Organic photovoltaics have attracted significant interest over the last decade due to their promise as clean low-cost alternatives to large-scale electric power generation such as coal-fired power, natural gas, and nuclear power. Many believe power conversion efficiency targets of 10-15% must be reached before commercialization is possible. Consequently, understanding the loss mechanisms which currently limit efficiencies to 4-5% is crucial to identify paths to reach higher efficiencies. In this work, we investigate the dominant loss mechanisms in some of the leading organic photovoltaic architectures.;In the first class of architectures, which include planar heterojunctions and bulk heterojunctions with large domains, efficiencies are primarily limited by the distance photogenerated excitations (excitons) can be transported (termed the exciton diffusion length) to a heterojunction where the excitons may dissociate. We will discuss how to properly measure the exciton diffusion length focusing on the effects of optical interference and of energy transfer when using fullerenes as quenching layers and show how this explains the variety of diffusion lengths reported for the same material.;After understanding that disorder and defects limit exciton diffusion lengths, we suggest some approaches to overcome this. We then extensively investigate the use of long-range resonant energy transfer to increase exciton harvesting. Using simulations and experiments as support, we discuss how energy transfer can be engineered into architectures to increase the distance excitons can be harvested. In an experimental model system, DOW Red/PTPTB, we will show how the distance excitons are harvested can be increased by almost an order of magnitude up to 27 nm from a heterojunction and give design rules and extensions of this concept for future architectures.;After understanding exciton harvesting limitations we will look at other losses that are present in planar heterojunctions. One of the primary losses that puts stringent requirements on the charge carrier mobilities in these cells is the recombination losses due to space charge build up at the heterojunction. Because electrons are confined to the acceptor and holes to the donor, net charge density always exists even when mobilities are matched, in contrast to bulk heterojunctions wherein matched mobilities lead to zero net charge. This net charge creates an electric field which opposes the built-in field and limits the current that can be carried away from this heterojunction. Using simulations we show that for relevant current densities charge carrier mobilities must be higher than 10-4 cm2/V.s to avoid significant losses due to space charge formation.;In the last part of this work, we will focus on the second class of architectures in which exciton harvesting is efficient. We will present a systematic analysis of one of the leading polymer:fullerene bulk heterojunction cells to show that losses in this architecture are due to charge recombination. Using optical measurements and simulations, exciton harvesting measurements, and device characteristics we will show that the dominant loss is likely due to field-dependent geminate recombination of the electron and hole pair created immediately following exciton dissociation. No losses in this system are seen due to bimolecular recombination or space charge which provides information on charge-carrier mobility targets necessary for the future design of high efficiency organic photovoltaics.
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机译：在过去的十年中，有机光伏已经引起了人们极大的兴趣，这是因为它们有望作为清洁大规模生产电力的低成本替代品，例如燃煤发电，天然气和核能。许多人认为，在实现商业化之前必须达到10-15％的功率转换效率目标。因此，了解当前将效率限制为4-5％的损耗机制对于确定实现更高效率的途径至关重要。在这项工作中，我们研究了一些领先的有机光伏体系结构中的主要损耗机制。在第一类体系结构中，包括平面异质结和具有大畴的本体异质结，效率主要受光生激发（激子）的距离限制可以将其运输（称为激子扩散长度）到激子可能解离的异质结。我们将讨论在使用富勒烯作为淬火层时，如何着眼于光学干涉和能量转移的影响，如何正确地测量激子扩散长度，并展示这如何解释相同材料报道的扩散长度的变化。缺陷限制了激子的扩散长度，我们提出了一些克服这一问题的方法。然后，我们广泛地研究了使用远程共振能量转移来增加激子的收获。使用模拟和实验作为支持，我们讨论了如何将能量传递设计到体系结构中以增加可收集激子的距离。在一个实验模型系统DOW Red / PTPTB中，我们将展示如何从异质结处将收获的激子距离增加近一个数量级，直至27 nm，并给出该概念的设计规则和扩展以用于未来的体系结构。了解激子收集的局限性之后，我们将研究平面异质结中存在的其他损耗。对这些电池中的载流子迁移率提出严格要求的主要损失之一是由于在异质结处积累的空间电荷引起的重组损失。因为电子被限制在受体上，空穴被限制在施主上，所以即使迁移率匹配，净电荷密度始终存在，这与体异质结相反，在主体异质结中，迁移率匹配导致净电荷为零。该净电荷会产生一个与内置场相反的电场，并限制可以从该异质结带走的电流。通过仿真我们表明，对于相关的电流密度，载流子迁移率必须高于10-4 cm2 / Vs，以避免由于空间电荷形成而造成的重大损失。在本工作的最后一部分，我们将重点介绍第二类架构激子的收获是有效的。我们将对一种主要的聚合物：富勒烯本体异质结电池进行系统分析，以表明这种结构的损耗是由于电荷重组所致。使用光学测量和模拟，激子收集测量以及器件特性，我们将显示出主要的损失很可能是由于激子解离后立即产生的电子和空穴对的场相关复极重组。由于双分子重组或空间电荷，该系统未见任何损失，这提供了有关未来高效有机光伏光伏设计所必需的载流子迁移率目标的信息。
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