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High-efficiency and stable hole transport materials (HTMs) play an essential role in high-performance planar perovskite solar cells (PSCs). 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobi-fluorene (Spiro-OMeTAD) is often used as HTMs in perovskite solar cells because of its excellent characteristics, such as energy level matching with perovskite, good film-forming ability, and high solubility. However, the accumulation and hydrolysis of the common additive Li-TFSI in Spiro-OMeTAD can cause voids/pinholes in the hole transport layer (HTL), which reduces the efficiency of the PSCs. In order to improve the functional characteristics of HTMs, in this work, we first used CsI as a dopant to modify the HTL and reduce the voids in the HTL. A small amount of CsI is introduced into Spiro-OMeTAD together with Li-TFSI and 4-tert-butylpyridine (TBP). It is found that CsI and TBP formed a complex, which prevented the rapid evaporation of TBP and eliminated some cracks in Spiro-OMeTAD. Moreover, the uniformly dispersed TBP inhibits the agglomeration of Li-TFSI in Spiro-OMeTAD, so that the effective oxidation reaction between Spiro-OMeTAD and air produces Spiro-OMeTAD+ in the oxidation state, thereby increasing the conductivity and adjusting the HTL energy. Correspondingly, the PCE of the planar PSC of the CsI-modified Spiro-OMeTAD is up to 13.31%. In contrast, the PSC without CsI modification showed a poor PCE of 10.01%. More importantly, the PSC of Spiro-OMeTAD treated with CsI has negligible hysteresis and excellent long-term stability. Our work provides a low-cost, simple, and effective method for improving the performance of hole transport materials and perovskite solar cells.

The power conversion efficiency (PCE) of solution-processed CdTe nanocrystals (NCs) solar cells has been significantly promoted in recent years due to the optimization of device design by advanced interface engineering techniques. However, further development of CdTe NC solar cells is still limited by the low open-circuit voltage (Voc) (mostly in range of 0.5–0.7 V), which is mainly attributed to the charge recombination at the CdTe/electrode interface. Herein, we demonstrate a high-efficiency CdTe NCs solar cell by using organic polymer poly[bis(4–phenyl)(2,4,6–trimethylphenyl)amine] (PTAA) as the hole transport layer (HTL) to decrease the interface recombination and enhance the Voc. The solar cell with the architecture of ITO/ZnO/CdS/CdSe/CdTe/PTAA/Au was fabricated via a layer-by-layer solution process. Experimental results show that PTAA offers better back contact for reducing interface resistance than the device without HTL. It is found that a dipole layer is produced between the CdTe NC thin film and the back contact electrode; thus the built–in electric field (Vbi) is reinforced, allowing more efficient carrier separation. By introducing the PTAA HTL in the device, the open–circuit voltage, short-circuit current density and the fill factor are simultaneously improved, leading to a high PCE of 6.95%, which is increased by 30% compared to that of the control device without HTL (5.3%). This work suggests that the widely used PTAA is preferred as the excellent HTL for achieving highly efficient CdTe NC solar cells.

Moiré superlattices provide a highly tuneable and versatile platform to explore novel quantum phases and exotic excited states ranging from correlated insulators to moiré excitons. Scanning tunnelling microscopy has played a key role in probing microscopic behaviours of the moiré correlated ground states at the atomic scale. However, imaging of quantum excited states in moiré heterostructures remains an outstanding challenge. Here we develop a photocurrent tunnelling microscopy technique that combines laser excitation and scanning tunnelling spectroscopy to directly visualize the electron and hole distribution within the photoexcited moiré exciton in twisted bilayer WS 2 . The tunnelling photocurrent alternates between positive and negative polarities at different locations within a single moiré unit cell. This alternating photocurrent originates from the in-plane charge transfer moiré exciton in twisted bilayer WS 2 , predicted by our GW -Bethe–Salpeter equation calculations, that emerges from the competition between the electron–hole Coulomb interaction and the moiré potential landscape. Our technique enables the exploration of photoexcited non-equilibrium moiré phenomena at the atomic scale. The authors combine laser excitation and scanning tunnelling spectroscopy to visualize the electron and hole distributions in photoexcited moiré excitons in twisted bilayer WS 2 . This photocurrent tunnelling microscopy approach enables the study of photoexcited non-equilibrium moiré phenomena at atomic scales.

Moiré materials with flat electronic bands provide a highly controllable quantum system for studies of strong-correlation physics and topology. In particular, angle-aligned heterobilayers of semiconducting transition metal dichalcogenides with large band offset realize the single-band Hubbard model. Introduction of a new layer degree of freedom is expected to foster richer interactions, enabling Hund’s physics, interlayer exciton condensation and new superconducting pairing mechanisms to name a few. Here we report competing electronic states in twisted AB-homobilayer WSe2, which realizes a bilayer Hubbard model in the weak interlayer hopping limit for holes. By layer-polarizing holes via a perpendicular electric field, we observe a crossover from an excitonic insulator to a charge-transfer insulator at a hole density of ν = 1 (in units of moiré density), a transition from a paramagnetic to an antiferromagnetic charge-transfer insulator at ν = 2 and evidence for a layer-selective Mott insulator at 1 < ν < 2. The unique coupling of charge and spin to external electric and magnetic fields also manifests a giant magnetoelectric response. Our results establish a new solid-state simulator for the bilayer Hubbard model Hamiltonian.Twisted WSe2 AB-homobilayers enable the realization of bilayer Hubbard models in the weak interlayer hopping limit.

The employment of cost-effective and durable structures is essential for the successful commercialization of perovskite solar cells (PSCs). Identifying a viable substitute for hole-selective materials (HSMs), which represent a significant expense in the production of PSCs, could provide a number of benefits. Carbon nanotube-based PSCs have shown promising potential as an alternative to conventional PSCs due to their unique properties such as excellent stability behavior to be potential for commercialization to produce green energy for human industries. One of the most crucial disadvantages of carbon derivatives as HSMs in CPSCs is their low hole mobility, which in the current study has been targeted. In the current study, to increase the efficiency of CPSCs, net carbon nanotubes (CNTs) were doped with nonmetallic fluorine via a facile synthesis method. It was found that introducing fluorine-doped CNTs (F-CNTs) as HSM for MAPbI 3 perovskite could reach up to an efficiency of 15.29%, higher than the efficiency of 13.70% in devices with a net CNT layer. By doping CNTs with fluorine, the charge-transfer resistance and series resistance are reduced, resulting in lower charge recombination at the perovskite/CNT interface Also, the CPSCs with F-CNT film were more stable in ambient air because the F-CNTs covered more of the perovskite layer. The future trend of CNT-based PSCs is expected to focus on improving their performance, and exploring their potential for various optoelectronics.

Dye-sensitized solar cells (DSSCs) have been intensely researched for more than two decades. Electrolyte formulations are one of the bottlenecks to their successful commercialization, since these result in trade-offs between the photovoltaic performance and long-term performance stability. The corrosive nature of the redox shuttles in the electrolytes is an additional limitation for industrial-scale production of DSSCs, especially with low cost metallic electrodes. Numerous electrolyte formulations have been developed and tested in various DSSC configurations to address the aforementioned challenges. Here, we comprehensively review the progress on the development and application of electrolytes for DSSCs. We particularly focus on the improvements that have been made in different types of electrolytes, which result in enhanced photovoltaic performance and long-term device stability of DSSCs. Several recently introduced electrolyte materials are reviewed, and the role of electrolytes in different DSSC device designs is critically assessed. To sum up, we provide an overview of recent trends in research on electrolytes for DSSCs and highlight the advantages and limitations of recently reported novel electrolyte compositions for producing low-cost and industrially scalable solar cell technology.

Two-dimensional semiconductors, such as transition metal dichalcogenides, have demonstrated tremendous promise for the development of highly tunable quantum devices. Realizing this potential requires low-resistance electrical contacts that perform well at low temperatures and low densities where quantum properties are relevant. Here we present a new device architecture for two-dimensional semiconductors that utilizes a charge-transfer layer to achieve large hole doping in the contact region, and implement this technique to measure the magnetotransport properties of high-purity monolayer WSe 2 . We measure a record-high hole mobility of 80,000 cm 2  V –1  s –1 and access channel carrier densities as low as 1.6 × 10 11  cm −2 , an order of magnitude lower than previously achievable. Our ability to realize transparent contact to high-mobility devices at low density enables transport measurements of correlation-driven quantum phases including the observation of a low-temperature metal–insulator transition in a density and temperature regime where Wigner crystal formation is expected and the observation of the fractional quantum Hall effect under large magnetic fields. The charge-transfer contact scheme enables the discovery and manipulation of new quantum phenomena in two-dimensional semiconductors and their heterostructures. By utilizing the van der Waals electron acceptor α-RuCl 3 , this study establishes a p-type connection with WSe 2 , facilitating a high hole mobility of 80,000 cm 2  V –1  s –1 for investigating quantum transport properties in a magnetic field of over 30 T.

Controlling the flow of thermal energy is crucial to numerous applications ranging from microelectronic devices to energy storage and energy conversion devices. Here, we report ultralow lattice thermal conductivities of solution-synthesized, single-crystalline all-inorganic halide perovskite nanowires composed of CsPbI₃ (0.45 ± 0.05 W·m−1·K−1), CsPbBr₃ (0.42 ± 0.04 W·m−1·K−1), and CsSnI₃ (0.38 ± 0.04 W·m−1·K−1). We attribute this ultralow thermal conductivity to the cluster rattling mechanism, wherein strong optical–acoustic phonon scatterings are driven by a mixture of 0D/1D/2D collective motions. Remarkably, CsSnI₃ possesses a rare combination of ultralow thermal conductivity, high electrical conductivity (282 S·cm−1), and high hole mobility (394 cm²·V−1·s−1). The unique thermal transport properties in all-inorganic halide perovskites hold promise for diverse applications such as phononic and thermoelectric devices. Furthermore, the insights obtained from this work suggest an opportunity to discover low thermal conductivity materials among unexplored inorganic crystals beyond caged and layered structures.

Use of inorganic charge transport layer has demonstrated relatively stable perovskite solar cells (PSCs). NiO x is the most widely used inorganic hole transport layer in inverted PSCs and different techniques and doping in this layer have been reported to improve the performance of these devices. This manuscript describes the synthesis of NiO x thin film which act as hole transport layer on glass substrate at the initial stage and PSC devices were fabricated at the secondary stage. Effect of post deposition annealing temperature on the composition, tuning of the work function and aligning it with perovskite work function increase the hole transport efficiency and improve the open circuit voltage of devices from 0.96 to 1.08 V is reported. Ultraviolet photoelectron spectroscopy results were used to check the change in work function and X-ray photoelectron spectroscopy to probe the underlying reason for improvement of devices are also included. The charge transfer efficiency is checked by the results of time resolved photoluminescence spectra is also given. Devices with NiO x as hole transport layer and ZnSe as electron transport layer are also described and performance of devices is also included in this manuscript.

Three acceptor-donor-acceptor (A-D-A)-type small donor molecules ( M1 , M2 and M3 ) were evaluated for optoelectronic properties through density functional theory calculations. These designed molecules consist of a dithieno [2,3-b:3,2-d] thiophene (DTT) donor group linked with 2-(3-methyl-4-oxothiazolidin-2-ylidine)malononitrile acceptor through three different bridge groups. The effect of the donor and three different bridge spacer groups on the designed molecules for opto-electronic properties was evaluated in comparison with the reference molecule R . The reorganization energies of the designed donor molecules suggest very good charge mobility property. The lower value of hole mobility (λ h ), as compared to electron mobility (λ e ), revealed that the three designed molecules are best for hole mobility. Frontier molecular orbital (FMO) surfaces confirm the transfer of charge from donor to acceptor unit during excitation. The designed molecules show relatively low HOMO values (in the range of −2.19 to −2.36 eV), with strong absorption in UV-Visible region in the range of 459 nm to 500 nm in chloroform solvent. Electron-hole binding energy results indicate that the designed molecule M2 contains the highest amount of charge, which may dissociate into separate charge easily. Among all the studied molecules, the highest open circuit voltage (V oc ) of 3.01 eV (with respect to HOMO donor –LUMO PC61BM ) was shown by M3 . The open circuit voltages (V oc ) of R , M1 , M2 and M3 were 2.91 eV, 3.01 eV, 2.77 eV and 3.02 eV, respectively. Graphical abstract Three newly acceptor donor acceptor (A-D-A)-based donor small molecules ( M1 , M2 and M3 ) were designed by taking dithieno [2,3-b:3,2-d]thiophene (DTT) as a donor group linked with acceptor 2-(3-methyl-4-oxothiazolidin-2-ylidine)malononitrile through three different bridge groups. All the designed molecule were compared with the well-known reference compound R. Optical properties, electronic properties, photophysical and excited state energy were calculated and compared with the well-known, and recently published, reference molecule R . All the newly designed molecules shows good optoelectronic properties with respect to R .