Semiconductors group

Abstract:

This dataset accompanies the publication "Re-defining non-tracking solar cell efficiency limits with directional spectral filters" published in ACS Photonics (10.1021/acsphotonics.4c02181). The data can be used to reproduce figures 2-4 in the main text and all plots with data in the supporting information (noting figure 1 in the main text is only schematics). All data was generated via home-built modelling codes. All files are in .CSV and easily readable. The abstract for the associated paper is as follows: Optical filters that respond to the wavelength and direction of incident light can be used to increase the efficiency of tracking solar cells. However, as tracking solar cells are more expensive to install and maintain, it is likely that non-tracking solar cells will remain the main product of the (terrestrial) solar cell industry.  Here we demonstrate that wavelength and directionally selective filters can also be used to increase the efficiency limit of non-tracking solar cells at the equator beyond what is currently understood by up to ~ 0.5 % (relative ~ 1.8 %). We also reveal that such filters can be used to regulate the energy output of solar cells throughout a day or year, and can reduce the thickness of the absorber layer by up to 40 %. We anticipate that similar gains would be seen at other latitudes. As this filter has complex wavelength-direction functionality, we present a proof-of-concept design based on Luneburg lenses, demonstrating these filters can be realized. Our results will enable solar cells with higher efficiency and more stable output while using less material.LNETv

Abstract:

FAPbI3 has emerged as a promising semiconductor for photovoltaic applications offering a suitable bandgap for single-junction cells and high chemical stability. However, device efficiency is negatively affected by intrinsic quantum confinement (QC) effects that manifest as additional peaks in the absorption spectra. Here, we show that aerosol-assisted crystallization is an effective method to improve crystallinity and suppresses regions exhibiting QC in FAPbI3. We demonstrate that films with minimized QC effects exhibit markedly enhanced optoelectronic properties, such as higher charge-carrier mobilities and recombination lifetimes. Films crystallized under an aerosol solvent flow of either a mixture of N, N-dimethylformamide and dimethyl sulfoxide or methylammonium thiocyanate vapor displayed reduced charge-carrier recombination losses and improved diffusion lengths compared to those of thermally annealed control films. Our study indicates clear correlations between suppression of QC features in absorption spectra with optimization of crystallinity and mitigation of internal strain, highlighting pathways toward high-performance solar cells.

Abstract:

2D lead halide perovskites (2DPs) offer chemical compatibility with 3D perovskites and enhanced stability, which are attractive for applications in photovoltaic and light‐emitting devices. However, such lowered structural dimensionality causes increased excitonic effects and highly anisotropic charge‐carrier transport. Determining the diffusivity of excitations, in particular for out‐of‐plane or inter‐layer transport, is therefore crucial, yet challenging to achieve. Here, an effective method is demonstrated for monitoring inter‐layer diffusion of photoexcitations in (PEA)2PbI4 thin films by tracking time‐dependent changes in photoluminescence spectra induced by photon reabsorption effects. Selective photoexcitation from either substrate‐ or air‐side of the films reveals differences in diffusion dynamics encountered through the film profile. Time‐dependent diffusion coefficients are extracted from spectral dynamics through a 1D diffusion model coupled with an interference correction for refractive index variations arising from the strong excitonic resonance of 2DPs. Such analysis, together with structural probes, shows that minute misalignment of 2DPs planes occurs at distances far from the substrate, where efficient in‐plane transport consequently overshadows the less efficient out‐of‐plane transport in the direction perpendicular to the substrate. Through detailed analysis, a low out‐of‐plane excitation diffusion coefficient of (0.26 ± 0.03) ×10−4 cm2 s−1 is determined, consistent with a diffusion anisotropy of ≈4 orders of magnitude.

Abstract:

Covalent organic frameworks (COFs), crystalline and porous conjugated structures, are of great interest for sustainable energy applications. Organic building blocks in COFs with suitable electronic properties can feature strong optical absorption, whereas the extended crystalline network can establish a band structure enabling long-range coherent transport. This peculiar combination of both molecular and solid-state materials properties makes COFs an interesting platform to study and ultimately utilize photoexcited charge carrier diffusion. Herein, we investigated the charge carrier diffusion in a two-dimensional COF thin film generated through condensation of the building blocks benzodithiophene-dialdehyde (BDT) and N,N,N′,N′-tetra(4-aminophenyl)benzene-1,4-diamine (W). We visualized the spatiotemporal evolution of photogenerated excited states in the 2D WBDT COF thin film using remote-detected time-resolved PL measurements (RDTR PL). Combined with optical pump terahertz probe (OPTP) studies, we identified two diffusive species dominating the process at different time scales. Initially, short-lived free charge carriers diffuse almost temperature-independently before relaxing into bound states at a rate of 0.7 ps^–1. Supported by theoretical simulations, these long-lived bound states were identified as excitons. We directly accessed the lateral exciton diffusion within the oriented and crystalline film, revealing remarkably high diffusion coefficients of up to 4 cm² s^–1 (200 K) and diffusion lengths of several hundreds of nanometers and across grain boundaries. Temperature-dependent exciton transport analysis showed contributions from both incoherent hopping and coherent band-like transport. In the transport model developed based on these findings, we discuss the complex impact of order and disorder on charge carrier diffusion within the WBDT COF thin film.