The laboratory for the “Thin Films and Thin Film Photovoltaics” at the University of Michigan-Shanghai Jiao Tong University Joint Institute focuses on fundamental and applied research on thin liquid and solid films that lead to the fabrication of emerging thin film devices.
Thin films and thin film devices have a ubiquitous presence in numerous conventional and emerging technologies. This is because of the recent advances in nanotechnology, the development of functional and smart materials, conducting polymers, molecular semiconductors, carbon nanotubes, and graphene, and the employment of unique properties of thin films and ultrathin films, such as high surface area, controlled nanostructure for effective charge transfer, and special physical and chemical properties,to develop new thin film devices. We therefore focuse on development of thin film photovolatics. We have recently started working on other similar thin film devices such as thin film transistors, thermoelectric devices, and so on. The thin film devices may consist of organic, inorganic, and composite thin layers, and share similar functionality, properties, and fabrication route. Therefore, due to the multidisciplinary nature of thin film devices, knowledge and advances already made in one area may be effectively applied to other similar areas. Owing to the importance of developing low-cost, scalable, and vacuum-free fabrication routes, we focus on solution-processed thin film devices.
The photovoltaic (PV) effect, i.e., creation of voltage or electric current in a material upon exposure to light, may be used to generate electricity through PV solar cells. The PV effect was first used in the specialty applications in the form of crystalline semiconductor solar panels. Due to the high cost of the vacuum-based fabrication process of such solar cells, research and development has been directed toward the invention and development of thin film inorganic and emerging inorganic/organic thin film PV solar cells in an attempt to fabricate solar cells out of less-expensive materials and processes.
Most emerging thin film solar cells are solution processed(solar cell materials are dissolved in a solvent and deposited in a substrate in ambient conditions using low-cost and vacuum-free methods, such as spin or spray coating), and therefore there is no need for expensive processes for their fabrication. Perovskite solar cell is the newest member of emerging solution-processed thin film solar cells. Within few years its efficiency has increased to over 20%. Polymer solar cell is another solution processed solar cell, which is rather mature and has been under investigations for a decade. The potential for large scale application of the perovskite and polymer thin film solar cells necessitates the development of cost-effective fabrication techniques and high photon to electron conversion efficiency solar cell materials.
Most solution-processed solar cells are fabricated by spin-coating, which is a lab-scale method. In this lab, we use spray coating, which is much faster, and can produce large area cells. Innovative ideas, such as imposing ultrasonic vibration on the substrate (recently developed in our lab) is employed to improve the device performance. Other innovative ideas such as doping the solar cell materials and layers with conductive agents, such as graphene and carbon nanotubes is employed also. Spray coating process may be performed in air, or if needed it is performed under nitrogen environment to suppress film degradation.
In addition to thin solid films, we also focus on thermal-fluid aspects of thin liquid films, instability, droplet dynamics and sprays and atomization. This is because in solution-processed methods, thin solid films usually form through drying of thin liquid films, and therefore, understanding of the underlying physical phenomena associated with thin liquid films and the coating process is essential for development of high performance thin film devices.