High resolution scanning optical imaging of a frozen polymer p-n junction

Abstract

Semiconductor homojunctions such as p-n or p-i-n junctions are the building blocks of many semiconductor devices such as diodes, photodetectors, transistors, or solar cells. The determination of junction depletion width is crucial for the design and realization of high-performance devices. The polymer analogue of a conventional p-n or p-i-n junction can be created by in situ electrochemical doping in a polymer light-emitting electrochemical cell (LEC). As a result of doping and junction formation, the LECs possess some highly desirable device characteristics. The LEC junction, however, is still poorly understood due to the difficulties of characterizing a dynamic-junction device. Here, we report concerted optical-beam-induced-current (OBIC) and scanning photoluminescence (PL) imaging studies of planar LECs that have been frozen to preserve the doping profile. By optimizing the cell composition, the electrode work function, and the turn-on conditions, we realize a long, straight, and highly emissive p-n junction with an interelectrode spacing of 700 μm. The extremely broad planar cell allows for time-lapse fluorescence imaging of the in situ electrochemical doping process and detailed scanning of the entire cell. A total of eighteen scans at seven locations along the junction have been performed using a versatile, custom cryogenic laser scanning apparatus. The Gaussian OBIC profiles yield an average 1/e2 junction width of only 1.5 μm, which is the smallest ever reported in a planar LEC. The controlled dedoping of the frozen device via warming cycles leads to an unexpectedly narrower OBIC profile, suggesting the presence and disappearance of fine structures at the edges of the frozen p-n junction. The results reported in this work provide new insight into the nature and structure of the LEC p-n junction. Since only about 0.2% of the entire device area is photoactive in response to an incident optical beam, the effective junction width (or volume) must be dramatically increased to realize a more efficient device.