Mid-IR Range Photosensitive Device Based on Chemically Deposited Nanocrystalline Chalcogenide Thin Films with Tunable Opto-electrical properties

Past several decades have driven modernization of technology and machinery. This modernization has pushed the limits of our technology and increased our dependence on the energy. Additional side-effect of this rapid growth has been an exponential increase in the generated heat of the modern machinery.

In most cases this waste heat is simply released into the environment. Numerous research groups have pursued an idea of capturing and converting the waste heat. Thermoelectric, pyroelectrics, organic Rankine cycle (ORC), and several other methods have been proposed to capture and convert the waste heat into electricity. Presently, all methods, however, have low conversion efficiency and are not economically feasible.

In this work we focused on a practical approach to convert mid-IR electromagnetic waves to electricity. It is based on inexpensive thin film technology utilizing a junction between a narrow bandgap lead salt and wide bandgap chalcogenide film.

Lead sulfide (PbS) was chosen as the narrow band semiconductors for the IR energy conversion. Lead salt photodetectors uniquely demonstrate high room temperature sensitivity to black-body sources in low temperatures and high internal quantum efficiency (QE). Further, the peculiar band structure of the lead salt allows for small Auger recombination and minimized losses.

Due to its favorable opto-electrical properties and band compatibility with PbS, cadmium sulfide (CdS) was chosen as the wide bandgap semiconductor for this work.

This work has shown that high quality nanocrystalline thin films of lead sulfide (PbS) and cadmium sulfide (CdS) can be grown cost efficiently using chemical bath deposition (CBD) method. Chemical bath seeding procedure was also developed in order to achieve reproducibility in the transport phenomena of the advanced materials. Seeding also allows these films to be deposited on any surface, including smooth flexible materials. Seeded kernels have shown to become the crystallization centers for both nanocrystalline films.

Opto-electrical properties of the films were tuned in such a way to make the materials useful in a broad-band of the IR spectrum. We have shown that altering the parameters of the chemical bath deposition alters the grains hence changing the transport characteristics of the materials. We have shown that parameters of the chemical bath deposition can be optimized to produce highly sensitive thin films tuned to a specific range of the electromagnetic spectra.

Novel transparent conducting oxide, Iridium (Ir) doped Titanium Oxide (TiO2) was developed in this work for the use in the optoelectronic device. Ir has been shown to be one of the most efficient dopants in thin films. Ir doped TiO2 has shown to have transport characteristics similar to those of the commonly used TCOs, with much higher optical transmittance in the infra-red range.

Lastly devices were manufactured from the developed materials. TiO2/CdS/PbS/Au heterojunctions were manufactured and showed photoresponsivity. Device efficiencies were shown to depend on the doping concentrations of all materials, TCO thicknesses, and electrode selection. Conversion rates of 3% were shown in the devices.

Two secondary contributions to the scientific body of knowledge were also achieved. First, crystalline structure has a profound effect on the opto-electrical properties of the film. Phase transitions from cubic to hexagonal crystals in CdS during heat treatment were observed. Hexagonal phase showed much more favorable opto-electrical properties. Additionally, film’s stored energy due to the developed stresses and strains was studied. When the stored energy is larger than the adhesive energy, the deposited film would peel off from the substrate. Low adhesion was observed in films with high stresses. Some of the stresses can be alleviated using het treatment, hence allowing for higher adhesion.

The result of this work show that a cost efficient method for the conversion of thermal energy into electricity is possible using advanced thin film materials. Conversion efficiencies and cost efficiency make this technology feasible for use in large scale applications.