Ligang Yuan

Charge transport in organic photovoltaic (OPV) devices is often characterized by space-charge limited currents (SCLC). However, this technique only probes the transport of charges residing at quasi-equilibrium energies in the disorder-broadened density of states (DOS). In contrast, in an operating OPV device the photogenerated carriers are typically created at higher energies in the DOS, followed by slow thermalization. Here, by ultrafast time-resolved experiments and simulations it is shown that in disordered polymer/fullerene and polymer/polymer OPVs, the mobility of photogenerated carriers significantly exceeds that of injected carriers probed by SCLC. Time-resolved charge transport in a polymer/polymer OPV device is measured with exceptionally high (picosecond) time resolution. The essential physics that SCLC fails to capture is that of photo­generated carrier thermalization, which boosts carrier mobility. It is predicted that only for materials with a sufficiently low energetic disorder, thermalization effects on carrier transport can be neglected. For a typical device thickness of 100 nm, the limiting energetic disorder is σ ≈71 (56) meV for maximum-power point (short-circuit) conditions, depending on the error one is willing to accept. As in typical OPV materials the disorder is usually larger, the results question the validity of the SCLC method to describe operating OPVs.

Detailed comparison of steady-state and ultrafast time-resolved experiments reveals that photogenerated carrier mobility in organic solar cells is significantly higher than that probed by space-charge limited currents (SCLCs). The SCLC method is only valid for materials with a sufficiently low energetic disorder, where photogenerated carrier thermalization can be neglected. The over-simplified use of quasi-equilibrium mobility data in literature requires re-evaluation.

Organometal halide perovskite materials have become a superstar in the photovoltaic (PV) field because of their advantageous properties, which boost the power conversion efficiency (PCE) of perovskite solar cells (PSCs) from about 3.8% to above 22% in just seven years. Most importantly, such promising achievement is mainly based on its low-cost and solution-processed fabrication technique. One of the most promising and famous approaches to fabricating perovskite is a two-step sequential deposition method because precursor (e.g., PbI₂) deposition is controllable, versatile, and flexible. Due to tremendous efforts, great progress has been achieved on the two-step sequential deposition method, which helps to promote the development of PSCs. Herein, the progresses on the two-step sequential deposition method of perovskite layers is reviewed thoroughly. At first, the reaction process and principle is introduced and discussed. Then, the research on the deposition techniques, structures, and compositions of precursors (the first step) is presented. Subsequently, the developments on the conversion techniques, conversion solutions, and growth of large crystals at the second step are introduced. Finally, four important issues on the two-step sequential deposition method will be stated, accompanied with proposed solutions.

The two-step sequential deposition method is one of the most important deposition methods for the organometal halide perovskite layer in photovoltaic application. So far, great progress has been made on this method, which helps to promote the development of PSCs. Herein, its recent achievements are reviewed and the important issues are highlighted while proposing the corresponding solutions.

Lead halide perovskite nanocrystals (NCs) with bright luminescence and broad spectral tunability are good candidates as smart probes for bioimaging, but suffer from hydrolysis even when exposed to atmosphere moisture. In this paper, a strategy is demonstrated by embedding CsPbX₃ (X = Cl, Br, I) NCs into microhemispheres (MHSs) of polystyrene matrix to prepare “water-resistant” NCs@MHSs hybrids as multicolor multiplexed optical coding agents. First, a facile room-temperature solution self-assembly approach to highly luminescent colloidal CsPbX₃ NCs is developed by injecting a stock solution of CsX⋅PbX₂ in N,N-dimethylformamide into dichloromethane. Polyvinyl pyrrolidone (PVP) is chosen as the capping ligand, which is physically adsorbed and wrapped on the surface of perovskite NCs to form a protective layer. The PVP protective layer not only leads to composition-tunable CsPbX₃ NCs with high quantum yields and narrow emission linewidths of 12–34 nm but also acts as an interfacial layer, making perovskite NCs compatible with polystyrene polymers and facilitating the next step to embed CsPbX₃ NCs into polymer MHSs. CsPbX₃ NCs@MHSs are demonstrated as multicolor luminescence probes in live cells with high stability and nontoxicity. Using ten intensity levels and seven-color NCs@MHSs that show non-overlapping spectra, it will be possible to individually tag about ten million cells.

A strategy to overcome the inherent vulnerability of perovskites to water is demonstrated by embedding CsPbX₃ nanocrystals (NCs) into microhemispheres (MHSs) of polystyrene matrix to prepare “water-resistant” NCs@MHSs hybrids. NCs@MHSs are demonstrated as multicolor luminescence probes in live cells with high stability and nontoxicity. Using ten intensity levels and seven-color NCs@MHSs, it will be possible to individually tag about ten million cells.

The solidification of hybrid perovskite (MAPbX₃, X = I, Br, Cl) inks is studied in situ by A. Amassian and co-workers in article 1604113, revealing a remarkably complex process mediated by strong solvent-solute interactions, the outcome of which is halide-dependent. The ink is shown to solidify within 15–20s of spinning, forming a highly solvated (60–70 vol%) precursor phase. Crystalline order of the precursor, or lack thereof, strongly impacts the morphological outcome of thermal conversion and the reproducibility of solar cells.

The recent success of organometallic halide perovskites (OHPs) in photovoltaic devices has triggered lots of corresponding research and many perovskite analogues have been developed to look for devices with comparable performance but better stability. Upon the preparation of all inorganic halide perovskite nanocrystals (IHP NCs), research activities have soared due to their better stability, ultrahigh photoluminescence quantum yield (PL QY), and composition dependent luminescence covering the whole visible region with narrow line-width. They are expected to be promising materials for next generation lighting and display, and many other applications. Within two years, a lot of interesting results have been observed. Here, the synthesis of IHPs is reviewed, and their progresses in optoelectronic devices and optical applications, such as light-emitting diodes (LEDs), photodetectors (PDs), solar cells (SCs), and lasing, is presented. Information and recent understanding of their crystal structures and morphology modulations are addressed. Finally, a brief outlook is given, highlighting the presently main problems and their possible solutions and future development directions.

All inorganic halide perovskite nanosystems are expected to be promising materials for next generation lighting, display, and many other applications. Within two years, lots of interesting results have been observed. The synthesis and progresses on optoelectronic devices and optical applications, such as LEDs, photodetectors, solar cells, and lasing, are summarized. Future directions, challenges and possible applications are also put forward.

A post-permeation method is constructed for fabricating bulk-heterojunction hybrid solar cells. Porous CdTe film is prepared by annealing the mixture solution of aqueous CdTe nanocrystals and cetyltrimethyl ammonium bromide, after which the post-permeation of polymer is employed. By this method, kinds of polymers can be applied regardless of the intermiscibility with the nanoparticles. The inorganic nanocrystals and the polymer can be treated under respective optimized annealing temperatures, which can facilitate the growth of nanocrystals without damaging the polymers. A high power conversion efficiency of 6.36% in the polymer/nanocrystals hybrid solar cells is obtained via systematical optimization.

Polymer/nanocrystals hybrid solar cells are made by a post-permeation method. By this method, various polymers can be applied regardless of the intermiscibility with the nanoparticles. The inorganic nanocrystal layer and the polymer layer can be treated under their optimized annealing temperatures, respectively. A high power conversion efficiency of 6.36% in polymer/nanocrystals hybrid solar cells is achieved.