Vertical Phase Separation Research Articles

19.5% Efficiency organic solar cells enabled by a direct C–H arylation-derived wide-bandgap small-molecule guest donor

A tin-free asymmetric small-molecule guest donor (oPhFO) with a wide bandgap allows PM6:oPhFO:BTP-ec9 ternary OSCs to reach a champion efficiency of 19.5% by optimizing blend morphology and vertical phase separation as well as reducing energy loss.

Synergistic control of multilength-scale morphology and vertical phase separation for high-efficiency organic solar cells

Controlling the morphology of organic solar cells (OSCs) presents a significant challenge due to their complex structure and composition.

A Water-Processed Mesoscale Structure Enables 18.5% Efficient Binary Layer-by-Layer Organic Solar Cells.

The two-step layer-by-layer (LBL) deposition of donor and acceptor films enables desired vertical phase separation and high performance in organic solar cells (OSCs), which becomes a promising technology for large-scale printing devices. However, limitations including the use of toxic solvents and unpredictable infiltration between donor and acceptor still hinder the commercial production of LBL OSCs. Herein, we developed a water-based nanoparticle (NP) ink containing donor polymer to construct a mesoscale structure that could be infiltrated with an acceptor solution. Using non-halogen o-xylene for acceptor deposition, the LBL strategy with a mesoscale structure delivered outstanding efficiencies of 18.5% for binary PM6:L8-BObased LBL OSCs. Enhanced charge carrier mobility and restricted trap states were observed in the meso-LBL devices with optimized vertical morphology. It is believed that the findings in this work will bring about more research interest and effort on eco-friendly processing in preparation for the industrial production of OSCs.

Manipulating Vertical Phase Separation Enables Pseudoplanar Heterojunction Organic Solar Cells Over 19% Efficiency via Ternary Polymerization.

Controlling vertical phase separation of the active layer to enable efficient exciton dissociation and charge carrier transport is crucial to boost power conversion efficiencies (PCEs) of pseudoplanar heterojunction (PPHJ) organic solar cells (OSCs). However, how to optimize the vertical phase separation of PPHJ OSCs via molecule design is rarely reported yet. Herein, ternary polymerization strategy is employed to develop a series of polymer donors, DL1-DL4, and regulate their solubility, molecular aggregation, molecular orientation, and miscibility, thus efficiently manipulating vertical phase separation in PPHJ OSCs. Among them, DL1 not only has enhanced solubility, inhibited molecular aggregation and partial edge-on orientation to facilitate acceptor molecules, Y6, to permeate into polymer layer and increase donor/acceptor interfaces, but also sustains high crystallinity and appropriate miscibility with Y6 to acquire ordered molecular packing, thus achieving optimized vertical phase separation to well juggle exciton dissociation and charge transport in PPHJ devices. Therefore, DL1/Y6 based PPHJ OSCs gain the best exciton dissociation probability, highest charge carrier mobilities and weakest charge recombination, and thus afford an impressive PCE of 19.10%, which is the record value for terpolymer donors. It demonstrates that ternary polymerization is an efficient method to optimize vertical phase separation in PPHJ OSCs for high PCEs.

Hot-Casting Strategy Empowers High-Boiling Solvent-Processed Organic Solar Cells with Over 18.5% Efficiency.

Most top-rank organic solar cells (OSCs) are manufactured by the halogenated solvent chloroform, which possesses a narrow processing window due to its low-boiling point. Herein, based on two high-boiling solvents, halogenated solvent chlorobenzene (CB) and non-halogenated green solvent ortho-xylene (OX), preparing active layers with the hot solution is put forward to enhance the performance of the OSCs. In situ test and morphological characterization clarify that the hot-casting strategy assists in the fast and synchronous molecular assembly of both donor and acceptor in the active layer, contributing to preferable donor/acceptor ratio, vertical phase separation, and molecular stacking, which is beneficial to charge generation and extraction. Based on the PM6:BO-4Cl, the hot-casting OSCs with a wide processing window achieve efficiencies of 18.03% in CB and 18.12% in OX, which are much higher than the devices processed with room temperature solution. Moreover, the hot-casting devices with PM6:BTP-eC9 deliver a remarkable fill factorof 80.31% and efficiencyof 18.52% in OX, representing the record value among binary devices with green solvent. This work demonstrates a facile strategy to manipulate the molecular distribution and arrangement for boosting the efficiency of OSCs with high-boiling solvents.

Strengthening the Hetero-Molecular Interactions in Giant Dimeric Acceptors Enables Efficient Organic Solar Cells.

Giant dimeric acceptor (G-Dimer) is becoming one of the most promising organic solar cell (OSC) materials because of its definite structure, long-term stability, and high efficiency. Strengthening the hetero-molecular interactions by monomer modification greatly influences the morphology and thus the device performance, but lacks investigation. Herein, two novel quinoxaline core-based G-Dimers, Dimer-QX and Dimer-2CF, are synthesized. By comparing trifluoromethyl-substituted Dimer-2CF and non-substituted Dimer-QX, the trifluoromethylation effect on the G-Dimer is investigated and revealed. The trifluoromethyl with strong electronegativity increases electrostatic potential and reduces surface energy of the G-Dimer, weakening the homo-molecular ordered packing but reinforcing the hetero-molecular interaction with the donor. The strong hetero-molecular interaction suppresses the fast assembly during the film formation, facilitating small domains with ordered molecular packing in the blend, which is a trade-off in conventional morphology control. Together with favorable vertical phase separation, efficient charge generation, and reduced bimolecular recombination are concurrently obtained. Hence, the Dimer-2CF-based OSCs obtain a cutting-edge efficiency of 19.02% with fill factor surpassing 80%, and an averaged extrapolated T80 of ≈12000h under continuous 80 °C heating. This study emphasizes the importance of hetero-molecular interaction and trifluoromethylation strategy, providing a facile strategy for designing highly efficient and stable OSC materials.

Achieving More Insight on the Dynamic Process from Conventional/Inverted Layer‐by‐Layer All‐Polymer Solar Cells with 17.24% or 12.41% Efficiency

Series of bulk heterojunction (BHJ) and layer‐by‐layer (LbL) all‐polymer solar cells (APSCs) were prepared with polymer PM6 as donor and polymer PY‐DT as acceptor based on conventional and inverted configuration. Benefiting from the sequential deposition strategy, the good vertical phase separation and more ordered molecular arrangement can be formed in the LbL APSCs. The conventional LbL APSCs exhibit an optimized power conversion efficiency (PCE) of 17.24% with a relatively large short circuit current density of 23.83 mA cm−2 and fill factor of 74.60%, photogenerated excitons near the indium tin oxide electrode can be efficiently utilized through energy transfer from PM6 to PY‐DT and the self‐absorption effect of PM6 for its long exciton diffuse distance. The 17.24% PCE of conventional LbL APSCs is higher than 16.72% of conventional BHJ APSCs, 14.59% of inverted BHJ APSCs and 12.41% of inverted LbL APSCs. The rather low PCE of 12.41% for the inverted LbL APSCs further indicates that the energy transfer from donor to acceptor and self‐absorption effect of donor should play a vital role in determining the performance of LbL APSCs. This work provides more insights on the exciton and carrier dynamic process in sequentially deposited active layer, providing more guidance for preparing efficient LbL APSCs.

Enhancing the Molecular Order and Vertical Component Distribution of the P3HT/O-IDTBR System during Layer-by-Layer Processing.

The molecular order and vertical component distribution are critical to enhance the charge transport in layer-by-layer (LbL) processed active layer. However, the excessive inter-diffusion between donor and acceptor layers during LbL processing irrepressibly reduces their ordered packing. Herein, a novel tactic to optimize the molecular order and vertical morphology of the active layer through suppressing the deep penetration of (5Z,5'Z)-5,5'-((7,7'-(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6 -b']dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene)) bis(3-ethyl-2-thioxothiazolidin-4-one) (O-IDTBR) to poly(3-hexylthiophene) (P3HT) film during LbL processing is proposed. This is enabled by inducing the formation of P3HT nanofibers through ultraviolet (UV) irradiation and solution aging. During the LbL processing, these nanofibers with high crystallinity reduce the damage of O-IDTBR solution to P3HT film and restrict the penetration of O-IDTBR into P3HT matrix. As a result, the P3HT nanofibers are preserved and the degree of vertical phase separation is enlarged in the LbL-processed film. Meanwhile, the molecular order of both components is enhanced. The resulting morphology that featured as intertwined P3HT nanofibers/O-IDTBR network efficiently promotes charge transport and extraction, boosting the power conversion efficiency (PCE) of the devices from 6.70±0.12% to 7.71±0.10%.

Enhancing Stability and Efficiency of Perovskite Solar Cells with a Bilayer Hole Transporting Layer of Nickel Phthalocyanine and Poly(3‐Hexylthiophene)

AbstractTo expedite the commercialization of perovskite solar cells (PSCs), researchers are exploring the feasibility of employing nickel phthalocyanine (NiPc) as a hole transport material (HTM) due to its cost‐effectiveness, excellent thermal stability, and suitability for solution coating. However, the low LUMO energy level of the NiPc may limit its ability to block photoelectrons generated in the perovskite layer from recombining with holes, which can reduce the overall efficiency of the solar cell. One solution is to use cascaded bilayers with HTMs that have relatively higher LUMO levels. In this study, a bilayer consisting of NiPc and poly(3‐hexylthiophene) (P3HT) is employed as the HTM, where the P3HT exhibits vertical phase separation during the coating process. By optimizing the mixing amount of P3HT into the NiPc, a record power conversion efficiency of 23.11%, the highest reported for NiPc‐based PSCs is achieved. Moreover, an excellent long‐term stability is demonstrated by encapsulating the PSC in polyisobutylene, with the device retaining 90% of its initial efficiency after exposure to 85 °C and 85% relative humidity for 1000 h.

Over 19.1% efficiency for sequentially spin-coated polymer solar cells by employing ternary strategy

Series of sequentially spin-coated polymer solar cells (SS-PSCs) are fabricated with donor layers containing PM1, D18 or PM1:D18 and sequentially spin-coated L8-BO acceptor layer. Photon harvesting of donor layers could be enhanced by optimizing the weight ratios of PM1 to D18 due to their distinct optical band-gap. The optimized ternary SS-PSCs exhibit a power conversion efficiency of 19.13%, resulting from the simultaneously enhanced short-circuit-current density (JSC) of 27.20 mA/cm2, open-circuit voltage (VOC) of 0.90 V and fill factor of 78.53% in comparison to those of PM1/L8-BO based SS-PSCs. The increased JSCs of ternary SS-PSCs is due to the enhanced photon harvesting and efficient exciton utilization assisted by an additional channel from D18 to PM1 through energy transfer. The increased VOC of optimal ternary SS-PSCs benefits from the little larger VOC of binary SS-PSCs with D18 as donor layer. The donor/acceptor interfacial energy is slightly increased by incorporating D18 in donor layer, which is conducive to form the optimized vertical phase separation to decrease charge recombination loss. This work indicates that ternary strategy is also a versatile and effective method in improving the performance of SS-PSCs.

Improving the Performance of Layer-by-Layer Processed Organic Solar Cells via Introducing a Wide-Bandgap Dopant into the Upper Acceptor Layer.

The layer-by-layer (LbL) solution-processed organic solar cells (OSCs) are conductive to achieve vertical phase separation, tunable donor-acceptor (D/A) interfaces, and favorable charge-transport pathways. In this work, a wide-bandgap component poly(9-vinylcarbazole) (PVK) is added to the upper electron acceptor layer to improve the performance of LbL-processed OSCs. Results show that the PVK component can adjust the film morphology, dope the electron acceptor, increase the electron concentration, and improve charge transport. Such n-type doping is verified by Seebeck coefficient measurement, ultraviolet photoelectron spectroscopy, and electron paramagnetic resonance characterization. In addition, the fluorescence intensity and exciton lifetime of the PVK-doped acceptor film are increased, thus being beneficial for exciton diffusion to the D/A interface. Therefore, the power conversion efficiency (PCE) of LbL OSCs increases when 2.50 wt.% PVK is employed in the electron acceptor layer of commonly-used high-efficiency system and a maximum value of 19.05% can be achieved. The role of PVK played in the active layer is different from those of additives and ternary components reported previously, so the results provide an alternative way to enhance the device performance of LbL-processed OSCs.

Enhancing the enrichment of conjugated polymer at surfaces of its elastomer blend film via increasing molecular weight to induce desired nanofibril network for highly stretchable polymer film

The vertical phase separation in conjugated polymer-elastomer blend systems plays a critical role in inducing desired film morphology of interconnected polymer nanofibrils for high-performance stretchable electronics. However, the precise control of vertical component distribution remains a great challenge due to the complex interactions among solutes, solvent and substrate in the blend system. Herein, we proposed a strategy to enhance the enrichment of polymer at film surfaces in the conjugated polymer-elastomer blend via increasing polymer molecular weight and systematically investigated the microstructural transition of blends under strain. For this purpose, a number of N2200 polymers with the number-average molecular weight (Mn) below (28, 57, 96 kDa) and above (143 kDa) the critical molecular weight (Mc) were used to blend with SEBS. With the increase of Mn, the phase separation between N2200 and SEBS along depth direction occurred at a greater extent due to the reduced polymer solubility and miscibility of the two components. Film-depth-dependent light absorption spectroscopy (FLAS) measurement shows that the content of N2200 concentrated at the film surfaces is dramatically increased from 66% to 85% as the Mn increases from 28 to 143 kDa, leading to the change of morphology from isolated aggregates to continuous and entangled nanofibril network within SEBS matrix. Under deformation, these nanofibrils formed in the 143 kDa blend film are able to maintain sufficient connections and the crystallites are seldom destroyed, thus guarantee efficient charge transport channels. The mobility of the 143 kDa blend film is 0.188 cm2 V−1 s−1, much higher than that of 28 kDa blend film (0.043 cm2 V−1 s−1) and does not decrease during stretching up to 150% strain.

Record indoor performance of organic photovoltaics with long-term stability enabled by self-assembled monolayer-based interface management

Though with the advent of the Internet-of-Things state-of-the-art organic photovoltaic (OPV) systems for harnessing indoor light energy have successfully been developed; however, the practical use of OPVs is limited owing to their low power conversion efficiency (PCE) and marginal understanding on the charge dynamics of OPVs under dim indoor lights. Herein, a record-high performance in indoor OPV system is secured by combining a 2-(9 H-carbazol-9-yl) phosphonic acid (2PACz)-processed indium tin oxide (ITO) and a 2PACz-mixed photoactive layer. Charge carrier dynamics of the 2PACz-mixed photoactive layer are systematically investigated to develop efficient indoor OPVs. Spontaneous vertical phase separation of photoreactive layers with 2PACz forms a vertical component distribution and dramatically improves carrier yield-mobility product which yields suppression of trap-assisted recombination and leaking current in the indoor OPVs. Also, phosphonic acid groups-based 2PACz-treated ITO leads to induce a sufficiently large work function owing to a vacuum-level shift, thereby enabling efficient energy-level matching to achieve charge selection enhancement at the hole-selective interface. The champion OPV (∼ 36% PCE under indoor lights) system maintains 95% of its initial efficiency after 1000 h of operation in ambient air. Our findings highlight the tremendous potential of indoor OPVs for simultaneously achieving high efficiency and ambient shelf-lifetime.

Solid Additive-Assisted Layer-by-Layer Processing for 19% Efficiency Binary Organic Solar Cells

HighlightsA solid additive-assisted layer-by-layer (SAA-LBL) processing was developed to facilitate the inter-diffusion between polymer donor and acceptor and optimize the morphology of quasi-planar heterojunction for high-performance organic solar cells (OSCs).The pre-phase separation between fatty acid and polymer donor, easily tuned via controlling the cohesive energy, is critical to form the desired vertical phase-separation morphology.The SAA-LBL is generally applicable to various OSC systems, and enables the record efficiency of 19.02% among the binary OSCs.

Layer‐by‐Layer‐Processed Organic Solar Cells with 18.02% Efficiency Enabled by Regulating the Aggregation of Bottom Polymers

The fabrication of organic solar cells (OSCs) by a layer‐by‐layer (LBL) method has attracted growing attention in recent years. As already known, the pre‐aggregates of conjugated polymers in solution have a profound impact on their microstructure morphology in films. Herein, by simply controlling the solution temperature and annealing processes, the pre‐aggregation behavior of D18 polymer in solution can be fine‐tuned and the microstructure of D18 bottom layer is well manipulated. The optimized D18 bottom layer can effectively regulate L8‐BO upper‐layer‐forming suitable networks for efficient charge transportation. In addition, a vertical phase separation with a special D/D:A/A structure (P‐i‐N‐type component distribution) is also formed. As a result, compared to the 16.43% power conversion efficiency (PCE) of the bulk heterojunction devices, such control enables bilayer OSC devices based on the polymer D18 and L8‐BO to deliver an enhanced PCE of 18.02% with simultaneously improved short‐circuit current density, open‐circuit voltage, and fill factor. It is also demonstrated in these results that the LBL deposition process utilizing the pre‐aggregation of polymer and its fiber‐network‐forming ability is a very promising approach to improve charge dynamics, suppress carrier recombination, and fabricate highly efficient OSCs.

Layer‐by‐Layer‐Processed Ternary All‐Polymer Organic Solar Cells with 17.74% Efficiency Enabled by Introducing a Designed Narrow‐Bandgap Guest Polymer Acceptor

Obtaining an admirably modified vertical phase separation of the active layer for all‐polymer solar cells (all‐PSCs) to facilitate charge generation, charge transfer and transport properties are a prerequisite for achieving high performance. Herein, the active layer of all‐PSCs is finely manipulated by combining a ternary blend strategy with a layer‐by‐layer (LbL) process. Based on the LbL‐processed PM6/PYT‐1S1Se host binary all‐PSCs, a chlorinated polymer acceptor PYT‐1S1Se‐4Cl is designed and introduced into the host system for rationally controlling blend morphology with ordered molecular stacking and suitable vertical distribution. The optimized bulk microstructure of the ternary system is not only beneficial to the charge generation and charge transport properties, but also can significantly reduce the nonradiative energy loss that occurs in the LbL‐type ternary blend. Thus, the LbL‐type PM6/(PYT‐1S1Se:PYT‐1S1Se‐4Cl) ternary all‐PSCs exhibit a promising power conversion efficiency (PCE) of 17.74%, which is higher than the corresponding binary systems (PCE = 16.86% for PM6/PYT‐1S1Se and PCE = 15.83% for PM6/PYT‐1S1Se‐4Cl), indicating the special merits of material design and processing technology. Overall, a promising combinatorial method for the morphological regulation of all‐polymer systems is demonstrated that contributes to enhanced efficiency.

Approaching 18% efficiency of ternary layer-by-layer polymer solar cells with alloyed acceptors

In this work, ternary layer-by-layer (LbL) polymer solar cells (PSCs) were prepared with D18-Cl/Y6 as host system and MAZ-2 as the second acceptor. The open circuit voltage (VOC) of LbL PSCs can be increased with the addition of MAZ-2, signifying the alloy state formation between Y6 and MAZ-2 with good compatibility. A power conversion efficiency (PCE) of 17.98% can be achieved in the optimal ternary LbL PSCs with acceptor layer prepared from Y6:MAZ-2 (10:1, wt/wt) blend solution, accompanying with synchronously improved short circuit current density (JSC) of 27.10 mA cm−2, open circuit voltage (VOC) of 0.88 V and FF of 75.39% as compared to those of host binary system. The JSC improvement of ternary LbL PSCs should be closely related to more efficient exciton utilization through the additional channel of intermolecular energy transfer from MAZ-2 to Y6. The interfacial energy between donor and acceptor layer can be increased by adding MAZ-2 with lower compatibility with D18-Cl, which may lead to the weakened interpenetration of acceptor into donor layer to form more ideal vertical phase separation for efficient charge transport and collection. This work provides a guidance to select the third component having appropriate compatibility with host materials for realizing performance improvement of LbL PSCs.

Highly Conductive Ultrathin Layers of Conjugated Polymers for Metal-Free Coplanar Transistors with Single-Polymer Transport Layers.

Although metal or oxide conductive films are widely used as electrodes of electronic devices, organic electrodes would be more favorable for next-generation organic electronics. Here, using some model conjugated polymers as examples, we report a class of highly conductive and optically transparent polymer ultrathin layers. Vertical phase separation of semiconductor/insulator blends leads to a highly ordered two-dimensional (2D) ultrathin layer of conjugated-polymer chains on the insulator. Afterwards, the thermally evaporated dopants on the ultrathin layer lead to a conductivity of up to 103 S cm-1 and a sheet resistance 103 Ω/square for a model conjugated polymer poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophenes) (PBTTT). The high conductivity is due to the high hole mobility (∼ 20 cm2 V-1 s-1), although doping-induced charge density is still in the moderate range of 1020 cm-3 with a 1 nm thick dopant. Metal-free monolithic coplanar field-effect transistors using the same conjugated-polymer ultrathin layer with alternatively doped regions as electrodes and a semiconductor layer are realized. The field-effect mobility of this monolithic transistor is over 2 cm2 V-1 s-1 for PBTTT, one order higher than that of the conventional PBTTT transistor using metal electrodes. The optical transparency of the single conjugated-polymer transport layer is over 90%, demonstrating a bright future for all-organic transparent electronics.

Controlling Kinetic Quenching Depth Toward High‐Performance and Photo‐Stable Organic Solar Cells Printed from a Non‐Halogenated Solvent

AbstractThe investigation of drying dynamics and kinetic quenching depth related degradation of high‐performance photoactive materials with scalable coating techniques demands significant research attention. Herein, film formation kinetics regulated crystallinity, preferential orientation and vertical phase separation of the active layers is revealed, which then further affects exciton diffusion, dissociation, charge‐transport and recombination processes. By suppressing over aggregation/crystallization in slow drying process and overcoming quenching of disordered liquid phase in fast‐dried films, the optimized PM6:BTP‐eC9‐based organic solar cells obtain power conversion efficiencies up to 16.81%. In addition, photoluminescence lifetime distribution is found to be an alternative probe for kinetic quenching depth that governs the degradation rate. This study provides valuable insights into the control of thin film formation kinetics during scalable processing and develops an effective way to associate the kinetic quenching depth with morphological degradation for mechanistic understanding of long‐term stability.

Non‐Fused Ring Acceptors Achieving over 15.6% Efficiency Organic Solar Cell by Long Exciton Diffusion Length of Alloy‐Like Phase and Vertical Phase Separation Induced by Hole Transport Layer (Adv. Energy Mater. 6/2023)

Alloy-Like Acceptors Non-fused ring electron acceptors are becoming attractive in organic solar cells due to their low cost and easy synthesis. In article number 2203402, Baomin Xu, Aung Ko Ko Kyaw, and co-workers demonstrate a ternary blend using two in-house designed non-fused ring electron acceptors and a polymer donor to form an alloy-like acceptor and vertical phase separation in the bulk-heterojunction.