Ligang Yuan

Unlike widely used postsynthetic halide exchange for CsPbX₃ (X is halide) perovskite nanocrystals (NCs), cation exchange of Pb is of a great challenge due to the rigid nature of the Pb cationic sublattice. Actually, cation exchange has more potential for rendering NCs with peculiar properties. Herein, a novel halide exchange-driven cation exchange (HEDCE) strategy is developed to prepare dually emitting Mn-doped CsPb(Cl/Br)₃ NCs via postsynthetic replacement of partial Pb in preformed perovskite NCs. The basic idea for HEDCE is that the partial cation exchange of Pb by Mn has a large probability to occur as a concomitant result for opening the rigid halide octahedron structure around Pb during halide exchange. Compared to traditional ionic exchange, HEDCE is featured by proceeding of halide exchange and cation exchange at the same time and lattice site. The time and space requirements make only MnCl₂ molecules (rather than mixture of Mn and Cl ions) capable of doping into perovskite NCs. This special molecular doping nature results in a series of unusual phenomenon, including long reaction time, core–shell structured mid states with triple emission bands, and dopant molecules composition-dependent doping process. As-prepared dual-emitting Mn-doped CsPb(Cl/Br)₃ NCs are available for ratiometric temperature sensing.

A novel halide exchange-driven cation exchange strategy is developed to prepare dually emitting Mn-doped CsPb(Cl/Br)₃ nanocrystals (NCs). The key is the simultaneous reactions of halide exchange and cation exchange at the same time and lattice site, which can be satisfied only by direct doping of MnCl₂ molecules into NCs.

High-efficiency small-molecule-based organic photovoltaics (SM-OPVs) using two electron donors (p-DTS(FBTTh₂)₂ and ZnP) with distinctively different absorption and structural features are reported. Such a combination works well and synergically improves device short-circuit current density (J_sc) to 17.99 mA cm⁻² and fill factor (FF) to 77.19%, yielding a milestone efficiency of 11%. To the best of our knowledge, this is the highest power conversion efficiency reported for SM-OPVs to date and the first time to combine high J_sc over 17 mA cm⁻² and high FF over 77% into one SM-OPV. The strategy of using multicomponent materials, with a selecting role of balancing varied electronic and structural necessities can be an important route to further developing higher performance devices. This development is important, which broadens the dimension and versatility of existing materials without much chemistry input.

High-efficiency all-small-molecule organic solar cells using two electron donors with distinctively different absorption and structural features are reported. Such a combination works well and synergically improves device current and fill factor, yielding a milestone efficiency of 10.97%.

Organic–inorganic hybrid perovskites (OIHPs) are new photoactive layer candidates for lightweight and flexible solar cells due to their low-temperature process capability; however, the reported efficiency of flexible OIHP devices is far behind those achieved on rigid glass substrates. Here, it is revealed that the limiting factor is the different perovskite film deposition conditions required to form the same film morphology on flexible substrates. An optimized perovskite film composition needs a different precursor ratio, which is found to be essential for the formation of high-quality perovskite films with longer radiative carrier recombination lifetime, smaller density of trap states, reduced precursor residue, and uniform and pin-hole free films. A record efficiency of 18.1% is achieved for the flexible perovskite solar-cell devices made on an indium tin oxide/poly(ethylene terephthalate) substrate via a low temperature (≤100 °C) solution process.

A different precursor ratio is found to be essential for the formation of high-quality perovskite films on flexible substrates compared to those formed on rigid substrates. A high efficiency of 18.1% is achieved for flexible perovskite solar-cell devices made on an indium tin oxide/poly(ethylene terephthalate) substrate via a low-temperature (≤100 °C) solution process.

This paper presents a systematic study of the influence of electron-transport materials on the operation stability of the inverted perovskite solar cells under both laboratory indoor and the natural outdoor conditions in the Negev desert. It is shown that all devices incorporating a Phenyl C₆₁ Butyric Acid Methyl ester ([60]PCBM) layer undergo rapid degradation under illumination without exposure to oxygen and moisture. Time-of-flight secondary ion mass spectrometry depth profiling reveals that volatile products from the decomposition of methylammonium lead iodide (MAPbI₃) films diffuse through the [60]PCBM layer, go all the way toward the top metal electrode, and induce its severe corrosion with the formation of an interfacial AgI layer. On the contrary, alternative electron-transport material based on the perylendiimide derivative provides good isolation for the MAPbI₃ films preventing their decomposition and resulting in significantly improved device operation stability. The obtained results strongly suggest that the current approach to design inverted perovskite solar cells should evolve with respect to the replacement of the commonly used fullerene-based electron-transport layers with other types of materials (e.g., functionalized perylene diimides). It is believed that these findings pave a way toward substantial improvements in the stability of the perovskite solar cells, which are essential for successful commercialization of this photovoltaic technology.

Diffusion of CH₃NH₃I and other volatile products of photodegradation of CH₃NH₃PbI₃ into the [60]PCBM electron-transport layer represents the key failure mechanism of inverted hybrid perovskite solar cells.

In article number 1601935, Andriy Zakutayev and co-workers study trade-offs in thin film solar cells with layered photovoltaic absorbers. Using high-throughput combinatorial experimental methods, electron collection in such photovoltaic devices is found to be enhanced by a drift in the electric field, which however also reduces the solar cell voltage.

The photovoltaic performance of polymer solar cells can be dramatically improved upon the incorporation of a nonfullerene electron acceptor (NFA) as the third component. In article number 1602127, Lei Ying, Fei Huang, and co-workers develop a new NFA that can lead to obviously improved performance by virtue of cascade energy transfer. This cover describes a frame of such ternary device, where the three components are highlighted in different colors.

Perovskite solar cells have emerged as a promising technique for low-cost, light weight, and highly efficient photovoltaics. However, they still largely rely on 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) to serve as hole-transporting materials (HTMs). Here, a series of HTMs with small molecular weight is designed, which are constructed on a spiro core involving phenylpyrazole and a second heteroaromatics, i.e., xanthene (O atom), thioxanthene (S atom), and acridine (N atom). Through varying from phenylpyrazole substituted xanthene (PPyra-XA), thioxanthene (PPyra-TXA), to acridine (PPyra-ACD), their optical and electrochemical properties, hole mobilities, and the photovoltaic performance are optimized. As a consequence, PPyra-TXA based device exhibits the highest power conversion efficiency (PCE) of 18.06%, outperforming that of Spiro-OMeTAD (16.15%), which could be attributed to the enhancement of hole mobility exerted by the thioxanthene. In addition, the dopant-free device shows PCE of 11.7%. These results open a new direction for designing spiro-HTMs by simple modification of chemical structures.

Perovskite solar cells bearing spiro-phenylpyrazole-9,9′-thioxanthene exhibit power conversion efficiency of 18.06%, outperforming that of Spiro-OMeTAD (16.15%), which could be attributed to the enhancement of hole mobility exerted by the thioxanthene.

A wide-bandgap polymer, (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(2,5-(methyl thiophene carboxylate))]) (3MT-Th), is synthesized to obtain a complementary broad range absorption when harmonized with 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC). The synthesized regiorandom 3MT-Th polymer shows good solubility in nonhalogenated solvents. A film of 3MT-Th:ITIC can be employed for forming an active layer in a polymer solar cell (PSC), with the blend solution containing toluene with 0.25% diphenylether as a nonhalogenated additive. The corresponding PSC devices display a power conversion efficiency of 9.73%. Moreover, the 3MT-Th-based PSCs exhibit excellent shelf-life time of over 1000 h and are operationally stable under continuous light illumination. Therefore, methyl thiophene-3-carboxylate in 3MT-Th is a promising new accepting unit for constructing p-type polymers used for high-performance nonfullerene-type PSCs.

A wide-bandgap polymer, (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(2,5-(methyl thiophene carboxylate))]) (3MT-Th), displays a high efficiency of 9.73% with 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene acceptor using toluene as a nonhalogenated solvent. The 3MT-Th-based PSCs exhibit excellent shelf-life stability of over 1000 h and operationally stable under continuous light illumination compared with the PTB7-Th-based PSCs.

Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low-temperature photoluminescence of formamidinium lead triiodide (HC(NH₂)₂PbI₃) is investigated. A power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate-equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of ≈3 meV. Charge-carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open-circuit voltage losses commonly observed in these materials. In this high-quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose band-structure picture is moderated by a modest degree of energetic disorder.

Low-temperature measurements of the photoluminescence from HC(NH₂)₂PbI₃ thin films are presented. The emission exhibits a power-law intensity decay with time after excitation, and an additional low-energy peak displaying an anomalous Stokes shift. These phenomena demonstrate that charge–carrier recombination in this perovskite is mediated by a band tail with characteristic energy 3 meV, determined from a rate-equation model and Monte Carlo simulation.

High-performance, multicolor perovskite light-emitting diodes are fabricated in a two-step solution process by Liang-Sheng Liao, Wei Lei, Qiaoliang Bao, and co-workers in article number 1606874. A specially designed n-type semiconductor consisting of Ca-doped ZnO nanoparticles is used as the electron transport layer (ETL). Combining a tunable ETL with a solution process pushes perovskite-based materials a step closer to practical application in multicolor light-emitting devices.

A novel architecture made of a CsPbX₃/ZnS quantum dot heterostructure is reported, by Xiaosheng Tang, Miao Zhou, and co-workers in article number 1604085, by combining material synthesis, characterization, optical measurements and density functional theory based first-principles calculations. This architecture presents high crystal quality, enhanced structural stability and tunable optoelectronic properties, providing an exciting playground for future exploration of novel and high-performance optoelectronic devices.