Probing Nanoscale Charge Transport Mechanisms in Quasi-2D Halide Perovskites for Photovoltaic Applications.

Quasi-2D layered halide perovskites (quasi-2DLPs) have emerged as promising materials for photovoltaic (PV) applications owing to their advantageous bandgap for absorbing visible light and the improved stability they enable. Their charge transport mechanism is heavily influenced by the grain orientation of their crystals as well as their nanostructures, such as grain boundaries (GBs) and edge states─the formation of which is inevitable in polycrystalline quasi-2DLP thin films. Despite their importance, the impact of these features on charge transport remains unexplored. In this study, we conduct a detailed investigation on polycrystalline quasi-2DLP thin films and devices, carefully analyzing how grain orientation and nanostructures influence charge transport. Employing nondestructive atomic force microscopy (AFM) topography, along with transient absorption spectroscopy (TAS) and grazing-incidence wide-angle X-ray scattering (GIWAXS), we obtained significant insights regarding the phase purity, crystallographic information, and morphologies of these films. Moreover, our systematic investigation using AFM-based techniques, including Kelvin probe force microscopy (KPFM) and conductive AFM (c-AFM), elucidates the roles played by GBs and edge states in shaping charge transport behavior. In particular, the local band structure along the GBs and edge states within both vertical and parallel grains was found to selectively repel electrons and holes, thus facilitating charge carrier separation. These findings provide perspectives for the development of high-performance quasi-2DLP PV devices and highlight potential approaches that can leverage the intrinsic properties of quasi-2DLPs to advance the performance of perovskite solar cells.

Highly Efficient Wide Bandgap Perovskite Solar Cells With Tunneling Junction by Self-Assembled 2D Dielectric Layer.

Reducing non-radiative recombination and addressing band alignment mismatches at interfaces remain major challenges in achieving high-performance wide-bandgap perovskite solar cells. This study proposes the self-organization of a thin two-dimensional (2D) perovskite BA2PbBr4 layer beneath a wide-bandgap three-dimensional (3D) perovskite Cs0.17FA0.83Pb(I0.6Br0.4)3, forming a 2D/3D bilayer structure on a tin oxide (SnO2) layer. This process is driven by interactions between the oxygen vacancies on the SnO2 surface and hydrogen atoms of the n-butylammonium cation, aiding the self-assembly of the BA2PbBr4 2D layer. The 2D perovskite acts as a tunneling layer between SnO2 and the 3D perovskite, neutralizing the energy level mismatch and reducing non-radiative recombination. This results in high power conversion efficiencies of 21.54% and 19.16% for wide-bandgap perovskite solar cells with bandgaps of 1.7 and 1.8 eV, with open-circuit voltages over 1.3 V under 1-Sun illumination. Furthermore, an impressive efficiency of over 43% is achieved under indoor conditions, specifically under 200 lux white light-emitting diode light, yielding an output voltage exceeding 1 V. The device also demonstrates enhanced stability, lasting up to 1,200 hours.

Revealing the Dynamics of the Thermal Reaction between Copper and Mixed Halide Perovskite Solar Cells.

Copper (Cu) is present not only in the electrode for inverted-structure halide perovskite solar cells (PSCs) but also in transport layers such as copper iodide (CuI), copper thiocyanate (CuSCN), and copper phthalocyanine (CuPc) alternatives to spiro-OMeTAD due to their improved thermal stability. While Cu or Cu-incorporated materials have been effectively utilized in halide perovskites, there is a lack of thorough investigation on the direct reaction between Cu and a perovskite under thermal stress. In this study, we investigated the thermal reaction between Cu and a perovskite as well as the degradation mechanism by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Kelvin probe force microscopy (KPFM). The results show that high temperatures of 100 °C induce Cu to be incorporated into the perovskite lattice by forming "Cu-rich yet organic A-site-poor" perovskites, (CuxA1-x)PbX3, near the grain boundaries, which result in device performance degradation.

Transparent Electrodes with Enhanced Infrared Transmittance for Semitransparent and Four-Terminal Tandem Perovskite Solar Cells.

This report addresses indium oxide doped with titanium and tantulum with high near-infrared transparency to potentially replace the conventional indium tin oxide transparent electrode used in semitransparent perovskite devices and top cells of tandem devices. The high near-infrared transparency of this electrode is possibly explained by the lower carrier concentration, suggesting less defect sites that may sacrifice its optical transparency. Incorporating this transparent electrode into semitransparent perovskite solar cells for both the top and bottom electrodes improved the device performance through possible reduction of interfacial defect sites and modification in energy alignment. With this indium oxide-based semitransparent perovskite top cell, we also demonstrated four-terminal perovskite-silicon tandem configurations with improved photocurrent response in the bottom silicon cell.

Microstructural Evaluation of Phase Instability in Large Bandgap Metal Halide Perovskites.

The optoelectronic performance of organic-inorganic halide perovskite (OIHP)-based devices has been improved in recent years. Particularly, solar cells fabricated using mixed-cations and mixed-halides have outperformed their single-cation and single-halide counterparts. Yet, a systematic evaluation of the microstructural behavior of mixed perovskites is missing despite their known composition-dependent photoinstability. Here, we explore microstructural inhomogeneity in (FAPbI3)x(MAPbBr3)1-x using advanced scanning probe microscopy techniques. Contact potential difference (CPD) maps measured by Kelvin probe force microscopy show an increased fraction of grains exhibiting a low CPD with flat topography as MAPbBr3 concentration is increased. The higher portion of low CPD contributes to asymmetric CPD distribution curves. Chemical analysis reveals these grains being rich in MA, Pb, and I. The composition-dependent phase segregation upon illumination, reflected on the emergence of a low-energy peak emission in the original photoluminescence spectra, arises from the formation of such grains with flat topology. Bias-dependent piezo-response force microscopy measurements, in these grains, further confirm vigorous ion migration and cause a hysteretic piezo-response. Our results, therefore, provide insights into the microstructural evaluation of phase segregation and ion migration in OIHPs pointing toward process optimization as a mean to further enhance their optoelectronic performance.