Ultrafast Exciton Dissociation Followed by Nongeminate Charge Recombination in PCDTBT:PCBM Photovoltaic Blends
Ultrafast Exciton Dissociation Followed by Nongeminate Charge Recombination in PCDTBT:PCBM Photovoltaic Blends
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DOI：
10.1021/ja201837e
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发表时间：
2011-06-22
影响因子：
15
通讯作者：
Laquai, Frederic
中科院分区：
文献类型：
--
作者：
Etzold, Fabian;Howard, Ian A.;Laquai, Frederic
许多研究小组已经对本体异质结聚合物：通常用于有机光伏器件的富勒烯共混薄膜中电荷产生和复合的精确机制和动力学进行了深入研究，但仍然存在争议。特别是界面电荷转移（CT）态在自由电荷载流子生成中的作用，这是理解器件功能的重要一步，仍在积极讨论中。在本文中，我们提出了基于原型聚咔唑的光伏供体聚合物原始薄膜中激子动力学的直接光学探针，即聚[N-11''-henicosanyl-2,7-咔唑-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-苯并噻二唑)] (PCDTBT)，以及使用亚甲基富勒烯 (PC61BM) 作为电子受体的铸态和退火光伏共混薄膜中的电荷产生和复合动力学。与早期的研究相比，我们使用宽带（500-1100 nm）瞬态吸收光谱，包括以前未观察到但非常重要的 2 ns 到 1 ins 之间的时间范围，这使我们不仅能够观察整个电荷载流子复合动力学，而且还能量化现有的衰减通道。我们确定超快激子解离发生在混合物中，并产生两个独立的产物池，即库仑束缚电荷转移（CT）态和未束缚（自由）电荷载流子。在先前报道的聚（3-己基噻吩）:PCBM 模型（Howard, I. A. 等人 J. Am. Chem. Soc. 2010, 132, 14866）的框架内分析了重组动力学，该模型基于 CT 态的伴随双子重组和自由电荷载流子的非双子重组。结果表明，只有大约 11% 的初始光激发产生仅通过快速纳秒双子复合进行重组的界面 CT 态，因此对光电流没有贡献，而大约 89% 的激子在超快时间尺度上产生自由电荷载流子，然后对提取的光电流做出贡献。尽管自由电荷产量很高，但器件的功率转换效率仍然中等，约为 3.0%。这主要是由于设备的填充因子低造成的。我们将低填充因子与原始聚合物和聚合物：富勒烯共混物中存在的显着能量紊乱联系起来。在前者中，我们观察到激子发射（荧光）的显着光谱弛豫，在后者中观察到极化子引起的基态漂白，这意味着激子和载流子的态密度（DOS）因原始PCDTBT及其与PCBM的混合物中的能量无序而显着拓宽。这种无序导致太阳能电池中的电荷捕获，进而导致更高的载流子浓度和更显着的非成对复合。非成对复合对器件的IV曲线有显着影响，即其与载流子提取的竞争导致器件光电流具有更强的偏压依赖性，进而导致器件填充因子较差。此外，我们的结果证明了超快自由载流子生成和抑制界面 CT 态形成的重要性，并对常用的 Braun-Onsager 模型描述聚合物：富勒烯有机光伏器件中光电流的偏压依赖性的适用性提出了质疑。
The precise mechanism and dynamics of charge generation and recombination in bulk heterojunction polymer: fullerene blend films typically used in organic photovoltaic devices have been intensively studied by many research groups, but nonetheless remain debated. In particular the role of interfacial charge-transfer (CT) states in the generation of free charge carriers, an important step for the understanding of device function, is still under active discussion. In this article we present direct optical probes of the exciton dynamics in pristine films of a prototypic polycarbazole-based photovoltaic donor polymer, namely poly[N-11 ''-henicosanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), as well as the charge generation and recombination dynamics in as-cast and annealed photovoltaic blend films using methanofullerene (PC61BM) as electron acceptor. In contrast to earlier studies we use broadband (500-1100 nm) transient absorption spectroscopy including the previously unobserved but very important time range between 2 ns and 1 ins, which allows us not only to observe the entire charge carrier recombination dynamics but also to quantify the existing decay channels. We determine that ultrafast exciton dissociation occurs in blends and leads to two separate pools of products, namely Coulombically bound charge-transfer (CT) states and unbound (free) charge carriers. The recombination dynamics are analyzed within the framework of a previously reported model for poly(3-hexylthiophene):PCBM (Howard, I. A. et al. J. Am. Chem. Soc. 2010, 132, 14866) based on concomitant geminate recombination of CT states and nongeminate recombination of free charge carriers. The results reveal that only similar to 11% of the initial photoexcitations generate interfacial CT states that recombine exclusively by fast nanosecond geminate recombination and thus do not contribute to the photocurrent, whereas similar to 89% of excitons create free charge carriers on an ultrafast time scale that then contribute to the extracted photocurrent. Despite the high yield of free charges the power conversion efficiency of devices remains moderate at about 3.0%. This is largely a consequence of the low fill factor of devices. We relate the low fill factor to significant energetic disorder present in the pristine polymer and in the polymer: fullerene blends. In the former we observed a significant spectral relaxation of exciton emission (fluorescence) and in the latter of the polaron-induced ground-state bleaching, implying that the density of states (DOS) for both excitons and charge carriers is significantly broadened by energetic disorder in pristine PCDTBT and in its blend with PCBM. This disorder leads to charge trapping in solar cells, which in turn causes higher carrier concentrations and more significant nongeminate recombination. The nongeminate recombination has a significant impact on the IV curves of devices, namely its competition with charge carrier extraction causes a stronger bias dependence of the photocurrent of devices, in turn leading to the poor device fill factor. In addition our results demonstrate the importance of ultrafast free carrier generation and suppression of interfacial CT-state formation and question the applicability of the often used Braun-Onsager model to describe the bias dependence of the photocurrent in polymer: fullerene organic photovoltaic devices.