An all-solid-state wavelength-dependent bipolar photodetector (WBPD), composed of a single semiconductor, has demonstrated a faster response time than existing WBPDs that are comprised of hetero-nanostructures. The novel WBPD has also demonstrated tunable switching wavelengths. To speed response time in the novel photodetector, researchers at Toyota Central R&D Labs used a tungsten disulphide film with front and rear ends modified by oxidation and sulpherization. The band structure of the semiconductor film had the ability to increase or decrease at both the rear and front surfaces, which allowed it to form a U (or upside down U) shape, making the film capable of carrying photocurrents with wavelength-dependent switching. Schematic illustration of the device structure of the WBPD. λ1 indicates shorter-wavelength incident light, λ2 indicates longer-wavelength incident light. The researchers also exploited the dependency between wavelength and depth of photon penetration in semiconductor material. Because shorter wavelengths are more readily absorbed than long, shorter wavelengths have a higher distribution of excited electrons near the front surface; whereas longer wavelengths have greater distribution at a deeper level of penetration into the material. When drift and diffusion processes are taken into account, this phenomenon causes photocurrents in opposite directions, for short- and long- wavelength incident light. The researchers showed that the threshold wavelength at which the photocurrent polarity changed could be tuned by choosing a suitable thickness for the device. The device thickness affected the distribution of photoexcited carriers as a result of the wavelength-dependent absorption, allowing tunable switching wavelengths. In existing WBPD devices, the switchable photocurrent polarity behaviors are caused by differences in the optical properties of the two materials that comprise the device; and the transition wavelengths of the output polarity are limited by the optical properties of the two types of materials. The low carrier mobility in liquid electrolytes, which are used in many current WBPD devices, can slow switching response time. For current and future optoelectronic devices such as logic gates, extremely high response times will be required. An all-solid-state WBPD (i.e., a WBPD without any electrolytes) may have significant advantages over WBPDs using molecules and electrolytes. “Optoelectronic sensors that can switch their photocurrent direction based on the wavelength of incident light are an important building block in novel optical logic gates, color sensors, and photocatalysts,” said Takashi Ikuno and Masaki Hasegawa, researchers at the Toyota Central R&D Labs. The research was published in Applied Physics Express (doi:10.7567/APEX.9.062201).