Unraveling the Molecular Size Effect on Surface Engineering of Perovskite Solar Cells.

Surface engineering in perovskite solar cells, especially for the upper surface of perovskite, is widely studied. However, most of these studies have primarily focused on the interaction between additive functional groups and perovskite point defects, neglecting the influence of other parts of additive molecules. Herein, additives with -NH3 + functional group are introduced at the perovskite surface to suppress surface defects. The chain lengths of these additives vary to conduct a detailed investigation into the impact of molecular size. The results indicate that the propane-1,3-diamine dihydroiodide (PDAI2 ), which possesses the most suitable size, exhibited obvious optimization effects. Whereas the molecules, methylenediamine dihydroiodide (MDAI2 ) and pentane-1,5-diamine dihydroiodide (PentDAI2 ) with unsuitable size, lead to a deterioration in device performance. The PDAI2 -treated devices achieved a certified power conversion efficiency (PCE) of 25.81% and the unencapsulated devices retained over 80% of their initial PCE after 600 h AM1.5 illumination.

Template-Assisted Synthesis of a Large-Area Ordered Perovskite Nanowire Array for a High-Performance Photodetector.

One-dimensional (1D) organic-inorganic hybrid perovskite nanowires (NWs) with well-defined structures possess superior optical and electrical properties for optoelectronic applications. However, most of the perovskite NWs are synthesized in air, which makes the NWs susceptible to water vapor, resulting in large amounts of grain boundaries or surface defects. Here, a template-assisted antisolvent crystallization (TAAC) method is designed to fabricate CH3NH3PbBr3 NWs and arrays. It is found that the as-synthesized NW array has designable shapes, low crystal defects, and ordered alignment, which is attributed to the sequestration of water and oxygen in air by the introduction of acetonitrile vapor. The photodetector based on the NWs exhibits an excellent response to light illumination. Under the illumination of a 532 nm laser with 0.1 µW and a bias of -1 V, the responsivity and detectivity of the device reach 1.55 A/W and 1.21 × 1012 Jones, respectively. The transient absorption spectrum (TAS) shows a distinct ground state bleaching signal only at 527 nm, which corresponds to the absorption peak induced by the interband transition of CH3NH3PbBr3. Narrow absorption peaks (a few nanometers) indicate that the energy-level structures of CH3NH3PbBr3 NWs only have a few impurity-level-induced transitions leading to additional optical loss. This work provides an effective and simple strategy to achieve high-quality CH3NH3PbBr3 NWs, which exhibit potential application in photodetection.

Fabrication of Transferable and Micro/Nanostructured Superhydrophobic Surfaces Using Demolding and iCVD Processes.

Superhydrophobic surfaces possess enormous potential in various applications on account of their versatile functionalities. However, artificial superhydrophobic surfaces with ultralow solid/liquid adhesion often require complicated structure fabrication and surface fluorination processes. Here, we designed a superhydrophobic surface possessed of micro/nanoscale structures by employing facile and low-cost demolding and initiated chemical vapor deposition (iCVD) processes. The achieved micro/nanostructured superhydrophobic surface has a maximum static contact angle of ∼170°, a roll-off angle and contact angle hysteresis below 1°, ultralow solid/liquid adhesion for water droplets, and maintains excellent superhydrophobicity after exposure to strongly corrosive species, like strong acid/base and salt solutions, for 60 h. This reasonability-designed method of creating the superhydrophobic surface could provide valuable guidelines for the manufacture of transferable superhydrophobic surfaces and facilitate potential applications extending from optoelectronic devices to self-cleaning materials, such as solar cells, windows, and electronic displays.

Synergistic Effects of Bipolar Additives on Grain Boundary-Mediated Charge Transport for Efficient Carbon-Based Inorganic Perovskite Solar Cells.

Carbon-based all-inorganic CsPbIxBr3-x perovskite solar cells offer high stability against heat and humidity and a suitable band gap for tandem and semitransparent photovoltaics. In CsPbIxBr3-x perovskite films, the defects at grain boundaries (GBs) cause charge trapping, reducing the efficiency of the cell. Electronic deactivation of GB has been a conventional strategy to suppress the trapping, but at the cost of charge carrier transport through the boundaries. Here, we turn the GBs into benign charge transport pathways with the aid of bipolar charge transport semiconductors, namely, Ti3C2TX (MXene) and Spiro-OMeTAD, respectively. Thanks to the synergistic effects of both n- and p-type transport media, the charge transport is improved and balanced at the GBs. As a result, the cells achieve an efficiency of 12.7%, the highest among all low-temperature-processed carbon-based inorganic perovskite solar cells. Benign GBs also lead to enhanced light and aging stabilities. Our work demonstrates a proof-of-concept strategy of benign electronic modulation of GBs for solution-processed perovskite solar cells.

Efficient Organic Solar Cells Enabled by Simple Non-Fused Electron Donors with Low Synthetic Complexity.

Fused-ring electron donors boost the efficiency of organic solar cells (OSCs), but they suffer from high cost and low yield for their large synthetic complexity (SC > 30%). Herein, the authors develop a series of simple non-fused-ring electron donors, PF1 and PF2, which alternately consist of furan-3-carboxylate and 2,2'-bithiophene. Note that PF1 and PF2 present very small SC of 9.7% for their inexpensive raw materials, facile synthesis, and high synthetic yield. Compared to their all-thiophene-backbone counterpart PT-E, two new polymers feature larger conjugated plane, resulting in higher hole mobility for them, especially a value up to ≈10-4 cm2 V-1 ·s for PF2 with longer alkyl side chain. Meanwhile, PF1 and PF2 exhibit larger dielectric constant and deeper electronic energy level versus PT-E. Benefiting from the better physicochemical properties, the efficiencies of PF1- and PF2-based devices are improved by ≈16.7% and ≈71.3% relative to that PT-E-based devices, respectively. Furthermore, the optimized PF2-based devices with introducing PC71 BM as the third component deliver a higher efficiency of 12.40%. The work not only indicates that furan-3-carboxylate is a simple yet efficient building block for constructing non-fused-ring polymers but also provides a promising electron donor PF2 for the low-cost production of OSCs.

Quantum Dot Interface-Mediated CsPbIBr2 Film Growth and Passivation for Efficient Carbon-Based Solar Cells.

CsPbIxBry-based all-inorganic perovskite materials are a potential candidate for stable semitransparent and tandem structured photovoltaic devices. However, poor film (morphological and crystalline) quality and interfacial recombination lead consequently to a decline in the photoelectric conversion performance of the applied solar cells. In this work, we incorporated PbS quantum dots (QDs) at the interface of electron transporting layer (ETL) SnO2 and perovskite to modulate the crystallization of CsPbIBr2 and the interfacial charge dynamics in carbon-based solar cells. The as-casted PbS QDs behave as seeds for lattice-matching the epitaxial growth of pinhole-free CsPbIBr2 films. The modified films with reduced defect density exhibit facilitated carrier transfer and suppressed charge recombination at the ETL/perovskite interface, contributing to an enhanced device efficiency from 7.00 to 9.09% and increased reproducibility and ambient stability. This strategic method of QD-assisted lattice-matched epitaxial growth is promising to prepare high-quality perovskite films for efficient perovskite solar cells.

Spatioselective Growth on Homogenous Semiconductor Substrates by Surface State Modulation.

Nanofabrication schemes usually suffer challenges in direct growth on complex nanostructured substrates. We provide a new technology that allows for the convenient, selective growth of complex nanostructures directly on three-dimensional (3D) homogeneous semiconductor substrates. The nature of the selectivity is derived from surface states modulated electrochemical deposition. Metals, metal oxides, and compound semiconductor structures can be prepared with high fidelity over a wide scale range from tens of nanometers to hundreds of microns. The utility of the process for photoelectrochemical applications is demonstrated by selectively decorating the sidewalls and tips of silicon microwires with cuprous oxide and cobalt oxides catalysts, respectively. Our findings indicate a new selective fabrication concept applied for homogeneous 3D semiconductor substrates, which is of high promise in community of photoelectronics, photoelectrochemistry, photonics, microelectronics, etc.

Facile Synthesis of Defect-Modified Thin-Layered and Porous g-C3N4 with Synergetic Improvement for Photocatalytic H2 Production.

Modulating and optimizing the diverse parameters of photocatalysts synergistically as well as exerting these advantages fully in photocatalytic reactions are crucial for the sufficient utilization of solar energy but still face various challenges. Herein, a novel and facile urea- and KOH-assisted thermal polymerization (UKATP) strategy is first developed for the preparation of defect-modified thin-layered and porous g-C3N4 (DTLP-CN), wherein the thickness of g-C3N4 was dramatically decreased, and cyano groups, nitrogen vacancies, and mesopores were simultaneously introduced into g-C3N4. Importantly, the roles of thickness, pores, and defects can be targetedly modulated and optimized by changing the mass ratio of urea, KOH, and melamine. This can remarkably increase the specific area, improve the light-harvesting capability, and enhance separation efficiency of photoexcited charge carriers, strengthening the mass transfer in g-C3N4. Consequently, the photocatalytic hydrogen evolution efficiency of the DTLP-CN (1.557 mmol h-1 g-1, λ > 420 nm) was significantly improved more than 48.5 times with the highest average apparent quantum yield (AQY) of 18.5% and reached as high as 0.82% at 500 nm. This work provides an effective strategy for synergistically regulating the properties of g-C3N4, and opens a new horizon to design g-C3N4-based catalysts for highly efficient solar-energy conversion.

Realization of Moisture-Resistive Perovskite Films for Highly Efficient Solar Cells Using Molecule Incorporation.

The development of highly crystalline perovskite films with large crystal grains and few surface defects is attractive to obtain high-performance perovskite solar cells (PSCs) with good device stability. Herein, we simultaneously improve the power conversion efficiency (PCE) and humid stability of the CH3NH3PbI3 (CH3NH3 = MA) device by incorporating small organic molecule IT-4F into the perovskite film and using a buffer layer of PFN-Br. The presence of IT-4F in the perovskite film can successfully improve crystallinity and enhance the grain size, leading to reduced trap states and longer lifetime of the charge carrier, and make the perovskite film hydrophobic. Meanwhile, as a buffer layer, PFN-Br can accelerate the separation of excitons and promote the transfer process of electrons from the active layer to the cathode. As a consequence, the PSCs exhibit a remarkably improved PCE of 20.55% with reduced device hysteresis. Moreover, the moisture-resistive film-based devices retain about 80% of their initial efficiency after 30 days of storage in relative humidity of 10-30% without encapsulation.

Ultrafast plasmonic lasing from a metal/semiconductor interface.

To date, plasmonic nanowire lasers mostly adopt hybrid plasmonic waveguides, while there is a lack of study in terms of the confinement effect and the corresponding ultrafast dynamics of non-hybridized plasmonic lasers. Here, we report ultrafast plasmonic nanowire lasers composed of a single CH3NH3PbBr3 nanowire on a silver film without any insulating layer at room temperature. The non-hybridized plasmonic nanowire lasers exhibit ultrafast lasing dynamics with around 1.9 ps decay rate and 1 ps peak response time. Such values are among the best ones ever reported. Interestingly, the threshold of the non-hybridized plasmonic nanowire lasers is in the same order as that of their hybrid counterparts. The low threshold is due to the ultra-flat single-crystal silver films and high-quality single-crystal perovskite nanowires. The non-hybridized plasmonic lasing in CH3NH3PbBr3 nanowires originates from the stimulated emission of an electron-hole plasma based on our experiments. This work deepens the understanding of non-hybridized plasmonic lasers and paves the way to design electric pump plasmonic lasers by getting rid of insulating layers.

Nonfullerene Ternary Organic Solar Cell with Effective Charge Transfer between Two Acceptors.

High power conversion efficiency can be realized by using a ternary bulk heterojunction with complementary absorption spectra in organic solar cells. However, as the development of nonfullerene acceptors with a broad absorption spectrum makes the absorption efficiency of the photovoltaic devices close to optimal, such a strategy needs modifying. In particular, charge transfer between the two acceptors is necessary to be considered. Herein, we purposely design a ternary system based on PTB7-Th:COi8DFIC:ITIC-4F. Though the presence of ITIC-4F in PTB7-Th:COi8DFIC could not broaden the absorption spectrum obviously, the formed cascade-energy-level alignment is beneficial for promoting and balancing exciton separation and charge transport between the donor and two acceptors and even between the acceptors. Insights into the charge transport route in the completed system are provided via using the techniques including photoluminescence spectroscopy and pump-probe photoconductivity spectroscopy. This work provides a new idea for designing highly efficient ternary organic solar cells.

Optimizing the component ratio of PEDOT:PSS by water rinse for high efficiency organic solar cells over 16.7.

For the state-of-the-art organic solar cells (OSCs), PEDOT:PSS is the most popularly used hole transport material for the conventional structure. However, it still suffers from several disadvantages, such as low conductivity and harm to ITO due to the acidic PSS. Herein, a simple method is introduced to enhance the conductivity and remove the additional PSS by water rinsing the PEDOT:PSS films. The photovoltaic devices based on the water rinsed PEDOT:PSS present a dramatic improvement in efficiency from 15.98% to 16.75% in comparison to that of the untreated counterparts. Systematic characterization and analysis reveal that although part of the PEDOT:PSS is washed away, it still leaves a smoother film and the ratio of PEDOT to PSS is higher than before in the remaining films. It can greatly improve the conductivity and reduce the damage to substrates. This study demonstrates that finely modifying the charge transport materials to improve conductivity and reduce defeats has great potential for boosting the efficiency of OSCs.

A wrinkled structure with broadband and omnidirectional light-trapping abilities for improving the performance of organic solar cells with low defect density.

Fabricating thin film solar cells on the light-trapping structures is an effective way to improve the absorption of the active layer. Here, we report a non-fullerene organic solar cell based on a PBDB-T:ITIC active layer, a wrinkled metal rear electrode, and a MoO3/Ag/ZnS front transparent electrode. Optical characterization shows that the wrinkled metal structure can remarkably increase the absorption of the active layer in a broadband range. The resulting device shows a power conversion efficiency of 8.2%, which increases by 4.6% compared to that of the flat counterpart. Apart from higher absorption, the improved performance can also be ascribed to the efficient charge transport and collection in the device due to the lower defect density, larger interfacial area, and smaller active layer thickness. A device with a power conversion efficiency of 10.19% based on the flat ITO/glass substrate is also achieved. Accordingly, a power conversion efficiency of about 10.66% is predicted under the condition where the wrinkled rear electrode and the ITO front electrode are employed. In addition, the power conversion efficiency of the wrinkled device could increase by about 50.48% compared to that of the flat device under an incident angle of 60 °C, illustrating that a better omnidirectional ability is achieved.

Multiple-engineering controlled growth of tunable-bandgap perovskite nanowires for high performance photodetectors.

Controllable growth of perovskite nanowires is very important for various applications in optical and electrical devices. Although significant progress has been achieved in the solution method, a deep understanding of the mechanics of growing perovskite nanowires is still lacking. Herein, we developed an electrochemical method for growing the perovskite nanowires and studied the growth processes systematically. The initial nucleation and crystal growth could be controlled by simply varying the additive solvents, thus leading to two stable size ratio distributions of the perovskite nanowires. Further, with compositional engineering, the bandgap of the perovskites could be tuned from 1.59 eV to 3.04 eV. All the as-grown perovskite nanowires displayed a unique structure with high crystallization quality, contributing to a very high responsivity of 2.1 A W-1 and a large on/off ratio of 5 × 103 for the photodetectors based on the CH3NH3PbBr3 nanowires. All of these findings demonstrate that the optimized solution method offers a new approach to synthesize perovskite nanowires for applications in photoelectric devices.

Insights into the Influence of Work Functions of Cathodes on Efficiencies of Perovskite Solar Cells.

Though various efforts on modification of electrodes are still undertaken to improve the efficiency of perovskite solar cells, attributing to the large scope of these methods, it is of significance to unveil the working principle systematically. Herein, inverted perovskite solar cells based on indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/CH3 NH3 PbI3 /phenyl-C61-butyric acid methyl ester (PC61 BM)/buffer metal/Al are constructed. Through the choice of different buffer metals to tune work function of the cathode, the contact nature of the active layer with the cathode could be manipulated well. In comparison with the device using Au/Al as the electrode that shows an unfavorable band bending for conducting the excited electrons to the cathode, the one with Ca/Al presents a dramatically improved efficiency over 17.1%, ascribed to the favorable band bending at the interface of the cathode with the active layer. Details for tuning the band bending and the corresponding charge transfer mechanism are given in a systematic manner. Thus, a general guideline for constructing perovskite photovoltaic devices efficiently is provided.

Efficient Organic/Inorganic Hybrid Solar Cell Integrating Polymer Nanowires and Inorganic Nanotetrapods.

Constructing a highly efficient bulk-heterojunction is of critical importance to the hybrid organic/inorganic solar cells. Here in this work, we introduce a novel hybrid architecture containing P3HT nanowire and CdSe nanotetrapod as bicontinuous charge channels for holes and electrons, respectively. Compared to the traditionally applied P3HT molecules, the well crystallized P3HT nanowires qualify an enhanced light absorption at the long wavelength as well as strengthened charge carrier transport in the hybrid active layer. Accordingly, based on efficient dissociation of photogenerated excitons, the interpercolation of these two nano-building blocks allows a photovoltaic conversion efficiency of 1.7% in the hybrid solar cell, up to 42% enhancement compared to the reference solar cell with traditional P3HT molecules as electron donor. Our work provides a promising hybrid structure for efficient organic/inorganic bulk-heterojunction solar cells.

Recent Development in ITO-free Flexible Polymer Solar Cells.

Polymer solar cells have shown good prospect for development due to their advantages of low-cost, light-weight, solution processable fabrication, and mechanical flexibility. Their compatibility with the industrial roll-to-roll manufacturing process makes it superior to other kind of solar cells. Normally, indium tin oxide (ITO) is adopted as the transparent electrode in polymer solar cells, which combines good conductivity and transparency. However, some intrinsic weaknesses of ITO restrict its large scale applications in the future, including a high fabrication price using high temperature vacuum deposition method, scarcity of indium, brittleness and scaling up of resistance with the increase of area. Some substitutes to ITO have emerged in recent years, which can be used in flexible polymer solar cells. This article provides the review on recent progress using other transparent electrodes, including carbon nanotubes, graphene, metal nanowires and nanogrids, conductive polymer, and some other electrodes. Device stability is also discussed briefly.

Organic-inorganic perovskite plasmonic nanowire lasers with a low threshold and a good thermal stability.

Plasmonic nanolasers have ushered in a paradigm of deep sub-wavelength coherent optical sources with ultrafast dynamics that exploit the strong confinement capabilities of metals. Although these devices are usually associated with higher thresholds due to absorption in metals, the high gain inorganic II-VI and III-V semiconductor materials have allowed the realization of plasmonic nanolasers operating under ambient conditions. In this work, we introduce single-crystalline lead halide perovskite (CH3NH3PbI3) nanowires as an organic-inorganic semiconducting gain material to the plasmonic laser community. We demonstrate plasmonic laser action using a hybrid geometry whereby the perovskite nanowires are placed on a silver substrate with an insulating spacer layer. We report relatively low threshold operation under ambient conditions (13.5 µJ cm-2), and the devices work well even at temperatures up to 43.6 °C. The demonstration highlights the high optical gain achievable in perovskite materials and thus provides a solution to high gain materials for plasmonic devices.

The Effect of UV-Ozone Treatment on the Performance of Plasmonic Organic Solar Cells.

Silver nanoparticles (Ag NPs) solution was spin-coated on indium-tin oxide (ITO) glass substrates prior to spin-coating poly(3,4-ethylenedioxythiophene):poly(styrene- sulfonate) ( PEDOT: PSS) for the plasmonic solar cells. The sequence of spin-coating of Ag and UV-ozone treatment resulted in different device performance. For devices in which Ag NPs solution was spin-coated on ITO substrates before UV-ozone treatment, power conversion efficiency increased from 3.4% to 3.7%, while the power conversion efficiency decreased if Ag NPs solution was spin-coated after UV-ozone treatment. In both cases, the short-circuit current density increased, and the open-circuit voltage remained relatively constant. The variation of power conversion efficiency mainly depended on the changing of the fill factor, which is related to film morphology of the devices. AFM measurements of PEDOT: PSS films were taken to study the influence of film morphology on device performance.