Research On The Influence Of Bulk Trap Depth On The Performance Of Organic Photodetectors

In the past three decades,organic photodetectors(OPDs)based on organic photodiode architectures have been greatly developed.Replacing inorganic photodiode arrays with OPDs is a promising strategy toward more cost-effective,lighter and more sophisticated detectors.The introduction and application of the concept of bulk heterojunction guarantees the excellent photoelectric conversion efficiency of the devices,but also brings about excessive noise current,which limits the specific detection rate and linear dynamic range of the device for a long period of time.Previous pervasive dark current suppression strategies have focused on device structure or layer thickness tuning,which has resulted in increased process complexity and decreased optical responsitivity.At the same time,the dark current of organic photodiodes in standard architectures has been widely varying in the literatures,and the disagreement implies that our understanding of the factors governing the dark current is quite limited.To address these issues,this thesis proposes a conduction mechanism for the bulk trap acting on the injected carriers by analyzing the intrinsic trap states of organic semiconductor films.In the work of this thesis,we have successfully adjusted the energy level depth of the intrinsic trap of the semiconductor film by process means to achieve effective control of the dark current density while protecting the device responsivity.As a result,a reasonable evaluation of the intrinsic dark current of the device is obtained and a novel dark current reduction strategy is proposed.The details of the study are as follows:(1)The organic semiconductor films were prepared by high-temperature annealing to remove water to obtain a near trap-free system.Solvent modification supplemented by low temperature annealing to remove residual solvent is proposed to enhance the energy depth of traps inherent in organic semiconductor films.The trap density is adjusted by changing the solvent concentration.After the effective adjustment of the intrinsic trap state,the current-voltage characteristics of the organic photodiodes prepared from this active layer are investigated under light and dark conditions,as well as their performance parameters.(2)High-performance all-polymer organic photodetectors were prepared based on the newly proposed strategy.Based on the proven inverted device architecture,the optimized device achieves a specific detection rate of up to 6.23×10¹² Jones at-2 V bias,where the dark current density drops to 1.4×10^-5 m A cm^-2 and the maximum responsivity reaches 0.42 A W^-1.This specific detection rate is 49 times higher compared to the original device.(3)The mechanism of the action of the bulk trap in the active layer film under the organic photodiode architecture to limit the mobility of the injected carriers is proposed.The composition analysis of the device dark current is added to the consideration of the trap-limited carrier transport regime,and a reasonable evaluation of the intrinsic dark current order of magnitude of the device is obtained,which complements the dark current theory of organic photodiodes.A novel optimization strategy for organic photodetectors is proposed based on this mechanism.In summary,the theoretical addition of dark currents to organic photodetectors in this thesis helps to open new approaches for constructing high performance OPD toward practical applications.