Manipulating Polymer Backbone Configuration via Halogenated Asymmetric End-Groups Enables Over 18% Efficiency All-Polymer Solar Cells

Dithienoquinoxalineimide-Based Polymer Donor Enables All-Polymer Solar Cells Over 19 % Efficiency

All-polymer solar cells (all-PSCs) have been regarded as one of the most promising candidates for commercial applications owing to their outstanding advantages such as mechanical flexibility, light weight and stable film morphology. However, compared to large amount of new-emerging excellent polymer acceptors, the development of high-performance polymer donor lags behind. Herein, a new D-π-A type polymer donor, namely QQ1, was developed based on dithienoquinoxalineimide (DTQI) as the A unit, benzodithiophene with thiophene-conjugated side chains (BDTT) as the D unit, and alkyl-thiophene as the π-bridge, respectively. QQ1 not only possesses a strong dipole moment, but also shows a wide band gap of 1.80 eV and a deep HOMO energy level of -5.47 eV, even without halogen substituents that are commonly indispensable for high-performance polymer donors. When blended with a classic polymer acceptor PY-IT, the QQ1-based all-PSC delivers an outstanding PCE of 18.81 %. After the introduction of F-BTA3 as the third component, a record PCE of 19.20 % was obtained, the highest value reported so far for all-PSCs. The impressive photovoltaic performance originates from broad absorption range, reduced energy loss, and compact π-π stacking. These results provide new insight in the rational design of novel nonhalogenated polymer donors for further development of all-PSCs.

A Difluoro-Methoxylated Ending-Group Asymmetric Small Molecule Acceptor Lead Efficient Binary Organic Photovoltaic Blend

Developing a new end group for synthesizing asymmetric small molecule acceptors (SMAs) is crucial for achieving high-performance organic photovoltaics (OPVs). Herein, an asymmetric small molecule acceptor, BTP-BO-4FO, featuring a new difluoro-methoxylated end-group is reported. Compared to its symmetric counterpart L8-BO, BTP-BO-4FO exhibits an upshifted energy level, larger dipole moment, and more sequential crystallinity. By adopting two representative and widely available solvent additives (1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO)), the device based on PM6:BTP-BO-4FO (CN) photovoltaic blend demonstrates a power conversion efficiency (PCE) of 18.62% with an excellent open-circuit voltage (VOC) of 0.933 V, which surpasses the optimal result of L8-BO. The PCE of 18.62% realizes the best efficiencies for binary OPVs based on SMAs with asymmetric end groups. A series of investigations reveal that optimized PM6:BTP-BO-4FO film demonstrates similar molecular packing motif and fibrillar phase distribution as PM6:L8-BO (DIO) does, resulting in comparable recombination dynamics, thus, similar fill factor. Besides, it is found PM6:BTP-BO-4FO possesses more efficient charge generation, which yields better VOC-JSC balance. This study provides a new ending group that enables a cutting-edge efficiency in asymmetric SMA-based OPVs, enriching the material library and shed light on further design ideas.

Tunable Donor Aggregation Dominance in a Ternary Matrix of All-Polymer Blends with Improved Efficiency and Stability

Using two structurally similar polymer acceptors in constructing high-efficiency ternary all-polymer solar cells is a widely acknowledged strategy; however, the focus thus far has not been on how polymer acceptor(s) would tune the aggregation of polymer donors, and furthermore film morphology and device performance (efficiency and stability). Herein, it is reported that matching of the celebrity acceptor PY-IT and the donor PBQx-TCl results in enhanced H-aggregation in PBQx-TCl, which can be finely tuned by controlling the amount of the second acceptor PY-IV. Consequently, the efficiency-optimized PY-IV weight ratio (0.2/1.2) leads to a state-of-the-art power conversion efficiency of 18.81%, wherein light-illuminated operational stability is also enhanced along with well-protected thermal stability. Such enhancements in the efficiency and operational and thermal stabilities of solar cells can be attributed to morphology optimization and the desired glass transition temperature of the target active layer based on comprehensive characterization. In addition to being a high-power conversion efficiency case for all-polymer solar cells, these enhancements are also a successful attempt for using combined acceptors to tune donor aggregation toward optimal morphology, which provides a theoretical basis for the construction of other types of organic photovoltaics beyond all-polymer solar cells.

Efficient and Stable All-Polymer Solar Cells Enabled by Dual Working Mechanism

Ternary strategy with integration characteristics and adaptability is a simple and effective method for blooming of the performance of photovoltaic devices. Herein, a novel wideband gap polymer donor PBB2-Hs is synthesized as the guest component to optimize all-polymer solar cells (all-PSCs). High-energy photon absorption and long exciton lifetime of PBB2-Hs constitute efficient energy transfer. Good miscibility and cascade energy levels promote the formation of alloy-like structure between PBB2-Hs and host system. The dual working mechanisms greatly improve photon capture and charge transfer in active layers. Additionally, the introduction of PBB2-Hs also optimizes the ordered molecular stacking of acceptors and suppresses molecular peristalsis. Upon adding 15 wt% PBB2-Hs, the ternary all-PSC achieved a champion efficiency of 17.66%, and can still maintain 82% photostability (24 h) and 91% storage stability (1000 h) of the original PCE. Moreover, the strong molecular stacking and entanglement between PBB2-Hs and the host material increased the elongation at break of ternary blend film by 1.6 times (16.2%), allowing the flexible device to maintain 83% of the original efficiency after 800 bends (R = 5 mm). This work highlights the effectiveness of guest polymer on simultaneously improving photovoltaic performance, photostability and mechanical stability in all-PSCs.

The role of interfacial donor-acceptor percolation in efficient and stable all-polymer solar cells

Polymerization of Y6-type acceptor molecules leads to bulk-heterojunction organic solar cells with both high power-conversion efficiency and device stability, but the underlying mechanism remains unclear. Here we show that the exciton recombination dynamics of polymerized Y6-type acceptors (Y6-PAs) strongly depends on the degree of aggregation. While the fast exciton recombination rate in aggregated Y6-PA competes with electron-hole separation at the donor-acceptor (D-A) interface, the much-suppressed exciton recombination rate in dispersed Y6-PA is sufficient to allow efficient free charge generation. Indeed, our experimental results and theoretical simulations reveal that Y6-PAs have larger miscibility with the donor polymer than Y6-type small molecular acceptors, leading to D-A percolation that effectively prevents the formation of Y6-PA aggregates at the interface. Besides enabling high charge generation efficiency, the interfacial D-A percolation also improves the thermodynamic stability of the blend morphology, as evident by the reduced device "burn-in" loss upon solar illumination.

Delayed Crystallization Kinetics Allowing High-Efficiency All-Polymer Photovoltaics with Superior Upscaled Manufacturing

Though encouraging performance is achieved in small-area organic photovoltaics (OPVs), reducing efficiency loss when evoluted to large-area modules is an important but unsolved issue. Considering that polymer materials show benefits in film-forming processability and mechanical robustness, a high-efficiency all-polymer OPV module is demonstrated in this work. First, a ternary blend consisting of two polymer donors, PM6 and PBQx-TCl, and one polymer acceptor, PY-IT, is developed, with which triplet state recombination is suppressed for a reduced energy loss, thus allowing a higher voltage; and donor-acceptor miscibility is compromised for enhanced charge transport, thus resulting in improved photocurrent and fill factor; all these contribute to a champion efficiency of 19% for all-polymer OPVs. Second, the delayed crystallization kinetics from solution to film solidification is achieved that gives a longer operation time window for optimized blend morphology in large-area module, thus relieving the loss of fill factor and allowing a record efficiency of 16.26% on an upscaled module with an area of 19.3 cm2 . Besides, this all-polymer system also shows excellent mechanical stability. This work demonstrates that all-polymer ternary systems are capable of solving the upscaled manufacturing issue, thereby enabling high-efficiency OPV modules.

Central Core Substitutions and Film-Formation Process Optimization Enable Approaching 19% Efficiency All-Polymer Solar Cells

Molecular interactions and film-formation processes greatly impact the blend film morphology and device performances of all-polymer solar cells (all-PSCs). Molecular structure, such as the central cores of polymer acceptors, would significantly influence this process. Herein, the central core substitutions of polymer acceptors are adjusted and three quinoxaline (Qx)-fused-core-based materials, PQx1, PQx2, and PQx3 are synthesized. The molecular aggregation ability and intermolecular interaction are systematically regulated, which subsequently influence the film-formation process and determine the resulting blend film morphology. As a result, PQx3, with favorable aggregation ability and moderate interaction with polymer donor PM6, achieves efficient all-PSCs with a high power conversion efficiency (PCE) of 17.60%, which could be further improved to 18.06% after carefully optimizing device annealing and interface layer. This impressive PCE is one of the highest values for binary all-PSCs based on the classical polymer donor PM6. PYF-T-o is also involved in promoting light utilization, and the resulting ternary device shows an impressive PCE of 18.82%. In addition, PM6:PQx3-based devices exhibit high film-thickness tolerance, superior stability, and considerable potential for large-scale devices (16.23% in 1 cm2 device). These results highlight the importance of structure optimization of polymer acceptors and film-formation process control for obtaining efficient and stable all-PSCs.

A Polymer Acceptor with Grafted Small Molecule Acceptor Unit for Efficient All Polymer Organic Solar Cells

A polymer acceptor, named PX-1, is  designed and synthesized using a polymerization strategy with grafted small molecule acceptors. This design approach allows for the freedom of end groups while maintaining efficient terminal packing, enhancing π-π interactions, and facilitating charge transport. All-polymer organic solar cells based on PM6: PX-1 demonstrate a promising efficiency of 13.55%. The result presents an alternative pathway for the design of high-efficiency polymer acceptors through the careful regulation of small molecule acceptor monomers and linker units.

Improving the efficiency of ternary organic solar cells by reducing energy loss

Effectively reducing the voltage loss in organic solar cells (OSCs) is critical to improving the power conversion efficiency (PCE) of OSCs. In this study, highly efficient ternary OSCs were constructed by adding a non-fullerene acceptor Qx2 with a high open-circuit voltage (VOC) and low energy loss (Eloss) into PM6:m-BTP-PhC6 based binary devices. The third component Qx2 shows slightly complementary absorption with m-BTP-PhC6 and also optimizes the molecular packing, orientation, and morphology of the active layer. Moreover, the incorporation of Qx2 reduced the energetic disorder and improved the electroluminescence quantum efficiency, which suppresses the Eloss and further leads to a higher VOC than the PM6:m-BTP-PhC6 binary blend. Consequently, synergetic enhancements of VOC, short circuit current (JSC), and fill factor (FF) are realized, resulting in the PCE of 18.60%. This work shows that the selection of the appropriate third component has positive implications for reducing Eloss and improving the PCE of OSCs.

Critical Role of Non-Halogenated Solvent Additives in Eco-Friendly and Efficient All-Polymer Solar Cells

Organic solar cells (OSCs) demonstrating high power conversion efficiencies have been mostly fabricated using halogenated solvents, which are highly toxic and harmful to humans and the environment. Recently, non-halogenated solvents have emerged as a potential alternative. However, there has been limited success in attaining an optimal morphology when non-halogenated solvents (typically o-xylene (XY)) were used. To address this issue, we studied the dependence of the photovoltaic properties of all-polymer solar cells (APSCs) on various high-boiling-point non-halogenated additives. We synthesized PTB7-Th and PNDI2HD-T polymers that are soluble in XY and fabricated PTB7-Th:PNDI2HD-T-based APSCs using XY with five additives: 1,2,4-trimethylbenzene (TMB), indane (IN), tetralin (TN), diphenyl ether (DPE), and dibenzyl ether (DBE). The photovoltaic performance was determined in the following order: XY + IN < XY + TMB < XY + DBE ≤ XY only < XY + DPE < XY + TN. Interestingly, all APSCs processed with an XY solvent system had better photovoltaic properties than APSCs processed with chloroform solution containing 1,8-diiodooctane (CF + DIO). The key reasons for these differences were unraveled using transient photovoltage and two-dimensional grazing incidence X-ray diffraction experiments. The charge lifetimes of APSCs based on XY + TN and XY + DPE were the longest, and their long lifetime was strongly associated with the polymer blend film morphology; the polymer domain sizes were in the nanoscale range, and the blend film surfaces were smoother, as the PTB7-Th polymer domains assumed an untangled, evenly distributed, and internetworked morphology. Our results demonstrate that the use of an additive with an optimal boiling point facilitates the development of polymer blends with a favorable morphology and can contribute to the widespread use of eco-friendly APSCs.