Optimization of Hole and Electron Transport Layer for Highly Efficient Lead-Free Cs2TiBr6-Based Perovskite Solar Cell

The methylammonium lead halide solar cell has attracted a great deal of attention due to its lightweight, low cost, and simple fabrication and processing. Despite these advantages, these cells are still far from commercialization because of their lead-based toxicity. Among lead-free perovskites, cesium-titanium (IV) bromide (Cs2TiBr6) is considered one of the best alternatives, but it faces a lack of higher PCE (power conversion efficiency) due to the unavailability of the matched hole and electron transport layers. Therefore, in this study, the ideal hole and electron transport layer parameters for the Cs2TiBr6-based solar cell were determined and discussed based on a simulation through SCAPS-1D software. It was observed that the maximum PCE of 20.4% could be achieved by using the proper hole and electron transport layers with optimized parameters such as energy bandgap, electron affinity, doping density, and thickness. Unfortunately, no hole and electron transport material with the required electronic structure was found. Then, polymer NPB and CeOx were selected as hole and electron transport layers, respectively, based on their closed electronic structure compared to the simulation results, and, hence, the maximum PCE was found as ~17.94% for the proposed CeOx/Cs2TiBr6/NPB solar cell.

Analysis of the combined effect of long-term heat light soaking and KF/NaF post-deposition treatment on the open-circuit voltage loss in CIGS solar cells

Heat light soaking (HLS) has been known to impact the photovoltaic parameters of Cu(In,Ga)Se2 (CIGS) solar cells for a long time. Recently, the focus shifted to the effect of the procedure on alkali fluoride-treated CIGS. Here, we investigate the impact of long-term HLS on the open-circuit (VOC) loss in high-efficiency CIGS with potassium fluoride (KF) and sodium fluoride (NaF) post-deposition treatment (PDT). HLS is shown to increase the net doping density, however, the subsequent improvement of the VOC is lower than expected. Using an analysis based on the SQ theory, we show that HLS reduces the nonradiative recombination rate in the bulk but increases the one at the interface. We present a model to explain the increase of interface recombination. We further demonstrate that a combination of HLS and KF/NaF-PDT is necessary to enhance the positive impacts of HLS and mitigate the detrimental ones leading to high-efficiency CIGS devices (22%).

Unveiling charge dynamics of visible light absorbing oxysulfide for efficient overall water splitting

Oxysulfide semiconductor, Y2Ti2O5S2, has recently discovered its exciting potential for visible-light-induced overall water splitting, and therefore, imperatively requires the probing of unknown fundamental charge loss pathways to engineer the photoactivity enhancement. Herein, transient diffuse reflectance spectroscopy measurements are coupled with theoretical calculations to unveil the nanosecond to microsecond time range dynamics of the photogenerated charge carriers. In early nanosecond range, the pump-fluence-dependent decay dynamics of the absorption signal is originated from the bimolecular recombination of mobile charge carriers, in contrast, the power-law decay kinetics in late microsecond range is dominated by hole detrapping from exponential tail trap states of valence band. A well-calibrated theoretical model estimates various efficiency limiting material parameters like recombination rate constant, n-type doping density and tail-states parameters. Compared to metal oxides, longer effective carrier lifetime ~6 ns is demonstrated. Different design routes are proposed to realize efficiency beyond 10% for commercial solar-to-hydrogen production from oxysulfide photocatalysts.

Defect concentration and Δn change in light- and elevated temperature- induced degradation

The wide variety of silicon materials used by various groups to investigate LeTID make it difficult to directly compare the defect concentrations (Nt) using the typical normalised defect density (NDD) metric. Here, we propose a new formulation for a relative defect concentration (β) as a correction for NDD that allows flexibility to perform lifetime analysis at arbitrary injection levels (Δn), away from the required ratio between Δn and the background doping density (Ndop) for NDD of Δn/N dop = 0.1. As such, β allows for a meaningful comparison of the maximum degradation extent between different samples in different studies and also gives a more accurate representative value to estimate the defect concentration. It also allows an extraction at the cross-over point in the undesirable presence of iron, or flexibility to reduce the impact of modulation in surface passivation. Although the accurate determination of β at a given Δn requires knowledge of the capture cross-section ratio (k), the injection-independent property of the β formulation allows a self-consistent determination of k. Experimental verification is also demonstrated for boron-oxygen related defects and LeTID defects, yielding k-values of 10.6 ± 3.2 and 30.7 ± 4.0, respectively, which are within the ranges reported in the literature. With this, when extracting the defect density at different Δn ranging between 1014 /cm3 to 1015 /cm3 with Ndop = 9.1 ×1015 /cm3, the error is less than 12% using β, allowing for a greatly improved understanding of the defect concentration in a material.

Laser enabled ultra-high doping patterns in graphene for rewritable photodetectors

Chemical doping has been extensively studied for control of charge carrier polarity and concentration in two-dimensional (2D) van der Waals materials. However, conventional routes by substitutional doping or absorbed molecules suffer from degradation of the electrical mobility due to structural disorder, while the maximum doping density is set by the solubility limit of dopants. Here, we show that laser assisted chlorination can achieve high doping concentration (> 3×1013 cm− 2) in graphene monolayer with minimal mobility drop, while holding reversibility and spatial selectivity. Such superior doping scheme is enabled by two lasers with selected photon energies and geometric configurations, resulting to high Cl coverage ratio (C2Cl) and subsequent local Cl-removal without damaging graphene. Based on this method, we demonstrate rewritable graphene photodetector, manifesting high quality reversible doping patterns in graphene. We believe that the presented results offer a new route for chemical doping of 2D materials that may enable exotic optoelectronic applications.

Dipolar excitonic insulator in a moiré lattice

Two-dimensional (2D) moiré materials provide a highly-controllable solid-state platform for studies of correlated quantum phenomena. To date, experimental studies have focused on the correlated electronic states; the correlated bosonic states in moiré materials have remained practically unexplored. Here, we report a correlated dipolar excitonic insulator--a charge insulating state driven by exciton formation--in a Coulomb-coupled WSe2 monolayer and WSe2/WS2 moiré bilayer at total hole doping density equal to the moiré density. The system is a Mott insulator when all the holes reside in the moiré layer. Under an out-of-plane electric field, the holes can be continuously transferred to the WSe2 monolayer, but remain strongly bound to the empty moiré sites. This is effectively an interlayer exciton fluid in the moiré lattice under a particle-hole transformation. We identify the phase space and determine the charge gap energy of the excitonic insulating state by optical spectroscopy and capacitance measurements, respectively. We further observe the emergence of local magnetic moments in the WSe2 monolayer induced by the strong interlayer Coulomb correlation. Our result paves the path for realizing correlated bosonic quantum phenomena described by the Bose-Hubbard model in a solid-state system.