Organic field-effect devices as tool to characterize the bipolar transport in polymer-fullerene blends: The case of P3HT-PCBM
Organic field-effect devices as tool to characterize the bipolar transport in polymer-fullerene blends: The case of P3HT-PCBM
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DOI：
10.1002/adfm.200700124
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发表时间：
2007-11-05
影响因子：
19
通讯作者：
Brabec, Christoph
中科院分区：
文献类型：
--
作者：
Morana, Mauro;Koers, Peter;Brabec, Christoph
在有机体异质结太阳能电池(OBHJ)中，混合物的形态与单个组分的电荷传输性质相结合，控制着提取的光电流。有机场效应晶体管(OFET)已被证明是一种在多种情况下评价单极载流子输运性质的有力工具。在我们的工作中，我们将OFET的概念推广到聚合物-富勒烯共混物中双极输运性质的评价，并提出了一种提高评价准确性的方法。该方法基于对在相同共混物上制备的MOS结构的电容-电压(C-V)的测量，并提供了关于体异质结的补充信息，而不是用FET获得的信息。通过太阳能电池的电流-电压行为与聚合物分子量不同的不同溶剂制备的复合材料的双极迁移率之间的关联，研究了太阳能电池在光伏应用中的相关性。特别是比较了由邻二甲苯(OX)生产的太阳能电池的传输特性和由氯苯(CB)溶液浇铸的太阳能电池的传输特性。对于P3HT-PCBM共混物，迁移率与电填充因子和功率性能之间存在一致的相关性。双极载流子迁移率的显著不对称性，以及依赖于M-w值的低电子迁移率，影响了厚型邻二甲苯浇注器件的性能。在用氯苯加工的器件中，较慢的载流子具有较高的迁移率，并且检测到的小的电损最终更多地与空间电荷的形成以及最终的表面复合有关。这导致了几乎与厚度无关的高效电荷收集。我们报告了慢载流子类型(电子或空穴)及其迁移率与特定的分子量和溶剂组合的关系。迁移率数据和太阳电池性能符合器件模型的预测，仅基于来自施主-受主的内建电场下的载流子漂移。
In organic bulk heterojunction solar cells (oBHJ) the blend morphology in combination with the charge transport properties of the individual components controls the extracted photocurrent. The organic field-effect transistor (OFET) has been proved as a powerful instrument to evaluate the unipolar carrier transport properties in a wide range of cases. In our work we extend the OFET concept to the evaluation of the bipolar transport properties in polymer-fullerenes blends and propose a method to improve the accuracy of the evaluation. The method is based on capacitance-voltage (C-V) measurements on MOS structures prepared on the same blends and delivers complementary information on the bulk heterojunction to the one obtained with FETs. The relevance for photovoltaic applications is investigated through the correlation between the current-voltage behavior of solar cells and the bipolar mobility for composites with varying polymer molecular weight and processed from different solvents. In particular the transport features of solar cells produced from o-Xylene (oX), a non chlorinated solvent more suitable to production requirements, have been compared to the one of devices cast from Chlorobenzene (CB) solution. For the P3HT-PCBM blend a consistent correlation between the mobility and the electrical fill factor and power performance was found. A significant asymmetry in the bipolar carrier mobility, together with low electron mobility dependent on the M-w value, affects the performances of thick o-Xylene cast devices. In the case of devices processed from Chlorobenzene the slower carrier has higher mobility and the small electrical losses detected are eventually more related to the formation of space-charge and eventually to surface recombination. This results in an efficient charge collection that is almost thickness independent. We report a dependence of the slow-carrier type (electrons or holes) and their mobility on the specific combination of molecular weight and solvent. The mobility data and the solar cell performance coherently fit to the prediction of a device model only based on the drift of carriers under the built-in electric field originated in the donor-acceptor oBHJ.