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Molecular engineering and sequential cosensitization for preventing the “trade-off” effect with photovoltaic enhancement
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0001-6508-8355
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2017 (English)In: Chemical Science, ISSN 2041-6520, E-ISSN 2041-6539, Vol. 8, no 3, 2115-2124 p.Article in journal (Refereed) Published
Abstract [en]

In dye-sensitized solar cells (DSSCs), it is essential to use rational molecular design to obtain promising photosensitizers with well-matched energy levels and narrow optical band gaps. However, the “trade-off” effect between the photocurrent and photovoltage is still a challenge. Here we report four benzoxidazole based D-A-π-A metal-free organic dyes (WS-66, WS-67, WS-68 and WS-69) with different combinations of π-spacer units and anchoring-acceptor groups. Either extending the π-spacer or enhancing the electron acceptor can efficiently modulate the molecular energy levels, leading to a red-shift in the absorption spectra. The optimal dye, WS-69, containing a cyclopentadithiophene (CPDT) spacer and cyanoacetic acid acceptor, shows the narrowest energy band gap, which displays a very high photocurrent density of 19.39 mA cm−2, but suffers from a relatively low photovoltage of 696 mV, along with the so-called deleterious “trade-off” effect. A cosensitization strategy is further adopted for enhancing the device performance. Optimization of the dye loading sequence is found to be capable of simultaneously improving the photocurrent and photovoltage, and distinctly preventing the “trade-off” effect. The superior cosensitized cell exhibits an excellent power-conversion efficiency (PCE) of 10.09% under one-sun irradiation, and 11.12% under 0.3 sun irradiation, which constitutes a great achievement in that the efficiency of a pure metal-free organic dye with iodine electrolyte can exceed 11% even under relatively weak light irradiation. In contrast with the previous cosensitization strategy which mostly focused on compensation of light-harvesting, we propose a novel cosensitization architecture, in which the large molecular-sized, high photocurrent dye WS-69 takes charge of broadening the light-harvesting region to generate a high short-circuit current (JSC) while the small molecular-sized, high photovoltage dye WS-5 is responsible for retarding charge recombination to generate a high open-circuit voltage (VOC). In addition, adsorption amount and photo-stability studies suggest that the cyano group in the anchoring acceptor is important for the stability since it is beneficial towards decreasing the LUMO levels and enhancing the binding of dyes onto TiO2 nanocrystals.

Place, publisher, year, edition, pages
Royal Society of Chemistry , 2017. Vol. 8, no 3, 2115-2124 p.
Keyword [en]
Dye-sensitized solar cells, Electrolytes, Energy gap, Irradiation, Open circuit voltage, Photocurrents, Photosensitizers, Photovoltaic effects, Solar cells, Adsorption amounts, Charge recombinations, Device performance, Metal free organic dyes, Molecular energy levels, Molecular engineering, Photocurrent density, Power conversion efficiencies, Economic and social effects
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-207408DOI: 10.1039/c6sc03938cISI: 000395906900053ScopusID: 2-s2.0-85014112583OAI: oai:DiVA.org:kth-207408DiVA: diva2:1103669
Note

Funding details: 21421004, NSFC, National Natural Science Foundation of China; Funding details: WJ1416005, NSFC, National Natural Science Foundation of China; Funding text: This work was supported by the NSFC for Creative Research Groups (21421004) and Distinguished Young Scholars (21325625), NSFC/China, Oriental Scholarship, Fundamental Research Funds for the Central Universities (WJ1416005), Scientific Committee of Shanghai (15XD1501400), and Programme of Introducing Talents of Discipline to Universities (B16017).

QC 20170530

Available from: 2017-05-30 Created: 2017-05-30 Last updated: 2017-05-30Bibliographically approved

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