Unlocking the Complexity of Organic Solar Cell Materials: Insights from Spectroscopy and Modeling

Organic solar cells (OSCs) have emerged as a promising technology. OSCs are built using organic materials that can convert sunlight into electricity. Over the years, significant progress has been made in improving the efficiency of OSCs, thanks to advancements in materials science and engineering. A new study published in the peer-reviewed Journal Materials Horizons and led by Professor Harald Ade from North Carolina State University and Professor Chad Risko from the University of Kentucky, the research conducted by graduate student Somayeh Kashani and Dr. Zhen Wang investigated some of the critical aspects of OSC materials. Indeed, the evolution of OSCs has been remarkable, with state-of-the-art devices now achieving efficiencies exceeding 19%. A pivotal moment in this progress was the introduction of non-fullerene small molecule acceptors (NF-SMAs) in OSCs, exemplified by the development of ITIC. These NF-SMAs, in conjunction with polymer donors, have enabled significant improvements in OSC efficiency. Much of this success is attributed to the rapid development of ladder-type, fused-ring cores that can be precisely tuned through molecular engineering.

There has been extensive research in the molecular engineering of these NF-SMAs. Researchers have explored a wide range of acceptor (A) and donor (D) building blocks, creating diverse core structures with varying p-bridges connecting the A end-group to the D core. Additionally, side-chain modifications are employed to manipulate the electronic and optical properties of these materials, with a profound impact on their thin-film manifestations. Characterizing the properties of these materials is a complex task. Reorganization energies and energetic disorder, both stemming from structural changes and conformational disorder, are critical factors in achieving high performance OSCs with minimal voltage losses. Accurately measuring these properties is challenging and often requires specialized techniques, including high-sensitivity photocurrent measurements, simulations, and modeling. In this context, optical absorption spectroscopy emerges as a valuable tool for studying the optoelectronic properties of materials. However, analyzing absorption spectra is not without its challenges. The absorption and emission spectra of organic materials can exhibit vibrational progressions due to electron-phonon coupling, described by Franck-Condon (FC) models. Yet, the complexity of these spectra often extends beyond the capabilities of simple FC models, particularly when dealing with materials in solution or solid thin films. To overcome these challenges, the authors used multi-parameter FC (MFC) analyses of absorption spectra in dilute solutions, coupled with density functional theory (DFT) and time-dependent DFT calculations. This new approach allowed them to dissect the contributions to absorption spectra and gain valuable insights into the reorganization energy and conformation populations of NF-SMAs.

The authors classified NF-SMAs into three distinct groups based on their molecular structures and absorption spectra: Firstly, ITIC-Like Structures which are Linear molecules with a single dominant conformation and one effective vibrational mode. Secondly, Y6-Like Structures which are curved molecules with one dominant conformation but with two effective vibrational modes and two electronic transitions. Thirdly, EH-IDTBR-Like Structures which are molecules with extra p-bridges, exhibiting a higher degree of conformational diversity with multiple conformations and one effective vibrational mode.

The researchers’ analysis showed that the reorganization energy and conformational diversity play crucial roles in determining the performance of OSC materials. For instance, Y6, with its low reorganization energy and single dominant conformation, exhibits a larger exciton diffusion length and smaller non-radiative voltage losses, which contribute to its success as a top performer in OSCs.

Indeed, understanding these molecular properties has significant implications for the design and development of OSC materials. Small reorganization energies, low electronic disorder, and conformational uniformity contribute to the success of NF-SMAs like Y6, resulting in high-performance OSCs with minimal voltage losses. This research underscores the importance of molecular design guidelines based on spectroscopic insights to drive further advancements in organic solar cell technology.

In conclusion, through a combination of spectroscopy and modeling techniques, Professor Harald Ade, Professor Chad Risko, and their research team have unraveled the complexities of NF-SMAs and their impact on OSC performance. Their findings offer valuable guidance for researchers and engineers striving to develop more efficient and sustainable solar energy solutions, ultimately contributing to a cleaner and greener future.