Tress Wolfgang, Beyer Beatrice, Ashari Astani Negar, Gao Feng, Meloni Simone, Rothlisberger Ursula
Biomolecular and Organic Electronics, IFM, Linköping University , 58183 Linköping, Sweden.
Fraunhofer Institute for Electron Beam, Plasma Technology and COMEDD (FEP) , Maria-Reiche-Strasse 2, 01109 Dresden, Germany.
J Phys Chem Lett. 2016 Oct 6;7(19):3936-3944. doi: 10.1021/acs.jpclett.6b01962. Epub 2016 Sep 22.
Efficient organic solar cells are based on (electron) donor-acceptor heterojunctions. An optically generated excited molecular state (exciton) is dissociated at this junction, forming a charge-transfer (CT) state in an intermediate step before the electron and hole are completely separated. The observed highly efficient dissociation of this Coulombically bound state raises the question on the dissociation mechanism. Here, we show that the observed high quantum yields of charge carrier generation and CT state dissociation are due to extended (and consequently weakly bound) CT states visible in absorption and emission spectra and first-principles calculations. Identifying a new geminate-pair loss mechanism via donor excimers, we find that the hole on the small-molecule donor is not localized on a single molecule and charge separation is correlated with the energetic offset between excimer and CT states. Thus, the charges upon interface charge transfer and even in the case of back-transfer and recombination are less localized than commonly assumed.
高效有机太阳能电池基于（电子）供体-受体异质结。光生激子状态（激子）在该结处解离，在电子和空穴完全分离之前，会在中间步骤形成电荷转移（CT）态。这种库仑束缚态的高效解离引发了关于解离机制的问题。在此，我们表明，观察到的高量子产率的电荷载流子生成和CT态解离是由于吸收光谱和发射光谱以及第一性原理计算中可见的扩展（因此束缚较弱）CT态。通过供体准分子确定了一种新的双生对损失机制，我们发现小分子供体上的空穴并非局限于单个分子，电荷分离与准分子和CT态之间的能量偏移相关。因此，界面电荷转移时的电荷，甚至在反向转移和复合的情况下，其局域性都比通常假设的要低。
