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Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance

Abby Casey, Stoichko Dimitrov Orcid Logo, Pabitra Shakya-Tuladhar, Zhuping Fei, Nguyen Malgorzata, Yang Han, Thomas D. Anthopoulos, James R. Durrant, Martin Heeney

Chemistry of Materials, Volume: 28, Issue: 14, Pages: 5110 - 5120

Swansea University Author: Stoichko Dimitrov Orcid Logo

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DOI (Published version): 10.1021/acs.chemmater.6b02030

Abstract

We report a systematic study into the effects of cyano substitution on the electron accepting ability of the common acceptor 4,7-bis(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT). We describe the synthesis of DTBT monomers with either 0, 1, or 2 cyano groups on the BT unit and their corresponding cop...

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Published in: Chemistry of Materials
ISSN: 0897-4756 1520-5002
Published: 2016
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URI: https://cronfa.swan.ac.uk/Record/cronfa31791
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We describe the synthesis of DTBT monomers with either 0, 1, or 2 cyano groups on the BT unit and their corresponding copolymers with the electron rich donor dithienogermole (DTG). The presence of the cyano group is found to have a strong influence on the optoelectronic properties of the resulting donor&#x2013;acceptor polymers, with the optical band gap red-shifting by approximately 0.15 eV per cyano substituent. We find that the polymer electron affinity is significantly increased by &#x223C;0.25 eV upon addition of each cyano group, while the ionization potential is less strongly affected, increasing by less than 0.1 eV per cyano substituent. In organic photovoltaic (OPV) devices power conversion efficiencies (PCE) are almost doubled from around 3.5% for the unsubstituted BT polymer to over 6.5% for the monocyano substituted BT polymer. However, the PCE drops to less than 1% for the dicyano substituted BT polymer. These differences are mainly related to differences in the photocurrent, which varies by 1 order of magnitude between the best (1CN) and worst devices (2CN). The origin of this variation in the photocurrent was investigated by studying the charge generation properties of the photoactive polymer&#x2013;fullerene blends using fluorescence and transient absorption spectroscopic techniques. These measurements revealed that the improved photocurrent of 1CN in comparison to 0CN was due to improved light harvesting properties while maintaining a high exciton dissociation yield. The addition of one cyano group to the BT unit optimized the position of the polymer LUMO level closer to that of the electron acceptor PC71BM, such that the polymer&#x2019;s light harvesting properties were improved without sacrificing either the exciton dissociation yield or device VOC. We also identify that the drop in performance for the 2CN polymer is caused by very limited yields of electron transfer from the polymer to the fullerene, likely caused by poor orbital energy level alignment with the fullerene acceptor (PC71BM). This work highlights the impact that small changes in chemical structure can have on the optoelectronic and device properties of semiconducting polymer. 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spelling 2017-07-07T11:56:19.8406882 v2 31791 2017-01-27 Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance 9fc26ec1b8655cd0d66f7196a924fe14 0000-0002-1564-7080 Stoichko Dimitrov Stoichko Dimitrov true false 2017-01-27 EEN We report a systematic study into the effects of cyano substitution on the electron accepting ability of the common acceptor 4,7-bis(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT). We describe the synthesis of DTBT monomers with either 0, 1, or 2 cyano groups on the BT unit and their corresponding copolymers with the electron rich donor dithienogermole (DTG). The presence of the cyano group is found to have a strong influence on the optoelectronic properties of the resulting donor–acceptor polymers, with the optical band gap red-shifting by approximately 0.15 eV per cyano substituent. We find that the polymer electron affinity is significantly increased by ∼0.25 eV upon addition of each cyano group, while the ionization potential is less strongly affected, increasing by less than 0.1 eV per cyano substituent. In organic photovoltaic (OPV) devices power conversion efficiencies (PCE) are almost doubled from around 3.5% for the unsubstituted BT polymer to over 6.5% for the monocyano substituted BT polymer. However, the PCE drops to less than 1% for the dicyano substituted BT polymer. These differences are mainly related to differences in the photocurrent, which varies by 1 order of magnitude between the best (1CN) and worst devices (2CN). The origin of this variation in the photocurrent was investigated by studying the charge generation properties of the photoactive polymer–fullerene blends using fluorescence and transient absorption spectroscopic techniques. These measurements revealed that the improved photocurrent of 1CN in comparison to 0CN was due to improved light harvesting properties while maintaining a high exciton dissociation yield. The addition of one cyano group to the BT unit optimized the position of the polymer LUMO level closer to that of the electron acceptor PC71BM, such that the polymer’s light harvesting properties were improved without sacrificing either the exciton dissociation yield or device VOC. We also identify that the drop in performance for the 2CN polymer is caused by very limited yields of electron transfer from the polymer to the fullerene, likely caused by poor orbital energy level alignment with the fullerene acceptor (PC71BM). This work highlights the impact that small changes in chemical structure can have on the optoelectronic and device properties of semiconducting polymer. In particular this work highlights the effect of LUMO–LUMO offset on the excited state dynamics of polymer–fullerene blends. Journal Article Chemistry of Materials 28 14 5110 5120 0897-4756 1520-5002 3 7 2016 2016-07-03 10.1021/acs.chemmater.6b02030 http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&amp;SrcAuth=ORCID&amp;SrcApp=OrcidOrg&amp;DestLinkType=FullRecord&amp;DestApp=WOS_CPL&amp;KeyUT=WOS:000380576700029&amp;KeyUID=WOS:000380576700029 COLLEGE NANME Engineering COLLEGE CODE EEN Swansea University 2017-07-07T11:56:19.8406882 2017-01-27T13:45:41.9244562 Faculty of Science and Engineering School of Engineering and Applied Sciences - Uncategorised Abby Casey 1 Stoichko Dimitrov 0000-0002-1564-7080 2 Pabitra Shakya-Tuladhar 3 Zhuping Fei 4 Nguyen Malgorzata 5 Yang Han 6 Thomas D. Anthopoulos 7 James R. Durrant 8 Martin Heeney 9 0031791-01022017140932.pdf casey2017.pdf 2017-02-01T14:09:32.3270000 Output 3562399 application/pdf Version of Record true 2017-02-01T00:00:00.0000000 false
title Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance
spellingShingle Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance
Stoichko Dimitrov
title_short Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance
title_full Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance
title_fullStr Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance
title_full_unstemmed Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance
title_sort Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance
author_id_str_mv 9fc26ec1b8655cd0d66f7196a924fe14
author_id_fullname_str_mv 9fc26ec1b8655cd0d66f7196a924fe14_***_Stoichko Dimitrov
author Stoichko Dimitrov
author2 Abby Casey
Stoichko Dimitrov
Pabitra Shakya-Tuladhar
Zhuping Fei
Nguyen Malgorzata
Yang Han
Thomas D. Anthopoulos
James R. Durrant
Martin Heeney
format Journal article
container_title Chemistry of Materials
container_volume 28
container_issue 14
container_start_page 5110
publishDate 2016
institution Swansea University
issn 0897-4756
1520-5002
doi_str_mv 10.1021/acs.chemmater.6b02030
college_str Faculty of Science and Engineering
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hierarchy_top_id facultyofscienceandengineering
hierarchy_top_title Faculty of Science and Engineering
hierarchy_parent_id facultyofscienceandengineering
hierarchy_parent_title Faculty of Science and Engineering
department_str School of Engineering and Applied Sciences - Uncategorised{{{_:::_}}}Faculty of Science and Engineering{{{_:::_}}}School of Engineering and Applied Sciences - Uncategorised
url http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&amp;SrcAuth=ORCID&amp;SrcApp=OrcidOrg&amp;DestLinkType=FullRecord&amp;DestApp=WOS_CPL&amp;KeyUT=WOS:000380576700029&amp;KeyUID=WOS:000380576700029
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description We report a systematic study into the effects of cyano substitution on the electron accepting ability of the common acceptor 4,7-bis(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT). We describe the synthesis of DTBT monomers with either 0, 1, or 2 cyano groups on the BT unit and their corresponding copolymers with the electron rich donor dithienogermole (DTG). The presence of the cyano group is found to have a strong influence on the optoelectronic properties of the resulting donor–acceptor polymers, with the optical band gap red-shifting by approximately 0.15 eV per cyano substituent. We find that the polymer electron affinity is significantly increased by ∼0.25 eV upon addition of each cyano group, while the ionization potential is less strongly affected, increasing by less than 0.1 eV per cyano substituent. In organic photovoltaic (OPV) devices power conversion efficiencies (PCE) are almost doubled from around 3.5% for the unsubstituted BT polymer to over 6.5% for the monocyano substituted BT polymer. However, the PCE drops to less than 1% for the dicyano substituted BT polymer. These differences are mainly related to differences in the photocurrent, which varies by 1 order of magnitude between the best (1CN) and worst devices (2CN). The origin of this variation in the photocurrent was investigated by studying the charge generation properties of the photoactive polymer–fullerene blends using fluorescence and transient absorption spectroscopic techniques. These measurements revealed that the improved photocurrent of 1CN in comparison to 0CN was due to improved light harvesting properties while maintaining a high exciton dissociation yield. The addition of one cyano group to the BT unit optimized the position of the polymer LUMO level closer to that of the electron acceptor PC71BM, such that the polymer’s light harvesting properties were improved without sacrificing either the exciton dissociation yield or device VOC. We also identify that the drop in performance for the 2CN polymer is caused by very limited yields of electron transfer from the polymer to the fullerene, likely caused by poor orbital energy level alignment with the fullerene acceptor (PC71BM). This work highlights the impact that small changes in chemical structure can have on the optoelectronic and device properties of semiconducting polymer. In particular this work highlights the effect of LUMO–LUMO offset on the excited state dynamics of polymer–fullerene blends.
published_date 2016-07-03T03:38:51Z
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score 11.012678

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