Study On The Photostability Of Organic Photovoltaics And Their Applications For Opto-electronic Information

Organic solar cells(OSCs)and organic photodetectors(OPDs)share the same principle so they can be regarded as organic photovoltaic devices in a broad sense.As compared to OPDs,OSCs get more intensive research interest and the field continues to mature,leading to the realization of 18%power conversion efficiency(PCE)in single-junction device and paving the way for employing OSCs as an efficient solar photons conversion device in preliminary applications.At the same time,OPDs have attracted increasing basic research and innovative research interest due to the extended spectral,the improved performance and promising application potential,where important figure-of-merit performances,such as the dark current(J_dark),the linear dynamic range(LDR),the-3d B cut-off frequency(f_{3d B}),are used to evaluate the performance of the OPDs.With comparable device performance to their inorganic counterpart in most sensitive laboratory-scale devices,from practical considerations,it is naturally feasible to utilize OPDs for innovative applications.Therefore,the first two chapters of this dissertation mainly focus on the study of OPDs for wearable medical device and optical communication applications.In the last chapter of this dissertation,in view of the unusual light stability observed in the material systems(PBDB-T:IT-M,PBDB-T:IT-4F)used for applications in light signal detection,the origin of the phenomenon are investigated by means of various techniques,including impedance spectroscopy,dielectric spectra,etc.In brief,this dissertation dedicated to the photostability of organic photovoltaics and their applications for opto-electronic information.The dissertation is organized as follows:In the Chapter 1,the working principle of organic photovoltaics is addressed.Moreover,the device physics relating performance parameters to material properties in organic photovoltaics are elaborated.Besides,the research progress of OPDs in photoplethysmography signal detection and optical communication is reviewed.In consideration of the flexible characteristics in wearable electronics and its compatibility to human skin,we demonstrate a simple but feasible and useful OPDs array architecture in the Chapter 2,where a series of individual OPDs are connected in series.The ITO film on glass substrate of 1.5 cm² is divided into six ITO films with the same area and the fabricated OPDs array on a piece of ITO-coated glass composes of six independent sub-devices.With this array architecture,the output photovoltage can be effectively strengthened,as such the photo detecting ability of a photodiode is greatly improved.In addition,because the device operates at 0 V bias,additional power supply is not required.We further show that this array architecture has a good compatibility with flexible substrates,hence can be an emerging candidate for wearable electronics.More importantly,light sensing systems based on this OPDs array architecture can enjoy significantly decreased complexity of a whole system by getting rid of the transimpedance amplifier.Therefore,this array architecture may have tremendous potential for practice applications,such as photoplethysmography(PPG)test for monitoring the pulse and determining the heart rate.In the Chapter 3,we describe a method to fabricate differential-response OPDs upon incident-specific illumination.The OPD with a planar heterojunction structure(PHJ)of ITO/donor/acceptor/semitransparent Ag is developed for wavelength differential detection application.By employing an modulated exciton profile resulted from an intrinsic charge collection narrowing principle that allow for fine-tuning the charge carrier photogeneration profile and thereby varied external quantum efficiency(EQE)in OPDs,an OPD capable of differential-response from double-side is realized.The OPD exhibits responsivity of 51 m A W^-1/2 m A W^-1at ranges from 300 to 650 nm and 11 m A W^-1/131 m A W^-1 at ranges from 650 to850 nm under Ag-top/ITO-bottom illumination condition,respectively.The resultant devices show totally different strong photo response in visible and NIR spectral region,respectively,when light is incident from the side of the ITO bottom electrode or from the side of the semitransparent Ag top electrode.We further utilize the double-side differential-response OPDs in a visible light communication(VLC)system for data transmission,in which analog audio signal is successfully transmitted with high fidelity.This double-side differential-response OPDs are expected to play a unique role in new type of optical communication systems.In the former two chapters,the photostability phenomena are found in the PBDB-T:IT-M and PBDB-T:IT-4F devices,i.e.,irradiating OSCs with light can achieve the desired devices characteristics and this effect is called light-soaking effect.For example,the typical inverted PBDB-T:IT-M device with an initial PCE of 8.92%requires 140 s light illumination to reach its maximum efficiency of 9.43%.In Chapter 4,the impact of the space charge polarization on the light-soaking is traced experimentally through the determination of dielectric spectra and explored by recombination characterization through charge carrier dynamics study.On the basis of the analysis of dielectric spectra,we find the dielectric changes are closely related to the stability of devices.Moreover,study of charge carrier dynamics in the device under operation shows that light-soaking and the subsequent space charge polarization result in improved mobility and charge carrier lifetime,thus responsible for the improved device performance.