ZiQi Sun

Sn-based perovskites are promising Pb-free photovoltaic materials with an ideal 1.3 eV bandgap. However, to date, Sn-based thin film perovskite solar cells have yielded relatively low power conversion efficiencies (PCEs). This is traced to their poor photophysical properties (i.e., short diffusion lengths (<30 nm) and two orders of magnitude higher defect densities) than Pb-based systems. Herein, it is revealed that melt-synthesized cesium tin iodide (CsSnI₃) ingots containing high-quality large single crystal (SC) grains transcend these fundamental limitations. Through detailed optical spectroscopy, their inherently superior properties are uncovered, with bulk carrier lifetimes reaching 6.6 ns, doping concentrations of around 4.5 × 10¹⁷ cm⁻³, and minority-carrier diffusion lengths approaching 1 µm, as compared to their polycrystalline counterparts having ≈54 ps, ≈9.2 × 10¹⁸ cm⁻³, and ≈16 nm, respectively. CsSnI₃ SCs also exhibit very low surface recombination velocity of ≈2 × 10³ cm s⁻¹, similar to Pb-based perovskites. Importantly, these key parameters are comparable to high-performance p-type photovoltaic materials (e.g., InP crystals). The findings predict a PCE of ≈23% for optimized CsSnI₃ SCs solar cells, highlighting their great potential.

Pb-free CsSnI₃ single crystal possesses superior optoelectronic properties compared to its polycrystalline thin film counterparts for photovoltaic application, uncovered using detailed optical spectroscopy, with a bulk carrier lifetimes of around 6.6 ns, doping concentrations of ≈4.5 × 10¹⁷ cm⁻³, minority-carrier diffusion lengths approaching 1 µm, and surface recombination velocity of <2 × 10³ cm s⁻¹.

Organometallic perovskites, solution-processable materials with outstanding optoelectronic properties and high index of refraction, provide a platform for all-dielectric metamaterials operating at visible frequencies. Perovskite metasurfaces with structural coloring tunable across visible frequencies are realized through subwavelength structuring. Moreover, a threefold increase of the luminescence yield and comparable reduction of luminescence decay time are observed.

2D layered materials based p–n junctions are fundamental building block for enabling new functional device applications with high efficiency. However, due to the lack of controllable doping technique, state-of-the-art 2D p–n junctions are predominantly made of van der Waals heterostructures or electrostatic gated junctions. Here, the authors report the demonstration of a spatially controlled aluminum doping technique that enables a p–n homojunction diode to be realized within a single 2D black phosphorus nanosheet for high performance photovoltaic application. The diode achieves a near-unity ideality factor of 1.001 along with an on/off ratio of ≈5.6 × 10³ at a low bias of 2 V, allowing for low-power dynamic current rectification without signal decay or overshoot. When operated under a photovoltaic regime, the diode's dark current can be significantly suppressed. The presence of a built-in electric field additionally gives rise to temporal short-circuit current and open-circuit voltage under zero external bias, indicative of its enriched functionalities for self-powered photovoltaic and high signal-to-noise photodetection applications.

2D p–n junction is a fundamental building block for nanoelectronics devices, which has been predominantly realized using van der Waals heterostructures or electrostatic-gated junctions due to the lack of controllable doping techniques. Here, near-ideal black phosphorus p–n homojunction diodes are achieved by novel and facile Al-atom doping, paving the way toward high performance photovoltaic applications.

The synthesis and growth of perovskite films with controlled crystallinity and microstructure for highly efficient and stable solar cells is a critical issue. In this work, thiourea is introduced into the CH₃NH₃PbI₃ precursor with two-step sequential ethyl acetate (EA) interfacial processing. This is shown for the first time to grow compact microsized and monolithically grained perovskite films. X-ray diffraction patterns and infrared spectroscopy are used to prove that thiourea significantly impacts the perovskite crystallinity and morphology by forming the intermediate phase MAI·PbI₂·SC(NH₂)₂. Afterward, the residual thiourea which coursed charge recombination is completely extracted by the sequential EA processing. The product has improved light harvesting, suppressed defect state, and enhanced charge separation and transport. The sequentially EA processed perovskite solar cells offer an impressive 18.46% power conversion efficiency and excellent stability in ambient air. More importantly, the EA postprocessed perovskite solar cells also have excellent voltage response under ultraweak light (0.05% sun) with promising utility in photodetectors and photoelectric sensors.

A new perovskite precursor and a two-step antisolvent processing method are utilized to grow monolithically grained perovskite films. The as-prepared films have achieved enhanced light absorption, suppressed surface defect level, and accelerated charge separation and transport. The power conversion efficiency of the solar cells reaches 18.46% with much optimized stability, repeatability, and voltage responsibility.

TCO-free thin film solar cell exist in three configurations: i) front-side illuminated planar, ii) back-side illuminated planar, and iii) fiber-shaped solar cells (FSSCs). On page 8855, W. Guo, X. Y. Liu, and co-workers review the advances in the field, focusing on flexible TCO-free TCEs, including carbon nanotubes (CNTs), graphene, metallic NW/nanotroughs, metallic grids, conducting polymers, metallic fiber and carbon based fibers.

Hybrid materials in optoelectronic devices can generate new functionality or provide synergistic effects that enhance the properties of each component. Here, high-performance phototransistors with broad spectral responsivity in UV–vis–near-infrared (NIR) regions, using gold nanorods (Au NRs)-decorated n-type organic semiconductor and N,N′-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI) nanowires (NWs) are reported. By way of the synergistic effect of the excellent photo-conducting characteristics of single-crystalline BPE-PTCDI NW and the light scattering and localized surface plasmon resonances (LSPR) of Au NRs, the hybrid system provides new photo-detectivity in the NIR spectral region. In the UV–vis region, hybrid nanomaterial-based phototransistors exhibit significantly enhanced photo-responsive properties with a photo-responsivity (R) of 7.70 × 10⁵ A W⁻¹ and external quantum efficiency (EQE) of 1.42 × 10⁸% at the minimum light intensity of 2.5 µW cm⁻², which are at least tenfold greater than those of pristine BPE-PTCDI NW-based ones and comparable to those of high-performance inorganic material-based devices. While a pristine BPE-PTCDI NW-based photodetector is insensitive to the NIR spectral region, the hybrid NW-based phototransistor shows an R of 10.7 A W⁻¹ and EQE of 1.35 × 10³% under 980 nm wavelength-NIR illumination. This work demonstrates a viable approach to high-performance photo-detecting systems with broad spectral responsivity.

Optoelectrical performances of phototransistors based on gold nanorods-decorated n-type single-crystalline organic semiconductor nanowires are reported. Owing to the synergy among the excellent photo-conducting characteristics of organic nanowire, the scattering effect and localized surface plasmon resonances of gold nanorods, the hybrid system exhibits broad spectral responsivity with unprecedented performances that are comparable to those of inorganic materials-based phototransistors.

The device efficiency of polymer:fullerene bulk heterojunction solar cells has recently surpassed 11%, as a result of synergistic efforts among chemists, physicists, and engineers. Since polymers are unequivocally the “heart” of this emerging technology, their design and synthesis have consistently played the key role in the device efficiency enhancement. In this article, the first focus is a discussion on molecular engineering (e.g., backbone, side chains, and substituents), then the discussion moves on to polymer engineering (e.g., molecular weight). Examples are primarily selected from the authors contributions; yet other significant discoveries/developments are also included to put the discussion in a broader context. Given that the synthesis, morphology, and device physics are inherently related in explaining the measured device output parameters (J_sc, V_oc and FF), we will attempt to apply an integrated and comprehensive approach (synthesis, morphology, and device physics) to elucidate the fundamental, underlying principles that govern the device characteristics, in particular, in the context of disclosing structure-property correlations. Such correlations are crucial to the design and synthesis of next generation materials to further improve the device efficiency.

Recent progress (2012–2016) in polymer:fullerene bulk-heterojunction solar cells is reviewed. The intrinsic complexity of such solar cells urges the community to apply an integrated and comprehensive approach – including synthesis, morphology, and device physics – to elucidate the fundamental underlying principles that govern the device performance, in particular, in the context of disclosing structure–property correlations.

In article 1604044, T. Wang and co-workers report compositional and surface modifications to low-temperature-processed TiO₂ films as electron transport layers in inverted polymer solar cells. This approach not only increases the power conversion efficiency of photovoltaic devices to 10.5%, but more importantly, eliminates the light-soaking problem that is commonly observed in polymer solar cells employing metal oxides as the charge-transport layers.

Despite their outstanding photovoltaic performance, organic–inorganic perovskite solar cells still face severe stability issues and limitations in their device dimension. Further development of perovskite solar cells therefore requires a deeper understanding of loss mechanisms, in particular, concerning the origin and impact of trap states. Here, different surface properties of submicrometer sized CH₃NH₃PbI₃ particles are studied as a model system by photoluminescence spectroscopy to investigate the impact of the perovskite crystal surface on photoexcited states. Comparison of single crystals with either isolating or electron-rich surface passivation indicates the presence of positively charged surface trap states that can be passivated in case of the latter. These surface trap states cause enhanced nonradiative recombination at room temperature, which is a loss mechanism for solar cell performance. In the orthorhombic phase, the origin of multiple emission peaks is identified as the recombination of free and bound excitons, whose population ratio critically depends on trap state properties. The dynamics of exciton trapping at 50 K are observed on a time-scale of tens of picoseconds by a simultaneous population decrease and increase of free and bound excitons, respectively. These results emphasize the potential of surface passivation to further improve the performance of perovskite solar cells.

A comparison of electron-rich and isolating surface passivation of submicrometer sized CH₃NH₃PbI₃ single crystals reveals the presence of positively charged trap states at the crystal surface. Photoluminescence measurements at different temperatures demonstrate that these surface trap states cause surface lattice distortions and nonradiative recombination at room temperature and energy transfer from free excitons to bound excitons at low temperatures.

Unique insights into magnetotransport in 20 nm ligand-free La_0.67Sr_0.33MnO₃ perovskite nanocrystals of nearly perfect crystalline quality reveal a chemically altered 0.8 nm thick surface layer that triggers exceptionally large magnetoresistance at low temperature, independently of the spin polarization of the ferromagnetic core. This discovery shows how the nanoscale impacts magnetotransport in a material widely spread as electrode in hybrid spintronic devices.

Triplet excitons form in quasi-2D hybrid inorganic–organic perovskites and diffuse over 100 nm before radiating with >11% photoluminescence quantum efficiency (PLQE) at low temperatures.