ZiQi Sun

Mixing elements on the B site in ABO₃ results perovskite with narrow band gap. Ba₂FeNbO₆ exhibits band gap of 2.29 eV by using two elements. SrTiO₃-Ba₂FeNbO₆ solid solution displays a good visible light photocatalytic activity by promoting the charge separation process.

In this work, different from the commonly explored strategy of incorporating a smaller cation, MA⁺ and Cs⁺ into FAPbI₃ lattice to improve efficiency and stability, it is revealed that the introduction of phenylethylammonium iodide (PEAI) into FAPbI₃ perovksite to form mixed cation FA_xPEA_1–xPbI₃ can effectively enhance both phase and ambient stability of FAPbI₃ as well as the resulting performance of the derived devices. From our experimental and theoretical calculation results, it is proposed that the larger PEA cation is capable of assembling on both the lattice surface and grain boundaries to form quais-3D perovskite structures. The surrounding of PEA⁺ ions at the crystal grain boundaries not only can serve as molecular locks to tighten FAPbI₃ domains but also passivate the surface defects to improve both phase and moisture stablity. Consequently, a high-performance (PCE:17.7%) and ambient stable FAPbI₃ solar cell could be developed.

The introduction of a bulkier phenylethylammonium cation into FAPbI₃ is revealed to effectively enhance both phase and ambient stability of FAPbI₃. The larger hydrophobic cation is proposed to assemble on lattice surface and grain boundaries to form quasi-3D perovskite structures, which tightens FAPbI₃ domains and passivates surface defects, leading to a efficient and stable FAPbI₃ based solar cell.

Photoinduced charge generation (PCG) dynamics are notoriously difficult to correlate with specific molecular properties in device relevant polymer:fullerene organic photovoltaic blend films due to the highly complex nature of the solid state blend morphology. Here, this study uses six judiciously selected trifluoromethylfullerenes blended with the prototypical polymer poly(3-hexylthiophene) and measure the PCG dynamics in 50 fs–500 ns time scales with time-resolved microwave conductivity and femtosecond transient absorption spectroscopy. The isomeric purity and thorough chemical characterization of the fullerenes used in this study allow for a detailed correlation between molecular properties, driving force, local intermolecular electronic coupling and, ultimately, the efficiency of PCG yield. The findings 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 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.

Effective engineering of surface ligands in semiconductor nanocrystals can facilitate the electronic interaction between the individual nanocrystals, making them promising for low-cost optoelectronic applications. Here, the use of high purity Cu₂ZnSnS₄ (CZTS) nanocrystals as the photoactive layer and hole-transporting material is reported in low-temperature solution-processed solar cells. The high purity CZTS nanocrystals are prepared by engineering the surface ligands of CZTS nanocrystals, capped originally with the long-chain organic ligand oleylamine. After ligand removal, CZTS nanocrystals show substantial improvement in photoconductivity and mobility, displaying also an appreciable photoresponse in a simple heterojunction solar cell architecture. More notably, CZTS nanocrystals exhibit excellent hole-transporting properties as interface layer in perovskite solar cells, yielding power conversion efficiency (PCE) of 15.4% with excellent fill factor (FF) of 81%. These findings underscore the importance of removing undesired surface ligands in nanocrystalline optoelectronic devices, and demonstrate the great potential of CZTS nanocrystals as both active and passive material for the realization of low-cost efficient solar cells.

Effective ligand engineering of Cu₂ZnSnS₄ (CZTS) nanocrystal surface is demonstrated to lead to improvement of the electrical properties of the thin film. Furthermore, surface-modified CZTS nanocrystals exhibit excellent hole transporting properties as the interface layer in a perovskite solar cell. The perfomance of perovskite devices is shown to vary from 12.2% to 15.4% as a function of ligand removal.

Enhancing open-circuit voltage in CH₃NH₃PbI₃(Cl) perovskite solar cells has become a major challenge for approaching the theoretical limit of the power conversion efficiency. Here, for the first time, it is demonstrated that the synergistic effect of PbI₂ passivation and chlorine incorporation via controlling the molar ratio of PbI₂, PbCl₂ (or MACl), and MAI in the precursor solutions, boosts the open-circuit voltage of CH₃NH₃PbI₃(Cl) perovskite solar cells over 1.15 V in both mesoscopic and inverted planar perovskite solar cells. Such high open-circuit voltage can be attributed to the enhanced photoluminescence emission and carrier lifetime associated with the reduced trap densities. The morphology and composition analysis using scanning electron microscopy, X-ray diffraction measurements, and energy dispersive X-ray spectroscopy confirm the high quality of the optimized CH₃NH₃PbI₃(Cl) perovskite film. On this basis, record-high efficiencies of 16.6% for nonmetal-electrode all-solution-processed perovskite solar cells and 18.4% for inverted planar perovskite solar cells are achieved.

A synergistic effect of PbI₂ passivation and chlorine incorporation induces suppression of non-radiative recombination in a perovskite film. This leads to a high open-circuit voltage exceeding 1.15 V in both mesoscopic and inverted planar CH₃NH₃PbI₃(Cl)-based perovskite solar cells.

Side-chain fluorination of polymers is demonstrated as a highly effective strategy to improve the efficiency of all-polymer solar cells from 2.93% (nonfluorinated P1) to 7.13% (fluorinated P2). This significant enhancement is achieved by synergistic improvements in open-circuit voltage, charge generation, and charge transport, as fluorination of the donor polymer optimizes the band alignment and the film morphology.

Two different nonfullerene acceptors and one copolymer are used to fabricate ternary organic solar cells (OSCs). The two acceptors show unique interactions that reduce crystallinity and form a homogeneous mixed phase in the blend film, leading to a high efficiency of ≈10.3%, the highest performance reported for nonfullerene ternary blends. This work provides a new approach to fabricate high-performance OSCs.

A white light-emitting diode (0.33, 0.33) is fabricated using perovskite quantum dot/silica composites. It is shown to have greatly improved stability.

The large energy loss (>0.6 eV) in polymer solar cells (PSCs) is limiting the improvement of photovoltage. In article number 1600148, Xiaofeng Xu, Wei Ma, Ergang Wang, and co-workers report that a low band gap (1.49 eV) polymer attains a high photovoltage of 1.0 V with efficiency of 6.7% in PSCs. It represents an impressively low energy loss of 0.49 eV, which challenges the current paradigm and reveals the potential to further enhance the performance of PSCs.

The chemical structure of conjugated polymers plays an important role in determining their physical properties that, in turn, dictates their performance in photovoltaic devices. 5-Fluoro-2,1,3-benzothiadiazole, an asymmetric unit, is incorporated into a thiophene-based polymer backbone to generate a hole conducting polymers with controlled regioregularity. A high dipole moment is seen in regioregular polymers, which have a tighter interchain stacking that promotes the formation of a morphology in bulk heterojunction blends with improved power conversion efficiencies. Aliphatic side chain substitution is systematically varied to understand the influence of side chain length and symmetry on the morphology and resultant performance. This side chain modification is found to influence crystal orientation and the phase separated morphology. Using the asymmetric side chain substitution with regioregularity of the main chain, an optimized power conversion efficiency of 9.06% is achieved, with an open circuit voltage of 0.72 V, a short circuit current of 19.63 mA cm⁻², and a fill factor over 65%. These results demonstrate that the local chemical environment can dramatically influence the physical properties of the resultant material.

The unidirectional regioregular conjugated polymers using 5-fluoro-2,1,3-benzothiadiazole (FBT) asymmetric unit were synthesized. Compared with the regiorandom control polymers, the regioregular polymers with a unidirectional fluorine atom alignment can lead to a progressive dipole moment along the backbone. The regioregular FBT polymers exhibit tighter interchain stacking and better morphology in bulk heterojunction blends, giving rise to improved power conversion efficiency.

Narrow bandgap (1.37–1.46 eV) polymers incorporating a head-to-head linkage containing 3-alkoxy-3′-alkyl-2,2′-bithiophene are synthesized. The head-to-head linkage enables polymers with sufficient solubility and the noncovalent sulfur–oxygen interaction affords polymers with high degree of backbone planarity and film ordering. When integrated into polymer solar cells, the polymers show a promising power conversion efficiency approaching 10%.

The addition of Sr²⁺ in CH₃NH₃PbI₃ perovskite films enhances the charge carrier collection efficiency of solar cells leading to very high fill factors, up to 85%. The charge carrier lifetime of Sr²⁺-containing perovskites is in excess of 40 μs, longer than those reported for perovskite single crystals.