Transient Photoconductivity And Transient Photoinduced Absorption Spectroscopy Studies On Ultrafast Photocarrier Processes In Organic Solar Cells

Organic solar cell (OSC) technology is one of the most promising new generation solar celltechnologies and therefore it is a hot topic in research. Compared to inorganic solar cells, OSCshave some unique advantages, however the low performance is still a disadvantage. In order tofurther improve the performance of OSCs, deeper understanding on the OSC workingmechanisms must be obtained. Among the various problems in mechanism studies, three aremost important: The generation, the recombination and the transport properties of photocarriersin OSCs.Critical processes involving these three mechanisms mentioned above usually happenwithin the timescales of several hundred femtoseconds to several nanoseconds. The aim of thisthesis is to capture and study these ultrafast processes with various transient detectionexperimental techniques. Namely, we studied the generation, the recombination and the transportproperties of photocarriers in organic solar cells through sub-nanosecond transientphotoconductivity (ns-TPC) and femtosecond transient photoinduced absorption spectroscopy(fs-TAS). Additionally, we studied the application of novel electron selective layer materials inorganic light emitting devices (OLEDs) and OSCs. For convenience the thesis is divided intofive parts.In the first part we studied the photocarrier transport properties in two leading molecularbulk-heterojunction (BHJ) systems through temperature dependent ns-TPC measurements. Wefind that in these systems shallow trap states are distributed at relatively narrow energy range,giving rise to time-independent carrier mobility in the nanosecond time regime that evidentlycontributes to the high performance of molecular donor based OSC devices compared to thepolymer donor based counterparts.The second part we studied the the dynamics of charge generation in another novel highperforming molecular OSC system with fs-TAS measurements. The optimized BHJ OSC systemshows behavior observed in many other BHJ systems: the majority of charges are generated atsub-picosecond time scales, and a slower contribution from incoherently diffusing excitons isobserved at low pump fluence. In a separate experiment, the role of bulk heterojunction materialmorphology on the process of ultrafast charge generation was investigated with bilayers madewith solution processed donor molecules on a photopolymerized acceptor layer. We find that the majority of carriers are again produced at short time scales, ruling out the idea that sub-picosecond charge generation can be understood wholly in terms of localized excitons. Weevaluate possible causes of this behavior and propose that the excited state is highly delocalizedon short time scales, providing ample probability density at the donor/acceptor interface.In the third part we first adapted the fs-TAS system to enable femtosecond transientphotoinduced reflectivity (fs-TRS) measurement, and then we systematically studied the chargecarrier dynamics of five highly efficient BHJ systems in actual OSC devices with the newlydeveloped fs-TRS technique. We find that the charge generation process in devices is similar tothe process in thin-films as measured by fs-TAS, which proves that delocalized excited statesalso exist in actual OSC devices. It is also found that the charge generation dynamics in thehighly efficient OSC devices are not dependent on electric field, which effectively rules out theexistence of geminate recombination process in these BHJ systems.In the fourth part we applied two novel crosslinkable non-ionic water/alcohol solubleconjugated polymers (WSCPs) as independent electron selective layer in inverted OLED deviceswithout the aid of n-type oxide materials. The devices achieved high performance anddemonstrated the potential of this kind of WSCPs in inverted OLED devices and points towardpromising applications in OSCs.In the fifth part we applied five distinctive WSCP materials with different free ionic groupsas electron selective layer in both conventional structure and inverted structure OSC devices. Wesystematically studied the influence of ionic groups on the performance of the OSCs and foundthat in the conventional structure devices the ones with zwitterionic WSCP or non-ionic WSCPhave the best performance while in the inverted structure devices the ones with free negative ioncontaining WSCP have the best performance. We also studied the mechanisms of electronselective function of the five WSCPs from two perspectives: the ion redistribution effect andinterfacial dipole effect. We found that both effects contribute to the high performance of WSCPincorporated OSC devices.