Efficient Polymer Solar Cells with Thin Active Layers Based on Alternating Polyfluorene Copolymer/Fullerene Bulk Heterojunctions

Abstract

Polymer solar cells have evolved as a promising costeffective alternative to inorganic-based solar cells [1,2] due to their potential to be low-cost, light-weight, and flexible. Since the discovery of ultrafast photoinduced charge transfer from a conjugated polymer to fullerene molecules, followed by the introduction of the bulk heterojunction (BHJ) concept, [3] intensive research with potential materials has been carried out as future photovoltaic (PV) technology. [4][5][6][7][8] Two organic materials with distinct donor and acceptor properties are required to form a heterojunction in the bulk film, which is often achieved by solution processing. In such a case, the BHJ not only provides abundant donor/acceptor interfaces for charge separation, but also forms an interpenetrating network for charge transport. [8,9] Highly efficient polymer solar cells based on poly(3-hexylthiosphene) (P3HT) and [6,6]-phenyl C 61 butyric acid methyl ester (PC 61 BM) have been reported with power conversion efficiencies of 4-5%. [6,[10][11][12] The two most decisive parameters regarding polymer-solar-cell efficiencies are the open-circuit voltage (V oc ) and the short-circuit current (J sc ). J sc is mostly determined by the light absorption ability of the material, the charge-separation efficiency, and the high and balanced carrier mobilities. On the other hand, V oc is limited by the difference in the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor, where a small V oc (as compared to the photon energy) represents a smaller driving force for the PV process. For the P3HT:PC 61 BM system, the V oc is around 0.6 V, which significantly limits the overall device efficiency. An effective method to improve the V oc of polymer solar cells is to manipulate the HOMO level of the donor and/or LUMO level of the acceptor.[13] Until now, fullerene derivatives have proved to be one of the best and most commonly used electron acceptors. Fortunately, it is convenient to change the band gap and energy levels of the donor material by modifying the chemical structure to achieve a high V oc . [13,14] Amongst various polymers, poly{[2,7-(9-(20-ethylhexyl)-9-hexylfluorene])-alt- [5,50-(40, 70-di-2-thienyl-20,10,30-benzothiadiazole)]} (PFDTBT) has a deep HOMO level, which leads to a large V oc when blended with PC 61 BM. Svensson et al. [15] have reported polymer PV cells with a V oc of 1 V based on alternating copolymer PFDTBT blended with PC 61 BM. Moreover, Inganäs et al.[16] reported a systematic study of PV cells using four different fluorene copolymers by varying the length of the alkyl side chain and chemical structure, exhibiting power conversion efficiencies above 2-3%. Unfortunately, in their case, the low photocurrent becomes a major limiting factor in achieving higher efficiencies, suggesting low carrier mobilities.In this study, poly{[2,7-(9,9-bis-(2-ethylhexyl)-fluorene)]-alt-[5,5-(4,7-di-2 0 -thienyl-2,1,3-benzothiadiazole)]} (BisEH-PFDTBT) and poly{[2,7-(9,9-bis-(3,...

Synthesis and photovoltaic properties of new small molecules with rhodanine derivative as the end-capped blocks

“…Herein, two acceptordonor-acceptor (A-D-A) type molecules DCAO3TIDT and DCNR3TIDT (Scheme 1) with IDT as the core D unit, CAO and CNR as the A unit, and trithiophene as the linker have been designed and synthesized. To reduce the steric effect, there is no any side chain in the trithiophene unit [37,38]. As expected, the ability of electron withdrawing of CNR is stronger than CAO, and DCNR3TIDT shows a broad absorption than that of DCAO3TIDT, which could improve the short circuit current density (J sc ) and result in higher PCE for DCNR3TIDT-based OSC.…”

Hydrophilic poly-ether side-chained benzodithiophene-based homopolymer for solar cells and field-effect transistors

“…The relationship between side chain bulkiness and the measured PCEs was exposed. PF-DTBT with ethylhexyl and hexyl substitutes on fluorene unit exhibited PCE of 2.2 %, while BisDMO-PF-DTBT with 3,7-dimethyl-octyl substitutes showed a higher PCE of 4.5 % [17,18]. The effects of side chain have also been systematically explored on one of the best polymer photovoltaic materials to date, P3HT [19][20][21][22].…”

Synthesis, characterization, and optical properties of poly[2-(4-(2,2′-bithiophen-5-yl)phenyl)-4-(4-alkoxyphenyl)-6-phenylpyridine]s

“…The longer chain have lower conductance, because the electron transfer is less and caught effect will be more. [21][22][23][24] In polymers, if the backbone chains are close together, electrons and holes can be transferred from one another. So, long chains have both advantage and disadvantage and the length of these chains must be optimized.…”