Yinglong Yang

Boron Doping of Multiwalled Carbon Nanotubes Significantly Enhances Hole Extraction in Carbon-Based Perovskite Solar Cells

Compared to the conventional perovskite solar cells (PSCs) containing hole-transport materials (HTM), carbon materials based HTM-free PSCs (C-PSCs) have often suffered from inferior power conversion efficiencies (PCEs) arising at least partially from the inefficient hole extraction at the perovskite-carbon interface. Here, we show that boron (B) doping of multiwalled carbon nanotubes (B-MWNTs) electrodes are superior in enabling enhanced hole extraction and transport by increasing work function, carrier concentration, and conductivity of MWNTs. The C-PSCs prepared using the B-MWNTs as the counter electrodes to extract and transport hole carriers have achieved remarkably higher performances than that with the undoped MWNTs, with the resulting PCE being considerably improved from 10.70% (average of 9.58%) to 14.60% (average of 13.70%). Significantly, these cells show negligible hysteretic behavior. Moreover, by coating a thin layer of insulating aluminum oxide (Al2O3) on the mesoporous TiO2 film as a physical barrier to substantially reduce the charge losses, the PCE has been further pushed to 15.23% (average 14.20%). Finally, the impressive durability and stability of the prepared C-PSCs were also testified under various conditions, including long-term air exposure, heat treatment, and high humidity.

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.

An amorphous precursor route to the conformable oriented crystallization of CH3NH3PbBr3 in mesoporous scaffolds: toward efficient and thermally stable carbon-based perovskite solar cells

CH3NH3PbBr3 (MAPbBr3)-perovskite solar cells (Br-PSCs) have attracted much attention due to their green-long wavelength transparency and high open-circuit voltage originating from their large bandgap (2.2 eV). However, the efficiency of carbon-based Br-PSCs without organic hole transport materials (HTM) is still low due to the inappropriate quality of MAPbBr3 deposited in a relatively thick porous scaffold. Herein, an amorphous precursor route based on a two-step sequential method is exploited to conformably and seamlessly grow MAPbBr3 in the TiO2 porous scaffold. In the first step, the amorphous Pb–Br precursor containing a large amount of DMF molecules was prepared by lowering the post-treatment temperature to 25 °C, affording full pore filling and smooth surface capping. The conversion to MAPbBr3 in the second step was accelerated by the molecular exchange between DMF and MABr in IPA solution. Moreover, by solvent engineering through the addition of non-polar cyclohexane into the MABr IPA solution, the molecular exchange process was tuned in such a way to separate the nucleation and growth of MAPbBr3 crystals, leading to the preferential [001] orientation with an even surface finishing and the subsequent light absorption enhancement and trap state reduction. Using MAPbBr3 films in carbon-based PSCs has boosted their efficiency to 8.09% (Voc = 1.35 V), a record value for HTM-free Br-PSCs, also comparable to that of the best HTM-based Br-PSCs. Significantly, non-encapsulated devices showed no efficiency decay after storage in dry air (25–30 °C and 10–20% humidity) for 90 days. What is more, the efficiency was retained up to about 90% after storage for 15 days under high heat stress (air, 80 °C and 50–85% humidity).

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.

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.