Analyzing local exciton generation profiles as a means to extract transport lengths in organic solar cells
- Analyzing local exciton generation profiles as a means to extract transport lengths in organic solar cells
- John R. Tumbleston; 고두현; Edward T. Samulski; Rene Lopez
- Issue Date
- Physical review B, Condensed matter and materials physics
- VOL 82, NO 20, 205325-1-205325-8
- In this work, we determine the carrier-transport lengths of electrons and holes ( Le,h) for bulk heterojunction BHJ organic solar cells using a method applicable to functional devices. By linking the local exciton generation profile [ G (x)] in the photoactive layer to photocurrent losses, we are able to determine the onset of bimolecular recombination, which is the dominate loss process of free carrier transport. Even though many factors affect photocurrent generation, we single out bimolecular recombination by measuring the scaling of photocurrent with light intensity as a function of applied voltage. For the common BHJ system, annealed poly-3-hexylthiophene: [6,6] -phenyl-C61-butyric acid methyl ester ( P3HT:PCBM) , a minimum for Le in PCBM is found to be 340 nm while Lh is estimated to be 90 nm for P3HT. The relationship between G (x) and carrier transport is further exemplified by demonstrating a scaling exponent below that for traditional space-chargelimited photocurrent. Likewise, by incorporating a drift/diffusion model, an intuitive link between G (x) and charge transport is established where recombination is shown to occur in regions of the photoactive layer far from the electrode of the slowest carrier species. Finally, the consequences of Le,h on device design for operation under 1 Sun conditions are described.
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