Understanding And Modulating The Charge Transport Properties Of Organic Semiconductors

With unique features distinct from their inorganic counterparts,organic semiconductors（OSCs）are promising candidates for flexible,wearable,low-cost,and large-area electronics.All organic（opto）electronic devices share a common underpinning,i.e.,their performance vitally depends on the process of charge transport within theπ-conjugated materials.Although the basic theoretical framework of organic electronics has been established,the understanding of the charge transport in OSCs remain immature and controversial until now.As a consequence,some fundamental scientific issues are elusive and practical charge transport control strategies are missing,restricting the scientific and technological progress in this field.This thesis focuses on the charge transport,a basic scientific issue in organic electronics,to unravel the innately extrinsic state of OSCs and the mechanism of thermal-activated charge transport,and thus to develop performance modulation strategies for organic electronic devices.The main contents are as below.Firstly,the purified OSCs are generally believed intrinsic（undoped）,a cornerstone that underpins the understanding and utilizing of their physicochemical properties,but this tenet conflicts with experimental observations.Here,we reveal that OSCs are actually extrinsic,and develop a generally applicable de-doping method（i.e.,soft plasma）to approach their intrinsic state by eliminating the trace amounts(～10¹⁵ cm^-3)dopants.This finding not only clarifies previously unexplained organic electronics phenomena,but also implies that vast（opto）electronic properties of OSCs need to be re-investigated/re-understood.This work may shed light on the conduction mechanisms and guide the development of organic electronics.Secondly,temperature dependence of charge transport directly determines the performance and applicability of OSCs.Although we have known that the conduction of OSCs is thermal-activated,its temperature dependence varies widely in previous reports,making researchers confused.Here,a model is developed to depict and modulate temperature dependence of thermal-activated transport.This model shows that temperature dependence in organic devices is governed by the potential barrier at grain boundaries,and can be effectively modulated through precisely tuning the effective height of the potential barrier（i.e.,potential barrier engineering）.Specifically,the charge transport exhibits strong temperature dependence when the effective potential barrier height reaches maximum at the point of grain size near to twice of Debye length,and larger or smaller grain size reduce the effective potential barrier height,rendering devices relatively thermostable.This model shows a good applicability for OSCs.This work unravels the mechanism of thermal-activated charge transport in OSCs,and provides guidance to the design of organic devices with tailorable temperature dependence.Finally,based on the understanding of charge transport in Chapter 2 and 3,we develop two strategies to control the performance of organic field-effect transistors（OFETs）towards functional applications.1)A controllable doping/de-doping technology is developed to tune oxygen acceptor density,allowing the accurate modulation of the essential properties of OFETs in a nondestructive way（e.g.,polarity,mobility,conductivity and threshold voltage）.This strategy opens the feasibility for tailorable electrical parameters of organic devices towards special requirements.2)Then,based on the model in Chapter 3,the potential barrier engineering strategy is employed to control the temperature dependence of OSCs,achieving the high thermal-stable and thermal-sensitive OFETs,respectively.This strategy dramatically expands the property space of OSCs,showing great potential for heat-resistant components and temperature sensors.This work gives OSCs a fantastic flexibility in electronics,making the performance,once unavailable,possible.