Yingzhi Jin

The dependence of performance on composition in organic solar cells based on PffBT4T‐2DT polymer with O‐IDTBR or O‐IDFBR as a nonfullerene acceptor is investigated. The effect on morphology is discussed in terms of the interplay between immiscibility, inferred from phase behavior, and polymer aggregation. Morphology is optimized when polymer crystallite interconnectivity and size are balanced.

Abstract

The temperature‐dependent aggregation behavior of PffBT4T polymers used in organic solar cells plays a critical role in the formation of a favorable morphology in fullerene‐based devices. However, there is little investigation into the impact of donor/acceptor ratio on morphology tuning, especially for nonfullerene acceptors (NFAs). Herein, the influence of composition on morphology is reported for blends of PffBT4T‐2DT with two NFAs, O‐IDTBR and O‐IDFBR. The monotectic phase behavior inferred from differential scanning calorimetry provides qualitative insight into the interplay between solid–liquid and liquid–liquid demixing. Transient absorption spectroscopy suggests that geminate recombination dominates charge decay and that the decay rate is insensitive to composition, corroborated by negligible changes in open‐circuit voltage. Exciton lifetimes are also insensitive to composition, which is attributed to the signal being dominated by acceptor excitons which are formed and decay in domains of similar size and purity irrespective of composition. A hierarchical morphology is observed, where the composition dependence of size scales and scattering intensity from resonant soft X‐ray scattering (R‐SoXS) is dominated by variations in volume fractions of polymer/polymer‐rich domains. Results suggest an optimal morphology where polymer crystallite size and connectivity are balanced, ensuring a high probability of hole extraction via such domains.

Polymer aggregation and miscibility have been demonstrated to influence photovoltaic performance in nonfullerene‐based organic solar cells. Polymers having a strong tendency to aggregate are herein found to undergo aggregation prior to liquid–liquid phase separation and have a higher miscibility with nonfullerene acceptors, resulting in mixed donor–acceptor domains, stronger PL quenching, and a higher exciton dissociation efficiency.

Abstract

Understanding the correlation between polymer aggregation, miscibility, and device performance is important to establish a set of chemistry design rules for donor polymers with nonfullerene acceptors (NFAs). Employing a donor polymer with strong temperature‐dependent aggregation, namely PffBT4T‐2OD [poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3″′‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2″′‐quaterthiophen‐5,5‐diyl)], also known as PCE‐11 as a base polymer, five copolymer derivatives having a different thiophene linker composition are blended with the common NFA O‐IDTBR to investigate their photovoltaic performance. While the donor polymers have similar optoelectronic properties, it is found that the device power conversion efficiency changes drastically from 1.8% to 8.7% as a function of thiophene content in the donor polymer. Results of structural characterization show that polymer aggregation and miscibility with O‐IDTBR are a strong function of the chemical composition, leading to different donor–acceptor blend morphology. Polymers having a strong tendency to aggregate are found to undergo fast aggregation prior to liquid–liquid phase separation and have a higher miscibility with NFA. These properties result in smaller mixed donor–acceptor domains, stronger PL quenching, and more efficient exciton dissociation in the resulting cells. This work indicates the importance of both polymer aggregation and donor–acceptor interaction on the formation of bulk heterojunctions in polymer:NFA blends.

An elastic‐impact‐based nonresonant hybridized generator is designed for biomechanical energy harvesting at low frequency vibrations. Introducing a soft magnetic material‐based composite film as a flux‐concentrator and nanowire/nanofiber‐based triboelectric materials significantly improves the output performance of the hybridized generator. A customized power management circuit in the hybrid generator is designed to serve as a universal power source for smart electronics.

Abstract

The fast growth of smart electronics requires novel solutions to power them sustainably. Portable power sources capable of harvesting biomechanical energy are a promising modern approach to reduce battery dependency. Herein, a novel elastic impact‐based nonresonant hybridized generator (EINR‐HG) is reported to effectively harvest biomechanical energy from diverse human activities outdoors. Through the rational integration of a nonlinear electromagnetic generator with two contact‐mode triboelectric nanogenerators, the proposed EINR‐HG generates hybrid electrical output simultaneously under the same mechanical excitations. By introducing a flux‐concentrator with a nanowire‐nanofiber surface modification, significant improvement in the energy harvesting efficiency of the EINR‐HG is achieved. After optimizing using simulations and vibration tests, the as‐fabricated EINR‐HG delivers an outstanding normalized power density of 3.13 mW cm⁻³ g⁻² across a matching resistance of 1.5 kΩ at 6 Hz under 1 g acceleration. Under human motion testing, the EINR‐HG generates an optimal output power of 131.4 mW with horizontal handshaking. With a customized power management circuit, the EINR‐HG serves as a universal power source that successfully drives commercial smart electronics, including smart bands and smartphones. This work shows the massive potential of biomechanical energy‐driven hybridized generators for powering personal electronics and portable healthcare monitoring devices.

Azobenzene‐containing polymers with polymer chain entanglements have improved mechanical properties and reprocessability. Robust, stretchable, and flexible photoactuators are fabricated using the polymers. The polymer actuators show photoinduced reversible bending because of trans–cis photoisomerization. The polymers exhibit photoinduced reversible solid‐to‐liquid transitions that enable repairing and reprocessing of the polymer actuators with light.

Abstract

Photoactuators based on liquid crystal elastomers or networks are smart materials that show photoinduced motions. However, their crosslinked networks make their repair or reprocessing difficult. Here, a healable and reprocessable photoactuator is fabricated using entangled high‐molecular‐weight azobenzene‐containing polymers (azopolymers) that are non‐crosslinked. A series of linear liquid crystal azopolymers with different molecular weights are synthesized. The low‐molecular‐weight azopolymers (5–53 kg mol⁻¹) cannot form freestanding photoactuators because their polymer chains lack entanglements, which makes them hard and brittle. In contrast, flexible and stretchable actuators are fabricated using high‐molecular‐weight azopolymers (80–100 kg mol⁻¹) that exhibit good processability because of the polymer chain entanglements. The azopolymer photoactuators show photoinduced bending based on photoinduced trans–cis isomerization of the azopolymers on the irradiated side. The experiments show not only photoinduced phase transitions or changes in the order parameters but also photoinduced solid‐to‐liquid transition of the azopolymers resulting in shape changes and mechanical responses. Thus, photoinduced solid‐to‐liquid transition is a new mechanism for the design of photoactuators. Moreover, the azopolymer photoactuators are healable and reprocessable via solution processing or light irradiation. Healability and reprocessability prolong lifetimes of photoactuators are important for materials reusage and recycling, and represent a new strategy for the preparation of smart materials.

Highly efficient single‐doped thermally activated delayed fluorescence (TADF) WOLDs are developed with single emissive layers, in which the steric hindrance and mismatched frontier molecular orbital energy levels of yellow TADF dopants and blue TADF matrixes are utilized to restrain the excessive energy transfer, supporting the high‐quality cool white, pure white and warm white emissions with the 100% exciton utilization efficiencies.

Abstract

Despite their merits of environmental friendliness, low cost, and large‐scale production, thermally activated delayed fluorescence (TADF) based white organic light‐emitting diodes (WOLEDs) for daily lighting applications still face the formidable challenges of structural simplification and controllable exciton allocation. Here, the state‐of‐the‐art full‐TADF WOLEDs with features of the single‐doped single emissive layers (EMLs) and ultrasimple trilayer structure are demonstrated. The EMLs are binary systems as yellow TADF emitter (4CzTPNBu) doped blue TADF matrix (ptBCzPO₂TPTZ) with the large steric hindrance and mismatched frontier molecular orbital energy levels to effectively restrain excessive blue‐to‐yellow triplet exciton transfer and host‐dopant interaction induced triplet quenching. Simultaneously, Förster resonance energy transfer is utilized to optimize exciton allocation for the balance of blue and yellow emissions, giving rise to the photoluminescence quantum yield beyond 90%. Consequently, these single‐doped EMLs endow their cool white, pure white, and warm white diodes with the high‐quality and ultrastable white light and the 100% exciton utilization efficiencies through the extremely simple structures, making them competent for the diverse daily lighting applications.

Through a strategy of embedding cyclohexane‐1,4‐dione into the thieno[3,4‐b]thiophene unit, a highly electron‐deficient core (TTDO) is synthesized, and the corresponding donor polymer (PBTT‐F) is also developed. The nonfullerene photovoltaic device based on this new donor polymer exhibits an outstanding PCE of 16.1% with a very high fill factor of 77.1%, which demonstrates it a very promising donor for high‐performance solar cells.

Abstract

It is of great significance to develop efficient donor polymers during the rapid development of acceptor materials for nonfullerene bulk‐heterojunction (BHJ) polymer solar cells. Herein, a new donor polymer, named PBTT‐F, based on a strongly electron‐deficient core (5,7‐dibromo‐2,3‐bis(2‐ethylhexyl)benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione, TTDO), is developed through the design of cyclohexane‐1,4‐dione embedded into a thieno[3,4‐b]thiophene (TT) unit. When blended with the acceptor Y6, the PBTT‐F‐based photovoltaic device exhibits an outstanding power conversion efficiency (PCE) of 16.1% with a very high fill factor (FF) of 77.1%. This polymer also shows high efficiency for a thick‐film device, with a PCE of ≈14.2% being realized for an active layer thickness of 190 nm. In addition, the PBTT‐F‐based polymer solar cells also show good stability after storage for ≈700 h in a glove box, with a high PCE of ≈14.8%, which obviously shows that this kind of polymer is very promising for future commercial applications. This work provides a unique strategy for the molecular synthesis of donor polymers, and these results demonstrate that PBTT‐F is a very promising donor polymer for use in polymer solar cells, providing an alternative choice for a variety of fullerene‐free acceptor materials for the research community.

Nature Photonics, Published online: 20 January 2020; doi:10.1038/s41566-019-0573-5

Two hydroxyl‐functionalized nonfullerene acceptors, IT‐OH and IT‐DOH, are synthesized, showing improved molecular arrangements and crystallinity compared to the parent ITIC molecule. This is attributed to intermolecular hydrogen bonds elongating conjugated planes and thus leading to long‐range‐ordered structures via π–π stacking. A best efficiency of 12.5% is achieved from the IT‐DOH‐based nonannealed solar cell with good device stability.

Abstract

Various substituents have been incorporated into nonfullerene acceptors (NFAs) to modulate absorption scopes and energy levels for boosting efficiencies of organic solar cells (OSCs). The manipulation of the NFAs' molecular order and crystallinity via those substitutions is equally crucial to OSC performances, which yet remains interesting and challenging. The hydroxyl group, which can potentially form strong intermolecular hydrogen bonds (H‐bonds) for improving molecular arrangements, has, however, never been considered. Herein, two hydroxyl‐functionalized NFAs, IT‐OH with one hydroxyl and IT‐DOH with two hydroxyls, are synthesized to tune the molecular packing and crystallinity. The ordered molecular arrangement and higher crystallinity are observed for the IT‐OH and IT‐DOH than the parent ITIC. This is assigned to the formation of intermolecular H‐bonds induced by the hydroxyls, which elongates molecular conjugated planes leading to long‐range‐ordered structures via π–π stacking. By the appropriate crystallinity and miscibility with donor polymer, an IT‐DOH‐based nonannealed OSC affords an efficiency of 12.5% with good device stability. This work provides a promising strategy to tune the molecular packing and crystallinity to design NFAs by introducing hydroxyl groups.

A new nonfullerene acceptor (TOBDT) with asymmetrical side chains is rationally designed and reported. Compared to the PM6:TOBDT binary devices, the as‐cast ternary devices based on PM6:IDIC:TOBDT obtain improved J _sc, V _oc, and fill factor values simultaneously, leading to a high power conversion efficiency of 14.0%, which is among the best reported for as‐cast fullerene‐free ternary devices.

Abstract

A new small‐molecule nonfullerene acceptor based on the benzo[1,2‐b:4,5‐b′]dithiophene (BDT) fused central core with asymmetrical alkoxy and thienyl side chains, namely TOBDT, is designed and synthesized. The alkoxy unit helps narrow the bandgap, and thienyl side chain helps enhance the intermolecular interaction. As a result, TOBDT is suitable to match the deep‐lying highest occupied molecular orbital (HOMO) of polymer donor PM6. Then, a strong crystalline acceptor IDIC is introduced as the third component to fabricate as‐cast nonfullerene ternary devices to achieve absorption and morphology control. Addition of IDIC not only mixes well with TOBDT but modulates the morphology of the blend film, which helps to balance the charge transport properties and reduce the photovoltage loss of ternary devices. All these contribute to synergetic improvement of J _sc, V _oc, and fill factor parameters, leading to a power conversion efficiency of 14.0% for the as‐cast fullerene‐free ternary device.

A new nonfullerene acceptor (TOBDT) with asymmetrical side chains is rationally designed and reported. Compared to the PM6:TOBDT binary devices, the as‐cast ternary devices based on PM6:IDIC:TOBDT obtain improved J _sc, V _oc, and fill factor values simultaneously, leading to a high power conversion efficiency of 14.0%, which is among the best reported for as‐cast fullerene‐free ternary devices.

Abstract

A new small‐molecule nonfullerene acceptor based on the benzo[1,2‐b:4,5‐b′]dithiophene (BDT) fused central core with asymmetrical alkoxy and thienyl side chains, namely TOBDT, is designed and synthesized. The alkoxy unit helps narrow the bandgap, and thienyl side chain helps enhance the intermolecular interaction. As a result, TOBDT is suitable to match the deep‐lying highest occupied molecular orbital (HOMO) of polymer donor PM6. Then, a strong crystalline acceptor IDIC is introduced as the third component to fabricate as‐cast nonfullerene ternary devices to achieve absorption and morphology control. Addition of IDIC not only mixes well with TOBDT but modulates the morphology of the blend film, which helps to balance the charge transport properties and reduce the photovoltage loss of ternary devices. All these contribute to synergetic improvement of J _sc, V _oc, and fill factor parameters, leading to a power conversion efficiency of 14.0% for the as‐cast fullerene‐free ternary device.

Lignin as a reinforced binder is incorporated into cellulose fibers by successive infiltration and mechanical hot‐pressing. The resulting lignin‐cellulose composite exhibits an outstanding isotropic tensile strength and good water stability, while its thermostability and UV‐blocking performance are also improved. This biodegradable and sustainable composite with both components from natural wood represents a promising alternative that can potentially replace the nonbiodegradable plastics.

Abstract

Plastic waste has been increasingly transferred from land into the ocean and has accumulated within the food chain, causing a great threat to the environment and human health, indicating that fabricating an eco‐friendly and biodegradable replacement is urgent. Paper made of cellulose is attractive in terms of its favorable biodegradability, resource abundance, large manufacturing scale, and low material cost, but is usually hindered by its inferior stability against water and poor mechanical strength for plastic replacement. Here, inspired by the reinforcement principle of cellulose and lignin in natural wood, a strong and hydrostable cellulosic material is developed by integrating lignin into the cellulose. Lignin as a reinforced matrix is incorporated to the cellulose fiber scaffold by successive infiltration and mechanical hot‐pressing treatments. The resulting lignin‐cellulose composite exhibits an outstanding isotropic tensile strength of 200 MPa, which is significantly higher than that of conventional cellulose paper (40 MPa) and some commercial petroleum‐based plastics. Additionally, the composite demonstrates a superior wet strength of 50 MPa. Adding lignin also improves the thermostability and UV‐blocking performance of cellulose paper. The demonstrated lignin‐cellulose composite is biodegradable and eco‐friendly with both components from natural wood, which represents a promising alternative that can potentially replace the nonbiodegradable plastics.

In article number https://doi.org/10.1002/adma.2018058431805843, Yongsheng Chen and Yongsheng Liu review integrated perovskite/bulk‐heterojunction (BHJ) organic solar cells (IPOSCs), which have recently emerged, and which could take the advantage of tandem cells using both perovskite and near‐infrared organic materials for wide‐range sunlight absorption. Combined with the reserved high open‐circuit voltage (V _OC), efficiencies close to or even exceeding the Shockley–Queisser limit of single‐junction cells are expected.

The efficiency of photocurrent generation is studied in the high‐efficiency nonfullerene PM6:Y6 blend, using a combination of field‐ and temperature‐dependent optoelectronic measurements. These experiments reveal barrierless free charge generation, despite a small driving force. Theoretical modeling suggests the existence of a large electrostatic interfacial field, which pushes charges away from the donor–acceptor interface.

Abstract

Organic solar cells are currently experiencing a second golden age thanks to the development of novel non‐fullerene acceptors (NFAs). Surprisingly, some of these blends exhibit high efficiencies despite a low energy offset at the heterojunction. Herein, free charge generation in the high‐performance blend of the donor polymer PM6 with the NFA Y6 is thoroughly investigated as a function of internal field, temperature and excitation energy. Results show that photocurrent generation is essentially barrierless with near‐unity efficiency, regardless of excitation energy. Efficient charge separation is maintained over a wide temperature range, down to 100 K, despite the small driving force for charge generation. Studies on a blend with a low concentration of the NFA, measurements of the energetic disorder, and theoretical modeling suggest that CT state dissociation is assisted by the electrostatic interfacial field which for Y6 is large enough to compensate the Coulomb dissociation barrier.

Organic solar cells based on ternary systems have promise as high efficiency and high stability systems. Finding the performance sweet‐spot is, however, a monumental task. In article number https://doi.org/10.1002/aenm.2019024171902417, Mariano Campoy‐Quiles and co‐workers present a simple high‐throughput method to map the ternary parameter space and efficiently discover optimum conditions.

Semiconducting polymers based on naphthobisthiadiazole (NTz) are designed and synthesized by introducing fluorine atoms on the NTz and bithiophene moieties. The impact of induced noncovalent sulfur–fluorine interaction position on the electronic structures, ordering structures, and photovoltaic performance is systematically studied. The newly developed polymer exhibits the power conversion efficiency of 10.8% that is one of the highest values for polymer/fullerene organic solar cells.

Abstract

Controlling the energetics and backbone order of semiconducting polymers is essential for the performance improvement of polymer‐based solar cells. The use of fluorine as the substituent for the backbone is known to effectively deepen the molecular orbital energy levels and coplanarize the backbone by noncovalent interactions with sulfur of the thiophene ring. In this work, novel semiconducting polymers are designed and synthesized based on difluoronaphthobisthiadiazole (FNTz) as a new family of naphthobisthiadiazole (NTz)–quaterthiophene copolymer systems, which are one of the highest performing polymers in solar cells. The effect of the fluorination position on the energetics and backbone order is systematically studied. It is found that the dependence of the solar cell fill factor on the active layer thickness is very sensitive to the fluorination position. It is thus further investigated and discussed how the structural features of the polymers influence the photovoltaic parameters as well as the diode characteristics and bimolecular recombination. Further, the polymer with fluorine on both the naphthobisthiadiazole and quaterthiophene moieties exhibits a quite high power conversion efficiency of 10.8% in solar cells in combination with a fullerene. It is believed that the results would offer new insights into the development of semiconducting polymers.

High‐efficiency, solution‐processed, hybrid tandem photovoltaic devices are demonstrated employing colloidal quantum dot (CQD) and organic bulk heterojunction as an active layer for front‐ and back‐cell, respectively. Notable efficiency of 12.82% is achieved, which is the highest among the reported CQD‐based solar cells, including single‐junction devices and tandem devices.

Abstract

While colloidal quantum dot photovoltaic devices (CQDPVs) can achieve a power conversion efficiency (PCE) of ≈12%, their insufficient optical absorption in the near‐infrared (NIR) regime impairs efficient utilization of the full spectrum of visible light. Here, high‐efficiency, solution‐processed, hybrid series, tandem photovoltaic devices are developed featuring CQDs and organic bulk heterojunction (BHJ) photoactive materials for front‐ and back‐cells, respectively. The organic BHJ back‐cell efficiently harvests the transmitted NIR photons from the CQD front‐cell, which reinforces the photon‐to‐current conversion at 350–1000 nm wavelengths. Optimizing the short‐circuit current density balance of each sub‐cell and creating a near ideal series connection using an intermediate layer achieve a PCE (12.82%) that is superior to that of each single‐junction device (11.17% and 11.02% for the CQD and organic BHJ device, respectively). Notably, the PCE of the hybrid tandem device is the highest among the reported CQDPVs, including single‐junction devices and tandem devices. The hybrid tandem device also exhibits almost negligible degradation after air storage for 3 months. This study suggests a potential route to improve the performance of CQDPVs by proper hybridization with NIR‐absorbing photoactive materials.

A systematic study of intrinsic light‐induced degradation pathways of conjugated polymers and small molecules under anoxic conditions is reported. It is shown that light induces a facile crosslinking of all investigated organic semiconductors, as revealed by gel permeation chromatography. The photochemical instability of all common absorber molecules can be considered to be a major limitation for reaching long operation lifetimes in organic photovoltaics.

Abstract

In view of a rapid development and increase in efficiency of organic solar cells, reaching their long‐term operational stability represents now one of the main challenges to be addressed on the way toward commercialization of this photovoltaic technology. However, intrinsic degradation pathways occurring in organic solar cells under realistic operational conditions remain poorly understood. The light‐induced dimerization of the fullerene‐based acceptor materials discovered recently is considered to be one of the main causes for burn‐in degradation of organic solar cells. In this work, it is shown that not only the fullerene derivatives but also different types of conjugated polymers and small molecules undergo similar light‐induced crosslinking regardless of their chemical composition and structure. In the case of conjugated polymers, crosslinking of macromolecules leads to a rapid increase in their molecular weight and consequent loss of solubility, which can be revealed in a straightforward way by gel permeation chromatography analysis via a reduction/loss of signal and/or smaller retention times. Results of this work, thus, shift the paradigm of research in the field toward designing a new generation of organic absorbers with enhanced intrinsic photochemical stability in order to reach practically useful operation lifetimes required for successful commercialization of organic photovoltaics.

Novel cathode interlayers (CILs) are developed by tailoring an organic electron acceptor, viz. ITIC. A high efficiency of 16.6% is achieved in an organic solar cell with S‐3 as the CIL. It is demonstrated that the difference of electrostatic surface potential between the CIL molecule and the polymer donor can promote exciton dissociation, contributing to additional charge generation.

Abstract

With the rapid advance of organic photovoltaic materials, the energy level structure, active layer morphology, and fabrication procedure of organic solar cells (OSCs) are changed significantly. Thus, the photoelectronic properties of many traditional electrode interlayers have become unsuitable for modifying new active layers; this limits the further enhancement in OSC efficiencies. Herein, a new design strategy of tailoring the end‐capping unit, ITIC, to develop a cathode interlayer (CIL) material for achieving high power conversion efficiency (PCE) in OSCs is demonstrated. The excellent electron accepting capacity, suitable energy level, and good film‐forming ability endow the S‐3 molecule with an outstanding electron extraction property. A device with S‐3 shows a PCE of 16.6%, which is among the top values in the field of OSCs. More importantly, it is demonstrated that the electrostatic potential difference between the CIL molecule and the polymer donor plays a crucial role in promoting exciton dissociation at the CIL/active layer interface, contributing to additional charge generation; this is crucial for enhancement of the current density. The results of this work not only develop a new design strategy for high‐performance CIL, but also demonstrate a reliable approach of density functional theory (DFT) calculation to predict the effect of the CIL chemical structure on exciton dissociation in OSCs.

A proper vertical phase separation and purer phases of donor and acceptor are finely controlled by sequential blade‐casting strategy in the PTB7‐Th:FOIC‐based organic solar cell, resulting in simultaneous enhancement of efficiency, stability, and mechanical properties.

Abstract

As a predominant fabrication method of organic solar cells (OSCs), casting of a bulk heterojunction (BHJ) structure presents overwhelming advantages for achieving higher power conversion efficiency (PCE). However, long‐term stability and mechanical strength are significantly crucial to realize large‐area and flexible devices. Here, controlling blend film morphology is considered as an effective way toward co‐optimizing device performance, stability, and mechanical properties. A PCE of 12.27% for a P‐i‐N‐structured OSC processed by sequential blade casting (SBC) is reported. The device not only outperforms the as‐cast BHJ devices (11.01%), but also shows impressive stability and mechanical properties. The authors corroborate such enhancements with improved vertical phase separation and purer phases toward more efficient transport and collection of charges. Moreover, adaptation of SBC strategy here will result in thermodynamically favorable nanostructures toward more stable film morphology, and thus improving the stability and mechanical properties of the devices. Such co‐optimization of OSCs will pave ways toward realizing the highly efficient, large‐area, flexible devices for future endeavors.

Exciton dissociation into free charges is the most important optoelectronic process in organic solar cells, which is driven by the energy‐level offset of electron‐donor and ‐acceptor materials. With the continuously tunable HOMO level achieved by organic semiconducting alloys, the minimum HOMO offset of ≈40 meV is shown to be necessary to achieve the most efficient exciton dissociation and photovoltaic performance.

Abstract

Current research indicates that exciton dissociation into free charge carriers can be achieved in material combinations with the highest occupied molecular orbital (HOMO) offset lowered to 0 eV in non‐fullerene organic solar cells. However, the quantitative relationship between the HOMO offset and exciton dissociation has not been established because of the difficulty in achieving continuously tunable HOMO offsets. Here, the binary blends of PTQ10:ZITI‐S and PTQ10:ZITI‐N are combined to form the positive and negative HOMO offsets of 0.20 and −0.07 eV, respectively. While the PTQ10:ZITI‐S binary blend delivers a decent power conversion efficiency (PCE) of 10.69% with a short‐circuit current (J _sc) of 16.94 mA cm⁻², the PTQ10:ZITI‐N with the negative offset shows a much lower PCE of 7.06% mainly because of the low J _sc of 12.03 mA cm⁻². Because the tunable HOMO levels can be realized in organic semiconducting alloys based on ZITI‐N and ZITI‐S acceptors, the transformation of the HOMO energy offset from negative to positive values is achieved in the PTQ10:ZITIN:ZITI‐S ternary blends, delivering much‐improved PCEs up to 13.26% with a significant, 74% enhancement of J _sc to 20.93 mA cm⁻². With detailed investigations, the study reveals that the minimum HOMO offset of ≈40 meV is required to achieve the most‐efficient exciton dissociation and photovoltaic performance.

A novel methodology to efficiently screen ternary blends for organic photovoltaics is presented. Ternary blend libraries based on lateral gradients are generated. These are then, colocally and hyperspectrally imaged in terms of performance and composition. This ultrafast approach uncovers complex efficiency landscapes and helps to navigate the ternary phase space.

Abstract

Organic solar cells based on ternary active layers can lead to higher power conversion efficiencies than corresponding binaries, and improved stability. The parameter space for optimization of multicomponent systems is considerably more complex than that of binaries, due to both, a larger number of parameters (e.g., two relative compositions rather than one) and intricate morphology–property correlations. Most experimental reports to date reasonably limit themselves to a relatively narrow subset of compositions (e.g., the 1:1 donor/s:acceptor/s trajectory). This work advances a methodology that allows exploration of a large fraction of the ternary phase space employing only a few (<10) samples. Each sample is produced by a designed sequential deposition of the constituent inks, and results in compositions gradients with ≈5000 points/sample that cover about 15%–25% of the phase space. These effective ternary libraries are then colocally imaged by a combination of photovoltaic techniques (laser and white light photocurrent maps) and spectroscopic techniques (Raman, photoluminescence, absorption). The generality of the methodology is demonstrated by investigating three ternary systems, namely PBDB‐T:ITIC:PC₇₀BM, PTB7‐Th:ITIC:PC₇₀BM, and P3HT:O‐IDFBR:O‐IDTBR. Complex performance‐structure landscapes through the ternary diagram as well as the emergence of several performance maxima are discovered.

Various definitions of band gaps are used in the perovskite solar cell community as a reference to analyze losses in open‐circuit voltage. This essay proposes a band‐gap independent method to reference voltages that is easy to implement and a meta‐analysis of literature data to illustrate the state of the art and development of voltage losses in perovskite solar cells.

Abstract

Open‐circuit voltages of lead‐halide perovskite solar cells are improving rapidly and are approaching the thermodynamic limit. Since many different perovskite compositions with different bandgap energies are actively being investigated, it is not straightforward to compare the open‐circuit voltages between these devices as long as a consistent method of referencing is missing. For the purpose of comparing open‐circuit voltages and identifying outstanding values, it is imperative to use a unique, generally accepted way of calculating the thermodynamic limit, which is currently not the case. Here a meta‐analysis of methods to determine the bandgap and a radiative limit for open‐circuit voltage is presented. The differences between the methods are analyzed and an easily applicable approach based on the solar cell quantum efficiency as a general reference is proposed.