Perovskite photodetectors are used in anti-interference optical communications

• Research highlights

  1. A frequency-selective photodetector composed of a perovskite film with a 2D–3D–2D structure and two photodiodes in a back-to-back structure is reported. ;

  
  

  2. Achieved frequency-selective optical response from 0.8 to 9.7MHz. And ultra-fast response of less than 20 nanoseconds in the 3 × 3 mm2 active area;

  
  

  3. Demonstrates frequency-selective optical response in a single back-to-back structure device without the need for external system integration, providing promising application possibilities for spatially coupled optical communications.

  
  

  1. Problems faced by optical communications

  Optical communications have received widespread attention for their applications in data transmission and acquisition, which typically require light emission, transmission channels, and light detectors. However, not all optical communication applications have closed optical transmission channels. Due to the limitations of factors such as air scattering and solar radiation interference, when the optical signal is transmitted to the target detector, the signal will be greatly weakened and accompanied by extensive wide-spectrum interference. Although wavelength or frequency selection techniques are commonly used to solve this problem, wavelength selection strategies cannot fully function in the surrounding broad spectrum interference, while frequency-selected optical responses seem to be more adaptable in extracting target signals. As space-coupled optical communication applications, ground-to-space and distributed optical fidelity (Li-Fi) communications have high requirements on cost and system complexity. Unfortunately, frequency selectivity always relies on peripheral circuits, which inevitably increases the size and complexity of the receiver, hinders the development of integration, and cannot meet the needs of ground-to-air and distributed Li-Fi communications. payload and cost requirements. Therefore, there is an urgent need to achieve signal selectivity without integrating the system.

  
  

  2. Introduction to results

  The team of Li Liang and Sun Haoxuan of Suzhou University reported a frequency-selective photodetector composed of a perovskite film with a 2D–3D–2D structure and two photodiodes in a back-to-back architecture. These photodiodes show different response speeds, with the net current at different frequencies depending on the sum of the two reverse current values. Due to the vertical "V" shape energy distribution of the perovskite, a frequency-selective optical response from 0.8 to 9.7MHz (center frequency is 3.0MHz) is achieved. In addition, at the bottom of the “V”-shaped energy band, the accumulated electrons significantly accelerate the recombination process, achieving an ultrafast response shorter than 20 nanoseconds in the 3 × 3 mm2 active area. Thanks to the frequency-selective light response and fast response, character and video data can be transmitted in real time even under strong interference from light-emitting diodes (LEDs) (source intensity 454 mW cm2).
  

  
  

  3. Results and Discussion

  Point 1: Design principles of photodetector equipment and characterization of perovskites

  As shown in Figure 1a, by designing two opposing photodiodes with different response speeds, the net current should be output only within the time interval Δt. It is worth noting that in spatial coupling scenarios, most interfering light sources are either constant or low-frequency signals driven by solar radiation or power lines. Under constant incident light, if the electric potentials of equal amplitude but opposite directions generated by two opposing photodiodes can be precisely controlled, the external circuit will display a zero current signal. When low-frequency (less than the center frequency) interference is incident, although the device current cannot reach absolute zero response, the current amplitude is greatly suppressed. At the center frequency, a slow-responding photodiode only outputs a lower amplitude signal, while a fast photodiode can still reach maximum output. Therefore, maximum net output can be achieved. The low-frequency perturbation produced by the interfering light is negligible compared to the effective signal transmission near the center frequency. At higher frequencies (greater than the center frequency), neither photodiode can respond in time, causing the net current amplitude to decrease until it reaches zero again. By adjusting the distribution and intensity of the field built into the photodiode, device properties such as center frequency can be controlled. In previous work, the authors found that severe phase separation exists in 2D perovskites, and that the phase composition distribution can be controlled by using different 2D salts, solvents, and preparation methods.

  
  

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• Figure 1 Device structure and characterization of perovskite film

  
  

  Using a simple one-step solution spin coating supplemented by hot casting, highly oriented (tBBA)2MA2Pb3I10 (i.e. N=3, where tBBA+ and MA+ are 4-tert-butylphenylmethylammonium and methylammonium ions respectively) can be prepared 2D perovskite. This simple yet effective approach to modulating the distribution of perovskite components is ideally suited to control the vertical built-in field of photodiodes. As shown in Figure 1b, the complete device architecture consists of an approximately 700 nm thick heterogeneous photoactive N3 perovskite layer containing poly(3,4-ethylenedioxide) sandwiched between two hole transport layers (HTLs). Thiophene): poly(styrenesulfonic acid) (PEDOT:PSS) and poly(3-hexylthiophene-2,5-diyl) (P3HT). Notably, the thickness of perovskite, PEDOT:PSS, and P3HT is critical to achieve frequency-selective photoresponse.

  
  

  Figure 1c shows the X-ray diffraction (XRD) pattern of the N3 perovskite film. The strong diffraction peaks at 14.2° and 28.5° correspond to the (111) and (202) crystal planes, respectively, and their presence indicates high crystallinity. The peak intensity ratio (I(202)/I(111)) is 0.95, indicating that the 2D perovskite grows perpendicular to the substrate. The absorption spectra in Figure 1d show the transition from perovskite/air (front) to perovskite/glass (back) for 2D perovskites (n = 2, 3, 4) and 3D perovskites (n = ∞) coexistence, the perovskite phase distribution needs further analysis. As shown in Figure 1e, only the emission peak of 3D perovskite appears in the photoluminescence (PL) on the front side, while multiple emission peaks of 2D perovskite appear on the back side; these results indicate that the 2D perovskite is mainly on the back side, while 3D perovskite on the front. However, the front part of the film is not entirely composed of 3D perovskite. The lack of PL signal for the thin film 2D perovskite component may be due to effective carrier separation on the front side. Furthermore, as the incident angle increases, the intensity of the 2D perovskite peak remains almost unchanged, while the intensity of the 3D perovskite peak increases significantly. This observation means that the middle region is mainly composed of 3D perovskite, while the surface region is rich in 2D perovskite. Through GIWAXS, GIXRD and PL spectra, it was confirmed that the perovskite film exhibits a vertical 2D–3D–2D composition distribution.

  
  

  Point 2: Optimization of frequency-selective light response

  The key to achieving frequency-selective photoresponse is to modulate the electric field by controlling the thickness and phase composition of each layer. The layer thickness can be finely tuned by varying the precursor concentration. Using a 1:1 (volume ratio) diluted PEDOT:PSS stock solution, a 0.8 M perovskite precursor solution and 20 mg mL-1 of P3HT, a minimum EQE of less than 1.5% can be achieved in the visible range, corresponding to AM The integrated current under 1.5 G irradiation is as low as 0.0925 mA cm-2 (see Figure 2a). A laser with a wavelength of 450 nm is used as the communication source. At a low frequency of 1 Hz, the device produces no current (see Figure 2b). The type of 2D ammonium salt significantly affects the center frequency. Choose ethylammonium iodide (EAI), butylammonium iodide (BAI), benzyl ammonium iodide (PMAI), phenylethyl ammonium iodide (PEAI), octyl ammonium iodide (OAI), Decamonium iodide (DAI) and 4-tert-butylbenzyl ammonium iodide (tBBAI) were used as appropriate 2D ammonium salts to modulate the center frequency. Among them, the perovskite films of BAI, PEAI and tBBAI show good crystallinity and crystal orientation. 2D ammonium salt-based devices exhibit a distinct -3 dB cutoff frequency. Devices based on MAI, PMAI and BAI lose frequency selective response characteristics due to significant leakage current in the low frequency region. In contrast, devices based on EAI, PEAI and tBBAI exhibit more obvious frequency selective characteristics. The center frequency of EAI-based devices is only 0.2 MHz. Additionally, the response bandwidth of PEAI-based devices is lower than 1 MHz due to rise/fall times of 0.3/0.5 microseconds, which is much slower than tBBAI-based devices. Therefore, the range of the center frequency can be adjusted by modifying the 2D ammonium salt. Due to the fastest response time, tBBAI was selected to manufacture the final frequency-selective device using 2D ammonium salts for optical communications.

  
  

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• Figure 2 Anti-interference and frequency selective optical response detection

  
  

  At a high frequency of 6 MHz, the N3 perovskite device produced a photocurrent of 25.6 μA (Figure 2c), while at low frequency the photocurrent was close to 0 (Figure 2b). The enlarged pattern in Figure 2d shows fast edges at 19.7/18.3 ns, indicating rapid signal transmission. Figure 2e shows the statistical distribution of rise and fall times for 36 devices. All devices respond quickly within 100 ns, with most response times around 20 ns. As shown in Figure 2f, the -3 dB cutoff frequency is calculated in the range of 0.8–9.7 MHz, which meets the frequency range requirements of free-space optical communication.

  
  

  Point 3: Charge Carrier Dynamics and Energy Bands

  In order to explore the influence of the back-to-back device structure on the vertical electric field and carrier distribution behavior, cross-sectional Kelvin probe force microscopy (KPFM) was used to map and observe the changes in the contact potential difference (CPD) measured under dark and light conditions. By calculating the first and second derivatives of CPD, the vertical electric field and charge density distribution profiles were determined, as shown in Figure 3a–h. When the device is exposed to light, the CPD of the entire perovskite layer decreases, indicating that the photogenerated electrons are captured by the perovskite layer. This result can be attributed to the intrinsic 2D-3D-2D phase structure of perovskite, which generates a “V”-shaped built-in electric field. Electron blocking materials (P3HT and PEDOT:PSS) on both sides of the perovskite layer further reduce the CPD, resulting in enhanced capture of photoelectrons within the perovskite film. The accumulation of electrons within the perovskite layer provides the basis for the rapid reduction of the photoresponsive edge. Notably, the CPD change at the P3HT–perovskite interface is particularly pronounced; this result suggests that this region serves as the main electron storage site following the competition between the two reverse potentials. It is worth noting that the PEDOT:PSS side exhibits a larger potential difference than P3HT under both dark and light conditions. When the device is exposed to light, the vacuum level change (ΔEvac) indicates that the junction electric field on the P3HT side almost disappears; therefore, the PEDOT:PSS/perovskite junction electric field becomes dominant (see Supplementary Figure 19). When the device is in the dark state, two prominent peaks appear in the spectra of the two HTL–perovskite interfaces and no flat high electric field is observed; therefore, the perovskite is completely depleted.

  
  

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• Figure 4 Demonstration of free space optical communication

  
  

  Because video data has higher information density than character data, video data transmission requires a higher frequency than character data transmission. Devices based on 2D-3D-2D have fast enough response speed and fully meet the requirements for high-density information transmission (see Figure 4c, d). Amid interference caused by continuous illumination, the device accurately transmits video signals even with low-frequency LED light source intensities of 55.1, 170 and 454 mW cm-2. When the light source intensity finally reaches 910 mW cm-2, the video signal begins to be distorted; this result indicates the device's strong anti-interference signal transmission capability (Supplementary Video 5). It is worth noting that subtle video jitter was observed in the presence of flicker interference. As shown in Supplementary Figure 31, the square wave interference undergoes wavelet transformation, showing many high-frequency components in the MHz range. These high-frequency signals are then captured by the detector, causing interference during the video transmission, as evidenced by the fact that when pure low-frequency sinusoidal light is used as the interfering signal, the signal is not interfered with (Supplementary Video 6). Due to the insufficient response speed of traditional structural devices, a commercial high-speed silicon photodetector (model: S6968, Hamamatsu) was used as a reference. While video transmission is possible, even slight external interference can cause momentary signal loss,

  
  

  4. Summary

  
  

  In this work, the researchers fabricated a device in a back-to-back structure (Ag/P3HT/perovskite/PEDOT:PSS/ITO) to cancel the currents of two photodiodes. The resulting single device achieved frequency Selective light response. Through a simple one-step spin-coating process, 2D perovskite (tBBA) 2MA2Pb3I10 shows a vertical 2D–3D–2D phase composition distribution. In addition to the thickness adjustment of the dual HTLs, the front and rear field strengths are balanced to eliminate the device's photoresponse under continuous illumination. The device exhibits a fast response of 19.7/18.3 ns over a response range of 0.8–9.7 MHz. Despite the interference caused by the LED light source, the video data can still be accurately transmitted, proving the device's good anti-interference transmission capabilities. By replacing integrated systems with a single device, complexity and cost can be significantly reduced, allowing greater flexibility and diversity in designing free-space optical communications devices.

  
  

  5. References

  Min, L., Sun, H., Guo, L. et al. Frequency-selective perovskite photodetector for anti-interference optical communications. Nat Commun 15, 2066 (2024).

  
  

  https://doi.org/10.1038/s41467-024-46468-5