Highly Crystalline and Low Bandgap Donor Polymers for Efficient Polymer Solar Cells
Abstract
Polymer solar cells (PSCs) have recently attracted great attention because of their potential advantages, including flexibility, light weight, and low fabrication cost.[1 4] Photon absorption by conjugated polymers often creates bound electron/hole pairs (i.e., excitons). Charge collection, therefore, requires dissociation of the excitons, a process which is known to be favorable at the interface between conjugated polymers as donors and C60 derivatives as acceptors. Since [6,6]-phenyl-C61-butyric acid ester (PCBM) is still one of the best electron acceptors, most recent efforts have focused on the development of low bandgap donor polymers.[5 22] Examples of low bandgap polymers include conjugated polymers having electron-donating (D) and electron-accepting (A) moieties in the main chain.[21,22] The bandgap of the D- -A polymers can be effectively reduced through intramolecular charge transfer while their physical properties can be readily tuned by tailoring structures of the D and A moieties and/or the linking bridge. An ideal donor polymer should have a broad absorption spectrum for an efficient solar photon harvesting, a low HOMO level for a high opencircuit voltage (VOC), a good compatibility with PCBM for the formation of a bi-continuous network (i.e., bulk heterojunction, BHJ) with a large interface area, and a high hole mobility for an efficient transportation of the photo-generated charge carriers.[ 4] When designing donor polymers, therefore, a delicate balance between the crystallinity and the solubility needs to be taken into consideration.[5] Highly crystalline conjugated polymers with closely packed polymer chains in the solid state often show high hole mobility, but low solubility in organic solvents. Although the high hole mobility is an asset to PSCs, highly crystalline polymers with poor solution-processability rarely form the desired bi-continuous morphology with PCBM, and hence usually exhibit poor photovoltaic performance.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 01, 2012
- Accession Number
- ADA578741
Entities
People
- Chris Bailey
- Hyosung Choi
- Jin Y. Kim
- Jun Liu
- Liming Dai
- Michael Durstock
Organizations
- Air Force Research Laboratory