JIALINGBO

Spiro‐OMeTAD has been widely used as a promising hole conductor for metal halide perovskite solar cells due to its ability to deliver highly efficient devices. However, additives such as lithium salt and O₂ exposure are still required to modify the electrical properties due to the poor conductivity of pristine spiro‐OMeTAD. In article number https://doi.org/10.1002/aenm.2019015191901519, Jianfeng Lu, Udo Bach and co‐workers employ the oxidized form of spiro‐OMeTAD as a dopant to improve the efficiency of the spiro‐OMeTAD‐based lithium‐free perovskite solar cells from 10% to 19.3% while simultaneously enhancing the device stability.

Homojunction bilayer electron transport layers (ETLs) are developed by stacking Sb‐doped SnO₂ (Sb‐SnO₂) and SnO₂ ETLs via a low‐temperature process. The perovskite solar cells with the Sb‐SnO₂/SnO₂ bilayer ETLs achieve the best power conversion efficiency of 20.73%. Due to Sb‐SnO₂/SnO₂, the bilayer ETL with a cascade energy arrangement enhances charge separation and reduces carrier recombination.

Energy alignment between electron transport layers (ETLs) and perovskite has a strong influence on the device performance of perovskite solar cells (PSCs). Two approaches are deployed to tune the energy level of ETLs: 1) doping ETLs with aliovalent metal cations and 2) constructing heterojunction bilayers with different materials. However, the abrupt interfaces in the heterojunction bilayers introduce undesirable carrier recombination. Herein, a homojunction bilayer ETL is developed by stacking Sb‐doped SnO₂ (Sb‐SnO₂) and SnO₂ ETLs via low‐temperature spin‐coating processes. The energy levels of ETLs are tuned by the incorporation of Sb and altering stacking orders. Bilayer ETL of Sb‐SnO₂/SnO₂ with cascade energy alignment promotes the best power conversion efficiency of 20.73%, surpassing single‐layer ETLs of SnO₂ (18.23%) and Sb‐SnO₂ (19.15%), whereas the SnO₂/Sb‐SnO₂ bilayer with barricade energy alignment receives the poorest device performance. The cascade bilayer ETL facilitates charge separation and suppresses carrier recombination in PSCs, which is verified by photoluminescence, conductivity, and impedance characterizations. The homojunction bilayer ETLs with adjustable energy levels open a new direction for interface engineering toward efficient PSCs.

A new method is developed to synthesize SnO _x ‐Cl colloids and to realize an in situ and spontaneous ion‐exchange reaction during the perovskite film crystallization process. It is found that such ion exchange can perfectly passivate the interface defects and reduce energy loss at the interface.

Abstract

Interface engineering is of great concern in photovoltaic devices. For the solution‐processed perovskite solar cells, the modification of the bottom surface of the perovskite layer is a challenge due to solvent incompatibility. Herein, a Cl‐containing tin‐based electron transport layer; SnO _x ‐Cl, is designed to realize an in situ, spontaneous ion‐exchange reaction at the interface of SnO _x ‐Cl/MAPbI₃. The interfacial ion rearrangement not only effectively passivates the physical contact defects, but, at the same time, the diffusion of Cl ions in the perovskite film also causes longitudinal grain growth and further reduces the grain boundary density. As a result, an efficiency of 20.32% is achieved with an extremely high open‐circuit voltage of 1.19 V. This versatile design of the underlying carrier transport layer provides a new way to improve the performance of perovskite solar cells and other optoelectronic devices.

In article number https://doi.org/10.1002/adfm.2019023191902319, featuring a synchrotron‐based in situ X‐ray diffraction technique, Gang Chen and co‐workers investigate the sequential deposition process of FAPbI₃ from a smooth PbI₂ film to a trilayer PbI₂‐FAI composite film and finally to the oriented perovskite film. The effects of the additive ions of Br⁻, Cl⁻, and MA⁺ on the crystallization dynamics and grain orientation of the resultant perovskite films are also surveyed.

Efficient singlet oxygen generation, near‐infrared aggregation‐induced emission, and broad absorption are achieved in one single molecule of TBTC8 through precise molecular design. The polymer‐encapsulated TBTC8 nanoparticles are demonstrated to show promising results for photodynamic anticancer therapy.

Abstract

Owing to efficient singlet oxygen (¹O₂) generation in aggregate state, photosensitizers (PSs) with aggregation‐induced emission (AIE) have attracted much research interests in photodynamic therapy (PDT). In addition to high ¹O₂ generation efficiency, strong molar absorption in long‐wavelength range and near‐infrared (NIR) emission are also highly desirable, but difficult to achieve for AIE PSs since the twisted structures in AIE moieties usually lead to absorption and emission in short‐wavelength range. In this contribution, through acceptor engineering, a new AIE PS of TBT is designed to show aggregation‐induced NIR emission centered at 810 nm, broad absorption in the range between 300 and 700 nm with a large molar absorption coefficient and a high ¹O₂ generation efficiency under white light irradiation. Further, donor engineering by attaching two branched flexible chains to TBT yielded TBTC8, which circumvented the strong intermolecular interactions of TBT in nanoparticles (NPs), yielding TBTC8 NPs with optimized overall performance in ¹O₂ generation, absorption, and emission. Subsequent PDT results in both in vitro and in vivo studies indicate that TBTC8 NPs are promising candidates in practical application.

A rational core–shell design of open air low temperature in situ processable CsPbI₃ quasi‐nanocrystals is proposed. A bifunctional ligand 4‐fluorophenethylammonium iodide and new compound H₂PbI₄ increase crystal stability, charge extraction, and assist divalent ion doping, respectively. The best p‐i‐n solar cell with 13.4% efficiency can retain 72% beyond 500 h in ambient air without encapsulation.

Abstract

As a promising alternative, inorganic perovskite nanocrystals allow reinforced stability of photovoltaic device. Unfortunately, directly assembling these nanocrystals into film is uncontrollable. Instead, in situ assembling technology under low temperature in open air is attractive but limited due to the tendency of nonperovskite transition. The adverse shell ligands and unstable core lattices are known as the fundamental problems. In order to address this issue, here proposed is a rational core–shell design: 1) with respect to ligands, a new one, 4‐fluorophenethylammonium iodide, is used to enhance bonding force and charge coupling between ligands and nanocrystals; 2) with respect to lattices, a novel compound H₂PbI₄ is employed to assist divalent ion (Mn²⁺) doping into perovskite lattices. By low temperature in situ processing CsPbI₃ quasi‐nanocrystal film, the highest power conversion efficiency of 13.4% for p‐i‐n solar cells is achieved, which retains 92% after 500 h in ambient air. The current study underlines the significance of rational hierarchical design of inorganic perovskite nanocrystals, especially for low temperature in situ processable electronic devices.

CsPbBr₃ perovskite quantum dots (QDs)‐loaded poly(methyl methacrylate) composite microspheres are easily prepared through in situ polymerization of methyl methacrylate in the presence of quantum dots in hexane. This method is very facile and the CsPbBr₃ perovskite QDs are evenly incorporated into the microspheres. Protected by the microsphere, the water and storage stability of CsPbBr₃ quantum dots are greatly improved.

Abstract

In this paper, a facile synthesis of water‐resistant CsPbBr₃ perovskite quantum dots (PQDs) loaded poly(methyl methacrylate) (PMMA) composite microspheres (CsPbBr₃@PMMA) is reported. The method is based on the precipitation polymerization of methyl methacrylate in hexane in the presence of CsPbBr₃ PQDs and stabilizer. The CsPbBr₃@PMMA microspheres show a tunable size and a narrow size distribution, with the CsPbBr₃ PQDs being uniformly dispersed in the PMMA microspheres. The effective incorporation of PQDs is attributed to the strong coordination interactions between Pb ions on the surface of PQDs and carbonyl groups (CO) from PMMA. Based on this mechanism, multicolor composite microspheres can be easily prepared through absorbing CsPbX₃ (X = Cl, Br, I) PQDs into blank PMMA microspheres. Protected by the PMMA microspheres, the imbedded CsPbBr₃ PQDs show improved water resistance and storage stability. Further, a wide‐color‐gamut (129%) white light‐emitting diode (LED) is demonstrated by combining the green‐emitting CsPbBr₃@PMMA composite microspheres and red‐emitting K₂SiF₆: Mn⁴⁺ with a blue LED, which enables to be used as backlights for liquid crystal displays.

Herein, a facile method to improve the long‐term stability of perovskite solar cells using N719 dye molecules as additives is presented. Perovskite‐dye hybrid films show better crystallinity, enhanced light absorption, and boosted moisture stability. Due to the greatly retarded hydration process, the degradation process of perovskite‐dye solar cells is three times longer than that for pristine devices.

Metal‐halide perovskite solar cells (PSC) have shown great success in achieving high efficiencies but less satisfaction in achieving long‐term stability. Perovskites are prone to forming perovskite hydrates in humid environments, which leads to the decomposition of the perovskite materials. Herein, a common and cheap dye molecule, called cis‐di(thiocyanato)bis(2,2‐bipyridyl4,4‐dicarboxylate)ruthenium(II), denoted as N719, is introduced into mixed‐cation mixed‐halide perovskites for better moisture stability. It is discovered that the N719 molecules form perovskite‐dye complexes in the precursor solution, leading to larger grains and better film crystallinity by slowing down the crystallization process. Fourier‐transform infrared spectroscopy and X‐ray diffraction characterizations suggest that the N719 molecules exist in the crystallized perovskite films but are not incorporated into the perovskite crystal lattice. The presence of N719 molecules in perovskite films greatly retards the formation of perovskite hydrates due to a three‐times‐increased water migration barrier. Owing to these improvements, nonencapsulated N719‐PSC retain over 80% of their original efficiencies after aging under a high relative humidity of 60% for 250 h, which is three times longer than that for pristine cells. A cheap and effective route for controlling the perovskite crystallization process and improving the stability of PSC without sacrificing device efficiency is represented.

Grain boundaries (GBs) play an important role in most polycrystalline solar cells. In this essay, three important questions are explored: Do GBs affect: 1) recombination and thus open‐circuit voltage? Not dramatically, if at all; 2) current–voltage hysteresis? Most studies show that hysteresis is dominated by defects at GBs; and 3) long‐term durability? Yes, GBs definitely help increase the rate of perovskite degradation.

Abstract

Grain boundaries (GBs) play an important role in most polycrystalline solar cells. In perovskite solar cells, the research community is just starting to understand their effects on performance and long‐term durability. In this essay, three important questions are explored: Do GBs affect: 1) recombination and thus open‐circuit voltage? Not dramatically, if at all; 2) current–voltage hysteresis? Most studies show that hysteresis is dominated by defects at GBs; and 3) long‐term durability? Yes, GBs definitely help increase the rate of perovskite degradation. In this essay, the latest reports are summarized and the authors' perspective on this very important subject is given.

A novel method is developed for fabricating high‐quality Ruddlesden–Popper perovskite films by directly using commercially available organic amines, avoiding extra chemical synthesis processing of organic ammonium halides. This new approach results in similar (and even better) device performance for both solar cells and light‐emitting diodes when compared with control devices fabricated from organic ammonium halides.

Abstract

Ruddlesden–Popper perovskites (RPPs), consisting of alternating organic spacer layers and inorganic layers, have emerged as a promising alternative to 3D perovskites for both photovoltaic and light‐emitting applications. The organic spacer layers provide a wide range of new possibilities to tune the properties and even provide new functionalities for RPPs. However, the preparation of state‐of‐the‐art RPPs requires organic ammonium halides as the starting materials, which need to be ex situ synthesized. A novel approach to prepare high‐quality RPP films through in situ formation of organic spacer cations from amines is presented. Compared with control devices fabricated from organic ammonium halides, this new approach results in similar (and even better) device performance for both solar cells and light‐emitting diodes. High‐quality RPP films are fabricated based on different types of amines, demonstrating the universality of the approach. This approach not only represents a new pathway to fabricate efficient devices based on RPPs, but also provides an effective method to screen new organic spacers with further improved performance.

A fullerene derivative, [6,6]‐phenyl‐C₆₁‐butyric acid‐N,N‐dimethyl‐3‐(2‐thienyl)propanam ester (PCBB‐S‐N), is designed and synthesized to correct defects in electron‐transporting layers (ETLs) and perovskite films. Its use leads to a promising power conversion efficiency (PCE) of 21.08% for perovskite solar cells. Importantly, devices containing PCBB‐S‐N simultaneously realize excellent thermal stability and water resistance.

Abstract

The poor long‐term stability of organic–inorganic hybrid halide perovskite solar cells (pero‐SCs) remains a big challenge for their commercialization. Although strategies such as encapsulation, doping, and passivation have been reported, there remains a lack of understanding of the water resistance and thermal stability of pero‐SCs. A fullerene derivative, [6,6]‐phenyl‐C₆₁‐butyric acid‐N,N‐dimethyl‐3‐(2‐thienyl)propanam ester (PCBB‐S‐N) containing a functional sulfur atom and C_60, is synthesized and employed as electron transporting layer (ETL)/intermediary layer to targetedly heal the multitype defects in pero‐SCs or assist the growth of ETL, such as [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), in planar p‐i‐n pero‐SCs. The repaired pero‐SCs can not only dramatically improve their power conversion efficiencies, but also address stability issues under moisture and high temperature. The corresponding mechanism of PCBB‐S‐N with targeted therapy effect in a device is systematically investigated by both experiments and theoretical calculation. This work demonstrates that the proposed fullerene derivative with finely tuned chemical structure can be a promising ETL candidate or intermediary to approach stable and efficient planar p‐i‐n pero‐SCs.

In article no. 1900192, Ai Ping Chen, Shuang Yang, Yu Hou, and co‐workers employ a versatile alkaline earth metals doping strategy to engineer the electronic structure of NiO_x contacts for inverted planar perovskite solar cells, in which the champion device demonstrates a power conversion efficiency of 19.49% with a high open circuit voltage of 1.14 V. Alkaline earth metals doping can significantly optimize the electrical properties by deepening the valence band maximum and enhancing the hole conductivity.

Diboron‐treated SnO₂ exhibits some Sn³⁺ species, which serve as electron donors with more n‐type nature, resulting in the higher Fermi level on the surface of SnO₂, promoting electron extraction and reducing carrier recombination in the electron transport layer (ETL)/perovskite interface. A power‐conversion efficiency of 22.04% is obtained in an n‐i‐p structure perovskite solar cell.

Energy‐level modulation between perovskite and carrier transport layers to obtain a promoted carrier extraction and reduced charge recombination is an effective way to achieve high‐efficiency perovskite solar cells. Here, diboron is used as an effective interfacial modifier between SnO₂ and perovskite. By taking advantage of the higher Fermi level on the surface of SnO₂ after diboron treatment, a power‐conversion efficiency of 22.04% in a solar cell device based on two‐step solution‐processed planar n‐i‐p structure is obtained. With the help of thorough characterizations, it is argued that the diboron‐treated SnO₂ exhibits some Sn³⁺ species, which serve as electron donors with a more n‐type nature, promoting electron extraction and reducing carrier recombination in the electron transport layer (ETL)/perovskite interface. Further analysis speculates that the formation of surface diboron–oxygen Lewis pair induces a reducing state of diboron complexes, resulting in the spontaneous electron redistribution and the formation of Sn³⁺−O^–• species. This provides an effective chemical approach to tune the energy alignment between the oxide ETL and absorber.

Herein, a facile strategy that can carry out double passivation to improve the performance of perovskite solar cells (PSCs) is demonstrated. By using the dilute halide salt PEABr solution to treat the perovskite film, PbI₂ can precipitate from the perovskite. Both PEABr and PbI₂ can passivate the perovskite film; double passivation improves the performance of PSCs significantly.

Material passivation is essential to enhance the quality of perovskite materials and boost the performance of perovskite solar cells (PSCs). However, most of the previous reports only paid attention to improving the quality of perovskite films by adopting single passivation. Here, a facile strategy that can carry out double passivation to improve the performance of PSCs is demonstrated. By using the dilute halide salt PEABr solution to treat the perovskite film, PbI₂ can precipitate from the perovskite. Both PEABr and PbI₂ can passivate the perovskite film, and by combining PEABr and PbI₂, the double passivation improves the performance of PSCs significantly. Very high short‐circuit current density of 24.30 mA cm⁻², open‐circuit voltage of 1.10 V, and fill factor of 79.75% are achieved which lead to a surprising efficiency of 21.32% for the passivated device. The improved efficiency is mainly according to the available surface passivation of the perovskite material, leading to repressed nonradiative recombination and unhindered charge collection. In addition, the passivated device exhibits better power conversion efficiency stability relative to the control device.

Photocurrent–voltage hysteresis in perovskite solar cells (PSCs) induced by ion migration combined with nonradiative recombination near the interface depends on perovskite composition and device structure. Among the methods used in the attempt to reduce the hysteresis, potassium‐ion doping is found to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition.

Abstract

Current‐density–voltage (J–V) hysteresis in perovskite solar cells (PSCs) is a critical issue because it is related to power conversion efficiency and stability. Although parameters affecting the hysteresis have been already reported and reviewed, little investigation is reported on scan‐direction‐dependent J–V curves depending on perovskite composition. This review investigates J–V hysteric behaviors depending on perovskite composition in normal mesoscopic and planar structure. In addition, methodologies toward hysteresis‐free PSCs are proposed. There is a specific trend in hysteresis in terms of J–V curve shape depending on composition. Ion migration combined with nonradiative recombination near interfaces plays a critical role in generating hysteresis. Interfacial engineering is found to be an effective method to reduce the hysteresis; however, bulk defect engineering is the most promising method to remove the hysteresis. Among the studied methods, KI doping is proved to be a universal approach toward hysteresis‐free PSCs regardless of perovskite composition. It is proposed from the current studies that engineering of perovskite film near the electron transporting layer (ETL) and the hole transporting layer (HTL) is of vital importance for achieving hysteresis‐free PSCs and extremely high efficiency.

The interface and bulk defects of perovskite solar cells are distinguished and quantified, and are for the first time traced in situ using an expanded admittance model. A fullerene derivative [6, 6]‐phenyl‐C61‐butyric acid (PCBA) is introduced into the TiO₂/perovskite interface to release the interface stress.

Abstract

The stability issue that is obstructing commercialization of the perovskite solar cell is widely recognized, and tremendous effort has been dedicated to solving this issue. However, beyond the apparent thermal and moisture stability, more intrinsic semiconductor mechanisms regarding defect behavior have yet to be explored and understood. Herein, defects are quantified; especially interface defects, within the cell to reveal their impact on device performance and especially stability. Both the bulk and interface defects are distinguished and traced in situ using an expanded admittance model when the cell degrades in its efficiency under illumination or voltage. The electric field‐induced interface, rather than bulk defects, is found to have a direct correlation to stability. Releasing the interface strain using a fullerene derivative is an effective way to suppress interface defect formation and improve stability. Overall, this work provides a quantitative approach to probing the semiconductor mechanism behind the stability issue, and the inherent correlation discovered here among the electric field, interface strain, interface defects, and cell stability has important implications for ongoing device stability engineering.

With the synthesis of two novel hole transport materials, the inverted planar perovskite solar cell achieves a high fill factor of 81.7%, with an efficiency exceeding 19%. More importantly, a highly possible correlation between the molecular packing, hole mobility, and device performance is revealed, which provides some insights for the rational design of hole transport materials.

Abstract

In this paper, two novel D‐π‐D hole‐transporting materials (HTM) are reported, abbreviated as BDT‐PTZ and BDT‐POZ, which consist of 4,8‐di(hexylthio)‐benzo[1,2‐b:4,5‐b′]dithiophene (BDT) as π‐conjugated linker, and N‐(6‐bromohexyl) phenothiazine (PTZ)/N‐(6‐bromohexyl) phenoxazine (POZ) as donor units. The above two HTMs are deployed in p‐i‐n perovskite solar cells (PSCs) as dopant‐free HT layers, exhibiting excellent power conversion efficiencies of 18.26% and 19.16%, respectively. Particularly, BDT‐POZ demonstrates a superior fill factor of 81.7%, which is consistent with its more efficient hole extraction and transport verified via steady‐state/transient fluorescence spectra and space‐charge‐limited current technique. Single‐crystal X‐ray diffraction characterization implies these two molecules present diverse packing tendencies, which may account for various interfacial hole‐transport ability in PSCs.

1,2,4‐triazole is a stable and efficient aromatic compound having triamine structure that can improve the bond strength and electronic properties of perovskite with the reduced carrier traps. Proper alloying of 1,2,4‐triazole greatly stabilizes triple‐cation perovskite, allowing extremely high stability under 85 °C/85% relative humidity for 700 h and a high power conversion efficiency of 20.9% with spiro‐OMeTAD as a hole‐transporting material.

Abstract

Operational stability of perovskite solar cells has been a challenge from the beginning of perovskite research. In general, humidity and heat are the most well‐known degradation sources for perovskites, requiring ideal design of perovskite chemistry to withstand them. Although triple‐cation perovskite (Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃) has been already introduced as the stable perovskite material, the high reactivity of methylammonium and formamidinium in the cation sites demands further modification. Herein, 1,2,4‐triazole is suggested as an effective cation solute to improve the performance and stability of perovskite solar cells. 1,2,4‐Triazole is an aromatic cation with low dipole moment that is stable under humidity and heat. It also possesses three nitrogen atoms, forming additional hydrogen bonds in the lattice, stabilizing the material. In this study, the solar cell utilizing 1,2,4‐triazole alloying achieves a power conversion efficiency of 20.9% with superior stability under extreme condition (85 °C/85% of relative humidity (RH), encapsulated) for 700 h. The 1,2,4‐triazole‐alloyed perovskite exhibits reduced trap density and film roughness and enhanced carrier lifetime with electrical conductivity, suggesting an ideal perovskite structure for efficient and stable optoelectronic applications.

A thin layer of C₆₀ pyrrolidine tris‐acid is found essential for achieving high efficiency with planar solar cells of Sn‐based perovskites. As a result, a power conversion efficiency of 7.40% is achieved for FASnI₃ solar cells with a planar n–i–p architecture. For the first time, highly efficient Sn‐based hybrid perovskite solar cells on n–i–p architecture is achieved.

Abstract

For solar cell applications, Sn‐based hybrid perovskites have drawn particular interest due to their environmental friendliness. Here, a thin layer of C₆₀ pyrrolidine tris‐acid (CPTA) is found essential for achieving high efficiency with planar solar cells of Sn‐based perovskites. As a result, a power conversion efficiency of 7.40% is achieved for {en}FASnI₃ solar cells with a planar n–i–p architecture, and the device exhibits excellent stability in air. For the first time, highly efficient Sn‐based hybrid perovskite solar cells on n–i–p architecture are achieved. A V _oc of 0.72 V is highlighted as the highest V _oc ever reported for FASnI₃ solar cells.

A systematic study of structurally similar fullerene derivatives shows that even minor modifications in their structure have a strong impact on their performance as electron transport layer (ETL) materials for perovskite solar cells. The best ETL significantly improves ambient stability of the devices for >800 h presumably due to an optimal size/shape of the solubilizing addend enabling compact molecular packing.

It is known that the operation lifetime of perovskite solar cells can be extended by orders of magnitude if properly selected hole‐transport and electron transport layers provide good isolation for the perovskite absorber preventing evaporation of volatile species (e.g., photoinduced) from the active layer and blocking the diffusion of aggressive moisture and oxygen from the surrounding environment. Herein, a systematic study of a family of structurally similar fullerene derivatives as electron transport layer (ETL) materials for p‐i‐n perovskite solar cells is presented. It is shown that even minor modifications of the molecular structure of the fullerene derivatives have a strong impact on their electrical performance and, particularly, ambient stability of the devices. Indeed, an optimally functionalized fullerene derivative applied as an ETL enables stable operation of perovskite solar cells when exposed to air for >800 h, which is manifested in retention of 90% of the original photovoltaic performance. In contrast, the reference devices with phenyl‐C₆₁‐butyric acid methyl ester as the ETL degraded almost completely within less than 100 h of air exposure. Most probably, the side chains of the best‐performing fullerene ETL materials are filling the gaps between the carbon spheres, thus preventing the diffusion of oxygen and moisture inside the device.