Controlled epitaxial growth of perovskite single‑crystal heterojunction arrays for self‑powered imaging

As the demand for high-resolution imaging, advanced displays, and integrated optical systems continues to accelerate, conventional polycrystalline perovskite devices face fundamental limitations in trap-state density, carrier mobility, and pixel-to-pixel uniformity. Now, researchers from Beihang University and the Chinese Academy of Sciences, led by Professor Wenqiang Wu, Professor Hui Lu, and Professor Yang Yu, together with collaborators from The Hong Kong Polytechnic University, have presented a breakthrough selective epitaxial growth strategy that bridges the gap between bulk single-crystal fabrication and large-scale array integration.

Why This Strategy Matters

Traditional perovskite heterojunctions have been successfully demonstrated on bulk crystals or in layered films, but fabricating large-scale single-crystal heterojunction arrays remains a critical unmet challenge. The gap between bulk fabrication and array integration requires miniaturizing and spatially confining the liquid-phase heterojunction formation process—while ensuring every single pixel maintains identical crystallographic orientation. Randomly oriented crystals lead to severe pixel-to-pixel variations that degrade imaging quality. The novel selective epitaxial growth approach overcomes this limitation by combining patterned polymer templates for spatial definition with single-crystal substrates for crystallographic guidance, enabling precise control over pixel size, arrangement angle, and crystal orientation across the entire array.

Innovative Design and Mechanism

The fabrication process begins with high-quality single-crystal perovskite films grown on ITO glass via spatial confinement, serving as epitaxial substrates. A patterned Parylene-C template is then deposited to define pixel positions and dimensions. The key innovation lies in the three-stage epitaxial growth dynamics: upon precursor introduction, halide ion exchange creates a graded transition layer (e.g., MAPb_3xBr_3(1-x) on MAPbBr₃ surface) that facilitates lattice-matched nucleation and homoepitaxial growth of target crystals along the [100] direction. This mechanism ensures coherent crystal locking across the entire array, with the substrate guiding orientation while the template defines geometry.

The method demonstrates versatile control over geometric and crystallographic properties. Pixel dimensions scale linearly with template opening sizes—30 μm openings generate ~45 μm pixels ensuring 100% coverage. Array rotation angles precisely replicate template orientations (0° to 45° demonstrated). Crystal habits transform from pyramidal to cubic via DPSI additive modulation, while substrate surface orientations ((001), (111), (110)) directly dictate pixel morphologies, all while preserving epitaxial relationships confirmed by SAED patterns.

Outstanding Performance

The resulting MAPbCl₃/MAPbBr₃ and MAPbBr₃/MAPbI₃ single-crystal heterojunction arrays exhibit exceptional quality: atomically smooth surfaces (roughness ~2.08 nm), sharp edges, and uniform orientation across 44×44 arrays within 1.5×1.5 mm² areas. EDS line scans reveal ~4 μm graded transition regions with gradual halide concentration changes, confirming the ion-exchange-mediated epitaxial mechanism.

Self-powered photodetector arrays (8×8 pixels) based on these heterojunctions deliver remarkable metrics: specific detectivity of 6.0×10¹¹ Jones, weak-light detection limit of 9 nW cm^-2, and peak responsivity of 2.4 mA W^-1. The type-II band alignment creates a built-in electric field of 0.99 eV at the interface, enabling efficient charge separation and zero-bias operation—confirmed by accelerated carrier extraction in TRPL and surface potential mapping via KPFM. The devices exhibit fast response (rise time: 22 ms; fall time: 173 ms) and excellent stability: only 10% photocurrent decay after 7,000 s continuous operation, and minimal degradation after four weeks ambient exposure without encapsulation.

Imaging Applications and Future Outlook

The 8×8 self-powered array demonstrates exceptional imaging capability. Dark current distribution is highly concentrated (32.96±14.17 pA average), and photocurrent remains uniform across all pixels under varying intensities. Negligible electrical crosstalk between adjacent pixels is confirmed—even on continuous substrates. When an "H"-shaped light pattern is projected, the array reconstructs clear images with continuous multi-level response from 0 to 1.291 mW cm^-2. Scaling to 16×16 arrays achieves spatial resolution of 8.0 lp mm^-1 (MTF = 0.2), accurately resolving complex "Christmas tree" patterns with sharp geometric outlines.

This work establishes a robust, simple, and versatile method for heterogeneous integration of perovskite single crystals toward high-performance optoelectronic applications. By simultaneously solving the challenges of spatial confinement and crystallographic control, the selective epitaxial growth strategy opens promising avenues for next-generation self-powered imaging systems, neuromorphic vision sensors, and integrated photonic circuits combining ultrahigh sensitivity, zero-bias operation, and long-term stability.

Stay tuned for more groundbreaking research from this collaborative team at Beihang University, the Chinese Academy of Sciences, and The Hong Kong Polytechnic University!