Publications

Journal article

Kirchartz T, Nelson J, Rau U, 2016,

Reciprocity between Charge Injection and Extraction and Its Influence on the Interpretation of Electroluminescence Spectra in Organic Solar Cells

, PHYSICAL REVIEW APPLIED, Vol: 5, ISSN: 2331-7019

Journal article

Leguy AMA, Azarhoosh P, Alonso MI, Campoy-Quiles M, Weber OJ, Yao J, Bryant D, Weller MT, Nelson J, Walsh A, van Schilfgaarde M, Barnes PRFet al., 2016,

Experimental and theoretical optical properties of methylammonium lead halide perovskites

, Nanoscale, Vol: 8, Pages: 6317-6327, ISSN: 2040-3372

The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3]+ cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS).

Journal article

Emmott CJM, Moia D, Sandwell P, Ekins-Daukes N, Hosel M, Lukoschek L, Amarasinghe C, Krebs FC, Nelson Jet al., 2016,

In-situ, long-term operational stability of organic photovoltaics for off-grid applications in Africa

, Solar Energy Materials and Solar Cells, Vol: 149, Pages: 284-293, ISSN: 0927-0248

Cite

Journal article

Ben Dkhil S, Gaceur M, Dachraoui W, Hannani D, Fall S, Brunel F, Wang M, Poize G, Mawyin J, Shupyk I, Martini C, Shilova E, Fages F, Ishwara T, Nelson J, Watanabe T, Yoshimoto N, Margeat O, Videlot-Ackermann C, Ackermann Jet al., 2016,

P-type semiconductor surfactant modified zinc oxide nanorods for hybrid bulk heterojunction solar cells

, Solar Energy Materials and Solar Cells, Vol: 159, Pages: 608-616, ISSN: 0927-0248

In this work, hybrid bulk heterojunction solar cells based on surfactant-modified zinc oxide nanorods (ZnO NRs) blended with poly-(3-hexylthiophene) (P3HT) are presented. (E)-2-cyano-3-(5′-(4-(dibutylamino)styryl)-2,2′-bithiophen-5-yl)acrylic acid (1), a p-type semiconductor, is used as grafted interfacial surfactant on ZnO NRs, named 1-ZnO NRs, in order to improve simultaneously the nanoscale morphology of the hybrid polymer blend as well as the electronic properties of the heterojunction interface. Our studies reveal that the ligand modification of ZnO NRs leads to strongly improved aggregate free P3HT/ZnO blends that show five time increased power conversion efficiency and corresponding photo-generated charge carrier transport compared to untreated ZnO NRs. From transient absorption spectroscopy, it was found that recombination kinetics were similar in the device using untreated ZnO and modified 1-ZnO NRs, respectively, pointing to a major impact of the ligand in the improvement of the blend morphology. Corresponding device optimization led to improvements of FF and Voc to values comparable to P3HT blends using fullerene acceptors, but photocurrent density of the P3HT/1-ZnO solar cells was found low even after optimization. The latter could be addressed to destruction of long range organization of P3HT induced by the presence of the ZnO NRs as well as low electron transport inside the blend.

Conference paper

Hayes SC, Pieridou G, Vezie M, Few S, Bronstein H, Meager I, McCulloch I, Nelson Jet al., 2016,

Effect of molecular weight on the vibronic structure of a diketopyrrolopyrrole polymer

, Conference on Physical Chemistry of Interfaces and Nanomaterials XV, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, ISSN: 0277-786X

Journal article

Vaissier V, Frost JM, Barnes PRF, Nelson Jet al., 2015,

Influence of Intermolecular Interactions on the Reorganization Energy of Charge Transfer between Surface-Attached Dye Molecules

, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 119, Pages: 24337-24341, ISSN: 1932-7447

The parameters controlling the kinetics ofintermolecular charge transfer are traditionally estimatedfrom electronic structure calculations on the charge donorand charge acceptor in isolation. Here, we show that thisprocedure results in inaccuracies for hole transfer between apair of organic dye molecules by comparing charge-constraineddensity functional theory (DFT) calculations on a dyecation/neutral dye pair to the conventional DFT calculationson the isolated molecules. We quantify the error made in thereorganization energy of hole exchange between dye molecules(λi). We choose three indolene-based organic dyes with application to dye-sensitized solar cells, namely, D149, D102, and D131,for which experimental values of λ are available. We find that, although highly system dependent, the intermolecular interactionbetween the charge donor and acceptor can lead to a 0.25 eV change in λi, illustrating the limitations of the widely used originalmethod in predicting the rate of charge transfer.

Journal article

Guilbert AAY, Urbina A, Abad J, Diaz-Paniagua C, Batallan F, Seydel T, Zbiri M, Garcia-Sakai V, Nelson Jet al., 2015,

Temperature-dependent dynamics of polyalkylthiophene conjugated polymers: a combined neutron scattering and simulation study

, Chemistry of Materials, Vol: 27, Pages: 7652-7661, ISSN: 1520-5002

The dynamics of conjugated polymers are known to influence the performance of optoelectronic devices. Polyalkylthiophenes are a widely studied class of conjugated polymers, which exhibit a glass transition around room temperature and consequently are sensitive to temperature variations. We studied the dynamics of two polyalkylthiophenes of different side chain lengths (hexyl and octyl) as a function of temperature, by comparing their quasi-elastic neutron scattering (QENS) with molecular dynamics simulations (MD). We found a good agreement between the simulated and experimental data within the explored time window (of ∼4 ns), demonstrating that the force fields used in MD simulations are appropriate and that the QENS technique can be used as a validation of such force fields. Using MD allows us to identify and to assign contributions to the QENS signal from different parts of the polymers and to determine the activation energies of the different motions.

Journal article

Moia D, Leijtens T, Noel N, Snaith HJ, Nelson J, Barnes PRFet al., 2015,

Dye Monolayers Used as the Hole Transporting Medium in Dye-Sensitized Solar Cells

, ADVANCED MATERIALS, Vol: 27, Pages: 5889-5894, ISSN: 0935-9648

Journal article

Wheeler, Deledalle F, Tokmoldin N, Kirchartz T, Nelson J, Durrant Jet al., 2015,

Influence of Surface Recombination on Charge-Carrier Kinetics in Organic Bulk Heterojunction Solar Cells with Nickel Oxide Interlayers

, Physical review applied, Vol: 4, ISSN: 2331-7019

The choice of electrode for organic photovoltaics is known to be of importance to both device stability and performance, especially regarding the open-circuit voltage (VOC). Here we show that the work function of a nickel oxide anode, varied using an O2 plasma treatment, has a considerable influence on the open-circuit voltage VOC of an organic solar cell. We probe recombination in the devices using transient photovoltage and charge extraction to determine the lifetime as a function of charge-carrier concentration and compare the experimental results with numerical drift-diffusion simulations. This combination of experiment and simulations allows us to conclude that the variations in VOC are due to a change in surface recombination, localized at the NiO anode, although only a small change in carrier lifetime is observed.

Journal article

Baran D, Vezie MS, Gasparini N, Deledalle F, Yao J, Schroeder BC, Bronstein H, Ameri T, Kirchartz T, McCulloch I, Nelson J, Brabec CJet al., 2015,

Role of Polymer Fractionation in Energetic Losses and Charge Carrier Lifetimes of Polymer: Fullerene Solar Cells

, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 119, Pages: 19668-19673, ISSN: 1932-7447

Author Web Link
Cite
Citations: 20

Journal article

Moia D, Cappel UB, Leijtens T, Li X, Telford AM, Snaith HJ, O'Regan BC, Nelson J, Barnes PRFet al., 2015,

The Role of Hole Transport between Dyes in Solid-State Dye-Sensitized Solar Cells

, Journal of Physical Chemistry C, Vol: 119, Pages: 18975-18985, ISSN: 1932-7447

Journal article

Yao J, Kirchartz T, Vezie MS, Faist MA, Gong W, He Z, Wu H, Troughton J, Watson T, Bryant D, Nelson Jet al., 2015,

Quantifying losses in open-circuit voltage in solution-processable solar cells

, Physical Review Applied, Vol: 4, ISSN: 2331-7019

The maximum open-circuit voltage of a solar cell can be evaluated in terms of its ability to emit light. We herein verify the reciprocity relation between the electroluminescence spectrum and subband-gap quantum efficiency spectrum for several photovoltaic technologies at different stages of commercial development, including inorganic, organic, and a type of methyl-ammonium lead- halide CH3NH3PbI3−xClx perovskite solar cells. Based on the detailed balance theory and reciprocity relations between light emission and light absorption, voltage losses at open circuit are quantified and assigned to specific mechanisms, namely, absorption edge broadening and nonradiative recombination. The voltage loss due to nonradiative recombination is low for inorganic solar cells (0.04–0.21 V), while for organic solar cell devices it is larger but surprisingly uniform, with values of 0.34–0.44 V for a range of material combinations. We show that, in CH3NH3PbI3−xClx perovskite solar cells that exhibit hysteresis, the loss to nonradiative recombination varies substantially with voltage scan conditions. We then show that for different solar cell technologies there is a roughly linear relation between the power conversion efficiency and the voltage loss due to nonradiative recombination.

Journal article

Leguy AMA, Frost JM, McMahon AP, Sakai VG, Kockelmann W, Law C, Li X, Foglia F, Walsh A, O'Regan BC, Nelson J, Cabral JT, Barnes PRFet al., 2015,

Corrigendum: The dynamics of methylammonium ions in hybrid organic–inorganic perovskite solar cells

, Nature Communications, Vol: 6, ISSN: 2041-1723

Journal article

Leguy AMA, Frost JM, McMahon AP, Sakai VG, Kochelmann W, Law C, Li X, Foglia F, Walsh A, O'Regan BC, Nelson J, Cabral JT, Barnes PRFet al., 2015,

The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells

, Nature Communications, Vol: 6, ISSN: 2041-1723

Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH3NH3+ ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ~14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1–200-ps time window. Monte Carlo simulations of interacting CH3NH3+ dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3+ in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3+ to screen a device’s built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ~0.1–1 ms, faster than most observed hysteresis.

Journal article

Leguy AMA, Hu Y, Campoy-Quiles M, Isabel Alonso M, Weber OJ, Azarhoosh P, van Schilfgaarde M, Weller MT, Bein T, Nelson J, Docampo P, Barnes PRFet al., 2015,

Reversible hydration of CH(3)NH(3)Pbl(3) in films, single crystals, and solar cells

, Chemistry of Materials, Vol: 27, Pages: 3397-3407, ISSN: 0897-4756

Solar cells composed of methylammonium lead iodide perovskite (MAPI) are notorious for their sensitivity to moisture. We show that (i) hydrated crystal phases are formed when MAPI is exposed to water vapor at room temperature and (ii) these phase changes are fully reversed when the material is subsequently dried. The reversible formation of CH3NH3PbI3·H2O followed by (CH3NH3)4PbI6·2H2O (upon long exposure times) was observed using time-resolved XRD and ellipsometry of thin films prepared using “solvent engineering”, single crystals, and state-of-the-art solar cells. In contrast to water vapor, the presence of liquid water results in the irreversible decomposition of MAPI to form PbI2. MAPI changes from dark brown to transparent on hydration; the precise optical constants of CH3NH3PbI3·H2O formed on single crystals were determined, with a bandgap at 3.1 eV. Using the single-crystal optical constants and thin-film ellipsometry measurements, the time-dependent changes to MAPI films exposed to moisture were modeled. The results suggest that the monohydrate phase forms independent of the depth in the film, suggesting rapid transport of water molecules along grain boundaries. Vapor-phase hydration of an unencapsulated solar cell (initially Jsc ≈ 19 mA cm–2 and Voc ≈ 1.05 V at 1 sun) resulted in more than a 90% drop in short-circuit photocurrent and ∼200 mV loss in open-circuit potential; however, these losses were fully reversed after the device was exposed to dry nitrogen for 6 h. Hysteresis in the current–voltage characteristics was significantly increased after this dehydration, which may be related to changes in the defect density and morphology of MAPI following recrystallization from the hydrate. Based on our observations, we suggest that irreversible decomposition of MAPI in the presence of water vapor only occurs significantly once a grain has been fully converted to the monohydrate phase.

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

Author Web Link
Cite
Citations: 46

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

Emmott CJM, Roehr JA, Campoy-Quiles M, Kirchartz T, Urbina A, Ekins-Daukes NJ, Nelson Jet al., 2015,

Organic photovoltaic greenhouses: a unique application for semi-transparent PV?

, Energy & Environmental Science, Vol: 8, Pages: 1317-1328, ISSN: 1754-5706

Organic photovoltaics are an emerging solar power technology which embody properties such astransparency, flexibility, and rapid, roll to roll manufacture, opening the potential for unique nicheapplications. We report a detailed techno-economic analysis of one such application, namely thephotovoltaic greenhouse, and discuss whether the unique properties of the technology can provideadvantages over conventional photovoltaics. The potential for spectral selectivity through the choice ofOPV materials is evaluated for the case of a photovoltaic greenhouse. The action spectrum of typicalgreenhouse crops is used to determine the impact on crop growth of blocking different spectral rangesfrom the crops. Transfer matrix optical modelling is used to assess the efficiency and spectrally resolvedtransparency of a variety of commercially available semi-conducting polymer materials, in addition to anon-commercial low-band-gap material with absorption outside that required for crop growth. Economicanalysis suggests there could be a huge potential for OPV greenhouses if aggressive cost targets can bemet. Technical analysis shows that semi-transparent OPV devices may struggle to perform better thanopaque crystalline silicon with partial coverage, however, OPV devices using the low-band-gap materialPMDPP3T, as well as a high efficiency mid-band-gap polymer PCDTBT, can demonstrate improvedperformance in comparison to opaque, flexible thin-film modules such as CIGS. These results stress theimportance of developing new, highly transparent electrode and interlayer materials, along with highefficiency active layers, if the full potential of this application is going to be realised.

Abstract
Cite

Journal article

Few S, Frost JM, Nelson J, 2015,

Models of charge pair generation in organic solar cells

, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 17, Pages: 2311-2325, ISSN: 1463-9076

Author Web Link
Cite
Citations: 139

Journal article

Dattani R, Gibson KF, Few S, Borg AJ, DiMaggio PA, Nelson J, Kazarian SG, Cabral JTet al., 2015,

Fullerene oxidation and clustering in solution induced by light

, Journal of Colloid and Interface Science, Vol: 446, Pages: 24-30, ISSN: 1095-7103

We investigate the environmental stability of fullerene solutions by static and dynamic light scattering, FTIR, NMR and mass spectroscopies, and quantum chemical calculations. We find that visible light exposure of fullerene solutions in toluene, a good solvent, under ambient laboratory conditions results in C60 oxidation to form fullerene epoxides, and subsequently causes fullerene clustering in solution. The clusters grow with time, even in absence of further illumination, and can reach dimensions from ≈100 nm to the μm scale over ≈1 day. Static light scattering suggests that resulting aggregates are fractal, with a characteristic power law (df) that increases from approximately 1.3 to 2.0 during light exposure. The clusters are bound by weak Coulombic interactions and are found to be reversible, disintegrating by mechanical agitation and thermal stress, and reforming over time. Our findings are relevant to the solution processing of composites and organic photovoltaics, whose reproducibility and performance requires control of fullerene solution stability under storage conditions.

Journal article

MacLachlan AJ, Rath T, Cappel UB, Dowland SA, Amenitsch H, Knall A-C, Buchmaier C, Trimmel G, Nelson J, Haque SAet al., 2015,

Polymer/Nanocrystal Hybrid Solar Cells: Influence of Molecular Precursor Design on Film Nanomorphology, Charge Generation and Device Performance

, ADVANCED FUNCTIONAL MATERIALS, Vol: 25, Pages: 409-420, ISSN: 1616-301X

Journal article

Holliday S, Ashraf RS, Nielsen CB, Kirkus M, Roehr JA, Tan C-H, Collado-Fregoso E, Knall A-C, Durrant JR, Nelson J, McCulloch Iet al., 2015,

A rhodanine flanked nonfullerene acceptor for solution-processed organic photovoltaics

, Journal of the American Chemical Society, Vol: 137, Pages: 898-904, ISSN: 0002-7863

A novel small molecule, FBR, bearing 3-ethylrhodanine flanking groups was synthesized as a nonfullerene electron acceptor for solution-processed bulk heterojunction organic photovoltaics (OPV). A straightforward synthesis route was employed, offering the potential for large scale preparation of this material. Inverted OPV devices employing poly(3-hexylthiophene) (P3HT) as the donor polymer and FBR as the acceptor gave power conversion efficiencies (PCE) up to 4.1%. Transient and steady state optical spectroscopies indicated efficient, ultrafast charge generation and efficient photocurrent generation from both donor and acceptor. Ultrafast transient absorption spectroscopy was used to investigate polaron generation efficiency as well as recombination dynamics. It was determined that the P3HT:FBR blend is highly intermixed, leading to increased charge generation relative to comparative devices with P3HT:PC60BM, but also faster recombination due to a nonideal morphology in which, in contrast to P3HT:PC60BM devices, the acceptor does not aggregate enough to create appropriate percolation pathways that prevent fast nongeminate recombination. Despite this nonoptimal morphology the P3HT:FBR devices exhibit better performance than P3HT:PC60BM devices, used as control, demonstrating that this acceptor shows great promise for further optimization.

Journal article

Steiner F, Foster S, Losquin A, Labram J, Anthopoulos TD, Frost JM, Nelson Jet al., 2015,

Distinguishing the influence of structural and energetic disorder on electron transport in fullerene multi-adducts

, MATERIALS HORIZONS, Vol: 2, Pages: 113-119, ISSN: 2051-6347

Author Web Link
Cite
Citations: 43

Journal article

Manke F, Frost JM, Vaissier V, Nelson J, Barnes PRFet al., 2015,

Influence of a nearby substrate on the reorganization energy of hole exchange between dye molecules

, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 17, Pages: 7345-7354, ISSN: 1463-9076

Journal article

Hermerschmidt F, Kalogirou AS, Min J, Zissimou GA, Tuladhar SM, Ameri T, Faber H, Itskos G, Choulis SA, Anthopoulos TD, Bradley DDC, Nelson J, Brabec CJ, Koutentis PAet al., 2015,

4<i>H</i>-1,2,6-Thiadiazin-4-one-containing small molecule donors and additive effects on their performance in solution-processed organic solar cells

, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 3, Pages: 2358-2365, ISSN: 2050-7526

Author Web Link
Cite
Citations: 26

Journal article

Wade J, Steiner F, Niedzialek D, James DT, Jung Y, Yun D-J, Bradley DDC, Nelson J, Kim J-Set al., 2014,

Charge mobility anisotropy of functionalized pentacenes in organic field effect transistors fabricated by solution processing

, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 2, Pages: 10110-10115, ISSN: 2050-7526

Journal article

Guilbert AAY, Schmidt M, Bruno A, Yao J, King S, Tuladhar SM, Kirchartz T, Alonso MI, Goni AR, Stingelin N, Haque SA, Campoy-Quiles M, Nelson Jet al., 2014,

Spectroscopic Evaluation of Mixing and Crystallinity of Fullerenes in Bulk Heterojunctions

, ADVANCED FUNCTIONAL MATERIALS, Vol: 24, Pages: 6972-6980, ISSN: 1616-301X

Author Web Link
Cite
Citations: 26

Journal article

Madsen MV, Gevorgyan SA, Pacios R, Ajuria J, Etxebarria I, Kettle J, Bristow ND, Neophytou M, Choulis SA, Roman LS, Yohannes T, Cester A, Cheng P, Zhan X, Wu J, Xie Z, Tu W-C, He J-H, Fell CJ, Anderson K, Hermenau M, Bartesaghi D, Koster LJA, Machui F, Gonzalez-Valls I, Lira-Cantu M, Khlyabich PP, Thompson BC, Gupta R, Shanmugam K, Kulkarni GU, Galagan Y, Urbina A, Abad J, Roesch R, Hoppe H, Morvillo P, Bobeico E, Panaitescu E, Menon L, Luo Q, Wu Z, Ma C, Hambarian A, Melikyan V, Hambsch M, Burn PL, Meredith P, Rath T, Dunst S, Trimmel G, Bardizza G, Muellejans H, Goryachev AE, Misra RK, Katz EA, Takagi K, Magaino S, Saito H, Aoki D, Sommeling PM, Kroon JM, Vangerven T, Manca J, Kesters J, Maes W, Bobkova OD, Trukhanov VA, Paraschuk DY, Castro FA, Blakesley J, Tuladhar SM, Roehr JA, Nelson J, Xia J, Parlak EA, Tumay TA, Egelhaaf H-J, Tanenbaum DM, Ferguson GM, Carpenter R, Chen H, Zimmermann B, Hirsch L, Wantz G, Sun Z, Singh P, Bapat C, Offermans T, Krebs FCet al., 2014,