ZiQi Sun

The commercialization of solar fuel devices requires the development of novel engineered photoelectrodes for water splitting applications which are based on redundant, cheap, and environmentally friendly materials. In the current study, a combination of titanium dioxide (TiO₂) and zinc oxide (ZnO) onto nanotextured silicon is utilized for a composite electrode with the aim to overcome the individual shortcomings of the respective materials. The properties of conformal coverage of TiO₂ and ZnO layers are designed on the atomic scale by the atomic layer deposition technique. The resulting photoanode shows not only promising stability but also nine times higher photocurrents than an equivalent photoanode with a pure TiO₂ encapsulation onto the nanostructured silicon. Density functional theory calculations indicate that segregation of TiO₂ at the ZnO surfaces is favorable and leads to the stabilization of the ZnO layers in water environments. In conclusion, the novel designed composite material constitutes a promising base for a stable and effective photoanode for the water oxidation reaction.

A novel photoanode for water oxidation based on a composite TiO₂-ZnO encapsulation engineered for stabilized photo­electroactivity of the nanostructured silicon substrate is developed. Density functional theory is applied to obtain realistic structural models of the TiO₂-ZnO composite on the atomic scale and the way in which the configurational modifications lead to a better performance of the electrodes is investigated.

Three small molecules as front cell donors for tandem cells are thoroughly evaluated and a high power conversion efficiency of 11.47% is achieved, which demonstrates that the oligomer-like small molecules offer a good choice for high-performance tandem solar cells.

Methylammonium bromide is used as an additive in the lead acetate-based precursor solution for perovskite solar cells by T. P. Russell, R. Zhu, and co-workers on page 3508. This is found to significantly improved perovskite film quality and optoelectronic properties. The optimized device delivers a power conversion efficiency of 18.3%, holding promise for highly efficient perovskite solar cells.

The interfacial microstructure in organic bulk heterojunction solar cells can dictate photovoltaic performance. By controlling the chemical interactions of bulk heterojunction components with specific surfaces, the electrical pathways at interfaces can be precisely varied to achieve suitable electronic properties across such interfaces. This is studied by J. J. Jasieniak and co-workers on page 3944.

Within the past few years, the record efficiency of inorganic–organic perovskite solar cell (PSC) has improved rapidly up to over 20%. However, the viability of commercialization of the PSC technology has been seriously questioned due to the moisture- and thermal-induced instabilities. Here, it is demonstrated that these issues may be mitigated via cell structure design and contact engineering. By employing the hole-conductor layer-free cell structure and a bi-layer back contact consisting of a carbon/CH₃NH₃I composite layer and a compact hydrophobic carbon layer, the PSCs have shown excellent stability, inhibiting moisture ingression and heat-induced perovskite degradation. It is found that, the unique bi-layer contact enables the optimization of perovskite absorbers during thermal stress. As a result, instead of degradation, the devices present enhanced performance under heating at 100 °C for 30 min. The best-performing cell shows a final efficiency of 13.6% from an initial efficiency of 11.3% after thermal stress. Upon encapsulation, these cells can even retain 90% of the initial efficiencies after water exposure and over 100% initial efficiency under thermal stress at 150 °C for half an hour. This approach provides a facile way for stabilizing the PSCs and opens a door for viable commercialization of the emerging PSC technology.

To mitigate the instability of perovskite solar cells via device structure design and contact engineering, a hole-conductor layer-free structure of device and a unique bi-layer hybrid carbon back contact are adopted. The cells not only achieve an enhanced efficiency of 13.6% from the initial 11.3% after 100 °C treatment, but also exhibit excellent moist and thermal stability.

Recently, intensive studies on the role of water molecule in the formation of organic–inorganic perovskite film have been reported. However, not only the contradictive phenomena but also the complex processing technique has hindered the widespread use of water molecule in perovskite preparation. Here the hydration water is introduced into the precursors instead of water. By precisely controlling the content of hydration water, a smoother and more uniform perovskite film is obtained through a simple one-step spin coating method. The improvement of perovskite film quality leads to highly efficient planar perovskite solar cells. Summing up the device studies and the investigation of morphology, crystallization, and optical properties, the impact of water molecule in the formation of perovskite crystal and consequences of device performance is understood. Due to its universal adaptability and simplified process, precise control of hydration water is therefore of great utility to high quality perovskite films fabrication and large-scale production of this upcoming photovoltaic technology.

The content of hydration water is carefully controlled in precursor solution. The findings reveal that the hydration water has played an essential role in the formation of perovskite film. With an optimum amount of hydration water, a high quality perovskite film with better uniformity and smoother surface is formed. Based on it, an efficient perovskite solar cell is fabricated.

Tunneling contacts made of any insulating polymers, a champion technology in silicon solar cells, are shown to increase the stabilized efficiency of perovskite solar cells (PSCs) to 20.3%. The tunneling layers spatially separate photo-generated electrons and holes at the perovskite-cathode interface and reduce charge recombination. The tunneling layers made of hydrophobic polymers also significantly enhance the resistance of PSCs to water-caused damage.

Mixed Pb–In perovskite solar cells are fabricated by using lead(II) chloride and indium(III) chloride with methylammonium iodide. A maximum power conversion efficiency as high as 17.55% is achieved owing to the high quality of perovskites with multiple ordered crystal orientations.

Hall effect measurements in CH₃NH₃PbBr₃ single crystals reveal that the charge-carrier mobility follows an inverse-temperature power-law dependence, μ ∝ T^−γ, with the power exponent γ = 1.4 ± 0.1 in the cubic phase, indicating an acoustic-phonon-dominated carrier scattering, and γ = 0.5 ± 0.1 in the tetragonal phase, suggesting another dominant mechanism, such as a piezoelectric or space-charge scattering.

Hybrid organic–inorganic halide-perovskite-based solar cells have achieved notable progress. A hot topic in this field is exploring inexpensive, stable and effective hole-transporting materials (HTMs) in order to improve the device performance and be favorable for large-scale production in the future. The HTMs have been proven to be an important component of perovskite solar cells, which can form selective contact being favorable for reducing charge recombination and effective hole collection, thus resulting in the enhancement of the open-circuit voltage and the fill factor. Here, an overview of the design and development of HTMs is given, mainly divided into conductive polymers, inorganic p-type semiconductors in inverted-structure-based planar perovskite solar cells. The influences of their mobility, work function and film property on device performance are discussed.

Hole-transporting materials in inverted planar perovskite solar cells have been widely studied in the past years. Most commonly these are p-type wide band-gap semiconductors which can be mainly divided into conductive polymers and inorganic p-type semiconductors. Their energy levels and chemical structures are summarized and the effects of their properties on the device performance are discussed in detail.

A new fluorinated polythiophene (PT) derivative, PBDD-ff4T, is designed and synthesized. The PBDD-ff4T/PC₇₁BM-based device without any extra treatment exhibits a high efficiency of 9.2%, under the irradiation of AM 1.5G, 100 mW cm⁻², which is the highest power conversion efficiency reported for PT derivative-based polymer solar cells.

A triphenylamine based molecular “butterfly” is developed as dopant-free hole-transport material for perovskite solar cells exhibiting excellent power conversion efficiency of 16.3% which is comparable to the state-of-the-art doped 2,2′,7,7′-tetrakis(N,N′-di-p-methoxy-phenylamine)-9,9′-spirobifluorene (spiro-OMeTAD). Moreover, the device is much more stable than that of spiro-OMeTAD based device under light irradiation.

A lateral photodetector based on the bilayer composite film of a perovskite and a conjugated polymer is reported. It exhibits significantly enhanced responsivity in the UV–vis region and sensitive photoresponse in the near-IR (NIR) region at a low applied voltage. This broadband photodetector also shows excellent mechanical flexibility and improved environmental stability.

Halide perovskites are a rapidly developing class of medium-bandgap semiconductors which, to date, have been popularized on account of their remarkable success in solid-state heterojunction solar cells raising the photovoltaic efficiency to 20% within the last 5 years. As the physical properties of the materials are being explored, it is becoming apparent that the photovoltaic performance of the halide perovskites is just but one aspect of the wealth of opportunities that these compounds offer as high-performance semiconductors. From unique optical and electrical properties stemming from their characteristic electronic structure to highly efficient real-life technological applications, halide perovskites constitute a brand new class of materials with exotic properties awaiting discovery. The nature of halide perovskites from the materials' viewpoint is discussed here, enlisting the most important classes of the compounds and describing their most exciting properties. The topics covered focus on the optical and electrical properties highlighting some of the milestone achievements reported to date but also addressing controversies in the vastly expanding halide perovskite literature.

Halide perovskites are a promising new class of semiconductors that deliver clean energy from inexpensive inorganic materials. Perovskite materials compete in conversion efficiency with classical inorganic semiconductors and promise a bright future toward exploitation of cheap, clean energy resources. Understanding how these materials actually work poses the main challenge in contemporary materials science research.

Highly crystalline SnO₂ is demonstrated to serve as a stable and robust electron-transporting layer for high-performance perovskite solar cells. Benefiting from its high crystallinity, the relatively thick SnO₂ electron-transporting layer (≈120 nm) provides a respectable electron-transporting property to yield a promising power conversion efficiency (PCE)(18.8%) Over 90% of the initial PCE can be retained after 30 d storage in ambient with ≈70% relative humidity.

4,4′-Biphenol (BPO), a common, cheap chemical, is employed as a “molecular lock” in blends of fluorine-containing polymer or small molecule donors and fullerene acceptors to lock donors via hydrogen bond formed between the donor and BPO. The molecular lock is a versatile key to enhance the efficiency and stability of organic solar cells simultaneously.