Two-Dimensional BDT-Based Wide Band Gap Polymer Donor for Efficient Non-Fullerene Organic Solar Cells

In this paper, to study the two-dimensional (2D) conjugated side chain effect on the polymer donor for the non-fullerene polymer solar cells (NFPSCs), the thiophenothiophene (TT) side-chained benzodithiophene (BDT) and bithiophene copolymer (PBDTDT-TT) has been prepared by the Stille coupling reaction and compared with the copolymer (PBDTDT-O) with an alkoxy side group and the same polymer backbone. The 2D conjugated polymer PBDTDT-TT exhibits better thermal stability and wider and red-shifted ultraviolet–visible absorption spectra. These two copolymers were used as the donor materials blended with a new non-fullerene receptor ITM, successfully fabricating the positive devices with an ITO/PEDOT:PSS/polymer:ITM/PFN-Br/Al structure. Compared with the PBDTDT-O:ITM blend film, the PBDTDT-TT:ITM blend film shows a lower LUMO energy level, a narrower optical band gap, and a higher electron and hole mobility in the devices. The best power conversion efficiency (PCE) is 8.43% for the PBDTDT-TT polymer, while the PCE for the PBDTDT-O polymer is <1%. Extending the conjugated plane through the 2D conjugated side chain can help the PBDTDT-TT-based PSC devices show higher exciton dissociation efficiencies of 85.36% [compared with that of PBDTDT-O-based device (13.12%)] and thereby enhance the photovoltaic performance of the NFPSCs

An Easily Accessible Cathode Buffer Layer for Achieving Multiple High Performance Polymer Photovoltaic Cells

Here we report a successful efficiency improvement strategy in both conventional and inverted polymer solar cells (PSCs) based on multiple polymer blends, using a feasible and commercially available cathode buffer layer (CBL), namely barium hydroxide [Ba­(OH)₂], to modify the photoactive blend and cathode contacts. High performance PSCs with an identical Ba­(OH)₂ buffer layer were fabricated based on the multiple light-harvesting PBDT-TS1:PC₇₁BM, PffBT4T-2OD:PC₇₁BM, and PBDT-TS1:N2200 blends. The conventional PSC with Ba­(OH)₂ as the CBL showed a higher power conversion efficiency (PCE) of 9.65% based on the PBDT-TS1:PC₇₁BM system under the illumination of 100 mW/cm². For the inverted cells based on the PffBT4T-2OD:PC₇₁BM system, the PCE can be improved from 4.26% (without CBL) to 9.02% after inserting the Ba­(OH)₂ buffer layer. More importantly, the Ba­(OH)₂ buffer layer presents similar positive effects in the conventional and inverted all-polymer devices based on a new combination, i.e., the PBDT-TS1:N2200 system. The dramatic enhancement in device performance resulted from the suitable work function of Ba­(OH)₂, extremely high transmittance, and excellent film-forming capability. Therefore, inserting Ba­(OH)₂ as the CBL is a simple, low-cost, and widely applicable method to simultaneously improve the conventional and inverted photovoltaic device performance

Structure–Activity Relationship between Crystallinity and Carrier Transport of Two-Dimensional Donor Units in Organic Solar Cells

Benzo[1,2-b:4,5-b′]­dithiophene (BDT) and its derivatives have made important contributions to constructing high-performance polymers. However, it is difficult to clarify the real role of donor units due to the interference of strong electronegativity and crystallinity of acceptor units in the D–A copolymer. Here, we design a cyclohexane-substituted dithieno[3,2-f:2′,3′-h]­quinoxaline (DTQ)-based acceptor unit with successfully destroyed crystallinity and charge transport. Three donor-dominated materials PQH-BTF, PQH-BTCl, and PQH-BFCl are obtained. It is found that the materials exhibit obvious differences after destroying the crystallization and charge transport of the acceptor unit, and the real role of different two-dimensional donor units in designed polymers is confirmed. The backbone BDF exhibits much stronger intermolecular interactions compared to BDT, while the side chain ThF demonstrates a higher crystallization capacity than that of ThCl. More interestingly, it can be inferred that the molecular backbone is likely to construct miscible-phase crystallization (D–A crystal) while the side chain tends to demonstrate a capacity for pure-phase crystallization (D–D crystal) in a 2D donor system. Different crystallization leads to different exciton transport: pure-phase crystallization is conducive to the reduction of trap-assisted recombination, while miscible crystallization is beneficial to the reduction of bimolecular recombination. This work can help to choose donor units more accurately when preparing D–A copolymers

Fluorene Side-Chained Benzodithiophene Polymers for Low Energy Loss Solar Cells

Here we design and synthesize one novel fluorene side-chained benzodithiophene (BDT) monomer for polymer solar cells (PSCs) donor. By copolymerizing this monomer with 4,7-di­(thiophen-2-yl)-2,1,3-benzo­thiadiazole (DTBT) or 4,7-di­(4-(2-ethylhexyl)-2-thienyl)-5,6-difluoro-2,1,3-benzo­thiadiazole (DT<i>ff</i>BT), two donor–acceptor (D–A) conjugated polymers PFBDT–DTBT and PFBDT–DT<i>ff</i>BT are prepared. PSCs are prepared with these polymers as donor and PC₇₁BM as acceptor. The maximum power conversion efficiency (PCE) of the two polymers PFBDT–DTBT and PFBDT–DT<i>ff</i>BT based PSCs is 7.13% (<i>V</i>_OC = 0.90 V, <i>J</i>_SC = 13.26 mA cm^–2, and FF = 0.598) and 7.33% (<i>V</i>_OC = 0.96 V, <i>J</i>_SC = 13.24 mA cm^–2, and FF = 0.577). The UV–vis absorption and electrochemical cyclic voltammetry test results show that F atoms in DT<i>ff</i>BT unit present an obvious influence on intermolecular effect and molecular energy levels of polymers. Furthermore, the energy loss of two PSCs devices in this work is confirmed to be 0.78 and 0.71 eV, lower than most results based on BDT PSCs devices, which is critical to obtain high PCE PSCs devices with a decent trade-off between <i>J</i>_SC and <i>V</i>_OC

Single-Component Oligomer Nanoparticle-Based Size-Dependent Dual-Emission Modulation

Multichromophoric oligomers offer a versatile platform for nanoparticle multicolor fluorescent modulation. A donor–acceptor–donor (D–A–D) type oligomer (DDBTD), with blue emitting antenna and red emitting core, is chosen to assemble into fluorescent colloidal nanoparticles (FCNs) using a nanoprecipitation method. By modulating the DDBTD concentrations in good solvent, the DDBTD nanoparticles with average diameters ranging from sub-10 to 300 nm are obtained by the nanoprecipitation process in aqueous solution. Interestingly, multicolor photoluminescence (PL) can be realized from bright blue (∼440 nm) to rose red (∼630 nm) based on FCNs size control. The size-dependent PL originates from the aggregation-enhanced fluorescence resonance energy transfer (FRET) from diphenyl-aminofluorenyl antenna unit (blue emitter) to benzothiadiazole-based core (red emitter). Furthermore, the lifetime measurement of the FCNs in excited state shows a size-dependent behavior, which confirms that the size-dependent multicolor PL modulation is adjusted by FRET in the nanoscale oligomer. This work highlights the potential of the single-component multichromophoric oligomer FCNs for luminescent modulation applications

Investigation of Fluorination on Donor Moiety of Donor–Acceptor 4,7-Dithienyl­benzo­thiadiazole-Based Conjugated Polymers toward Enhanced Photovoltaic Efficiency

It is known that fluorination on π-conjugated donor–acceptor (D–A) polymers can significantly affect the optoelectronic properties and fluorination on A moiety has been well established for design of efficient photovoltaic materials. For example, polymers based on 4,7-dithienyl-5,6-difluorobenzo­thiadiazole (DT<i>ff</i>BT) have been intensively investigated and exhibited excellent performance, but the corresponding DTBT-based polymers without fluorine often display an unfavorable efficiency. With the purpose of improving photovoltaic efficiency of DTBT-based D–A polymers, we design three polymers PDTBT-T<i>xf</i>BT (<i>x</i> = 0, 1, 2) with fluorination on D moiety (T<i>xf</i>BT) and systematically investigate fluorination on the photophysical/electrochemical and photovoltaic properties. The results show that polymer solar cells (PSCs) based on PDTBT-TBT exhibit moderate power conversion efficiency (PCE) of 5.84%. However, the bis-fluorination on T<i>ff</i>BT moiety (PDTBT-T<i>ff</i>BT) can greatly enhance the molecular planarity and intermolecular interaction, improve the charge transport and heterojunction morphology, and further suppress the charge recombination losses. PSCs based on PDTBT-T<i>ff</i>BT demonstrate obviously improved photovoltaic efficiency with the best PCE up to 7.53% without any processing additives, which ranks among the top DTBT-based PSCs. However, it should be noted that unsymmetrical fluorination on T<i>f</i>BT moiety (PDTBT-T<i>f</i>BT) impairs the regularity of polymer backbone and intermolecular interaction, increases the recombination losses, and seriously reduces the short-circuit current density and efficiency (5.44%). The results exhibit that fluorination on D moiety is a helpful strategy for design high-performance photovoltaic materials and the regularity of fluorination is crucial to improving efficiencies

Intra- and Intermolecular Steric Hindrance Effects Induced Higher Open-Circuit Voltage and Power Conversion Efficiency

A pair of donor–acceptor polymers PBDT<i>h</i>DTBT and PBDT<i>ch</i>DTBT are synthesized, which share the same conjugated backbone, but are designed with hexyl and cyclohexyl side chains, respectively. The stronger steric hindrance of cyclohexyl endows PBDT<i>ch</i>DTBT a deeper lying HOMO energy level of −5.39 eV compared to −5.22 eV for PBDT<i>h</i>DTBT. However, PBDT<i>h</i>DTBT and PBDT<i>ch</i>DTBT exhibit a similar optical bandgap around 1.72 eV and a hole mobility around 10^–5 cm² V^–1 s^–1. Interestingly, the PBDT<i>ch</i>DTBT/PC₇₁BM blends exhibited higher hole mobility than PBDT<i>h</i>DTBT/PC₇₁BM after DIO was added. The higher hole mobility and fibrillar network in the active layer endows PBDT<i>ch</i>DTBT higher power conversion efficiency of 7.9%, together with simultaneously improved open-circuit voltage of 0.80 V, short-circuit current density of 13.50 mA cm^–2, and fill factor of 72.74% after a systemic study of their solar cell devices

Crystalline Medium-Bandgap Light-Harvesting Donor Material Based on <i>β-</i>Naphthalene Asymmetric-Modified Benzodithiophene Moiety toward Efficient Polymer Solar Cells

In this paper, we reported a crystalline p-type medium-bandgap conjugated D–A polymer <i>asy</i>-PBDBTN based on a symmetry-breaking-modified BDT moiety to combine the advantages of both one-dimension (1D) and two-dimension (2D) symmetric BDTs. Polymer <i>asy</i>-PBDBTN is a highly efficient light-harvesting donor material. Single BHJ PSCs exhibit PCE of 8.88% with PC₇₁BM as acceptor. Also, PCE values of 10.50% are achieved with the use of ITIC as an acceptor to couple <i>asy</i>-PBDBTN with <i>V</i>_OC of 0.942 V, <i>J</i>_SC of 16.81 mA cm^–2, and FF of 0.663. It is worth noting that lower energy loss is obtained in fullerene-free-based PSCs, which is essential to overcome the trade-off between <i>V</i>_OC and <i>J</i>_SC and boost these two parameters simultaneously for high photovoltaic performance. The combination process of additive and thermal annealing is critical to enhance and retain the π–π stacking behavior of donor and fullerene-free acceptor; as a result, the trap-assisted recombination was greatly suppressed. This work demonstrates a great prospect for the construction of the symmetry-breaking BDT-based D–A conjugated polymers toward high-performance PSCs, especially with fullerene-free acceptor material