Long photoexcited carrier lifetime in a stable and earth-abundant zinc polyphosphide

Technical Deep Dive: Long Photoexcited Carrier Lifetime in a Stable and Earth-Abundant Zinc Polyphosphide

Overview

This work reports the discovery of exceptionally long photoexcited carrier lifetime in monoclinic zinc polyphosphide (ZnP2), a material characterized by an unconventional polyphosphide bonding structure. ZnP2 exhibits bright band-to-band photoluminescence and carrier lifetimes ranging from 500 ns up to 1 μs, bridging the lifetime gap between direct-gap inorganic semiconductors and halide perovskites. This makes ZnP2 a promising candidate for optoelectronic applications such as photovoltaics, light-emitting diodes, and photodetectors.

Problem & Context

• Achieving long carrier lifetime is critical for high-performance optoelectronic devices like solar cells, LEDs, and photodetectors
• Conventional inorganic semiconductors typically suffer from short carrier lifetimes (≤200 ns) unless very carefully prepared
• Halide perovskites have shown remarkably long carrier lifetimes (up to 10 μs), enabling high efficiency, but face stability challenges
• An open question is whether inorganic semiconductors can achieve the favorable defect chemistry and long carrier lifetime of halide perovskites

Methodology

• Computational high-throughput screening of over 1,400 phosphide compounds to identify promising candidates based on intrinsic defect properties and carrier lifetime
• ZnP2 was identified as a leading candidate with a predicted long carrier lifetime due to its unique polyphosphide bonding structure
• Experimental synthesis and characterization of ZnP2 crystals using various growth methods
• Structural, chemical, and optical characterization including X-ray diffraction, photoluminescence, and carrier lifetime measurements

Data & Experimental Setup

• ZnP2 crystals synthesized using gas phase transport reactions, some with addition of K or Sn
• Structural characterization by powder X-ray diffraction, SEM-EDS, and Raman spectroscopy
• Stability testing in air, water, and acid
• Optical characterization by photoluminescence and various carrier lifetime measurement techniques:
  • Time-resolved photoluminescence (TRPL)
  • Time-resolved microwave conductivity (TRMC)
  • Phase fluorometry
  • DC photoconductive current decay

Results

• ZnP2 exhibits a direct bandgap of ~1.49 eV and bright band-to-band photoluminescence
• Carrier lifetimes measured by multiple techniques range from 500 ns to nearly 1 μs, significantly exceeding those of other inorganic semiconductors
• ZnP2 shows excellent environmental stability, with no degradation observed after air exposure, water immersion, or acid treatment
• Computational analysis indicates the long carrier lifetime is enabled by the polyphosphide bonding structure, which suppresses the formation of deep intrinsic defects

Interpretation

• The exceptional optoelectronic properties and stability of ZnP2, combined with its earth-abundant constituents, make it a promising candidate material for a variety of optoelectronic applications
• The work demonstrates that exploring unconventional inorganic materials with unusual bonding, such as polypnictides, can lead to the discovery of novel semiconductors with desirable properties

Limitations & Uncertainties

• The ZnP2 crystals were not single crystals, and additional scattering mechanisms may limit the measured carrier mobility compared to theoretical predictions
• The origin of the reduced carrier lifetime observed at longer excitation wavelengths is not fully understood and requires further investigation of the surface chemistry

What Comes Next

• Optimization of ZnP2 crystal growth to obtain high-quality single crystals
• Further studies on the surface passivation and defect engineering of ZnP2 to maximize carrier lifetime and mobility
• Investigation of the potential of ZnP2 in various optoelectronic device applications such as photovoltaics, light-emitting diodes, and photodetectors

Sources:

• [Source 1: Long photoexcited carrier lifetime in a stable and earth-abundant zinc polyphosphide]