Imperial College London

Dr Pabitra Shakya Tuladhar

Faculty of Natural SciencesDepartment of Chemistry

Research Technician in Organic Electronics
 
 
 
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Contact

 

+44 (0)20 7594 5710p.shakya

 
 
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Location

 

G22Molecular Sciences Research HubWhite City Campus

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Summary

 

Publications

Publication Type
Year
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24 results found

Wu J, Luke J, Lee HKH, Tuladhar PS, Cha H, Jang S-Y, Tsoi WC, Heeney M, Kang H, Lee K, Kirchartz T, Kim J-S, Durrant JRet al., 2019, Tail state limited photocurrent collection of thick photoactive layers in organic solar cells, Nature Communications, Vol: 10, ISSN: 2041-1723

Weanalyseorganic solar cells with four differentphotoactive blends exhibiting differing dependencies ofshort-circuit current upon photoactive layer thickness.These blends and devices are analysedbytransient optoelectronic techniques ofcarrier kinetics and densities, airphotoemission spectroscopyof material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We concludethat,for the device series studied, the photocurrent losswith thick active layersis primarilyassociatedwith the accumulation of photo-generated charge carriers in intra-bandgap tail states.This charge accumulation screens the device internal electricalfield, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail statedistribution anddensity and increase the maximal photoactive thickness forefficient operation. Ourwork suggests that selectingorganic photoactive layerswith a narrow distribution of tail states isa keyrequirement for the fabrication of efficient, high photocurrent, thick organic solar cells.

Journal article

Dimitrov SD, Azzouzi M, Wu J, Yao J, Dong Y, Tuladhar PS, Schroeder BC, Bittner ER, McCulloch I, Nelson J, Durrant JRet al., 2019, Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends., J Am Chem Soc, Vol: 141, Pages: 4634-4643

Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron-hole separation at organic donor-acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron-hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer-fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer-fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron-hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron-hole recombination is observed revealing the importance of spatially localized electron-hole pairs (bound CT states) in the electron-hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron-hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron-hole separation can be exp

Journal article

Dong Y, Cha H, Zhang J, Pastor E, Tuladhar PS, McCulloch I, Durrant JR, Bakulin AAet al., 2019, The binding energy and dynamics of charge-transfer states in organic photovoltaics with low driving force for charge separation, Journal of Chemical Physics, Vol: 150, ISSN: 0021-9606

Recent progress in organic photovoltaics (OPVs) has been enabled by optimization of the energetic driving force for charge separation, and thus maximization of open-circuit voltage, using non-fullerene acceptor (NFA) materials. In spite of this, the carrier dynamics and relative energies of the key states controlling the photophysics of these systems are still under debate. Herein, we report an in-depth ultrafast spectroscopic study of a representative OPV system based on a polymer donor PffBT4T-2OD and a small-molecule NFA EH-IDTBR. Global analysis of the transient absorption data reveals efficient energy transfer between donor and acceptor molecules. The extracted kinetics suggest that slow (∼15 ps) generation of charge carriers is followed by significant geminate recombination. This contrasts with the "reference" PffBT4T-2OD:PC71BM system where bimolecular recombination dominates. Using temperature-dependent pump-push-photocurrent spectroscopy, we estimate the activation energy for the dissociation of bound charge-transfer states in PffBT4T-2OD:EH-IDTBR to be 100 ± 6 meV. We also observe an additional activation energy of 14 ± 7 meV, which we assign to the de-trapping of mobile carriers. This work provides a comprehensive picture of photophysics in a system representing new generation of OPV blends with a small driving force for charge separation.

Journal article

Francas Forcada L, Burns E, Steier L, Cha H, Solà-Hernández L, Li X, Shakya Tuladhar P, Bofill R, García-Antón J, Sala X, Durrant Jet al., 2018, Rational design of a neutral pH functional and stable organic photocathode., Chemical Communications, Vol: 2018, ISSN: 1359-7345

In this work we lay out design guidelines for catalytically more efficient organic photocathodes achieving stable hydrogen production in neutral pH. We propose an organic photocathode architecture employing a NiO hole selective layer, a PCDTBT:PCBM bulk heterojunction, a compact TiO2 electron selective contact and a RuO2 nanoparticle catalyst. The role of each layer is discussed in terms of durability and function. With this strategically designed organic photocathode we obtain stable photocurrent densities for over 5 h and discuss routes for further performance improvement.

Journal article

Utzat H, Dimitroy SD, Wheeler S, Collado-Fregoso E, Tuladhar PS, Schroeder BC, McCulloch I, Durrant JRet al., 2017, Charge-Separation in Intermixed Polymer:PC70BM Photovoltaic Blends: Correlating Structural and Photophysical Length Scales as a Function of Blend Composition, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 121, Pages: 9790-9801, ISSN: 1932-7447

A key challenge in achieving control over photocurrent generation by bulk-heterojunction organic solar cells is understanding how the morphology of the active layer impacts charge separation and in particular the separation dynamics within molecularly intermixed donor–acceptor domains versus the dynamics between phase-segregated domains. This paper addresses this issue by studying blends and devices of the amorphous silicon–indacenodithiophene polymer SiIDT-DTBT and the acceptor PC70BM. By changing the blend composition, we modulate the size and density of the pure and intermixed domains on the nanometer length scale. Laser spectroscopic studies show that these changes in morphology correlate quantitatively with the changes in charge separation dynamics on the nanosecond time scale and with device photocurrent densities. At low fullerene compositions, where only a single, molecularly intermixed polymer–fullerene phase is observed, photoexcitation results in a ∼ 30% charge loss from geminate polaron pair recombination, which is further studied via light intensity experiments showing that the radius of the polaron pairs in the intermixed phase is 3–5 nm. At high fullerene compositions (≥67%), where the intermixed domains are 1–3 nm and the pure fullerene phases reach ∼4 nm, the geminate recombination is suppressed by the reduction of the intermixed phase, making the fullerene domains accessible for electron escape.

Journal article

Casey A, Green JP, Shakya Tuladhar P, Kirkus M, Han Y, Anthopoulos TD, Heeney MJet al., 2017, Cyano substituted benzotriazole based polymers for use in organic solar cells, Journal of Materials Chemistry A, Vol: 5, Pages: 6465-6470, ISSN: 2050-7496

A new synthetic route to the electron accepting di-cyano substituted benzo[d][1,2,3]triazole (BTz) monomer 2-(2-butyloctyl)-4,7-di(thiophen-2-yl)-2H-benzotriazole-5,6-dicarbonitrile (dTdCNBTz) is reported. The cyano substituents can be easily introduced to the BTz unit in one step via the nucleophilic aromatic substitution of the fluorine substituents of the fluorinated precursor 2-(2-butyloctyl)-4,7-di(thiophen-2-yl)-2H-benzotriazole-5,6-difluoro (dTdFBTz). Co-polymers were prepared with distannylated benzo[1,2-b:4,5-b′]dithiophene (BDT) monomers containing either 2-ethylhexylthienyl (T-EH) side chains or 2-butyloctylthienyl (T-BO) side chains via Stille coupling to yield the novel medium band gap polymers P1 and P2 respectively. Whilst the organic photovoltaic (OPV) performance of P1 was limited by a lack of solubility, the improved solubility of P2 resulted in promising device efficiencies of up to 6.9% in blends with PC61BM, with high open circuit voltages of 0.95 V.

Journal article

Collado Fregoso E, Deledalle F, Utzat H, Tuladhar PS, Dimitrov S, Gillett A, Tan C, Zhang W, McCulloch I, Durrant Jet al., 2016, Photophysical study of DPPTT-T/PC70BM blends and solar devices as a function of fullerene loading: an insight into EQE limitations of DPP-based polymers, Advanced Functional Materials, Vol: 27, ISSN: 1616-3028

Diketopyrrolopyrrole (DPP)-based polymers have been consistently used for the fabrication of solar cell devices and transistors, due to the existence of intermolecular short contacts,resulting in high electron and hole mobilities. However, they also often show limited external quantum efficiencies (EQEs). In this contribution we analyze the limitations on EQE by a combined study of exciton dissociation efficiency, charge separation and recombination kinetics in thin films and solar devices of a DPP-based donor polymer, DPPTT-T(thieno[3,2-b]thiophene-diketopyrrolopyrrolecopolymer)blended with varying weight fractions of the fullerene acceptor PC70BM. From the correlations between photoluminescence quenching (PLQ), transient absorption studies and EQEmeasurements, we concludethat the main limitation of photon-to-charge conversion in DPPTT-T/PC70BM devices is poor exciton dissociation. This exciton quenching limit is related to the low affinity/miscibility of the materials, as confirmed by WAXDdiffraction and transmission electron microscopy data, but also to the relatively short DPPTT-T singlet exciton lifetime,possibly associated with highnon-radiative losses. A further strategy to improve EQE in this class of polymers without sacrificing the good extractionproperties in optimized blends is therefore to limit those non-radiative decay processes.

Journal article

Deledalle F, Kirchartz T, Vezie MS, Campoy-Quiles M, Tuladhar PS, Nelson J, Durrant JRet al., 2015, Understanding the Effect of Unintentional Doping on Transport Optimization and Analysis in Efficient Organic Bulk-Heterojunction Solar Cells, PHYSICAL REVIEW X, Vol: 5, ISSN: 2160-3308

Journal article

Dimitrov SD, Wheeler S, Niedzialek D, Schroeder BC, Utzat H, Frost JM, Yao J, Gillett A, Tuladhar PS, McCulloch I, Nelson J, Durrant JRet al., 2015, Polaron pair mediated triplet generation in polymer/fullerene blends, Nature Communications, Vol: 6, ISSN: 2041-1723

Electron spin is a key consideration for the function of organic semiconductors in light-emitting diodes and solar cells, as well as spintronic applications relying on organic magnetoresistance. A mechanism for triplet excited state generation in such systems is by recombination of electron-hole pairs. However, the exact charge recombination mechanism, whether geminate or nongeminate and whether it involves spin-state mixing is not well understood. In this work, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related polymer-fullerene blends with differing polymer fluorination and photovoltaic performance. Using time-resolved laser spectroscopic techniques and quantum chemical calculations, we show that lower charge separation in the fluorinated system is associated with the formation of bound electron-hole pairs, which undergo spin-state mixing on the nanosecond timescale and subsequent geminate recombination to triplet excitons. We find that these bound electron-hole pairs can be dissociated by electric fields.

Journal article

Shoaee S, Deledalle F, Tuladhar PS, Shivanna R, Rajaram S, Narayan KS, Durrant JRet al., 2015, A Comparison of Charge Separation Dynamics in Organic Blend Films Employing Fullerene and Perylene Diimide Electron Acceptors, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 6, Pages: 201-205, ISSN: 1948-7185

Journal article

Fei Z, Ashraf RS, Han Y, Wang S, Yau CP, Tuladhar PS, Anthopoulos TD, Chabinyc ML, Heeney Met al., 2014, Diselenogermole as a novel donor monomer for low band gap polymers, Journal of Materials Chemistry A, Vol: 3, Pages: 1986-1994, ISSN: 2050-7496

Journal article

Gevorgyan SA, Corazza M, Madsen MV, Bardizza G, Pozza A, Muellejans H, Blakesley JC, Dibb GFA, Castro FA, Trigo JF, Guillen CM, Herrero JR, Morvillo P, Maglione MG, Minarini C, Roca F, Cros S, Seraine C, Law CH, Tuladhar PS, Durrant JR, Krebs FCet al., 2014, Interlaboratory indoor ageing of roll-to-roll and spin coated organic photovoltaic devices: Testing the ISOS tests, POLYMER DEGRADATION AND STABILITY, Vol: 109, Pages: 162-170, ISSN: 0141-3910

Journal article

Deledalle F, Shakya Tuladhar P, Nelson J, Durrant JR, Kirchartz Tet al., 2014, Understanding the Apparent Charge Density Dependence of Mobility and Lifetime in Organic Bulk Heterojunction Solar Cells, The Journal of Physical Chemistry C, Vol: 118, Pages: 8837-8842, ISSN: 1932-7447

Journal article

Gevorgyan SA, Zubillaga O, Maria Vega de Seoane J, Machado M, Parlak EA, Tore N, Voroshazi E, Aernouts T, Muellejans H, Bardizza G, Taylor N, Verhees W, Kroon JM, Morvillo P, Minarini C, Roca F, Castro FA, Cros S, Lechene B, Trigo JF, Guillen C, Herrero J, Zimmermann B, Sapkota SB, Veit C, Wuerfel U, Tuladhar PS, Durrant JR, Winter S, Rousu S, Valimaki M, Hinrichs V, Cowan SR, Olson DC, Sommer-Larsen P, Krebs FCet al., 2014, Round robin performance testing of organic photovoltaic devices, RENEWABLE ENERGY, Vol: 63, Pages: 376-387, ISSN: 0960-1481

Journal article

Planells M, Nikolka M, Hurhangee M, Tuladhar PS, White AJP, Durrant JR, Sirringhaus H, McCulloch Iet al., 2014, The effect of thiadiazole out-backbone displacement in indacenodithiophene semiconductor polymers, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 2, Pages: 8789-8795, ISSN: 2050-7526

Journal article

Huang Z, Fregoso EC, Dimitrov S, Tuladhar PS, Soon YW, Bronstein H, Meager I, Zhang W, McCulloch I, Durrant JRet al., 2014, Optimisation of diketopyrrolopyrrole:fullerene solar cell performance through control of polymer molecular weight and thermal annealing, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 2, Pages: 19282-19289, ISSN: 2050-7488

Journal article

Bronstein H, Hurhangee M, Fregoso EC, Beatrup D, Soon YW, Huang Z, Hadipour A, Tuladhar PS, Rossbauer S, Sohn E-H, Shoaee S, Dimitrov SD, Frost JM, Ashraf RS, Kirchartz T, Watkins SE, Song K, Anthopoulos T, Nelson J, Rand BP, Durrant JR, McCulloch Iet al., 2013, Isostructural, Deeper Highest Occupied Molecular Orbital Analogues of Poly(3-hexylthiophene) for High-Open Circuit Voltage Organic Solar Cells, CHEMISTRY OF MATERIALS, Vol: 25, Pages: 4239-4249, ISSN: 0897-4756

Journal article

Kirchartz T, Deledalle F, Tuladhar PS, Durrant JR, Nelson Jet al., 2013, On the Differences between Dark and Light Ideality Factor in Polymer:Fullerene Solar Cells, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 4, Pages: 2371-2376, ISSN: 1948-7185

Journal article

Shoaee S, Subramaniyan S, Xin H, Keiderling C, Tuladhar PS, Jamieson F, Jenekhe SA, Durrant JRet al., 2013, Charge Photogeneration for a Series of Thiazolo-Thiazole Donor Polymers Blended with the Fullerene Electron Acceptors PCBM and ICBA, ADVANCED FUNCTIONAL MATERIALS, Vol: 23, Pages: 3286-3298, ISSN: 1616-301X

Journal article

Bronstein H, Collado-Fregoso E, Hadipour A, Soon YW, Huang Z, Dimitrov SD, Ashraf RS, Rand BP, Watkins SE, Tuladhar PS, otherset al., 2013, Thieno [3, 2-b] thiophene-diketopyrrolopyrrole Containing Polymers for Inverted Solar Cells Devices with High Short Circuit Currents, Advanced Functional Materials

Journal article

Schroeder BC, Ashraf RS, Thomas S, White AJ, Biniek L, Nielsen CB, Zhang W, Huang Z, Tuladhar PS, Watkins SE, Anthopoulos TD, Durrant JR, McCulloch Iet al., 2012, Synthesis of novel thieno[3,2-b]thienobis(silolothiophene) based low bandgap polymers for organic photovoltaics., Chem Commun (Camb), Vol: 48, Pages: 7699-7701, ISSN: 1364-548X

Thieno[3,2-b]thienobis(silolothiophene), a new electron rich hexacyclic monomer has been synthesized and incorporated into three novel donor-acceptor low-bandgap polymers. By carefully choosing the acceptor co-monomer, the energy levels of the polymers could be modulated and high power conversion efficiencies of 5.52% were reached in OPV devices.

Journal article

Schroeder BC, Bronstein HA, Ashraf RS, Zhang W, Huang Z, Tuladhar PS, Anthopoulos TD, Durrant JR, McCulloch Iet al., 2012, Electronic structure tuning of new fused thieno[3,2-b]thieno bisthiophene based polymers via alkyl chain and Group IV heteroatom modulation, SPIE Optics & Photonics

Conference paper

Dimitrov SD, Nielsen CB, Shoaee S, Tuladhar PS, Du J, McCulloch I, Durrant JRet al., 2012, Efficient Charge Photogeneration by the Dissociation of PC70BM Excitons in Polymer/Fullerene Solar Cells, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 3, Pages: 140-144, ISSN: 1948-7185

Journal article

Bronstein H, Chen Z, Ashraf RS, Zhang W, Du J, Durrant JR, Tuladhar PS, Song K, Watkins SE, Geerts Y, Wienk MM, Janssen RAJ, Anthopoulos T, Sirringhaus H, Heeney M, McCulloch Iet al., 2011, Thieno[3,2-b]thiophene-Diketopyrrolopyrrole-Containing Polymers for High-Performance Organic Field-Effect Transistors and Organic Photovoltaic Devices, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 133, Pages: 3272-3275, ISSN: 0002-7863

Journal article

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