Anomalous 3D nanoscale photoconduction in hybrid perovskite semiconductors revealed by tomographic atomic force microscopy

Hybrid perovskite semiconductors are promising materials for various optoelectronic applications, such as solar cells and photodetectors. However, the role of grain boundaries (GBs) in their photogenerated carrier transport remains unclear. In conventional inorganic semiconductors, GBs are often considered detrimental, impeding carrier flow. However, the situation is far more nuanced in hybrid perovskites. The complexity arises from the inherent challenges in probing the properties of GBs. Experimental investigations typically focus on the top surface, failing to capture depth-dependent inhomogeneities and potentially affected by topographic artifacts. This study addresses these limitations by introducing a novel approach using tomographic atomic force microscopy (T-AFM). This technique allows for fully three-dimensional (3D) mapping of photogenerated carrier transport at the nanoscale within hybrid perovskite thin films. The ability to visualize the 3D distribution of photocurrents and the properties of GBs provides an unprecedented level of detail, allowing researchers to move beyond surface-level observations and uncover previously hidden conduction pathways that are crucial to optimizing the performance of these promising materials. Understanding the complex interplay between GBs and charge carrier transport is essential for enhancing the efficiency and stability of hybrid perovskite-based devices. This knowledge is crucial for designing future high-performance optoelectronic devices.

Previous studies on the effect of grain boundaries in hybrid perovskite solar cells have yielded conflicting results, reporting enhanced, reduced, or equal photocurrents at grain boundaries compared to grains. These discrepancies stem from limitations in experimental techniques that primarily focused on surface-level characterization. Surface-sensitive techniques, such as Kelvin probe force microscopy (KPFM), can be influenced by surface roughness and artifacts. The inherent surface sensitivity of many prior studies led to a lack of understanding of the depth-dependent behavior of charge transport. This paper directly addresses these limitations by presenting a 3D technique capable of characterizing the interior of the perovskite film, allowing a more thorough understanding of the relationship between grain boundaries and charge transport. Previous studies have also highlighted the impact of grain orientation and facets on photocurrent measurements, adding to the complexity of understanding the behavior of grain boundaries in these materials.

The researchers utilized a unique approach combining tomographic atomic force microscopy (T-AFM) and photoconductive atomic force microscopy (pc-AFM). The T-AFM technique, which involves repeated scanning with a conductive diamond probe, allows for the nanoscale removal of material, effectively polishing the surface to reveal the interior structures of the hybrid perovskite thin film. Pc-AFM measurements were then performed on the polished surface at various depths, generating 3D photocurrent maps. The methylammonium lead iodide (MAPbI3) thin films were prepared by spin-coating a perovskite precursor solution onto a fluorine-doped tin oxide (FTO) substrate. The spin-coating process was followed by annealing in a dimethylsulfoxide (DMSO) atmosphere. The use of a MACI additive during the preparation was intended to increase the grain size of the MAPbI3 thin films. The resulting films were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The T-AFM and pc-AFM measurements were conducted under ambient conditions using a MFP-3D IO-AFM, with a 650 nm diode laser for oblique top illumination or a broadband LED for back-illumination. The photocurrent maps were acquired at different depths to generate a 3D representation of the carrier transport. The analysis involved mapping the photocurrent-voltage (Iph-V) characteristics at different depths using pc-AFM scans with varying potential differences between the tip and the FTO back electrode. This enabled a pixel-by-pixel estimation of the product of carrier mobility (µ) and photogenerated carrier density (Nph), which is crucial for evaluating device performance. Control measurements were conducted using KPFM on both the as-grown and T-AFM polished surfaces to illustrate the influence of surface topography on photocurrent and surface potential measurements. The high spatial resolution of T-AFM and pc-AFM ensured reliable measurements of photocurrents in the grains and GBs across a range of applied biases, providing insights into the depth-dependent behavior of carrier transport. The use of a low-load force minimized any artifacts from tip-sample interaction, resulting in accurate mapping of photocurrent and potential distributions.

The 3D photocurrent mapping revealed a network of highly interconnected conducting channels at the grain boundaries (GBs). The analysis identified two distinct types of GBs: Type I GBs, which act as preferred channels for conduction, exhibiting enhanced carrier mobility; and Type II GBs, which show no significant enhancement in photocurrent compared to the adjacent grains. Type I GBs facilitate carrier transport through the film, potentially preempting lateral inter-grain carrier diffusion and enhancing vertical charge transport. Type II GBs, on the other hand, do not exhibit enhanced carrier transport properties and appear nearly indistinguishable from the surrounding grains in photocurrent maps. The quantitative analysis of the Iph-V characteristics allowed for a pixel-by-pixel estimation of the µNph product. The analysis revealed a range of µNph values across grains and GBs. Importantly, the enhanced µNph product was observed in Type I GBs, indicating higher carrier mobility at these boundaries. The tomographic approach was crucial in revealing Type I GBs located deep within the film, features invisible to conventional surface-based characterization. The researchers estimated that around 5% of GBs in the investigated films were of the Type II kind. The observed distinction between Type I and Type II GBs is not correlated with the load or direction of the AFM scan, further emphasizing the inherent differences in their conductive properties. Line scans across both Type I and Type II GBs confirmed their unique behavior across the entire probed voltage range (0.6–1.1 V). The 3D analysis of the T-AFM data allowed for clear identification and visualization of these two GB types within the 3D structure of the film. The study also showed that the variations in photocurrent and effective carrier mobility are consistent with previous reports on MAPbI3 thin films, but the 3D approach allowed for a far more detailed and accurate analysis, eliminating the ambiguity introduced by surface artifacts. Overall, the study confirms the benefits of 3D tomographic studies for understanding the complex nature of carrier transport in hybrid perovskite thin films.

The findings of this study significantly advance our understanding of carrier transport in hybrid perovskite semiconductors. The 3D mapping of photocurrent using T-AFM provides direct evidence of GBs acting as interconnected pathways for charge carriers, contrasting with the often-assumed detrimental role of GBs in conventional semiconductors. The identification of two distinct GB types highlights the complexity of the GB landscape in these materials and the need for a three-dimensional approach to understand their role in optoelectronic device performance. The discovery of Type I GBs acting as highly conductive pathways could lead to the development of strategies for enhancing charge carrier transport in hybrid perovskite-based devices. Conversely, the presence of Type II GBs provides valuable insights for designing devices where lateral or surface transport is preferred. The relative proportion of Type I and II GBs might be crucial depending on the device architecture (lateral vs. vertical). The observed variations in carrier mobility suggest the potential for chemical tailoring of GBs to further enhance device performance. Future research directions might involve targeted manipulation of GB chemistry to control the density and type of GBs present, leading to the optimization of specific properties and improved device efficiency. Further studies using complementary techniques such as atomic-resolution electron microscopy and advanced spectroscopic methods could provide additional insights into the nature and origin of these distinct GB types.

This study demonstrates the power of tomographic atomic force microscopy in unraveling the 3D nanoscale photoconduction mechanisms in hybrid perovskite thin films. The identification of two distinct grain boundary types with contrasting carrier transport properties has significant implications for optimizing the design and performance of perovskite-based optoelectronic devices. Future research will focus on leveraging these findings to engineer the grain boundary network for improved device performance. The development of techniques to control and tailor the GB properties will be essential for further progress in the field.

The study primarily focused on MAPbI3 thin films prepared using a specific method. The generalizability of these findings to other hybrid perovskite compositions and fabrication methods needs further investigation. While the T-AFM technique minimizes surface artifacts, it does introduce some degree of surface modification. Further studies using non-destructive techniques are valuable to confirm these results. The study did not explore the long-term stability of these distinct GB types under operating conditions. The study did not investigate the impact of different film thickness or grain size on the observed GB types and their relative prevalence.