We report highly efficient all-polymer solar cells with power conversion efficiencies of over 4.5% by highly intermixed blends of PTB7-Th donor and P(NDI2OD-T2) acceptor polymers. The low interfacial tension and the face-on π–π stackings of the all-polymer blends afforded desired nanophase morphology, which facilitates efficient charge transport from the active layer to each electrode. In addition, the incorporation of 1,8-diiodooctane additives was able to tune the degree of crystallinity and orientation of P(NDI2OD-T2) acceptors, resulting in remarkable enhancement of electron mobility, external quantum efficiency, and JSC values.

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High-Performance All-Polymer Solar Cells Based on Face-on Stacked Polymer Blends with Low Interfacial Tension

The development of polymerized small‐molecule acceptors has boosted the power conversion efficiencies (PCEs) of all‐polymer organic photovoltaic (OPV) cells to 17%. However, the polymer donors suitable for all‐polymer OPV cells are still lacking, restricting the further improvement of their PCEs. Herein, a new polymer donor named PQM‐Cl is designed and its photovoltaic performance is explored. The negative electrostatic potential and low average local ionization energy distribution of the PQM‐Cl surface enable efficient charge generation and transfer process. When blending with a well‐used polymer acceptor, PY‐IT, the PQM‐Cl‐based devices deliver an impressive PCE of 18.0% with a superior fill factor of 80.7%, both of which are the highest values for all‐polymer OPV cells. The relevant measurements demonstrate that PQM‐Cl‐based films possess excellent mechanical and flexible properties. As such, PQM‐Cl‐based flexible photovoltaic cells are fabricated and an excellent PCE of 16.5% with high mechanical stability is displayed. These results demonstrate that PQM‐Cl is a potential candidate for all‐polymer OPV cells and provide insights into the design of polymer donors for high‐efficient all‐polymer OPV cells.

A New Polymer Donor Enables Binary All‐Polymer Organic Photovoltaic Cells with 18% Efficiency and Excellent Mechanical Robustness

Solution-processed organic photovoltaics (OPVs) have the superiorities of light weight, low cost, easy fabrication, high mechanical flexibility and good semitransparency, enabling the great potential in next-generation photovoltaic technology. All-small-molecule OPVs...

Recent Progress on All-Small-Molecule Organic Photovoltaics

The rapid development of low bandgap polymer acceptors has promoted the efficiency up to ≈17% for all‐polymer solar cells (all‐PSCs). Nevertheless, the polymeric blend film, core to the photoelectric conversion of all‐PSCs, has not been thoroughly understood in terms of the influence and regulatory factors of mechanical properties, which hinders the advances in flexible and wearable applications. Herein, a range of characterization methods is combined to investigate the mechanical properties, miscibility, and film microstructure of the blends based on several representative polymer donors (PTzBI‐Si, PTVT‐T, PM6 and PTQ10) and a benchmark polymer acceptor N2200, and to further reveal the miscibility‐property relationships of the miscibility property. The results stress that fracture behaviors and elastic moduli of these blends with varied compositions show different changing trends, which are affected by molecular interactions and aggregated structure of the blends. The elastic moduli of the four all‐polymer blends can be nicely predicted by different models that are deduced from macromolecular mechanics. Most crucially, the correlations between elastic modulus, morphology, and miscibility of all‐polymer blends are elucidated for the first time. The derived relationships is validated with another high‐efficiency blend and will be the key to the successful fabrication of mechanically robust and stretchable all‐PSCs with high efficiency.

Unraveling the Correlations between Mechanical Properties, Miscibility, and Film Microstructure in All‐Polymer Photovoltaic Cells

Non-fullerene acceptors (NFAs) have been widely developed to improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs) The highest PCE has reached over 18% in binary bulk heterojunction (BHJ) NFA-OSCs, approaching the threshold for commercial applications, while the most stable NFA-OSCs have been inferred with a potential lifetime of 22 years The instability of NFA-OSCs has not been paid enough attention and becomes one of the major barriers to their commercialization Herein, we present a systematic discussion on degradation mechanisms of NFA-OSCs, in which we focused on the fundamentals of instability in NFA-OSCs, including photo-oxidation, morphology instability, and interlayer induced instability Meanwhile, the recent progress and promising strategies to improve the stability of NFA-OSCs are presented in the themes of their molecular design, a third component in the blend film and passivation of the interlayer We believe that a study on their stability should be the next focus in the field of NFA-OSCs

Stability: next focus in organic solar cells based on non-fullerene acceptors

All‐polymer solar cells (all‐PSCs) have drawn growing attention and achieved tremendous progress recently, but their power conversion efficiency (PCE) still lags behind small‐molecule‐acceptor (SMA)‐based PSCs due to the relative difficulty on morphology control of polymer photoactive blends. Here, low‐cost PTQ10 is introduced as a second polymer donor (a third component) into the PM6:PY‐IT blend to finely tune the energy‐level matching and microscopic morphology of the polymer blend photoactive layer. The addition of PTQ10 decreases the π–π stacking distance, and increases the π–π stacking coherence length and the ordered face‐on molecular packing orientation, which improves the charge separation and transport in the photoactive layer. Moreover, the deeper highest occupied molecular orbital energy level of the PTQ10 polymer donor than PM6 leads to higher open‐circuit voltage of the ternary all‐PSCs. As a result, a PCE of 16.52% is achieved for ternary all‐PSCs, which is one of the highest PCEs for all‐PSCs. In addition, the ternary devices exhibit a high tolerance of the photoactive layer thickness with high PCEs of 15.27% and 13.91% at photoactive layer thickness of ≈205 and ≈306 nm, respectively, which are the highest PCEs so far for all‐PSCs with a thick photoactive layer.

16.52% Efficiency All‐Polymer Solar Cells with High Tolerance of the Photoactive Layer Thickness

Optimization of morphology and precise control of miscibility between donors and acceptors play an important role in improving the power conversion efficiencies (PCEs) of all-small-molecule organic solar cells (SM-OSCs). Besides device optimization, methods such as additives and thermal annealing are applied for finely tuning bulk-heterojunction morphology; strategies of molecular design are also the key to achieve efficient phase separation. Here, a series of A-D-A-type small-molecule donors (SM4, SM8, and SM12) based on benzodithiophene units were synthesized with different lengths of alkylthio side chains to regulate crystallinity, and their miscibility with the acceptor (BO-4Cl) was investigated. Consequently, SM4 with a short alkylthio substituent had a high crystallization propensity, leading to the oversized molecular domains and the poor morphology of the active layer. Meanwhile, SM12 with a longer alkylthio substituent showed weak crystallinity, causing a relatively looser π-π stacking and thus adversely affecting charge-carrier transport. The SM-OSC based on the small-molecule donor SM8 with a mid-length alkylthio substituent achieved a better PCE over 13%, which was attributed to a more harmonious blend miscibility without sacrificing carrier-charge transport. Eventually, the modulation of phase separation and miscibility via controlling the lateral side chains has proven its potential in optimizing the blend morphology to aid the development of highly efficient SM-OSCs.

Fine-Tuning Miscibility and π-π Stacking by Alkylthio Side Chains of Donor Molecules Enables High-Performance All-Small-Molecule Organic Solar Cells.

The high‐performance organic solar cells (OSCs) tend to choose the polymers with high molecular weight as donors, which easily produce good crystallinity to facilitate intermolecular charge transfer. However, these polymers usually accompanied by the low solubility and synthetic difficulty, increasing batch‐to‐batch variations. The proposal of conjugated mesopolymers (molar mass (Mn) in 1–10 kDa) can overcome these problems. Herein, a new mesopolymer, MePBDFClH as donor material is designed and synthesized, and firstly applied in OSCs. As a comparison, other lower molecular weight mesopolymer of MePBDFClL and higher molecular weight polymer of PBDFCl with same structure are also prepared and investigated. Because of its appropriate phase separation and miscibility in the blend film, the MePBDFClH exhibits the highest power conversion efficiency (PCE) of 15.06% among the three materials. Meanwhile, the champion PCE is a new record for benzo[1,2‐b:4,5‐b′]difuran‐based photovoltaic materials. Importantly, comparing to the pronounced PCE decrease of polymer PBDFCl by about 12%, a slightly PCE difference for mespolymer MePBDFClL is only less than 5%, reducing the batch‐to‐batch variation. This work not only suggests that the benzo[1,2‐b:4,5‐b′]difuran unit is a promising electron‐donating core but also shows that the mesopolymers have great potentials to produce the low‐differentiated and high‐performance organic photovoltaic materials.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/advs.202105430

Conjugated Mesopolymer Achieving 15% Efficiency Single‐Junction Organic Solar Cells

Effects of the Center Units of Small-Molecule Donors on the Morphology, Photovoltaic Performance, and Device Stability of All-Small-Molecule Organic Solar Cells

The application of high-performance organic solar cells (OSCs) requires high efficiency, high stability and low-cost devices. Herein, we propose a low boiling-point and low-cost additive 1,4-difluorobenzene (DFB) with a boiling point of 88.8 °C to optimize the morphology of the active layer of the OSCs based on conjugated polymer J71 as a donor and n-type small molecule m-ITTC as an acceptor. The DFB additive treatment enhances intermolecular π–π stacking of the photovoltaic materials, which is conducive to charge carrier separation and transport. The power conversion efficiency (PCE) of the OSCs based on J71:m-ITTC with the DFB additive treatment increased to 12.38% from 11.01% for the device without the additive treatment. Furthermore, the OSCs with the additive treatment also showed higher stability. The results indicate that 1,4-difluorobenzene will be a promising low cost additive for commercial application of OSCs.

Shaman Li

Papers

16.52% Efficiency All‐Polymer Solar Cells with High Tolerance of the Photoactive Layer Thickness

Fine-Tuning Miscibility and π-π Stacking by Alkylthio Side Chains of Donor Molecules Enables High-Performance All-Small-Molecule Organic Solar Cells.

Conjugated Mesopolymer Achieving 15% Efficiency Single‐Junction Organic Solar Cells

Effects of the Center Units of Small-Molecule Donors on the Morphology, Photovoltaic Performance, and Device Stability of All-Small-Molecule Organic Solar Cells

A low boiling-point and low-cost fluorinated additive improves the efficiency and stability of organic solar cells