Technical: The main focus of this research project is on the determination of the electronic structure of three-dimensional heterostructure excitonic solar cells based on blends and thin films of nanoparticles, polymers, dyes and small molecular materials. Such solar cells promise low production costs, while offering low weight and mechanical flexibility. The electronic structure of materials interfaces in the solar cells is crucial for exciton dissociation, recombination prevention, and charge transfer. In order to further increase efficiency and open circuit voltage, it is essential to understand the electronic structure of these interfaces. The main analytical techniques employed in this project include photoemission and inverse photoemission spectroscopies, which allow the direct measurement of charge injection barriers at molecular heterointerfaces. The enabling preparative technique for these experiments is in-vacuum electrospray deposition, which has been established in recent years through previous efforts in the PI's group. This unique combination of photoemission spectroscopy, inverse photoemission spectroscopy, and electrospray deposition in the same experimental system allows the characterization of occupied and unoccupied states at macro-molecular interfaces at the same time. Additionally, feasibility of spray-based fabrication of solar cell structures using the electrospray deposition at atmospheric pressure is investigated in this project. The electrostatics offers ways for controlling the microscopic morphology of deposited films.
The project addresses basic research issues in a topical area of materials science with high technological relevance. The research project aims to improve the understanding of heterojunction solar cells based on organic materials and nanoparticles and at the same time seeks a new method for solar cell fabrication. On the educational side, graduate and undergraduate students are trained while participating in the cutting-edge scientific research of a strategically important area. The PI especially focuses on recruiting minority and female students for this research project and has an established record of involving undergraduate students, including those from the underrepresented groups, in research.
Summary of Outcomes: The objective for this project was to investigate the electronic structure of materials interfaces relevant for plastic solar cells. In such solar cells photons are converted into electrical energy via the generation of electron-hole (holes can be viewed as positive charges in difference to the negatively charged electrons) pairs. In order to generate a useful current through a load these electrons and holes need to be spatially separated and forced to go to the opposing sides of the solar cell structure. This is achieved through specially designed interfaces between electron and hole transporting materials which prevent the recombination of electron hole pairs (i.e. the neutralization of electrons and holes before they can do useful work in the load) after the impinging photons generated them. The hypothesis of the project was to develop a systematic rule which would allow to predict the properties of such materials interfaces formed between electronic polymer materials themselves as well as their interfaces to inorganic electrode materials used for charge extraction from the cell. In order to carry out these experiments a new set-up need to be designed and implemented in our lab. This "inverse photoemission spectroscopy (IPES)" set-up involved the development of a Geiger counter for photon detection and the implementation of control electronics. In combination with a low-energy electron gun this Geiger counter is able to detect photons emitted from a sample surface when electrons are absorbed into the sample. This allows the characterization of unoccupied electronic states within the sample. In combination with conventional photoelectron spectroscopy, which measures electrons emitted from the sample by means of x-ray and ultraviolet light exposure a complete picture of the electronic structure of a sample surface can be gained. Both methods are very surface sensitive, i.e. only a few atomic layers are "seen" during such measurements. This enables the investigation of interfaces between two materials A and B. Such experiments are carried out by first characterizing the surface of material A, and then depositing atomically thin layers of materials B in several steps on the surface. In-between deposition steps measurements are taken. This yields a series of spectra that show the transition from the electronic structure of material A to that of material B. From this series of spectra the electronic structure at the interface between A and B can be determined. The most significant information that can be gleaned from this is the amount of energy it takes to bring an electron or a hole from one side to the other. This information is crucial for the determination of a given A-B materials combination will result in successful separation of electrons and holes (i.e. in this case the interface has sufficiently high energy barriers to keep the electrons and holes apart despite their mutual attraction due to their opposing charges) or not. In our experiments we investigated a number of polymer materials and determined their "charge neutrality level" which is a measure for their capacity to attract electrons or holes. The results of these experiments revealed that polymer materials obey the "induced density of interface states (IDIS)" model which was determined earlier for small molecular materials. This allows the prediction of the electronic structure of polymer materials, which considerably simplifies the search for materials combinations that have promising properties for the use in plastic solar cells.
