Slow geminate-charge-pair recombination dynamics at polymer: Fullerene heterojunctions in efficient organic solar cells (original) (raw)
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Advanced Materials, 2014
be bound by Coulombic interaction. Direct evidence for the formation of strongly correlated interfacial charge pairs came from electron spin resonance studies on polymer-fullerene blends. As most donor-acceptor blends exhibit additional sub-bandgap absorption and emission features, it was proposed that these interfacial excitations are charge transfer states (CT states) coupled to the ground state via radiative transitions. The reasons for highly effi cient charge generation despite the existence of Coulombically bound geminate pairs is subject of ongoing debate. Correlations have been found between the free carrier generation yield and the energy difference
Two-step charge photogeneration dynamics in polymer/fullerene blends for photovoltaic applications
Physical Review B, 2012
We measured the picoseconds (ps) transient dynamics of photoexcitations in blends of poly(3-hexyl-thiophene) (P3HT; donors-D) and fullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM; acceptor-A), using the transient pump/probe photomodulation technique in an unprecedented broad spectral range from 0.25 to 2.5 eV, and compared the results with organic solar cell performance based on the same blends. In D-A blends with maximum domain separation such as regio-regular P3HT/PCBM with (1.2:1) weight ratio having solar cell power conversion efficiency of ∼4%, we found that, although the photogenerated intrachain excitons in the polymer nano-domains decay within ∼10 ps, no charge polarons are generated on their expense up to ∼2 ns. Instead, there is a buildup of charge transfer (CT) excitons at the D-A interfaces having the same kinetics as the exciton decay, which dissociate into separate polarons in the D and A domains at a much later time ( 1 ns). This two-step charge photogeneration process may be typical in organic bulk heterojunction cells. Although the CT excitons are photogenerated on the exciton expense much faster in D-A blends having smaller domain size such as in regio-random P3HT/PCBM, their dissociation is less efficient because of larger binding energy. This explains the poor solar cell power conversion efficiency (<0.1%) based on this blend. Our results support the two-step charge photogeneration mechanism in polymer/fullerene blends and emphasize the important role of the CT binding energy in generating free charge polarons in organic solar cells.
Advanced Energy Materials, 2012
While much synthetic research has focused on the development of lower bandgap materials with absorption spectra that better match the solar irradiation spectrum, [ 1 , 2 , 5 ] new materials will also be required which maximize the photovoltage obtained from absorbed photons. Many organic photovoltaic materials have optical bandgaps in the range of 1.7-2.0 eV, which make them suitable for the large bandgap junction of a tandem solar cell. However, the highest effi ciency allorganic tandem solar cells reported thus far only obtain open-circuit voltages (V oc 's) of 0.9 V or less from the large bandgap junction. [ 4 , 6 ] Internal quantum effi ciencies (IQEs) approaching 100% have been demonstrated in single junction polymer:fullerene BHJ solar cells. [ 3 , 7 ] In order to generate and extract charges with high quantum effi ciency from bound photogenerated excitons, a heterojunction is employed where the ionization potential and electron affi nity of the fullerene are substantially higher than for the polymer, providing suffi cient free energy (Δ G) for charge transfer and exciton dissociation. Consequently, in the most effi cient BHJ devices, the V oc is 0.7-1.0 V less than the optical bandgap potential of the individual materials, E g / q. [ 1-3 , 5 , 7 ] The C 60 and C 70 fullerene derivatives currently employed in organic solar cells with efficiencies above 4% have an optical bandgap of 1.7 eV. Although there have been reports of polymer:fullerene devices with V oc values above 1.0 V, [ 8-10 ] and as high as 1.15 V, [ 11 ] all of these Polymer:fullerene solar cells are demonstrated with power conversion effi ciencies over 7% with blends of PBDTTPD and PC 61 BM. These devices achieve open-circuit voltages (V oc) of 0.945 V and internal quantum effi ciencies of 88%, making them an ideal candidate for the large bandgap junction in tandem solar cells. V oc 's above 1.0 V are obtained when the polymer is blended with multiadduct fullerenes; however, the photocurrent and fi ll factor are greatly reduced. In PBDTTPD blends with multiadduct fullerene ICBA, fullerene emission is observed in the photoluminescence and electroluminescence spectra, indicating that excitons are recombining on ICBA. Voltage-dependent, steady state and time-resolved photoluminescence measurements indicate that energy transfer occurs from PBDTTPD to ICBA and that back hole transfer from ICBA to PBDTTPD is ineffi cient. By analyzing the absorption and emission spectra from fullerene and charge transfer excitons, we estimate a driving free energy of-0.14 ± 0.06 eV is required for effi cient hole transfer. These results suggest that the driving force for hole transfer may be too small for effi cient current generation in polymer:fullerene solar cells with V oc values above 1.0 V and that non-fullerene acceptor materials with large optical gaps (> 1.7 eV) may be required to achieve both near unity internal quantum effi ciencies and values of V oc exceeding 1.0 V.
Applied Physics Letters, 2010
We correlate carrier recombination via charge transfer excitons ͑CTEs͒ with the short circuit current, J sc , in polymer/fullerene solar cells. Near infrared photoluminescence spectroscopy of CTE in three blends differing for the fullerene acceptor, gives unique insights into solar cell characteristics. The energetic position of the CTE is directly correlated with the open-circuit voltage, V oc , and more important J sc decreases with increasing CTE emission intensity. CTE emission intensity is discussed from the perspective of blend morphology. The work points out the fundamental role of CTE recombination and how optical spectroscopy can be used to derive information on solar cell performances.
Free Carrier Generation in Fullerene Acceptors and Its Effect on Polymer Photovoltaics
Journal of Physical Chemistry C
Early research on C 60 led to the discovery that the absorption of photons with energy greater than 2.35 eV by bulk C 60 produces free charge carriers at room temperature. We find that not only is this also true for many of the soluble fullerene derivatives commonly used in organic photovoltaics, but also that the presence of these free carriers has significant implications for the modeling, characterization, and performance of devices made with these materials. We demonstrate that the discrepancy between absorption and quantum efficiency spectra in P3HT:PCBM is due to recombination of such free carriers in large PCBM domains before they can be separated at a donor/acceptor interface. Since most theories assume that all free charges result from the separation of excitons at a donor/acceptor interface, the presence of free carrier generation in fullerenes can have a significant impact on the interpretation of data generated by numerous fielddependent techniques.