ICMCTF2013 Session C4-1: Thin Film Growth and Characterization for Optoelectronic Devices

Due to the advantages of low cost, fast and large-area production, electrodeposition has become one of the mainstream fabrication methods of the CIGS solar cells. In this study, CIGS film was formed using one-step electrochemical deposition at various deposition potential voltages in solution with diverse pH values. M ultiple material analyses such as X-ray diffraction (XRD) analysis, scanning electron microscope (SEM) images, energy diffraction spectrometer (EDX), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) were used to examine the electro-deposition parameters. The results reveal that the Ga could not be deposited on the substrate in stirred conditions. In unstirred conditions, quaternary CuInGaSe compound could be formed with various Cu/In/Ga/Se ratios grown in various deposition parameters. The experimental results indicate that the pH value of the solution and electric potential play important roles on determining the composition of the CIGS film. UV spectrometer was performed to measure the bandgap of the CIGS thin film. The electrodeposited CIGS film shows promises for future solar cell applications.

A new class of dilute magnetic semiconductor (DMS, CdMnS) thin film based photoelectrochemical cell is presented. DMS thin films are synthesized on both glass and stainless steel substrates using a chemical growth process. The preparation parameters (such as growth temperature, time, reaction pH, precursor concentrations, etc) were optimized to yield characteristically oriented films. The layer thickness was found to be decreased with an increase in Mn²⁺ concentration. The composition of the as-grown samples was determined by an EDS technique. The electrochemical cells were then formed out of these series of films as the active photoelectrodes, an electrolyte and a counter electrode. The cells were then characterized through their dark and photosensing properties. The other cell parameters were determined from these studies and the cell performance has been evaluated with a special reference to Mn²⁺ concentration in CdS. A significant enhancement in performance has been observed for a cell with electrode composition of x = 0.01.

We have recently demonstrated that intentional doping of CdO results in an ideal uncompensated material with extremely low resistivity (ρ<10^-4Ω-cm) and an excellent transmission window in the range from 400 nm to >1500 nm, making this material an ideal transparent conductor for photovoltaics with low band gap absorbers [1]. These exceptional electrical and optical properties were obtained typically in polycrystalline CdO synthesized by a variety of methods including pulsed laser deposition (PLD), filtered cathodic arc deposition and RF sputtering. Here, we present a systematic study of the electrical and optical properties of CdO thin films grown by RF sputtering. In particular we studied the effects of defects in undoped and In-doped CdO samples under different deposition and annealing conditions. We found that at low growth temperatures (<200^oC), a fast growth rate tends to trap both oxygen vacancies and compensating defects in the film resulting in materials with high electron concentration of >2x10²⁰/cm³ and relatively low mobility (~30-50 cm²/V-s). Reducing the growth rate to ~2-3 nm/min results in high quality material with electron concentration of ~1x10²⁰/cm³ and mobility >100 cm²/Vs. In order to identify the dominating defects in the films, we have also carried out thermal annealing in N₂ or O₂ ambient. Annealing in O₂ ambient consistently reduces electron concentration and increases mobility, suggesting that the dominating defects in sputtered CdO films are oxygen vacancies. Sputtering with an Ar/O₂ (80%/20%) plasma results in high resistivity CdO films ( ρ>10^-2Ω-cm) with low electron concentration of 10¹⁸/cm³, most likely due to the increased incorporation of excess O resulting in anon-stoichiometric material. However such material is not stable and an increase in the electron concentration is observed at brief annealing as short as 1 second at 300^oC, suggesting outdiffusion of O and increase of the O vacancy concentration. This conclusion is further supported by more pronounced annealing effects observed in thinner samples. Intentional doping with In donors leads to an increase of both the electron concentration and the mobility. With proper doping CdO films with electron concentration of more than 10²¹ cm^-3 and electron mobility higher than 100 cm²/Vs can be achieved, indicating greatly reduced ionized impurity scattering effects in in this material. We will also discuss the effect of defects and intentional doping on optical properties of as grown and annealed CdO films.

[1] K.M Yu, et al., J. Appl. Phys. 111 123505 (2012).

This work reports the synthesis and characterization of N-doped TiO₂ mesoporous beads prepared by a two-cycle rapid microwave-assisted hydrothermal method using three different types of nitrogen dopants: diaminohezane, triethylamine, urea. In the first cycle, TiO₂ mesoporous beads with controlled structures were synthesized at 200°C. The obtained beads were then subjected to a second cycle of microwave-assistaed hydrothermal process for dpoing with one of the aforementioned dopants. The sue of a secon cycle is to maintain the integrity of the beads, which otherwise would be easily destroyed if the synthesis and doping processes are carried out at the same time. The crystalline structure of the N-doped TiO₂ was examinedusing X-ray diffraction. The surface state and structure were investigated using X-ray photoelectron spectroscopy and scanning electron microscopy, respectively. The absorption of N-doped TiO₂ in the range of visible light was confirmed using UV-Vis spectroscopy. The self-assembled N-doped TiO2 redshift in adsorption edge up to 500nm. The obatined TiO₂ was also dissloved in methyl blue solution to function photocatalyst and the catalytic activity was determined. The photocatalytic activity of all N-doped TiO2 can be found the N-doped TiO2 used dianimohexane as the nitrogen dopant decompose the organic pollution more complete and rapid than others .

Nickel oxide (NiO) has been used in photoelectrochromic devices due to its p-type intrinsic behavior. Titanium oxide (TiO₂) is an n-type semiconductor with excellent photocatalytic properties. In this work, Ni and Ti films were deposited by the PLD technique at room temperature and oxidized at 550ºC in air for 2 hours to form NiO and TiO₂, respectively. The third harmonic (λ= 355 nm) of a YAG:Nd pulsed laser operated at an energy fluence of 3 J/cm² and 10 Hz repetition rate was used for the laser ablation experiments. Microstructural development and chemical composition of the films were analyzed by SEM, XRD, AES/XPS and XRD techniques. Hall Effect and Van der Pauw measurements confirmed p- and n-type behavior of NiO and TiO₂ films, respectively. Heterojunctions have been fabricated by deposition of p-type NiO on n-type TiO₂ on ITO coated glass substrates. The relationship between electrical and photocatalytic properties of NiO and TiO2 thin films will be discussed.

Support from DGAPA-UNAM (Grant No. IN114010) and CONACYT (Grant No. 100555) and technical assistance by E. Aparicio, D. Dominguez and I. Gradilla are acknowledged.