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      High performance polymer solar cells with as-prepared zirconium acetylacetonate film as cathode buffer layer

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          Abstract

          Low-work-function active metals are commonly used as cathode in polymer solar cells (PSCs), but sensitivity of the active metals towards moisture and oxygen results in poor stability of the devices. Therefore, solution-proceessable and stable cathode buffer layer is of great importance for the application of PSCs. Here we demonstrate high performance PSCs by employing as-prepared zirconium acetylacetonate (a-ZrAcac) film spin-cast from its ethanol solution as cathode buffer layer. The PSCs based on a low bandgap polymer PBDTBDD as donor and PC 60BM as acceptor with a-ZrAcac/Al cathode demonstrated an average power conversion efficiency (PCE) of 8.75% which is significantly improved than that of the devices with traditional Ca/Al cathode. The improved photovoltaic performance is benefitted from the decreased series resistance and enhanced light harvest of the PSCs with the a-ZrAcac/Al cathode. The results indicate that a-ZrAcac is a promising high performance cathode buffer layer for fabricating large area flexible PSCs.

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          Molecular design of photovoltaic materials for polymer solar cells: toward suitable electronic energy levels and broad absorption.

          Bulk heterojunction (BHJ) polymer solar cells (PSCs) sandwich a blend layer of conjugated polymer donor and fullerene derivative acceptor between a transparent ITO positive electrode and a low work function metal negative electrode. In comparison with traditional inorganic semiconductor solar cells, PSCs offer a simpler device structure, easier fabrication, lower cost, and lighter weight, and these structures can be fabricated into flexible devices. But currently the power conversion efficiency (PCE) of the PSCs is not sufficient for future commercialization. The polymer donors and fullerene derivative acceptors are the key photovoltaic materials that will need to be optimized for high-performance PSCs. In this Account, I discuss the basic requirements and scientific issues in the molecular design of high efficiency photovoltaic molecules. I also summarize recent progress in electronic energy level engineering and absorption spectral broadening of the donor and acceptor photovoltaic materials by my research group and others. For high-efficiency conjugated polymer donors, key requirements are a narrower energy bandgap (E(g)) and broad absorption, relatively lower-lying HOMO (the highest occupied molecular orbital) level, and higher hole mobility. There are three strategies to meet these requirements: D-A copolymerization for narrower E(g) and lower-lying HOMO, substitution with electron-withdrawing groups for lower-lying HOMO, and two-dimensional conjugation for broad absorption and higher hole mobility. Moreover, better main chain planarity and less side chain steric hindrance could strengthen π-π stacking and increase hole mobility. Furthermore, the molecular weight of the polymers also influences their photovoltaic performance. To produce high efficiency photovoltaic polymers, researchers should attempt to increase molecular weight while maintaining solubility. High-efficiency D-A copolymers have been obtained by using benzodithiophene (BDT), dithienosilole (DTS), or indacenodithiophene (IDT) donor unit and benzothiadiazole (BT), thienopyrrole-dione (TPD), or thiazolothiazole (TTz) acceptor units. The BDT unit with two thienyl conjugated side chains is a highly promising unit in constructing high-efficiency copolymer donor materials. The electron-withdrawing groups of ester, ketone, fluorine, or sulfonyl can effectively tune the HOMO energy levels downward. To improve the performance of fullerene derivative acceptors, researchers will need to strengthen absorption in the visible spectrum, upshift the LUMO (the lowest unoccupied molecular orbital) energy level, and increase the electron mobility. [6,6]-Phenyl-C(71)-butyric acid methyl ester (PC(70)BM) is superior to [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) because C(70) absorbs visible light more efficiently. Indene-C(60) bisadduct (ICBA) and Indene-C(70) bisadduct (IC(70)BA) show 0.17 and 0.19 eV higher LUMO energy levels, respectively, than PCBM, due to the electron-rich character of indene and the effect of bisadduct. ICBA and IC(70)BA are excellent acceptors for the P3HT-based PSCs.
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            Simultaneous enhancement of open-circuit voltage, short-circuit current density, and fill factor in polymer solar cells.

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              Development of novel conjugated donor polymers for high-efficiency bulk-heterojunction photovoltaic devices.

              Solar cells are one attractive method for harnessing inexhaustible clean energy from the sun. Organic photovoltaic technology is emerging as a potential competitor to silicon-based photovoltaic cells (PVCs), and their power conversion efficiencies (PCE) can now exceed 6%. Polymeric bulk-heterojunction (BHJ) PVCs, whose photoactive layer is composed of a blend of bicontinuous and interpenetrating donors and acceptors, can maximize interfacial area between the donor and the acceptor. Classic polymer donors, such as dialkoxy-substituted poly(para-phenylene vinylene)s (PPVs) and poly(3-hexylthiophene) (P3HT), have been widely investigated. However, advances in synthetic methodology provide new avenues for the development of novel conjugated polymer donors with improved power conversion efficiencies. Recently, researchers have achieved great advances in this area. This Account primarily focuses on novel donor polymers that have shown power conversion efficiencies greater than 1%. 2,1,3-Benzothiadiazole, thiophene, thieno[3,4-b]pyrazine, quinoxaline, and silole have emerged as useful heterocycles for constructing a variety of conjugated polymers for photovoltaic applications. We summarize useful information, such as molecular weights, absorption, bandgap, energy levels, and their photovoltaic performances with detailed device parameters (see comparison tables), about these novel donor polymers. We use statistical summaries to evaluate several important parameter relationships among these polymer donors including open-circuit voltage versus HOMO, power conversion efficiency versus bandgap, and power conversion efficiency versus hole mobility. Further statistical analysis of the data listed in these tables may guide further structural design and evaluation of polymer donor materials.
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                Author and article information

                Journal
                Sci Rep
                Sci Rep
                Scientific Reports
                Nature Publishing Group
                2045-2322
                15 April 2014
                2014
                : 4
                : 4691
                Affiliations
                [1 ]State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, Beijing Key Laboratory of Energy Security and Clean Utilization, North China Electric Power University , Beijing 102206, China
                [2 ]Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, China
                Author notes
                Article
                srep04691
                10.1038/srep04691
                3986729
                24732976
                d730858c-48ed-4c49-ac94-ab4574b9109e
                Copyright © 2014, Macmillan Publishers Limited. All rights reserved

                This work is licensed under a Creative Commons Attribution 3.0 Unported License. The images in this article are included in the article's Creative Commons license, unless indicated otherwise in the image credit; if the image is not included under the Creative Commons license, users will need to obtain permission from the license holder in order to reproduce the image. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/

                History
                : 17 October 2013
                : 28 March 2014
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