Polymer Electron Acceptors with Conjugated Side Chains for Improved Photovoltaic Performance

Organoboron molecules and polymers for organic solar cell applications

Organic solar cells (OSCs) are emerging as a new photovoltaic technology with the great advantages of low cost, light-weight, flexibility and semi-transparency. They are promising for portable energy-conversion products and building-integrated photovoltaics. Organoboron chemistry offers an important toolbox to design novel organic/polymer optoelectronic materials and to tune their optoelectronic properties for OSC applications. At present, organoboron small molecules and polymers have become an important class of organic photovoltaic materials. Power conversion efficiencies (PCEs) of 16% and 14% have been realized with organoboron polymer electron donors and electron acceptors, respectively. In this review, we summarize the research progress in various kinds of organoboron photovoltaic materials for OSC applications, including organoboron small molecular electron donors, organoboron small molecular electron acceptors, organoboron polymer electron donors and organoboron polymer electron acceptors. This review also discusses how to tune their opto-electronic properties and active layer morphology for enhancing OSC device performance. We also offer our insight into the opportunities and challenges in improving the OSC device performance of organoboron photovoltaic materials.

Polymer Acceptors Containing B←N Units for Organic Photovoltaics

ConspectusOrganic photovoltaics (OPVs), in which blend films of organic or polymer electron donor and electron acceptor are used as the active layer, are a promising photovoltaic technology with the great advantages of solution processing, low cost, and flexibility. The development of small molecular or polymer electron acceptors has boosted power conversion efficiency (PCE) of OPVs from 10% to 18%. Among them, polymer acceptors have the merits of superior morphology stability and excellent mechanical properties. However, owing to the key requirement of very low-lying LUMO/HOMO energy levels for polymer acceptors, very few conjugated polymers can work as polymer acceptors in OPVs. The majority of polymer electron acceptors are based on strong electron-withdrawing imide units or cyano substituents. Since 2015, conjugated polymers containing the boron-nitrogen coordination bond (B←N) have emerged as a new kind of polymer electron acceptor with excellent photovoltaic performance in various kinds of organic photovoltaic devices. In this Account, we summarize our research progress on polymer acceptors containing B←N units.At first, we introduce the principle of B←N to greatly down shift LUMO/HOMO energy levels, which enables B←N to be used to design polymer acceptors. Then we describe the two molecular design strategies for polymer acceptors containing B←N units. For high-efficiency OPVs, polymer acceptors should have wide absorption spectra, proper LUMO/HOMO energy levels, high electron mobility, and good donor/acceptor blend morphology. We discuss how to use molecular design to finely tune the absorption spectra, energy levels, and electron mobility of the B←N-containing polymer acceptors. We also discuss how to improve the phase separation morphology of the blends of these polymer acceptors with small molecular donors or polymer donors. These improvements lead to excellent performance of the polymer acceptors containing B←N units in three kinds of organic photovoltaic devices. The small molecular donor/polymer acceptor type organic solar cells show excellent thermal stability and PCE of 8.0%, which is the highest value reported so far. The all-polymer solar cells exhibit PCE of 10.1%. The all-polymer indoor photovoltaics show PCE as high as 27.4% under fluorescent lamp illumination at 2000 lx. This PCE is fairly comparable to those of the best organic or inorganic indoor photovoltaics. These results provide a solid foundation for future advances. Finally, we propose that great attention should be paid to further PCE enhancement of OPVs and indoor photovoltaic applications of this new emerging kind of polymer acceptor.

Low-Bandgap n-Type Polymer Based on a Fused-DAD-Type Heptacyclic Ring for All-Polymer Solar Cell Application with a Power Conversion Efficiency of 10.7

An n-type polymer (A701) is designed and synthesized with an alternative A'-DAD-A'-D' backbone, where 1,1-dicyanomethylene-3-indanone (IC), dithienothiophen[3,2-b]-pyrrolobenzothiadiazole (TPBT), and benzodithiophene (BDT) are used as A', DAD, and D' units, respectively. A701 shows enhanced light absorption with a narrow bandgap of 1.42 eV and a high absorption coefficient of 6.85 × 10⁴ cm^-1 at 780 nm. It displays an uplifted LUMO (the lowest unoccupied molecular orbital) level of -3.80 eV. By introducing a high point solvent additive of 1,8-diiodooctane (DIO), all-polymer solar cells (all-PSCs) based on the PBDB-T:A701 blend exhibit efficient exciton dissociation, enhanced charge transport, and decreased bimolecular recombination. Thus, a high open-circuit voltage (V_OC) of 0.92 V, a short-circuit current (J_SC) of 18.27 mA cm^-2, and a fill factor (FF) of 0.64 are attained, affording an impressive power conversion efficiency (PCE) of 10.70%. The low voltage loss of 0.50 V and high efficiency of 10.7% are among the top values for all-PSCs. Our results indicate that the fused DAD-type heptacyclic ring can be utilized to construct not only nonfullerene small molecular acceptors but also promising polymer acceptors.

BODIPY bearing alkylthienyl side chains: a new building block to design conjugated polymers with near infrared absorption for organic photovoltaics

A new benzene-fused BODIPY unit for designing polymer donors with near-infrared absorption for organic photovoltaics.

8.78% Efficient All-Polymer Solar Cells Enabled by Polymer Acceptors Based on a B←N Embedded Electron-Deficient Unit

In the field of all-polymer solar cells (all-PSCs), all efficient polymer acceptors that exhibit efficiencies beyond 8% are based on either imide or dicyanoethylene. To boost the development of this promising solar cell type, creating novel electron-deficient units to build high-performance polymer acceptors is critical. A novel electron-deficient unit containing B←N bonds, namely, BNIDT, is synthesized. Systematic investigation of BNIDT reveals desirable properties including good coplanarity, favorable single-crystal structure, narrowed bandgap and downshifted energy levels, and extended absorption profiles. By copolymerizing BNIDT with thiophene and 3,4-difluorothiophene, two novel conjugated polymers named BN-T and BN-2fT are developed, respectively. It is shown that these polymers possess wide absorption spectra covering 350-800 nm, low-lying energy levels, and ambipolar film-transistor characteristics. Using PBDB-T as the donor and BN-2fT as the acceptor, all-PSCs afford an encouraging efficiency of 8.78%, which is the highest for all-PSCs excluding the devices based on imide and dicyanoethylene-type acceptors. Considering that the structure of BNIDT is totally different from these classical units, this work opens up a new class of electron-deficient unit for constructing efficient polymer acceptors that can realize efficiencies beyond 8% for the first time.

Recent Advances in n-Type Polymers for All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) based on n- and p-type polymers have emerged as promising alternatives to fullerene-based solar cells due to their unique advantages such as good chemical and electronic adjustability, and better thermal and photochemical stabilities. Rapid advances have been made in the development of n-type polymers consisting of various electron acceptor units for all-PSCs. So far, more than 200 n-type polymer acceptors have been reported. In the last seven years, the power conversion efficiency (PCE) of all-PSCs rapidly increased and has now surpassed 10%, meaning they are approaching the performance of state-of-the-art solar cells using fullerene derivatives as acceptors. This review discusses the design criteria, synthesis, and structure-property relationships of n-type polymers that have been used in all-PSCs. Additionally, it highlights the recent progress toward photovoltaic performance enhancement of binary, ternary, and tandem all-PSCs. Finally, the challenges and prospects for further development of all-PSCs are briefly considered.

BN Embedded Polycyclic π-Conjugated Systems: Synthesis, Optoelectronic Properties, and Photovoltaic Applications

In the periodic table of elements, boron (B, atomic number, 5) and nitrogen (N, atomic number, 7) are neighboring to the carbon (C, atomic number, 6). Thus, the total electronic number of two carbons (12) is equal to the electronic sum of one boron (5) and one nitrogen (7). Accordingly, replacing two carbons with one boron and one nitrogen in a π-conjugated structure gives an isoelectronic system, i.e., the BN perturbed π-conjugated system, comparing to their all-carbon analogs. The BN embedded π-conjugated systems have unique properties, e.g., optical absorption, emission, energy levels, bandgaps, and packing order in contrast to their all-carbon analogs and have been intensively studied in terms of novel synthesis, photophysical characterizations, and electronic applications in recent years. In this review, we try to summarize the synthesis methods, optoelectronic properties, and progress in organic photovoltaic (OPV) applications of the representative BN embedded polycyclic π-conjugated systems. Firstly, the narrative will be commenced with a general introduction to the BN units, i.e., B←N coordination bond, B-N covalent bond, and N-B←N group. Then, the representative synthesis strategies toward π-conjugated systems containing B←N coordination bond, B-N covalent bond, and N-B←N group will be summarized. Afterwards, the frontier orbital energy levels, optical absorption, packing order in solid state, charge transportation ability, and photovoltaic performances of typical BN embedded π-conjugated systems will be discussed. Finally, a prospect will be proposed on the OPV materials of BN doped π-conjugated systems, especially their potential applications to the small molecules organic solar cells.

Adjusting the energy levels and bandgaps of conjugated polymers via Lewis acid–base reactions

Stoichiometry of the Lewis acid–base coordination between polymers and BCF and the effects on the optoelectronic properties.

Hexafluoroquinoxaline Based Polymer for Nonfullerene Solar Cells Reaching 9.4% Efficiency

Through introducing six fluorine atoms onto quinoxaline (Qx), a new electron acceptor unit-hexafluoroquinoxaline (HFQx) is first synthesized. On the basis of this unit, we synthesize a new donor-acceptor (D-A) copolymer (HFQx-T), which is composed of a benzodithiophene (BDT) derivative donor block and an HFQx accepting block. The strong electron-withdrawing properties of fluorine atoms increase significantly the open-circuit voltage (V_oc) by tuning the highest occupied molecular orbital (HOMO) energy level. In addition, fluorine atoms enhance the absorption coefficient of the conjugated copolymer and change the film morphology, which implies an increase of the short-circuit current density (J_sc) and fill factor (FF). Indeed, the HFQx-T:ITIC blended film achieves an impressive power conversion efficiency (PCE) of 9.4% with large short-current density (J_sc) of 15.60 mA/cm², high V_oc of 0.92 V, and FF of 65% via two step annealing (thermal annealing (TA) and solvent vapor annealing (SVA) treatments). The excellent results obtained show that the new copolymer HFQx-T synthesized could be a promising candidate for organic photovoltaics.

Recyclable and efficient polyurethane-Ir catalysts for direct borylation of aromatic compounds

Four polyurethanes comprising 2,2′-bipyridyl moieties incorporated in the main chain were synthesized as novel polymer ligands for the Ir(i)-catalyzed direct borylation of aromatic compounds.