Chen Weijie

It is understood that defects of the atomic arrangement of the lattice in 2D molybdenum disulfide (MoS₂) grown by chemical vapor deposition (CVD) can have a profound effect on the electronic and optical properties. Beyond these it is a major prerequisite to also understand the fundamental effect of such defects on phonon transport, to guarantee the successful integration of MoS₂ into the solid-state devices. A comprehensive joint experiment-theory investigation to explore the effect of lattice defects on the thermal transport of the suspended MoS₂ monolayer grown by CVD is presented. The measured room temperature thermal conductivity values are 30 ± 3.3 and 35.5 ± 3 W m⁻¹ K⁻¹ for two samples, which are more than two times smaller than that of their exfoliated counterpart. High-resolution transmission electron microscopy shows that these CVD-grown samples are polycrystalline in nature with low angle grain boundaries, which is primarily responsible for their reduced thermal conductivity. Higher degree of polycrystallinity and aging effects also result in smoother temperature dependency of thermal conductivity (κ) at temperatures below 100 K. First-principles lattice dynamics simulations are carried out to understand the role of defects such as isotopes, vacancies, and grain boundaries on the phonon scattering rates of our CVD-grown samples.

The lattice structure–thermal conductivity relation of MoS₂ monolayer grown by chemical vapor deposition is investigated in a wide temperature range using a suspended microdevice with integrated resistance thermometers. Higher degree of polycrystallinity leads to smoother temperature dependency at temperatures below 100 K. The observations are explained by the first-principles lattice dynamics calculations.

Graphene field effect transistor sensitized by a layer of semiconductor (sensitizer/GFET) is a device structure that is investigated extensively for ultrasensitive photodetection. Among others, organometallic perovskite semiconductor sensitizer has the advantages of long carrier lifetime and solution processable. A further step to improve the responsivity is to design a structure that can promote electron–hole separation and selective carrier trapping in the sensitizer. Here, the use of a hybrid perovskite–organic bulk heterojunction (BHJ) as the light sensitizer to achieve this goal is demonstrated. Our spectroscopy and device measurements show that the CH₃NH₃PbI₃–PCBM BHJ/GFET device has improved charge separation yield and carrier lifetime as compared to a reference device with a CH₃NH₃PbI₃ sensitizer only. The key to these enhancement is the presence of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), which acts as charge separation and electron trapping sites, resulting in a 30-fold increase in the photoresponsivity. This work shows that the use of a small amount of electron or hole acceptors in the sensitizer layer can be an effective strategy for improving and tuning the photoresponsivity of sensitizer/GFET photodetectors.

Mixing a small amount of electron acceptors ([6,6]-phenyl-C₆₁-butyric acid methyl ester) with hybrid organometallic perovskite significantly increases the photoresponsivity of perovskite–graphene photodetectors. It is found that the electron acceptors act as effective sites for both charge separation and electron trapping, which enhances the photoinduced change in the conductivity of the graphene channel.

Improving the fill factor (FF) is known as a challenging issue in organic solar cells (OSCs). Herein, a strategy of extending the conjugated area of end-group is proposed for the molecular design of acceptor–donor–acceptor (A–D–A)-type small molecule acceptor (SMA), and an indaceno[1,2-b:5,6-b′]dithiophene-based SMA, namely IDTN, by end-capping with the naphthyl fused 2-(3-oxocyclopentylidene)malononitrile is synthesized. Benefiting from the π-conjugation extension by fusing two phenyls, IDTN shows stronger molecular aggregation, more ordered packing structure, thus over one order of magnitude higher electron mobility relative to its counterpart. By utilizing the fluorinated polymer (PBDB-TF) as the electron donor, the corresponding device exhibits a high efficiency of 12.2% with a record-high FF of 0.78, which is approaching the theoretical limit of OSCs. Compared with the reference molecule, such a high FF in the IDTN system can be mainly attributed to the more ordered π–π packing of acceptor aggregates, higher domain purity and symmetric carrier transport in the blend. Hence, enlarging the conjugated area of the terminal-group in these A–D–A-type SMAs is a promising approach not only for enhancing the electron mobility, but also for improving the blend morphology, and both of them are conducive to the fill-factor breakthrough.

By extending the conjugated area of the end-group, a newly designed A–D–A–type small-molecule acceptor, namely IDTN, exhibits dense and ordered packing, and therefore, the electron mobility of the IDTN is over one order of magnitude higher than that of its counterpart. When blended with the donor polymer PBDB-TF, a high efficiency of 12.2% with an outstanding fill factor of 0.78 is achieved.

A novel patterning method for perovskite thin films is developed by Dae-Hyeong Kim and co-workers, which is described in article number 1702902. The patterning method (spin-on-patterning) is based on the thermodynamically preferred dewetting behavior of the perovskite precursor solution during the spin-coating process. By using this method, a high-performance, ultrathin, and deformable perovskite-on-silicon multiplexed image sensor array is successfully achieved.

CdS thin films are a promising electron transport layer in PbS colloidal quantum dot (CQD) photovoltaic devices. Some traditional deposition techniques, such as chemical bath deposition and RF (radio frequency) magnetron sputtering, have been employed to fabricate CdS films and CdS/PbS CQD heterojunction photovoltaic devices. However, their power conversion efficiencies (PCEs) are moderate compared with ZnO/PbS and TiO₂/PbS heterojunction CQD solar cells. Here, efficiencies have been improved substantially by employing solution-processed CdS thin films from a single-source precursor. The CdS film is deposited by a straightforward spin-coating and annealing process, which is a simple, low-cost, and high-material-usage fabrication process compared to chemical bath deposition and RF magnetron sputtering. The best CdS/PbS CQD heterojunction solar cell is fabricated using an optimized deposition and air-annealing process achieved over 8% PCE, demonstrating the great potential of CdS thin films fabricated by the single-source precursor for PbS CQDs solar cells.

A heterojunction PbS quantum dot solar cell with an efficiency over 8% is achieved by optimizing CdS electron transport layer deposited by a simple single-source precursor spin-coating process. The optimized band alignment of the device improves the short circuit current and fill factor.

Compact TiO₂ is widely used as an electron transport material in planar-perovskite solar cells. However, TiO₂-based planar-perovskite solar cells exhibit low efficiencies due to intrinsic problems such as the unsuitable conduction band energy and low electron extraction ability of TiO₂. Herein, the planar TiO₂ electron transport layer (ETL) of perovskite solar cells is modified with ionic salt CuI via a simple one-step spin-coating process. The p-type nature of the CuI islands on the TiO₂ surface leads to modification of the TiO₂ band alignment, resulting in barrier-free contacts and increased open-circuit voltage. It is found that the polarity of the CuI-modified TiO₂ surface can pull electrons to the interface between the perovskite and the TiO₂, which improves electron extraction and reduces nonradiative recombination. The CuI solution concentration is varied to control the electron extraction of the modified TiO₂ ETL, and the optimized device shows a high efficiency of 19.0%. In addition, the optimized device shows negligible hysteresis, which is believed to be due to the removal of trap sites and effective electron extraction by CuI-modified TiO₂. These results demonstrate the hitherto unknown effect of p-type ionic salts on electron transport material.

It is revealed that the CuI islands on the TiO₂ electron transport layer can induce change of polarity increasing electron extraction, establish barrier-free band alignment with perovskite, and reduce the trap sites. These changes of interface properties induce power conversion efficiency of 19.0% perovskite solar cell with negligible hysteresis.

Although antimony sulfoiodide (SbSI) exhibits very interesting properties including high photoconductivity, ferroelectricity, and piezoelectricity, it is not applied to solar cells. Meanwhile, SbSI is predominantly prepared as a powder using a high-temperature, high-pressure system. Herein, the fabrication of solar cells utilizing SbSI as light harvesters is reported for the first time to the best of knowledge. SbSI is prepared by solution processing, followed by annealing under mild temperature conditions by a reaction between antimony trisulfide, which is deposited by chemical bath deposition on a mesoporous TiO₂ electrode and antimony triiodide, under air at a low temperature (90 °C) without any external pressure. The solar cells fabricated using SbSI exhibit a power conversion efficiency of 3.05% under standard illumination conditions of 100 mW cm⁻².

Solar cells with the configuration of FTO (fluorine-doped SnO₂)/TiO₂ blocking layer/mesoporous TiO₂/SbSI/hole-transporting material/Au are demonstrated for the first time. The cells fabricated using TiO₂ as an electron-transporting layer and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] as a hole-transporting layer exhibit a power conversion efficiency of 3.05% under full illumination of air mass 1.5G.

High-density, crystalline, and freestanding PbI₂ and MAPbI₃ nanosheets are synthesized by Johnny C. Ho and co-workers in article number 1702759, on a large scale through the nucleation of microplanes on substrates with a rough surface by manipulating the microenvironment during physical vapor deposition. When configured into photodetectors, they exhibit efficient photodetection with excellent performance in responsivity and detectivity.

In this work, highly efficient ternary-blend organic solar cells (TB-OSCs) are reported based on a low-bandgap copolymer of PTB7-Th, a medium-bandgap copolymer of PBDB-T, and a wide-bandgap small molecule of SFBRCN. The ternary-blend layer exhibits a good complementary absorption in the range of 300–800 nm, in which PTB7-Th and PBDB-T have excellent miscibility with each other and a desirable phase separation with SFBRCN. In such devices, there exist multiple energy transfer pathways from PBDB-T to PTB7-Th, and from SFBRCN to the above two polymer donors. The hole-back transfer from PTB7-Th to PBDB-T and multiple electron transfers between the acceptor and the donor materials are also observed for elevating the whole device performance. After systematically optimizing the weight ratio of PBDB-T:PTB7-Th:SFBRCN, a champion power conversion efficiency (PCE) of 12.27% is finally achieved with an open-circuit voltage (V_oc) of 0.93 V, a short-circuit current density (J_sc) of 17.86 mA cm⁻², and a fill factor of 73.9%, which is the highest value for the ternary OSCs reported so far. Importantly, the TB-OSCs exhibit a broad composition tolerance with a high PCE over 10% throughout the whole blend ratios.

Highly efficient ternary-blend nonfullerene organic solar cells based on two copolymer donors and one electron acceptor are fabricated and evaluated. The multiple energy and charge-transfer pathways in this ternary system enable the power conversion efficiency to reach 12.27%, which is a new record for ternary-blend organic solar cells at present. These devices also exhibit a broad composition tolerance.

Low temperature solution processed planar-structure perovskite solar cells gain great attention recently, while their power conversions are still lower than that of high temperature mesoporous counterpart. Previous reports are mainly focused on perovskite morphology control and interface engineering to improve performance. Here, this study systematically investigates the effect of precise stoichiometry, especially the PbI₂ contents on device performance including efficiency, hysteresis and stability. This study finds that a moderate residual of PbI₂ can deliver stable and high efficiency of solar cells without hysteresis, while too much residual PbI₂ will lead to serious hysteresis and poor transit stability. Solar cells with the efficiencies of 21.6% in small size (0.0737 cm²) and 20.1% in large size (1 cm²) with moderate residual PbI₂ in perovskite layer are obtained. The certificated efficiency for small size shows the efficiency of 20.9%, which is the highest efficiency ever recorded in planar-structure perovskite solar cells, showing the planar-structure perovskite solar cells are very promising.

Planar-structure perovskite solar cells with efficiencies of 21.6% in small size (0.0737 cm²) and 20.1% in large size (1 cm²) with moderate residual PbI₂ in perovskite layer are obtained. The certificated efficiency for small size shows the efficiency of 20.9%, which is the highest efficiency ever recorded in planar-structure perovskite solar cells.