Understanding energetic disorder in electron-deficient-core-based non-fullerene solar cells

Abstract

Recent advances in material design for organic solar cells (OSCs) are primarily focused on developing near-infrared nonfullerene acceptors, typically A-DA′D-A type acceptors (where A abbreviates an electron-withdrawing moiety and D, an electron-donor moiety), to achieve high external quantum efficiency while maintaining low voltage loss. However, the charge transport is still constrained by unfavorable molecular conformations, resulting in high energetic disorder and limiting the device performance. Here, a facile design strategy is reported by introducing the "wing" (alkyl chains) at the terminal of the DA′D central core of the A-DA′D-A type acceptor to achieve a favorable and ordered molecular orientation and therefore facilitate charge carrier transport. Benefitting from the reduced disorder, the electron mobilities could be significantly enhanced for the "wing"-containing molecules. By carefully changing the length of alkyl chains, the mobility of acceptor has been tuned to match with that of donor, leading to a minimized charge imbalance factor and a high fill factor (FF). We further provide useful design strategies for highly efficient OSCs with high FF.

An A-D-A′-D-A type unfused nonfullerene acceptor for organic solar cells with approaching 14% efficiency

“…(3) The 4-and 7-positions are easy to carry out bromination reaction, and then more expanded intermediates can be obtained through cross-coupling reactions. Due to the above-mentioned chemical versatility, the BTz unit has been widely used in high-performance polymer donors (represented by J-series polymers [21]) and FRAs (represented by Y-series small molecules [22]). In the spirit of further manipulating the A′(D′) unit and exploring better UFAs, we think BTz moiety may be a promising candidate for UFAs.…”

The Influence of Photo‐Induced Space Charge and Energetic Disorder on the Indoor and Outdoor Performance of Organic Solar Cells

“…In recent years, organic solar cells with low Urbach energies have exhibited record power conversion efficiencies highlighting the importance of energetic disorder on the performance of organic solar cells. [ 6,15,53,54 ] One of the key loss mechanisms in the presence of such tail states is the accumulation of space charge in the defect states, when charge‐generation is nonuniform in the device. In this work, we use drift‐diffusion simulations to visualize how this space charge builds up with increasing light intensity and how the electric field is altered in the process.…”

“…[ 43–48 ] Experimental data detecting tail states acquired from external quantum efficiency measurements or photothermal deflection spectroscopy can usually be well reproduced with an exponential density of band tail states. [ 15,49–54 ] Figure S1a, Supporting Information schematically illustrates the density of defect states defined by its characteristic slope E U , the Urbach energy. [ 55 ] Typical values for the Urbach energy can be found between 30 and 80 meV [ 37,43,45,46,56 ] with state of the art material systems reaching Urbach energies close to thermal energy.…”

“…[ 55 ] Typical values for the Urbach energy can be found between 30 and 80 meV [ 37,43,45,46,56 ] with state of the art material systems reaching Urbach energies close to thermal energy. [ 6,15,53,54 ] When a defect state near the HOMO level is occupied by a hole, its positive charge is localized at the position of the trap. The same principle holds for electrons that occupy the defect states near the LUMO level.…”

Two Compatible Polymer Donors Enabling Ternary Organic Solar Cells with a Small Nonradiative Energy Loss and Broad Composition Tolerance

“…However, such a system suffers from the intrinsic shortcomings of fullerene and its derivatives, e.g., poor absorption in the visible region and large energy loss (E loss ). [29][30][31] Recently, remarkable achievements have been made in the synthesis of novel narrow-bandgap (E g ) FREAs, as exemplified by 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno [2,3-d:2 0 ,3 0 -d 0 ]-s-indaceno[1,2-b:5,6-b 0 ]dithiophene (ITIC) and Y6, [32,33] overcoming the aforementioned drawbacks and thus promoting the development of OSCs. The well-tailored FREAs with tunable optical and electrical properties broaden materials for achieving high-performance ternary OSCs with a D:A1:A2 DOI: 10.1002/solr.202000396 High-performance nonfullerene ternary organic solar cells (OSCs) with two polymer donors are less frequently reported because of the limited numbers of efficient polymer donors with good compatibility.…”