Exploring Charge Generation and Recombination in Dilute-Donor Organic Solar Cell Blends Using Ultrafast Transient Absorption Spectroscopy

Understanding the interplay between film morphology, photophysics, and its relation to device performance in bulk heterojunction organic photovoltaics remains challenging.<br/>Using the well-defined morphology of both vapordeposited neat C₆₀, as well as a6T:C₆₀ and TAPC:C₆₀ blends, where the dilute-donor blends (&lt;10% donor) show a higher efficiency that the completely intermixed (50% donor) blends, we see the morphological effects on charge generation, separation and recombination on an ultra-fast time scale. With the use of ultrafast transient spectroscopy we are able to follow each step of the charge generation process and give an explanation for the unusual efficiency trends in these organic solar cells.<br/>First, we explore the nature of Frenkel and Charge Transfer excitons in neat C₆₀ and then how photocurrent is generated over the entire fullerene absorption range in the blends, giving advantage to the dilute-donor blend with more C₆₀. [1,2]<br/>Second, we put a magnifying glass in the charge separation process by using a novel technique to show an energy cascade between interfacial and bulk C₆₀ electrons that assists free charge generation in the TAPC:C₆₀ blends, and then being able to separate and model the dynamics between interfacial CT state and separated charges in the a6T:C₆₀ blends. [2]<br/>Third, we identify a fast (&lt;1 ns) recombination channel, where free electrons recombine with trapped holes on isolated donor molecules which should harm the performance of dilute solar cells. We, however, give evidence that this recombination is mitigated when electrons are rapidly extracted in efficient devices, reminding us that more than just the active layer needs to be considered when designing efficient organic solar cells. [2]<br/>[1] J. Phys. Chem. Lett. 2018, 9, 8, 1885–1892<br/>[2] J. Phys. Chem. Lett. 2020, 11, 14, 5610–5617