Joseph Michel Mbengue

Photo-induced degradation of lead halide perovskite solar cells caused by the hole transport layer/metal electrode interface

Lead halide perovskite solar cells (PSCs) suffer from poor long-term stability, especially due to photo-induced degradation, as PSCs function under continuous sunlight. However, the origins of this instability have not been clearly explored. Herein, the photo-induced degradation of PSCs with mesoporous and planar architectures are investigated, respectively, and the main origin is proved to be correlated with the hole transport material (HTM)/metal (Au) electrode interface. The solar irradiation of PSCs causes significant deterioration of device performance, with the efficiency decreasing from approximately 18% to 2.46% for planar PSCs in 180 min. Electrical analysis of the PSCs and XPS measurements show that the deteriorated performance is induced by retarded carrier extraction from the HTM to the Au electrode, due to a broken interface binding. Accordingly, in situ renewal of the Au electrode was found to cause notable recovery (approximately 80%) of the device performance of both mesoporous and planar PSCs. In comparison, the material degradation of perovskite and the TiO2/perovskite interface were also studied; however, these showed minor effects on the photo-induced degradation of PSCs. These results indicate that the photo-induced degradation of PSCs is mainly caused by the HTM/Au interface. This study provides an important insight into the photo-induced degradation of PSCs, and is crucial for the fabrication of highly photo-stable PSCs.

Reduced surface defects of organometallic perovskite by thermal annealing for highly efficient perovskite solar cells

The surface defects of the organometallic perovskite play an important role in the photovoltaic performance of solar cells, which depress the conversion efficiency and cause photocurrent hysteresis. As a key step in fabricating perovskite solar cells, the heating step possibly modifies the surface defects, which in turn leads to the modified device performance. In this context, the surface defects of the organometallic perovskite (CH3NH3PbI3) and their modification by thermal annealing are investigated. It is revealed that the surface defects create electron traps which can be reduced by thermal annealing. Consequently, the perovskite solar cells exhibit improved conversion efficiency from 10.9% to 17.1% with the thermal annealing temperature of the perovskite increasing from 60 °C to 130 °C. The photocurrent hysteresis of the solar cells is also subdued. This work provides further insights into the function of thermal annealing by modifying surface defects, which also favors the exploration of the perovskite solar cells with high-efficiency and eliminated photocurrent hysteresis.

Dual function interfacial layer for highly efficient and stable lead halide perovskite solar cells

The trap states and the intrinsic nature of polycrystalline organometallic perovskites cause carrier losses in perovskite solar cells (PSCs) through carrier recombination at the surface and subsurface of the perovskites, leading to lowered conversion efficiency. Herein, to reduce the carrier losses, an intelligent approach concerning surface passivation and interfacial doping of the perovskite is proposed by introducing an F4TCNQ interfacial layer. The trap states at the perovskite surface are efficiently suppressed, leading to a homogenous surface potential of perovskite, which avoids the surface carrier recombination. The Fermi level of the perovskite is shifted to its valence band by 0.2 eV, inducing an energy barrier for electron diffusion and contributing directly to a minimized carrier recombination at the subsurface of the perovskite film. Consequently, the performance of the PSCs is remarkably improved, with the average efficiency increased from 14.3 ± 0.9% to 16.4 ± 1.0% (with a maximum efficiency of 18.1%). Moreover, the PSCs with the dual function interfacial layer show enhanced long-term stability in ambient air without device encapsulation.

A TiO2 embedded structure for perovskite solar cells with anomalous grain growth and effective electron extraction

The structure of the perovskite solar cells (PSCs) is either mesoporous or planar. Here, a novel structure for highly efficient and stable PSCs is proposed, i.e., an embedded structure, which combines the advantages of the mesoporous and planar structures. The embedded structure utilizes a TiO2 nanoparticle embedded perovskite (CH3NH3PbI3) film as the absorption layer. The presence of TiO2 nanoparticles in the perovskite film could improve the electron extraction, and promote the formation of a compact perovskite layer with large grains. Consequently, the performance of the PSCs is significantly improved with the efficiency increasing from 16.6% for the planar structure to 19.2% for the embedded structure, which is the best performance of the MAPbI3-based PSCs. Furthermore, the TiO2 embedded perovskite films present better long-term stability than the pristine perovskite films, and the corresponding PSCs, which have no other chemical modifications, also show excellent stability with efficiency approaching 80% (for average) or 90% (for the best) after being exposed to air for 28 days without encapsulation.

Degradation of organometallic perovskite solar cells induced by trap states

The degradation of organometallic perovskite solar cells (PSCs) is the key bottleneck hampering their development, which is typically ascribed to the decomposition of perovskite (CH3NH3PbI3). In this work, the degradation of PSCs is observed to be significant, with the decrease in efficiency from 18.2% to 11.5% in ambient air for 7 days. However, no obvious decomposition or structural evolution of the perovskite was observed, except the notable degradation phenomenon of the device. The degradation of PSCs derives from deteriorated photocurrent and fill factor, which are proven to be induced by increased trap states for enlarged carrier recombination in degraded PSCs. The increased trap states in PSCs over storage time are probably induced by the increased defects at the surface of perovskite. The trap states induced degradation provides a physical insight into the degradation mechanisms of PSCs. Moreover, as the investigations were performed on real PSCs instead of individual perovskite films, the finding...

Anatase/TiO2-B hybrid microspheres constructed from ultrathin nanosheets: facile synthesis and application for fast lithium ion storage

Anatase/TiO2-B hybrids are considered to be promising anode materials for lithium ion storage. Here, a modified synthesis process for the fabrication of anatase/TiO2-B hybrid TiO2 spheres is proposed. Compared to the conventional two-step method employing strong base and acid solutions with titanate as the intermediate, the modified process proposed here employs a one-step reaction which is more facile and moderate. The as-prepared anatase/TiO2-B hybrid TiO2 spheres are assembled from ultrathin anatase TiO2 nanosheets embedded with TiO2-B nanodomains, which take the advantages of both the anatase and TiO2-B phases, possessing a large surface area and a hybrid crystalline structure, which are beneficial for fast diffusion and reversible storage of the lithium ions. Therefore, the anodes with the anatase/TiO2-B hybrid TiO2 spheres have a capacity of 101 mA h g−1 even at a current density of 20C and good cycling stability. This work provides a facile process for the fabrication of TiO2 nanostructures with the TiO2-B phase, which also implies the potential application of anatase/TiO2-B hybrid spheres in many fields including lithium ion batteries and other electrochemical technologies.

Co-catalytic mechanism of Au and Ag in silicon etching to fabricate novel nanostructures

Metal-assisted chemical etching is a very popular method of fabricating silicon nanostructures. For the dissolution and recrystallization of Ag and the inertia of Au, we present a co-catalytic mechanism of silicon etching using non-overlapping Au and Ag nanofilms. Meanwhile, two kinds of novel nanostructures are obtained by changing the relative positions of the two nanofilms. The results show that regardless of whether the Ag or Au nanofilm is the upper layer, the Ag nanofilm will first react to etch the silicon substrate into silicon nanowires (Si NWs). Afterwards, the Au nanofilm will re-etch the Si NWs into thick pillars (Ag upper) or ultrathin porous Si NWs (Au upper). It should be noted that the vertical etching rates of the two layers are not observably different, in contrast to when the Au and Ag nanofilms are used separately, in which the vertical etching rate of the Ag nanofilm is much higher than that of the Au nanofilm. This occurs because the subsequent re-etching process by the Au nanofilm is conducted at multiple surfaces because Au can generate excessive holes during the decomposition of hydrogen peroxide. Furthermore, this study presents a feasible method for the fabrication of individual, thick (∼200 nm) silicon pillars and ultrathin (∼25 nm) porous Si NWs. These insights are significant for the syntheses of many nanostructures.

Exact comprehensive equations for the photon management properties of silicon nanowire.

Unique photon management (PM) properties of silicon nanowire (SiNW) make it an attractive building block for a host of nanowire photonic devices including photodetectors, chemical and gas sensors, waveguides, optical switches, solar cells, and lasers. However, the lack of efficient equations for the quantitative estimation of the SiNW’s PM properties limits the rational design of such devices. Herein, we establish comprehensive equations to evaluate several important performance features for the PM properties of SiNW, based on theoretical simulations. Firstly, the relationships between the resonant wavelengths (RW), where SiNW can harvest light most effectively, and the size of SiNW are formulized. Then, equations for the light-harvesting efficiency at RW, which determines the single-frequency performance limit of SiNW-based photonic devices, are established. Finally, equations for the light-harvesting efficiency of SiNW in full-spectrum, which are of great significance in photovoltaics, are established. Furthermore, using these equations, we have derived four extra formulas to estimate the optimal size of SiNW in light-harvesting. These equations can reproduce majority of the reported experimental and theoretical results with only ~5% error deviations. Our study fills up a gap in quantitatively predicting the SiNW’s PM properties, which will contribute significantly to its practical applications.

DFT Investigations on the CVD Growth of Graphene

The chemical vapor deposition technique is the most popular for preparing highquality graphene. Surface energy will dominate the nucleation process of graphene; thus, the surface energy problems involved in thin film growth are introduced first. The experimental tools to describe the growth process in detail are insufficient. So, a mass of simulation investigations, which can give out a very fine description of the surface atomic process, have been carried out on this topic. We mainly summarized the density functional theory works in unearthing the graphene nuclei process and mechanisms. In addition, some studies using molecular dynamics methods are also listed. Such a summary will be helpful to stimulate future experimental efforts on graphene synthesis.