Unraveling the Correlation Between Structure and Performance by Optical Techniques on Organic Solar Cells

Organic based photovoltaics present nowadays a bright future, due to its suitability for the integration on systems related with the IoT, its compatibility with lightweight and flexible substrate and the possibility of being deposited by easily scale-up techniques such as ink-jet printing. Furthermore, the development of Non-Fullerene Acceptors (NFA) has led to a jump on performances, approaching the 20% milestone. Nevertheless, there is still a lack of knowledge on fundamental properties of the devices that can compromise the successful transition of this technology into the industrial step.<br/>In particular, there is still not a clear understanding of how the materials arrange themselves in the active layer and which impact it has on its optical and electrical transport properties and thud on the final performance of the functioning devices. In order to investigate the structural arrangement long time-consuming techniques such as synchrotron X-ray diffraction or Transmission Electron Microscopy (TEM) must be used. This, compound with the fact that samples have to be prepared on special conditions suitable for the measurement set-up limits the amount of information we can actually extract that will apply to the functioning device. In this work, we propose to use optical nondestructive techniques to explore the structural properties of the active layer on an actual device and compare with the electrical transport properties and performances.<br/>For this, we analyze solar cells made by liquid deposition techniques with active layers composed by different NFA/polymers at different post-annealing temperatures. These devices have been studied with raman spectroscopy to determine the structural organization of the active layer, revealing the importance of NFA and polymer compatibility for enhancing a better microstructural arrangement of both materials. Photoluminescence measurements have also been carried both on separate materials and on solar cells to quantify the effective charge separation and its correlation with the active layer structure. Finally, Photovoltage Decay measurements allow us to have an estimation of the recombination states on these devices.