Improved materials stability and morphology in organic photovoltaic cells

塑料太阳能电池使用有机或聚合物半导体材料，而不是更传统的材料，如硅或碲化镉。塑料太阳能电池的效率，也被称为有机光伏（OPV），在过去十年中急剧增加，从百分之几增加到超过12%，使其性能与更成熟的太阳能电池技术具有竞争力。塑料太阳能电池具有显著降低太阳能发电价格的潜力，但目前它们的寿命相对较短，低成本生产尚未得到充分证明。较短的寿命（与20-25年的行业标准相比）是由于使用不稳定的反应性材料，低成本生产将需要廉价的半导体材料和制造技术。本文旨在通过设计一个科学的OPV电特性模型来解决这些问题，以提高我们对OPV工作原理和局限性的理解。这个模型将指导我们在其他两个项目中选择材料和设备结构，2)开发高效率和长期稳定的太阳能电池新分子，3)开发使用塑料半导体层压膜的低成本制造新方法。**为了指导我们对材料和结构的选择，我们必须从第一性原理物理学的角度了解OPV技术的操作和基本局限性。这些问题对于传统太阳能电池来说已经有了很好的理解，但是OPV电池的操作要复杂得多，需要新的模型。了解这些限制将使我们能够把精力集中在太阳能电池的性能上，在那里可以实现最大的改进。**每个高效OPV电池都使用富勒烯衍生分子作为“电子受体”。虽然在早期的器件中使用了一些非富勒烯，但富勒烯由于其优异的电子传递和高电子亲和性而取得了成功，使其得到了广泛的应用。不幸的是，有几个原因导致富勒烯不适合成功的商业OPV技术。首先，富勒烯在氧气和光的存在下非常不稳定。由此产生的光氧化大大降低了它们的电性能，并降低了太阳能电池的效率。其次，富勒烯需要大量的能量来生产，而可持续的太阳能电池技术应该需要尽可能少的能量，以最大限度地减少能源回报时间。最后，富勒烯吸收光的能力很差，所以我们必须依靠电子供体材料来收集太阳光子。通过开发非富勒烯受体，我们的目标是生产稳定、更可持续、更高效的塑料太阳能电池。**生产OPV电池的最低成本方法是基于溶液的，其中含有塑料半导体的“墨水”用于覆盖电池。不幸的是，这限制了可以生产的电池的复杂性（因此效率），因为试图在已经涂好的薄膜上涂上第二种墨水通常会溶解第一种墨水。我们建议开发新的制造技术，其中复杂的，高效的，多层电池是由层压半导体薄膜，而不是油墨涂层。在这个过程中，油墨被用来涂在一个临时的表面上，这个表面被称为印章。油墨干了，薄膜被转移或层压到含有先前沉积薄膜的第二表面上。这种干转移工艺避免了溶解早期层的问题，并且可以形成复杂的多层膜。这项技术将使高效多层OPV电池的低成本生产成为可能。

Plastic solar cells use organic or polymer semiconductor materials instead of more traditional materials, such as silicon or cadmium telluride. Efficiencies of plastic solar cells, also known as organic photovoltaics (OPV), have increased drastically over the last decade, from a few percent to over 12%, making their performance competitive with more established solar cell technologies. Plastic solar cells have the potential to significantly reduce the price of solar-generated electricity, but they currently suffer from relatively short lifetimes and low-cost production has yet to be fully demonstrated. The short lifetimes (compared to the industry standard of 20-25 years) are due to the use of unstable, reactive materials, and low-cost production would require both inexpensive semiconductor materials and fabrication techniques. This proposal aims to address these issues by 1) designing a scientific model of OPV electrical characteristics to improve our understanding of the operation and limitations of OPV. This model will guide our materials and device structure choices in the other two initiatives, 2) developing new molecules for solar cells that exhibit high efficiencies and long-term stability, and 3) developing new low-cost manufacturing methods using laminated films of plastic semiconductors.**To guide our choices of materials and structures, it is important that we understand, from a first-principles physics point of view, the operation and fundamental limitations of OPV technology. These issues have been well-understood for traditional solar cells for many years, but the operation of OPV cells is significantly more complicated, and new models are required. Understanding these limitations will allow us to focus our efforts on properties of the solar cells where the most improvement can be realized.**Every high-efficiency OPV cell uses a fullerene-derived molecule as the "electron acceptor". Although a few non-fullerenes were used in early devices, the success of fullerenes due to their excellent electron transport, and high electron affinity, has made their use wide-spread. Unfortunately, there are several reasons why fullerenes are not suitable for a successful commercial OPV technology. First, fullerenes are very unstable in the presence of oxygen and light. The resulting photo-oxidation drastically diminishes their electrical properties, and degrades solar cell efficiency. Second, fullerenes require a lot of energy to produce, and a sustainable solar cell technology should require as little energy as possible, to minimize the energy payback time. Finally, fullerenes absorb light very poorly, so we must rely on the electron donor material to harvest the solar photons. By developing non-fullerene acceptors, we aim to produce stable, more sustainable, and more efficient plastic solar cells.**The lowest cost methods for producing OPV cells are solution-based, where an "ink" containing the plastic semiconductor is used to coat the cell. Unfortunately, this limits the complexity (and therefore efficiency) of cells that can be produced, as trying to coat a second ink on top of an already-coated film will typically dissolve the first. We propose to develop new fabrication techniques, where complex, efficient, multilayer cells are produced by laminating semiconductor films, rather than ink-coating. In this procedure, the ink is used to coat a temporary surface called a stamp. The ink dries, and the film is transferred, or laminated, onto a second surface that contains previously deposited films. This dry-transfer process avoids the problems of dissolving earlier layers, and complex multilayer films can be formed. This technology will enable low-cost production of efficient multilayer OPV cells.