Filterless narrowband visible photodetectors

Journal name:
Nature Photonics
Volume:
9,
Pages:
687–694
Year published:
DOI:
doi:10.1038/nphoton.2015.175
Received
Accepted
Published online

Abstract

Wavelength-selective light detection is crucial for many applications, including imaging and machine vision. Narrowband spectral responses are required for colour discrimination, and current systems use broadband photodiodes combined with optical filters. This approach increases the architectural complexity and limits the quality of colour sensing. Here we report a method for tuning the spectral response to give filterless, narrowband red, green and blue photodiodes. The devices have simple planar junction architectures with the photoactive layer being a solution-processed mixture of either an organohalide perovskite or lead halide semiconductor and an organic (macro)molecule. The organic (macro)molecules modify the optical and electrical properties of the photodiode and facilitate charge collection narrowing of the device's external quantum efficiency. These red, green and blue photodiodes all possess full-width at half-maxima of <100 nm and performance metrics suitable for many imaging applications.

At a glance

Figures

  1. Working principles of CCN photodiodes.
    Figure 1: Working principles of CCN photodiodes.

    The absorption coefficient profile of an active layer of defined thickness with separate high and low absorption (α) regions (solid blue line). In the high-α region, the Beer–Lambert law dominates the light absorption, and carriers are mainly generated near the transparent electrode (we term this ‘surface generation’). This increases the recombination and reduces the charge collection efficiency ηcoll due to strongly imbalanced electron/hole transport. In the low-α region, cavity effects influence the light absorption significantly for optically thick junctions, and carriers are generated in the volume of the active layer (‘volume generation’). The EQE can be shaped by manipulating ηcoll, that is, the IQE10. The FWHM can be controlled by tuning the two onsets of α (reported as FWHM ≈ λonset1 − λonset2). Inset: the simple homojunction photodiode structure, comprising a photoactive layer and interlayers between the cathode and anode.

  2. Optical gap tunability and addition of an organic molecular component to organohalide perovskite semiconductors.
    Figure 2: Optical gap tunability and addition of an organic molecular component to organohalide perovskite semiconductors.

    a, The optical gap of the organohalide perovskite can be tuned by changing the ratio of PbI2 and PbBr2 during film preparation. This approach can be used to adjust absorption onset 1 (λonset1) of the narrowband CCN photodiodes of Fig. 1. b, Addition of Rhodamine B allows absorption onset 2 (λonset2) to be adjusted. c, Comparison of XRD spectra for organohalide perovskite (CH3NH3PbI2Br) with or without Rhodamine B. The films containing Rhodamine B possess attenuated diffraction intensity (∼20 times lower) and broader peaks. This indicates that, by adding Rhodamine B, the growth of larger crystals is hindered and the crystal size is much smaller compared with neat organohalide perovskite films.

  3. Working mechanism and performance of the red narrowband CCN photodiodes.
    Figure 3: Working mechanism and performance of the red narrowband CCN photodiodes.

    a, Optical field distributions in the red narrowband photodiodes (film thicknesses of 500 nm CH3NH3PbI2Br and 60 nm C60) for four wavelengths: for λ < 600 nm (the Beer–Lambert region) photons cannot penetrate the whole of the film and carriers are surface generated, and for 700 nm > λ > 600 nm (the cavity region) the photocarriers are volume-generated. b, EQEs of narrowband red CCN photodiodes with various junction thicknesses at a reverse bias of −0.5 V. The thinnest junction delivers an almost broadband response because surface- and volume-generated carriers are collected. By increasing the junction thickness, surface generation and volume generation are distinguished and the EQE at shorter wavelengths (in the Beer–Lambert region) is suppressed. c, Current-density–voltage (J–V) measurements of the red narrowband CCN photodiodes under different illumination colours with similar irradiances (∼50 mW cm−2). All J–Vs show fill factors of ∼50%, indicating similar charge transport efficiencies for the longer-lived carriers. d, IPC measurements of red narrowband photodiodes (zero bias) under various irradiance intensities and three different laser wavelengths. Deviation of the photocurrent from linearity as a function of input irradiance indicates the onset of significant bimolecular recombination, which occurs at more than an order of magnitude earlier for blue and green wavelengths (surface-generated carriers).

  4. Device performance and bandwidth tunability of red narrowband photodiodes.
    Figure 4: Device performance and bandwidth tunability of red narrowband photodiodes.

    a, EQEs at −0.5 V of the red narrowband photodiodes fabricated with different ratios of PbI2 and PbBr2. The long-wavelength edge of the photoresponse window can be controlled by the semiconductor optical gap (λonset1). b, Light and dark JV curves of the devices from a. The dark current at −0.5 V is <5 × 10−8 A cm–2. Each circle is an average of the measured dark current over 3 seconds with the error bars being the standard deviation. c, Measured specific detectivity D* of an optimized red narrowband photodiode at −0.5 V (1.9 × 1011 Jones with FWHM <80 nm).

  5. Device performance summary of red, green and blue narrowband CCN photodiodes.
    Figure 5: Device performance summary of red, green and blue narrowband CCN photodiodes.

    a, EQE spectra at −0.5 V of optimized narrowband photodiodes and related junction absorption coefficients clearly showing how the CCN concept can be realized across the visible spectrum. b, Linear dynamic range (LDR) of the optimized narrowband photodiodes measured at −0.5 V. The red photodetector shows a linear response of >6 orders of magnitude versus irradiance intensity, whereas the green and blue photodetectors exhibit ∼5 orders of magnitude of linear response. c, Frequency response (speed) of optimized narrowband RGB photodiodes at −0.5 V. The green photodetector shows f−3dB ≈ 144 kHz and the red and blue photodetectors f−3dB values of ∼297 kHz and 345 kHz, respectively—all more than sufficient for most imaging applications. NB, narrowband.

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Author information

Affiliations

  1. Centre for Organic Photonics & Electronics, School of Chemistry and Molecular Biosciences, and School of Mathematics and Physics, The University of Queensland, Brisbane, Queensland, Australia 4072

    • Qianqian Lin,
    • Ardalan Armin,
    • Paul L. Burn &
    • Paul Meredith

Contributions

Q.L. characterized the perovskite films and fabricated the devices. Q.L. and A.A. tested the devices and all authors interpreted the data. P.L.B. and P.M. supervised the project. All authors contributed to preparation of the manuscript. All authors have given approval to the final version of the manuscript.

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The authors declare no competing financial interests.

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