Ligang Yuan

Paintable carbon electrode-based perovskite solar cells (PSCs) are of particular interest due to their material and fabrication process costs, as well as their moisture stability. However, printing the carbon paste on the perovskite layer limits the quality of the interface between the perovskite layer and carbon electrode. Herein, an attempt to enhance the performance of the paintable carbon-based PSCs is made using a modified solvent dripping method that involves dripping of the carbon nanotubes (CNTs), which is dispersed in chlorobenzene solution. This method allows CNTs to penetrate into both the perovskite film and carbon electrode, facilitating fast hole transport between the two layers. Furthermore, this method is results in increased open circuit voltage (V_oc) and fill factor (FF), providing better contact at the perovskite/carbon interfaces. The best devices made with CNT dripping show 13.57% power conversion efficiency and hysteresis-free performance.

A novel approach for efficient carbon-based perovskite solar cells is explored using a modified solvent engineering method, which involves dripping of carbon nanotubes (CNT)-dispersed in a chlorobenzene solution, while spin-coating a perovskite precursor solution. This method resolves the intrinsically low-quality of the perovskite/carbon interface. In addition, the CNTs act as pathways for fast hole extraction to the carbon electrode.

Hybrid organic/inorganic perovskite solar cells are invigorating the photovoltaic community due to their remarkable properties and efficiency. However, many perovskite solar cells show an undesirable current–voltage (I–V) hysteresis in their forward and reverse voltage scans, working to the detriment of device characterization and performance. This hysteresis likely arises from slow ion migration in the bulk perovskite active layer to interfaces which may induce charge trapping. It is shown that interfacial chemistry between the perovskite and charge transport layer plays a critical role in ion transport and I–V hysteresis in perovskite-based devices. Specifically, phenylene vinylene polymers containing cationic, zwitterionic, or anionic pendent groups are utilized to fabricate charge transport layers with specific interfacial ionic functionalities. The interfacial-adsorbing boundary induced by the zwitterionic polymer in contact with the perovskite increases the local ion concentration, which is responsible for the observed I–V hysteresis. Moreover, the ion adsorbing properties of this interface are exploited for perovskite-based memristors. This fundamental study of I–V hysteresis in perovskite-based devices introduces a new mechanism for inducing memristor behavior by interfacial ion adsorption.

Ion migration at perovskite interfaces is investigated by varying the interface with cationic, anionic, and zwitterionic functionalities. The zwitterionic polymer interlayer generates an adsorbing boundary at the interface, increasing the local ion concentration, causing current–voltage (I–V) hysteresis in perovskite-based devices. This fundamental study of perovskite I–V hysteresis introduces a new mechanism for device memristor behavior by interfacial ion adsorption.

Fullerene derivatives, which possess extraordinary geometric shapes and high electron affinity, have attracted significant attention for thin film technologies. This study demonstrates an important photovoltaic application using carboxyl-functionalized carbon buckyballs, C60 pyrrolidine tris-acid (CPTA), to fabricate electron transport layers (ETLs) that replace traditional metal oxide-based ETLs in efficient and stable n-i-p-structured planar perovskite solar cells (PSCs). The uniform CPTA film is covalently anchored onto the surface of indium tin oxide (ITO), significantly suppressing hysteresis and enhancing the flexural strength in the CPTA-modified PSCs. Moreover, solution-processable CPTA-based ETLs also enable the fabrication of lightweight flexible PSCs. The maximum-performing device structures composed of ITO/CPTA/CH₃NH₃PbI₃/2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD)/Au yield power conversion efficiencies of more than 18% on glass substrates and up to 17% on flexible substrates. These results indicate that the CPTA layers provide new opportunities for solution-processed organic ETLs by substantially simplifying the procedure for fabricating PSCs for portable applications.

A low-temperature and solution-processed polar C60 pyrrolidine tris-acid (CPTA) electron transport layer (ETL) with conformal morphology, deposited on an indium tin oxide surface through an esterification step, is used to produce hysteresis-free, bendable, and durable perovskite solar cells. Our results suggest that CPTA is a promising candidate to replace metal oxides and shed light on employing these easily fabricated ETLs in other portable photovoltaic technologies.

Stable oxide topological insulators (TIs) have been sought for years, but none have been found; whereas heavier (selenides, tellurides) chalcogenides can be TIs. The basic contradiction between topological insulation and thermodynamic stability is pointed out, offering a narrow window of opportunity. The electronic motif is first identified and can achieve topological band inversion in ABO₃ as a lone-pair, electron-rich B atom (e.g., Te, I, Bi) at the octahedral site. Then, twelve ABO₃ compounds are designed in the assumed cubic perovskite structure, which satisfy this electronic motif and are indeed found by density function theory calculations to be TIs. Next, it is illustrated that poorly screened ionic oxides with large inversion energies undergo energy-lowering atomic distortions that destabilize the cubic TI phase and remove band inversion. The coexistence windows of topological band inversion and structure stability can nevertheless be expanded under moderate pressures (15 and 35 GPa, respectively, for BaTeO₃ and RbIO₃). This study traces the principles needed to design stable oxide topological insulators at ambient pressures as a) a search for oxides with small inversion energies; b) design of large inversion-energy oxide TIs that can be stabilized by pressure; and c) a search for covalent oxides where TI-removing atomic displacements can be effectively screened out.

ABO₃ perovskites where B is an electron-rich element would be topological insulators if the cubic structure could be stabilized. But in such band-inverted topological insulators, depopulation of bonding valence states and occupation of antibonding conduction states raises the energy, leading, in some cases, to structural distortions that are poorly shielded in oxides. In the distorted structure the band inversion is reversed.

Most of the high-performance all-polymer solar cells (all-PSCs) reported to date are based on polymer donor and polymer acceptor pairs with largely overlapped light absorption properties, which seriously limits the efficiency of all-PSCs. This study reports the development of a series of random copolymer donors possessing complementary light absorption with the naphthalenediimide-based polymer acceptor P(NDI2HD-T2) for highly efficient all-PSCs. By controlling the molar ratio of the electron-rich benzodithiophene (BDTT) and electron-deficient fluorinated-thienothiophene (TT-F) units, a series of polymer donors with BDTT:TT-F ratios of 1:1 (P1), 3:1 (P2), 5:1 (P3), and 7:1 (P4) are prepared. The synthetic control of polymer composition allows for precise tuning of the light absorption properties of these new polymer donors, enabling optimization of light absorption properties to complement those of the P(NDI2HD-T2) acceptor. Copolymer P1 is found to be the optimal polymer donor for the fullerene-based solar cells due to its high light absorption, whereas the highest power conversion efficiency of 6.81% is achieved for the all-PSCs with P3, which has the most complementary light absorption with P(NDI2HD-T2).

A series of poly(benzodithiophene-r-fluorinated-thienothiophene) [P(BDTT-r-TT-F)] random copolymers with tunable light absorption characteristics are developed by controlling the ratios of electron-rich BDTT and electron-deficient TT-F units. All-polymer solar cells (all-PSCs) fabricated from these polymer donors and the P(NDI2HD-T2) acceptor achieve efficiencies of up to 6.8% by optimizing the complementary light absorption of the polymer donor and acceptor.

Organic–inorganic hybrid perovskite solar cells are attracting the attention of researchers owing to the high level of performance they exhibit in photovoltaic device applications. However, the attainment of an even higher level of performance is hindered by their anomalous current–voltage (I–V) hysteresis behavior. Even though experimental and theoretical studies have suggested that the perovskite materials may have a ferroelectric nature, it is still far from being fully understood. In this study, the origin of the hysteresis behavior in CH₃NH₃PbI₃ perovskite thin films is investigated. The behavior of ferroelectricity using piezoresponse force microscopy is first examined. Then, by comparing the scan-rate-dependent nano/macroscopic I–V curves, it is found that ion migration assisted by the grain boundaries is a dominant origin of I–V hysteresis from a macroscopic viewpoint. Consequently, the observations suggest that, even though ferroelectricity exists in the CH₃NH₃PbI₃ perovskite materials, ion migration primarily contributes to the macroscopic I–V hysteresis. The presented results can provide fundamental guidelines to the resolution of hysteresis issues in organic–inorganic hybrid perovskite materials.

This study reports on the origin of the hysteresis in CH₃NH₃PbI₃ perovskite thin films. The presence of the ferroelectricity is demonstrated, and further, ion migration assisted by the grain boundaries is demonstrated as a dominant origin of macroscopic I–V hysteresis. Consequently, the observations suggest that although ferroelectricity exists, ion migration primarily contributes to the macroscopic I–V hysteresis.

The field of conjugated polymers has expanded in the last years considerably and impressive performance, both in field effect transistors and photovoltaic devices has been achieved. After the initial emphasis on improving the performance, more emphasis is recently given to fundamental studies on structure formation. Therefore, this review concentrates on systematic correlation studies of structure formation in solution, in bulk and thin films as well as in photovoltaic blends of donor-type π-conjugated polymers. The main focus is on the correlation of structure, morphology and molecular chain orientation as a function of macromolecular properties such as molecular weight, dispersity, non-covalent intramolecular and intermolecular interactions, solvent interactions and innovative processing techniques. The tools applied for elucidating fundamental information of structure formation and orientation mainly consist of optical spectroscopy and scattering techniques (SAXS/WAXS/GIWAXS). Since the field of conjugated polymers is very vast in terms of chemical structural diversity, only selected examples of donor polymers are covered here and the emerging class of n-type conjugated polymers are not included. The focus is not on the structural variation or their performance in solar cells or transistors in terms of record efficiencies, but on the systematic studies leading to a structure-property correlation in donor polymers.

A systematic correlation of structure formation of donor-type π-conjugated polymers in solution using optical spectroscopy and in bulk or thin films as well as in photovoltaic blends using scattering techniques (SAXS, WAXS, GIWAXS) is given. The main focus is on the correlation of structures, morphology and molecular chain orientation as a function of molecular weight, dispersity, intramolecular and intermolecular interactions and innovative processing techniques.

In article number 1701048, Jang-Sik Lee and Bohee Hwang report an approach for designing high density memory devices utilizing organolead halide perovskite (OHP) materials with a cross-point array structure which is deposited by sequential vapor deposition. The OHP-based nanoscale memory devices exhibit remarkably fast switching speed, low operating voltage, and long data retention. OHPs which are a key material in solar cells have a great potential to be used in high-density information storage devices.

Understanding and controlling defect generation and movement in organic-inorganic lead halide perovskite materials will not only lead to more stable devices but also new device concepts. In article number 1700527, Wei D. Lu and co-workers reveal the formation, migration and annihilation of iodine vacancies in CH₃NH₃PbI₃ films under an electric field and/or light illumination. An electric-write and light-erase memory device is demonstrated.

The π-conjugated organic small molecule 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl) benzenamine] (TAPC) has been explored as an efficient hole transport material to replace poly(3,4-ethylenedio-xythiophene):poly(styrenesulfonate) (PEDOT:PSS) in the preparation of p-i-n type CH₃NH₃PbI₃ perovskite solar cells. Smooth, uniform, and hydrophobic TAPC hole transport layers can be facilely deposited through solution casting without the need for any dopants. The power conversion efficiency of perovskite solar cells shows very weak TAPC layer thickness dependence across the range from 5 to 90 nm. Thermal annealing enables improved hole conductivity and efficient charge transport through an increase in TAPC crystallinity. The perovskite photoactive layer cast onto thermally annealed TAPC displays large grains and low residual PbI₂, leading to a high charge recombination resistance. After optimization, a stabilized power conversion efficiency of 18.80% is achieved with marginal hysteresis, much higher than the value of 12.90% achieved using PEDOT:PSS. The TAPC-based devices also demonstrate superior stability compared with the PEDOT:PSS-based devices when stored in ambient circumstances, with a relatively high humidity ranging from 50 to 85%.

Conjugated molecule 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl) benzenamine] (TAPC) has been explored to replace poly(3,4-ethylenedio-xythiophene):poly(styrenesulfonate) in perovskite solar cells. The CH₃NH₃PbI₃ solar cells are hysteresis-free, with marginal dependence on the thickness of TAPC, and achieve a power conversion efficiency of 18.8 over 12.9% as a result of increased J_sc, V_oc, and fill factor.

This study reports the development of copper(I) thiocyanate (CuSCN) hole-transport layers (HTLs) processed from aqueous ammonia as a novel alternative to conventional n-alkyl sulfide solvents. Wide bandgap (3.4–3.9 eV) and ultrathin (3–5 nm) layers of CuSCN are formed when the aqueous CuSCN–ammine complex solution is spin-cast in air and annealed at 100 °C. X-ray photoelectron spectroscopy confirms the high compositional purity of the formed CuSCN layers, while the high-resolution valence band spectra agree with first-principles calculations. Study of the hole-transport properties using field-effect transistor measurements reveals that the aqueous-processed CuSCN layers exhibit a fivefold higher hole mobility than films processed from diethyl sulfide solutions with the maximum values approaching 0.1 cm² V⁻¹ s⁻¹. A further interesting characteristic is the low surface roughness of the resulting CuSCN layers, which in the case of solar cells helps to planarize the indium tin oxide anode. Organic bulk heterojunction and planar organometal halide perovskite solar cells based on aqueous-processed CuSCN HTLs yield power conversion efficiency of 10.7% and 17.5%, respectively. Importantly, aqueous-processed CuSCN-based cells consistently outperform devices based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate HTLs. This is the first report on CuSCN films and devices processed via an aqueous-based synthetic route that is compatible with high-throughput manufacturing and paves the way for further developments.

Dissolution of copper thiocyanate (CuSCN) in aqueous ammonia enables processing of superior quality hole-transporting layers at low temperature in ambient air. Transistors based on these CuSCN layers exhibit mobilities close to 0.1 cm² V⁻¹ s⁻¹, while solar cells incorporating CuSCN interlayers yield power conversion efficiencies of 10.7% and 17.5% for organic bulk heterojunction and organometal halide cells, respectively.