Deciphering the Effects of Molecular Dipole Moments on the Photovoltaic Performance of Organic Solar Cells

The dielectronic constant of organic semiconductor materials is directly related to its molecule dipole moment, which can be used to guide the design of high-performance organic photovoltaic materials. Herein, two isomeric small molecule acceptors, ANDT-2F and CNDT-2F, are designed and synthesized by using the electron localization effect of alkoxy in different positions of naphthalene. It is found that the axisymmetric ANDT-2F exhibits a larger dipole moment, which can improve exciton dissociation and charge generation efficiencies due to the strong intramolecular charge transfer effect, resulting in the higher photovoltaic performance of devices. Moreover, PBDB-T:ANDT-2F blend film exhibits larger and more balanced hole and electron mobility as well as nanoscale phase separation due to the favorable miscibility. As a result, the optimized device based on axisymmetric ANDT-2F shows a JSC of 21.30 mA cm-2 , an FF of 66.21%, and a power conversion energy of 12.13%, higher than that of centrosymmetric CNDT-2F-based device. This work provides important implications for designing and synthesizing efficient organic photovoltaic materials by tuning their dipole moment.

Asymmetric Non-Fullerene Small Molecule Acceptor with Unidirectional Non-Fused π-Bridge and Extended Terminal Group for High-Efficiency Organic Solar Cells

We designed and synthesized an asymmetric non-fullerene small molecule acceptor (NF-SMA) IDT-TNIC with an A-D-π-A structure, based on an indacenodithiophene (IDT) central core, with a unidirectional non-fused alkylthio-thiophene (T) π-bridge, and 2-(3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile (NIC) extended terminal groups. IDT-TNIC molecules still maintain a good coplanar structure, which benefits from the non-covalent conformational locks (NCL) between O···S and S···S. The asymmetric structure increases the molecular dipole moment, and the extended terminal group broadens the absorption of the material, resulting in an excellent photovoltaic performance of IDT-TNIC. The photovoltaic device, based on PBDB-T:IDT-TNIC, exhibits an energetic PCE of 11.32% with a high Voc of 0.87 V, high Jsc of 19.85 mA cm-2, and a low energy loss of 0.57 eV. More importantly, IDT-TNICs with asymmetric structures show a superior property compared to symmetric IDT-Ns. The results demonstrate that it is an effectual strategy to enhance the properties of asymmetric A-D-π-A-based NF-SMAs with non-fused NCL π-bridges and extended terminal groups.

Recent Progress in the Design of Fused-Ring Non-Fullerene Acceptors-Relations between Molecular Structure and Optical, Electronic, and Photovoltaic Properties

Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018-2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.

An asymmetric end-capping strategy enables a new non-fullerene acceptor for organic solar cells with efficiency over 10

Two different terminal groups, rhodanine-flanked benzo[c][1,2,5]thiadiazole (BR) and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IM2F), were connected to an indaceno[1,2-b:5,6-b']dithiophene (IDT) core to construct a new non-fullerene acceptor (IDTBF). Solar cells based on this acceptor exhibited promising photovoltaic performances with a power conversion efficiency (PCE) of up to 10.43%.

Novel Nitrogen-Containing Heterocyclic Non-Fullerene Acceptors for Organic PhotovoltaicCells: Different End-Capping Groups Leading to a Big Difference of Power Conversion Efficiencies

Novel cores for high performance nonfullerene acceptors (NFAs) remain to be developed. In this work, two new n-type nitrogen-containing organic heterocyclic NFAs, namely, BDTN-BF and BDTN-Th, were designed and synthesized based on a new seven fused-ring core (BDTN) with two different end-capping groups. As a result, BDTN-BF possessed similar absorption spectra in solution and solid state to BDTN-Th, but a slightly higher maximum molar extinction coefficient. Manufacturing the polymer solar cells with PM6 as the donor, the photovoltaic performance of BDTN-BF and BDTN-Th was investigated. The PM6:BDTN-BF-based device achieved the highest power conversion efficiency (PCE) of 11.54% with a high J_sc of 20.20 mA cm^-2, a fill factor (FF) of 61.46%, and a large V_oc of 0.93 V, and the energy loss (E_loss) was calculated to be 0.48 eV. Comparatively, the PM6:BDTN-Th-based device achieved the maximum PCE value of only 3.53% because of inadequate J_sc and FF. The higher J_sc and FF for the PM6:BDTN-BF-based device was mainly due to the effective electron transfer from PM6 to BDTN-BF, more balanced μ_h/μ_e, higher electron mobility of the neat film, better charge collection and dissociation efficiency, and more favorable morphology. These results demonstrate that the acceptors with nearly identical absorption spectra could result in a significant difference in photovoltaic performance, which stress the importance of end-capping units. Furthermore, few NFA-based devices achieve large V_oc and high J_sc simultaneously as one based on PM6:BDTN-BF, indicating that nitrogen hybridization of NFAs may be an efficient strategy to realize high and balanced V_oc and J_sc.