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

By Mei-Hsin Chen, Jianhui Hou, Z. Hong, Guanwen Yang, S. Sista, Li‐Min Chen, Yang Yang. 2009

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Adv. Mater. 2009, 21, 1–5 2009 WILEY-VCH Verlag Gmb Polymer solar cells have evolved as a promising costeffective alternative to inorganic-based solar cells 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, intensive research with potential materials has been carried out as future photovoltaic (PV) technology. 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. Highly efficient polymer solar cells based on poly(3hexylthiosphene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) have been reported with power conversion efficiencies of 4–5%. The two most decisive parameters regarding polymer-solar-cell efficiencies are the open-circuit voltage (Voc) and the short-circuit current (Jsc). Jsc 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, Voc 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 Voc (as compared to the photon energy) represents a smaller driving force for the PV process. For the P3HT:PC61BM system, the Voc is around 0.6 V, which significantly limits the overall device efficiency. An effective method to improve the Voc of polymer solar cells is to manipulate the HOMO level of the donor and/or LUMO level of the acceptor. 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 donormaterial bymodifying the chemical structure to achieve a high Voc. [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 Voc when blended with PC61BM. Svensson et al. [15] have reported polymer PV cells with a Voc of 1V based on alternating copolymer PFDTBT blendedwith PC61BM.Moreover, Inganas 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-20-thienyl-2,1,3-benzothiadiazole)]} (BisEH-PFDTBT) and poly{[2,7-(9,9-bis-(3,7-dimethyl-octyl)-fluorene)]-alt-[5,5-(4,7di-20-thienyl-2,1,3-benzothiadiazole)]} (BisDMO-PFDTBT), which have the same polymer backbone as PFDTBT but different side chains, were studied in order to achieve higher efficiency values, as well as to investigate the side-chain effects. BisDMO-PFDTBT has proven to be a promising candidate as a donor material for high efficiency polymer BHJ solar cells. Under simulated solar illumination of AM 1.5G (100mWcm ), the BisDMO-PFDTBT blended with (6,6)-phenyl-C71-butyric acidmethyl ester (PC71BM) achieveda maximum power conversion efficiency (PCE) of up to 4.5% with a thin active-layer thickness of only 47 nm. The device exhibited an open-circuit voltage (Voc) of 1V, a short-circuit current (Jsc) of 9.1mAcm , and a reasonably high external quantum efficiency (EQE) exceeding 50% over the entire visible range, with an EQE maxima of 67% at 380nm. When considering the polymer design on the molecular level, the two main factors relating to the ease of material processibility and PV performance are the polymer solubility in common organic solvents and the hole mobility, respectively. In order to obtain good solubility, it is important to have long side chains attached on the polymer backbone. Note that in our experiment, the attached side chains are saturated alkyl groups that have little influence on the molecular energy levels of the donor material. Therefore, it allows us to explore the interchain interaction, particularly the charge-hopping effect. In addition, bulky side chains have a negative effect on the carrier mobility, since interchain hopping of charge carriers requires a favorable overlapping of the electron wave function of adjacent conjugated units on the polymer main chains. Apparently, if the non-

Subfields: Electrical and Electronic Engineering, Polymers and Plastics

View Paper: https://www.semanticscholar.org/paper/07721cafaaf3b11e6fcadb3e58c7775d7d579b13

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