Photocurrent spectra from PbS photovoltaic infrared detectors using silver nanowires as plasmonic nano antenna electrodes

Metal nanowires have shown potential application as electrodes in the optoelectronic field owing to their high conductivity and transparency properties. This makes them a promising candidate for operation at mid- and far-infrared range. In particular, low-cost silver nanowires can be accommodated in flexible designs as opposed to their transparent oxide counterparts. Recently, silver nanoparticles were used to enhance the photocurrent in the weakly absorbed spectral range based on light scattering and trapping. This is mostly experienced in thin-film solar cells and photodetectors. Additionally, such effects have been successfully maximized by the plasmonic back reflector in the solar cell structure.

Researchers have realized that metallic nanowires can also be used as nanoantennas otherwise known as plasmonic antennas. These antennas generated by volume currents inside metal nanowires are generally different from the standard antenna modes by surface currents. On the other hand, the plasmonic antenna modes have their spectral ranges corresponding to visible light rather than radio-frequency. Even though the metallic nanowires have been effectively used as transparent electrodes in photodetectors, their applicability as plasmonic antennas for enhancing the photocurrent spectral within the weak absorption range is not fully explored in the existing literature. To this end, narrow bandgap lead sulfide has been identified as a promising solution for infrared photodetectors and multi-junction solar cells. Among the available methods for lead sulfide production, chemical bath deposition and electrochemical deposition are commonly used. Additionally, most of the available photovoltaic lead sulfide devices have been demonstrated using different heterojunctions by shining the incident light from the bottom surface rather than directly on the lead sulfide film.

To this effect, a group of researchers at Chungnam National University: Dr. Jungdong Kim, Emmanuel K. Ampadu (PhD student) and led by professor Eunsoon Oh together with Dr. Won Jun Choi from Korea Institute of Science and Technology explored Schottky photovoltaic infrared detectors using lead sulfide films, deposited by chemical bath deposition technique. This was an extension of the initial study that majorly investigated the Schottky characteristics of lead sulfide nanowires with a variety of metals. Silver nanowires were utilized as top transparent electrodes while the bottom metal electrode formed the Schottky contacts with the films. They demonstrated the applicability of the silver nanowires as plasmonic nanoantenna electrodes. Their work is currently published in the journal, Nanotechnology.

The research team recorded a large film crystallite size of 0.2 µm in the lateral direction, 1 µm in the vertical direction and high mobility value of 60cm² V^-1 s^-1 at room temperature. Consequently, silver nanowires worked as plasmonic antennas for generating surface plasmons whose effects played a significant role in amplifying the electric field thus enhancing the photocurrent. This was, however, more pronounced when the silver nanowire was located near the constructive interference. For instance, a cut-off wavelength of approximately 3µm was attained at room temperature while at 100K, a pronounced photocurrent peak was obtained. It was worth noting that the amplification ratio was majorly influenced by the film thickness and the nanowire segment lengths. Regarding the extensive application of metal nanowires, the plastic nanoantenna modes can be used to enhance device efficiency at desired wavelengths.