Ochsmann Julian R, Chandran Deepak, Gehrig Dominik W, Anwar Husna, Madathil Pramod Kandoth, Lee Kwang-Sup, Laquai Frédéric
Max Planck Research Group for Organic Optoelectronics, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany.
Department of Advanced Materials, Hannam University, Daejeon, 305-811, Korea.
Macromol Rapid Commun. 2015 Jun;36(11):1122-8. doi: 10.1002/marc.201400714. Epub 2015 Apr 29.
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.
通过覆盖可见-近红外光谱和飞秒-微秒动态范围的宽带瞬态吸收（TA）泵浦-探测实验，研究了两种结构相关的基于低带隙二酮吡咯并吡咯（DPP）的给体-受体共聚物的原始薄膜中的激子动力学以及使用DPP共聚物：PC71 BM共混物的本体异质结太阳能电池中的光物理过程。实验揭示了原始聚合物薄膜中令人惊讶的短激子寿命以及快速的三重态形成。通过多变量曲线分辨率分析对TA数据进行深入分析表明，在与富勒烯作为受体的共混物中，超快激子解离产生电荷载流子，然后这些电荷载流子在亚纳秒时间尺度上迅速复合。此外，在脉冲激光激发产生的载流子密度下，电荷载流子复合导致聚合物三重态的大量形成。实际上，在亚纳秒时间尺度上观察到几乎定量的三重态形成。然而，亚纳秒时间尺度上的定量三重态形成与器件的功率转换效率不一致，这表明三重态形成在这些共混物中是一个强度依赖的过程，并且在太阳光照条件下会减少，因为自由电荷载流子可以从器件中的光活性层中提取出来。
