Synthesis and Photovoltaic Properties of D–A Copolymers Based on Dithienosilole and Benzotriazole

What We have Learnt from PM6:Y6

Over the past three years, remarkable advancements in organic solar cells (OSCs) have emerged, propelled by the introduction of Y6-an innovative A-DA'D-A type small molecule non-fullerene acceptor (NFA). This review provides a critical discussion of the current knowledge about the structural and physical properties of the PM6:Y6 material combination in relation to its photovoltaic performance. The design principles of PM6 and Y6 are discussed, covering charge transfer, transport, and recombination mechanisms. Then, the authors delve into blend morphology and degradation mechanisms before considering commercialization. The current state of the art is presented, while also discussing unresolved contentious issues, such as the blend energetics, the pathways of free charge generation, and the role of triplet states in recombination. As such, this review aims to provide a comprehensive understanding of the PM6:Y6 material combination and its potential for further development in the field of organic solar cells. By addressing both the successes and challenges associated with this system, this review contributes to the ongoing research efforts toward achieving more efficient and stable organic solar cells.

Fused Benzotriazole A-Unit Constructs a D-π-A Polymer Donor for Efficient Organic Photovoltaics

Herein, we originally developed a fused ring building block as an acceptor unit, namely, 2,6,10-trihydro-carbazole[3,4-c:5,6-c]bis[1,2,5]-triazole (CTA), through fusing two benzotriazoles (BTA) with a pyrrole ring. A p-type polymer PE93 containing the CTA unit exhibits relatively high molecular energy levels and excellent luminescent properties. The PE93:BTA76-based solar cell obtained a device efficiency of 12.16%, with a VOC of 0.94 V and a low nonradiative recombination loss of 0.18 eV. The results suggest that the CTA unit is an efficient acceptor unit to achieve excellent photovoltaic performance.

Indacenodithieno[3,2-b]thiophene-Based Wide Bandgap D-π-A Copolymer for Nonfullerene Organic Solar Cells

Herein, a wide-bandgap (2.02 eV) donor-π-acceptor (D-π-A) polymer PIDTT-DTffBTA, composed of a rigid indacenodithieno[3,2-b]thiophene (IDTT) and fluorinated benzo[d][1,2,3]triazole (ffBTA) units as D and A units, respectively, is synthesized. In comparison with its analogue benzodithiophene-alt-benzotriazole copolymer J52 with classic benzodithiophene (BDT) as the D unit, PIDTT-DTffBTA demonstrates a lower-lying HOMO energy level and higher carrier mobilities when paired with a nonfullerene acceptor (NFA) Y6 based on a ladder-type dithienothiophen[3.2-b]-pyrrolobenzothiadiazole central unit. Thus, PIDTT-DTffBTA:Y6 based organic solar cells (OSCs) exhibit an improved power conversion efficiency (PCE) of 11.05% than that of J52:Y6 (7.15%), which is also the highest value for IDTT-based photovoltaic polymers. This result proves that the IDTT unit is also a promising building block to construct not only NFAs but also p-type photovoltaic polymers.

Low-Energy-Loss Polymer Solar Cells with 14.52% Efficiency Enabled by Wide-Band-Gap Copolymers

Two wide band-gap copolymers poly[4,8-bis(5-(2-butylhexylthio)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-TZNT] (PBDTS-TZNT) and poly[4,8-bis(4-fluoro-5-(2-butylhexylthio)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-TZNT] (PBDTSF-TZNT) based on naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) (TZNT) and benzo[1,2-b:4,5-b']dithiophene (BDT) with different conjugated side chains have been developed for efficient nonfullerene polymer solar cells (NF-PSCs). The rigid planar backbone of BDT and TZNT units imparted high crystallinity and good molecular stacking property to these copolymers. Using 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']-dithiophene (ITIC) as the acceptor, PBDTSF-TZNT devices showed a high Voc of 0.98 V with an Eloss of 0.61 eV. On selecting 3,9-bis(2-methylene-(5,6-difluoro-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']-dithiophene (IT-4F) instead of ITIC, the devices maintained the high Voc of 0.93 V with an even lower Eloss of 0.59 eV. The combination of the above-mentioned low Eloss, broadened absorption, better matched energy level, improved crystallinity, and fine-tuned morphology promoted the power conversion efficiency (PCE) of PBDTSF-TZNT:IT-4F devices from 12.16% to 13.25%. Homo-tandem devices based on PBDTSF-TZNT:IT-4F subcells further enhanced the light-harvesting ability and boosted the PCE of 14.52%, which is the best value for homo-tandem NF-PSCs at present.

Effects of Inserting Thiophene as a π-Bridge on the Properties of Naphthalene Diimide-alt-Fused Thiophene Copolymers

On the basis of naphthalene diimide (NDI) units connected to thiophene (T), thienothiophene (TT), or dithienothiophene (DTT) units via a thiophene π-bridge, three new copolymers-PDTNDI-T, PDTNDI-TT, and PDTNDI-DTT, respectively-were synthesized and used in the fabrication of all-polymer solar cells (all-PSCs). The relationships between the structures of the polymers and their optoelectronic properties and photovoltaic performances as electron acceptors in all-PSCs were investigated in detail. As the number of copolymerized heteroaromatic rings in the DTNDI-based polymers increased, the power conversion efficiencies of the resulting all-PSCs were found to decrease. This decreasing trend in the photovoltaic performance is opposite to the results reported previously for NDI-based polymers lacking the thiophene π-bridge and naphthodithiophene diimide-based polymers. In addition, the three polymers were found to exhibit distinct molecular orientations: a face-on orientation for PDTNDI-T and edge-on orientations for PDTNDI-TT and PDTNDI-DTT. Our results indicate that large fused aromatic rings are not necessarily advantageous in the design of NDI-based polymers containing π-conjugated bridges.

Medium Bandgap D-A Type Photovoltaic Polymers Based on an Asymmetric Dithienopyran Donor and a Benzotriazole Acceptor

Conjugated polymers based on the donor of an asymmetric 5H-dithieno[3,2-b:2',3'-d]pyran (DTPa) and the acceptors of benzo[d][1,2,3]triazole (BTA) or di-fluorinated benzo[d][1,2,3]triazole (ffBTA) with thiophene as π-bridge were designed and synthesized. Two asymmetric-building-block-containing polymers (ABC-polymers) possess a strong and broad absorption in the range of 300⁻750 nm and medium optical bandgap of 1.73 and 1.77 eV for PDTPa-TBTA and PDTPa-TffBTA, respectively. Polymer solar cells using PDTPa-TBTA as donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor exhibited power conversion efficiencies (PCE) of 2.22% with a Voc of 0.58 V, a Jsc of 6.04 mA/cm², and an FF of 63.41%. The introduction of fluorine substituents on the BTA unit evidently influenced the optical and photovoltaic properties. Interestingly, although the HOMO energy level indeed decreased, PDTPa-TffBTA showed a decreased Voc of 0.52 V in solar cells. Combined with an obviously enhanced Jsc of 10.23 mA/cm², and an outstanding FF of 0.64, the PCE of solar cells based on PDTPa-TffBTA was improved by nearly 55%, reached 3.43%. Our results indicate that the BTA unit can be used to construct ABC polymers with a medium bandgap, and the introduction of fluorine on the BTA unit is also effective in improving the photovoltaic performance.

Precise Side-Chain Engineering of Thienylenevinylene-Benzotriazole-Based Conjugated Polymers with Coplanar Backbone for Organic Field Effect Transistors and CMOS-like Inverters

Two donor-acceptor (D-A) alternating conjugated polymers based on thienylenevinylene-benzotriazole (TV-BTz), PTV6B with a linear side chain and PTVEhB with a branched side chain, were synthesized and characterized for organic field effect transistors (OFETs) and complementary metal-oxide-semiconductor (CMOS)-like inverters. According to density functional theory (DFT), polymers based on TV-BTz exhibit a coplanar and rigid structure with no significant twists, which could cause to an increase in charge-carrier mobility in OFETs. Alternating alkyl side chains of the polymers impacted neither the band gap nor the energy level. However, it significantly affected the morphology and crystallinity when the polymer films were thermally annealed. To investigate the effect of thermal annealing on the morphology and crystallinity, we characterized the polymer films using atomic force microscopy (AFM) and 2D-grazing incidence X-ray diffraction (2D-GIWAXD). Fibrillary morphologies with larger domains and increased crystallinity were observed in the polymer films after thermal annealing. These polymers exhibited improved charge-carrier mobilities in annealed films at 200 °C and demonstrated optimal OFET device performance with p-type transport characteristics with charge-carrier mobilities of 1.51 cm²/(V s) (PTV6B) and 2.58 cm²/(V s) (PTVEhB). Furthermore, CMOS-like inorganic (ZnO)-organic (PTVEhB) hybrid bilayer inverter showed that the inverting voltage (V_inv) was positioned near the ideal switching point at half (1/2) of supplied voltage (V_DD) due to fairly balanced p- and n-channels.

N-Alkylthienopyrroledione versus benzothiadiazole pulling units in push–pull copolymers used for photovoltaic applications: density functional theory study

Low-band-gap push–pull copolymers are promising donor materials for bulk heterojunction organic solar cells.

Low bandgap semiconducting polymers for polymeric photovoltaics

This review highlights the design rules for low bandgap semiconducting polymers, with the overview of their applications in polymer solar cells and polymer photodetectors.

D-π-A-π-A Strategy to Design Benzothiadiazole-carbazole-based Conjugated Polymer with High Solar Cell Voltage and Enhanced Photocurrent

The theoretical calculations are used to find that D-π-A-π-A style conjugated polymer PC-TBTBT is more efficient for solar cells application than the D-π-A analog PC-TBT because the D-π-A-π-A structure has a narrower band gap and higher molar absorption coefficient and redshift spectrum. Motivated by the theoretical prediction, 5,6-bis(octyloxy)-2,1,3-benzothiadiazole and 2,7-carbazole are adopted to synthesize the D-π-A-π-A style PC-TBTBT (M(w) = 31.1 kDa) and D-π-A analog PC-TBT (M(w) = 87.5 kDa) by Suzuki coupling reaction. Experimental results confirm that D-π-A-π-A PC-TBTBT-based solar cell shows a power conversion efficiency (PCE) of 4.74% with high V(OC) of 0.99 V and enhanced J(SC) of 9.70 mA cm(-2). The PCE and J(SC) achieve improvements of 17% and 26%, respectively, compared to the D-π-A PC-TBT-based solar cell.

Direct C–H arylation synthesis of (DD′AD′DA′)-constituted alternating polymers with low bandgaps and their photovoltaic performance

The alternating copolymers possess a planar backbone structure and high intramolecular charge transfer, and they contain a strong push–pull (D–A) system and gave a power conversion efficiency up to 2.36%.

High-crystalline medium-band-gap polymers consisting of benzodithiophene and benzotriazole derivatives for organic photovoltaic cells

Two semiconducting conjugated polymers were synthesized via Stille polymerization. The structures combined unsubstituted or (triisopropylsilyl)ethynyl (TIPS)-substituted 2,6-bis(trimethylstannyl)benzo[1,2-b:4.5-b']dithiophene (BDT) as a donor unit and benzotriazole with a symmetrically branched alkyl side chain (DTBTz) as an acceptor unit. We investigated the effects of the different BDT moieties on the optical, electrochemical, and photovoltaic properties of the polymers and the film crystallinities and carrier mobilities. The optical-band-gap energies were measured to be 1.97 and 1.95 eV for PBDT-DTBTz and PTIPSBDT-DTBTz, respectively. Bulk heterojunction photovoltaic devices were fabricated and power conversion efficiencies of 5.5% and 2.9% were found for the PTIPSBDT-DTBTz- and PBDT-DTBTz-based devices, respectively. This difference was explained by the more optimal morphology and higher carrier mobility in the PTIPSBDT-DTBTz-based devices. This work demonstrates that, under the appropriate processing conditions, TIPS groups can change the molecular ordering and lower the highest occupied molecular orbital level, providing the potential for improved solar cell performance.