• Energy Research

An accelerated deposition test facility was operated with three different coal ash species to study the effect of ash composition on deposition rate and spatial distribution. The facility seeds a combusting (natural gas) flow with 10–20 micron mass mean diameter coal ash particulate. The particulate-laden combustor exhaust is accelerated through a rectangular-to-annular transition duct and expands to ambient pressure through a nozzle guide vane annular sector. For the present study, the annular cascade consisted of two CFM56 aero-engine vane doublets; comprising three full passages and two half passages of flow. The inlet Mach number (0.1) and gas temperature (1100°C) are representative of operating turbines. Ash samples were tested from the three major coal ranks: lignite, subbituminous, and bituminous. Investigations over a range of inlet gas temperatures from 900°C to 1120°C showed that deposition increased with temperature, though the threshold for deposition varied with ash type. Deposition levels varied with coal rank, with lignite producing the largest deposits at the lowest temperature. Regions of heightened deposition were noted; the leading edge and pressure surface being particularly implicated. Scanning electron microscopy was used to identify deposit structure. For a limited subset of tests, film cooling was employed at nominal design operating conditions but provided minimal protection in cases of severe deposition.

Simplifying the manufacturing processes of renewable energy technologies is crucial to lowering the barriers to commercialization. In this context, to improve the manufacturability of perovskite solar cells (PSCs), we have developed a one-step solution-coating procedure in which the hole-selective contact and perovskite light absorber spontaneously form, resulting in efficient inverted PSCs. We observed that phosphonic or carboxylic acids, incorporated into perovskite precursor solutions, self-assemble on the indium tin oxide substrate during perovskite film processing. They form a robust self-assembled monolayer as an excellent hole-selective contact while the perovskite crystallizes. Our approach solves wettability issues and simplifies device fabrication, advancing the manufacturability of PSCs. Our PSC devices with positive-intrinsic-negative (p-i-n) geometry show a power conversion efficiency of 24.5% and retain >90% of their initial efficiency after 1,200 h of operating at the maximum power point under continuous illumination. The approach shows good generality as it is compatible with different self-assembled monolayer molecular systems, perovskites, solvents and processing methods. Improving the manufacturability of perovskite solar cells is key to their deployment. Zheng et al. report a one-step deposition of the hole-selective and absorber layers that addresses wettability issues and simplifies the fabrication process.

Perovskite solar cells (PSCs) based on thin films of organolead trihalide perovskites (OTPs) hold unprecedented promise for low-cost, high-efficiency photovoltaics (PVs) of the future. While PV performance parameters of PSCs, such as short circuit current, open circuit voltage, and maximum power, are always measured at the macroscopic scale, it is necessary to probe such photoresponses at the nanoscale to gain key insights into the fundamental PV mechanisms and their localized dependence on the OTP thin-film microstructure. Here we use photoconductive atomic force microscopy spectroscopy to map for the first time variations of PV performance at the nanoscale for planar PSCs based on hole-transport-layer free methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) thin films. These results reveal substantial variations in the photoresponse that correlate with thin-film microstructural features such as intragrain planar defects, grains, grain boundaries, and notably also grain-aggregates. The insights gained into such microstructure-localized PV mechanisms are essential for guiding microstructural tailoring of OTP films for improved PV performance in future PSCs.

AbstractWe report herein the discovery of methylamine (CH3NH2) induced defect‐healing (MIDH) of CH3NH3PbI3 perovskite thin films based on their ultrafast (seconds), reversible chemical reaction with CH3NH2 gas at room temperature. The key to this healing behavior is the formation and spreading of an intermediate CH3NH3PbI3⋅xCH3NH2 liquid phase during this unusual perovskite–gas interaction. We demonstrate the versatility and scalability of the MIDH process, and show dramatic enhancement in the performance of perovskite solar cells (PSCs) with MIDH. This study represents a new direction in the formation of defect‐free films of hybrid perovskites.

Mesoscopic perovskite–polymer solar cells based on solution-processed NH2CHNH2PbI(3−x)Clxwas firstly reported and the effect of annealing temperature on perovskite formation and photovoltaic performance was studied.

A new generation of solid-state photovoltaics is being made possible by the use of organometal-trihalide perovskite materials. While some of these materials are expected to be ferroelectric, almost nothing is known about their ferroelectric properties experimentally. Using piezoforce microscopy (PFM), here we show unambiguously, for the first time, the presence of ferroelectric domains in high-quality β-CH3NH3PbI3 perovskite thin films that have been synthesized using a new solution-processing method. The size of the ferroelectric domains is found to be about the size of the grains (∼100 nm). We also present evidence for the reversible switching of the ferroelectric domains by poling with DC biases. This suggests the importance of further PFM investigations into the local ferroelectric behavior of hybrid perovskites, in particular in situ photoeffects. Such investigations could contribute toward the basic understanding of photovoltaic mechanisms in perovskite-based solar cells, which is essential for the further enhancement of the performance of these promising photovoltaics.