Tailoring the Morphology's Microevolution for Binary All-Polymer Solar Cells Processed by Aromatic Hydrocarbon Solvent with 16.22% Efficiency

A-A Strategy Enables Desirable Performance of All-Polymer Solar Cells Fabricated with Nonhalogenated Solvents

Most polymer acceptors have been designed by applying a D (electron-rich unit)-A (electron-deficient unit) strategy, which are principally processed with halogenated solvents to fabricate all-polymer solar cells (all-PSCs). Two novel polymer acceptors, containing an A-A type backbone, were designed and synthesized, which can be readily dissolved in o-xylene. The polymer PY-FBTA, comprising a Y6 derivative as the first A unit and a benzotriazole derivative as the second A unit, shows smaller dihedral angles in the backbone, stronger molecular interactions, higher LUMO level, more complementary absorption spectrum, and better morphology with PM6 than the polymer PY-DPP comprising a diketopyrrolopyrrole derivative as the second A unit. Accordingly, the PM6:PY-FBTA all-PSC achieves a higher PCE of 13.95% than the all-PSC based on PM6:PY-DPP (9.51%) for thoroughly improved Jsc (22.34 mA cm-2), Voc (0.963 V), and FF (64.84%) values, which are fabricated with o-xylene as the solvent. This work demonstrates that the A-A structure is a desirable strategy for designing polymer acceptors for efficient all-PSCs prepared with nonhalogenated solvents.

Green-Processed Non-Fullerene Organic Solar Cells Based on Y-Series Acceptors

The development of environmentally friendly and sustainable processes for the production of high-performance organic solar cells (OSCs) has become a critical research area. Currently, Y-series electron acceptors are widely used in high-performance OSCs, achieving power conversion efficiencies above 19%. However, these acceptors have large fused conjugated backbones that are well-soluble in halogenated solvents, such as chloroform and chlorobenzene, but have poor solubility in non-halogenated green solvents. To overcome this challenge, recent studies have focused on developing green-processed OSCs that use non-chlorinated and non-aromatic solvents to dissolve bulk-heterojunction photoactive layers based on Y-series electron acceptors, enabling environmentally friendly fabrication. In this comprehensive review, an overview of recent progress in green-processed OSCs based on Y-series acceptors is provided, covering the determination of Hansen solubility parameters, the use of non-chlorinated solvents, and the dispersion of conjugated nanoparticles in water/alcohol. It is hoped that the timely review will inspire researchers to develop new ideas and approaches in this important field, ultimately leading to the practical application of OSCs.

Triggering the Donor-Acceptor Phase Segregation with Solid Additives Enables 16.5% Efficiency in All-Polymer Solar Cells

All-polymer solar cells have attracted considerable research interest due to their superior morphological stabilities, stretchability, and mechanical durability. However, the morphology optimization of the all-polymer bulk heterojunctions remains challenging due to the two long conjugated polymer chains, limiting its power conversion efficiency. Herein, we focus on the donor-acceptor phase segregation of an all-polymer active layer composed of PM6/PY-IT, a state-of-the-art all-polymer combination, by the introduction of volatile solid additives. Especially with 1,3-dibromo-5-chlorobenzene (DBCl) as the processing additive, we could effectively tune the miscibility between PM6 and PY-IT and thus optimize the phase segregation of the polymer donor and acceptor. Due to the synergetic effects on the favorable phase segregation and desired donor-acceptor distribution, the DBCl-treated devices feature the evident improvement of charge transport and collection, accompanied by the suppressed trap-assisted charge recombination. We consequently achieved a champion device efficiency of 16.5% (16.4% averaged), which is a 13% improvement compared with the control device without DBCl (14.6%). Our results highlight the importance of altering the miscibility of the polymer donor-acceptor pairs for practical applications of high-performance all-polymer solar cells.

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.

Formation of Efficient Quasi-All-Polymer Solar Cells by Synergistic Effect of the Ternary Strategy and Solid Additives

All-polymer solar cells (all-PSCs) have been widely studied owing to their unique mechanical flexibility and stability. However, all-PSCs have a lower efficiency than small-molecule acceptor-based PSCs. In the work, a ternary quasi-all-polymer solar cell (Q-all-PSC) using a synergy of the ternary strategy and solid additive engineering is reported. The introduction of PC₇₁BM can not only match the energy level of the photoactive materials with an improved open circuit voltage (V_OC) of the ternary devices but also enhance photon capture, which can improve short circuit current density. It is found that there is effective charge transfer between PC₇₁BM and PY-IT, which can form an electron transport channel and promote efficient charge transport. Moreover, the introduction of PC₇₁BM made the PM6/PY-IT/PC₇₁BM ternary blends more crystalline while slightly reducing phase separation, resulting in a suitable domain size. Importantly, by introducing a high dielectric-constant PFBEK solid additive as the fasten matrix, the Q-all-PSC's efficiency can reach 16.42%. This method provides a new idea for future research on all-polymer solar cells.

Simple Solvent Treatment Enabled Improved PEDOT:PSS Performance toward Highly Efficient Binary Organic Solar Cells

PSS is the most popular hole-transporting material (HTM) for conventional structural organic solar cell (OSC) devices, whose performance is of great importance for realizing high power conversion efficiency (PCE). However, its performance in OSC devices has been continuously challenged by various replacing materials and different doping strategies, for better conductivity, work function, and surface property. Here, we report a simple dopant-free method to tune the phase separation of the PEDOT:PSS layer, which results in better charge transport and extraction in devices. Specifically, high PCEs for binary polymer-small-molecule (>18%) and polymer-polymer (>17%) systems are simultaneously achieved. This work engineeringly provides encouraging improvement for OSC device performance with easy modification and scientifically offers insights into tuning the property of the PEDOT:PSS layer.