Nanoscale surface potential variation correlates with local S/Se ratio in solution-processed CZTSSe solar cells

Identification the Role of Grain Boundaries in Polycrystalline Photovoltaics via Advanced Atomic Force Microscope

Atomicforce microscopy (AFM)-based scanning probing techniques, including Kelvinprobe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM), have been widely applied to investigate thelocal electromagnetic, physical, or molecular characteristics of functional materials on a microscopic scale. The microscopic inhomogeneities of the electronic properties of polycrystalline photovoltaic materials can be examined by these advanced AFM techniques, which bridge the local properties of materials to overall device performance and guide the optimization of the photovoltaic devices. In this review, the critical roles of local optoelectronic heterogeneities, especially at grain interiors (GIs) and grain boundaries (GBs) of polycrystalline photovoltaic materials, including versatile polycrystalline silicon, inorganic compound materials, and emerging halide perovskites, studied by KPFM and C-AFM, are systematically identified. How the band alignment and electrical properties of GIs and GBs affect the carrier transport behavior are discussed from the respective of photovoltaic research. Further exploiting the potential of such AFM-based techniques upon a summary of their up-to-date applications in polycrystalline photovoltaic materials is beneficial to acomprehensive understanding of the design and manipulation principles of thenovel solar cells and facilitating the development of the next-generation photovoltaics and optoelectronics.

Achieving Low VOC-deficit Characteristics in Cu2ZnSn(S,Se)4 Solar Cells through Improved Carrier Separation

Kesterite-based thin-film solar cells (TFSCs) have recently gained significant attention in the photovoltaic (PV) sector for their elemental earth abundance and low toxicity. An inclusive study from the past reveals basic knowledge about the grain boundary (GB) and grain interior (GI) interface. However, the compositional dependency of the surface potential within GBs and GIs remains unclear. The present work provides insights into the surface potential of the bulk and GB interfaces. The tin (Sn) composition is sensitive to the absorber morphology, and therefore, it significantly impacts absorber and device properties. The absorber morphology improves with the formation of larger grains as the Sn content increases. Additionally, the presence of Sn(S,Se)₂ and increased [Zn_Cu + V_Cu] A-type defect cluster density are observed, validated through Raman analysis. The secondary ion mass spectroscopy analysis reveals the altered distribution of sulfur (S) and sodium (Na) with higher near-surface accumulation. The synergistic outcome of the increased density of defects and the accumulation of S near the interface provides a larger GB and GI difference and expedites carrier separation improvement. Consequently, at an optimum compositional ratio of Cu/(Zn+Sn) = ∼0.6, the power conversion efficiency (PCE) is significantly improved from 6.42 to 11.04% with a record open-circuit voltage (V_OC) deficit of 537 mV.

Plasmon-enhanced photocatalytic activity of Na0.9Mg0.45Ti3.55O8 loaded with noble metals directly observed with scanning Kelvin probe microscopy

The noble metals Au, Ag and Pt were loaded onto Na_0.9Mg_0.45Ti_3.55O₈ (NMTO) using a chemical bath deposition method devised in our recent work for the first time. The composite photocatalysts exhibit more effective photodegradation of methylene blue, due to the Schottky barrier built between NMTO and noble metal. Hot electrons generated during localized surface plasmon processes in metal nanoparticles transfer to the semiconductor, manifesting as a depression of surface potential directly detectable by scanning Kelvin probe microscopy. The key factor responsible for the improved ability of semiconductor-based photocatalysts is charge separation. The most effective weight concentrations of Au, Ag and Pt loaded onto NMTO were found to be 5.00%, 12.6% and 5.55% respectively. NMTO loaded with noble metals shows good photostability and recyclability for the degradation of methylene blue. A possible mechanism for the photodegradation of methylene blue over NMTO loaded with noble metals is proposed. This work highlights the potential application of NMTO-based photocatalysts, and provides an effective method to detect localized surface plasmons.

Identifying Nanoscale Structure-Function Relationships Using Multimodal Atomic Force Microscopy, Dimensionality Reduction, and Regression Techniques

Correlating nanoscale chemical specificity with operational physics is a long-standing goal of functional scanning probe microscopy (SPM). We employ a data analytic approach combining multiple microscopy modes using compositional information in infrared vibrational excitation maps acquired via photoinduced force microscopy (PiFM) with electrical information from conductive atomic force microscopy. We study a model polymer blend comprising insulating poly(methyl methacrylate) (PMMA) and semiconducting poly(3-hexylthiophene) (P3HT). We show that PiFM spectra are different from FTIR spectra but can still be used to identify local composition. We use principal component analysis to extract statistically significant principal components and principal component regression to predict local current and identify local polymer composition. In doing so, we observe evidence of semiconducting P3HT within PMMA aggregates. These methods are generalizable to correlated SPM data and provide a meaningful technique for extracting complex compositional information that is impossible to measure from any one technique.

Cation Substitution in Earth-Abundant Kesterite Photovoltaic Materials

As a promising candidate for low-cost and environmentally friendly thin-film photovoltaics, the emerging kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have experienced rapid advances over the past decade. However, the record efficiency of CZTSSe solar cells (12.6%) is still significantly lower than those of its predecessors Cu(In,Ga)Se2 (CIGS) and CdTe thin-film solar cells. This record has remained for several years. The main obstacle for this stagnation is unanimously attributed to the large open-circuit voltage (VOC) deficit. In addition to cation disordering and the associated band tailing, unpassivated interface defects and undesirable energy band alignment are two other culprits that account for the large VOC deficit in kesterite solar cells. To capture the great potential of kesterite solar cells as prospective earth-abundant photovoltaic technology, current research focuses on cation substitution for CZTSSe-based materials. The aim here is to examine recent efforts to overcome the VOC limit of kesterite solar cells by cation substitution and to further illuminate several emerging prospective strategies, including: i) suppressing the cation disordering by distant isoelectronic cation substitution, ii) optimizing the junction band alignment and constructing a graded bandgap in absorber, and iii) engineering the interface defects and enhancing the junction band bending.

Local Observation of Phase Segregation in Mixed-Halide Perovskite

Mixed-halide perovskites have emerged as promising materials for optoelectronics due to their tunable band gap in the entire visible region. A challenge remains, however, in the photoinduced phase segregation, narrowing the band gap of mixed-halide perovskites under illumination thus restricting applications. Here, we use a combination of spatially resolved and bulk measurements to give an in-depth insight into this important yet unclear phenomenon. We demonstrate that photoinduced phase segregation in mixed-halide perovskites selectively occurs at the grain boundaries rather than within the grain centers by using shear-force scanning probe microscopy in combination with confocal optical spectroscopy. Such difference is further evidenced by light-biased bulk Fourier-transform photocurrent spectroscopy, which shows the iodine-rich domain as a minority phase coexisting with the homogeneously mixed phase during illumination. By mapping the surface potential of mixed-halide perovskites, we evidence the higher concentration of positive space charge near the grain boundary possibly provides the initial driving force for phase segregation, while entropic mixing dominates the reverse process. Our work offers detailed insight into the microscopic processes occurring at the boundary of crystalline perovskite grains and will support the development of better passivation strategies, ultimately allowing the processing of more environmentally stable perovskite films.

Functional Scanning Probe Imaging of Nanostructured Solar Energy Materials

From hybrid perovskites to semiconducting polymer/fullerene blends for organic photovoltaics, many new materials being explored for energy harvesting and storage exhibit performance characteristics that depend sensitively on their nanoscale morphology. At the same time, rapid advances in the capability and accessibility of scanning probe microscopy methods over the past decade have made it possible to study processing/structure/function relationships ranging from photocurrent collection to photocarrier lifetimes with resolutions on the scale of tens of nanometers or better. Importantly, such scanning probe methods offer the potential to combine measurements of local structure with local function, and they can be implemented to study materials in situ or devices in operando to better understand how materials evolve in time in response to an external stimulus or environmental perturbation. This Account highlights recent advances in the development and application of scanning probe microscopy methods that can help address such questions while filling key gaps between the capabilities of conventional electron microscopy and newer super-resolution optical methods. Focusing on semiconductor materials for solar energy applications, we highlight a range of electrical and optoelectronic scanning probe microscopy methods that exploit the local dynamics of an atomic force microscope tip to probe key properties of the solar cell material or device structure. We discuss how it is possible to extract relevant device properties using noncontact scanning probe methods as well as how these properties guide materials development. Specifically, we discuss intensity-modulated scanning Kelvin probe microscopy (IM-SKPM), time-resolved electrostatic force microscopy (trEFM), frequency-modulated electrostatic force microscopy (FM-EFM), and cantilever ringdown imaging. We explain these developments in the context of classic atomic force microscopy (AFM) methods that exploit the physics of cantilever motion and photocarrier generation to provide robust, nanoscale measurements of materials physics that are correlated with device operation. We predict that the multidimensional data sets made possible by these types of methods will become increasingly important as advances in data science expand capabilities and opportunities for image correlation and discovery.

Seeing is believing: atomic force microscopy imaging for nanomaterial research

Atomic force microscopy can image nanomaterial properties such as the topography, elasticity, adhesion, friction, electrical properties, and magnetism.

Low-Temperature Solution-Processed Kesterite Solar Cell Based on in Situ Deposition of Ultrathin Absorber Layer

The production of high-performance, solution-processed kesterite Cu2ZnSn(Sx,Se1-x)4 (CZTSSe) solar cells typically relies on high-temperature crystallization processes in chalcogen-containing atmosphere and often on the use of environmentally harmful solvents, which could hinder the widespread adoption of this technology. We report a method for processing selenium free Cu2ZnSnS4 (CZTS) solar cells based on a short annealing step at temperatures as low as 350 °C using a molecular based precursor, fully avoiding highly toxic solvents and high-temperature sulfurization. We show that a simple device structure consisting of ITO/CZTS/CdS/Al and comprising an extremely thin absorber layer (∼110 nm) achieves a current density of 8.6 mA/cm(2). Over the course of 400 days under ambient conditions encapsulated devices retain close to 100% of their original efficiency. Using impedance spectroscopy and photoinduced charge carrier extraction by linearly increasing voltage (photo-CELIV), we demonstrate that reduced charge carrier mobility is one limiting parameter of low-temperature CZTS photovoltaics. These results may inform less energy demanding strategies for the production of CZTS optoelectronic layers compatible with large-scale processing techniques.