| Autor: |
Thomson SAJ; Organic Semiconductor Centre, SUPA, School of Physics & Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK., Niklas J; Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA., Mardis KL; Department of Chemistry and Physics, Chicago State University, Chicago, Illinois 60628, USA., Mallares C; Department of Chemistry and Physics, Chicago State University, Chicago, Illinois 60628, USA., Samuel IDW; Organic Semiconductor Centre, SUPA, School of Physics & Astronomy, University of St Andrews, St Andrews, KY16 9SS, UK., Poluektov OG; Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA. |
| Abstrakt: |
Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh 2 ) 2 , DTS(F 2 BTTh 2 ) 2 , DTS(PTTh 2 ) 2 , DTG(FBTTh 2 ) 2 and DTG(F 2 BTTh 2 ) 2 ) with the fullerene derivative PC 61 BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh 2 ) 2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh 2 ) 2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. The higher BET triplet exciton population in the DTS(PTTh 2 ) 2 blend is in accordance with the slower charge separation dynamics observed in this blend. |
