I07-Surface & interface diffraction
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Donghui
Li
,
Xiaolong
Chen
,
Jinglong
Cai
,
Wei
Li
,
Mengxue
Chen
,
Yuchao
Mao
,
Baocai
Du
,
Joel A.
Smith
,
Rachel C.
Kilbride
,
Mary E.
O'Kane
,
Xue
Zhang
,
Yuan
Zhuang
,
Pang
Wang
,
Hui
Wang
,
Dan
Liu
,
Richard A. L.
Jones
,
David G.
Lidzey
,
Tao
Wang
Diamond Proposal Number(s):
[22651]
Abstract: Optimizing the components and morphology within the photoactive layer of organic solar cells (OSCs) can significantly enhance their power conversion efficiency (PCE). A new A-D-A type non-fullerene acceptor IDMIC-4F is designed and synthesized in this work, and is employed as the third component to prepare high performance ternary solar cells. IDMIC-4F can form fibrils after solution casting, and the presence of this fibrillar structure in the PBDB-T-2F:BTP-4F host confines the growth of donors and acceptors into fine domains, as well as acting as transport channels to enhance electron mobility. Single junction ternary devices incorporating 10 wt% IDMIC-4F exhibit enhanced light absorption and balanced carrier mobility, and achieve a maximum PCE of 16.6% compared to 15.7% for the binary device, which is a remarkable efficiency for OSCs reported in literature. This non-fullerene acceptor fibril network strategy is a promising method to improve the photovoltaic performance of ternary OSCs.
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Feb 2020
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I07-Surface & interface diffraction
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Wei
Li
,
Zuo
Xiao
,
Joel As.
Smith
,
Jinlong
Cai
,
Donghui
Li
,
Rachel C.
Kilbride
,
Emma L. K.
Spooner
,
Onkar S.
Game
,
Xianyi
Meng
,
Dan
Liu
,
Richard A. L.
Jones
,
David G.
Lidzey
,
Liming
Ding
,
Tao
Wang
Diamond Proposal Number(s):
[20419]
Abstract: Traditional single-junction binary organic solar cells suffer from narrow absorption windows, limiting their ability to harvest photons. One promising approach to avoid this issue is through the construction of a ternary system to enhance the spectral response and efficiency. However, the complex morphology and photophysical processes within ternary blends leave the criteria of an effective third component unclear, and so they remain a challenge. In this work, we report on the fabrication of PTB7-Th:COi8DFIC-based ternary solar cells with enhanced efficiency by employing either a polymer donor or a nonfullerene acceptor as the third component. We demonstrate that the third component is highly associated with the condensed state of the host acceptor and is the primary factor in determining efficiency improvement. The π-π stacking molecular packing of COi8DFIC helps to maintain the optimal phase separation within the ternary blends and improves both the hole and electron charge mobilities, resulting in enhanced power conversion efficiency of over 14%, compared to 13.1% in binary devices. We also found an excessive amount of polymer donor or nonfullerene acceptor increases the phase separation and encourages lamellar crystallization with the host acceptor domain, resulting in reduced light-harvesting and external quantum efficiencies at long wavelengths. Our results provide a rational guide to selecting the third component to fabricate high-performance nonfullerene-based ternary solar cells.
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Dec 2019
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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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Mengxue
Chen
,
Zhuohan
Zhang
,
Wei
Li
,
Jinlong
Cai
,
Jiangsheng
Yu
,
Emma L. K.
Spooner
,
Rachel C.
Kilbride
,
Donghui
Li
,
Baocai
Du
,
Robert S.
Gurney
,
Dan
Liu
,
Weihua
Tang
,
David G.
Lidzey
,
Tao
Wang
Diamond Proposal Number(s):
[20419]
Abstract: Fluorinated non-fullerene acceptors (NFAs) usually have planar backbone and a higher tendency to crystallize compared to their non-fluorinated counterparts, which leads to enhanced charge mobility in organic solar cells (OSCs). However, this self-organization behavior may result in excessive phase separation with electron donors and thereby deteriorate device efficiency. Herein, we demonstrate an effective approach to tune the molecular organization of a fluorinated NFA (INPIC-4F), and its phase separation with the donor PBDB-T, by varying the casting solvent. A prolonged film drying time encourages the crystallization of INPIC-4F into spherulites and consequently results in excessive phase separation, leading to a low device power conversion efficiency (PCE) of 8.1%. Contrarily, a drying time leads to fine mixed domains with inefficient charge transport properties, resulting in a moderate device PCE of 11.4%. An intermediate film drying time results in the formation of face-on π-π stacked PBDB-T and INPIC-4F domains with continuous phase-separated networks, which facilitates light absorption, exciton dissociation as well as balanced charge transport towards the electrode, and achieves a remarkable PCE of 13.1%. This work provides a rational guide for optimizing the molecular ordering of NFAs and electron donors for high device efficiency.
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Sep 2019
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I07-Surface & interface diffraction
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Baocai
Du
,
Renyong
Geng
,
Wei
Li
,
Donghui
Li
,
Yuchao
Mao
,
Mengxue
Chen
,
Xue
Zhang
,
Joel A.
Smith
,
Rachel C.
Kilbride
,
Mary E.
O'Kane
,
Dan
Liu
,
David G.
Lidzey
,
Weihua
Tang
,
Tao
Wang
Diamond Proposal Number(s):
[22651]
Abstract: The insufficient phase separation between polymer donors and non-fullerene acceptors (NFAs) featuring with low-structural orders disrupts efficient charge transport and increases charge recombination, consequently limits the maximum achievable power conversion efficiency (PCE) of organic solar cells (OSCs). Herein, an NFA IT-M has been added as the third component into the PBDB-T:m-INPOIC OSCs, and is shown to effectively tune the phase separation between donor and acceptor molecules, although all components in the ternary system exhibit low degrees of structural orders. The incorporation of 10 wt% IT-M into a PBDB-T:m-INPOIC binary host blend appreciably increases the length scale of phase separation, creating continuous pathways which increase and balance charge transport. This leads to an enhanced photovoltaic performance from 12.8% in the binary cell to 13.9% for the ternary cell with simultaneously improved open-circuit voltage, short-circuit current and fill factor. This work highlights the beneficial role of ternary components in controlling the morphology of the active layer for high performance OSCs.
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Sep 2019
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I07-Surface & interface diffraction
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Wei
Li
,
Zuo
Xiao
,
Jinlong
Cai
,
Joel A.
Smith
,
Emma L. K.
Spooner
,
Rachel C.
Kilbride
,
Onkar S.
Game
,
Xianyi
Meng
,
Donghui
Li
,
Huijun
Zhang
,
Mengxue
Chen
,
Robert S.
Gurney
,
Dan
Liu
,
Richard A. L.
Jones
,
David
Lidzey
,
Liming
Ding
,
Tao
Wang
Diamond Proposal Number(s):
[120419]
Abstract: The chemical structure of non-fullerene acceptors (NFAs) affects their light-harvesting capabilities, energy levels and molecular orders, all of which play a crucial role in determining the efficiency of organic solar cells (OSCs). In this work, we have systematically investigated a series of ladder-type NFAs having different carbon-oxygen-bridged electron-donating cores, and revealed the effects of core structures and film casting conditions on molecular ordering and performance of OSCs. We found that NFAs containing the thieno [3,2-b]thiophene centered, 6 or 8 fused rings (i.e. COi6DFIC, COi8DFIC) exhibit narrower optical band gaps than NFAs containing the benzene centered, 5 or 7 fused rings (i.e. COi5DFIC, COi7DFIC). NFAs containing less fused rings in the carbon-oxygen-bridged core (i.e. COi5DFIC and COi6DFIC) exhibit edge-on molecular orientation in the blends with face-on oriented PTB7-Th donor, and result in low device efficiency. Although NFAs containing more fused rings (i.e. COi7DFIC and COi8DFIC) possess a pronounced flat-on lamellar crystalline structure in the pure state, the crystallization can be reduced when blending with PTB7-Th and under hot-substrate casting, while the lamella in COi8DFIC can be effectively suppressed to form face-on H- and J-type aggregates, leading to enhanced efficiency.
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Jul 2019
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I07-Surface & interface diffraction
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Mengxue
Chen
,
Dan
Liu
,
Wei
Liu
,
Robert S.
Gurney
,
Donghui
Li
,
Jinlong
Cai
,
Emma L. K.
Spooner
,
Rachel C.
Kilbride
,
James D.
Mcgettrick
,
Trystan M.
Watson
,
Zhe
Li
,
Richard A. L.
Jones
,
David G.
Lidzey
,
Tao
Wang
Diamond Proposal Number(s):
[20419]
Abstract: Fluorination of conjugated molecules has been established as an effective structural modification strategy to influence properties, and has attracted extensive attention in organic solar cells (OSCs). Here, we have investigated optoelectronic and photovoltaic property changes of OSCs made of polymer donors with the non-fullerene acceptors (NFAs) ITIC and IEICO and their fluorinated counterparts IT-4F and IEICO-4F. Device studies show that fluorinated NFAs lead to reduced Voc but increased Jsc and FF, and therefore the ultimate influence to efficiency depends on the compensation of Voc loss and gains of Jsc and FF. Fluorination lowers energy levels of NFAs, reduces their electronic bandgaps and red-shifts the absorption spectra. The impact of fluorination on the molecular order depends on the specific NFA, with the conversion of ITIC to IT-4F reduces structural order, which can be reversed after blending with the donor PBDB-T. Contrastingly, IEICO-4F presents stronger π−π stacking after fluorination from IEICO, and this is further strengthened after blending with the donor PTB7-Th. The photovoltaic blends universally present a donor-rich surface region which can promote charge transport and collection towards anode in inverted OSCs. The fluorination of NFAs, however, reduces the fraction of donors in this donor-rich region, consequently encourage the intermixing of donor/acceptor for efficient charge generation.
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Jun 2019
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I07-Surface & interface diffraction
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Wei
Li
,
Mengxue
Chen
,
Jinlong
Cai
,
Emma L. K.
Spooner
,
Huijun
Zhang
,
Robert S.
Gurney
,
Dan
Liu
,
Zuo
Xiao
,
David G.
Lidzey
,
Liming
Ding
,
Tao
Wang
Diamond Proposal Number(s):
[20419]
Abstract: Adjusting molecular ordering, orientation, and nanoscale morphology within the photoactive layer of polymer:non-fullerene organic solar cells is crucial in achieving high power-conversion efficiency (PCE). Herein, we demonstrate that the molecular ordering and orientation of the n-type small-molecule acceptor COi8DFIC can be tuned from flat-on and edge-on lamellar crystalline to H- and J-type π-π stacking during the solution-casting process, resulting in broadened photon absorption and fine phase separation with the electron donor PTB7-Th. This favorable morphology with face-on π-π stacked electron donors and acceptors promotes efficient exciton dissociation at the donor/acceptor interface, together with enhanced and balanced carrier mobility. The enhanced short-circuit current density and fill factor lead to the achievement of a maximum PCE of 13.8% in binary, single-junction PTB7-Th:COi8DFIC non-fullerene polymer solar cells while also exhibiting superior stability.
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Dec 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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