Correlating molecular morphology with optoelectronic function in solar cells based on low band-gap copolymer:fullerene blends

Abstract

We review recent progress in the development of organic bulk heterojunction (BHJ) solar cells employing donor-acceptor copolymers as the electron-donor and fullerene derivatives as the electron-acceptor. We discuss the role of the donor and acceptor moieties, side-chains, bridging units and atomic substitutions of the copolymers on their optoelectronic functionality. The physical properties, e.g. molecular conformation, miscibility, phase-separated lateral and vertical morphology, of various photovoltaic blends prepared via solution casting and post-treatments are reviewed and correlated with photovoltaic device performance. Factors influencing the morphological stability of polymer:fullerene BHJ thin-films are briefly discussed. Finally, we address the use of thin organic interlayers to increase the efficiency of BHJ solar cells. Tao Wang received his B.Sc. in Polymer Materials (2002) and M.Sc. in Materials Science (2005). He obtained his Ph.D. in So Condensed Matter Physics from the University of Surrey (UK) in Feb. 2009, under the supervision of Prof. Joe Keddie. He then took a post-doc research associate position in the same group until Oct. 2009 when he moved to the Department of Physics and Astronomy at the University of Sheffield (UK). At Sheffield, he works with Prof. Richard Jones and Prof. David Lidzey on morphology characterization and device optimization of organic solar cells. His research interests are polymer physics and polymer electronics. Andrew Pearson received his MSci (Physics, 2008) from the University of Nottingham (UK). Thereaer, he joined the research group of Professor David Lidzey at the University of Sheffield (UK) to study for his Ph.D. in Organic Semiconductor Physics, which was awarded in 2012. His postgraduate research focussed on the rational understanding of common processing techniques used in the fabrication of polymer:fullerene blend lms for solar cell applications. Upon graduating, he has stayed at Sheffield as a post-doctoral research associate, to continue his work on the structural and spectroscopic characterisation of polymer:fullerene blend lms.

Investigation of Radical and Cationic Cross‐Linking in High‐Efficiency, Low Band Gap Solar Cell Polymers

“…[ 2,3 ] This morphology has been typically achieved by the manipulation of coating conditions, after processing thermal or solvent annealing and by the choice of solvent and processing additives. [ 2,3 ] In many cases, the nanostructure of the blend can be considered kinetically trapped rather than thermodynamically preferred.…”

Theoretical Approach Towards Rational Design and Characterization of Benzo[1,2-b:5-B’]dithiophene (BDT)-Based (A-D-A) Small Molecules of Relevance for High Performance Solar Cells

“…This result denotes that the HOMO energy levels of the designed SMs 2-5, having more coplanar configuration across the backbone than 1, are very close to the HOMO energy of an ideal polymer (~-5.4 eV), providing them with good long-term stability in air and their BHJ photovoltaic devices with high V OC [32,57]. However, the LUMO energy levels of the molecules 2-5 are between -3.40 eV and -3.57 eV, lower by about 0.26 to 0.39 eV than that of 1.…”

“…Previous studies have demonstrated that the crystalline character of BDT based D-A copolymers and SMs can be adjusted by increasing the number of nitrogen atoms in the acceptor unit [32,33]. Benefited from the good properties of SM 2, we have subsequently designed SMs 3 and 4 by retaining the core acceptor unit of 2 and modifying the core donor BDTT-S by substituting some carbon atoms by nitrogen atoms.…”

Solid State Physicochemical Properties and Applications of Organic and Metallo- Organic Fullerene Derivatives

“…While using donor-fullerene dyads as single active components provides excellent excitons dissociation yields, it also limits the interface between donating and accepting domains to the molecular size, leading to low overall efficiency. In fact, fullerene-based photovoltaic devices do not use molecular dyads as active material, but rather rely on fullerenecontaining polymer blends to enhance electron and hole transport to opposite electrodes by means of a bi-continuous interpenetrating network [198], leading to what is known as heterojunction polymer solar cells. The conjugated polymers act as electron donors, and functionalized fullerenes such as methanofullerenes and indene-fullerene derivatives [181,199,200,201] as acceptor molecules: after creation of excitons by absorption of light, they are separated by electron transfer from the donor's LUMO to the lower-lying fullerene LUMO.…”