I07-Surface & interface diffraction
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Abstract: The nanoscale morphology within the photoactive layer of organic solar cells is critical in determining the power conversion efficiency (PCE). Here, we draw attention to the roles of molecular arrangement, and domain size in improving performance, which can be tuned by subjecting the photovoltaic materials to solvent vapor annealing (SVA). In our PTB7-Th:ITIC devices, the PCE can be improved by exposing the device to solvent vapor for 60 s after solution casting. The solvent vapor prolongs reorganizational time and increases molecular ordering and domain size/phase separation, which are sub-optimal in pristine PTB7-Th:ITIC blend films. This improved morphology results in better charge mobility, reduced recombination, and ultimately an improved PCE from 7.1% to 7.9% when using CS2 as the annealing solvent. This simple SVA technique can be applied to a range of OPV systems where the molecular ordering is inferior within the as-cast photoactive layer.
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Oct 2019
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[7223]
Open Access
Abstract: Efficient vacuum‐processed organic light‐emitting diodes are fabricated using a carbene–metal–amide material, CMA1. An electroluminescence (EL) external quantum efficiency of 23% is achieved in a host‐free emissive layer comprising pure CMA1. Furthermore external quantum efficiencies of up to 26.9% are achieved in host–guest emissive layers. EL spectra are found to depend on both the emissive‐layer doping concentration and the choice of host material, enabling tuning of emission color from mid‐green (Commission Internationale de l'Éclairage co‐ordinates [0.24, 0.46]) to sky blue ([0.22 0.35]) without changing dopant. This tuning is achieved without compromising luminescence efficiency (>80%) while maintaining a short radiative lifetime of triplets (<1 μs).
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Jul 2018
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I07-Surface & interface diffraction
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Wei
Li
,
Jinlong
Cai
,
Yu
Yan
,
Feilong
Cai
,
Sunsun
Li
,
Robert S.
Gurney
,
Dan
Liu
,
James D.
Mcgettrick
,
Trystan M.
Watson
,
Zhe
Li
,
Andrew J.
Pearson
,
David G.
Lidzey
,
Jianhui
Hou
,
Tao
Wang
Diamond Proposal Number(s):
[17223]
Abstract: In this work, the correlation between three‐dimensional morphology and device performance of PBDB‐T:IT‐M non‐fullerene organic solar cells is investigated. We found that a PBDB‐T‐rich top layer formed when the PBDB‐T:IT‐M film is cast on PEDOT:PSS, indicating a vertical component distribution that will hinder electron transport toward the cathode in a conventional device. This PBDB‐T‐rich top layer remained upon low‐temperature annealing at 80 °C, but disappeared when the annealing temperature is raised, resulting in an optimum annealing temperature of 160 °C for conventional devices as the removal of this polymer‐rich layer facilitates electron transport toward the top cathode layer. The PBDB‐T‐rich layer is also found in the surface region of the PBDB‐T:IT‐M film cast on a TiO2 substrate, but in this case it remained after thermal annealing at 80 or 160 °C, leading to a favorable vertical stratification for efficient charge collection in inverted devices. Although thermal annealing can enhance the crystallinity of PBDB‐T:IT‐M blend and lead to improved charge mobility, the correlation length of the PBDB‐T component increased excessively under annealing at 160 °C. Further, the packing of IT‐M clusters became loose upon high temperature annealing, an effect we believe results in more diffuse interfaces with PBDB‐T that result in reduced charge separation efficiency, consequently reducing the short‐circuit current in the inverted devices.
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Jun 2018
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[17223]
Abstract: Nanoscale morphology has been established as one of the controlling factors in the device performance of bulk heterojunction polymer solar cells. We report in this work morphology changes in both lateral and vertical directions in PffBT4T-2OD:PC71BM solar cells, as well as their effects on device performance. Thermal annealing was found to increase the crystallinity of PffBT4T-2OD and domain size of PC71BM clusters without any observable impact on vertical component redistribution, whilst methanol rinsing reduces the crystallinity of PffBT4T-2OD, encourages the migration of PC71BM towards the mixed polymer-rich phase as well as towards the film surface on both PEDOT:PSS and TiO2 substrates. The polymer-rich surface region in vacuum- and thermal annealing- treated conventional devices obstructs electron injection towards the cathode, and reduces the maximum achievable device efficiency, whilst this polymer-rich surface region is beneficial in the inverted devices. However, although a PC71BM-rich region will locate at the cathode or anode interface upon methanol rinsing treatment in conventional and inverted devices respectively, holes can still be effectively injected from both sides the device to ensure effective charge transport, as supported by a number of optoelectronic property investigations.
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Jun 2018
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I09-Surface and Interface Structural Analysis
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Mojtaba
Abdi-Jalebi
,
Zahra
Andaji-Garmaroudi
,
Stefania
Cacovich
,
Camille
Stavrakas
,
Bertrand
Philippe
,
Johannes M.
Richter
,
Mejd
Alsari
,
Edward P.
Booker
,
Eline M.
Hutter
,
Andrew J.
Pearson
,
Samuele
Lilliu
,
Tom J.
Savenije
,
Hakan
Rensmo
,
Giorgio
Divitini
,
Caterina
Ducati
,
Richard H.
Friend
,
Samuel D.
Stranks
Diamond Proposal Number(s):
[15841]
Abstract: Metal halide perovskites are of great interest for various high-performance optoelectronic applications. The ability to tune the perovskite bandgap continuously by modifying the chemical composition opens up applications for perovskites as coloured emitters, in building-integrated photovoltaics, and as components of tandem photovoltaics to increase the power conversion efficiency. Nevertheless, performance is limited by non-radiative losses, with luminescence yields in state-of-the-art perovskite solar cells still far from 100 per cent under standard solar illumination conditions. Furthermore, in mixed halide perovskite systems designed for continuous bandgap tunability (bandgaps of approximately 1.7 to 1.9 electronvolts), photoinduced ion segregation leads to bandgap instabilities. Here we demonstrate substantial mitigation of both non-radiative losses and photoinduced ion migration in perovskite films and interfaces by decorating the surfaces and grain boundaries with passivating potassium halide layers. We demonstrate external photoluminescence quantum yields of 66 per cent, which translate to internal yields that exceed 95 per cent. The high luminescence yields are achieved while maintaining high mobilities of more than 40 square centimetres per volt per second, providing the elusive combination of both high luminescence and excellent charge transport. When interfaced with electrodes in a solar cell device stack, the external luminescence yield—a quantity that must be maximized to obtain high efficiency—remains as high as 15 per cent, indicating very clean interfaces. We also demonstrate the inhibition of transient photoinduced ion-migration processes across a wide range of mixed halide perovskite bandgaps in materials that exhibit bandgap instabilities when unpassivated. We validate these results in fully operating solar cells. Our work represents an important advance in the construction of tunable metal halide perovskite films and interfaces that can approach the efficiency limits in tandem solar cells, coloured-light-emitting diodes and other optoelectronic applications.
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Mar 2018
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[14701]
Open Access
Abstract: Crystallizable, high-mobility conjugated polymers have been employed as secondary donor materials in ternary polymer solar cells in order to improve device efficiency by broadening their spectral response range and enhancing charge dissociation and transport. Here, contrasting effects of two crystallizable polymers, namely, PffBT4T-2OD and PDPP2TBT, in determining the efficiency improvements in PTB7-Th:PC71BM host blends are demonstrated. A notable power conversion efficiency of 11% can be obtained by introducing 10% PffBT4T-2OD (relative to PTB7-Th), while the efficiency of PDPP2TBT-incorporated ternary devices decreases dramatically despite an enhancement in hole mobility and light absorption. Blend morphology studies suggest that both PffBT4T-2OD and PDPP2TBT are well dissolved within the host PTB7-Th phase and facilitate an increased degree of phase separation between polymer and fullerene domains. While negligible charge transfer is determined in binary blends of each polymer mixture, effective energy transfer is identified from PffBT4T-2OD to PTB7-Th that contributes to an improvement in ternary blend device efficiency. In contrast, energy transfer from PTB7-Th to PDPP2TBT worsens the efficiency of the ternary device due to inefficient charge dissociation between PDPP2TBT and PC71BM.
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Dec 2017
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I07-Surface & interface diffraction
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Edward P.
Booker
,
Tudor H.
Thomas
,
Claudio
Quarti
,
Michael R.
Stanton
,
Cameron D.
Dashwood
,
Alexander J.
Gillett
,
Johannes M.
Richter
,
Andrew J.
Pearson
,
Nathaniel J. L. K.
Davis
,
Henning
Sirringhaus
,
Michael B.
Price
,
Neil C.
Greenham
,
David
Beljonne
,
Sian E.
Dutton
,
Felix
Deschler
Diamond Proposal Number(s):
[14886]
Open Access
Abstract: We investigate the origin of the broadband visible emission in layered hybrid lead-halide perovskites and its connection with structural and photophysical properties. We study <001> oriented thin films of hexylammonium (HA) lead iodide, (C6H16N)2PbI4, and dodecylammonium (DA) lead iodide, (C12H28N)2PbI4 by combining first-principles simulations with time-resolved photoluminescence, steady-state absorption and X-ray diffraction measurements on cooling from 300 K to 4 K. Ultrafast transient absorption and photoluminescence measurements are used to track the formation and recombination of emissive states. In addition to the excitonic photoluminescence near the absorption edge, we find a red-shifted, broadband (full-width at half maximum of about 0.4 eV), emission band below 200 K, similar to emission from <110> oriented bromide 2D perovskites at room temperature. The lifetime of this sub-bandgap emission exceeds that of the excitonic transition by orders of magnitude. We use X-ray diffraction measurements to study the changes in crystal lattice with temperature. We report changes in the octahedral tilt and lattice spacing in both materials, together with a phase change around 200 K in DA2PbI4. DFT simulations of the HA2PbI4 crystal structure indicate that the low-energy emission is due to interstitial iodide and related Frenkel defects. Our results demonstrate that white-light emission is not limited to <110> oriented bromide 2D perovskites but a general property of this class of system and highlight the importance of defect control for the formation of low-energy emissive sites, which can provide a pathway to design tailored white-light emitters.
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Nov 2017
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[11676]
Abstract: Photovoltaics made from organic-inorganic hybrid perovskite semiconductors are attracting significant interest due to their ability to harvest sunlight with remarkable efficiency. The presence of lead in the best performing devices raises concerns regarding their toxicity, a problem that may create barriers to commercialization. Hybrid perovskites with reduced lead content are being investigated to overcome this issue and here we evaluate bismuth as a possible lead substitute. For a series of hybrid perovskite films with the general composition CH3NH3(Pb yBi1-y)I3-xCl x, we characterize their optical and structural properties using UV-Vis spectroscopy, scanning electron microscopy and grazing incidence wide angle X-ray scattering. We show that they form crystalline structures with an optical band gap, around 2 eV for CH3NH3BiI3. However, preliminary solar cell tests show low power conversion efficiencies (<0.01%) due to both incomplete precursor conversion and material de-wetting from the substrate. The overall outcome is severely limited photocurrent. With current processing methods the general applicability of hybrid bismuth perovskites in photovoltaics may be limited.
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May 2017
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I07-Surface & interface diffraction
I12-JEEP: Joint Engineering, Environmental and Processing
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Alex J.
Barker
,
Aditya
Sadhanala
,
Felix
Deschler
,
Marina
Gandini
,
Satyaprasad P.
Senanayak
,
Phoebe M.
Pearce
,
Edoardo
Mosconi
,
Andrew
Pearson
,
Yue
Wu
,
Ajay Ram
Srimath Kandada
,
Tomas
Leitjens
,
Filippo
De Angelis
,
Sian E.
Dutton
,
Annamaria
Petrozza
,
Richard H.
Friend
Diamond Proposal Number(s):
[11454, 12436]
Abstract: Solution processable lead halide perovskites show immense promise for use in photovoltaics and other optoelectronic applications. The ability to tune their bandgap by alloying various halide anions (for example in CH3NH3Pb(I1-xBrx)3, 0
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May 2017
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I07-Surface & interface diffraction
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Maxim
Tabachnyk
,
Arfa H.
Karani
,
Katharina
Broch
,
Luis M.
Pazos-Outón
,
James
Xiao
,
Tom C.
Jellicoe
,
Jiri
Novak
,
David
Harkin
,
Andrew J.
Pearson
,
Akshay
Rao
,
Neil C.
Greenham
,
Marcus L.
Böhm
,
Richard H.
Friend
Open Access
Abstract: Carrier multiplication using singlet exciton fission (SF) to generate a pair of spin-triplet excitons from a single optical excitation has been highlighted as a promising approach to boost the photocurrent in photovoltaics (PVs) thereby allowing PV operation beyond the Shockley-Queisser limit. The applicability of many efficient fission materials, however, is limited due to their poor solubility. For instance, while acene-based organics such as pentacene (Pc) show high SF yields (up to200%), the plain acene backbone renders the organic molecule insoluble in common organic solvents. Previous approaches adding solubilizing side groups such as bis(tri-iso-propylsilylethynyl) to the Pc core resulted in low vertical carrier mobilities due to reduction of the transfer integrals via steric hindrance, which prevented high efficiencies in PVs. Here we show how to achieve good solubility while retaining the advantages of molecular Pc by using a soluble precursor route. The precursor fully converts into molecular Pc through thermal removal of the solubilizing side groups upon annealing above 150 °C in the solid state. The annealed precursor shows small differences in the crystallinity compared to evaporated thin films of Pc, indicating that the Pc adopts the bulk rather than surface polytype. Furthermore, we identify identical SF properties such as sub-100 fs fission time and equally long triplet lifetimes in both samples.
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Nov 2016
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