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Enhancing the Performance of Solution-Processed BulkHeterojunction Solar Cells Using Hydrogen-Bonding-Induced SelfOrganization of Small Molecules Kwan Hang Lam,†,‡ Thelese Ru Bao Foong,‡ Zi En Ooi,‡ Jie Zhang,‡ Andrew Clive Grimsdale,† and Yeng Ming Lam*,†,§,∇ †
School of Materials Science and Engineering, Nanyang Technological University, Blk N4.1, Nanyang Avenue, Singapore 639798 Institute of Materials Research and Engineering (IMRE), Agency for Science Technology and Research (A*STAR), 3 Research Link, Singapore 117602 § Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, 50 Nanyang Drive, Singapore 637533 ∇ Institute of Materials for Electronic Engineering II, RWTH-Aachen, Sommerfeldstraße 24, D-52074 Aachen, Germany ‡
S Supporting Information *
ABSTRACT: Small-molecule solar-cell performance is highly sensitive to the crystallinity and intermolecular connectivity of the molecules. In order to enhance the crystallinity for the solution-processed small molecule, it is possible to make use of carboxylic acid end-functional groups to drive hydrogen-bonding-induced π−π stacking of conjugated molecules. Herein, we report the synthesis and characterization of quarterthiophenes with carboxylic acid as end groups. The formation of hydrogen bonds between neighboring acid groups gives rise to a pseudo-polymeric structure in the molecules, which leads to substantial improvement in the organization and crystallinity of the active layers. This resulted in a four-fold increase in the hole mobility and a two-fold improvement in the performance of the solar cell device for the acidfunctionalized molecule, compared to its ester analogue. More importantly, optimal device performance for the acid-functionalized molecule was achieved for the as-cast film, thereby reducing the reliance on thermal annealing and solvent additives. KEYWORDS: small-molecule organic solar cells, solution processable, self-assembly, hydrogen bonding, quarterthiophene, carboxylic acid
1. INTRODUCTION Research into new materials for organic photovoltaics (OPVs) have focused mainly on polymeric semiconductors, because of their good film-forming properties, high charge carrier mobility, and device performance.1−6 The bulk-heterojunction (BHJ) device architecture remains ubiquitous, as it gives rise to large interfacial area between the donor and acceptor phases, which leads to higher exciton splitting efficiency and, therefore, an increase in charge generation.7−11 However, synthesis of semiconducting polymers has been plagued by issues such as batch-to-batch reproducibility, difficulty in purification, and high polydispersity in molecular weight. Hence, there has been growing interest in using small molecules as an alternative class of active donor materials for OPVs. In contrast to polymers, small molecules are generally easier to purify and its molecular weight is monodispersed in nature. However, the device performance of solution-processed smallmolecule-based OPVs generally has been lagging behind that of its polymeric counterpart. Although with increased effort invested device efficiencies have improved,2,12−16 the issue of © 2013 American Chemical Society
poor connectivity between the molecules has not been properly addressed. This lack in intermolecular connectivity will lead to lowered charge carrier mobility and device performance. For polymeric systems, it is well-established that high molecular weight (MW) are crucial for good solar-cell performance.17−20 This is due to the improved interchain connectivity and enhanced charge mobility along the polymer chains. These same requirements also apply to small-molecule systems. Lee et al. recently re-emphasized the importance of interconnectivity between small molecules by showing that pyrene, when attached at an appropriate carbon position, can facilitate the self-assembly and interconnectivity of small molecules, thereby improving solar-cell performance.21 It is believed that, by incorporating hydrogen bonding into solution-processable small molecules, a similar effect can be achieved. The hydrogen-bond-induced π−π interaction can lead to enhance Received: September 28, 2013 Accepted: November 11, 2013 Published: November 11, 2013 13265
dx.doi.org/10.1021/am4042614 | ACS Appl. Mater. Interfaces 2013, 5, 13265−13274
ACS Applied Materials & Interfaces
Research Article
Figure 1. Chemical structures of quarterthiophene with diester groups (QT-ES) and quarterthiophene with diacid groups (QT-DA). were then subjected to UV-ozone cleaning for 10 min at 100 °C prior to the spin coating of a 40-nm-thick poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (CLEVIOS P VP Al 4083, H.C. Starck) hole transporting layer. Annealing of the PEDOT:PSS layer was done in a nitrogen-filled glove box at 140 °C for 10 min. Subsequently, blends of the active materials were spin-coated on top of the PEDOT:PSS layer in the glove box. Cathode layers (aluminum with thicknesses of ∼80 nm) were evaporated through a shadow mask at a pressure of
