Probing the Interfacial Energetics in Organic Semiconductor Blends

In the realm of shrinking new-generation electronic and photonic devices, the role of functional thin-film interfaces is pivotal for optimizing device design. For example, the energetics of materials play a critical role in charge transfer at the interface between functional materials with dissimilar properties, as in the case of organic photovoltaics (OPVs).^[1]Yet, a clear correlation between material energetics at the donor (<i>D) - </i>acceptor<i> (A)</i> interface and key device parameters is missing. This is primarily because of a lack of criteria for choosing the most suitable method to determine the ionization energy (<i>IE</i>) and electron affinity (<i>EA</i>) of organic semiconductors that could precisely predict the energy level alignment at the <i>D/A</i> interfaces. Consequently, the development of state-of-the-art donor and non-fullerene acceptor materials has led to contradictory claims about the role of energetic offsets at the <i>D-A</i> interface. This makes it difficult to establish design rules for the development of future materials.<br/>In this work, we systematically investigate the frontier molecular orbital energies of organic semiconductors and their molecularly mixed blends, as well as the impact of different solvents and molecular orientation on the resultant energies, via different probing techniques; in particular, low-energy inverse photoelectron spectroscopy (<i>LE-IPES), </i>which is a relatively novel and non-destructive technique that enables the direct determination of EA of organic semiconductors with high precision. By characterizing and fabricating thin films and devices based on over a dozen different D/A blends, we demonstrate the significant differences that have been an ongoing debate in the field for energetic losses and device performances.^[2]<br/>We show that the <i>IE</i> and <i>EA</i> values measured using ultraviolet photoelectron spectroscopy (<i>UPS)</i> and <i>LE-IPES</i> are the most relevant in understanding the charge generation mechanism in OPVs. We further demonstrate how the energy levels of organic semiconductors evolve when blended with energetically dissimilar materials,^[3] and the impact of morphology and phase segregation on the resultant energetics. Probing a range of small molecule - small molecule and polymer: small molecule D/A blends, we show that the photovoltaic gap <i>E_pv</i> (<i>IE_donor-EA_acceptor</i>) measured from neat materials can be insufficient in some cases, for establishing material-property relationships in solar cells. By controlling the D/A ratio in molecular blend films, we probe the changes induced in <i>E_pv</i> of D/A blends as a result of intermixing, intermolecular, and electrostatic interactions between the D/A materials. We rationalize these findings to the resultant photovoltaic parameters and voltage losses in OPVs.<br/>Lastly, from OPVs based on six different <i>D-A</i> blends having systematically varying <i>IE</i>-offsets (Δ<i>IE)</i>, we convincingly demonstrate that Δ<i>IE</i> plays a crucial role in charge generation. In contrast to earlier works, we show that a vanishing Δ<i>IE</i> is detrimental to device performance.<br/>Overall, these findings establish a solid base for reliably evaluating material energetics and interfacial properties toward interpreting property-performance relationships in solution-processed OPVs.<br/><br/>[1] S. Karuthedath, J. Gorenflot, Y. Firdaus, N. Chaturvedi, C. S. P. De Castro, G. T. Harrison, J. I. Khan, A. Markina, A. H. Balawi, T. A. D. Peña, W. Liu, R.-Z. Liang, A. Sharma, S. H. K. Paleti, W. Zhang, Y. Lin, E. Alarousu, D. H. Anjum, P. M. Beaujuge, S. De Wolf, I. McCulloch, T. D. Anthopoulos, D. Baran, D. Andrienko, F. Laquai, <i>Nature Materials</i> <b>2021</b>, 20, 378.<br/>[2] J. Bertrandie, J. Han, C. De Castro, E. Yengel, J. Gorenflot, T. Anthopoulos, F. Laquai, A. Sharma, D. Baran, <i>Advanced Materials</i> <b>2022</b>, 2202575.<br/>[3] X.e. Li, Q. Zhang, J. Yu, Y. Xu, R. Zhang, C. Wang, H. Zhang, S. Fabiano, X. Liu, J. Hou, F. Gao, M. Fahlman, Nature Communications, 2022, 13, 2046.