Dilute-Donor Organic Solar Cells: Breaking the Fullerene Monopoly

太阳能是最具吸引力的可再生能源之一，推动了基础研究，使低成本技术能够适应各种应用。基于有机小分子和聚合物的有机光伏太阳能电池(OPV)是一个潜在的合适选择。OPV是由环保材料制成的轻质材料，可以通过廉价的方法制造。这项技术为将可再生电力生产能力整合到建筑基础设施中提供了一条轻便、灵活的平台。OPV技术还可以为室内显示器、汽车传感器、健康监测器和物联网发电。一种新型的OPV，稀释供体有机太阳能电池，最近被证明克服了以前的技术缺陷，如电池的电流与电压之间的权衡。到目前为止，对稀释供体OPV如何发电的基本科学理解是有限的。该项目将调查已建立的基于富勒烯的稀释型OPV系统，然后利用这些信息合理地设计采用其他材料的新系统，以提高性能和太阳能转换效率。除了技术进步，该项目还包括针对高中生和本科生的能源科学教育活动，为研究生提供的职业指导，以及通过与德克萨斯州达拉斯大学科学和工程教育中心的合作，面向德克萨斯州北部地区的K-12学生和普通公众的外联活动。这一基础研究项目的重点是扩大对稀释型有机光伏材料中光生载流子输运动力学的理解。在目前高性能的OPV系统中，有源层是基于体异质结(BHJ)纳米结构，即供体(D)和受体(A)分子的大致相等的混合物。基于BHJ的电池有两个基本缺陷：(1)短路电流密度和开路电压之间存在权衡；(2)性能对纳米级形貌高度敏感，到目前为止，这种形貌已经通过反复试验进行了优化。最近，一种绕过短路电流密度和开路电压之间权衡的OPV系统引起了人们的关注。基于富勒烯的OSC，也被称为稀施体OPV，只含有少量嵌入在富勒烯基质中的施主材料，在保持高开路电压的同时显示出相当大的短路电流密度。虽然稀释型施主体系中的高电压值被普遍认为是由金属电极和受主基质之间的肖特基势垒高度决定的，但没有空穴渗流路径的高电流密度的起源是未知的。本项目开展了一项联合的实验和理论模型研究，以研究稀释型OPV系统中载流子的光产生、传输和复合。到目前为止，所有已知的稀释体OPV研究都使用富勒烯受体作为骨架材料。然而，富勒烯的低吸收系数和低载流子迁移率是一个根本的弱点。为了解决这个问题，该项目将研究具有较小带隙和较高载流子迁移率的双极材料作为基质材料，以分别增加光吸收和改善载流子传输。将使用技术计算机辅助设计结合动力学蒙特卡罗模拟进行漂移扩散计算，以补充实验活动。蒙特卡罗分析将揭示稀释型OPV中各种材料性质和器件结构的相对重要性背后的基本原理。理论结果将有助于指导材料的实验选择。这一合作将导致高短路电流密度和高开路电压的稀释型供体OPV的合理设计。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Solar energy is among the most attractive renewable energy sources, motivating fundamental research to enable low-cost technologies that are adaptable for a variety of applications. Organic photovoltaic solar cells (OPVs), based on small organic molecules and polymers, are a potentially suitable option. OPVs are lightweight, made of environmentally benign materials, and can be manufactured by inexpensive methods. This technology provides a path towards a lightweight, flexible platform for integration of renewable electricity production capacity into building infrastructure. OPV technology can also generate electricity for indoor displays, automobile sensors, health monitors, and the internet of things. A new kind of OPV, the dilute-donor organic solar cell, has recently been demonstrated to overcome previous technological shortcomings such as the cell's current-versus-voltage trade off. To date, there is limited fundamental scientific understanding of how electricity is generated in dilute-donor OPVs. This project will investigate the established fullerene-based dilute-donor OPV systems and then use this information to rationally design new systems with other materials for improved performance and solar conversion efficiency. In addition to technical advancements, the project includes educational activities on energy science for high school and undergraduate students, career guidance for graduate students, and outreach activities to K-12 students and the general public in the Northern Texas area through a partnership with UT Dallas Science and Engineering Education Center. The focus of this fundamental research project is to expand the understanding of photogenerated charge carrier transport dynamics in dilute donor organic photovoltaics. The active layers in current high-performing OPV systems are based on the bulk heterojunction (BHJ) nanostructure, i.e. a blend of donor (D) and acceptor (A) molecules in roughly equal amounts. BHJ based cells have two fundamental drawbacks: (1) there is a trade-off between short circuit current density and open circuit voltage; and (2) the performance is highly sensitive to the nanoscale morphology, which to date has been optimized by trial and error. Recently, a different kind of OPV system which circumvents the trade-off between short circuit current density and open circuit voltage has gained attention. Fullerene-based OSCs, also called dilute-donor OPVs, contain only a small amount of donor material embedded in the fullerene matrix and show a substantial short circuit current density while maintaining a high open circuit voltage. While the high voltage value in the dilute-donor system is largely accepted to be determined by the Schottky barrier height between the metal electrode and the acceptor matrix, the origin of a high current density without percolating paths for holes was unknown. This project lays out a joint experimental and theoretical modeling study to investigate the photo-generation, transport, and recombination of carriers in dilute-donor OPV systems. So far, all known dilute-donor OPV studies use fullerene acceptors as the matrix material. However, the low absorption coefficient and low carrier mobilities of fullerenes are a fundamental weakness. To address this, the project will investigate ambipolar materials with smaller band gaps and higher carrier mobilities as matrix materials, to increase light absorption and improve carrier transport, respectively. Drift-diffusion calculations using Technology Computer-Aided Design combined with kinetic Monte Carlo simulations will be performed to complement the experimental activities. The Monte Carlo analysis will reveal the fundamentals behind the relative importance of various material properties and device structures in dilute-donor OPVs. The theoretical results will help guide the experimental selection of materials. The joint effort will lead to the rational design of dilute-donor OPVs with high short circuit current density and high open circuit voltage.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.