The sun represents the most abundant potential source of sustainable energy on earth. Solar cells that use organic conducting polymers to convert light to electricity ? organic photovoltaic (OPV) devices - offer a potentially low-cost route for renewable electricity production. However, in order to achieve parity with other solar photovoltaic technologies, organic solar cells must increase their power conversion efficiency. This project will incorporate light-active materials into the organic polymer solar cell device to promote a quantum mechanical process called triplet fusion. It is believed that triplet fusion can increase the voltage of the power output, leading to higher solar energy conversion efficiency. This concept is innovative from a scientific point of view because it will focus on increasing the voltage potential, not the photocurrent, leading to a pathway to increase solar cell efficiency through an increase in thermodynamic efficiency. The educational activities associated with this project include participation in the Princeton University Materials Academy that teaches hands-on materials science to under-represented high school students from the Trenton, New Jersey, and multi-generational outreach through the Nano Materials Science Day at a local library that encourages students, teachers, and parents to meet with solar scientists and participate in hands-on demos to make and test simple solar cell devices.
The overall goal of the research is to develop a new, organic polymer-based excitonic solar photovoltaic device architecture that has the potential exceed classical thermodynamic power conversion efficiency limits through the process of triplet fusion. In triplet fusion, two excitons formed from two low-energy sub band gap photons are required to produce one higher-energy photon. The integration of sensitized phosphorescent materials into the organic photovoltaic (OPV) device architecture offers the means to achieve triplet fusion. In a phosphor-sensitized OPV device, absorption and singlet formation occurs primarily on a fluorescent host donor. This initial absorption event is followed by energy transfer to a phosphorescent guest present in the donor layer at low concentration. Excitons on the phosphorescent guest then transfer to the triplet level of the host, permitting the population of the long-lived triplet state in the fluorescent material. This process of triplet fusion will be used to create free charge carriers with higher energy than the exciton from which the carriers originated. In essence, this device configuration represents the molecular analogue to an intermediate band solar cell, but possesses similar limiting efficiencies as singlet fission-based solar cells. The main distinction between the triplet fusion device and a conventional intermediate band solar cell is that, in the triplet fusion device, the intermediate band is satisfied via the fusion of two low energy triplet excitons into a higher energy singlet exciton. Furthermore, whereas the singlet fission device looks to increase photocurrent, the one based upon triplet fusion looks to increase the photovoltage, leading to fundamental enhancement of the thermodynamic efficiency. To explore this new phenomenon, the research has two major objectives. The first objective is to demonstrate the functionality of the phosphor-sensitized OPV device for achieving triplet fusion, and the second objective is to understand efficiency-limiting mechanisms involved with multiple exciton devices through comprehensive electrical and optical characterization. Ultimately, the demonstration of a triplet fusion OPV device adds another option to exceed Shockley-Queisser limits and inspire work toward high efficiency organic solar cells.
