Bipolar charge transport

Mobility data on bipolar charge-transport materials are still rare. Some bipolar molecules with balanced mobilities have been developed [267], but the mobilities are low (10 6—10 8 cm2/Vs). Up to now, no low molecular material is known that exhibits both high electron and hole conductivity in the amorphous state, but it is believed that it will be only a matter of time. One alternative approach, however, is to use blends of hole and electron transporting materials [268]. 

Peng, Z., et al. 2000. Towards highly photoluminescent and bipolar charge-transporting conjugated polymers. Macromol Symp 154 245. 

Blends of semiconducting polymers can also be used as active layers to achieve bipolar charge transport. Since controlling the morphology of polymer blends is challenging, there are only few reports based on polymer-polymer composite bipolar devices. 

Oligofluorenes are exceptional in their charge transport properties, exhibiting relatively high bipolar charge-carrier mobilities in comparison to other non-crystalline organic materials. An interesting study of the dependence... 

The properties of the host material generally determine device performance. Depending on the charge-transporting properties of the host, recombination can occur at the interface with the blocking layer or in the bulk of the EML. A bipolar host can promote bulk recombination, but can also lead to a situation where the recombination occurs at both the EML HTL and EML IETL interfaces, in particular, when the electric field and the mobility of charge carriers in the EML are high. ... 

Similarly, research conducted on materials and fabrication methods for GDL and bipolar plates aim to tune their properties in order to improve the fuel cell performance. It is clear that the current trend is the integration of the MEA components in order to improve the architecture of the triple phase boundary region and, consequently, the mass and charge transport. 

Photoeonductive polymers can be -type (hole-transporting), -type (eleetron-transporting) or bipolar (capable of transporting both holes and eleetrons). To date, all practical photoeonductive charge-transporting polymers used eommercially are -type. 

The lack of the bipolar transport in binary blends of polymers for most of the compositions shows that controlling the thin-film morphology is challenging. Therefore, the key issue is to realize an interpenetrating and bicontinuous networks of binary polymer blends in order to establish ambipolar charge transport. 

In the case of weakly FeCla doped PPPs, the dc conductivity (Cdc) was proportional to T n 10). This result, which is very different from Mott s relationship, was interpreted as being due to hopping and fitted a relationship involving a slightly modified Kivelson model [228] between polarons and bipolar-ons. With more heavily doped PPPs, the charge transport mechanism changed dramatically, and Cdc was then given by Mott s relationship corre-... 

Charge injection, 367, 380, 388, 392-394, 411, 428 unipolar, 411 positive, 412-416 negative, 416-417 bipolar homo-, 417 Charge recombination, 69-72 Charge transport, 435-340, 504-508 Conduction band effects... 

The charge transport equations in electrolyte, electrodes, and current collector or bipolar plates are derived based on charge balance. 

Photoconductive and charge-transporting polymers can be either p-type or n-type, depending on the majority carrier. Only in the special cases discussed below can polymers be bipolar, i.e. capable of trans-... 

In an HBT the charge carriers from an emitter layer are transported across a thin base layer and coUected by a third layer called the coUector. A small base current is present which iacludes the carriers that did not successfully cross the base layer from the emitter to the coUector. The FET is a unipolar device making use of a single charge carrier in each device, either electrons or holes. The HBT is a bipolar device, using both electrons and holes in each device. The emitter and coUector layers are doped the same polarity n- or -type), with the base being the opposite polarity (p- or n-ty- e). An HBT with a n-ty e emitter is referred to as a n—p—n device ap—n—p device has a -type emitter. The n—p—n transistors are typicaUy faster and have been the focus of more research. For the sake of simplicity, the foUowing discussion wiU focus on n—p—n transistors. 

The analytic theory outlined above provides valuable insight into the factors that determine the efficiency of OI.EDs. However, there is no completely analytical solution that includes diffusive transport of carriers, field-dependent mobilities, and specific injection mechanisms. Therefore, numerical simulations have been undertaken in order to provide quantitative solutions to the general case of the bipolar current problem for typical parameters of OLED materials [144—1481. Emphasis was given to the influence of charge injection and transport on OLED performance. 1. Campbell et at. [I47 found that, for Richardson-Dushman thermionic emission from a barrier height lower than 0.4 eV, the contact is able to supply... 

---