Search

Polymer light-emitting electrochemical cells (PLECs) and light-emitting diodes (PLEDs) receive interest for large-area lighting and signage applications. During operation of a PLEC, a p-i-n junction develops where electrons and holes are injected into the film and are transported along n- and p-doped regions to the intrinsic (i) region, where they recombine under light emission. Conceptually, this resembles the PLED device architecture equipped with Ohmic charge-injection and transport layers. The similarity between the i-region of the PLEC and the emissive layer of the PLED is obvious; however, implication of this has not been examined in detail so far. For example, for PLEDs it is known that electron traps hinder the electron transport, and that hole trap formation dictates the long-term durability. Here, for PLECs the electrical and optical response to electrical driving and breaks are studied, the current response to external light irradiation is probed, and degradation is followed with long-term absorption and capacitance measurements. The electron traps in PLECs are identified and it is found that hole trap formation limits the device lifetime, in the same manner as established for PLEDs. It is concluded that charge traps in semiconducting polymers present important, but so far overlooked intrinsic performance limitations for PLECs.

Organic upconversion devices (OUCs) consist of an organic infrared photodetector and an organic visible light-emitting diode (OLED), connected in series. OUCs convert photons from the infrared to the visible and are of use in applications such as process control or imaging. Many applications require a fast OUC response speed, namely the ability to accurately detect in the visible a rapidly changing infrared signal. Here, high image-contrast, narrowband OUCs are reported that convert near-infrared (NIR) light at 980 and 976 nm with a full-width at half maximum of 130 nm into visible light. Transient photocurrent measurements show that the response speed decreases when lowering the NIR light intensity. This is contrary to conventional organic photodetectors that show the opposite speed-versus-light trend. It is further found that the response speed increases (when using a phosphorescent OLED) or decreases (for a fluorescent OLED) when increasing the driving voltage. To understand these surprising results, an analysis by numerical simulation is conducted. Results show that the response speed behavior is primarily determined by the electron mobility in the OLED. It is proposed that the low electron drift velocity in the emitter layer sets a fundamental limit to the response speed of OUCs.

Semiconducting polymers are being studied intensively for optoelectronic device applications, including solution-processed light-emitting diodes (PLEDs). Charge traps in polymers limit the charge transport and thus the PLED efficiency. It is firmly established that electron transport is hindered by the presence of the universal electron trap density, whereas hole trap formation governs the long-term degradation of PLEDs. Here, the response of PLEDs to electrical driving and breaks covering the timescale from microseconds to (a few) hours is studied, thus focusing on electron traps. As reference polymer, a phenyl-substituted poly(para-phenylene vinylene) (PPV) copolymer termed super yellow (SY) is used. Three different traps with depths between approximate to 0.4 and 0.7 eV, and a total trap site density of approximate to 2 x 10(17) cm(-3) are identified. Surprisingly, filling of deep traps takes minutes to hours, at odds with the common notion that charge trapping is complete after a few hundred microseconds. The slow trap filling feature for PLEDs is confirmed using poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene (MEH-PPV) and poly(3-hexylthiophene) (P3HT) as active materials. This unusual phenomenon is explained with trap deactivation upon detrapping and slow trap reactivation. The results provide useful insight to pinpoint the chemical nature of the universal electron traps in semiconducting polymers.