Yingzhi Jin

Nature Energy, Published online: 10 November 2022; doi:10.1038/s41560-022-01155-x

A polyfluoroalkyl-containing guest acceptor (EH-C₈F₁₇) enables self-stratification in the active layer of bulk-heterojunction organic solar cells. The favorable vertical phase separation and molecular stacking increases the mobility, increases carrier lifetimes, and reduces trap-assisted recombination, leading to significantly improved device performance.

Abstract

The elaborate control of the vertical phase distribution within an active layer is critical to ensuring the high performance of organic solar cells (OSCs), but is challenging. Herein, a self-stratification active layer is realised by adding a novel polyfluoroalkyl-containing non-fullerene small-molecule acceptor (NFSMA), EH-C₈F₁₇, as the guest into PM6:BTP-eC9 blend. A favourable vertical morphology was obtained with an upper acceptor-enriched thin layer and a lower undisturbed bulk heterojunction layer. Consequently, a power conversion efficiency of 18.03 % was achieved, higher than the efficiency of 17.40 % for the device without EH-C₈F₁₇. Additionally, benefiting from the improved charge transport and collection realised by this self-stratification strategy, the OSC with a thickness of 350 nm had an impressive PCE of 16.89 %. The results of the study indicate that polyfluoroalkyl-containing NFSMA-assisted self-stratification within the active layer is effective for realising an ideal morphology for high-performance OSCs.

Sandwich-structured perovskite solar cells (SS-PSCs) are constructed by coating protection layer at the back of device, to tune the neutral plane place into the brittle perovskite films. Benefiting from the water-resisting Al₂O₃/polyethylene terephthalate (PET), the SS-PSCs demonstrate high environment stability. More importantly, the SS-PSCs exhibit excellent bending and folding stability, which is one of the best mechanical stable devices.

Foldable perovskite solar cells (PSCs) have promising applications in self-powered wearable and portable electronic devices. However, the folding stability requires further improvement through mitigating the strain in brittle layers under folding deformation. Herein, sandwich-structured PSCs (SS-PSCs) are constructed by coating a protection layer at the back of the device, to tune the neutral plane (NP) place into the brittle perovskite films. To accurately regulating the NP place, the effect of protective layer thickness on the strain distribution in PSCs is simulated. The SS-PSCs are experimentally prepared using Al₂O₃/polyethylene terephthalate (PET) protection layer with optimum thickness, combined with ultrathin silver foldable electrodes. Benefitting from the water-resisting Al₂O₃/PET, the SS-PSCs demonstrate high environment stability which maintains 85.1% of original efficiency after 700 h in humidity of 50% ± 10%. More importantly, the SS-PSCs exhibit excellent mechanical stability, which maintains >90% of the initial efficiency after bending with 4000 cycles at radius of 0.2 mm, and even 67.2% of the initial efficiency after 50 direct folding cycles. As far as it is known, it is one of the best mechanical stable devices. The findings pave the way for realizing not only foldable PSCs but also other foldable optoelectronic devices, such as sensors and organic light emitting devices.

Polymer solar cells are shown with a hole transport layer based on the same polymer as the one used in the bulk heterojunction. By doping, its work function is strongly affected to maximize the open-circuit voltage. This homojunction approach is promising and applies to recent trending semiconducting polymers with deep-lying energetics.

Hole transporting layers (HTL) in polymer solar cells remain a subject of importance to enable enhanced efficiency and stability compared to the benchmark PEDOT:PSS. The design of an interlayer based on the same polymer as the one used in the bulk heterojunction (BHJ) is reported here. In this HTL, the polymer is doped, thus forming a so-called homojunction. The conductivity of PTQ10 doped with magic blue is optimized for varying dopant concentrations. The resulting solar cells show competing power conversion efficiency as the widely used PEDOT:PSS and improved stability. This strategy opens the route toward the development of deep-lying work function HTL and is promising for future BHJ materials with deep-lying highest occupied molecular orbital polymers.

Ternary polymer solar cells (PSCs) are fabricated with polymer PM6 as donor, and two fluorinated and chlorinated nonfullerene acceptors (L8-BO and eC9-2Cl) with similar chemical structures as acceptors. Incorporating the chlorinated material eC9-2Cl can effectively broaden the absorption spectrum, elevate the open-circuit voltage, and improve the crystallinity and charge transport, leading to a high power conversion efficiency of 18.66%.

Organic semiconducting materials with fluorine or chlorine substitution are commonly designed to prepare efficient polymer solar cells (PSCs) through finely modulating the energy levels and absorption spectra. Herein, two small molecular acceptors with fluorine substitution L8-BO or chlorine substitution eC9-2Cl are selected to prepare ternary PSCs with PM6 as polymer donor. The optimal ternary PSCs exhibit a power conversion efficiency of 18.66%, benefiting from the simultaneously increased open-circuit voltage (V _OC) of 0.89 V, short-circuit current density (J _SC) of 26.63 mA cm⁻², and fill factor of 78.74% when the content of chlorine substitution eC9-2Cl is about 20 wt% in acceptors. Fluorinated and chlorinated substitutions can improve the V _OC or J _SC of the corresponding binary PSCs, which can be recombined into efficient ternary PSCs through optimizing their content in acceptors. Chlorinated materials have special crystalline conditions due to the specific features of the chlorine atoms with large atomic radius and the empty 3d orbits compared to the corresponding fluorinated materials. Adding an appropriate amount of the chlorine substitution eC9-2Cl further enhances the crystallization and intermolecular interaction of the ternary PSCs, which is beneficial for charge transport in the active layer and for device performance improvement.

The electron transport and photovoltaic performance of double-cable polymers (DCPs) with pendent rylene diimides suffer from the discontinuous acceptor network. Introducing an appropriate number of twisted dimeric rylene diimides in the DCPs can enable donor:acceptor bicontinuous networks via short contacts, paving the way toward independent, efficient, balanced charge transport for efficient DCP-based organic solar cells.

Double-cable polymers (DCPs) with donors and acceptors in the same molecular matrix exhibit excellent stability and have great potential in the practical application of organic solar cells (OSCs). However, such chemical structures of DCPs make the morphology and charge transport very complicated. Here, the packing structures and charge transport channels are revealed for the DCPs with pendant acceptors of rylene diimides. Because of strong π–π overlap, the number of neighboring acceptors is limited and the acceptors cannot form a percolation network for electron transport. Importantly, introducing twisted dimeric rylene diimides can significantly increase the number of neighbors and connectivity for the acceptors via short contacts. Moreover, donor:acceptor bicontinuous networks can be achieved by tuning the dimerization ratio. This opens an avenue toward independent, efficient hole, and electron transport for efficient DCP-based OSCs.

A combined homo hydrocarbon solvent sequential deposition process is demonstrated for all-polymer solar cells (all-PSCs) fabrication. A champion power conversion efficiency (PCE) of 17.7% with a remarkably high fill factor of 77.7% is achieved for rigid all-PSCs based on PM6:PY-V-γ, while the corresponding flexible device also enables an impressive PCE of 14.5% with excellent storage, thermal, photo, and mechanical stabilities.

Abstract

All-polymer solar cells (all-PSCs) have achieved impressive progress in photovoltaic performance and stabilities recently. However, their power conversion efficiencies (PCEs) still trail that of small-molecular acceptor-based organic solar cells (>19%) mainly because of the inferior fill factor (FF). Herein, a combined homo hydrocarbon solvent and sequential deposition (SD) strategy is presented to boost the FF of rigid all-PSCs to 77.7% and achieve a superior PCE of 17.7% with excellent stability, which is among the highest efficiencies reported for all-PSCs thus far. Meanwhile, a remarkable PCE of 14.5% is realized for flexible all-PSCs with outstanding mechanical stability. The blend film morphologies measurements suggest that the SD method enables the formation of an ideal pseudo-bilayer film with bicontinuous interdigitated structure and ordered polymer packing. The numerical simulation result indicates that the FF enhancement mainly results from the efficient exciton diffusion dynamics, increased carrier mobilities, and more balanced electron/hole mobility ratio induced by the developed SD method. This is also confirmed by the FF loss analysis, which manifests that the reduced series resistance and increased shunt resistance are the main reasons for the reduction of FF loss. This work provides a promising strategy to fabricate highly efficient and stable all-PSCs to promote their future development and practical manufacturing.

A solvent-induced anti-aggregation (SIAA) strategy is proposed to cope with the severe aggregation action of the small-molecule electron-transporting layer via the mixing of ethanol and trifluoroethanol solvents at an optimal volume ratio. A champion power conversion efficiency of 19.0% is yielded based on the PM6:L8-BO system after the SIAA treatment.

Abstract

The severe aggregation property of the small molecule electron-transporting layer (ETL) not only deteriorates the photovoltaic performance and operational reliability but also constrains its compatibility with large-scale coating techniques. Herein, by applying N,N′-Bis{3-[3-(Dimethylamino)propylamino]propyl}perylene-3,4,9,10-tetracarboxylic diimide (PDINN) (a well-known ETL) as a demo, a solvent-induced anti-aggregation (SIAA) strategy is proposed to cope with these hurdles via the mixing of ethanol and trifluoroethanol solvents at an optimal volume ratio. In situ photoluminescence and dynamic light scattering synergistically reveals the suppressed aggregation behavior of the SIAA-treated PDINN dispersion during the film-forming process. Owing to this amendment, the film quality and electron-transport capability of the PDINN layer are remarkably enhanced. In consequence, based on the PM6:L8-BO system, a champion power conversion efficiency (PCE) of 19.0% together with an impressive fill factor of 80.6% is harvested. A 1 cm² device with an excellent PCE of 16.6% is also fabricated using the doctor-blading SIAA-treated PDINN ink. More strikingly, this SIAA treatment impels better reliability under long-term shelf-lifetime and thermal stress periods. This work provides a promising and tractable approach to address the inherent self-aggregation issue of electron-transporting materials, which is beneficial for the development of efficient and stable organic optoelectronic devices.

Two benzotriazole-based polymer acceptors, PTz-BO and PTz-C11, featuring the same molecular backbone and different side chain are synthesized. Compared to PTz-C11, PTz-BO shows slightly blueshifted absorption and a higher lowest unoccupied molecular orbitals energy level. The ternary all-polymer solar cells (all-PSCs) based on a combination of PTz-C11 and PTz-BO yield a high efficiency of 16.58%, representing the highest efficiency reported for benzotriazole-based all-PSCs thus far.

Abstract

The power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs) have already exceeded 17%. However, the limited absorption range of an all-polymer system results in significantly reduced short-circuit current density (J _sc), which eventually influences the PCE improvement. To broaden the light absorption of polymer acceptors, herein, benzotriazole is introduced in the core unit of small molecule acceptors and thus two narrow-bandgap polymer acceptors named PTz-BO and PTz-C11 featuring the same molecular backbone and different side-chain length are synthesized. Compared with PTz-C11, the PTz-BO based-all PSCs deliver a slightly reduced J _sc, a large open-circuit voltage (V _oc) and a low voltage loss below 0.50 V. Moreover, ternary all-PSCs are constructed by introducing PTz-C11 as a guest component. Benefiting from the reduced recombination, improved exciton generation and dissociation, and balanced charge transport, a high efficiency of 16.58% is obtained for the ternary all-PSCs, with a high J _sc over 25 mA cm⁻² without sacrificing the V _oc. Such result represents the highest efficiency reported for benzotriazole-based all-PSCs in the literature thus far. This work demonstrates the great potential of benzotriazole for the synthesis of efficient narrow-bandgap polymer acceptors.

Hierarchical and porous lignocellulose-based hydrogels are fabricated for high-performance and salt-resistant solar steam generation. The presence of lignin tunes the water into an intermediate state and reduces the evaporation enthalpy of the water in the hydrogels, which endows the evaporator with an evaporation rate of 1.84 kg m⁻² h⁻¹ and a photothermal conversion efficiency of 86.5% under 1 sun irradiation.

Abstract

Solar-driven interfacial evaporation is an important approach for solving the issue of freshwater scarcity. However, the practical application of solar steam generation is hindered by high fabrication cost and environmental concerns regarding the petroleum-based materials. Herein, lignocellulose (cellulose-lignin composite) hydrogel (LCG) and lignin-derived carbon (LC) are used as the substrate and photothermal material, respectively, to construct a fully lignocellulose-based double-layered hydrogel (LC@LCG) evaporator. Results indicate that LC has an ultrahigh specific surface area and full-spectrum solar absorption of 98%. The presence of lignin can improve the hydrophilicity and maintain the capillary channels of the hydrogel, which tunes water into an intermediate state and reduces the vaporization enthalpy of water. Moreover, it ensures a high water transport rate in the hydrogel. Based on these advantages, the evaporation rate and photothermal conversion efficiency of hydrogel evaporator reach 1.84 kg m⁻² h⁻¹ under one sun and 86.5%, respectively. The lignocellulosic hydrogel evaporator could remove >99.95% of primary metal ions from seawater to generate fresh water, and shows outstanding salt resistance, durability, and long-term stability for desalination. This study demonstrates an eco-friendly and economic solution for continuous freshwater production from seawater using a fully lignocellulosic biomass-based hydrogel evaporator.

The nanoimprint lithography combined with donor/acceptor sequential deposition dual-functionalized regulation strategy can optimize the vertical composition gradient of the active layer. High-quality PM6 nanograting is fabricated, which is beneficial to precisely controlling vertical bicontinuous donor/acceptor network and building directional charge transport channels, thus endowing the imprinted pseudo-planar heterojunction organic solar cells with an improved power conversion efficiency of 17.36%.

Abstract

Although suitable vertical phase separation morphology in organic solar cells (OSCs) can be obtained by the donor/acceptor sequential deposition (SD) method, the lack of precisely adjusting vertical composition gradient and molecular crystallinity is a key limitation. Here, nanoimprint lithography (NIL) combined with SD dual-functionalized regulation strategy is first used to fabricate high-performance pseudo-planar heterojunction (PPHJ) OSCs, which is conducive to constructing vertical bi-continuous donor/acceptor network to provide sufficient charge separation interface area and orderly charge transport channels. PM6 donor with regular periodic nanograting structure and improved crystallinity is formed via NIL, effectively avoiding the erosion problem ascribed from the subsequent depositing of the Y6 acceptor. Furthermore, the finite-different time-domain (FDTD) measurement is employed to confirm the vertical composition gradient of the donor/acceptor, revealing a strong regular light absorption and reduced voltage loss. As a result, the best-imprinted device enables a power conversion efficiency as high as 17.36%, which is higher than the control SD-based device (15.46%). It is the first time to obtain high-quality PM6 nanograting by NIL, which can provide an avenue to form favorable phase separation morphology and adjust the vertical composition gradient for the high-performance PPHJ OSCs.

As the conjugation length of the intermediate donor block increases, the power conversion efficiency and operation photostability improved simultaneously. The PB-b-PY-4 device yields a higher efficiency of 13.28% than those of other conjugated block copolymers, which is also one of the highest values for single-component organic solar cells (SCOSCs) reported to date. This study demonstrates the effectiveness of the conjugated length of intermediate D-blocks in designing high-performance conjugated donor-acceptor block copolymers, which paves the way toward developing high-performance and more stable SCOSCs.

Abstract

Batch-to-batch variation widely exists in conjugated donor-acceptor (D-A) block copolymer materials and plays a crucial role in photovoltaic performance of organic solar cells (OSCs). To investigate the influence of conjugated-length of the intermediate block on the performance of single-component OSC (SCOSCs), herein, four batches of conjugated block copolymers (CBCs, PB-b-PY-1, PB-b-PY-2, PB-b-PY-3, and PB-b-PY-4) are synthesized, which possess different D/A block lengths. As the conjugation length of intermediate D-block increases, the lamellar packing order of these CBCs shows a monotonically increasing trend, leading to stronger absorption spectra in the visible region, efficient charge transfer, and suppressed carrier recombination loss. Consequently, the PB-b-PY-4 device yields a higher efficiency of 13.28% than those of other CBCs. This study demonstrates the effectiveness of the conjugated length of intermediate D-blocks in designing high-performance conjugated D-A block copolymers, which paves the way toward developing high-performance SCOSCs.

The atomic optimization is conducted by replacing sulfur in FO-2Cl with selenium, thus affording FOSe-2Cl. A systematical investigation to reveal effects of selenium on energy levels, absorption, charge transfer dynamics and photovoltaic performance performed. Ultimately, the PM6:FOSe-2Cl-based device achieved an improved power conversion efficiency of 15.94% and a reduced energy loss of 0.670 eV.

Abstract

Atomic replacement on platforms of nonfullerene acceptor (NFA) with already excellent performance is expected to further optimize the energy levels, absorptions, and even charge transfer dynamics of NFAs effectively without greatly destroying their superior molecular conformations. On the basis of high-performance F-series NFAs, the structural optimization at atomic level is performed by replacing sulfur atoms in FO-2Cl with selenium atoms, thus affording a new NFA labeled as FOSe-2Cl. FOSe-2Cl not only inherits the good planar configuration of FO-2Cl, but also exhibits more suitable energy levels, redshifted absorption, enhanced molecular packing, and accelerative charge transfer/transport dynamics compared with those of FO-2Cl. With a widely used polymer PM6 as the donor, organic solar cell (OSC) based on FOSe-2Cl affords a significantly improved power conversion efficiency (PCE) of 15.94% with a reduced energy loss (E _loss) of 0.670 eV, with respect to that of FO-2Cl-based OSC with a PCE of 14.94% and E _loss of 0.706 eV. The result represents the best performance reported to date for pyran-fused NFAs and F-series NFAs-based binary OSCs, providing another promising platform to achieve the state-of-the-art OSCs in addition to the well-known Y-series NFAs.

The multiscale morphology and stretch-induced microstructural evolution of the high-efficiency ternary organic solar cell blends are probed in real-time with multiple synchrotron X-ray scattering techniques. The morphology–stretchability relations are established with stretchable metrics being predictable. This work sets a methodology for uncovering the regulation mechanisms of morphology, mechanics, and performance of stretchable organic electronics.

Abstract

The stretchability and stretch-induced structural evolution of organic solar cells (OSCs) are pivotal for their collapsible, portable, and wearable applications, and they are mainly affected by the complex morphology of active layers. Herein, a highly ductile conjugated polymer P(NDI2OD-T2) is incorporated into the active layers of high-efficiency OSCs based on nonfullerene small molecule acceptors to simultaneously investigate the morphological, mechanical, and photovoltaic properties and structural evolution under stretching of ternary blend films with various acceptor contents. The structural robustness of the blend films is indicated by their stretch-induced structural evolution, which is monitored in real-time by a combination of in situ wide/small angle X-ray scattering. It is found that adding the soft P(NDI2OD-T2) can enhance the stretchability and structural robustness of ternary blend films by more entangled chains and tie chains to dissipate strain. Furthermore, the stretchability of the ternary blends can be superbly predicted by a 3D equivalent box model. This work provides instructive insight and guidance for designing stretchable electronics and predicting the stretchability of multicomponent blends.

Highly efficient and stable flexible inverted perovskite solar cells are developed through modifying the interface between perovskite and hole transport layer via pentylammonium acetate molecule, which achieve a record power conversion efficiency of 23.68% (0.08 cm², certified: 23.35%) and excellent mechanical stability.

Abstract

Among the emerging photovoltaic technologies, rigid perovskite solar cells (PSCs) have made tremendous development owing to their exceptional power conversion efficiency (PCE) of up to 25.7%. However, the record PCE of flexible PSCs (≈22.4%) still lags far behind their rigid counterparts and their mechanical stabilities are also not satisfactory. Herein, through modifying the interface between perovskite and hole transport layer via pentylammonium acetate (PenAAc) molecule a highly efficient and stable flexible inverted PSC is reported. Through synthetic manipulation of anion and cation, it is shown that the PenA⁺ and Ac⁻ have strong chemical binding with both acceptor and donor defects of surface-terminating ends on perovskite films. The PenAAc-modified flexible PSCs achieve a record PCE of 23.68% (0.08 cm², certified: 23.35%) with a high open-circuit voltage (V _OC) of 1.17 V. Large-area devices (1.0 cm²) also realized an exceptional PCE of 21.52%. Moreover, the fabricated devices show excellent stability under mechanical bending, with PCE remaining above 91% of the original PCE even after 5000 bends.

A high-performance flexible photodetector is realized by gating an organic electrochemical transistor with a perovskite solar cell, outperforming previously reported flexible photodetectors, which can be attributed to the high transconductance of the transistor. The device can monitor photoplethysmogram signals and peripheral oxygen saturation under ambient light and even remotely. This novel device design demonstrates great potential in emerging wearable optoelectronics.

Abstract

Photodetectors (PDs) are the building block of various imaging and sensing applications. However, commercially available PDs based on crystalline inorganic semiconductors cannot meet the requirements of emerging wearable/implantable applications due to their rigidity and fragility, which creates the need for flexible devices. Here, a high-performance flexible PD is presented by gating an organic electrochemical transistor (OECT) with a perovskite solar cell. Due to the ultrahigh transconductance of the OECT, the device demonstrates a high gain of ≈10⁶, a fast response time of 67 µs and an ultrahigh detectivity of 6.7 × 10¹⁷ Jones to light signals under a low working voltage (≤0.6 V). Thanks to the ultrahigh sensitivity and fast response, the device can track photoplethysmogram signals and peripheral oxygen saturation under ambient light and even provide contactless remote sensing, offering a low-power and convenient way for continuous vital signs monitoring. This work offers a novel strategy for realizing high-performance flexible PDs that are promising for low-power, user-friendly and wearable optoelectronics.

A simple and environmentally friendly method, through boric acid, to remove the residual amine in zinc oxide (ZnO) without decomposing ZnO is reported. Consequently, the optimized tandem organic solar cells (OSCs) output of 19.56% power conversion efficiency and long-term illumination stability of OSCs are significantly improved.

Abstract

Owing to outstanding optoelectronic properties and simple preparation, zinc oxide (ZnO) has widely been used in organic solar cells (OSCs). Although versatile cathode interface materials have been designed in past, ZnO remains indispensable owing to its excellent overall performance. Therefore, solving the persistent problem of residual amine reacting with non-fullerene acceptors will make ZnO superior over other materials, and thus improve the performance and energy budget of OSCs. Herein, a simple, effective, and economical method for removing residual amine in ZnO without distorting ZnO is reported. By accurately comparing the alkalinities of ZnO and residual amine, boric acid (BA) is selected as the amine-removing agent because of its suitable acidic dissociation constant. Moreover, the high water solubility of BA ensures that the post-cleaning process can be easily performed. The work function, electron extraction, and stability of cathode interface layer are optimized through rinsing them with BA. Consequently, the power conversion efficiency (PCE) and stability of OSCs under long-term illumination are significantly improved. The optimal 0.04 and 1.00 cm² single-junction OSCs are based on PBDB-TF:HDO-4Cl:BTP-eC9 bulk heterojunction output 18.40% and 17.42% efficiencies, respectively. Furthermore, tandem OSCs based on the BA-treated ZnO exhibit a 19.56% PCE, demonstrating the reliability of this method.

The thermodynamic relaxation of small-molecule acceptors (SMAs) in its blend with polymer donor raises concerns related to the long-term operational stability of polymer solar cells. With flexible spacers to restrict the motion of individual SMAs, the tethered SMAs show higher efficiency, and, most important, large glass transition temperatures to suppress the thermodynamic relaxation in mixed domains.

Abstract

For polymer solar cells (PSCs), the mixture of polymer donors and small-molecule acceptors (SMAs) is fine-tuned to realize a favorable kinetically trapped morphology and thus a commercially viable device efficiency. However, the thermodynamic relaxation of the mixed domains within the blend raises concerns related to the long-term operational stability of the devices, especially in the record-holding Y-series SMAs. Here, a new class of dimeric Y6-based SMAs tethered with differential flexible spacers is reported to regulate their aggregation and relaxation behavior. In their polymer blends with PM6, it is found that they favor an improved structural order relative to that of Y6 counterpart. Most importantly, the tethered SMAs show large glass transition temperatures to suppress the thermodynamic relaxation in mixed domains. For the high-performing dimeric blend, an unprecedented open circuit voltage of 0.87 V is realized with a conversion efficiency of 17.85%, while those of regular Y6-base devices only reach 0.84 V and 16.93%, respectively. Most importantly, the dimer-based device possesses substantially reduced burn-in efficiency loss, retaining more than 80% of the initial efficiency after operating at the maximum power point under continuous illumination for 700 h. The tethering approach provides a new direction to develop PSCs with high efficiency and excellent operating stability.