Highly Efficient and Scalable p-i-n Perovskite Solar Cells Enabled by Poly-metallocene Interfaces.

Inverted p-i-n perovskite solar cells (PSCs) are easy to process but need improved interface characteristics with reduced energy loss to prevent efficiency drops when increasing the active photovoltaic area. Here, we report a series of poly ferrocenyl molecules that can modulate the perovskite surface enabling the construction of small- and large-area PSCs. We found that the perovskite-ferrocenyl interaction forms a hybrid complex with enhanced surface coordination strength and activated electronic states, leading to lower interfacial nonradiative recombination and charge transport resistance losses. The resulting PSCs achieve an enhanced efficiency of up to 26.08% for small-area devices and 24.51% for large-area devices (1.0208 cm2). Moreover, the large-area PSCs maintain >92% of the initial efficiency after 2000 h of continuous operation at the maximum power point under 1-sun illumination and 65 °C.

Charge Management Enables Efficient Spontaneous Chromatic Adaptation Bipolar Photodetector.

Solution-processed photodetectors have emerged as promising candidates for next-generation of visible-near infrared (vis-NIR) photodetectors. This is attributed to their ease of processing, compatibility with flexible substrates, and the ability to tune their detection properties by integrating complementary photoresponsive semiconductors. However, the limited performance continues to hinder their further development, primarily influenced by the difference of charge transport properties between perovskite and organic semiconductors. In this work, a perovskite-organic bipolar photodetectors (PDs) is introduced with multispectral responsivity, achieved by effectively managing charges in perovskite and a ternary organic heterojunction. The ternary heterojunction, incorporating a designed NIR guest acceptor, exhibits a faster charge transfer rate and longer carrier diffusion length than the binary heterojunction. By achieving a more balanced carrier dynamic between the perovskite and organic components, the PD achieves a low dark current of 3.74 nA cm-2 at -0.2 V, a fast response speed of <10 µs, and a detectivity of exceeding 1012 Jones. Furthermore, a bioinspired retinotopic system for spontaneous chromatic adaptation is achieved without any optical filter. This charge management strategy opens up possibilities for surpassing the limitations of photodetection and enables the realization of high-purity, compact image sensors with exceptional spatial resolution and accurate color reproduction.

Green-solvent Processable Dopant-free Hole Transporting Materials for Inverted Perovskite Solar Cells.

The commercialization of perovskite solar cells (PVSCs) urgently requires the development of green-solvent processable dopant-free hole transporting materials (HTMs). However, strong intermolecular interactions that ensure high hole mobility always compromise the solubility and film-forming ability in green solvents. Herein, we show a simple but effective design strategy to solve this trade-off, that is, constructing star-shaped D-A-D structure. The resulting HTMs (BTP1-2) can be processed by green solvent of 2-methylanisole (2MA), a kind of food additive, and show high hole mobility and multiple defect passivation effects. An impressive efficiency of 24.34 % has been achieved for 2MA-processed BTP1 based inverted PVSCs, the highest value for green-solvent processable HTMs so far. Moreover, it is manifested that the charge separation of D-A type HTMs at the photoinduced excited state can help to passivate the defects of perovskites, indicating a new HTM design insight.

Rashba Band Splitting and Bulk Photovoltaic Effect Induced by Halogen Bonds in Hybrid Layered Perovskites.

Non-covalent interactions play an essential role in directing the self-assembly of hybrid organic-inorganic crystals. In hybrid halide perovskites, hydrogen bonding has been the paramount non-covalent interaction. Here, we show another non-covalent interaction, namely, the halogen bond interaction, that directs a symmetry-breaking assembly in a new series of two-dimensional (2D) perovskites (ICH2 CH2 NH3 )2 (CH3 NH3 )n-1 Pbn I3n+1 (n is the layer thickness, n=1-4). Structural analysis shows that the halogen bond strength varies with the layer thickness. For the odd number (n=1, 3) layered perovskites, stronger halogen interaction leads to centrosymmetric structures, whereas for the n=2 layered perovskites, weaker halogen bonds result in non-centrosymmetric structures. Transient reflection spectroscopy shows a suppressed radiative recombination rate (k2 ≈0) and prolonged spin lifetime for n=2 structure, suggesting an enhanced Rashba band splitting effect. The structural asymmetry is further confirmed with a reversible bulk photovoltaic effect. Our work provides a new design strategy to enable hybrid perovskites with emerging properties and functionalities associated with structural asymmetry.

Freestanding 2D NiFe Metal-Organic Framework Nanosheets: Facilitating Proton Transfer via Organic Ligands for Efficient Oxygen Evolution Reaction.

The oxygen evolution reaction (OER) is crucial to electrochemical hydrogen production. However, designing and fabricating efficient electrocatalysts still remains challenging. By confinedly coordinating organic ligands with metal species in layered double hydroxides (LDHs), an innovative LDHs-assisted approach is developed to facilely synthesize freestanding bimetallic 2D metal-organic framework nanosheets (2D MOF NSs), preserving the metallic components and activities in OER. Furthermore, the research has demonstrated that the incorporation of carboxyl organic ligands coordinated with metal atoms as proton transfer mediators endow 2D MOF NSs with efficient proton transfer during the electrochemical OHads  â†’ Oads transition. These freestanding NiFe-2D MOF NSs require a small overpotential of 260 mV for a current density of 10 mA cm-2 . When this strategy is applied to LDH nanosheets grown on nickel foam, the overpotential can be reduced to 221 mV. This outstanding OER activity supports the capability of multimetallic organic frameworks for the rational design of water oxidation electrocatalysts. This strategy provides a universal path to the synthesis of 2D MOF NSs that can be used as electrocatalysts directly.

Selenium-Containing Organic Photovoltaic Materials.

ConspectusOrganic photovoltaics (OPVs) with a photoactive layer containing a blend of organic donor and acceptor species are considered to be a promising technology for clean energy owing to their unique flexible form factor and good solution processability that can potentially address the scalability challenges. The delicate designs of both donors and acceptors have significantly enhanced the power conversion efficiency of OPVs to more than 18%. Nonfullerene small-molecule acceptors (NFAs) have played a critical role in enhancing the short-circuit current density (JSC) by efficiently harvesting near-infrared (NIR) sunlight. To take full advantage of the abundant NIR photons, the optical band gap of NFAs should be further reduced to improve the performance of OPVs. Incorporating highly polarizable selenium atoms onto the backbone of organic conjugated materials has been proven to be an effective way to decrease their optical band gap. For example, a selenium-substituted NFA recently developed by our group has attained a JSC of approximate 27.5 mA cm-2 in OPV devices, surpassing those of most emerging photovoltaic systems. Inspired by this advance, we concentrate on the topic of selenium-containing materials in this Account to incite readers' interest in further exploring this series of materials.In this Account, we first compare the differences among chalcogen heterocycles and discuss the influence of fundamental electronic behavior on the collective photoelectrical properties of the resulting materials. The superior features of selenium-substituted materials are summarized as follows: (1) The large covalent radius of selenium can diminish the π-orbital overlap, rendering enhanced quinoidal resonance character and a narrowed optical band gap of resulting materials. (2) The selenium atom is more polarizable than sulfur owing to its larger and looser outermost electron cloud, enabling enhanced intermolecular Se-Se interaction and increased charge carrier mobility of relevant materials in the solid state. We then focus on summarizing the design rules for various categories of selenium-containing materials including polymer donors, small-molecule acceptors, and polymer acceptors, especially those composed of ladder-type polycyclic units. The motivation for incorporating selenium atoms into these materials and the structure-property relationships were thoroughly elucidated. Specifically, we discuss the changes in the optical band gap, charge carrier mobility, and molecular packing induced by selenium substitution and correlate the effects of these changes with the exciton behaviors, energy loss, and nanoscale film morphology of corresponding OPV devices. Furthermore, we point out the intrinsic stability of selenium-containing materials under maximum-power-point tracking and long-term photo- or thermostress and indicate their potential use in semitransparent and tandem solar cells. At the end, the prospect of future research focuses and the possible applications of selenium-containing materials in the OPV field are discussed.

Designs from single junctions, heterojunctions to multijunctions for high-performance perovskite solar cells.

Hybrid metal-halide perovskite solar cells (PVSCs) have drawn unprecedented attention during the last decade due to their superior photovoltaic performance, facile and low-cost fabrication, and potential for roll-to-roll mass production and application for portable devices. Through collective composition, interface, and process engineering, a comprehensive understanding of the structure-property relationship and carrier dynamics of perovskites has been established to help achieve a very high certified power conversion efficiency (PCE) of 25.5%. Apart from material properties, the modified heterojunction design and device configuration evolution also play crucial roles in enhancing the efficiency. The adoption and/or modification of heterojunction structures have been demonstrated to effectively suppress the carrier recombination and potential losses in PVSCs. Moreover, the employment of multijunction structures has been shown to reduce thermalization losses, achieving a high PCE of 29.52% in perovskite/silicon tandem solar cells. Therefore, understanding the evolution of the device configuration of PVSCs from single junction, heterojunction to multijunction designs is helpful for the researchers in this field to further boost the PCE beyond 30%. Herein, we summarize the evolution and progress of the single junction, heterojunction and multijunction designs for high-performance PVSCs. A comprehensive review of the fundamentals and working principles of these designs is presented. We first introduce the basic working principles of single junction PVSCs and the intrinsic properties (such as crystallinity and defects) in perovskite films. Afterwards, the progress of diverse heterojunction designs and perovskite-based multijunction solar cells is synopsized and reviewed. Meanwhile, the challenges and strategies to further enhance the performance are also summarized. At the end, the perspectives on the future development of perovskite-based solar cells are provided. We hope this review can provide the readers with a quick catchup on this emerging solution-processable photovoltaic technology, which is currently at the transition stage towards commercialization.

High Efficiency (15.8%) All-Polymer Solar Cells Enabled by a Regioregular Narrow Bandgap Polymer Acceptor.

Despite the significant progresses made in all-polymer solar cells (all-PSCs) recently, the relatively low short-circuit current density (Jsc) and large energy loss are still quite difficult to overcome for further development. To address these challenges, we developed a new class of narrow-bandgap polymer acceptors incorporating a benzotriazole (BTz)-core fused-ring segment, named the PZT series. Compared to the commonly used benzothiadiazole (BT)-containing polymer PYT, the less electron-deficient BTz renders PZT derivatives with significantly red-shifted optical absorption and up-shifted energy levels, leading to simultaneously improved Jsc and open-circuit voltage in the resultant all-PSCs. More importantly, a regioregular PZT (PZT-γ) has been developed to achieve higher regiospecificity for avoiding the formation of isomers during polymerization. Benefiting from the more extended absorption, better backbone ordering, and more optimal blend morphology with donor component, PZT-γ-based all-PSCs exhibit a record-high power conversion efficiency of 15.8% with a greatly enhanced Jsc of 24.7 mA/cm2 and a low energy loss of 0.51 eV.

Highly Efficient and Rapid Inactivation of Coronavirus on Non-Metal Hydrophobic Laser-Induced Graphene in Mild Conditions.

The prevalence of COVID-19 has caused global dysfunction in terms of public health, sustainability, and socio-economy. While vaccination shows potential in containing the spread, the development of surfaces that effectively reduces virus transmission and infectivity is also imperative, especially amid the early stage of the pandemic. However, most virucidal surfaces are operated under harsh conditions, making them impractical or potentially unsafe for long-term use. Here, it is reported that laser-induced graphene (LIG) without any metal additives shows marvelous antiviral capacities for coronavirus. Under low solar irradiation, the virucidal efficacy of the hydrophobic LIG (HLIG) against HCoV-OC43 and HCoV-229E can achieve 97.5% and 95%, respectively. The photothermal effect and the hydrophobicity of the HLIG synergistically contribute to the superior inactivation capacity. The stable antiviral performance of HLIG enables its multiple uses, showing advantages in energy saving and environmental protection. This work discloses a potential method for antiviral applications and has implications for the future development of antiviral materials.

Multi-Selenophene-Containing Narrow Bandgap Polymer Acceptors for All-Polymer Solar Cells with over 15 % Efficiency and High Reproducibility.

All-polymer solar cells (all-PSCs) progressed tremendously due to recent advances in polymerized small molecule acceptors (PSMAs), and their power conversion efficiencies (PCEs) have exceeded 15 %. However, the practical applications of all-PSCs are still restricted by a lack of PSMAs with a broad absorption, high electron mobility, low energy loss, and good batch-to-batch reproducibility. A multi-selenophene-containing PSMA, PFY-3Se, was developed based on a selenophene-fused SMA framework and a selenophene π-spacer. Compared to its thiophene analogue PFY-0Se, PFY-3Se shows a ≈30 nm red-shifted absorption, increased electron mobility, and improved intermolecular interaction. In all-PSCs, PFY-3Se achieved an impressive PCE of 15.1 % with both high short-circuit current density of 23.6 mA cm-2 and high fill factor of 0.737, and a low energy loss, which are among the best values in all-PSCs reported to date and much better than PFY-0Se (PCE=13.0 %). Notably, PFY-3Se maintains similarly good batch-to-batch properties for realizing reproducible device performance, which is the first reported and also very rare for the PSMAs. Moreover, the PFY-3Se-based all-PSCs show low dependence of PCE on device area (0.045-1.0 cm2 ) and active layer thickness (110-250 nm), indicating the great potential toward practical applications.

Efficient Inverted Perovskite Solar Cells with Low Voltage Loss Achieved by a Pyridine-Based Dopant-Free Polymer Semiconductor.

Currently, the performance improvement for inverted perovskite solar cells (PVSCs) is mainly limited by the high open circuit voltage (VOC ) loss caused by detrimental non-radiative recombination (NRR) processes. Herein, we report a simple and efficient way to simultaneously reduce the NRR processes inside perovskites and at the interface by rationally designing a new pyridine-based polymer hole-transporting material (HTM), PPY2, which exhibits suitable energy levels with perovskites, high hole mobility, effective passivation of the uncoordinated Pb2+ and iodide defects, as well as the capability of promoting the formation of high-quality polycrystalline perovskite films. In absence of any dopants, the inverted PVSCs using PPY2 as the HTM deliver an encouraging PCE up to 22.41 % with a small VOC loss (0.40 V), among the best device performances for inverted PVSCs reported so far. Furthermore, PPY2-based unencapsulated devices show an excellent long-term photostability, and over 97 % of its initial PCE can be maintained after one sun constant illumination for 500 h.

Synergistical Dipole-Dipole Interaction Induced Self-Assembly of Phenoxazine-Based Hole-Transporting Materials for Efficient and Stable Inverted Perovskite Solar Cells.

Delicately designed dopant-free hole-transporting materials (HTMs) with ordered structure have become one of the major strategies to achieve high-performance perovskite solar cells (PSCs). In this work, we report two donor-π linker-donor (D-π-D) HTMs, N01 and N02, which consist of facilely synthesized 4,8-di(n-hexyloxy)-benzo[1,2-b:4,5-b']dithiophene as a π linker, with 10-bromohexyl-10H-phenoxazine and 10-hexyl-10H-phenoxazine as donors, respectively. The N01 molecules form a two-dimensional conjugated network governed by C-H⋅⋅⋅O and C-H⋅⋅⋅Br interaction between phenoxazine donors, and synchronously construct a three-dimension lamellar structure with the aid of interlaminar π-π interaction. Consequently, N01 as a dopant-free small-molecule HTM exhibits a higher intrinsic hole mobility and more favorable interfacial properties for hole transport, hole extraction and perovskite growth, enabling an inverted PSC to achieve a very impressive power conversion efficiency of 21.85 %.

A Non-fullerene Acceptor with Enhanced Intermolecular π-Core Interaction for High-Performance Organic Solar Cells.

Understanding the molecular structure and self-assembly of thiadiazole-derived non-fullerene acceptors (NFAs) is very critical for elucidating the origin of their extraordinary charge generation and transport properties that enable high power conversion efficiencies to be achieved in these systems. A comprehensive crystallographic study on a state-of-the-art NFA, Y6, and its selenium analog, CH1007, has been conducted which revealed that the face-to-face π-core interaction induced by benzo[2,1,3]thiadiazole S-N-containing moieties plays a significant role in governing the molecular geometries and unique packing of Y6 and CH1007 to ensure their superior charge-transport properties. Moreover, benefitting from the red-shifted optical absorption via selenium substitution, photovoltaic devices based on a PM6:CH1007:PC71BM ternary blend delivered an exceptionally high short-circuit current of 27.48 mA/cm2 and a power conversion efficiency of 17.08%.

Regulating Surface Termination for Efficient Inverted Perovskite Solar Cells with Greater Than 23% Efficiency.

Passivating surface and bulk defects of perovskite films has been proven to be an effective way to minimize nonradiative recombination losses in perovskite solar cells (PVSCs). The lattice interference and perturbation of atomic periodicity at the perovskite surfaces often significantly affect the material properties and device efficiencies. By tailoring the terminal groups on the perovskite surface and modifying the surface chemical environment, the defects can be reduced to enhance the photovoltaic performance and stability of derived PVSCs. Here, we report a rationally designed bifunctional molecule, piperazinium iodide (PI), containing both R2NH and R2NH2+ groups on the same six-membered ring, behaving both as an electron donor and an electron acceptor to react with different surface-terminating ends on perovskite films. The resulting perovskite films after defect passivation show released surface residual stress, suppressed nonradiative recombination loss, and more n-type characteristics for sufficient energy transfer. Consequently, charge recombination is significantly suppressed to result in a high open-circuit voltage (VOC) of 1.17 V and a reduced VOC loss of 0.33 V. A very high power conversion efficiency (PCE) of 23.37% (with 22.75% certified) could be achieved, which is the highest value reported for inverted PVSCs. Our work reveals a very effective way of using rationally designed bifunctional molecules to simultaneously enhance the device performance and stability.

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells.

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.

Dopant-Free Squaraine-Based Polymeric Hole-Transporting Materials with Comprehensive Passivation Effects for Efficient All-Inorganic Perovskite Solar Cells.

Development of high-performance dopant-free hole-transporting materials (HTMs) with comprehensive passivation effects is highly desirable for all-inorganic perovskite solar cells (PVSCs). Squaraines (SQs) could be a candidate for dopant-free HTMs as they are natural passivators for perovskites. One major limitation of SQs is their relatively low hole mobility. Herein we demonstrate that polymerizing SQs into pseudo two dimensional (2D) p-π conjugated polymers could overcome this problem. By rationally using N,N-diarylanilinosquaraines as the comonomers, the resulting polysquaraine HTMs not only exhibit suitable energy levels and efficient passivation effects, but also achieve very high hole mobility close to 0.01 cm-2 V-1 s-1 . Thus as dopant-free HTMs for α-CsPbI2 Br-based all-inorganic PVSCs, the best PCE reached is 15.5 %, outperforming those of the doped-Spiro-OMeTAD (14.4 %) based control devices and among the best for all-inorganic PVSCs.

Rational Design of Dipolar Chromophore as an Efficient Dopant-Free Hole-Transporting Material for Perovskite Solar Cells.

In this paper, an electron donor-acceptor (D-A) substituted dipolar chromophore (BTPA-TCNE) is developed to serve as an efficient dopant-free hole-transporting material (HTM) for perovskite solar cells (PVSCs). BTPA-TCNE is synthesized via a simple reaction between a triphenylamine-based Michler's base and tetracyanoethylene. This chromophore possesses a zwitterionic resonance structure in the ground state, as evidenced by X-ray crystallography and transient absorption spectroscopies. Moreover, BTPA-TCNE shows an antiparallel molecular packing (i.e., centrosymmetric dimers) in its crystalline state, which cancels out its overall molecular dipole moment to facilitate charge transport. As a result, BTPA-TCNE can be employed as an effective dopant-free HTM to realize an efficient (PCE ≈ 17.0%) PVSC in the conventional n-i-p configuration, outperforming the control device with doped spiro-OMeTAD HTM.

Hexaazatrinaphthylene Derivatives: Efficient Electron-Transporting Materials with Tunable Energy Levels for Inverted Perovskite Solar Cells.

Hexaazatrinaphthylene (HATNA) derivatives have been successfully shown to function as efficient electron-transporting materials (ETMs) for perovskite solar cells (PVSCs). The cells demonstrate a superior power conversion efficiency (PCE) of 17.6 % with negligible hysteresis. This study provides one of the first nonfullerene small-molecule-based ETMs for high-performance p-i-n PVSCs.

Mesoporous SnO2 single crystals as an effective electron collector for perovskite solar cells.

Mesoporous single crystals are prized for their fast electron transport and high surface area. Here we report the first synthesis of mesoporous SnO2 single crystals (SnO2 MSCs) by a simple silica-templated hydrothermal method, and its application in solution-processed perovskite solar cells (PSCs). A relatively low efficiency (3.76%) was obtained due to the strong charge recombination at the SnO2/perovskite interface. However, by coating a thin TiO2 barrier layer on SnO2via TiCl4 treatment, we were able to achieve an 8.54% power conversion efficiency (PCE). A dynamics study using impedance spectroscopy revealed a much lower transport resistance for the SnO2 MSC-based solar cells than for the TiO2 nanocrystal PSCs, but a stronger recombination. Significantly, the thin TiO2 coating layer on SnO2 considerably reduced the recombination while largely maintaining the superior electron-transport properties.

Efficiency enhancement of perovskite solar cells through fast electron extraction: the role of graphene quantum dots.

We report on a significant power conversion efficiency improvement of perovskite solar cells from 8.81% to 10.15% due to insertion of an ultrathin graphene quantum dots (GQDs) layer between perovskite and TiO2. A strong quenching of perovskite photoluminescence was observed at ∼760 nm upon the addition of the GQDs, which is pronouncedly correlated with the increase of the IPCE and the APCE of the respective cells. From the transient absorption measurements, the improved cell efficiency can be attributed to the much faster electron extraction with the presence of GQDs (90-106 ps) than without their presence (260-307 ps). This work highlights that GQDs can act as a superfast electron tunnel for optoelectronic devices.