Yingzhi Jin

The new generation of all-small-molecule organic solar cells (ASM-OSCs) utilizing small-molecule donors and Y-series nonfullerene acceptors has shown great progress in recent years, which provides distinctly different scenery from fullerene- and ITIC-series-based ASM-OSCs. The materials design strategy, morphology formation mechanism, and potential challenges for ASM-OSCs are systematically summarized.

Abstract

The emerging Y-series nonfullerene acceptors (Y-NFA) has prompted the rapid progress of power conversion efficiency (PCE) of all-small-molecule organic solar cells (ASM-OSCs) from around 12% to 17%. The excellent PCE improvement benefits from not only the outstanding properties of Y-series acceptors but also the successful development of small-molecule donors. The short-circuit current density, fill factor, and nonradiative recombination are all optimized to the unprecedented values, providing a scenery that is obviously different from the ITIC-series based ASM-OSCs. In this review, OSCs utilizing small-molecule donors and Y-NFA are summarized and classified in order to provide an up-to-date development overview and give an insight on structure–property correlation. Then, the characteristics of bulk-heterojunction (BHJ) formation of ASM-OSCs are discussed and compared with that of polymer-based OSCs. Finally, the challenges and outlook on designing ground-breaking small-molecule donor and forming an ideal BHJ morphology are discussed.

In situ absorption measurement is used to investigate the aggregation behavior of acceptors during slot-die-coating. The 1 cm² flexible device can reach a power conversion efficiency of 13.70%, with excellent shelf stability and upscaling ability. The connected modules (180 cm²) can effectively power a smartphone, showing great potential for future applications.

Abstract

Slot-die coating is recognized as the most compatible method for the roll-to-roll (R2R) processing of large-area flexible organic solar cells (OSCs). However, the photovoltaic performance of large-area flexible OSC lags significantly behind that of traditional spin-coating devices. In this work, two acceptors, Qx-1 and Qx-2, show quite different film-formation kinetics in the slot-die coating process. In situ absorption spectroscopy indicates that the excessive crystallinity of Qx-2 provides early phase separation and early aggregation, resulting in oversized crystal domains. Consequently, the PM6:Qx-1-based 1 cm² flexible device exhibits an excellent power conversion efficiency (PCE) of 13.70%, which is the best performance among the slot-die-coated flexible devices; in contrast, the PM6:Qx-2 blend shows a pretty poor efficiency, which is lower than 1%. Moreover, the 30 cm² modules based on PM6:Qx-1, containing six 5 cm² sub-cells, exhibit a PCE of 12.20%. After being stored in a glove box for over 6000 h, the PCE remains at 103% of its initial values, indicating excellent shelf stability. Therefore, these results show a promising future strategy for the upscaling fabrication of flexible large-area OSCs.

PdSe₂ quantum dots exhibit great potential in optoelectronics. Herein, a strategy to boost the performance of organic solar cells is realized by incorporating PdSe₂ quantum dots into the PEDOT:PSS. Due to improved conductivity, light scattering, and hole transport properties of the hybrid hole transport layer, the efficiency of the devices based on D18:Y6 is improved from 16.63% to 18.12%.

Palladium diselenide as a thus-far rarely studied group-10 transition metal dichalcogenide, has exhibited great potential in the field of optoelectronics due to their superior conductivity, high carrier mobility, and excellent stability. However, few works are reported on the application of low-dimensional PdSe₂ in organic solar cells (OSCs). Herein, PdSe₂ quantum dots (QDs) are introduced into the high-efficient nonfullerene OSCs for the first time by incorporating the solution-processable QDs in the hole transport layer (HTL) of the OSCs, leading to a dramatic efficiency enhancement of the devices. Upon PdSe₂ QD addition, the maximum efficiency of the devices based on different active layer blends, including D18:Y6 and PM6:L8-BO, is improved to 18.12%, and 18.29%, with efficiency enhancement of 8.96% and 6.90%, respectively, compared with that of the reference devices. The effects of PdSe₂ QDs on the optical absorption, photoluminescence spectrum of the hybrid HTL as well as the carrier mobility, electrochemical impedance spectroscopy, and transient photocurrent/transient photovoltage of the devices are deeply investigated, revealing the efficiency enhancement of the OSCs with PdSe₂ QDs addition results from the improved conductivity, light scattering, and hole transport properties of the hybrid HTL embedded with PdSe₂ QDs.

The morphology and the molecular crystallinity of an all-polymer blend have been finely modulated by using a ternary strategy. Benefiting from the favorable miscibility of the two acceptors and the higher regularity of PY-DT, the ternary blend features a well-defined fibrillar morphology and improved molecular ordering, which leads to an efficiency of 18.03%, representing the highest efficiency for all-polymer solar cells thus far.

Abstract

Although all-polymer solar cells (all-PSCs) show great commercialization prospects, their power conversion efficiencies (PCEs) still fall behind their small molecule acceptor-based counterparts. In all-polymer blends, the optimized morphology and high molecular ordering are difficult to achieve since there is troublesome competition between the crystallinity of the polymer donor and acceptor during the film-formation process. Therefore, it is challenging to improve the performance of all-PSCs. Herein, a ternary strategy is adopted to modulate the morphology and the molecular crystallinity of an all-polymer blend, in which PM6:PY-82 is selected as the host blend and PY-DT is employed as a guest component. Benefiting from the favorable miscibility of the two acceptors and the higher regularity of PY-DT, the ternary matrix features a well-defined fibrillar morphology and improved molecular ordering. Consequently, the champion PM6:PY-82:PY-DT device produces a record-high PCE of 18.03%, with simultaneously improved open-circuit voltage, short-circuit current and fill factor in comparison with the binary devices. High-performance large-area (1 cm²) and thick-film (300 nm) all-PSCs are also successfully fabricated with PCEs of 16.35% and 15.70%, respectively.Moreover, 16.5 cm² organic solar module affords an encouraging PCE of 13.84% when using the non-halogenated solvent , showing the great potential of “Lab-to-Fab” transition of all-PSCs.

A method for organic solar cell stability testing is presented. It combines wavelength-dependent high-intensity photodegradation with transient electrical measurements and drift-diffusion simulations. The method is able to identify and localize degradation mechanisms in full-stack devices. Furthermore, the strong wavelength dependence and the effect of light intensity on the degradation of PM6:Y6-based cells are discussed.

Abstract

A method for organic solar cell (OSC) stability testing is presented that aims to provide more unique insight into the causes of degradation patterns of OSCs. The method involves using monochromatic light at high irradiation doses to accelerate isolated degradation mechanisms while monitoring the device with a series of in-situ steady-state and transient electrical measurements. The experimental results are accompanied by drift-diffusion simulations to localize degradation pathways. PM6:Y6-based OSCs are tested, which are known to show a rather broad range of lifetimes as a function of device architecture, material batches, or degradation conditions. The experiments reveal a degradation mechanism that causes an increased trap-state density inside the PM6:Y6 layer. The transient simulations suggest that these states are formed at or around the interface between the PM6:Y6 and the electron transport layer. Furthermore, the surprisingly dominant impact of the illuminating wavelength on the degradation pattern is evidenced. Lastly, the degradation rate of the devices scales linearly with light intensity, making high intensity and spectrally selective degradation the most promising way to accelerate stability testing for the faster development of stable OSCs.

High performance tandem all-polymer solar cells are fabricated by employing two complementary absorbing polymerized small molecule acceptors (PSMAs), a wide-band gap PSMA PIDT in front cell with PM7 as polymer donor and a narrow-bandgap PSMA PY-IT in rear cell with PM6 as polymer donor. The optimized device demonstrates a high power conversion efficiency of 17.87% with V _oc reaching 2.00 V.

Abstract

All-polymer solar cells (all-PSCs) possess distinguished advantages of excellent morphology stability, thermal stability, and mechanical flexibility. Tandem solar cells, by stacking two sub-cells, can absorb more photons in a wider wavelength range and can reduce thermal losses. However, limitation of polymer acceptors with suitable bandgaps hinders the development of tandem all-PSCs. Herein, highly efficient tandem all-PSCs are fabricated by employing two polymerized small molecular acceptors (PSMAs) of wide bandgap PIDT (1.66 eV) in the front cell and narrow bandgap PY-IT (1.4 eV) in the rear cell. The two sub-cells with the polymer donors of PM7 in front cell and PM6 in rear cell show high open circuit voltage (V _oc) of 1.10 V for the front cell and 0.94 V for the rear cell. By rational device optimizations, the best power conversion efficiency of 17.87% is achieved for the tandem all-PSCs with high V _oc of 2.00 V. 17.87% is one of the highest efficiency for the all-PSCs, and 2.00 V is one of the highest V _oc for all the tandem organic solar cells. Moreover, the tandem all-PSCs show excellent thermal and light-soaking stability compared with their small-molecule counterparts. The results provide insight to the potential of bandgap tuning in PSMAs, and indicate that the tandem architecture is an effective strategy to boost performance of the all-PSCs.

A series of novel imide-functionalized nanographenes is investigated as acceptor components in inverted bulk-heterojunction solar cells in combination with donor polymer PM6. The best performance is observed for derivatives that are able to self-assemble into dimers, reaching power conversion efficiencies of up to 7.1%.

A series of novel imide-functionalized C₆₄ nanographenes is investigated as acceptor components in organic solar cells (OSCs) in combination with donor polymer PM6. These electron-poor molecules either prevail as a monomer or self-assemble into dimers in the OSC active layer depending on the chosen imide substituents. This allows for the controlled stacking of electron-poor and electron-rich π–scaffolds to establish a novel class of non-fullerene acceptor materials to tailor the bulk-heterojunction morphology of the OSCs. The best performance is observed for derivatives that are able to self-assemble into dimers, reaching power conversion efficiencies of up to 7.1%.

A low-temperature processed PC61BM:ZnO hybrid thin film is developed using a PC61BM-doped diethylzinc precursor. The structural, chemical, optical, and electrical characterizations of PC61BM-doped ZnO establish it as an efficient cathode buffer layer (CBL) for interface engineering in organic solar cells. The optimized energy-level alignment at the CBL interface is realized because of surface-rich self-assembled graded fullerene isomers.

In organic solar cells (OSCs), improving the properties at the interface of the bulk heterojunction (BHJ) photoactive layer and the buffer layer is desired, but achieving a high-quality interface between these two layers is inevitably challenging. Herein, a novel (PC61BM)-doped zinc oxide, that is, a (PC61BM-ZnO)_DEZ hybrid thin film, is proposed for application as a cathode buffer layer (CBL) for OSCs based on both fullerene-based acceptors and nonfullerene acceptors. These thin films require low annealing temperatures and can be deposited via a one-step solution-processable method using a hybrid precursor composed of PC61BM and diethylzinc solution (DEZ). The study reveals that the performance of devices with (PC61BM-ZnO)_DEZ hybrid thin films as a CBL exceeds that of devices with thin films of conventional ZnO as a CBL. Impedance analysis of the fabricated OSCs suggests improved charge collection efficiency in the devices with (PC61BM-ZnO)_DEZ thin film. Optical, electrical, morphological, and compositional characterizations of thin films used as CBLs confirm the presence of a PC61BM-rich phase with traces of d-C61 (fullerene dimer) at the surface of the hybrid (PC61BM-ZnO)_DEZ thin film. These phases help in tuning the interfacial properties by ensuring better coupling between the BHJ photoactive layer and the CBL.

A dopant-free polymeric hole transport material (HTM) is designed and synthesized using a thienothiophene group as a π-bridge to connect donors and acceptors, and remarkable power conversion efficiency over 23% and long-term stability are achieved in perovskite solar cells (PSCs), which offers a practical method to enhance photovoltaic performance of dopant-free HTM-based PSCs.

Abstract

Hole transport materials (HTMs) play essential roles in achieving high photovoltaic performance and long-term stability in the n–i–p structure of perovskite solar cell (PSC) devices. Recently, dopant-free polymeric materials as HTMs in PSCs have attracted considerable attention owing to high carrier mobility and excellent hydrophobicity. However, achieving similar efficiencies to those of doped small molecule HTMs such as Spiro-OMeTAD is a big challenge. Herein, a thienothiophene π-bridge is selected as a stabilizer and energy level regulator incorporated into a donor–acceptor-type HTM to synthesize a new polymer, Nap-SiBTA. The incorporation of the thienothiophene group improves the thermal stability and favors the high planarity and face-on orientation, promoting high charge carrier mobility and tunable optical band gap. Finally, the dopant-free polymer Nap-SiBTA-based PSC achieves an excellent power conversion efficiency (PCE) of 23.07% with a high fill factor of 80.85%. To the best of the authors’ knowledge, this is one of the best efficiencies in dopant-free HTM PSCs. Moreover, the unencapsulated device retains 93% of its initial PCE after 1000 h owing to the excellent hydrophobicity of Nap-SiBTA. This work provides a general and practical method to design dopant-free HTMs for the high efficiency and long-term stability of PSCs.

Fibrillization of non-fullerene acceptor L8-BO is realized by employing a conjugated fused-ring solvent additive 1-fluoronaphtalene that acts as the molecular bridge, which contributes to realize a power conversion efficiency of 19% in the pseudo-bulk heterojunction D18/L8-BO binary organic solar cell, featuring a high fill factor of 80% with improved charge transport.

Abstract

The structural order and aggregation of non-fullerene acceptors (NFA) are critical toward light absorption, phase separation, and charge transport properties of their photovoltaic blends with electron donors, and determine the power conversion efficiency (PCE) of the corresponding organic solar cells (OSCs). In this work, the fibrillization of small molecular NFA L8-BO with the assistance of fused-ring solvent additive 1-fluoronaphthalene (FN) to substantially improve device PCE is demonstrated. Molecular dynamics simulations show that FN attaches to the backbone of L8-BO as the molecular bridge to enhance the intermolecular packing , inducing 1D self-assembly of L8-BO into fine fibrils with a compact polycrystal structure. The L8-BO fibrils are incorporated into a pseudo-bulk heterojunction (P-BHJ) active layer with D18 as a donor, and show enhanced light absorption, charge transport, and collection properties, leading to enhanced PCE from 16.0% to an unprecedented 19.0% in the D18/L8-BO binary P-BHJ OSC, featuring a high fill factor of 80%. This work demonstrates a strategy for fibrillating NFAs toward the enhanced performance of OSCs.

Two non-fused ring electron acceptors (NFREAs) have been developed. The halogen substituents on the aromatic side chains, as the new structure design tools, not only facilitate the construction of 3D stacks in solid, but also optimize the optoelectronic properties of the NFREAs, leading to organic solar cells with 16.2 % efficiency and excellent operational stability.

Abstract

Searching the cost-effective organic semiconductors is strongly needed in order to facilitate the practice of organic solar cells (OSCs), yet to be fulfilled. Herein, we have succeeded in developing two non-fused ring electron acceptors (NFREAs), leading to the highest efficiency of 16.2 % for the NFREA derived OSCs. These OSCs exhibit the superior operational stabilities under one sun equivalent illumination without ultraviolet (UV) filtration. It is revealed that the modulation of halogen substituents on aromatic side chains, as the new structural tool to tune the intermolecular interaction and optoelectronic properties of acceptors, not only promotes the interlocked tic-tac-toe frame of three-dimensional stacks in solid, but also improves charge dynamics of acceptors to enable high-performance and stable OSCs.

We report an unprecedentedly high power- and energy-density supercapacitor through the chlorine respiration in porous carbon materials. Both electrochemical results and theoretical calculations show that porous carbon with pore size around 3 nm delivers the best chlorine evolution and adsorption performance.

Abstract

Supercapacitor represents an important electrical energy storage technology with high-power performance and superior cyclability. However, currently commercialized supercapacitors still suffer limited energy densities. Here we report an unprecedentedly respiring supercapacitor with chlorine gas iteratively re-inspires in porous carbon materials, that improves the energy density by orders of magnitude. Both electrochemical results and theoretical calculations show that porous carbon with pore size around 3 nm delivers the best chlorine evolution and adsorption performance. The respiring supercapacitor with multi-wall carbon nanotube as the cathode and NaTi₂(PO₄)₃ as the anode can store specific energy of 33 Wh kg⁻¹ with negligible capacity loss over 30 000 cycles. The energy density can be further improved to 53 Wh kg⁻¹ by replacing NaTi₂(PO₄)₃ with zinc anode. Furthermore, thanks to the extraordinary reaction kinetics of chlorine gas, this respiring supercapacitor performs an extremely high-power density of 50 000 W kg⁻¹.

The use of the transparent ZnO nanoparticles/Ag nanowire/ZnO nanoparticles electrode in an inverted organic solar cell improves the optical properties and shortens the lifetime of the polymer excitons. Improved performances compared to the indium tin oxide-based reference are recorded, including a 20% increase in short-circuit current. Despite the use of high-absorbing thick active layer, the plasmonic electrodes are efficient.

Transparent electrodes are a key component in the manufacturing of optoelectronic devices such as light-emitting diodes, touch screens, and solar cells. The transparent electrode commonly used in this field is based on indium tin oxide (ITO). It contains indium, which is an expensive, rare, difficult to extract, and depleting element, hence the need to find an alternative. Silver nanowire (AgNW) electrodes are one of the best alternatives due to their excellent electrical, optical, and mechanical properties. Herein, it is shown that embedding AgNWs between two layers of ZnO nanoparticles (ZnONPs) leads to superior optical and electrical performance. The validation of the ZnONPs/AgNWs/ZnONPs electrode using the reference PF2:PC70BM organic active layer, known to present a long carrier lifetime, in inverted solar cell architectures shows an increased absorption of the active layer. This enhancement is due to the electrical field resulting from plasmonic resonance because the absorption is proportional to the square of the electric field amplitude. Through photoluminescence spectroscopy, a shorter exciton lifetime in PF2 in the presence of AgNWs is also observed. All these processes lead to improved photovoltaic performance compared to ITO-based reference, with approximately 20% increase in photocurrent and overall power conversion efficiency.

Highly efficient green thick-film organic solar cells are realized by balancing the crystallinity of donor and acceptor via combining hot slot-die coating and ternary strategy.

Abstract

The power conversion efficiency (PCE) of organic solar cells (OSCs) has reached high values of over 19%. However, most of the high-efficiency OSCs are fabricated by spin-coating with toxic solvents and the optimal photoactive layer thickness is limited to 100 nm, limiting practical development of OSCs. It is a great challenge to obtain ideal morphology for high-efficiency thick-film OSCs when using non-halogenated solvents due to the unfavorable film formation kinetics. Herein, high-efficiency ternary thick-film (300 nm) OSCs with PCE of 15.4% based on PM6:BTR-Cl:CH1007 are fabricated by hot slot-die coating using non-halogenated solvent (o-xylene) in the air. Compared to PM6:BTR-Cl:Y6 blends, the stronger pre-aggregation of CH1007 in solution induces the earlier aggregation of CH1007 molecules and longer aggregation time, and thus results in high and balanced crystallinity of donors and acceptor in CH1007-based ternary film, which led to high-carrier mobility and suppressed charge recombination. The ternary strategy is further used to fabricate high-efficiency, thick-film, large-area, and flexible devices processed from non-halogenated solvents, paving the way for industrial development of OSCs.

In this work, well-designed polymer acceptor materials for layer-by-layer (LbL) processing are combined with a ternary strategy to improve the active layer morphology. The optimized LbL-type ternary system not only shows the best efficiency of 18.14% among the all-polymer solar cells but also exhibits insensitivity to the active layer thickness ranging from 82–180 nm.

Abstract

Achieving a finely tuned active layer morphology with a suitable vertical phase to facilitate both charge generation and charge transport has long been the main goal for pursuing the highly efficient bulk heterojunction all-polymer solar cells (all-PSCs). Herein, a solution to address the above challenge via synergistically combining the ternary blend strategy and the layer-by-layer (LbL) procedure is proposed. By introducing a synthesized polymer acceptor (P _A), PY-Cl, with higher crystallinity into the designed host acceptor PY-SSe-V, vertical phase distribution and molecular ordering of the LbL-type ternary all-PSCs can be improved in comparison to the LbL-type PM6/PY-SSe-V binary all-PSCs. The formation of the superior bulk microstructure can not only promote charge transport and extraction properties but also reduce energetic disorder and non-radiative recombination loss, thus improving all three photovoltaic parameters simultaneously. Consequently, the PM6/(PY-SSe-V:PY-Cl) ternary all-PSCs show the best efficiency of 18.14%, which is among the highest values reported to date for all-PSCs. This work provides a facile and effective LbL-type ternary strategy for obtaining high-efficiency all-PSCs.

A versatile and low-cost 4-chlorothiazole-based polymer donor PBTTz3Cl is designed and synthesized. PBTTz3Cl not only exhibits excellent photovoltaic performances with various small molecule acceptors (e.g., BTP-Ec9 and L8-BO), but also possesses a decent power conversion efficiency of 19.12% in ternary devices when blended with BTP-Ec9:L8-BO mixture.

Abstract

Benefiting from the emergence of narrow-band-gap small-molecule acceptors (SMAs), especially “Y” series, the power conversion efficiency (PCE) of polymer solar cells (PSCs) is rapidly improved. However, polymer donors with high efficiency, easy synthesis, and good universality are relatively scarce except PBDB-TF and D18. Herein, two polymer donors are designed and synthesized based on 4-chlorothiazole derivatives with simple structures, namely PTz3Cl and PBTTz3Cl. The OSCs based on PBTTz3Cl with slightly weaker intermolecular forces in comparison to PTz3Cl exhibits a decent PCE of 18.38% in blending with SMA L8-BO, owing to its strong donor/acceptor interaction with L8-BO, which shapes suitable phase separation morphology. Further research finds that PBTTz3Cl can exhibit excellent photovoltaic performances with various SMA materials, highlighting its universality. Based on this, ternary PSCs are designed where BTP-eC9 is introduced as a guest into the PBTTz3Cl:L8-BO host system. Thanks to further optimal blend morphology and more balanced charge transport, the PCE is improved up to 19.12%, which is among the highest values for PSCs. This work provides a new design of low-cost electron-deficient units for constructing highly versatile, high-performance polymer donors.

A series of conjugated polymers by side chain engineering and used as additive, which enable a record power conversion efficiency of 19.10% and decent high FF of 80.5% in layer-by-layer processed organic solar cells.

Abstract

The morphology plays a key role in determining the charge generation and collection process, thus impacting the performances of organic solar cells (OSCs). The limited selection pool of additives to optimize the morphology of OSCs, especially for the emerging layer-by-layer (LbL) OSCs, impeding the improvements of photovoltaic performances. Herein, a new method of using conjugated polymers as the additives to optimize the morphology for improving the photovoltaic performances of LbL-OSCs is reported. Four polymers of PH, PS, PF, and PCl are developed with different side chains. These polymers exhibit poor performances as donor materials and additives in the BHJ devices, due to the unsuitable energy level alignment and unfavorable molecular interactions. By contrast, they can be served as efficient additives to optimize the PM6 fibril matrix for facilitating the penetration of BTP-eC9 and forming an intertwined D/A bicontinuous network with a vertical segregation. Such morphology is optimized by side chain engineering, which enables the progressive improvement of the charge separation and collection. As a result, adding a small amount of PCl as the additive, the optimized morphology contributes to a champion PCE of 19.10% with a high FF of 80.5%.