Non-Fullerene Acceptors with an Extended π-Conjugated Core: Third Components in Ternary Blends for High-Efficiency, Post-Treatment-Free Organic Solar Cells

Probing the Effect of Photovoltaic Material on Voc in Ternary Polymer Solar Cells with Non-Fullerene Acceptors by Machine Learning

The power conversion efficiency (PCE) of ternary polymer solar cells (PSCs) with non-fullerene has a phenomenal increase in recent years. However, improving the open circuit voltage (V_oc) of ternary PSCs with non-fullerene still remains a challenge. Therefore, in this work, machine learning (ML) algorithms are employed, including eXtreme gradient boosting, K-nearest neighbor and random forest, to quantitatively analyze the impact mechanism of V_oc in ternary PSCs with the double acceptors from the two aspects of photovoltaic materials. In one aspect of photovoltaic materials, the doping concentration has the greatest impact on V_oc in ternary PSCs. Furthermore, the addition of the third component affects the energy offset between the donor and acceptor for increasing V_oc in ternary PSCs. More importantly, to obtain the maximum V_oc in ternary PSCs with the double acceptors, the HOMO and LUMO energy levels of the third component should be around (-5.7 ± 0.1) eV and (-3.6 ± 0.1) eV, respectively. In the other aspect of molecular descriptors and molecular fingerprints in the third component of ternary PSCs with the double acceptors, the hydrogen bond strength and aromatic ring structure of the third component have high impact on the V_oc of ternary PSCs. In partial dependence plot, it is clear that when the number of methyl groups is four and the number of carbonyl groups is two in the third component of acceptor, the V_oc of ternary PSCs with the double acceptors can be maximized. All of these findings provide valuable insights into the development of materials with high V_oc in ternary PSCs for saving time and cost.

Star-shape non-fullerene acceptor featuring an aza-triangulene core for organic solar cells

We present the simple synthesis of a star-shape non-fullerene acceptor (NFA) for application in organic solar cells. This NFA possesses a D(A)3 structure in which the electron-donating core is an aza-triangulene unit and we report the first crystal structure for a star shape NFA based on this motive. We fully characterized this molecule's optoelectronic properties in solution and thin films, investigating its photovoltaic properties when blended with PTB7-Th as the electron donor component. We demonstrate that the aza-triangulene core leads to a strong absorption in the visible range with an absorption edge going from 700 nm in solution to above 850 nm in the solid state. The transport properties of the pristine molecule were investigated in field effect transistors (OFETs) and in blends with PTB7-Th following a Space-Charge-Limited Current (SCLC) protocol. We found that the mobility of electrons measured in films deposited from o-xylene and chlorobenzene are quite similar (up to 2.70 × 10-4 cm2 V-1 s-1) and that the values are not significantly modified by thermal annealing. The new NFA combined with PTB7-Th in the active layer of inverted solar cells leads to a power conversion efficiency of around 6.3% (active area 0.16 cm2) when processed from non-chlorinated solvents without thermal annealing. Thanks to impedance spectroscopy measurements performed on the solar cells, we show that the charge collection efficiency of the devices is limited by the transport properties rather than by recombination kinetics. Finally, we investigated the stability of this new NFA in various conditions and show that the star-shape molecule is more resistant against photolysis in the presence and absence of oxygen than ITIC.

C-H Direct Arylation: A Robust Tool to Tailor the π-Conjugation Lengths of Non-Fullerene Acceptors

Facile synthesis without involvement of toxic reagents is of great significance in the practical application of photovoltaic materials. In this work, four acceptor-donor-acceptor (A-D-A) type unfused-ring acceptors (UFRAs) with stepwise extension in π-conjugation, i. e., CPFB-IC-n (n=1-4), involving cyclopentadithiophene (CPDT) and 1,4-difluorobenzene (DFB) as cores, are facilely synthesized by an atom-, step-economic and labor-saving method through direct arylation of C-H bond (DACH). Among them, CPFB-IC-4 has the longest conjugation lengths among the molecular UFRA ever reported. The dependence of optoelectronic properties and photovoltaic performances of CPFB-IC-n (n=1-4) on conjugation length were systematically investigated. CPFB-IC-2 with near zero highest occupied molecular orbital (HOMO) offsets (ΔE_HOMO =0.06 eV) achieves the highest power conversion efficiency (PCE), due to the significantly enhanced open voltage (V_OC ) and short current (J_SC ) caused by the balanced frontier molecular orbitals (FMOs) and complementary light absorption. Our work demonstrates that the optical properties and FMOs of UFRAs can be finely tuned by the stepwise elongation of conjugation lengths. Meanwhile, DACH coupling as a powerful tool here established will be a promising candidate for synthesizing high-performance oligomeric UFRAs.

Effects of Halogenation on the Benzotriazole Unit of Non-Fullerene Acceptors in Organic Solar Cells with High Voltages

Non-fullerene acceptors (NFAs) can be simply divided into three categories: A-D-A, A-DA'D-A, and A₂-A₁-D-A₁-A₂ according to their chemical structures. Benefiting from the easily modified 1,1-dicyanomethylene-3-indanone end groups, the halogenation on the first two types of materials has been proved to be very effective to modulate their optoelectronic properties and improve their photovoltaic performance. Hence, in this work, we systematically investigate the effect of halogenation on the classic NFA molecule of BTA3, which has the linear A₂-A₁-D-A₁-A₂-type backbone. After fluorination and chlorination, F-BTA3 and Cl-BTA3 have similar optical band gaps but lower energy levels than BTA3. When blending with a linear copolymer PE25 composed of benzodifuran and chlorinated benzotriazole (BTA) according to "Same-A-Strategy", the corresponding V_OC of the halogenated NFAs gradually decreases (1.13 V for F-BTA3 and 1.09 V for Cl-BTA3), compared with that of the BTA3-based device (V_OC = 1.22 V). This tendency mainly comes from the lower lowest unoccupied molecular orbital energy levels due to the strong electron-withdrawing ability of halogen atoms and the larger nonradiative energy loss. However, the power conversion efficiencies of the halogenated materials are slightly improved, from 9.08% for PE25: BTA3 to 10.45% for PE25: F-BTA3 and 10.75% for PE25: Cl-BTA, with the nonhalogenated solvent tetrahydrofuran as the processing solvent. The improved photovoltaic performance of F-BTA3 and Cl-BTA3 should come from the higher carrier mobility, weaker bimolecular recombination, and higher fluorescence quenching rate. This study illustrates that halogenation on the A₁ unit is a promising strategy for developing novel and effective A₂-A₁-D-A₁-A₂-type NFAs.

Recent Advances of Film–Forming Kinetics in Organic Solar Cells

Solution–processed organic solar cells (OSC) have been explored widely due to their low cost and convenience, and impressive power conversion efficiencies (PCEs) which have surpassed 18%. In particular, the optimization of film morphology, including the phase separation structure and crystallinity degree of donor and acceptor domains, is crucially important to the improvement in PCE. Considering that the film morphology optimization of many blends can be achieved by regulating the film–forming process, it is necessary to take note of the employment of solvents and additives used during film processing, as well as the film–forming conditions. Herein, we summarize the recent investigations about thin films and expect to give some guidance for its prospective progress. The different film morphologies are discussed in detail to reveal the relationship between the morphology and device performance. Then, the principle of morphology regulating is concluded with. Finally, a future controlling of the film morphology and development is briefly outlined, which may provide some guidance for further optimizing the device performance.