Polymer acceptors based on Y6 derivatives for all-polymer solar cells

Nonhalogenated Solvent-Processed Efficient Ternary All-Polymer Solar Cells Enabled by the Introduction of a Naphthyloxy Group into the Side Chain of Polymer Donors

Conjugated polymer donors are crucial for enhancing the power conversion efficiencies (PCEs) in all-polymer solar cells (All-PSCs) in nonhalogenated solvents. In this work, three wide-band-gap polymer donors (Sil-D1, Ph-Sil-D1, and Nap-Sil-D1) based on dithienobenzothiadiazole (DTBT) and benzodithiophene (BDT) donor moieties optimized by side chain engineering were designed and synthesized. Alkyl (Sil-D1), phenyloxy (Ph-Sil-D1), and naphthyloxy (Nap-Sil-D1) alkyl siloxane side chain units were incorporated into these polymer donors, respectively. Notably, the Nap-Sil-D1 polymer donor had a greater conjugation length, π-electron delocalization, and improved dipole moment. The deepest highest occupied molecular orbital level of Nap-Sil-D1, with a high absorption coefficient, showed better aggregation properties. In addition, reduced bimolecular recombination and trap-state density generated a high charge transfer to cause a significant enhancement of open-circuit voltage, current density, and fill factor values of 0.94 V, 25.5 mA/cm2, and 70.4%, respectively, for the Nap-Sil-D1-blended All-PSC ternary device (PM6:Nap-Sil-D1:PY-IT), with the highest PCE of 16.8% in the o-xylene solvent, compared to other polymers (Sil-D1 and Ph-Sil-D1) with PCEs of 15.5 and 16.2%. As a result, this optimized device architecture was found to be the most promising as a nonhalogenated solvent processed in additive-free ternary All-PSCs with good stability.

Unveiling the Influence of Linkers on Conformations of Oligomeric Acceptors for High-Performance Polymer Solar Cells

Conformational isomerism of organic photovoltaic materials has a profound impact on their molecular packing and therefore performance of polymer solar cells (PSCs). However, the conformations of oligomeric acceptors (OAs) are mostly predicted by simulations rather than experimental determinations. Herein, the stereochemical S-shaped structure of two dimeric-type acceptor molecules, V-DYIC and V-DYIC-4F, is first confirmed with different end groups (IC for V-DYIC and IC-2F for V-DYIC-4F), incorporating vinylene linkage for connecting the distinct state-of-the-art small molecule acceptor Y-segments. Through the synthetic control of fluorination sites adjacent to the vinyl-linker, S-shaped the conformation by NMR experiments is validated. Compared to the O-shaped dimer, S-shaped conformation results in enhanced lamellar order and reduced nonradiative recombination losses. The optimal acceptor, V-DYIC-4F, achieved a champion efficiency of 18.10% with the lowest energy loss of 0.556 eV in its devices paired with PM6 due to their efficient carrier transport, and suppressed recombination compared to other devices, being attributed to the synergistic effect of conformation and end group fluorination. The insights gained in this work contribute valuable knowledge of both synthetic control and structural determination of OAs, providing strategic design guidelines for the future development of dimeric acceptors toward high-efficiency PSCs.

Excited-State Dynamics in All-Polymer Blends with Polymerized Small-Molecule Acceptors

Polymerizing small-molecular acceptors (SMAs) is a promising route to construct high performance polymer acceptors of all-polymer solar cells (all-PSCs). After SMA polymerization, the microstructure of molecular packing is largely modified, which is essential in regulating the excited-state dynamics during the photon-to-current conversion. Nevertheless, the relationship between the molecular packing and excited-state dynamics in polymerized SMAs (PSMAs) remains poorly understood. Herein, the excited-state dynamics and molecular packing are investigated in the corresponding PSMA and SMA utilizing a combination of experimental and theoretical methods. This study finds that the charge separation from intra-moiety delocalized states (i-DEs) is much faster in blends with PSMAs, but the loosed π-π molecular packing suppresses the excitation conversion from the local excitation (LE) to the i-DE, leading to additional radiative losses from LEs. Moreover, the increased aggregations of PSMA in the blends decrease donor: acceptor interfaces, which reduces triplet losses from the bimolecular charge recombination. These findings suggest that excited-state dynamics may be manipulated by the molecular packing in blends with PSMAs to further optimize the performance of all-PSCs.

Melt blending crystallization regulating balanced nanodomains in efficient and scalable coating processed organic solar cells

The miscibility between active layer donors (D) and acceptors (A) is a key factor impeding the development of organic photovoltaics (OPVs) toward higher performance and large-area production. In this study, melt blending crystallization (MBC) was used to accomplish molecular-level blending and highly oriented crystallization in bulk heterojunction (BHJ) films realized by a scalable blade coating process, which increased the D/A contact area and provided sufficient exciton diffusion and dissociation. At the same time, the highly organized and balanced crystalline nanodomain structures permitted dissociated carriers to be efficiently transmitted and collected, resulting in significantly enhanced short-circuit current density, fill factor, and efficiency of the device by means of optimum melting temperature and quenching rates. The method can be simply incorporated into current efficient OPV material systems and achieve a device performance comparable to the best values. The blade-coating-processed PM6/IT-4F MBC devices achieved an efficiency of 13.86% in a small-area device and 11.48% in a large-area device. A power conversion efficiency (PCE) of 17.17% was obtained in PM6:BTP-BO-4F devices, and a PCE of 16.14% was acquired in PM6:Y6 devices.

Linear Regulating of Polymer Acceptor Aggregation with Short Alkyl Chain Units Enhances All-Polymer Solar Cells' Efficiency

The power conversion efficiency (PCE) of all-polymer solar cells (all-PSCs) has ascended rapidly arising from the development of polymerized small-molecule acceptor materials. However, numerous insulating long alkyl chains, which ensure the solubility of the polymer, result in inferior aggregation and charge mobility. Herein, this study proposes a facile random copolymerization strategy of two small molecule acceptor units with different lengths of alkyl side chains and synthesizes a series of polymer acceptors PYT-EHx, where x is the percentage of the short alkyl chain units. The aggregation strength and charge mobility of the acceptors rise linearly with increasing the proportion of short alkyl chain units. Thus, the PYT-EH20 reaches balanced aggregation with the star polymer donor PBDB-T, resulting in optimal morphology, fastest carrier transport, and reduced recombination and energy loss. Consequently, the PYT-EH20-based device yields a 14.8% PCE, a 16% improvement over the control PYT-EH0-based device, accompanied by an increase in open-circuit voltage (V_oc ), short-circuit current density (J_sc ), and fill factor (FF). This work demonstrates that the random copolymerization strategy with short alkyl chain insertion is an effective avenue for developing high-performance polymer acceptors, which facilitates further advances in the efficiency of all-PSCs.

Binary Blend All-Polymer Solar Cells with a Record Efficiency of 17.41% Enabled by Programmed Fluorination Both on Donor and Acceptor Blocks

Despite remarkable breakthrough made by virtue of "polymerized small-molecule acceptor (PSMA)" strategy recently, the limited selection pool of high-performance polymer acceptors and long-standing challenge in morphology control impede their further developments. Herein, three PSMAs of PYDT-2F, PYDT-3F, and PYDT-4F are developed by introducing different fluorine atoms on the end groups and/or bithiophene spacers to fine-tune their optoelectronic properties for high-performance PSMAs. The PSMAs exhibit narrow bandgap and energy levels that match well with PM6 donor. The fluorination promotes the crystallization of the polymer chain for enhanced electron mobility, which is further improved by following n-doping with benzyl viologen additive. Moreover, the miscibility is also improved by introducing more fluorine atoms, which promotes the intermixing with PM6 donor. Among them, PYDT-3F exhibits well-balanced high crystallinity and miscibility with PM6 donor; thus, the layer-by-layer processed PM6/PYDT-3F film obtains an optimal nanofibril morphology with submicron length and ≈23 nm width of fibrils, facilitating the charge separation and transport. The resulting PM6/PYDT-3F devices realizes a record high power conversion efficiency (PCE) of 17.41% and fill factor of 77.01%, higher than the PM6/PYDT-2F (PCE = 16.25%) and PM6/PYDT-4F (PCE = 16.77%) devices.

Fine-tuned Morphology Based on Two Well-miscible Polymer Donors Enables Higher Open-circuit Voltage and Enhanced Stability for Highly Efficient Ternary All-Polymer Solar Cells

Developing organic solar cells (OSCs) based on a ternary active layer by simply incorporating a third component is one of the most effective approaches to improve their photovoltaic performance. However, limited success has been achieved in all-polymer solar cells (all-PSCs). In this study, a ternary all-PSC with improved efficiency and stability is realized by using J71 as the third component to adjust the host system of PBDB-T:PG1. The deeper highest occupied molecular orbital (HOMO) energy level of J71 downshifts the mixed HOMO energy levels of donors. The two polymer donors (P_D s) have good miscibility and present Förster resonance energy transfer. When blended with PG1, the optimized morphology is obtained, showing enhanced crystallinity but meanwhile slightly reduced phase separation with improved exciton dissociation and collection efficiency, suppressed monomer and bimolecular recombination, and reduced energy loss (0.55 eV). Combining the benefits mentioned above, the ternary all-PSC exhibits an excellent efficiency of 12.8% with simultaneously elevated open-circuit voltage (0.96 V), short-circuit current density (18.4 mA cm^-2 ), and fill factor (72.2%) when compared with binary devices. Moreover, the optimized ternary all-PSC shows improved storage stability and thermal stability. This study demonstrates that the utilization of a ternary all-polymer system based on two well-miscible P_D s is an effective strategy to enhance the photovoltaic performance and stability of all-PSCs. This article is protected by copyright. All rights reserved.

Oligomeric Acceptor: A "Two-in-One" Strategy to Bridge Small Molecules and Polymers for Stable Solar Devices

Oligomeric acceptors are expected to combine the advantages of both highly developed small molecular and polymeric acceptors. However, organic solar cells (OSCs) based on oligomers lag far behind due to their slow development and low diversity. Here, three oligomeric acceptors were produced through oligomerization of small molecules. The dimer dBTICγ-EH achieved the best power conversion efficiencies (PCEs) of 14.48 % in bulk heterojunction devices and possessed a T80 (80 % of the initial PCE) lifetime of 1020 h under illumination, which were far better than that of small molecular and polymeric acceptors. More excitingly, it showed PCEs of 16.06 % in quasi-planar heterojunction (Q-PHJ) devices which is the highest value OSCs using oligomeric acceptors to date. These results suggest that oligomerization of small molecules is a promising strategy to achieve OSCs with optimized performance between the high efficiency and durable stability, and offer oligomeric materials a bright future in commercial applications.

Vinylene-Inserted Asymmetric Polymer Acceptor with Absorption Approaching 1000 nm for Versatile Applications in All-Polymer Solar Cells and Photomultiplication-Type Polymeric Photodetectors

The emerging polymerized small-molecule acceptors (PSMAs) with near-infrared (NIR) absorption have not only significantly boosted the power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs) but have also exhibited great potential for sensitive NIR polymeric photodetectors (PPDs). However, there is no report regarding PSMAs with photo-response that can approach 1000 nm, which is an important criterion for applications in NIR-responsive all-PSCs and PPDs. Herein, by unidirectionally inserting vinylene segments into a selenophene-rich polymer backbone to improve the electron-donating strength and quinoidal character, an asymmetric PSMA, namely, PY3Se-1V, was developed, which showed an extensively red-shifted absorption approaching 1000 nm. The PBDB-T:PY3Se-1V-based binary all-PSCs achieve a decent PCE of 13.2% and a record-high photocurrent density of 25.9 mA cm-2 due to the significantly broadened photo-response and efficient photon-to-electron conversion. More encouragingly, narrowband photomultiplication (PM)-type PPDs based on poly(3-hexylthiophene-2,5-diyl) (P3HT):PY3Se-1V were developed, delivering an exceptionally high external quantum efficiency of 3680% and a responsivity of 28 A W-1 at an NIR peak of 960 nm under -50 V bias, which is reported for the first time in PM-type PPDs with a response approaching 1000 nm.

Eliminating the Detrimental Effect of Secondary Doping on PEDOT : PSS Hole Transporting Material Performance

Secondary doping has a long history of use in conductivity enhancement in poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS). However, very little research has addressed its detrimental effect on application performance of PEDOT : PSS in organic solar cells. Herein, it was shown that the uneven drying of secondary dopant-water mixture results in a nonuniform/continuous film structure, causing severe damage to the device efficiencies (dropping about 8 and 23 % for poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PBDB-T) : 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) and poly[(2,6-(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b']dithio-phene))-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)-benzo[1,2-c:4,5-c']dithiophene-4,8-dione))](PM6) : (3,9-bis(1-oxo-2-methylene-3-(1,1-dicyanomethylene)-5,6-difluoroindanone)-5,5,11,11-tetrakis(4-n-hexylphenyl)-dithieno[2,3d:2',3'd']-s-indaceno[1,2b:5,6b']dithiophene (IT-4F) cells, respectively) and thermal stabilities. Moreover, a simple yet robust dialysis treatment was proposed to solve the issue of noncontinuity and retain the secondary doping's advantages of quinoid structure simultaneously, thus demonstrating a significant enhancement in device performance. This study will be of great importance to the future exploration of the next generation of post-treatment strategy.

Binary and Ternary Polymer Solar Cells Based on a Wide Bandgap D-A Copolymer Donor and Two Nonfullerene Acceptors with Complementary Absorption Spectral

A new wide-bandgap conjugated D-A polymer denoted as P106 with a medium acceptor dithieno [2,3-e;3'2'-g]isoindole-7,9 (8H) (DTID) unit and strong 2-dodecylbenzo[1,2-b:3,4-b':6,5-b"]trithiophene (3TB) donor units shows an optical bandgap of 2.04 and highest occupied molecular orbital energy level of -5.56 eV. P106 is used as the donor and two nonfullerene acceptors-medium bandgap DBTBT-IC and narrow band Y18-DMO-are used as acceptors for the construction of binary and ternary bulk heterojunction polymer solar cells. The optimized polymer solar cells based on P106 : DBTBT-IC and P106 : Y18-DMO exhibit power conversion efficiencies of 11.76 % and 14.07 %, respectively. The short-circuit current density (22.78 mA cm^-2 ) for the P106 : Y18-DMO device is higher than that for P106 : DBTBT-IC (18.56 mA cm^-2 ) one, which could be attributed to the more photon harvesting efficiency of the P106 : Y18-DMO active layer. In light of the high short-circuit current densities and fill factors for the Y18-DMO based device and the high value of open circuit voltage of the DBTBT-IC based device, ternary polymer solar cells are fabricated by using ternary active layer (P106 : DBTBT-IC : Y18-DMO) and achieve a power conversion efficiency of 16.49 % with low energy loss of 0.47 eV.