On the Origin of Hysteresis in Perovskite Solar Cells

The origin of hysteresis behavior is probed in perovskite solar cells (PSCs) with simultaneous measurements of cell open circuit voltage (V_oc) and photoluminescence intensity over time following illumination of the cell. It is shown, for the first time, that the transient changes in terminal voltage and luminescent intensity do not follow the relationship that would be predicted by the generalized Plank radiation law. A mechanism is proposed based on the presence of a resistive barrier to majority carrier flow at the interface between the perovskite film and the electron or hole transport layer, in combination with significant interface recombination. This results in a decoupling of the internal quasi‐Fermi level separation and the externally measured voltage. A simple numerical model is used to provide in‐principle validation for the proposed mechanism and it is confirmed that mobile ionic species are a likely candidate for creating the time‐varying majority carrier bottleneck by its reduced conductivity. The findings show that the V_oc of PSCs may be lower than the limit imposed by the cell luminescence efficiency, even under steady‐state conditions.     

Role of PCBM in the Suppression of Hysteresis in Perovskite Solar Cells

The power conversion efficiency of inorganic–organic hybrid lead halide perovskite solar cells (PSCs) is approaching that of those made from single crystalline silicon; however, they still experience problems such as hysteresis and photo/electrical‐field‐induced degradation. Evidences consistently show that ionic migration is critical for these detrimental behaviors, but direct in‐situ studies are still lacking to elucidate the respective kinetics. Three different PSCs incorporating phenyl‐C61‐butyric acid methyl ester (PCBM) and a polymerized form (PPCBM) is fabricated to clarify the function of fullerenes towards ionic migration in perovskites: 1) single perovskite layer, 2) perovskite/PCBM bilayer, 3) perovskite/PPCBM bilayer, where the fullerene molecules are covalently linked to a polymer backbone impeding fullerene inter‐diffusion. By employing wide‐field photoluminescence imaging microscopy, the migration of iodine ions/vacancies under an external electrical field is studied. The polymerized PPCBM layer barely suppresses ionic migration, whereas PCBM readily does. Temperature‐dependent chronoamperometric measurements demonstrate the reduction of activation energy with the aid of PCBM and X‐ray photoemission spectroscopy (XPS) measurements show that PCBM molecules are viable to diffuse into the perovskite layer and passivate iodine related defects. This passivation significantly reduces iodine ions/vacancies, leading to a reduction of built‐in field modulation and interfacial barriers.     

Morphological Control for High Performance,Solution‐Processed Planar Heterojunction Perovskite Solar Cells

Organometal trihalide perovskite based solar cells have exhibited the highest efficiencies to‐date when incorporated into mesostructured composites. However, thin solid films of a perovskite absorber should be capable of operating at the highest efficiency in a simple planar heterojunction configuration. Here, it is shown that film morphology is a critical issue in planar heterojunction CH₃NH₃PbI_3‐xCl_x solar cells. The morphology is carefully controlled by varying processing conditions, and it is demonstrated that the highest photocurrents are attainable only with the highest perovskite surface coverages. With optimized solution based film formation, power conversion efficiencies of up to 11.4% are achieved, the first report of efficiencies above 10% in fully thin‐film solution processed perovskite solar cells with no mesoporous layer.     

3D/2D Perovskite Single Crystals Heterojunction for Suppressed Ions Migration in Hard X-Ray Detection

Halide perovskites exhibit diverse properties depending on their compositions. However, integrating desired properties into one material is still challenging. Here, a facile solution-processed epitaxial growth method to grow 2D perovskite single crystal on top of 3D perovskite single crystal, which can passivate the surface defects for improved device performance is reported. Short formamidine (FA⁺) ions are replaced by long organic cations, which can fully align and cover the single crystal surface to prevent the ions migration or short FA⁺ ions volatilization. The thickness of epitaxial layer can be finely adjusted by controlling the growth time. The defect density of single crystals heterojunction is only 3.18 × 10⁹ cm⁻³, and the carrier mobility is 80.43 cm² V⁻¹ s⁻¹, which is greater than that of the control 3D perovskite single crystal. This study for the first time realized large area 3D/2D perovskite single crystals heterojunction, which suppressed ions migration and exhibited advanced performance in hard X-rays detection applications. This strategy also provides a way to grow large area 2D perovskite single crystal from solution processes.     

Tailoring Crystal Structure of FA0.83Cs0.17PbI3 Perovskite Through Guanidinium Doping for Enhanced Performance and Tunable Hysteresis of Planar Perovskite Solar Cells

Current–voltage hysteresis of perovskite solar cells (PSCs) has raised the concern of accurate performance measurement in practice. Although various theories have been proposed to elucidate this phenomenon, the origin of hysteresis is still an open question. Herein, the use of guanidinium cation (Gu⁺)‐dopant is demonstrated to tailor the crystal structure of mixed‐cation formamidinium‐cesium lead triiodide (FA_0.83Cs_0.17PbI₃) perovskite, resulting in an improved energy conversion efficiency and tunable current–voltage hysteresis characteristic in planar solar cells. Particularly, when the concentration of Gu‐dopant for the perovskite film increases, the normal hysteresis initially observed in the pristine PSC is first suppressed with 2%‐Gu‐dopant, then changed to inverted hysteresis with a higher Gu‐dopant. The hysteresis tunability behavior is attributed to the interplay of charge/ion accumulation and recombination at interfaces in the PSC. Furthermore, compared to the cell without Gu⁺‐dopant, the optimal content of 2% Gu⁺‐dopant also increases the device efficiency by 14%, reaching over 17% under one sun illumination.     

Nucleation Regulation and Anchoring of Halide Ions in All-Inorganic Perovskite Solar Cells Assisted by CuInSe2 Quantum Dots

Ionic feature of halide perovskites may lead to the formation of defect states in the polycrystalline films, which can deteriorate the device performance. To solve this issue, a low-temperature seed-assisted growth method can contribute to defect-passivated CsPbI_1.2Br_1.8 perovskite films by introducing CuInSe₂ quantum dots (QDs). As a result, the migration channels inside perovskite grains can be obviously suppressed, but few positive effects on the ions migration in grain boundaries. Conversely, this appearance demonstrate that the location of CuInSe₂ QDs is grain interior, but not grain boundary. Meanwhile, the lower defect density can help promoting device efficiency from 8.97% to 10.26%, which is among the high-efficiency level. Besides, this work can also provide an in-depth insight into the issue of photoinduced halide segregation.     

Control over Light Soaking Effect in All-Inorganic Perovskite Solar Cells

Light soaking (LS) has been reported to positively influence the device performance of perovskite solar cells (PSCs), which, however, could be potentially harmful to the loaded devices due to the unstable output. There are very few reports on controls over the LS effect, especially in all-inorganic PSCs. In this study, a remarkable LS induced performance enhancement of CsPb(I_1−xBr_x)₃ based PSCs is presented. In situ grazing-incidence wide-angle X-ray scattering measurements quantize the temperature increase under illumination and reveal a radiative heating-induced lattice expansion. The device curing time is shortened with the increased Br/I ratio, evidently correlated with their distinct mobility and activation energy. It is suggested that LS could promote the migration of halide ions, giving rise to notable defect passivation and thus device improvements. Based on these understandings, an effective means is proposed to suppress the LS effect, which is to incorporate slightly over-stochiometric PbI₂ in precursor, and a champion PCE of 18.14% in all-inorganic PSCs with significantly reduced device curing time is obtained.     

Surface Defects Management by In Situ Etching with Methanol for Efficient Inverted Inorganic Perovskite Solar Cells

Inorganic perovskite solar cells (IPSCs) have developed rapidly due to their good thermal stability and the bandgap suitable for perovskite/silicon tandem solar cells. However, the large open-circuit voltage (V_OC) deficit derived from the surface defects and the energy level structure mismatch impede the development of device performance, especially in the P-I-N structure IPSCs. Herein, an innovative in situ etching (ISE) treatment method is proposed to reduce surface defects through methanol without additional passivator. It is found that the perovskite films treated with methanol result in a slight excess of PbI₂ on the surface and inserted into the grain boundaries. Therefore, the successful decrease of surface defects by methanol and the passivation of grain boundary defects by PbI₂ greatly reduce the trap density of perovskite films. And the larger work function of PbI₂ contributes to the energy band of perovskite surface bending downward and forms gradient energy level alignment at the I/N interface, which accelerates extraction of charge carriers. As a result, the efficiency of CsPbI_2.85Br_0.15 inverted IPSC is enhanced from 16.00% to 19.34%, which is one of the mostly efficient IPSCs. This work provides an original idea without additional passivator to manage the defects of inorganic perovskite.     

In Situ Growth of 2D Perovskite Capping Layer for Stable and Efficient Perovskite Solar Cells

2D halide perovskites have recently been recognized as a promising avenue in perovskite solar cells (PSCs) in terms of encouraging stability and defect passivation effect. However, the efficiency (less than 15%) of ultrastable 2D Ruddlesden–Popper PSCs still lag far behind their traditional 3D perovskite counterparts. Here, a rationally designed 2D‐3D perovskite stacking‐layered architecture by in situ growing 2D PEA₂PbI₄ capping layers on top of 3D perovskite film, which drastically improves the stability of PSCs without compromising their high performance, is reported. Such a 2D perovskite capping layer induces larger Fermi‐level splitting in the 2D‐3D perovskite film under light illumination, resulting in an enhanced open‐circuit voltage (V_oc) and thus a higher efficiency of 18.51% in the 2D‐3D PSCs. Time‐resolved photoluminescence decay measurements indicate the facilitated hole extraction in the 2D‐3D stacking‐layered perovskite films, which is ascribed to the optimized energy band alignment and reduced nonradiative recombination at the subgap states. Benefiting from the high moisture resistivity as well as suppressed ion migration of the 2D perovskite, the 2D‐3D PSCs show significantly improved long‐term stability, retaining nearly 90% of the initial power conversion efficiency after 1000 h exposure in the ambient conditions with a high relative humidity level of 60 ± 10%.     

In Situ Inkjet Printing Strategy for Fabricating Perovskite Quantum Dot Patterns

Perovskite quantum dots are emerging as attractive materials for photonic and optoelectronic applications. Patterning is an important step to incorporate them into display, anti‐counterfeiting, and optical chip applications. In this work, an in situ inkjet printing strategy is demonstrated for fabricating perovskite quantum dots patterns by printing perovskite precursor solutions onto a polymeric layer. Importantly, this strategy can achieve bright photoluminescence with a quantum yield up to 80% and shows broad applicability to a variety of perovskites and polymers. Moreover, the as‐fabricated perovskite quantum dots patterns are composed of a microdisks array on the surface of polymeric layer. The size of these microdisks can be varied by adjusting the printing temperature. To demonstrate the potential use in display and advanced anti‐counterfeiting applications, color pixel patterns and 2D code pattern are fabricated by varying the precursor solutions. The combination of superior photoluminescence properties, simple process, and low cost makes the in situ inkjet printing strategy very promising for patterning perovskite quantum dots toward photonic integrations.     

Copper(I) Iodide as Hole‐Conductor in Planar Perovskite Solar Cells: Probing the Origin of J–V Hysteresis

Organic–inorganic lead halide perovskite solar cells are promising alternatives to silicon‐based cells due to their low material costs and high photovoltaic performance. In this work, thin continuous perovskite films are combined with copper(I) iodide (CuI) as inorganic hole‐conducting material to form a planar device architecture. A maximum conversion efficiency of 7.5% with an average efficiency of 5.8 ± 0.8% is achieved which, to our knowledge, is the highest reported efficiency for CuI‐based devices with a planar structure. In contrast to related planar 2,2′,7,7′‐tetrakis‐(N,N ‐di‐4‐methoxyphenylamino)‐9,9′‐spirobifluorene (spiro‐OMeTAD)‐based devices, the CuI‐based devices do not show a pronounced hysteresis when tested by scanning the potential in a forward and backward direction. The strong quenching of photoluminescence (PL) signal and comparatively fast decay of open‐circuit voltage demonstrates a more rapid removal of positive charge carriers from the perovskite layer when in contact with CuI compared to spiro‐OMeTAD. A slow response on a timescale of 10–100 s is observed for the spiro‐OMeTAD‐based devices. In comparison, the CuI‐based device displays a significantly faster response as determined through electrochemical impedance spectroscopy (EIS) and open‐circuit voltage decays (OCVDs). The characteristically slow kinetics measured through EIS and OCVD are linked directly to the current–voltage hysteresis.     

Composition and Energy Band–Modified Commercial FTO Substrate for In Situ Formed Highly Efficient Electron Transport Layer in Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) have received much attention and with them a power conversion efficiency (PCE) of over 22% has been achieved. Electron transport layers (ETLs) based on metal oxide materials play an important role in transferring electrons and reducing back recombination. However, existing fabrication approaches are generally waste too many materials and consume too much energy for commercial application. Here, a brand new plasma preannealing procedure is proposed that can replace the traditional ETL preparation process and alleviate the above‐mentioned problems. A pure SnO₂ phase in situ formed on the fluorine‐doped tin oxide (FTO) surface can be obtained at room temperature by only 15 min oxygen plasma assisted reaction without postheating treatment. It enables the precise control of compositions, defects, and energy levels of band at the surface of FTO substrate, resulting in a prominent PCE of 20.39% with excellent stability and reproducibility. This simple and efficient source‐free fabrication technology provides a versatile platform for the manufacture of PSCs in the future.     

Surface‐Modified Metallic Ti3C2Tx MXene as Electron Transport Layer for Planar Heterojunction Perovskite Solar Cells

MXenes are a large and rapidly expanding family of 2D materials that, owing to their unique optoelectronic properties and tunable surface termination, find a wide range of applications including energy storage and energy conversion. In this work, Ti₃C₂T_x MXene nanosheets are applied as a novel type of electron transport layer (ETL) in low‐temperature processed planar‐structured perovskite solar cells (PSCs). Interestingly, simple UV‐ozone treatment of the metallic Ti₃C₂T_x that increases the surface Ti? O bonds without any change in its bulk properties such as high electron mobility improves its suitability as an ETL. Improved electron transfer and suppressed recombination at the ETL/perovskite interface results in augmentation of the power conversion efficiency (PCE) from 5.00% in the case of Ti₃C₂T_x without UV‐ozone treatment to the champion PCE of 17.17%, achieved using the Ti₃C₂T_x film after 30 min of UV‐ozone treatment. As the first report on the use of pure MXene layer as an ETL in PSCs, this work shows the great potential of MXenes to be used in PSCs and displays their promise for applications in photovoltaic technology in general.     

In Situ Photosynthesis of an MAPbI3/CoP Hybrid Heterojunction for Efficient Photocatalytic Hydrogen Evolution

Halide perovskite like methylammonium lead iodide perovskite (MAPbI₃) with its prominent optoelectronic properties has triggered substantial concerns in photocatalytic H₂ evolution. In this work, to attain preferable photocatalytic performance, a MAPbI₃/cobalt phosphide (CoP) hybrid heterojunction is constructed by a facile in situ photosynthesis approach. Systematic investigations reveal that the CoP nanoparticle can work as co‐catalyst to not only extract photogenerated electrons effectively from MAPbI₃ to improve the photoinduced charge separation, but also facilitate the interfacial catalytic reaction. As a result, the as‐achieved MAPbI₃/CoP hybrid displays a superior H₂ evolution rate of 785.9 µmol h^?1 g^?1 in hydroiodic acid solution within 3 h, which is ≈8.0 times higher than that of pristine MAPbI₃. Furthermore, the H₂ evolution rate of MAPbI₃/CoP hybrid can reach 2087.5 µmol h^?1 g^?1 when the photocatalytic reaction time reaches 27 h. This study employs a facile in situ photosynthesis strategy to deposit the metal phosphide co‐catalyst on halide perovskite nanocrystals to conduct photocatalytic H₂ evolution reaction, which may stimulate the intensive investigation of perovskite/co‐catalyst hybrid systems for future photocatalytic applications.     

Revealing the Dynamics of Hybrid Metal Halide Perovskite Formation via Multimodal In Situ Probes

The exploration of the synthetic space of halide perovskites hinges on an enormous number of parameters requiring time‐consuming experimentation to decouple and optimize. Here, the formation of the prototype material CH₃NH₃PbI₃ (MAPbI₃) is investigated at different time and length scales using multimodal in situ measurements to monitor the evolution of crystalline phases, morphology, and photoluminescence as a function of the lead precursors. Kinetically fast formation of crystalline precursor phases already during the spin‐coat deposition is observed using lead iodide (PbI₂) or lead chloride (PbCl₂) routes. These precursor phases most likely template final MAPbI₃ film morphology. In particular, the emergence of the “needle‐like” structure is shown to appear before film annealing. In situ photoluminescence measurements suggest nanoscale nucleation followed by rapid nuclei densification and growth. Using this multimodal in situ approach, different formation pathways can be identified either via precursor phases in the PbI₂ and PbCl₂ routes or direct perovskite formation from molecular building blocks as observed in the lead acetate (PbAc₂) route. Correlation of in situ results with photovoltaic device performance demonstrates the power of in situ multimodal techniques, paves the way to a fast screening of synthetic parameters, and ultimately leads to controlled synthetic procedures that yield high‐efficiency devices.     

Electron‐Transport‐Layer‐Assisted Crystallization of Perovskite Films for High‐Efficiency Planar Heterojunction Solar Cells

Crystal engineering of CH₃NH₃PbI₃ perovskite materials through template‐directed nucleation and growth on PbI₂ nuclei dispersed in a polar fullerene (C₆₀ pyrrolidine tris‐acid, CPTA) electron transport layer (ETL) (CPTA:PbI₂) is proposed as a route for controlling crystallization kinetics and grain sizes. Chemical analysis of the CPTA:PbI₂ template confirms that CPTA carboxylic acid groups can form a monodentate or bidentate chelate with Pb(II), resulting in a lower nucleation barrier that promotes rapid formation of the tetragonal perovskite phase. Moreover, it is demonstrated that a uniform CH₃NH₃PbI₃ film with highly crystalline and large domain sizes can be realized by increasing the spacing between nuclei to retard perovskite crystal growth via careful control of the preferred nucleation site distribution in the CPTA:PbI₂ layer. The improved perovskite morphology possesses a long photoluminescence lifetime and efficient photocarrier transport/separation properties to eliminate the hysteresis effect. The corresponding planar heterojunction photovoltaic yields a high power conversion efficiency (PCE) of 20.20%, with a high fill factor (FF) of 81.13%. The average PCE and FF values for 30 devices are 19.03% ± 0.57% and 78.67% ± 2.13%, respectively. The results indicate that this ETL template‐assisted crystallization strategy can be applied to other organometal halide perovskite‐based systems.     

In Situ Crosslinkable Gelatin Hydrogels for Vasculogenic Induction and Delivery of Mesenchymal Stem Cells

Clinical trials utilizing mesenchymal stem cells (MSCs) for severe vascular diseases have highlighted the need to effectively engraft cells and promote pro‐angiogenic activity. A functional material accomplishing these two goals is an ideal solution as spatiotemporal and batch‐to‐batch variability in classical therapeutic delivery can be minimized, and tissue regeneration would begin rapidly at the implantation site. Gelatin may serve as a promising biomaterial due to its excellent biocompatibility, biodegradability, and non‐immuno/antigenicity. However, the dissolution of gelatin at body temperature and quick enzymatic degradation in vivo have limited its use thus far. To overcome these challenges, an injectable, in situ crosslinkable gelatin was developed by conjugating enzymatically crosslinkable hydroxyphenyl propionic acid (GHPA). When MSCs are cultured in 3D in vitro or injected in vivo in GHPA, spontaneous endothelial differentiation occurs, as evidenced by marked increases in endothlelial cell marker expressions (Flk1, Tie2, ANGPT1, vWF) in addition to forming an extensive perfusable vascular network after 2‐week subcutaneous implantation. Additionally, favorable host macrophage response is achieved with GHPA as shown by decreased iNOS and increased MRC1 expression. These results indicate GHPA as a promising soluble factor‐free cell delivery template which induces endothelial differentiation of MSCs with robust neovasculature formation and favorable host response.     

GSMBE生长SiGe/Si材料的原位掺杂控制技术

在特定温控下对掺杂气体分子的状态和活性进行控制 ,建立了一套具有自主知识产权的气源分子束外延工艺生长 Si Ge/Si材料的原位掺杂控制技术。采用该技术生长的 Si Ge/Si HBT外延材料 ,可将硼杂质较好地限制在 Si Ge合金基区内 ,并能有效地提高磷烷对 N型掺杂的浓度和外延硅层的生长速率 ,获得了理想 N、P型杂质分布的 Si Ge/Si HBT外延材料     

Merocyanine/C60 Planar Heterojunction Solar Cells: Effect of Dye Orientation on Exciton Dissociation and Solar Cell Performance

In this study the charge dissociation at the donor/acceptor heterointerface of thermally evaporated planar heterojunction merocyanine/C₆₀ organic solar cells is investigated. Deposition of the donor material on a heated substrate as well as post‐annealing of the complete devices at temperatures above the glass transition temperature of the donor material results in a twofold increase of the fill factor. An analytical model employing an electric‐field‐dependent exciton dissociation mechanism reveals that geminate recombination is limiting the performance of as‐deposited cells. Fourier‐transform infrared ellipsometry shows that, at temperatures above the glass transition temperature of the donor material, the orientation of the dye molecules in the donor films undergoes changes upon annealing. Based on this finding, the influence of the dye molecules’ orientations on the charge‐transfer state energies is calculated by quantum mechanical/molecular mechanics methods. The results of these detailed studies provide new insight into the exciton dissociation process in organic photovoltaic devices, and thus valuable guidelines for designing new donor materials.