMO energies show an excellent linear correlation with the open‐circuit voltage (Voc) of optimized solar cells in which the polymers form blends with a fullerene acceptor ([6,6]‐phenyl‐C61‐butyl acid methyl ester or [6,6]‐phenyl‐C71‐butyl acid methyl ester). Remarkably, the slope of the best linear fit is 0.75 ± 0.04, i.e., significantly less than unity. A weaker correlation with Voc is found for the HOMO energies obtained from UPS and DFT. Within the experimental error, the SWV and UPS data are correlated with a slope close to unity. The results show that electrochemically determined oxidation potentials provide an excellent method for predicting the Voc of bulk heterojunction solar cells, with absolute deviations less than 0.1 V. 相似文献
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Efficient Organic Solar Cells with Extremely High Open‐Circuit Voltages and Low Voltage Losses by Suppressing Nonradiative Recombination Losses 下载免费PDF全文
Xi Liu Xiaoyan Du Junyi Wang Chunhui Duan Xiaofeng Tang Thomas Heumueller Guogang Liu Yan Li Zhaohui Wang Jing Wang Feng Liu Ning Li Christoph J. Brabec Fei Huang Yong Cao 《Liver Transplantation》2018,8(26)
One of the most important factors that limits the efficiencies of bulk‐heterojunction organic solar cells (OSCs) is the modest open‐circuit voltage (Voc) due to their large voltage loss (Vloss) caused by significant nonradiative recombination loss. To boost the performance of OSCs toward their theoretical limit, developing high‐performance donor: acceptor systems featuring low Vloss with suppressed nonradiative recombination losses (<0.30 V) is desired. Herein, high performance OSCs based on a polymer donor benzodithiophene‐difluorobenzoxadiazole‐2‐decyltetradecyl (BDT‐ffBX‐DT) and perylenediimide‐based acceptors (PDI dimer with spirofluorene linker (SFPDI), PDI4, and PDI6) are reported which offer a high power conversion efficiency (PCE) of 7.5%, 56% external quantum efficiency associated with very high Voc (>1.10 V) and low Vloss (<0.60 V). A high Voc up to 1.23 V is achieved, which is among the highest values reported for OSCs with a PCE beyond 6%, to date. These attractive results are benefit from the suppressed nonradiative recombination voltage loss, which is as low as 0.20 V. This value is the lowest value for OSCs so far and is comparable to high performance crystalline silicon and perovskite solar cells. These results show that OSCs have the potential to achieve comparable Voc and voltage loss as inorganic photovoltaic technologies. 相似文献
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On the Impact of Contact Selectivity and Charge Transport on the Open‐Circuit Voltage of Organic Solar Cells 下载免费PDF全文
The selectivity of electrodes of solar cells is a critical factor that can limit the overall efficiency. If the selectivity of an electrode is not sufficient both electrons and holes recombine at its surface. In materials with poor transport properties such as in organic solar cells, these surface recombination currents are accompanied by large gradients of the quasi‐Fermi energies as the driving force. Experimental results from current–voltage characteristics, advanced photo‐ and electroluminescence as well as charge extraction of three different photoactive materials are shown and compared to drift‐diffusion simulations. It can be concluded that in cases of electrodes with reduced selectivity the decrease of the open‐circuit voltage can be divided into two distinct contributions, the reduction of the overall steady‐state charge carrier density and the gradients of the quasi‐Fermi energies. The results clearly show that for photoactive layers with poor transport properties, the gradient of the quasi‐Fermi energy in the vicinity of the contact is the main contribution to the loss in open‐circuit voltage. For imbalanced mobilities, this gives rise to the phenomenon that it is more challenging to realize a selective contact for the less mobile charge carrier, i.e., the hole contact in most organic solar cells. 相似文献
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Organic Salts as a Route to Energy Level Control in Low Bandgap,High Open‐Circuit Voltage Organic and Transparent Solar Cells that Approach the Excitonic Voltage Limit 下载免费PDF全文
John Suddard‐Bangsund Christopher J. Traverse Margaret Young Tyler J. Patrick Yimu Zhao Richard R. Lunt 《Liver Transplantation》2016,6(1)
A new series of organic salts with selective near‐infrared (NIR) harvesting to 950 nm is reported, and anion selection and blending is demonstrated to allow for fine tuning of the open‐circuit voltage. Extending photoresponse deeper into the NIR is a significant challenge facing small molecule organic photovoltaics, and recent demonstrations have been limited by open‐circuit voltages much lower than the theoretical and practical limits. This work presents molecular design strategies that enable facile tuning of energy level alignment and open‐circuit voltages in organic salt‐based photovoltaics. Anions are also shown to have a strong influence on exciton diffusion length. These insights provide a clear route toward achieving high efficiency transparent and panchromatic photovoltaics, and open up design opportunities to rapidly tailor molecules for new donor–acceptor systems. 相似文献
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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
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Understanding Open‐Circuit Voltage Loss through the Density of States in Organic Bulk Heterojunction Solar Cells 下载免费PDF全文
Samuel D. Collins Christopher M. Proctor Niva A. Ran Thuc‐Quyen Nguyen 《Liver Transplantation》2016,6(4)
The field of organic photovoltaics has recently produced highly efficient single‐junction cells with power conversion efficiency >10%, yet the open‐circuit voltage (VOC) remains relatively low in many high performing systems. An accurate picture of the density of states (DOS) in working solar cells is crucial to understanding the sources of voltage loss, but remains difficult to obtain experimentally. Here, the tail of the DOS is characterized in a number of small molecule bulk heterojunction solar cells from the charge density dependence of VOC, and is directly compared to the disorder present within donor and acceptor components as measured by Kelvin probe. Using these DOS distributions, the total energy loss relative to the charge transfer state energy (ECT)—ranging from ≈0.5 to 0.7 eV—is divided into contributions from energetic disorder and from charge recombination, and the extent to which these factors limit the VOC is assessed. 相似文献
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Xing Sheng Myoung Hee Yun Chen Zhang Ala'a M. Al‐Okaily Maria Masouraki Ling Shen Shuodao Wang William L. Wilson Jin Young Kim Placid Ferreira Xiuling Li Eli Yablonovitch John A. Rogers 《Liver Transplantation》2015,5(1)
Multijunction (MJ) solar cells have the potential to operate across the entire solar spectrum, for ultrahigh efficiencies in light to electricity conversion. Here an MJ cell architecture is presented that offers enhanced capabilities in photon recycling and photon extraction, compared to those of conventional devices. Ideally, each layer of a MJ cell should recycle and re‐emit its own luminescence to achieve the maximum possible voltage. This design involves materials with low refractive indices as interfaces between sub‐cells in the MJ structure. Experiments demonstrate that thin‐film GaAs devices printed on low‐index substrates exhibit improved photon recycling, leading to increased open‐circuit voltages (V oc), consistent with theoretical predictions. Additional systematic studies reveal important considerations in the thermal behavior of these structures under highly concentrated illumination. Particularly when combined with other optical elements such as anti‐reflective coatings, these architectures represent important aspects of design for solar cells that approach thermodynamic efficiency limits for full spectrum operation. 相似文献
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Shunsuke Yamamoto Akiko Orimo Hideo Ohkita Hiroaki Benten Shinzaburo Ito 《Liver Transplantation》2012,2(2):229-237
The origin of open‐circuit voltage (VOC) was studied for polymer solar cells based on a blend of poly(3‐hexylthiophene) (P3HT) and seven fullerene derivatives with different LUMO energy levels and side chains. The temperature dependence of J–V characteristics was analyzed by an equivalent circuit model. As a result, VOC increased with the decrease in the saturation current density J0 of the device. Furthermore, J0 was dependent on the activation energy EA for J0, which is related to the HOMO–LUMO energy gap between P3HT and fullerene. Interestingly, the pre‐exponential term J00 for J0 was larger for pristine fullerenes than for substituted fullerene derivatives, suggesting that the electronic coupling between molecules also has substantial impact on VOC. This is probably because the recombination is non‐diffusion‐lmilited reaction depending on electron transfer at the P3HT/fullerene interface. In summary, the origin of VOC is ascribed not only to the relative HOMO–LUMO energy gap but also to the electronic couplings between fullerene/fullerene and polymer/fullerene. 相似文献
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Xiaoyan Du Ole Lytken Manuela S. Killian Jiamin Cao Tobias Stubhan Mathieu Turbiez Patrik Schmuki Hans‐Peter Steinrück Liming Ding Rainer H. Fink Ning Li Christoph J. Brabec 《Liver Transplantation》2017,7(5)
Organic solar cells are promising in terms of full‐solution‐processing which enables low‐cost and large‐scale fabrication. While single‐junction solar cells have seen a boost in power conversion efficiency (PCE), multi‐junction solar cells are promising to further enhance the PCE. In all‐solution‐processed multi‐junction solar cells, interfacial losses are often encountered between hole‐transporting layer (HTL) and the active layers and therefore greatly limit the application of newly developed high‐performance donor and acceptor materials in multi‐junction solar cells. Here, the authors report on a systematic study of interface losses in both single‐junction and multi‐junction solar cells based on representative polymer donors and HTLs using electron spectroscopy and time‐of‐flight secondary ion mass spectrometry. It is found that a facile mixed HTL containing poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and MoO x nanoparticles successfully overcomes the interfacial losses in both single‐ and multi‐junction solar cells based on various active layers by reducing interface protonation, promoting better energy‐level alignment, and forming a dense and smooth layer. Solution‐processed single‐junction solar cells are demonstrated to reach the same performance as with evaporated MoO x (over 7%). Multi‐junction solar cells with polymers containing nitrogen atoms as the first layer and the mixed PEDOT:PSS and MoO x nanoparticles as hole extraction layer reach fill factor (FF) of over 60%, and PCE of over 8%, while the identical stack with pristine PEDOT:PSS or MoO x nanoparticles show FF smaller than 50% and PCE less than 5%. 相似文献
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Disorder‐Induced Open‐Circuit Voltage Losses in Organic Solar Cells During Photoinduced Burn‐In 下载免费PDF全文
Thomas Heumueller Timothy M. Burke William R. Mateker Isaac T. Sachs‐Quintana Koen Vandewal Christoph J. Brabec Michael D. McGehee 《Liver Transplantation》2015,5(14)
The photoinduced open‐circuit voltage (Voc) loss commonly observed in bulk heterojunction organic solar cells made from amorphous polymers is investigated. It is observed that the total charge carrier density and, importantly, the recombination dynamics are unchanged by photoinduced burn‐in. Charge extraction is used to monitor changes in the density of states (DOS) during degradation of the solar cells, and a broadening over time is observed. It is proposed that the Voc losses observed during burn‐in are caused by a redistribution of charge carriers in a broader DOS. The temperature and light intensity dependence of the Voc losses can be described with an analytical model that contains the amount of disorder broadening in a Gaussian DOS as the only fit parameter. Finally, the Voc loss in solar cells made from amorphous and crystalline polymers is compared and an increased stability observed in crystalline polymer solar cells is investigated. It is found that solar cells made from crystalline materials have a considerably higher charge carrier density than those with amorphous materials. The effects of a DOS broadening upon aging are suppressed in solar cells with crystalline materials due to their higher carrier density, making crystalline materials more stable against Voc losses during burn‐in. 相似文献