Organic Devices for Solar Energy Conversion and Storage

Flexible organic solar cells (OSCs) made on plastic substrates with traditional printing techniques can be semitransparent and have broader applications than their inorganic counterparts. After the power conversion efficiencies (PCEs) of bulkheterojunction (BHJ) OSCs reached 19 % with non-fullerene acceptors (NFAs), a major challenge to further increase PCEs to &gt;20% is how to reduce charge recombination in the BHJs. In the past 20 years, we have been working on optimizing processing of fullerene/non-fullerene OSCs and studying device physics to deepen understand on the mechanism of devices for enhancing the power conversion efficiency.<br/>In my presentation, I will introduce our recent progress on organic devices for solar energy conversion and storage.<br/><br/>1. Monitor morphology formation of BHJ<br/>To understand the BHJ morphology formation during solution processing, we studied three BHJ systems with the same donor PBDB-T and three different acceptors (PC₇₁BM; IT-M and N2200) via monitoring the morphology evolution during blade coating with a versatile in situ spectroscopy by recording photoluminescence (PL) and laser scattering (LS) signal, which indicated that the component with higher molecular weight dominates the blend film formation and the final morphology. (Liu et al., <i>Small Methods</i>, 2021, 2100585, https://doi.org/10.1002/smtd.202100585)<br/><br/>2. Dilute donor NFA OSCs<br/>The morphology and Donor/Acceptor ratio are crucial for PCEs of OSCs. In our recent work, we surprisingly found that the PCE is tolerant to PM6 contents ranging from 10 to 60 wt % in the OSCs based on PM6:Y6. Most surprisingly, a PCE over 10% was achieved in OSCs with only 10 wt% donor. The findings indicate low dependence of composition on charge generation, transport and extraction in NFA OSCs. (Yao et al., <i>J. Phys. Chem. Lett.</i> 2021, 12, 5039, https://doi.org/10.1021/acsaem.1c03017) It’s worth to note that even with large domains, the blends with low donor contents can still yield<i> J_sc</i> approaching 20 mA cm^-2. As the emerging NFAs have been demonstrated with long exciton diffusion length (Firdaus, Y. et al. <i>Nat. Commun. </i>2020,11, 5220), therefore, the charge generation process is more tolerant to the domain size. The findings are significant in the context of enabling large processing window, which is essential in a later scale-up stage, since it means lower sensitivity to compositional variations.<br/><br/>3. The Sunlight varies with time, season, and location on the earth. To extend the application of organic solar cells, storing electricity converted from the sunlight is necessary. Super-capacitors are components, which can store electricity. We developed hybrid super-capacitors based on MXene and polymers as well as integrating organic solar cells with super-capacitors (Photo-supercapacitors (PSCs)). Flexible semitransparent photovoltaic supercapacitors based on water-processed MXene electrodes were realized. The photovoltaic and storage performance of the PSC has potential applications as portable power units to charge low power consumption electronics/IoT. (L. Qin et al, J. Mater. Chem. A, 2020, 8, 5467, https://doi.org/10.1039/d0ta00687d)