Spatially resolved fourier transform impedance spectroscopy and applications to optoelectronics
Inventors
KELLEY, MATHEW L. • Greytak, Andrew B. • Chandrashekhar, M V S
Assignees
Publication Number
US-12152988-B2
Publication Date
2024-11-26
Expiration Date
2042-03-07
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Abstract
Spatially resolved Fourier Transform Impedance Spectroscopy (FTIS) is disclosed to spatially map and quickly build the frequency response of optoelectronic devices using optical probes. The transfer function of a linear system is the Fourier transform of its impulse response, which may be obtained from transient photocurrent measurements of devices such as photodetectors and solar cells. We apply FTIS to a PbS colloidal quantum dot (QD)/SiC heterojunction photodiode and corroborate results using intensity-modulated photocurrent spectroscopy. The cutoff frequencies of the QD/SiC devices were as high as ˜10 kHz, demonstrating their utility in advanced flexible and thin film electronics. The practical frequencies for FTIS lie in the mHz-kHz range, ideal for composite or novel materials such as QD films that are dominated by interfacial trap states.
Core Innovation
The invention discloses Spatially Resolved Fourier Transform Impedance Spectroscopy (FTIS), a methodology for rapidly characterizing the frequency response of optoelectronic devices and spatially mapping their frequency-dependent behaviors using optical probes. The technique acquires the transfer function of a linear system by performing a Fourier transform on the impulse response, which is obtained from transient photocurrent measurements of devices such as photodetectors and solar cells. This provides a fast alternative to commonly used time domain and frequency domain methods for evaluating the properties of composite interfaces found in optoelectronic devices.
The specific problem addressed is that existing time domain and frequency domain diagnostic techniques, such as impedance spectroscopy, intensity-modulated photocurrent spectroscopy, and deep-level transient spectroscopy, either require long measurement times, depend on temperature variations, or need small signal levels and are therefore not easily compatible with rapid, spatially correlated assessments. These limitations become especially significant for composite materials and novel device architectures, such as those employing quantum dot films, which are dominated by interfacial trap states and slow bandwidths.
FTIS overcomes these challenges by enabling the rapid building and spatial mapping of frequency responses from transient photocurrent data, suitable for practical frequency ranges in the mHz-kHz and MHz-KHz domains. The invention's method uses optical probes, such as laser light, to irradiate selected locations, records the transient photocurrent step response, computes the impulse response, and applies a Fourier transform to extract local frequency-dependent information efficiently. This enhances the throughput of optoelectronic device characterization and allows for high-resolution spatial mapping of interfacial properties and characteristic lengths, significantly advancing the diagnosis and optimization of optoelectronic devices with composite or novel materials.
Claims Coverage
The independent claims define three primary inventive features: (1) a methodology for determining frequency response using transient photocurrent and Fourier transform calculations, (2) an optical probe method for spatially mapping frequency response relative to a heterojunction, and (3) a diagnostic method for rapid spatial mapping of composite interfaces based on FTIS scanning.
Methodology for determining frequency response using transient photocurrent and Fourier transform calculations
- Turning on and off an electrical or optical source as an input to an optoelectronic device with a composite interface. - Measuring the resulting transient photocurrent from the device to generate measurement data. - Performing Fourier Transform calculations on the transient photocurrent measurement data to determine the frequency response of the device.
Optical probe method for spatially mapping frequency response at selected locations relative to a heterojunction
- Irradiating the optoelectronic device at a selected location with a laser light as a unit step input (u(t)). - Measuring the step response (s(t)) transient photocurrent at that location. - Computing the derivative of the step response (s(t)) to determine the impulse response h(t). - Computing the numerical Fourier transform of h(t) to determine the frequency response H(ω) at the selected location. - Repeating the above steps for each selected location to spatially map the frequency response of the device.
Diagnostic method for rapid spatial mapping of composite interfaces by FTIS scanning
- Applying a laser light to successive locations on the optoelectronic device. - Measuring the output for each successive location. - Performing Fourier Transform calculations on the outputs to accomplish FTIS scanning and provide the frequency response of the device.
These inventive features collectively provide a rapid and spatially resolved method for frequency response analysis of optoelectronic devices using optical transient measurements and Fourier transform processing, enabling efficient mapping and diagnostic capabilities for composite device interfaces.
Stated Advantages
FTIS allows for rapid (<3 seconds) acquisition of a device's frequency response compared to IMPS and related techniques, which can take several minutes or hours.
The method is compatible with relatively inexpensive hardware, with equipment costs significantly lower than conventional impedance analyzers or oscilloscopes.
FTIS can be conducted under ambient temperatures and is compatible with large signals, unlike methods requiring small-signal linearization or temperature-dependent measurements.
FTIS enables high-throughput, spatially correlated mapping of frequency response and characteristic interface parameters, ideal for industrial and research applications seeking rapid and comprehensive assessment.
Documented Applications
Rapid spatial mapping and diagnosis of frequency response in optoelectronic devices such as photodetectors, solar cells, and heterojunction devices.
Characterization of composite interfaces in devices like QD/SiC rectifying junction devices, QD/EG/SiC devices, as well as devices incorporating stripe contacts or printable quantum dot layers.
High-throughput characterization methods in photovoltaic, photodetector, display, and lighting industries, including use in industrial instrument development and research.
Extraction of interfacial capacitances and series/parallel resistances for advanced flexible and thin film electronics utilizing quantum dot and composite materials.
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