Alternating Polyfluorenes Collect Solar Light in Polymer Photovoltaics
- 16 October 2009
- journal article
- Published by American Chemical Society (ACS) in Accounts of Chemical Research
- Vol. 42 (11), 1731-1739
- https://doi.org/10.1021/ar900073s
Abstract
The effort to improve the energy conversion efficiency of polymer solar cells has led to the design of novel donor polymers. To improve open circuit photovoltages (OCVs) and the spectral coverage of the solar spectrum, researchers have looked for materials with high HOMO values, an easily modified electronic structure, and sufficient electronic transport within the polymers. One advance in design from our laboratories has been the development of a class of alternating polyfluorene copolymers (APFOs), which can be combined with fullerenes to make bulk heterojunction materials for photovoltaic conversion. This Account describes copolymers of fluorene that we designed to expand the range the optical absorption of solar cells to include wavelengths out to 1000 nm. In most cases, we combine these polymers with acceptors from the fullerene family, typically the phenyl C61 butyric acid methyl ester (PCBM) molecule, to generate solar cell materials. The synthesis of alternating copolymers of fluorene with various donor−acceptor−donor elements provides the opportunity to shift both HOMO and LUMO, which we have followed by electrochemical spectroscopy. Moving the LUMO of the APFOs farther from the vacuum level eventually leads to a situation where the driving force for photo-induced charge transfer from polymer donor to fullerene acceptor goes to zero, resulting in inefficient charge generation. Moving the HOMO level closer to the vacuum level reduces the OCV of devices made from bulk heterojunction blends. As we move the bandgap toward lower energies and increase the overlap of optical absorption with the solar spectrum, both these events eventually occur. In devices based on these APFO/fullerene blends, the performance depends on the OCV, the photocurrent under solar illumination, and the fill factor. The fill factor is influenced by electrical transport and charge generation. Optimizing these parameters requires new solutions to the perennial conflict between optically thin devices, where electrical extraction of charge is not a limitation, and the optically thick devices, where extraction of charge is hampered by trapping and recombination. As a result, we have developed methods to trap light in optically thin devices. When the thin film flexible solar cells are folded, multiple reflection between adjacent solar cells leads to a longer path length for the photon through the devices and considerable improvement of the optical dissipation in the active material. These optical tricks also enable an alternative route to tandem devices, where two different bandgap materials are located on adjacent folds. Thus light not absorbed in one cell is reflected onto the next cell to produce an effective optical series arrangement. Using experiments and simulations of the light trapping effects, we demonstrate power conversion efficiency enhancements of up to a factor of 1.8.Keywords
This publication has 57 references indexed in Scilit:
- Polymer‐Fullerene Bulk‐Heterojunction Solar CellsAdvanced Materials, 2009
- High photovoltage achieved in low band gap polymer solar cells by adjusting energy levels of a polymer with the LUMOs of fullerene derivativesJournal of Materials Chemistry, 2008
- Low‐Bandgap Alternating Fluorene Copolymer/Methanofullerene Heterojunctions in Efficient Near‐Infrared Polymer Solar CellsAdvanced Materials, 2006
- Infrared photocurrent spectral response from plastic solar cell with low-band-gap polyfluorene and fullerene derivativeApplied Physics Letters, 2004
- Barrier‐Free Electron–Hole Capture in Polymer Blend Heterojunction Light‐Emitting DiodesAdvanced Materials, 2003
- Semiconducting Polyfluorenes—Towards Reliable Structure-Property RelationshipsAdvanced Materials, 2002
- Polyfluorenes: Twenty years of progressJournal of Polymer Science Part A: Polymer Chemistry, 2001
- Laser action in polydialkylfluorene films: Influence of low-temperature thermal treatmentApplied Physics Letters, 1999
- Efficient Photoluminescence and Electroluminescence from a Soluble PolyfluoreneJournal of the American Chemical Society, 1996
- Synthesis of fusible and soluble conducting polyfluorene derivatives and their characteristicsJournal of Polymer Science Part A: Polymer Chemistry, 1993