Xiangyue Meng

Profiling the organic cation-dependent degradation of organolead halide perovskite solar cells

Operational stability is one of the main obstacles that may hold back the commercialization of perovskite solar cells (PVSCs). In this paper, we provide a detailed account of the ion migration accelerated PVSC degradation by comparatively studying perovskite materials with two different organic cations (methylammonium (MA+) and formamidinium (FA+)). Using time of flight secondary ion mass spectrometry (TOF-SIMS), we have uncovered the ion migration accelerated degradation of PVSCs at the device level. Not only did mobile iodide (I−) ions from the perovskite layer diffuse out, but Ag atoms/ions from the metal electrode also diffused into the perovskite layer, which resulted in severe device degradation. Besides, we identified I− species in the hole transport material (HTM) layer for even freshly prepared PVSC devices, which was responsible for the degradation of devices kept under inert conditions. This also testifies the existence of ion migration on the device level of PVSCs. Compared with MAPbI3, the ion migration process can slow down in FAPbI3 devices which accounts for a better stability of FAPbI3 devices. This work underscores the impact of organic cation substitution on PVSC degradation and provides solid evidence for mobile ion migration in perovskite materials and the consequent degradation in specific device settings such as the n–i–p type perovskite solar cells.

Hydrogen evolution electrocatalysis with binary-nonmetal transition metal compounds

The growing concern about global warming, environmental pollution and energy security has increased the demand for clean energy resources in place of fossil fuel. Cost-efficient generation of hydrogen from water splitting through electrocatalysis holds tremendous promise for clean energy. Central to electrocatalysis are efficient and robust electrocatalysts composed of earth-abundant elements, which are urgently needed for realizing low-cost and high-performance energy conversion devices. Transition metal compounds (TMCs) are a group of attractive noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER). The incorporation of foreign nonmetal atoms into TMCs is a way of controllable disorder engineering and modification of their electronic structure, and thus may realize the synergistic modulations of both activity and conductivity for efficient HER performance. In the last few years, the interest in binary-nonmetal TMCs as an efficient HER electrocatalyst has grown exponentially owing to their fascinating electronic structure and chemical properties. Here, we sum up the recent developments of binary-nonmetal TMCs in HER electrocatalysis from the viewpoint of their tunable physicochemical properties. In addition, we identify major challenges ahead in this area and refine viable strategies and future research directions that will effectively address the said challenges.

Low-temperature aqueous solution processed ZnO as an electron transporting layer for efficient perovskite solar cells

Lead halide perovskite solar cells (PSCs) typically use high-temperature processed TiO2 as electron transporting layers (ETLs). Here, we report a low temperature and aqueous solution processed route to prepare ZnO ETLs for perovskite solar cells. The ZnO ETL was prepared by spin coating a stable aqueous solution of an ammine–hydroxo zinc complex, [Zn(NH3)x](OH)2, followed by thermal annealing at a relatively low temperature of 150 °C. The as-prepared ZnO film was highly transparent and uniform. Moreover, the relatively low work function of the ZnO film led to a high open circuit voltage (1.07 V), indicating its promise as the electron transporting layer. To further improve the photovoltaic performance, urea was introduced between the ZnO layer and the perovskite layer to improve the perovskite crystal growth. As a result, the PCE surged to 14.6%. We further exploited the low-temperature processed ZnO film for flexible PSCs, which have shown good mechanical stability with a respectable PCE of 11.9%.

Pinning Down the Anomalous Light Soaking Effect toward High-Performance and Fast-Response Perovskite Solar Cells: The Ion-Migration-Induced Charge Accumulation

The light soaking effect (LSE) is widely known in perovskite solar cells (PVSCs), but its origin is still elusive. In this study, we show that in common with hysteresis, the LSE is owed to the ion migration in PVSCs. Driven by the photovoltage, the mobile ions in the perovskite materials (MA+/I-) migrate to the selective contacts, forming a boosted P-i-N junction resulting in enhanced charge separation. Besides, the mobile ions (MA+) can soften and suture the PCBM/perovskite interface and thus reduce the trap density, in keeping with a higher open-circuit voltage. Finally, almost LSE-free PVSCs can be prepared by using 0.1 wt % MAI-doped PCBM as the electron transport material, whereas overdoping (1 wt % MAI doping) makes the LSE even more pronounced due to excess mobile ions that need time to migrate to reach a new quasi-static state.

Tuning the A-site cation composition of FA perovskites for efficient and stable NiO-based p-i-n perovskite solar cells

Cation mixing has proved to be effective in stabilizing the high-temperature phase of formamidinium (FA)-based perovskites, affording high-performance n–i–p perovskite solar cells (PSCs). However, optimum cation mixing is found to be inapplicable directly to NiO p–i–n PSCs due to the energy band misalignment. In the present study, we reveal the role of mixing cesium (Cs), methylammonium (MA) and formamidinium (FA) in the energy band alignments and the crystallization of perovskites in such a device structure. By tuning the composition of mixed cations, we have significantly improved the energy band alignments of perovskites in p–i–n NiO-based PSCs. The relative amount of Cs to MA cations also plays a decisive role in shaping the nature of perovskite precursors, thus impacting the quality of the resulting perovskite layer in NiO p–i–n PSCs. These insights and the associated engineering efforts led to a significantly improved power conversion efficiency of 18.6% based on the NiO p–i–n PSCs, in addition to their superior ambient stability to typical n–i–p PSCs.

Versatility of Carbon Enables All Carbon Based Perovskite Solar Cells to Achieve High Efficiency and High Stability

Carbon-based perovskite solar cells (PVSCs) without hole transport materials are promising for their high stability and low cost, but the electron transporting layer (ETL) of TiO2 is notorious for inflicting hysteresis and instability. In view of its electron accepting ability, C60 is used to replace TiO2 for the ETL, forming a so-called all carbon based PVSC. With a device structure of fluorine-doped tin oxide (FTO)/C60 /methylammonium lead iodide (MAPbI3 )/carbon, a power conversion efficiency (PCE) is attained up to 15.38% without hysteresis, much higher than that of the TiO2 ones (12.06% with obvious hysteresis). The C60 ETL is found to effectively improve electron extraction, suppress charge recombination, and reduce the sub-bandgap states at the interface with MAPbI3 . Moreover, the all carbon based PVSCs are shown to resist moisture and ion migration, leading to a much higher operational stability under ambient, humid, and light-soaking conditions. To make it an even more genuine all carbon based PVSC, it is further attempted to use graphene as the transparent conductive electrode, reaping a PCE of 13.93%. The high performance of all carbon based PVSCs stems from the bonding flexibility and electronic versatility of carbon, promising commercial developments on account of their favorable balance of cost, efficiency, and stability.

Integration of inverse nanocone array based bismuth vanadate photoanodes and bandgap-tunable perovskite solar cells for efficient self-powered solar water splitting

Bismuth vanadate (BiVO4) has been regarded as a promising photoanode material for photoelectrochemical (PEC) water splitting owing to its rich elemental abundance and relatively narrow bandgap. However, the incompatibility of the penetration depth and short diffusion length limits its performance. To overcome this shortcoming, we develop a cost-effective stamping method to fabricate inverse nanocone array (ICA) substrates for supporting nanoporous Mo-doped BiVO4 films. The ICAs show a remarkable light trapping effect in such a way that the intensive light absorption region is advantageously shifted from the top of the active layer on a planar substrate to the bottom surrounded by the ICA, where charge separation is strikingly more efficient. By integrating the ICA-photoanode with a tailor-made, bandgap-adjustable perovskite solar cell, we devised a PEC-photovoltaic (PEC-PV) tandem device, which has achieved a self-powered STH efficiency of around 6.3%. Our study opens a new avenue for designing solar fuel devices with PEC-PV architectures.