给受共聚物链上与链间极化子的光谱特性

Spectral Signature of Intrachain and Interchain Polarons in Donor-Acceptor Copolymers

给受体共聚物具有重要的光伏效应,载流子在其中的传输机理一直备受关注.近来聚(吡咯并吡咯二酮-噻吩[3,2-b]并噻吩联二噻吩)[poly(DPP-DTT)]在其场效应晶体管器件中表现出高达10.5 cm2/(V?s)的空穴迁移率,有实验表明链上的极化子传输是主要途径,但也有光谱数据表明链上与链间极化子共存.单链或链间传输机制是目前大家关注的问题.本工作从计算化学的角度给出单链与双链极化子的光谱特征,为澄清载流子传输的机制提供依据.首先采用长程密度泛函理论计算不同结构的寡聚链的吸收光谱,通过与实验比较,找到能合理描述poly(DPP-DTT)单链中空穴/电子极化子及激子的有效模型.然后,通过格点能修正的方法,计算不同堆积情况下链间的电荷转移积分,阐述poly(DPP-DTT)链间不同堆积方式对极化子离域性的影响.转移积分大的空穴/电子双链极化子的理论吸收光谱与实验吻合.计算结果表明,在poly(DPP-DTT)中同时存在一维极化子和不同堆积结构的二维极化子,同时也说明载流子的一维和二维传输是同时进行的,而不同堆积结构会影响载流子链间的传输特性.

The charge transport mechanism in donor-acceptor conjugated copolymers has attracted significant attention in last few years due to the remarkable photovoltaic effects. Different from homopolymers, the donor-acceptor interaction makes the localization of electrons in copolymers more complicated. Polaron is the elementary excitation in polymer for charge transport, which can be detected by optical spectrum. Recently, poly（N-alkyl diketopyrrolo-pyrrole dithienylthieno-[3,2-b]-thiophene） [poly（DPP-DTT）] has shown great performance in organic field effect transistor, of which the hole mobility reached 10.5 cm2/（V.s）. There existed experimental evidence that the charge transport occurs in a single chain in some copolymer systems, while the optical spectra of poly（DPP-DTT） film indicated the existences of intrachain polaron and interchain polaron. We present here computational study on the optical signatures of the intrachain and interchain polarons, shedding light in clarifying the transport mechanism in poly（DPP-DTT）. Based on four possible intermolecular stacking models, we used the long-range corrected density functional theory to calculate the absorption spectra. DTT-DPP-DTT was identified as the appropriate model to describe the single-chain hole and electron polarons as well as exciton in poly（DPP-DTT）. We then built up four two-chain models with two DTT-DPP-DTT in different packing modes and applied the site-energy corrected method to compute the charge transfer integral between two-chains to identify the packing structure for generating interchain polarons. It was found that when DPP in one chain is in face to DPP or thieno-[3,2-b]-thiophene unit in another chain, both hole and electron interchain polaron can be generated due to the large intermolecular electron integral. The theoretical absorptions of interchain polaron are in good agreement with experiment. Therefore, the theoretical results prove the coexistence of 1D poloran and 2D polarons with different packing structures, and demonstrate that 1D and 2D charge transports are proceeding simultaneously in poly（DPP-DTT）, while the interchain transport is largely affected by the packing structure.