ZiQi Sun

Recently, newly engineered all-inorganic cesium lead halide perovskite nanocrystals (IPNCs) (CsPbX₃, X = Cl, Br, I) are discovered to possess superior optical gain properties appealing for solution-processed cost-effective lasers. Yet, the potential of such materials has not been exploited for practical laser devices, rendering the prospect as laser media elusive. Herein, the challenging but practically desirable vertical cavity surface emitting lasers (VCSELs) based on the CsPbX₃ IPNCs, featuring low threshold (9 µJ cm⁻²), directional output (beam divergence of ≈3.6°), and favorable stability, are realized for the first time. Notably, the lasing wavelength can be tuned across the red, green, and blue region maintaining comparable thresholds, which is promising in developing single-source-pumped full-color visible lasers. It is fully demonstrated that the characteristics of the VCSELs can be versatilely engineered by independent adjustment of the cavity and solution-processable nanocrystals. The results unambiguously reveal the feasibility of the emerging CsPbX₃ IPNCs as practical laser media and represent a significant leap toward CsPbX₃ IPNC-based laser devices.

All-inorganic halide perovskite nanocrystals (IPNCs) (CsPbX₃, X = Cl, Br, I) based vertical cavity surface emitting laser is realized for the first time. These laser devices operate at a very low threshold, such that quasi-steady-state pumping is feasible. The results highlight the emerging CsPbX₃ IPNCs as practical laser media and represent a significant leap toward practically desirable laser devices.

To improve the efficiency of existing perovskite solar cells (PSCs), a detailed understanding of the underlying device physics during their operation is essential. Here, a device model has been developed and validated that describes the operation of PSCs and quantitatively explains the role of contacts, the electron and hole transport layers, charge generation, drift and diffusion of charge carriers and recombination. The simulation to the experimental data of vacuum-deposited CH₃NH₃PbI₃ solar cells over multiple thicknesses has been fit and the device behavior under different operating conditions has been studied to delineate the influence of the external bias, charge-carrier mobilities, energetic barriers for charge injection/extraction and, different recombination channels on the solar cell performance. By doing so, a unique set of material parameters and physical processes that describe these solar cells is identified. Trap-assisted recombination at material interfaces is the dominant recombination channel limiting device performance and passivation of traps increases the power conversion efficiency (PCE) of these devices by 40%. Finally, guidelines to increase their performance have been issued and it is shown that a PCE beyond 25% is within reach.

A numerical model is developed and validated that describes the operation of perovskite solar cells and quantitatively explains the role of contacts, the charge transport layers, charge generation, drift and diffusion of carriers and recombination. By doing so, a unique set of material parameters and physical processes is identified that describes these solar cells. To increase their performance, some guidelines are issued.

Hole transport matertial (HTM) as charge selective layer in perovskite solar cells (PSCs) plays an important role in achieving high power conversion efficiency (PCE). It is known that the dopants and additives are necessary in the HTM in order to improve the hole conductivity of the HTM as well as to obtain high efficiency in PSCs, but the additives can potentially induce device instability and poor device reproducibility. In this work a new strategy to design dopant-free HTMs has been presented by modifying the HTM to include charged moieties which are accompanied with counter ions. The device based on this ionic HTM X44 dos not need any additional doping and the device shows an impressive PCE of 16.2%. Detailed characterization suggests that the incorporated counter ions in X44 can significantly affect the hole conductivity and the homogeneity of the formed HTM thin film. The superior photovoltaic performance for X44 is attributed to both efficient hole transport and effective interfacial hole transfer in the solar cell device. This work provides important insights as regards the future design of new and efficient dopant free HTMs for photovotaics or other optoelectronic applications.

A new strategy to design dopant-free hole transport materials (HTMs) by modifying the organic molecule to include charged moieties that are accompanied by counter ions is investigated. The introduced counter ions are highly beneficial for improving the conductivity of the HTM and the perovskite solar cell devices based on the designed ionic HTM show impressive power conversion efficiency of more than 16%.

An aqueous solution method is developed for the facile synthesis of Cl-containing SnSe nanoparticles in 10 g quantities per batch. The particle size and Cl concentration of the nanoparticles can be efficiently tuned as a function of reaction duration. Hot pressing produces n-type Cl-doped SnSe nanostructured compacts with thermoelectric power factors optimized via control of Cl dopant concentration. This approach, combining an energy-efficient solution synthesis with hot pressing, provides a simple, rapid, and low-cost route to high performance n-type SnSe thermoelectric materials.

An aqueous solution method is developed for the scalable synthesis of Cl-containing SnSe nanoparticles with tuneable particle size and Cl concentration. Hot pressing produces n-type Cl-doped SnSe nanostructured compacts with thermoelectric power factors optimized via control of Cl dopant concentration.

Nowadays, solvent additives are widely used in organic solar cells (OSCs) to tune the nano-morphology of the active blend film and enhance the device performance. With their help, power conversion efficiencies (PCEs) of OSCs have recently stepped over 10%. However, residual additive in the device can induce undesirable morphological change and also accelerate photo-oxidation degradation of the active blend film. Thereby, their involvements are actually unfavorable for practical applications. Here, a donor material PThBDTP is employed, and PThBDTP:PC71BM based OSCs are fabricated. A PCE of over 10% is achieved without using any additives and film post-treatments. The device displays a high open-circuit voltage of 0.977 V, a large short-circuit current density of 13.49 mA cm^-2, and a high fill factor of 76.3%. These results represent an important step towards developing high-efficiency additive-free OSCs.

Organic photovoltaic devices with PThBDTP as donor and PC₇₁BM as acceptor are fabricated, and a power conversion efficiency of over 10% is achieved without resorting to any additives and post treatments.

Photoinduced charge selective carrier extraction by linearly increasing voltage technique allows straightforward assessment of charge transport properties within planar and mesostructured perovskite solar cells with respect to light intensity and signal delay time. Charge sensitive device architecture is realized through implementation of insulating layer between the anode or cathode to prevent extraction of unwanted type of carriers. Resulting behavior of comparatively efficient mesoporous and planar solar cells exhibits well balanced charge transport with slight dependence of charge mobility on applied laser pulse fluence, for given pulse delay times. Very similar charge carrier mobilities are present within mesoporous devices, whereas holes trail approximately half an order of magnitude behind electrons in planar structured specimens. Moreover, dispersive transport is identified in the electron selective devices with titanium oxide electron transporter, suggesting considerable presence of trapping states at the perovskite interface, whereas no such behavior characterizes planar samples. Variation in delay time between laser pulse and extraction ramp only affects initial charge concentration present within the device, while transient outlay remains unchanged, indicating absence of film charging effect.

Charge selective extraction of photogenerated carriers by linearly increasing voltage is used to characterize planar and mesostructured perovskite solar cells. Inclusion of insulating layer into device structure prevents collection of unwanted carriers at corresponding electrode, provides valuable insight into charge transfer properties and pinpoints critical part of device responsible for trapping disorders.

Aimed at achieving ideal morphology, illuminating morphology–performance relationship, and further improving the power conversion efficiency (PCE) of ternary polymer solar cells (TSCs), a ternary system is designed based on PTB7-Th:PffBT4T-2OD:PC₇₁BM in this work. The PffBT4T-2OD owns large absorption cross section, proper energy levels, and good crystallinity, which enhances exciton generation, charge dissociation and transport and suppresses charge recombination, thus remarkably increasing the short-circuit current density (J_sc) and fill factor (FF). Finally, a notable PCE of 10.72% is obtained for the TSCs with 15% weight ratio of PffBT4T-2OD. As for the working mechanism, it confirmed the energy transfer from PffBT4T-2OD to PTB7-Th, which contributes to the improved exciton generation. And morphology characterization indicates that the devices with 15% PffBT4T-2OD possess both appropriate domain size (25 nm) and enhanced domain purity. Under this condition, it affords numerous D/A interface for exciton dissociation and good bicontinuous nanostructure for charge transport simultaneously. As a result, the device with 15% PffBT4T-2OD exhibits improved exciton generation, enhanced charge dissociation possibility, elevated hole mobility and inhibited charge recombination, leading to elevated J_sc (19.02 mA cm⁻²) and FF (72.62%) simultaneously. This work indicates that morphology optimization as well as energy transfer plays a significant role in improving TSC performance.

Ternary polymer solar cells based on PTB7-Th:PffBT4T-2OD:PC₇₁BM are designed according to the complementary properties of PTB7-Th and PffBT4T-2OD. The highest efficiency of 10.72% for this ternary system is achieved with 15% PffBT4T-2OD. As for the working mechanism, this ternary system paves the way to investigate the morphology–performance relationship. Moreover, the energy transfer is involved in ternary blend.

Perovskite-based photovoltaics have been rapidly developed, with record power conversion efficiencies now exceeding 22%. In order to rationally design efficient and stable perovskite solar cells, it is important to understand not only charge trapping and recombination events, but also processes occurring at the perovskite/transport material (TM) interface, such as charge transfer and interfacial recombination. In this work, time-resolved microwave conductivity measurements are performed to investigate these interfacial processes for methylammonium lead iodide and various state-of-the-art organic TMs. A global kinetic model is developed, which accurately describes both the dynamics of excess charges in the perovskite layer and transfer to charge-specific TMs. The authors conclude that for state-of-the-art materials, such as Spiro-OMeTAD and PCBM, the charge extraction efficiency is not significantly affected by intra-band gap traps for trap densities under 10¹⁵ cm^–3. Finally, the transfer rates to C60, PCBM, EDOT-OMeTPA, and Spiro-OMeTAD are sufficient to outcompete second order recombination under excitation densities representative for illumination by AM1.5.

The dynamics of charge transfer from CH₃NH₃PbI₃ to charge-specific transport materials are revealed using time-resolved microwave conductivity measurements and a global kinetic model. Hence, the authors find that the transfer rates to C60, PCBM, EDOT-OMeTPA, and Spiro-OMeTAD are sufficient to outcompete other recombination pathways at excitation densities representative for AM1.5, while this is not the case for bis-PCBM, ICBA, and H101.