|Support type:||Scholarships in Brazil - Post-Doctorate|
|Effective date (Start):||October 01, 2017|
|Effective date (End):||September 30, 2019|
|Field of knowledge:||Physical Sciences and Mathematics - Physics - Condensed Matter Physics|
|Principal Investigator:||Paulo Barbeitas Miranda|
|Home Institution:||Instituto de Física de São Carlos (IFSC). Universidade de São Paulo (USP). São Carlos , SP, Brazil|
Optoelectronic devices based on conjugated organic materials have become an attractive alternative to their inorganic counterparts due to their low cost and increased versatility, since they combine semiconducting properties with the processability and chemical diversity of polymers and small molecules. Organic solar cells, photodetectors and light-emitting diodes are examples of such devices. In particular, research on polymer-based solar cells has taken a groundbreaking momentum in the past decade, covering many different aspects, such as new device architectures, new materials, understanding (photo) physical mechanisms within the device, and long-term stability of these devices under ambient conditions. However, improvements in their performances and device life-time depend on a deep knowledge about the photophysical and electronic processes that occur in the materials. Although many different techniques for investigating physical processes occurring within the device have already been developed, several issues remain either relatively unexplored or investigated in a limited number of materials. One example is the role played by barriers for charge extraction at the electrodes, and charge recombination, in determining the overall device efficiency.In this proposal we aim at investigating the dynamics of charge recombination and extraction in organic solar cells, with a combination of transient photoinduced absorption spectroscopy (TAS) and transient photocurrent (TPC) measurements. TAS probes the charge density inside the organic active layer of the solar cell, while TPC measures the extracted charge density from the device. Comparing both measurements it is possible to determine how much photogenerated charge is lost within the active layer due to charge recombination, a process that is detrimental to the overall device efficiency. The candidate will initially get acquainted with our current continuous wave-photoinduced absorption (cw-PA) and TPC setups, then help in extending the cw-PA setup to perform TAS with ns to ms temporal resolution, and finally use these techniques to investigate charge recombination and extraction dynamics in organic solar cells.The candidate has previous experience with the fabrication and characterization of polymer-based solar cells, and has already investigated charge carrier generation and accumulation at the interfaces within the device based on PCDTBT:PCBM using the X-ray reflectrometry (XRR) method. He has shown that, in contrast to most studies in the literature on related systems, thermal annealing was not able to improve the efficiency of that solar cell. Extensive investigations by UV-Vis and Raman spectroscopy and XRR have linked changes in the electron density at the interface to the underperformance of the devices. Thus, it would also be interesting to use TAS mapping in a planar solar cell device to investigate the possibility of dynamical charge accumulation near the electrodes. Therefore, during this project the candidate will expand his knowledge and capabilities by using nanosecond transient absorption spectroscopy (nsTAS) to investigate charge carrier dynamics in operating solar cells.