Light Incoupling And Charge Carrier Extraction In Cavity-enhanced Organic Photodetectors

In the area of next-generation photodetector technology,organic photodetectors（OPDs）are an attractive study area because of their flexibility,lightweight,and ability to be processed in solutions.Similar to inorganic photodetectors,the spectral response range of OPD devices is often broad and devoid of wavelength selectivity.As a result,they cannot be used directly in applications like imaging and biomedical ones that demand wavelength resolution.Devices with narrowband OPD have been created to solve this problem.Typically,narrowband filters are integrated into OPD devices,or narrowband-absorbing organic semiconductors are developed.However,because of their high production costs and difficult tunable response wavelength,such narrowband OPD devices cannot satisfy the technical requirements of high-resolution spectral detection applications.Combining the OPD device with a Fabry-Perot resonant cavity is another method for achieving narrowband response.The optical microcavity effect exhibits extraordinarily high wavelength selectivity in such cavity-enhanced organic photodetectors by dramatically enhancing the organic active layer’s absorption at a particular wavelength while suppressing it at other wavelengths.Importantly,the length of the resonant cavity allows for continuous modulation of the response wavelength of cavity-enhanced OPD devices,making them extremely tunable.Consequently,there is great potential for advancement in the spectroscopic detection technique for cavity-enhanced OPD devices.Currently,further work needs to be done to improve the performance of cavity-enhanced OPD devices.The full width at half maximum（FWHM）of the response peak must be decreased,the device’s selectivity for the response wavelength must be improved,and the tunable range of the response wavelength（λm）must be further expanded.Additionally,from the viewpoint of device electronics,it is necessary to increase the photoresponsivity and quantum efficiency of the device while reducing its dark current and noise current.The ultimate objective is to increase the cavity-enhanced OPD devices’detectivity（D^*）for the response wavelength.This thesis focuses on these two issues of cavity-enhanced devices optical coupling and charge extraction,in order to meet this objective.The specific research contents are as follows:（1）The optical properties of the front and back reflectors,the active layer,and the interface layer of the cavity-enhanced OPD were clarified through optical simulations using the transfer matrix method（TMM）,and their relationship with the device’s photoresponse spectrum was examined.The results demonstrate that the wavelength selectivity of the device increases with the reflectivity（R₁,R₂）of the front and back reflectors and decreases with the active layer’s absorption coefficient（α）.However,the cavity-enhanced device’s external quantum efficiency（EQE）will be severely constrained if the reflectivity of the front reflector or theαof the active layer is too low.The wavelength selectivity of the device can be increased by reducing the optical parasitic absorption which occurs at the interface between the electrode and the active layer.Second,the tunable range of the cavity-enhanced OPD’s response wavelength（λ_m）is determined by the absorption range of the active layer and the cavity length,that is,the total thickness L of the interface layer and the active layer.The tunable range of the device’s response wavelength can be expanded by expanding L or the active layer’s absorption range.Theoretically,cavity-enhanced OPDs with superior optical performance can be built on the basis of our TMM simulation results.（2）To serve as the donor material for cavity-enhanced OPDs,we designed and synthesized PCDTPTSe,a polymer with a low bandgap and low absorption coefficient.We fabricated a number of high-performance cavity-enhanced OPDs using PCDTPTSe:PC₇₁BM as the active layer and Zn O as the interface optical spacer,guided by the TMM simulations.The devices reduce the FWHM of the response peak to 22 nm and achieve continuous tuning of the first-order response wavelength throughout a broad range of 660-1510 nm.The detectivity（D^*）of devices at the resonant frequency increases to 2×10¹¹ Jones.The devices additionally demonstrate a linear dynamic range of up to 140 d B and a response time as quick as 100 ns.Finally,using PCDTPTSe:PC₇₁BM,we created a variety of cavity-enhanced OPDs with various response wavelengths.We created a tiny near-infrared（NIR）spectrometer with a detection wavelength range of 660-1600 nm by mechanically combining these devices.A new method for the creative use of cavity-enhanced OPDs is successfully demonstrated by measuring the NIR absorption spectra of solvents like water,ethanol,and acetone.（3）Utilizing organic semiconductor materials,we built industrially acceptable resonant and non-resonant OPD devices utilizing non-halogen solvents.We investigated the effects of different non-halogen solvents on the morphology of the organic active layer,EQE,photoresponsivity,and dark current density（J_d）of the devices.According to our research,the PCDTPTSe:PC₇₁BM active layer exhibits superior donor-acceptor morphology when CS₂ is used as the solvent,which results in a lower J_d and higher EQE for the OPD device.As a result,the devices made with non-halogen solvents perform better than those made with halogen solvents,with a D^*value as high as 2.71×10¹⁰ Jones at 1315 nm.Further investigations show that using CS₂ as a solvent for making OPDs can improve carrier transport properties,leading to faster response rates for the OPD devices and being more advantageous for the commercial development of cavity-enhanced OPD devices.（4）We developed a processing method that tunes the work function of the ITO substrate electrode by combining oxygen plasma treatment with PEIE surface modification.In the following,OPD devices based on the PEIE/ITO electrode were produced,and the continuous adjustment of the device’s built-in electric field(E_bi)was accomplished by surface treatment of ITO.We also investigated the E_bi of the OPD device and the tunable range of the ITO electrode’s work function.According to the findings,the PEIE/ITO electrode work function can be continuously controlled between 4.0 and 5.2 e V by adjusting the duration and intensity of plasma surface treatment.When the treated electrode is used as an anode to prepare the OPD device,the E_bi increases and the device’s dark saturation current density decreases with increasing treatment time,leading to increased work function of ITO.The PEIE/ITO electrode can be used as a reliable anode to create standard OPD devices after being plasma treated for a substantial amount of time,and the device performance is comparable to that of devices utilizing conventional PEDOT:PSS-modified ITO anodes.The aforementioned findings demonstrate that the processing approach of oxygen plasma treatment coupled with PEIE surface modification has good practical value and a wide range of potential applications.It also offers a straightforward and universal experimental technique for creating high-performance cavity-enhanced OPD devices.