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The Role of Photon Energy in Free Charge Generation in Bulk Heterojunction Solar Cells
Author(s) -
Di Nuzzo Daniele,
Koster L. Jan Anton,
Gevaerts Veronique S.,
Meskers Stefan C. J.,
Janssen René A. J.
Publication year - 2014
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.201400416
Subject(s) - materials science , photocurrent , polymer solar cell , band gap , excitation , optoelectronics , heterojunction , organic solar cell , photon energy , solar cell , photon , polymer , optics , physics , quantum mechanics , composite material
To determine the role of photon energy on charge generation in bulk heterojunction solar cells, the bias voltage dependence of photocurrent for excitation with photon energies below and above the optical band gap is investigated in two structurally related polymer solar cells. Charges generated in (poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)] (C‐PCPDTBT):[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) solar cells via excitation of the low‐energy charge transfer (CT) state, situated below the optical band gap, need more voltage to be extracted than charges generated with excitation above the optical band gap. This indicates a lower effective binding energy of the photogenerated electrons and holes when the excitation is above the optical band gap than when excitation is to the bottom of the CT state. In blends of PCBM with the silicon‐analogue, poly[(4,4‐bis(2‐ethylhexyl)dithieno[3,2‐ b :2 ′ ,3 ′ ‐ d ]silole)‐2,6‐diyl‐ alt ‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (Si‐PCPDTBT), there is no effect of the photon energy on the electric field dependence of the dissociation efficiency of the CT state. C‐PCPDTBT and Si‐PCPDTBT have very similar electronic properties, but their blends with PCBM differ in the nanoscale phase separation. The morphology is coarser and more crystalline in Si‐PCPDTBT:PCBM blends. The results demonstrate that the nanomorphological properties of the bulk heterojunction are important for determining the effective binding energy in the generation of free charges at the heterojunction.