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

Ternary Blend Organic Solar Cells: Understanding the Morphology from Recent Progress

Ternary blend organic solar cells (TB-OSCs) incorporating multiple donor and/or acceptor materials into the active layer have emerged as a promising strategy to simultaneously improve the overall device parameters for realizing higher performances than binary devices. Whereas introducing multiple materials also results in a more complicated morphology than their binary blend counterparts. Understanding the morphology is crucially important for further improving the device performance of TB-OSC. This review introduces the solubility and miscibility parameters that affect the morphology of ternary blends. Then, this review summarizes the recent processes of morphology study on ternary blends from the aspects of molecular crystallinity, molecular packing orientation, domain size and purity, directly observation of morphology, vertical phase separation as well as morphological stability. Finally, summary and prospects of TB-OSCs are concluded.

Improved Performance of Ternary Solar Cells by Using BODIPY Triads

Two boron dipyrromethene (BODIPY) triads, namely BODIPY-1 and BODIPY-2, were synthesized and incorporated with poly-3-hexyl thiophene: (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) P3HT:PCBM. The photovoltaic performance of BODIPY:P3HT:PCBM ternary solar cells was increased, as compared to the control binary solar cells (P3HT:PCBM). The optimized power conversion efficiency (PCE) of BODIPY-1:P3HT:PCBM was improved from 2.22% to 3.43%. The enhancement of PCE was attributed to cascade charge transfer, an improved external quantum efficiency (EQE) with increased short circuit current (Jsc), and more homogeneous morphology in the ternary blend.

The effect of different aromatic conjugated bridges on optoelectronic properties of diketopyrrolopyrrole-based donor materials for organic photovoltaics

A series of twelve Acceptor-π-Donor-π-Acceptor (A-π-D-π-A) topology-based donor molecules, where diketopyrrolopyrrole (DPP) as donor core unit is connected through furan which acts as conjugated π-bridge (CB) to aromatic derivatives (Ar) as acceptor units, have been investigated by making substitutions in acceptor units by using density functional theory(DFT) and time-dependent density functional theory (TD-DFT) for organic solar cell applications. The comparative study of optoelectronic properties indicates that thiadiazole with pyridine units containing molecules (M6b) exhibit lower energy of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels than those of oxadiazole and pyridine containing units (M6b). Among our investigated donors, the smallest Eg of 1.60 eV was observed for both M6a and M6b with distinctive broad absorption at 843 and 857 nm, respectively. Overall, smaller electron transfer (λ_e) values in contrast to hole transfer (λ_h) demonstrate that these donor compounds would be best for λ_e. The calculated open circuit voltage (Voc) is 2.45 and 2.17 eV, regarding bisPCBM and PC60BM (phenyl-C61-butyric acid methyl ester) acceptors. Thus, these theoretical calculations not only endorse the deep consideration between the chemical structures and optoelectronic characteristics of the donor-acceptor systems but also suggest appropriate materials for high-performance Organic Photovoltaics (OPV). Graphical abstract.

Molecular Ordering and Performance of Ternary Nonfullerene Organic Solar Cells via Bar-Coating in Air with an Efficiency over 13

An in situ spectroscopy ellipsometry technique is utilized to probe the molecular ordering sequences of PBDB-T-2F/IT-4F/COi8DFIC ternary photovoltaic blends fabricated by bar-coating in air. The time-resolved dynamics show that the primary electron acceptor IT-4F aggregates ahead of the secondary acceptor COi8DFIC in the bar-coated photoactive layer, although the latter has much stronger crystallization ability. Wetting coefficient analysis supports that COi8DFIC locates at the interface between the host components PBDB-T-2F and IT-4F. We demonstrate that the suitable degree of phase separation with the presence of 20 wt % COi8DFIC facilitates exciton dissociation and charge transfer, leading to a remarkable power conversion efficiency of 13.2% as well as excellent stability of ternary organic solar cells (OSCs), which is among the highest reported efficiency for OSCs that were fabricated by scalable solution-casting in ambient conditions.

Synthesis of a new W-type of functional polymer for improving intermolecular charge transfer processes at donor/acceptor interfaces

Optimizing charge transfer (CT) processes at donor/acceptor interfaces is an important subject to improving photocurrent density. Geometries of functional polymers play important roles in design of new types of polymers, which were used as electron donor to improve effective separation of electron-hole pairs at donor/acceptor interfaces. In this article, a novel W-type of polymer, poly(1-[4-(9-(2-ethylhexyl)carbazole-3-yl)]phenylazo-2-phenylazoacenaphthylene), was synthesized by a Suzuki coupling reaction for improving interaction between polymers and electron acceptors to enhance intermolecular CT. Geometry of combination of the polymer and C60 shows that main-chain of the polymer could sufficiently touch C60 derivatives. The polymer exhibited a broadband light absorption at the wavelength range from 250 to 650 nm. Ultraviolet–visible spectra and cyclic voltammetry curve suggest that the highest occupied, lowest unoccupied molecular orbital energy levels, and energy gap values are −5.09, −3.18 and 1.91 eV. Fluorescence quenching experiments shows that 99.9% of emission fluorescence of the polymer was quenched by added C60. Therefore, excited electrons at the polymer would be completely transferred to C60 molecules. This article suggests a new W-type functional polymer for improving intermolecular CT processes at donor/acceptor interfaces.

Improved efficiency and thermal stability of ternary all-small-molecule organic solar cells by NCBA as a third component material

In this work, organic solar cells (OSCs) were fabricated with a blend of PC₇₁BM and p-DTS-(FBTTh₂)₂ employed as a binary photoactive layer and with a dihydronaphthyl-based C60 bisadduct (NCBA) small-molecule acceptor used as a third component material. We demonstrate that the short-circuit current density (J_SC), open-circuit voltage (V_OC), fill factor (FF), power conversion efficiency (PCE), and thermal stability can all be enhanced simultaneously. In addition, the crystallinity can be finely optimized and the photon harvesting ability was enhanced for short-wavelength light by adjusting the NCBA doping ratio, leading to efficient exciton dissociation and charge-carrier transport. At the same time, the ternary photoactive layer, with a small amount of NCBA as a third component material, reduced monomolecular recombination and bimolecular recombination under open-circuit and short-circuit conditions, respectively. Such a ternary structure with NCBA as a third component material helped enhance the crystallinity and fix the surface morphology of the photoactive layer, thus reducing the decay ratio while increasing the thermal annealing treatment time. Consequently, the PCE reaches 9.1% for ternary OSCs with a 12 wt% NCBA doping ratio in a blended acceptor, with 87.2% of the initial PCE value maintained after 100 h of thermal annealing treatment at 90 °C, which is much higher than that obtained for the PCE of binary OSCs.

Insights into photovoltaic properties of ternary organic solar cells from phase diagrams

The efficiency of ternary organic solar cells relies on the spontaneous establishment of a nanostructured network of donor and acceptor phases during film formation. A fundamental understanding of phase composition and arrangement and correlations to photovoltaic device parameters is of utmost relevance for both science and technology. We demonstrate a general approach to understanding solar cell behavior from simple thermodynamic principles. For two ternary blend systems we construct and model phase diagrams. Details of EQE and solar cell parameters can be understood from the phase behavior. Our blend system is composed of PC₇₀BM, PBDTTT-C and a near-infrared absorbing cyanine dye. Cyanine dyes are accompanied by counterions, which, in a first approximation, do not change the photophysical properties of the dye, but strongly influence the morphology of the ternary blend. We argue that counterion dissociation is responsible for different mixing behavior. For the dye with a hexafluorophosphate counterion a hierarchical morphology develops, the dye phase separates on a large scale from PC₇₀BM and cannot contribute to photocurrent. Differently, a cyanine dye with a TRISPHAT counterion shows partial miscibility with PC₇₀BM. A large two-phase region dictated by the PC₇₀BM: PBDTTT-C mixture is present and the dye greatly contributes to the short-circuit current.

High-Performance Organic Bulk-Heterojunction Solar Cells Based on Multiple-Donor or Multiple-Acceptor Components

Organic solar cells (OSCs) based on bulk heterojunction structures are promising candidates for next-generation solar cells. However, the narrow absorption bandwidth of organic semiconductors is a critical issue resulting in insufficient usage of the energy from the solar spectrum, and as a result, it hinders performance. Devices based on multiple-donor or multiple-acceptor components with complementary absorption spectra provide a solution to address this issue. OSCs based on multiple-donor or multiple-acceptor systems have achieved power conversion efficiencies over 12%. Moreover, the introduction of an additional component can further facilitate charge transfer and reduce charge recombination through cascade energy structure and optimized morphology. This progress report provides an overview of the recent progress in OSCs based on multiple-donor (polymer/polymer, polymer/dye, and polymer/small molecule) or multiple-acceptor (fullerene/fullerene, fullerene/nonfullerene, and nonfullerene/nonfullerene) components.

Improving the Compatibility of Donor Polymers in Efficient Ternary Organic Solar Cells via Post-Additive Soaking Treatment

In dual-donor ternary organic solar cells, the compatibility between the donor polymers plays important roles to control the conformational change and govern the photophysical behavior in the blend films. Here, we apply a post-additive soaking (PAS) approach to reconstruct the morphology in a ternary organic photovoltaic BHJ of PTB7-Th: PCDTBT: PC₇₁BM. The PAS-treated device has a maximum power conversion efficiency (PCE) of about 8.7% in this ternary system. From the analyses of GIWAXS and GISAXS, the superior device performance is attributed to the favorable nanomorphology with optimum crystallinity of PTB7-Th and good intermixing of PCDTBT with PTB7-Th:PC₇₁BM, leading to improved charge transport in the vertical direction. AFM and TRPL measurements clearly demonstrate PAS-treated film envisages a homogeneous distribution of smaller PC₇₁BM aggregates to facilitate the exciton dissociation and carrier extraction at the interface. The increased PCE ascribed to not only the enhancement of absorption and nonradiative Förster resonance energy transfer (FRET) between two donors (PCDTBT and PTB7-Th) but also the formation of a bicontinuous interpenetrating network of PC₇₁BM.