ZiQi Sun

The remarkable optoelectronic properties of hybrid halide perovskite absorbers have, in the past years, made the perovskite solar cell one of the most promising emerging photovoltaic technologies. The charge collecting layers are essential parts of this type of solar cell. Carbon nanotubes have emerged as a potential candidate to take on this role. Equipped with a range of highly beneficial properties including excellent charge transport characteristics, chemical inertness, as well as mechanical robustness, carbon nanotubes are able to both efficiently extract photogenerated charges, and improve the resilience and stability of a perovskite solar cell. Here, a concise overview of the current state-of-the-art of perovskite solar cells, in which carbon nanotubes are incorporated as a charge conduction layer, is provided.

Carbon nanotubes are increasingly growing into their role as successful charge-collection layers in perovskite solar cells. Their unique mechanical and chemical resilience combined with excellent charge transport characteristics sets them apart from conventional charge collection materials. Recent efforts that exploit these properties are described and the way forward is discussed.

In this work, both anode and cathode interfaces of p-i-n CH₃NH₃PbI₃ perovskite solar cells (PVSCs) are simultaneously modified to achieve large open-circuit voltage (V_oc) and fill factor (FF) for high performance semitransparent PVSCs (ST-PVSCs). At the anode, modified NiO serves as an efficient hole transport layer with appropriate surface property to promote the formation of smooth perovskite film with high coverage. At the cathode, a fullerene bisadduct, C₆₀(CH₂)(Ind), with a shallow lowest unoccupied molecular orbital level, is introduced to replace the commonly used phenyl-C₆₁-butyric acid methyl ester (PCBM) as an alternative electron transport layer in PVSCs for better energy level matching with the conduction band of the perovskite layer. Therefore, the V_oc, FF and power conversion efficiency (PCE) of the PVSCs increase from 1.05 V, 0.74 and 16.2% to 1.13 V, 0.80 and 18.1% when the PCBM is replaced by C₆₀(CH₂)(Ind). With the advantages of high V_oc and FF, ST-PVSCs are also fabricated using an ultrathin transparent Ag as cathode, showing an encouraging PCEs of 12.6% with corresponding average visible transmittance (AVT) over 20%. These are the highest PCEs reported for ST-PVSCs with similar AVTs paving the way for using ST-PVSCs as power generating windows.

The anode and cathode interfaces of CH₃NH₃PbI₃ perovskite solar cells are optimized simultaneously. The shallow lowest unoccupied molecular level of C₆₀(CH₂)(Ind) provides better energy level alignment and surface passivation. High performance opaque cell with a power conversion efficiency (PCE) of 18.1% (V_oc: 1.13 V, FF: 0.8) and semitransparent cell with a PCE of 12.6% (AVT>20%) are achieved.

The generation efficiency of long-lived photoinduced charges in polymer:fullerene bulk heterojunctions increases dramatically as the strength of intermolecular electronic coupling in the fullerene phase improves. Ultrafast geminate recombination is suppressed when electron delocalization within fullerene clusters is strong, while the opposite is observed when delocalization within the cluster is poor. In article number 1601427, Garry Rumbles and co-workers show that the molecular design of the fullerene not only determines inter-fullerene electronic coupling, but also influences the decay dynamics of free holes in the donor phase even when the polymer microstructure remains unchanged.

The influence of illumination on the long-term performance of planar structured perovskite solar cells (PSCs) is investigated using fast and spatially resolved luminescence imaging. The authors analyze the effect of illuminated current density–voltage (J–V) and light-soaking measurements on pristine PSCs by providing visual evidence for the spatial inhomogeneous evolution of device performance. Regions that are exposed to light initially produce stronger electroluminescence signals than surrounding unilluminated regions, mainly due to a lower contact resistance and, possibly, higher charge collection efficiency. Over a period of several days, however, these initially illuminated regions appear to degrade more quickly despite the device being stored in a dark, moisture- and oxygen-free environment. Using transmission electron microscopy, this accelerated degradation is attributed to delamination between the perovskite and the titanium dioxide (TiO₂) layer. An ion migration mechanism is proposed for this delamination process, which is in accordance with previous current–voltage hysteresis observations. These results provide evidence for the intrinsic instability of CH₃NH₃PbI₃-based devices under illumination and have major implications for the design of PSCs from the standpoint of long-term performance and stability.

Temporal behavior of a selectively illuminated region in perovskite solar cells is tracked using photoluminescence (PL) and electroluminescence (EL) imaging. Although initially the EL (PL) intensity shows noticeable enhancement (reduction) in the illuminated region, it deteriorates (improves) over time relative to the non-illuminated surroundings. Combined with cross-sectional transmission electron microscopy, bias-induced perovskite/TiO₂ interfacial electrical and/or physical decoupling is demonstrated.

Organic–inorganic perovskite photovoltaics are an emerging solar technology. Developing materials and processing techniques that can be implemented in large-scale manufacturing is extremely important for realizing the potential of commercialization. Here we report a hot-casting process with controlled Cl⁻ incorporation which enables high stability and high power-conversion-efficiencies (PCEs) of 18.2% for small area (0.09 cm²) and 15.4% for large-area (≈1 cm²) single solar cells. The enhanced performance versus tri-iodide perovskites can be ascribed to longer carrier diffusion lengths, improved uniformity of the perovskite film morphology, favorable perovskite crystallite orientation, a halide concentration gradient in the perovskite film, and reduced recombination by introducing Cl⁻. Additionally, Cl⁻ improves the device stability by passivating the reaction between I⁻ and the silver electrode. High-quality thin films deployed over a large-area 5 cm × 5 cm eight-cell module have been fabricated and exhibit an active-area PCE of 12.0%. The feasibility of material and processing strategies in industrial large-scale coating techniques is then shown by demonstrating a “dip-coating” process which shows promise for large throughput production of perovskite solar modules.

A hot-casting perovskite processing technique with controlled Cl⁻ incorporation affords a high power conversion efficiency of 18.2% for small-area (0.09 cm²), 15.4% for large-area (≈1 cm²) single solar cells, and 12.0% for large-area 5 cm × 5 cm eight-cell modules and compatibility with large throughput production techniques.

Efficient ternary polymer solar cells are constructed by incorporating an electron-deficient chromophore (5Z,5′Z)-5,5′-((7,7′-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(6-fluorobenzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (IFBR) as an additional component into the bulk-heterojunction film that consists of a wide-bandgap conjugated benzodithiophene-alt-difluorobenzo[1,2,3]triazole based copolymer and a fullerene acceptor. With respect to the binary blend films, the incorporation of a certain amount of IFBR leads to simultaneously enhanced absorption coefficient, obviously extended absorption band, and improved open-circuit voltage. Of particular interest is that devices based on ternary blend film exhibit much higher short-circuit current densities than the binary counterparts, which can be attributed to the extended absorption profiles, enhanced absorption coefficient, favorable film morphology, as well as formation of cascade energy level alignment that is favorable for charge transfer. Further investigation indicates that the ternary blend device exhibits much shorter charge carrier extraction time, obviously reduced trap density and suppressed trap-assisted recombination, which is favorable for achieving high short-circuit current. The combination of these beneficial aspects leads to a significantly improved power conversion efficiency of 8.11% for the ternary device, which is much higher than those obtained from the binary counterparts. These findings demonstrate that IFBR can be a promising electron-accepting material for the construction of ternary blend films toward high-performance polymer solar cells.

A nonfullerene acceptor (5Z,5′Z)-5,5′-((7,7′-(4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(6-fluorobenzo[c]-[1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) is introduced as an electron-cascade acceptor material in blends of a wide-bandgap donor poly(benzodithiophene-alt-difluorobenzo[1,2,3]triazole) and a fullerene acceptor [6,6]-phenyl-C₆₁-butyric acid methyl ester to fabricate ternary blend polymer solar cells (PSCs). The ternary device exhibits a power conversion efficiency of 8.11%, which is much higher than those obtained from the binary PSCs.

The electronic properties of single-layer hybrid perovskite BA₂MI₄ (M = Ge, Sn, and Pb) are theoretically investigated. The single-layer perovskites not only show tunable bandgaps, ranging from ≈1.5 to ≈2.0 eV, but also exhibit strong exciton effect. The desired properties of these atomically thin perovskites will motivate future experimental studies of 2D perovskites for improved functionality.

The use of laser-ablation-assisted solution synthesis of ligand-free perovskite semiconductor inks is demonstrated. The thin films exhibit no hysteresis under polarization and monotonic tunability of the semiconductor band/gap.

Easily accessible tetra-5-hexylthiophene-, tetra-5-hexyl-2,2′-bisthiophene-substituted zinc phthalocyanines (ZnPcs) and tetra-tert-butyl ZnPc are employed as hole-transporting materials in mixed-ion perovskite [HC(NH₂)₂]_0.85(CH₃NH₃)_0.15Pb(I_0.85Br_0.15)₃ solar cells, reaching the highest power conversion efficiency (PCE) so far for phthalocyanines. Results confirm that the photovoltaic performance is strongly influenced by both, the individual optoelectronic properties of ZnPcs and the aggregation of these tetrapyrrolic semiconductors in the solid thin film. The optimized devices exhibit PCE of 15.5% when using tetra-5-hexyl-2,2′-bisthiophene substituted ZnPcs, 13.3% for tetra-tert-butyl ZnPc, and a record 17.5% for tetra-5-hexylthiophene-based analogue under standard global 100 mW cm⁻² AM 1.5G illumination. These results boost up the potential of solution-processed ZnPc derivatives as stable and economic hole-transport materials for large-scale applications, opening new frontiers toward a realistic, efficient, and inexpensive energy production.

Easily accessible tetra-5-hexylthiophene-, tetra-5-hexyl-2,2′-bisthiophene-, and tetra-tert-butyl-substituted zinc phthalocyanines (ZnPcs) are employed as hole-transporting materials in mixed-ion perovskite [HC(NH₂)₂]_0.85(CH₃NH₃)_0.15Pb(I_0.85Br_0.15)₃ solar cells, reaching record power conversion efficiency of 17.5% under standard global AM 1.5G irradiation. The data highlight the potential large-scale applications of solution-processed ZnPc derivatives as stable and economic hole-transport materials for perovskite solar cells.

The use of polydopamine as a nitrogen containing precursor to generate catalytically active nitrogen-doped carbon (CN_x) materials on carbon nanotubes (CNTs) is reported. These N-doped CN_x/CNT materials display excellent electrocatalytic activity toward the reduction of triiodide electrolyte in dye-sensitized solar cells (DSSCs). Further, the influence of various synthesis parameters on the catalytic performance of CN_x/CNTs is investigated in detail. The best performing device fabricated with the CN_x/CNTs material delivers power conversion efficiency of 7.3%, which is comparable or slightly higher than that of Pt (7.1%) counter electrode-based DSSC. These CN_x/CNTs materials show great potential to address the issues associated with the Pt electrocatalyst including the high cost and scarcity.

The use of polydopamine as a nitrogen containing precursor to generate catalytically active nitrogen-doped carbon materials on carbon nanotubes is reported, which display excellent electrocatalytic activity toward the reduction of triiodide electrolyte in dye-sensitized solar cells and delivers a power conversion efficiency of 7.3%, which is comparable or slightly higher than that of using platinum (7.1%) as electrocatalyst.

The p-type inorganic semiconductor CuGaO₂ as a hole-transporting layer (HTL) in perovskite solar cells (PSCs) provides higher carrier mobility, better-energy level matching, and superior stability, as well as low-temperature processing technique. Compared to organic HTL, a very competitive PCE of 18.51% with long-term stability is achieved. This indicates that CuGaO₂ is a promising HTL for efficient and stable PSCs.

Gold–organic hybrids with various Au–aryl contacts are reviewed by L. Chi and H. Zhang on page 10492. Such hybrids can be prepared via on-surface reactions by using multi-halogenated precursors on gold substrates. STM/STS investigations allow a better understanding of the properties of the hybrids, which may offer a foundation for advanced synthesis of carbon-based functional materials and their practical applications in molecular electronics.