Yingzhi Jin

A combination of in‐ and ex situ measurements are used to monitor the degradation processes in mixed‐halide perovskite light‐emitting devices. Device performance degradation is found to arise from accumulation of bromide at the interface between the perovskite and electron injecting layer, causing nonradiative recombination. Potassium halides mitigate the ionic movement and degradation, leading to better stability of devices under operation.

Abstract

Halide perovskites have attracted substantial interest for their potential as disruptive display and lighting technologies. However, perovskite light‐emitting diodes (PeLEDs) are still hindered by poor operational stability. A fundamental understanding of the degradation processes is lacking but will be key to mitigating these pathways. Here, a combination of in operando and ex situ measurements to monitor the performance degradation of (Cs_0.06FA_0.79MA_0.15)Pb(I_0.85Br_0.15)₃ PeLEDs over time is used. Through device, nanoscale cross‐sectional chemical mapping, and optical spectroscopy measurements, it is revealed that the degraded performance arises from an irreversible accumulation of bromide content at one interface, which leads to barriers to injection of charge carriers and thus increased nonradiative recombination. This ionic segregation is impeded by passivating the perovskite films with potassium halides, which immobilizes the excess halide species. The passivated PeLEDs show enhanced external quantum efficiency (EQE) from 0.5% to 4.5% and, importantly, show significantly enhanced stability, with minimal performance roll‐off even at high current densities (>200 mA cm⁻²). The decay half‐life for the devices under continuous operation at peak EQE increases from <1 to ≈15 h through passivation, and ≈200 h under pulsed operation. The results provide generalized insight into degradation pathways in PeLEDs and highlight routes to overcome these challenges.

A rerouting crystallization pathway (RCP) is developed to suppress defects in vertically oriented 2D perovskites. Lower trap states, better homogeneity, and higher charge transport/collection efficiency are obtained due to the improved film quality. Solar cells using these RCP‐2D perovskite films show a highest efficiency of 18.5% with a high fill factor of 83.4% and exhibit superior environmental stability.

Abstract

Vertically oriented 2D perovskites exhibit promising optoelectronic properties and intrinsic stability, but their photovoltaic application is still limited by the low power conversion efficiency (PCE) compared to 3D analogs. Here, a new crystallization pathway (RCP) is reported to suppress defects in vertically oriented 2D perovskite caused by its over‐rapid self‐assembly behavior. By controlling the specific adsorption of an ammonium halide additive on different perovskite crystal planes, the dynamic preferred growth of (111) plane is intentionally restrained, and the minority (202) planes emerge as secondary nucleation sites to stimulate the creation of large grains. As the halogen‐regulated deprotonation of ammonium proceeds, the (111) crystal plane gradually recovers its growth dominance, and a vertically oriented 2D perovskite film finally forms with high homogeneity, reduced trap density of states, and desired carrier transport/collection kinetics. Solar cells using RCP‐2D films show a highly reproducible and stable PCE reaching 18.5% with a high fill factor of 83.4%. These findings provide critical missing information on simultaneously achieving highly oriented and less defective 2D perovskite films for excellent device performance.

Nature Energy, Published online: 09 November 2020; doi:10.1038/s41560-020-00721-5

Two-dimensional Ruddlesden–Popper layered metal-halide perovskites show better performance over three-dimensional versions, but are typically based on quantum wells with random width distribution. Liang et al. show that introducing molten salt spacers gives phase-pure quantum wells and improved solar cell performance.

Nature Energy, Published online: 11 November 2020; doi:10.1038/s41560-020-00732-2

The power conversion efficiency of organic solar cells has rapidly increased, yet significantly less attention has been paid to materials stability and device longevity. For organic solar cells to make an impact in the marketplace, researchers, funding agencies and journals should do more to address this crucial gap.

Significant progress has been made in non‐fullerene organic solar cells (OSCs) in recent years, including in materials development, device engineering, and mechanistic understanding. This review summarizes progress and offers some reflections on the emerging methods for enabling high efficiency and improved stability for non‐fullerene OSCs.

Abstract

The past three years have witnessed rapid growth in the field of organic solar cells (OSCs) based on non‐fullerene acceptors (NFAs), with intensive efforts being devoted to material development, device engineering, and understanding of device physics. The power conversion efficiency of single‐junction OSCs has now reached high values of over 18%. The boost in efficiency results from a combination of promising features in NFA OSCs, including efficient charge generation, good charge transport, and small voltage losses. In addition to efficiency, stability, which is another critical parameter for the commercialization of NFA OSCs, has also been investigated. This review summarizes recent advances in the field, highlights approaches for enhancing the efficiency and stability of NFA OSCs, and discusses possible strategies for further advances of NFA OSCs.

The chemical composition modulation and electric field‐induced ion migration of organic‐inorganic hybrid perovskites are utilized to fabricate performance‐enhanced triboelectric nanogenerators (TENGs). The chemical composition modulation induced conductive type conversion and electric field‐induced self‐doping on the surfaces enable controlled performance of the TENGs.

Abstract

In this paper, new strategies are proposed to design high‐performance organic–inorganic hybrid perovskite (PVK)‐based triboelectric nanogenerators (TENGs) via both chemical composition modulation and electric field‐induced ion migration in the films. Both composition variation and ion migration under electric field are found to change the type of conductivity of the perovskite films, then modify their surface potentials and electron affinities. These are utilized to fabricate PVK‐based TENGs in pairs with poly‐tetrafluoroethylene (PTFE) or nylon films, respectively. Results show that PVK films are able to work as either a positive or a negative tribo‐material depending on the tribo‐material pair used; the optimal performances are obtained for PTFE/PVK TENGs using a PVK film with a MAI/PbI₂ ratio of 2 and forward polarization, and for nylon/PVK TENGs using a PVK film with a MAI/PbI₂ ratio of 0.4 and reverse polarization, respectively. The maximum output voltage and peak power density of PTFE/PVK TENGs are about 979 V and 24 W m⁻², 2.5 and 6.5 times higher than those of TENGs with nonoptimal composition ratio or that are poorly polarized. This work provides a new material design method for high‐performance TENGs and a novel polarization strategy for TENG performance enhancement.

In article number 2001759, Kilwon Cho and co‐workers report the novel design of an organic spacer as a multifunctional additive for formamidinium lead tri‐iodide (FAPbI₃) perovskite solar cells. Low dimensional (LD) perovskites assembled by organic spacers not only protect the grain boundary of FAPbI₃ from moisture but also facilitate the nucleation and growth of FAPbI₃ at low temperature. An LD/ FAPbI₃ composite based solar cell exhibits power conversion efficiency of 21.25% retaining 80% of the initial efficiency after 500 hours without encapsulation.

Recent progress is reviewed in applying self‐assembled monolayers in perovskite solar cells to improve surface morphology, energy band alignment, reduced interfacial charge recombination, and the trap passivation mechanism. The opportunities for molecular design of self‐assembled monolayers in enhancing the power conversion efficiency and stability of perovskite solar cells are discussed.

Abstract

Due to a certified 25.2% high efficiency, low cost, and easy fabrication; perovskite solar cells (PSCs) are the focus of interest among the next‐generation photovoltaic technologies. Long‐term stability is one of the most challenging obstacles to bring technology from the lab to the market. In this review, applications of self‐assembled monolayers (SAMs) to enhance the power conversion efficiency (PCE) and stability of PSCs is discussed. In the first part, the introduction of SAMs, and deposition techniques applied to different PSC architectures are described. In the middle section, current efforts to utilize SAMs to fine‐tune the optoelectronic properties to enhance the PCE and stability are detailed. The improvements in surface morphology, energy band alignment, as well as reduced interfacial charge recombination induced by SAMs, and the trap passivation mechanism allowing optimal PCE and stability are described. A general outlook summarizing the importance of SAMs to the improvement of PSCs performance is also given, alongside a discussion of future opportunities and possible research directions.

Organic solar cells have the potential to become the cheapest form of electricity, even beating silicon solar cells, at least in principle. This article summarizes where the field is on its way towards successful large‐scale commercialization, highlighting research challenges, discussing the status of current and future applications as well as the environmental footprint of this renewable energy technology.

Abstract

Organic solar cells have the potential to become the cheapest form of electricity, beating even silicon photovoltaics. This article summarizes the state of the art in the field, highlighting research challenges, mainly the need for an efficiency increase as well as an improvement in long‐term stability. It discusses possible current and future applications, such as building integrated photovoltaics or portable electronics. Finally, the environmental footprint of this renewable energy technology is evaluated, highlighting the potential to be the energy generation technology with the lowest carbon footprint of all.

SnO₂ has been applied as an efficient electron transport layer for perovskite solar cells over the past few years. In this progress report, recent advances in SnO₂ modification toward high efficiency and stability are summarized from the perspective of the optimization strategies, and the remaining challenges as well as opportunities for future research are also discussed.

Abstract

The electron transport layer plays a key role in affecting the charge dynamics and photovoltaic parameters in perovskite solar cells. Compared to other counterparts, SnO₂ has unique advantages such as low temperature fabrication and high electron extraction ability, and it receives extra attentions from the research community since the first report. Planar‐type perovskite solar cells based on SnO₂ exhibit a simple architecture and state of art device can achieve a power conversion efficiency of over 23%, which can compete with traditional devices using mesoporous TiO₂. The modification engineering of SnO₂ has contributed significantly to the enhanced device performance during the past years. There is still great potential for further improvement in the efficiency and long‐term stability. Herein recent advances toward modifying the optoelectronic properties of SnO₂ from the perspective of the optimization strategies are summarized and the remaining challenges as well as opportunities for future research are discussed. The continuous efforts dedicated to this exciting field may pave the way for developing commercial perovskite solar cells.

This review summarizes the isomerization strategy of nonfullerene small‐molecule acceptors for organic solar cells, and discusses the key structure–property relationships in depth.

Abstract

Nonfullerene acceptors (NFAs) are a current focus of research on bulk‐heterojunction organic solar cells (OSCs), as they can exhibit strong absorption, suitably matched energy levels, and good stability. Isomerization affords a new material design strategy for nonfullerene small‐molecule acceptors (SMAs). In this article, the development of isomeric nonfullerene SMAs, including isomeric perylene diimide (PDI)‐based nonfullerene SMAs and isomeric acceptor–donor–acceptor (A–D–A)‐type nonfullerene SMAs, is reviewed. The general design principles for isomeric SMAs and the key structure–property relationships are comprehensively surveyed and discussed. The remaining challenges and promising future directions of isomeric nonfullerene acceptors are presented.

The novel use of cheap copper‐based corrole as hole transporting material in perovskite solar cells is shown by improving the device thermal stability of n–i–p mesoscopic architecture under prolonged 85 °C stress conditions. Corrole‐based devices show a remarkable power conversion efficiency above 16% by retaining more than 65% of the initial power conversion efficiency after 1000 h of thermal stress.

Abstract

Perovskite solar cells (PSCs) represent nowadays a promising starting point to develop a new efficient and low‐cost photovoltaic technology due to the demonstrated power conversion efficiency (PCE) exceeding 25% on small area devices. However, best reported devices suffer from stability issue under real working conditions thus slowing down the race for the commercialization. In particular, the hole transporting material commonly employed in mesoscopic n–i–p PSCs (nip‐mPSCs), namely spiro‐OMeTAD, is strongly corrupted when subjected to temperatures above 70 °C due to intrinsic thermal instability and because of the dopant employed to improve the hole mobility. In this work, the novel use of a copper‐based corrole as HTM is proposed to improve the device thermal stability of nip‐mPSCs under prolonged 85 °C stress conditions. Corrole‐based devices show remarkable PCE above 16% by retaining more than 65% of the initial PCE after 1000 h of thermal stress, while spiro‐OMeTAD cells abruptly lose more than 60% after the first 40 h. Once scaled‐up to large area modules, the proposed device structure can truly represent a possible way to pass thermal stress tests proposed by IEC‐61646 standards and, not less importantly, the high temperature required by the lamination process for panel production.

A two‐terminal lateral‐structured artificial synapse is designed and fabricated with CH₃NH₃PbBr₃ single‐crystalline thin platelets (SCTPs). This SCTP‐based artificial synapse can realize a series of synaptic functions, including paired‐pulse facilitation, spike‐dependent plasticity and potentiation/depression. Compared to the polycrystalline thin film‐based artificial synapses, the SCTP‐based artificial synapses can operate with a lower operating current and energy consumption.

Abstract

Polycrystalline organometal halide perovskite films have been recently exploited as the active layer in artificial synapses, demonstrating the basic functional emulation of biological synapses. However, for the implementation of neuromorphic computing and bioinspired intelligent systems, full synapse‐like functionality with a simple structure and extremely low energy consumption are of crucial importance. Here, a modified thickness‐confined surfactant‐assistant self‐assembly strategy is proposed to synthesize CH₃NH₃PbBr₃ single‐crystalline thin platelets (SCTPs) and a two‐terminal lateral‐structured synaptic device with ultralow operating current down to sub‐pA is fabricated. Essential synaptic behaviors are realized, including paired‐pulse facilitation, spike‐dependent plasticity, transition from sensory memory to short‐term memory and potentiation/depression. Furthermore, the activity‐dependent plasticity is also demonstrated on the SCTP‐based artificial synapse, which may enable nociceptors to detect intense external harm. These results provide a new protocol for designing lateral‐structured synaptic devices based on hybrid perovskite SCTPs and future neuromorphic bioelectronics.

Nonfullerene PBDB‐T:ITIC bulk heterojunction‐based photocathode affords a record high ideal ratiometric power‐saved efficiency up to 4.1% by optimizing the interface layers, and an unassisted water splitting with solar‐to‐hydrogen conversion efficiency of 1.1% is obtained through coupling this photocathode with BiVO₄ photoanode.

Abstract

Highly efficient photocathode with onset potential beyond 0.6 V is very desirable in order to construct photoelectrochemical (PEC) device for stand‐alone overall water splitting. In this work, it is reported a nonfullerene PBDB‐T:ITIC bulk heterojunction (BHJ) based photocathode delivering unprecedented positive onset potential of 0.8 V and optimal photocurrent density of −11.7 mA cm⁻² at 0 V in pH 1 solution. The yielded high ideal ratiometric power‐saved efficiency (Φ_saved,ideal) of 4.1% is a record among the previous organic photocathodes. When the electrolyte pH moves to neutral, the Φ_saved,ideal is still to be 3.2%, yet being twice more than the currently reported highest value. Pairing with TiO _x , the hole transport layer of CuO _x with more efficient charge carrier transportation and hole extraction ability excels MoO₃ and NiO _x , contributing to the highly efficient solar hydrogen production in both acidic and neutral solutions. Premixing the BHJ blend precursor with an anti‐oxidative agent is proved to be a beneficial strategy to improve the PEC stability by suppressing the photo‐oxidation degradation of organic polymer materials. Subsequently a conceptual PEC cell comprising of tandem CuO _x /BHJ/TiO _x /Pt photocathode and BiVO₄/NiFeO _x photoanode with a high solar‐to‐hydrogen efficiency up to 1.1% is fabricated in neutral solution without assistance of extra bias.

A clear solid‐state aggregation transition of the acceptor N3 is discovered, which enables a double‐annealing method that can fine‐tune aggregation and morphology. Compared with the 16.6% efficiency for PM6:N3:PC₇₁BM‐control devices, a higher efficiency of 17.6% is obtained through the improved protocol. The results provide a molecular design and engineering conundrum to achieve simultaneously low annealing temperatures, high efficiency, and stability.

Abstract

Thermal transition of organic solar cells (OSCs) constituent materials are often insufficiently researched, resulting in trial‐and‐error rather than rational approaches to annealing strategies to improve domain purity to enhance the power conversion efficiency. Despite the potential utility, little is known about the thermal transitions of the modern high‐performance acceptors Y6 and N3. Here, by using an optical method, it is discovered that the acceptor N3 has a clear solid‐state aggregation transition at 82 °C. This unusually low transition not only explains prior optimization protocols, but the transition informs and enables a double‐annealing method that can fine‐tune aggregation and the device morphology. Compared with 16.6% efficiency for PM6:N3:PC₇₁BM control devices, higher efficiency of 17.6% is obtained through the improved protocol. Morphology characterization with x‐ray scattering methods reveals the formation of a multilength scale morphology. Moreover, the double‐annealing method is illustrated and easily transferred and validated with Y6‐based devices, using the transition of Y6 at 102 °C. As a result, the PCE improved from 16.0% to 16.8%. Design of high‐performance acceptors with yet lower aggregation transitions might be required for OSCs to successfully transition to low thermal budget industrial processing methods where annealing temperatures on plastic substrates have to be kept low.

Highly efficient all‐inorganic perovskite solar cells based on CsPbI _x Br_{3‐ x} are fabricated through the introduction of a spontaneous interfacial manipulation method. A spontaneously formed ultrathin 2D perovskite top interface can not only eliminate interfacial defects but also effectively prevent moisture penetration. As a result, the device exhibits a power conversion efficiency of 18% with extended device stability.

Abstract

Cesium‐based all‐inorganic halide perovskites solar cells (PSCs) have recently attracted increasing attention. Currently, due to the existence of high defects density and unoptimized interfacial morphology, “state‐of‐the‐art” performances of all‐inorganic PSCs are still far away from their theoretical limits. Although commonly used two‐step passivation methods can effectively passivate the perovskite surface, they will inevitably detriment the original perovskite morphology due to the use of weak‐polarity solvents. This will potentially result in the unintentional doping, uncontrollable interfacial band alignment, and the additional defects formation. Hence, a spontaneous interfacial manipulation (SIM) method is developed to self‐organize a 2D/3D multidimensional perovskite top interface. It is demonstrated that the spontaneously formed ultrathin 2D perovskite can not only eliminate the interfacial defects, but also effectively prevent moisture penetration. As a result, a significant power conversion efficiency enhancement from 13.64% to over 18% is obtained along with greatly extended device lifetime, for CsPbI _x Br_{3‐ x} ‐based all‐inorganic PSC.

Two well‐regular polymer acceptors (PY‐IT and PY‐OT) with different polymerization sites are developed. For comparison, a random ternary copolymer (PY‐IOT) with the same ratio of the two acceptors is synthesized. All‐polymer solar cells (PSCs) based on PM6:PY‐IT achieve an excellent PCE of 15.05%, which is significantly higher than those based on PY‐OT (10.04%) and PY‐IOT (12.12%).

Abstract

Recent advances in the development of polymerized A–D–A‐type small‐molecule acceptors (SMAs) have promoted the power conversion efficiency (PCE) of all‐polymer solar cells (all‐PSCs) over 13%. However, the monomer of an SMA typically consists of a mixture of three isomers due to the regio‐isomeric brominated end groups (IC‐Br(in) and IC‐Br(out)). In this work, the two isomeric end groups are successfully separated, the regioisomeric issue is solved, and three polymer acceptors, named PY‐IT, PY‐OT, and PY‐IOT, are developed, where PY‐IOT is a random terpolymer with the same ratio of the two acceptors. Interestingly, from PY‐OT, PY‐IOT to PY‐IT, the absorption edge gradually redshifts and electron mobility progressively increases. Theory calculation indicates that the LUMOs are distributed on the entire molecular backbone of PY‐IT, contributing to the enhanced electron transport. Consequently, the PM6:PY‐IT system achieves an excellent PCE of 15.05%, significantly higher than those for PY‐OT (10.04%) and PY‐IOT (12.12%). Morphological and device characterization reveals that the highest PCE for the PY‐IT‐based device is the fruit of enhanced absorption, more balanced charge transport, and favorable morphology. This work demonstrates that the site of polymerization on SMAs strongly affects device performance, offering insights into the development of efficient polymer acceptors for all‐PSCs.

Through investigation of the underlying thermodynamic and kinetic aspects of non‐fullerene acceptor crystallization, the importance of diffusion coefficients and melting enthalpies in controlling the crystal growth rates is demonstrated, and it is revealed and that differences in halogenation can drastically change crystallization kinetics and device stability.

Abstract

With power conversion efficiency now over 17%, a long operational lifetime is essential for the successful application of organic solar cells. However, most non‐fullerene acceptors can crystallize and destroy devices, yet the fundamental underlying thermodynamic and kinetic aspects of acceptor crystallization have received limited attention. Here, room‐temperature (RT) diffusion coefficients of 3.4 × 10⁻²³ and 2.0 × 10⁻²² are measured for ITIC‐2Cl and ITIC‐2F, two state‐of‐the‐art non‐fullerene acceptors. The low coefficients are enough to provide for kinetic stabilization of the morphology against demixing at RT. Additionally profound differences in crystallization characteristics are discovered between ITIC‐2F and ITIC‐2Cl. The differences as observed by secondary‐ion mass spectrometry, differential scanning calorimetry (DSC), grazing‐incidence wide‐angle X‐ray scattering, and microscopy can be related directly to device degradation and are attributed to the significantly different nucleation and growth rates, with a difference in the growth rate of a factor of 12 at RT. ITIC‐4F and ITIC‐4Cl exhibit similar characteristics. The results reveal the importance of diffusion coefficients and melting enthalpies in controlling the growth rates, and that differences in halogenation can drastically change crystallization kinetics and device stability. It is furthermore delineated how low nucleation density and large growth rates can be inferred from DSC and microscopy experiments which could be used to guide molecular design for stability.

With synergistic optimization of the active layer morphology, flexible substrate properties, and processing temperature, large‐area flexible organic solar cells with high performance are achieved by the slot‐die coating process. The 1 cm² flexible devices produce an excellent power conversion efficiency (PCE) of 12.16%, and, for modules with an area of 25 cm², an extraordinary PCE of 10.09% is observed.

Abstract

Slot‐die coating is generally regarded as the most effective large‐scale methodology for the fabrication of organic solar cells (OSCs). However, the corresponding device performance significantly lags behind spin‐coated devices. Herein, the active layer morphology, flexible substrate properties, and the processing temperature are optimized synergistically to obtain high power conversion efficiency (PCE) for both the flexible single cells and the modules. As a result, the 1 cm² flexible devices produce an excellent PCE of 12.16% as compared to 12.37% for the spin‐coated small‐area (0.04 cm²) rigid devices. Likewise, for modules with an area of 25 cm², an extraordinary PCE of 10.09% is observed. Hence, efficiency losses associated with the upscaling are significantly reduced by the synergistic optimization. Moreover, after 1000 bending cycles at a bending radius of 10 mm, the flexible devices still produce over 99% of their initial PCE, whereas after being stored for over 6000 h in a glove box, the PCE reaches 103% of its initial value, indicating excellent device flexibility as well as superior shelf stability. These results, thus, are a promising confirmation the great potential for upscaling of large‐area OSCs in the near future.

9‐Fluorenone‐1‐carboxylic acid (FCA) is utilized to prolong the lifetime of photogenerated excitons in a nonfullerene acceptor (IT‐M) approximately twofold, ensuring longer exciton diffusion length and efficiency enhancement in organic photovoltaic devices. The prolongation arises from the discovered intermolecular vibrational coupling between the electronic excited state of IT‐M and the electronic ground state of FCA, thus suppressing the nonradiative decay.

Abstract

Exciton lifetime (τ) is crucial for the migration of excitons to donor/acceptor interfaces for subsequent charge separation in organic solar cells (OSCs); however, obvious prolongation of τ has rarely been achieved. Here, by introducing a solid additive 9‐fluorenone‐1‐carboxylic acid (FCA) into the active layer, which comprises a nonfullerene acceptor, 3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6/7‐methyl)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (IT‐M), τ is substantially prolonged from 491 to 928 ps, together with obvious increases in fluorescence intensity and quantum yield. Time‐resolved transient infrared spectra indicate the presence of an intermolecular vibrational coupling between the electronic excited state of IT‐M and the electronic ground state of FCA, which is first observed here and which can suppress the internal conversion process. IT‐M‐based OSCs display an improved short‐circuit current and fill factor after the addition of FCA. Thus, the power conversion efficiency is increased, particularly for devices with a large donor/acceptor ratio of 1:4, whose efficiency is increased by 56%. This study describes a novel method, which is also applicable to other nonfullerene acceptors, for further improving the performance of OSCs without affecting their morphology and light absorption properties.

The discovery of efficient n‐type organic thermoelectric materials is key to state‐of‐the art thermoelectric materials and can enable the design of efficient and fully organic thermoelectric generators. In article number 2002752, Dehua Hu, Xavier Crispin, Yuguang Ma, and co‐workers develop a novel n‐type perylene bisimide mixed‐ion–electron conductor with record‐high thermoelectric properties induced by the Soret effect.

The fabrication challenges of monolithic all‐perovskite tandem photovoltaics are detailed in a step‐by‐step, choose‐your‐own‐adventure fashion. The trade‐offs between sub‐cell efficiency and processing stability are highlighted and pros and cons are weighed. Through this detailed analysis, a few routes to reach >30% power conversion efficiency and the necessary work are identified.

Abstract

Metal halide perovskites (MHPs) have transfixed the photovoltaic (PV) community due to their outstanding and tunable optoelectronic properties coupled to demonstrations of high‐power conversion efficiencies (PCE) at a range of bandgaps. This has motivated the field to push perovskites to reach the highest possible performance. One way to increase the efficiency is by fabricating multijunction solar cells, which can split the solar spectrum, reducing thermalization loss. Low‐cost all‐perovskite tandems have a real chance to soon exceed 30% PCE, which could transform the PV industry. Achieving this goal requires the identification of perovskite sub‐cells that are both highly efficient and can be effectively integrated. Herein, it is discussed how to navigate the multiple‐choice adventure in choosing between the myriad of options and considerations present when deciding what perovskite materials, contact layers, and processing tools to use. Some of the potential fabrication pitfalls often encountered in MHP based tandem PVs are highlighted, so that they can hopefully be avoided in the future.

A bithiophene‐bridged D–A‐based 2D sp²‐carbon‐linked conjugated polymer (2D CCP‐Th) is synthesized via Knoevenagel polymerization. Benefiting from the robust sp²‐carbon‐linked conjugations and donor–acceptor structures, when employed as a photocathode for water reduction, 2D CCP‐Th demonstrates a superb saturated photocurrent density (5.5 µA cm⁻² at 0.3 V and 7.9 µA cm⁻² at 0 V vs reversible hydrogen electrode).

Abstract

Photoelectrochemical (PEC) water reduction, converting solar energy into environmentally friendly hydrogen fuel, requires delicate design and synthesis of semiconductors with appropriate bandgaps, suitable energy levels of the frontier orbitals, and high intrinsic charge mobility. In this work, the synthesis of a novel bithiophene‐bridged donor–acceptor‐based 2D sp²‐carbon‐linked conjugated polymer (2D CCP) is demonstrated. The Knoevenagel polymerization between the electron‐accepting building block 2,3,8,9,14,15‐hexa(4‐formylphenyl) diquinoxalino[2,3‐a:2′,3′‐c]phenazine (HATN‐6CHO) and the first electron‐donating linker 2,2′‐([2,2′‐bithiophene]‐5,5′‐diyl)diacetonitrile (ThDAN) provides the 2D CCP‐HATNThDAN (2D CCP‐Th). Compared with the corresponding biphenyl‐bridged 2D CCP‐HATN‐BDAN (2D CCP‐BD), the bithiophene‐based 2D CCP‐Th exhibits a wide light‐harvesting range (up to 674 nm), a optical energy gap (2.04 eV), and highest energy occupied molecular orbital–lowest unoccupied molecular orbital distributions for facilitated charge transfer, which make 2D CCP‐Th a promising candidate for PEC water reduction. As a result, 2D CCP‐Th presents a superb H₂‐evolution photocurrent density up to ≈7.9 µA cm⁻² at 0 V versus reversible hydrogen electrode, which is superior to the reported 2D covalent organic frameworks and most carbon nitride materials (0.09–6.0 µA cm⁻²). Density functional theory calculations identify the thiophene units and cyano substituents at the vinylene linkage as active sites for the evolution of H₂.

A state‐of‐the‐art n‐type organic semiconductor, PhC₂−BQQDI, which can be used to form single‐crystalline thin films by printing, is identified as a band‐transport material by means of variable‐temperature gated Hall effect measurements. The printed PhC₂−BQQDI single crystal is also used to demonstrate a high‐frequency transistor with a capability of 4.3 MHz under ambient atmosphere.

Abstract

Organic semiconductors (OSCs) have attracted growing attention for optoelectronic applications such as field‐effect transistors (FETs), and coherent (or band‐like) carrier transport properties in OSC single crystals (SCs) have been of interest as they can lead to high carrier mobilities. Recently, such p‐type OSC SCs compatible with a printing technology have been used to achieve high‐speed FETs; therefore, developments of n‐type counterparts may be promising for realizing high‐speed complementary organic circuits. Herein, coherent electron transport properties in a printed SC of a state‐of‐the‐art, air‐stable n‐type OSC, PhC₂−BQQDI, by means of variable‐temperature gated Hall effect measurements and X‐ray single‐crystal diffraction analyses in conjunction with band structure calculations, are reported. Furthermore, the SC FET is tested for high‐speed operations, which obtains a cutoff frequency of 4.3 MHz at an operation voltage of 20 V in air. Thus, PhC₂−BQQDI is shown as a new candidate for practical applications of SC‐based, organic complementary devices.

To fine‐tune the energy levels of polymer donors, a family of random polymers is synthesized, which shows favorable properties of aggregation and morphology. The performance of these polymers is less sensitive to their molecular weights compared with PM7. Thus, multiple cases of highly efficient nonfullerene organic solar cells are achieved with efficiencies between 16.0% and 17.1%.

Abstract

Developing high‐performance donor polymers is important for nonfullerene organic solar cells (NF‐OSCs), as state‐of‐the‐art nonfullerene acceptors can only perform well if they are coupled with a matching donor with suitable energy levels. However, there are very limited choices of donor polymers for NF‐OSCs, and the most commonly used ones are polymers named PM6 and PM7, which suffer from several problems. First, the performance of these polymers (particularly PM7) relies on precise control of their molecular weights. Also, their optimal morphology is extremely sensitive to any structural modification. In this work, a family of donor polymers is developed based on a random polymerization strategy. These polymers can achieve well‐controlled morphology and high‐performance with a variety of chemical structures and molecular weights. The polymer donors are D–A1–D–A2‐type random copolymers in which the D and A1 units are monomers originating from PM6 or PM7, while the A2 unit comprises an electron‐deficient core flanked by two thiophene rings with branched alkyl chains. Consequently, multiple cases of highly efficient NF‐OSCs are achieved with efficiencies between 16.0% and 17.1%. As the electron‐deficient cores can be changed to many other structural units, the strategy can easily expand the choices of high‐performance donor polymers for NF‐OSCs.

A bithiophene‐bridged D–A‐based 2D sp²‐carbon‐linked conjugated polymer (2D CCP‐Th) is synthesized via Knoevenagel polymerization. Benefiting from the robust sp²‐carbon‐linked conjugations and donor–acceptor structures, when employed as a photocathode for water reduction, 2D CCP‐Th demonstrates a superb saturated photocurrent density (5.5 µA cm⁻² at 0.3 V and 7.9 µA cm⁻² at 0 V vs reversible hydrogen electrode).

Abstract

Photoelectrochemical (PEC) water reduction, converting solar energy into environmentally friendly hydrogen fuel, requires delicate design and synthesis of semiconductors with appropriate bandgaps, suitable energy levels of the frontier orbitals, and high intrinsic charge mobility. In this work, the synthesis of a novel bithiophene‐bridged donor–acceptor‐based 2D sp²‐carbon‐linked conjugated polymer (2D CCP) is demonstrated. The Knoevenagel polymerization between the electron‐accepting building block 2,3,8,9,14,15‐hexa(4‐formylphenyl) diquinoxalino[2,3‐a:2′,3′‐c]phenazine (HATN‐6CHO) and the first electron‐donating linker 2,2′‐([2,2′‐bithiophene]‐5,5′‐diyl)diacetonitrile (ThDAN) provides the 2D CCP‐HATNThDAN (2D CCP‐Th). Compared with the corresponding biphenyl‐bridged 2D CCP‐HATN‐BDAN (2D CCP‐BD), the bithiophene‐based 2D CCP‐Th exhibits a wide light‐harvesting range (up to 674 nm), a optical energy gap (2.04 eV), and highest energy occupied molecular orbital–lowest unoccupied molecular orbital distributions for facilitated charge transfer, which make 2D CCP‐Th a promising candidate for PEC water reduction. As a result, 2D CCP‐Th presents a superb H₂‐evolution photocurrent density up to ≈7.9 µA cm⁻² at 0 V versus reversible hydrogen electrode, which is superior to the reported 2D covalent organic frameworks and most carbon nitride materials (0.09–6.0 µA cm⁻²). Density functional theory calculations identify the thiophene units and cyano substituents at the vinylene linkage as active sites for the evolution of H₂.