Efficiency Loss Mechanisms And Improvement Methods Of PbS-Based Infrared Up-Conversion Photodetectors

Infrared imaging technology has numerous applications in national economy and defense,such as night vision,range determination,biological imaging,wafer detection,national military security,and space science research.However,manufacturing infrared imagers based on traditional infrared detectors and read-out circuits can be difficult and expensive.The solution to this problem is the infrared up-conversion photodetector,which consists of an infrared light detector（PD）unit and a visible light emitting diode（LED）unit connected in series.This detector can directly convert infrared light signals into visible images that can be captured by the human eye and commercial digital equipment,thereby reducing the cost and complexity of manufacturing infrared imagers.The photon-photon conversion efficiency(η_p-p)of infrared up-conversion device is currently only about 6%,even when assuming the high energy conversion efficiency of PD and LED operating separately.Furthermore,the up-conversion device has a high working voltage.In order to switch on under infrared light,it often requires more than 2.5 V applied bias,and only when a high applied voltage is used does it produce a highη_p-p.This reduces the portability of the devices,since large room must be reserved for the power supply with large voltage output.The aforementioned issues restrict the advancement and applications of the infrared up-conversion device.Therefore,mechanisms of the efficiency loss and possible approaches for improving the performances of the infrared up-conversion detector are examined in this study.The key research contents are as follow:（1）This study first investigates the effect of PbS-EDT on the performance of the up-conversion device.The findings show that the presence of this layer can significantly reduce the dark current,while increase the hole injection barrier at the PD/QLED interface.To address the issue of high barrier for hole injection,the effects of various organic hole transport layers（TFB,Poly-TPD and PTAA）on device performance were investigated.The results reveal that TFB-based devices have a higher turn-on voltage,no matter whether there is infrared illumination or not.With PTAA replacing the TFB as interconnection layer,the photon-photon conversion efficiency at low voltage is significantly increased,and the turn-on voltage is significantly decreased.However,the turn-on voltage in dark also decreased a lot due to poor electron blocking,which reduces the contrast between dark and infrared illumination of an infrared up-conversion detector and thus limits its application prospects.Therefore,mixture of TFB and PTAA with blend ratio of 1:1 was used to achieve a balance between the hole injection and electron blocking.To get further understanding on the loss mechanism of infrared up-conversion devices,an equivalent circuit model is introduced to explain the voltage distributions between the PD unit,interconnection,and QLED unit of the up-conversion device.The efficiency loss mechanism of the up-convert device is then explored and clarified by evaluating experimental data of the up-conversion device with varied interconnection layers.Under low working bias（<6 V）,the main limits to the performance of the infrared up-conversion device include the too small effective bias allocated onto the PD unit and the too high barrier of injecting holes into the QLED unit at the interconnection,which reduce the quantum efficiencies of the PD unit and QLED unit,respectively.Under high working bias（～10 V）,the main limits to the performance of the infrared up-conversion device include the efficiency roll-off of the QLED unit and the re-absorption of the emitted visible light in PbS layers.（2）Based on the understanding of the loss mechanism of the infrared up-conversion device,we introduce an electrical dipole layer at the PD/QLED interface to improve the performances of the infrared up-conversion device,and relevant working mechanisms are investigated.Interfacial dipole layers based on Azetidinium Iodine and Azetidinium Bromium were prepared,respectively.The results show that both materials can significantly reduce the turn-on voltages of devices under infrared irradiation.With the interfacial dipole layer,the morphology of PbS-EDT is smoothed,and dark current of the device is reduced.Further characterizations using Kelvin Probe Force Microscopy（KPFM）and Capacitor-Voltage（C-V）measurements demonstrated that the successful introduction of interface dipoles into the PD/QLED interface increases the work function of the PbS-EDT films,and thus results in better energy level alignment between the PbS-EDT and TFB layers.Ultimately,an infrared up-conversion photodetector with zero barriers of hole injection is fabricated and achieves a 1.2V low turn-on voltage,leading to significant enhancement of photon conversion efficiency at low working voltage while maintaining efficiency at high voltage.