Imperial College London

DrJunYan

Faculty of Natural SciencesDepartment of Physics

Research Associate
 
 
 
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Contact

 

j.yan17

 
 
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Location

 

Huxley 704Blackett LaboratorySouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

16 results found

Yan J, Rezasoltani E, Azzouzi M, Eisner F, Nelson Jet al., 2021, Influence of static disorder of charge transfer state on voltage loss in organic photovoltaics, Nature Communications, Vol: 12, ISSN: 2041-1723

Spectroscopic measurements of charge transfer (CT) states provide valuable insight into the voltage losses in organic photovoltaics (OPVs). Correct interpretation of CT-state spectra depends on knowledge of the underlying broadening mechanisms, and the relative importance of molecular vibrational broadening and variations in the CT-state energy (static disorder). Here, we present a physical model, that obeys the principle of detailed balance between photon absorption and emission, of the impact of CT-state static disorder on voltage losses in OPVs. We demonstrate that neglect of CT-state disorder in the analysis of spectra may lead to incorrect estimation of voltage losses in OPV devices. We show, using measurements of polymer:non-fullerene blends of different composition, how our model can be used to infer variations in CT-state energy distribution that result from variations in film microstructure. This work highlights the potential impact of static disorder on the characteristics of disordered organic blend devices.

Journal article

Zhang M, Guo F, Zhou Q, Zhong T, Xiao B, Zou L, You Q, You B, Li Y, Liu X, Liu H, Yan J, Liu Jet al., 2021, Enhanced performance through trap states passivation in quantum dot light emitting diode, Journal of Luminescence, Vol: 234, Pages: 1-7, ISSN: 0022-2313

Device performance enhancement in quantum dot light-emitting diodes (QLEDs) is realized by adding a small amount of insulating polymer polymethyl methacrylate (PMMA) into the emitting quantum dot layer. Pool-Frenkel effect is observed through temperature-dependent current density-voltage experiments, indicating the important role of trap states, and the addition of the insulating PMMA helps to reduce the Poole-Frenkel barrier hence the trap depth. Reduced density and depth of trap states with PMMA are indeed observed through further capacitance measurements. This work contributes to a better understanding on the effects of traps in QLEDs.

Journal article

Yiwen W, Jinho L, Xueyan H, Labanti C, Jun Y, Amber P, Eva M, Jenny N, Ji-Seon K, Zhe Let al., 2021, Recent progress and Challenges toward highly stable nonfullerene acceptor‐based organic solar cells, Advanced Energy Materials, Vol: 11, ISSN: 1614-6832

Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have made significant breakthrough in their device performance, now achieving a power conversion efficiency of ≈18% for single junction devices, driven by the rapid development in their molecular design and device engineering in recent years. However, achieving long‐term stability remains a major challenge to overcome for their commercialization, due in large part to the current lack of understanding of their degradation mechanisms as well as the design rules for enhancing their stability. In this review, the recent progress in understanding the degradation mechanisms and enhancing the stability of high performance NFA‐based OSCs is a specific focus. First, an overview of the recent advances in the molecular design and device engineering of several classes of high performance NFA‐based OSCs for various targeted applications is provided, before presenting a critical review of the different degradation mechanisms identified through photochemical‐, photo‐, and morphological degradation pathways. Potential strategies to address these degradation mechanisms for further stability enhancement, from molecular design, interfacial engineering, and morphology control perspectives, are also discussed. Finally, an outlook is given highlighting the remaining key challenges toward achieving the long‐term stability of NFA‐OSCs.

Journal article

Rezasoltani E, Guilbert AAY, Yan J, Rodríguez-Martínez X, Azzouzi M, Eisner F, Tuladhar SM, Hamid Z, Wadsworth A, McCulloch I, Campoy-Quiles M, Nelson Jet al., 2020, Correlating the phase behavior with the device performance in binary poly-3-hexylthiophene: nonfullerene acceptor blend using optical probes of the microstructure, Chemistry of Materials, Vol: 32, Pages: 8294-8305, ISSN: 0897-4756

The performance of photovoltaic devices based on blends of conjugated polymers with nonfullerene acceptors depends on the phase behavior and microstructure of the binary, which in turn depends on the chemical structures of the molecular components and the blend composition. We investigate the correlation between the molecular structure, composition, phase behavior, and device performance of a model system consisting of semicrystalline poly-3-hexylthiophene (P3HT) as the donor polymer and three nonfullerene acceptors, two of which (O-IDTBR/EH-IDTBR) have a planar core with different side chains and one (O-IDFBR) of which has a twisted core. We combine differential scanning calorimetry with optical measurements including UV–Vis spectroscopy, photoluminescence, spectroscopic ellipsometry, and Raman spectroscopy and photovoltaic device performance measurements, all at varying blend composition. For P3HT:IDTBR blends, the crystallinity of polymer and acceptor is preserved over a wide composition range and the blend displays a eutectic phase behavior, with the optimum solar cell composition lying close to the eutectic composition. For P3HT:IDFBR blends, increasing acceptor content disrupts the polymer crystallinity, and the optimum device composition appears to be limited by polymer connectivity rather than being linked to the eutectic composition. The optical probes allow us to probe both the crystalline and amorphous phases, clearly revealing the compositions at which component mixing disrupts crystallinity.

Journal article

Xiao B, Calado P, Mackenzie R, Kirchartz T, Yan J, Nelson Jet al., 2020, Relationship between fill factor and light intensity in solar cells based on organic disordered semiconductors: The role of tail states, Physical Review Applied, Vol: 14, Pages: 024034 – 1-024034 – 17, ISSN: 2331-7019

The origin of the relationship between fill factor (FF) and light intensity (I) in organic disordered-semiconductor-based solar cells is studied. An analytical model describing the balance between transport and recombination of charge carriers, parameterized with a factor, Γm, is introduced to understand the FF-I relation, where higher values of Γm correlate to larger FFs. Comparing the effects of direct and tail-state-mediated recombination on the FF-I plot, we find that, for low-mobility systems, direct recombination with constant transport mobility can deliver only a negative dependence of Γm,dir on light intensity. By contrast, tail-state-mediated recombination with trapping and detrapping processes can produce a positive Γm,t versus sun dependency. The analytical model is validated by numerical drift-diffusion simulations. To further validate our model, two material systems that show opposite FF-I behavior are studied: poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-[4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl]} (PTB7-Th):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) devices show a negative FF-I relation, while PTB7-Th:(5Z,5′Z)-5,5′-{[7,7′ -(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl)]bis(methanylylidene)}bis(3-ethyl-2-thioxothiazolidin-4-one) (O-IDTBR) devices show a positive correlation. Optoelectronic measurements show that the O-IDTBR device presents a higher ideality factor, stronger trapping and detrapping behavior, and a higher density of trap states, relative to the PC71BM device, supporting the theoretical model. This work provides a comprehensive understanding of the correlation between FF and light intensity for disordered-semiconductor-based solar cells.

Journal article

Azzouzi M, Yan J, Kirchartz T, Liu K, Wang J, Wu H, Nelson Jet al., 2018, Non-radiative energy losses in bulk-heterojunction organic photovoltaics, Physical Review X, Vol: 8, ISSN: 2160-3308

The performance of solar cells based on molecular electronic materials is limited by relatively high nonradiative voltage losses. The primary pathway for nonradiative recombination in organic donor-acceptor heterojunction devices is believed to be the decay of a charge-transfer (CT) excited state to the ground state via energy transfer to vibrational modes. Recently, nonradiative voltage losses have been related to properties of the charge-transfer state such as the Franck-Condon factor describing the overlap of the CT and ground-state vibrational states and, therefore, to the energy of the CT state. However, experimental data do not always follow the trends suggested by the simple model. Here, we extend this recombination model to include other factors that influence the nonradiative decay-rate constant, and therefore the open-circuit voltage, but have not yet been explored in detail. We use the extended model to understand the observed behavior of series of small molecules:fullerene blend devices, where open-circuit voltage appears insensitive to nonradiative loss. The trend could be explained only in terms of a microstructure-dependent CT-state oscillator strength, showing that parameters other than CT-state energy can control nonradiative recombination. We present design rules for improving open-circuit voltage via the control of material parameters and propose a realistic limit to the power-conversion efficiency of organic solar cells.

Journal article

Liu S, Liang Q, Yan J, Wu H, Cao Yet al., 2018, Distinguishing limits on the fill factor in organic solar cells processed from different solvents: Charge recombination kinetics vs. charge extraction, ORGANIC ELECTRONICS, Vol: 59, Pages: 427-431, ISSN: 1566-1199

Journal article

Deng W, Gao K, Yan J, Liang Q, Xie Y, He Z, Wu H, Peng X, Cao Yet al., 2018, Origin of Reduced Open-Circuit Voltage in Highly Efficient Small-Molecule-Based Solar Cells upon Solvent Vapor Annealing, ACS APPLIED MATERIALS & INTERFACES, Vol: 10, Pages: 8141-8147, ISSN: 1944-8244

Journal article

Wadsworth A, Hamid Z, Bidwell M, Ashraf RS, Khan JI, Anjum DH, Cendra C, Yan J, Rezasoltani E, Guilbert AAY, Azzouzi M, Gasparini N, Bannock JH, Baran D, Wu H, de Mello JC, Brabec CJ, Salleo A, Nelson J, Laquai F, McCulloch Iet al., 2018, Progress in Poly (3-Hexylthiophene) Organic Solar Cells and the Influence of Its Molecular Weight on Device Performance, ADVANCED ENERGY MATERIALS, Vol: 8, ISSN: 1614-6832

Journal article

Xiao B, Zhang M, Yan J, Luo G, Gao K, Liu J, You Q, Wang H-B, Gao C, Zhao B, Zhao X, Wu H, Liu Fet al., 2017, High efficiency organic solar cells based on amorphous electron-donating polymer and modified fullerene acceptor, NANO ENERGY, Vol: 39, Pages: 478-488, ISSN: 2211-2855

Journal article

Yan J, Liang Q, Liu K, Miao J, Chen H, Liu S, He Z, Wu H, Wang J, Cao Yet al., 2017, Optimized Phase Separation and Reduced Geminate Recombination in High Fill Factor Small-Molecule Organic Solar Cells, ACS ENERGY LETTERS, Vol: 2, Pages: 14-21, ISSN: 2380-8195

Journal article

Wang J-L, Liu K-K, Yan J, Wu Z, Liu F, Xiao F, Chang Z-F, Wu H-B, Cao Y, Russell TPet al., 2016, Series of Multifluorine Substituted Oligomers for Organic Solar Cells with Efficiency over 9% and Fill Factor of 0.77 by Combination Thermal and Solvent Vapor Annealing, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 138, Pages: 7687-7697, ISSN: 0002-7863

Journal article

Wang J-L, Xiao F, Yan J, Wu Z, Liu K-K, Chang Z-F, Zhang R-B, Chen H, Wu H-B, Cao Yet al., 2016, Difluorobenzothiadiazole-Based Small-Molecule Organic Solar Cells with 8.7% Efficiency by Tuning of pi-Conjugated Spacers and Solvent Vapor Annealing, ADVANCED FUNCTIONAL MATERIALS, Vol: 26, Pages: 1803-1812, ISSN: 1616-301X

Journal article

Wang J-L, Xiao F, Yan J, Liu K-K, Chang Z-F, Zhang R-B, Wu H-B, Cao Yet al., 2016, Toward high performance indacenodithiophene-based small-molecule organic solar cells: investigation of the effect of fused aromatic bridges on the device performance, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 4, Pages: 2252-2262, ISSN: 2050-7488

Journal article

Chen H, Miao J, Yan J, He Z, Wu Het al., 2016, Improving Organic Solar Cells Efficiency Through a Two-Step Method Consisting of Solvent Vapor Annealing and Thermal Annealing, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, Vol: 22, ISSN: 1077-260X

Journal article

Yan J, Luo G, Xiao B, Wu H, He Z, Cao Yet al., 2015, Origin of high fill factor in polymer solar cells from semiconducting polymer with moderate charge carrier mobility, ORGANIC ELECTRONICS, Vol: 24, Pages: 125-130, ISSN: 1566-1199

Journal article

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