Solution-processable all-inorganic lead halide perovskites are under intensive attention due to their potential applications in low-cost high-performance optoelectronic devices such as photodetectors. However, solution processing usually generates structural and chemical defects which are detrimental to the photodetection performance of photodetectors. Here, a polymer additive of polyethylene glycol (PEG) was employed to passivate the localized defects in CsPbI 2 Br films through the Lewis acid−base interaction. The interfacial defects were passivated efficiently by introducing a trace amount of a PEG additive with a concentration of 0.4 mg mL −1 into the CsPbI 2 Br precursor solution, as suggested by the significantly reduced trap density of state, which was revealed using thermal admittance spectroscopy. Fourier transform infrared spectrum characterization showed that rather than Cs + or I − , a Lewis acid−base interaction was established between Pb 2+ and PEG to passivate the defects in the CsPbI 2 Br perovskite, which leads to large suppression of noise current. Both specific detectivity and linear dynamic range improved from 4.1 × 10 9 Jones and 73 dB to 2.2 × 10 11 Jones and 116 dB, respectively. Our work demonstrates the feasibility of employing an environmentally stable polymeric additive PEG to passivate defects for high photodetection performance in all-inorganic perovskite photodetectors.

In order to remove band-pass filters while maintaining narrow-band photodetection, three feasible approaches have been proposed. The first is the synthesis of narrow-band absorption of photoactive materials such as organic molecular materials with highly selective absorption characteristics. [7,8] Another one is the design of optic microcavity to intentionally enhance light absorption at a particular wavelength. [9,10] The third is the construction of thick films or bulk single crystals to tailor surface-charge recombination. Here, only long-wavelength light could generate photoexcited charges due to high light penetration length, whereas the shortwavelength photogenerated charges are lost as heat through charge recombination during the travel to the electrodes. [11,12] Lead halide perovskites are currently under intensive investigation owning to the exceptional optoelectronic properties that make them suitable for important applications in solar cells, light-emitting diodes, and photodetectors. [13][14][15][16][17][18][19][20][21] In particular, it has been shown that the photodetectors based on halide perovskite films with thicknesses larger than 10 µm can generate a significant narrow-band photoresponse with tunable spectralThe positive bias in theory narrows down the depletion region and thus results in significant charge injection, which should be detrimental to charge generation and collection performance for traditional photodetectors. Here, instead, it is found that the external quantum efficiency (EQE) is increased by more than 50 times when the photodetector is positively biased. A positive bias of +6 V drives ion migration of Br − and Cs + towards the anode and cathode, respectively, leading to self-doping within bulk single crystals to form an advantageous p-i-n junction for better charge collection in the devices. Meanwhile, the injected holes are allowed to tunnel through the cesium lead bromide/fullerene interface to reach the cathode which also significantly contributes to the enhancement of EQE in the forward-biased devices. The positively-biased narrow-band (full width at half maxima (FWHM) = 16 nm) photodetectors exhibit a specific detectivity of 6.5 × 10 10 Jones at 550 nm, along with the −3 dB cutoff frequency of 2776 Hz. By manipulating charge injection and ion migration using interfacial engineering, a class of non-traditional, positively-biased, and highly narrow-band photodetectors is demonstrated, which offers an alternative design strategy for imaging, biosensing, automatic control, and optical communication.

The performance of perovskite photodetectors was often tuned by changing the functional groups of polymeric additives, rather than their molecular weight to improve defect passivation. As a result of the steric effect of polymeric additives, we found that the molecular weight of poly(ethylene glycol) (PEG) additives played an important role in determining the perovskite crystal size, which increased from 470 to 550 nm with increasing molecular weight from 6k to 10k but declined back to 450 nm with further increasing molecular weight to 20k. The noise current, rather than the external quantum efficiency, was the dominant factor that can be tailored to improve the photodetection performance using PEG additives with different molecular weights. The photodetectors based on the 10k PEG additive exhibited a high specific detectivity of 1.9 × 10 11 Jones, a linear dynamic range of 119 dB, and a frequency response −3 dB of 11.57 kHz. This work demonstrates an alternative approach by tailoring the molecular weight of polymeric additives to optimize the morphology of perovskite films for improved performance in perovskite photodetectors and other perovskite optoelectronic devices.

The commercialization of perovskite solar cells is hindered by the poor thermal stability of organic–inorganic hybrid perovskite materials. Herein, we demonstrate that crystalline thermoplastic polymer additives, such as a mixture of polyethylene oxide (PEO, 100,000 MW) and polyethylene glycol (PEG, 12,000 MW), can improve the thermal stability of CH3NH3PbI3 (MAPbI3) perovskites and thereby enhance device stability. High-quality less-defect perovskite films were obtained by establishing a strong reaction between hydroxy groups in the PEO + PEG mixture and the uncoordinated Pb2+ in MAPbI3 perovskites, leading to a high power conversion efficiency of over 18% despite the presence of insulating thermoplastic polymers in the MAPbI3 film. More importantly, as compared with pristine MAPbI3 perovskite solar cells, the PEO + PEG-modified counterparts showed significantly improved stability under thermal treatment at 85 °C in ambient air with a relative humidity of 50–60%, remaining at nearly 71% of their initial efficiency values after 120 h. These demonstrations offer a feasible thermoplastic polymer additive engineering strategy to improve the thermal stability of perovskite solar cells.