Contrasting performance of donor-acceptor copolymer pairs in ternary blend solar cells and two-acceptor copolymers in binary blend solar cells

Donor-Acceptor Alternating Copolymer Compatibilizers for Thermally Stable, Mechanically Robust, and High-Performance Organic Solar Cells

Small-molecule acceptor (SMA)-based organic solar cells (OSCs) have achieved high power conversion efficiencies (PCEs), while their long-term stabilities remain to be improved to meet the requirements for real applications. Herein, we demonstrate the use of donor-acceptor alternating copolymer-type compatibilizers (DACCs) in high-performance SMA-based OSCs, enhancing their PCE, thermal stability, and mechanical robustness simultaneously. Detailed experimental and computational studies reveal that the addition of DACCs to polymer donor (P_D)-SMA blends effectively reduces P_D-SMA interfacial tensions and stabilizes the interfaces, preventing the coalescence of the phase-separated domains. As a result, desired morphologies with exceptional thermal stability and mechanical robustness are obtained for the P_D-SMA blends. The addition of 20 wt % DACCs affords OSCs with a PCE of 17.1% and a cohesive fracture energy (G_c) of 0.89 J m^-2, higher than those (PCE = 13.6% and G_c = 0.35 J m^-2) for the control OSCs without DACCs. Moreover, at an elevated temperature of 120 °C, the OSCs with 20 wt % DACC exhibit excellent morphological stability, retaining over 95% of the initial PCE after 300 h. In contrast, the control OSCs without the DACC rapidly degraded to below 60% of the initial PCE after 144 h.

Ternary Solar Cells Based on Two Small Molecule Donors with Same Conjugated Backbone: The Role of Good Miscibility and Hole Relay Process

Ternary organic solar cells (OSCs) are very attractive for further enhancing the power conversion efficiencies (PCEs) of binary ones but still with a single active layer. However, improving the PCEs is still challenging because a ternary cell with one more component is more complicated on phase separation behavior. If the two donors or two acceptors have similar chemical structures, good miscibility can be expected to reduce the try-and-error work. Herein, we report ternary devices based on two small molecule donors with the same backbone but different substituents. Whereas both binary devices show PCEs about 9%, the PCE of the ternary cells is enhanced to 10.17% with improved fill factor and short-circuit current values and external quantum efficiencies almost in the whole absorption wavelength region from 440 to 850 nm. The same backbone enables the donors miscible at molecular level, and the donor with a higher HOMO level plays hole relay process to facilitate the charge transportation in the ternary devices. Since side-chain engineering has been well performed to tune the active materials' energy levels in OSCs, our results suggest that their ternary systems are promising for further improving the binary cells' performance although their absorptions are not complementary.

A comparative study of the photovoltaic performances of terpolymers and ternary systems

Terpolymer systems were realized as a good strategy to combine two incompatible polymers as compared to ternary systems.

Influence of Surface Energy on Organic Alloy Formation in Ternary Blend Solar Cells Based on Two Donor Polymers

The compositional dependence of the open-circuit voltage (V_oc) in ternary blend bulk heterojunction (BHJ) solar cells is correlated with the miscibility of polymers, which may be influenced by a number of attributes, including crystallinity, the random copolymer effect, or surface energy. Four ternary blend systems featuring poly(3-hexylthiophene-co-3-(2-ethylhexyl)thiophene) (P3HT₇₅-co-EHT₂₅), poly(3-hexylthiophene-co-(hexyl-3-carboxylate)), herein referred to as poly(3-hexylthiophene-co-3-hexylesterthiophene) (P3HT₅₀-co-3HET₅₀), poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%), and an analog of P3HTT-DPP-10% with 40% of 3-hexylthiophene exchanged for 2-(2-methoxyethoxy)ethylthiophen-2-yl (3MEO-T) (featuring an electronically decoupled oligoether side-chain), referred to as P3HTTDPP-MEO40%, are explored in this work. All four polymers are semicrystalline and rich in rr-P3HT content and perform well in binary devices with PC₆₁BM. Except for P3HTTDPP-MEO40%, all polymers exhibit similar surface energies (∼21-22 mN/m). P3HTTDPP-MEO40% exhibits an elevated surface energy of around 26 mN/m. As a result, despite the similar optoelectronic properties and binary solar cell performance of P3HTTDPP-MEO40% compared to P3HTT-DPP-10%, the former exhibits a pinned V_oc in two different sets of ternary blend devices. This is a stark contrast to previous rr-P3HT-based systems and demonstrates that surface energy, and its influence on miscibility, plays a critical role in the formation of organic alloys and can supersede the influence of crystallinity, the random copolymer effect, similar backbone structures, and HOMO/LUMO considerations. Therefore, we confirm surface energy compatibility as a figure-of-merit for predicting the compositional dependence of the V_oc in ternary blend solar cells and highlight the importance of polymer miscibility in organic alloy formation.

Surface Energy Modification of Semi-Random P3HTT-DPP

Alkyl solubilizing side chains on conjugated polymers can serve as a handle for modifying polymer properties. Recently, oligo-ether and semifluoro alkyl side chains were utilized to tune the surface energy of random P3HT-based polymers without changing the optical and electronic properties. Here, this method is applied to semi-random poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP) and the subsequent polymer device, optical, electronic, structural, and thermal properties are characterized. P3HTMETT-DPP, bearing oligo-ether side chains, exhibited higher crystallinity, closer lamellar packing, and lower temperature thermal transitions. P3HTFHTT-DPP, featuring semifluoro alkyl side chains, presented reduced crystallinity, greater lamellar packing distances, and higher temperature thermal transitions. P3HTMETT-DPP performed similarly to P3HTT-DPP under identical processing conditions, whereas P3HTFHTT-DPP had greatly reduced J_SC due to lower polymer concentration necessitated by solubility.

Ternary Blend Composed of Two Organic Donors and One Acceptor for Active Layer of High-Performance Organic Solar Cells

Ternary blends composed of two donor absorbers with complementary absorptions provide an opportunity to enhance the short-circuit current and thus the power conversion efficiency (PCE) of organic solar cells. In addition to complementary absorption of two donors, ternary blends may exhibit favorable morphology for high-performance solar cells when one chooses properly the donor pair. For this purpose, we develop a ternary blend with two donors (diketopyrrolopyrrole-based polymer (PTDPP2T) and small molecule ((TDPP)2Ph)) and one acceptor (PC71BM). The solar cell made of a ternary blend with 10 wt % (TDPP)2Ph exhibits higher PCE of 7.49% as compared with the solar cells with binary blends, PTDPP2T:PC71BM (6.58%) and (TDPP)2Ph:PC71BM (3.21%). The higher PCE of the ternary blend solar cell is attributed mainly to complementary absorption of two donors. However, a further increase in (TDPP)2Ph content in the ternary blend (>10 wt %) decreases the PCE. The ternary blend with 10 wt % (TDPP)2Ph exhibits well-developed morphology with narrow-sized fibrils while the blend with 15 wt % (TDPP)2Ph shows phase separation with large-sized domains, demonstrating that the phase morphology and compatibility of ternary blend are important factors to achieve a high-performance solar cell made of ternary blends.

Conjugated donor–acceptor terpolymers entailing the Pechmann dye and dithienyl-diketopyrrolopyrrole as co-electron acceptors: tuning HOMO/LUMO energies and photovoltaic performances

Three new conjugated D–A terpolymers with the Pechmann dye derivative and dithienyl-diketopyrrolopyrrole as co-acceptors are presented for photovoltaic studies.

High-Performance Small Molecule/Polymer Ternary Organic Solar Cells Based on a Layer-By-Layer Process

The layer-by-layer process method, which had been used to fabricate a bilayer or bulk heterojunction organic solar cell, was developed to fabricate highly efficient ternary blend solar cells in which small molecules and polymers act as two donors. The active layer could be formed by incorporating the small molecules into the polymer based active layer via a layer-by-layer method: the small molecules were first coated on the surface of poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (

PEDOT

PSS), and then the mixed solution of polymer and fullerene derivative was spin-coated on top of a small molecule layer. In this method, the small molecules in crystalline state were effectively mixed in the active layer. Without further optimization of the morphology of the ternary blend, a high power conversion efficiency (PCE) of 8.76% was obtained with large short-circuit current density (Jsc) (17.24 mA cm(-2)) and fill factor (FF) (0.696). The high PCE resulted from not only enhanced light harvesting but also more balanced charge transport by incorporating small molecules.

7.7% Efficient All-Polymer Solar Cells

By controlling the polymer/polymer blend self-organization rate, all-polymer solar cells composed of a high-mobility, crystalline, naphthalene diimide-selenophene copolymer acceptor and a benzodithiophene-thieno[3,4-b]thiophene copolymer donor are achieved with a record 7.7% power conversion efficiency and a record short-circuit current density (18.8 mA cm(-2)).

Polymer-Polymer Förster Resonance Energy Transfer Significantly Boosts the Power Conversion Efficiency of Bulk-Heterojunction Solar Cells

Optically resonant donor polymers can exploit a wider range of the solar spectrum effectively without a complicated tandem design in an organic solar cell. Ultrafast Förster resonance energy transfer (FRET) in a polymer-polymer system that significantly improves the power conversion efficiency in bulk heterojunction polymer solar cells from 6.8% to 8.9% is demonstrated, thus paving the way to achieving 15% efficient solar cells.