Triplet state formation in photovoltaic blends of DPP-type copolymers and PC71 BM.
The exciton dynamics in pristine films of two structurally related low-bandgap diketopyrrolopyrrole (DPP)-based donor-acceptor copolymers and the photophysical processes in bulk heterojunction solar cells using DPP copolymer:PC71 BM blends are investigated by broadband transient absorption (TA) pump-probe experiments covering the vis-near-infrared spectral and fs-μs dynamic range. The experiments reveal surprisingly short exciton lifetimes in the pristine poly-mer films in conjunction with fast triplet state formation. An in-depth analysis of the TA data by multivariate curve resolution analysis shows that in blends with fullerene as acceptor ultrafast exciton dissociation creates charge carriers, which then rapidly recombine on the sub-ns timescale. Furthermore, at the carrier densities created by pulsed laser excitation the charge carrier recombination leads to a substantial population of the polymer triplet state. In fact, virtually quantitative formation of triplet states is observed on the sub-ns timescale. However, the quantitative triplet formation on the sub-ns timescale is not in line with the power conversion efficiencies of devices indicating that triplet state formation is an intensity-dependent process in these blends and is reduced under solar illumination conditions, as free charge carriers can be extracted from the photoactive layer in devices.
Max Planck Research Group for Organic Optoelectronics, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany.
Julian R
JR
Ochsmann
Department of Advanced Materials, Hannam University, Daejeon, 305-811, Korea.
Deepak
D
Chandran
Max Planck Research Group for Organic Optoelectronics, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany.
Dominik W
DW
Gehrig
Department of Physics, Mount Holyoke College, 50 College St, South Hadley, MA, 01075, USA.
Husna
H
Anwar
Department of Advanced Materials, Hannam University, Daejeon, 305-811, Korea.
Pramod Kandoth
PK
Madathil
Department of Advanced Materials, Hannam University, Daejeon, 305-811, Korea.
Kwang-Sup
KS
Lee
Max Planck Research Group for Organic Optoelectronics, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany.
Frédéric
F
Laquai
D037741:Fullerenes / Q000737:chemistry*; D007659:Ketones / Q000737:chemistry; D011108:Polymers / Q000737:chemistry*; D011758:Pyrroles / Q000737:chemistry; D012993:Solar Energy; D013056:Spectrophotometry, Ultraviolet
D037741:Fullerenes; D007659:Ketones; D011108:Polymers; D011758:Pyrroles; C000604894:diketopyrrolopyrrole dye
DPP polymers; charge generation; organic photovoltaics; transient absorption spectroscopy; triplet state formation