Charge Transfer States at Donor-Acceptor Heterojunctions: The role of morphology on dissociation efficiency and device lifetime.
Abstract
The generation of photocurrent at organic donor-acceptor junctions occurs via the formation of acharge transfer (CT) exciton, also known as the polaron pair. The exciton generated by opticalabsorption diffuses to the donor-acceptor junction where it is driven into the CT state that isCoulombically bound across the heterojunction. This ~indirect~ state is a precursor to the freecharge-separated state that results in photocurrent in the external circuit.Given the centrality of its importance in the photogeneration process, we must clearly understandthe mechanisms leading to the formation of polaron pairs. This understanding should lead to thedevelopment of donor-acceptor systems and film morphologies capable of achieving 100%internal quantum efficiency across the visible and near infrared spectral regions. Our group at theUniversity of Michigan has played a pioneering role in understanding such states, developingfundamental quantum and semi-classical theories that determine their static and dynamicproperties, along with the effects that morphology have on CT state dissociation. In this program,we propose to extend our theoretical and experimental approaches to gain a comprehensiveunderstanding of CT state photophysics, its dependence on materials combinations (includingnon-fullerene systems), and film morphology. We will use this understanding to achieve ~100%internal quantum efficiency at organic heterojunctions, thus guiding our choice of materials anddeposition conditions to achieve this major milestone in organic photovoltaics. A furtherobjective will be to determine the relationship between device lifetime and the photophysicalproperties of the CT state. Aging is well known to introduce morphological changes in devicestructure which can be sensitively probed via the properties of the CT states themselves.Hence, in this program we will endeavor to answer the following fundamental questions:1. What is the role of the CT state energy in its dissociation probability, and hence its efficiencyin photocurrent generation?2. Is there a combination of materials and morphologies that increases the rate of dissociationvs. recombination?3. What determines the donor-acceptor dilution ratio in mixed heterojunction active layers thatleads to the highest efficiency?4. What fundamental differences are there in the CT state dynamics of fullerene and nonfullereneacceptors?5. Can the control of morphology during growth, and donor-acceptor materials compositionslead to 100% internal quantum efficiency across the visible and into the near infrared?6. Can the CT state photophysics be used as an indicator of degradation processes in organicphotovoltaics, and ultimately can these processes be slowed via insights gained throughmonitoring the properties of the CT state over time?Successful answers to these questions is directed at breaking the apparent ~12% barrier~ to thepower conversion efficiency of organic photovoltaics, introducing new materials, filmmorphologies achieved during growth, and device architectures, all with the purpose ofincreasing the photon-to-electron conversion efficiency from the visible to the near infrared.Our focus is to ultimately develop high efficiency, stable and low cost solar power sources basedon organic photovoltaics. All services, including the Navy, have an increasing need to supplypower to its operational units. Organic photovoltaics have a lightweight, flexible and durableform factor such that they can be compactly stowed and easily transported and deployed inremote operating environments. Hence, our research has considerable long-term relevance to theNavy, as well as the several other armed services of the US government.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 03, 2017
- Source ID
- N000141712211
Entities
People
- Stephen R. Forrest
Organizations
- Board of Regents of the University of Michigan
- Office of Naval Research
- United States Navy