P. K. Nair

Cu2SnS3 and Cu4SnS4 Thin Films via Chemical Deposition for Photovoltaic Application

Thin films of copper sulfide (CuS, 200 nm thick) were deposited over thin films of tin sulfide (SnS, 180 nm thick) by sequential chemical deposition. The layers were heated in nitrogen atmosphere at 350 and 400°C. The grazing incidence X-ray diffraction analysis of these layers established the formation of thin films of ternary composition, Cu 2 SnS 3 and Cu 4 SnS 4 . Optical bandgaps of the films are direct, 0.95 eV for Cu 2 SnS 3 and 1.2 eV for Cu 4 SnS 4 , and the electronic transitions are of the forbidden type in both cases. The films are p-type, with electrical conductivities of 0.5-10 Ω ―1 cm ―1 and hole concentrations of 10 17 ―10 18 cm ―3 . Based on the optical absorption coefficients, the light generated current density (J L ) as a solar cell absorber was evaluated for these materials for air mass 1.5 (1000 W/m 2 ) global solar radiation. For a film thickness of 0.5 μm. Cu 2 SnS 3 and Cu 4 SnS 4 could offer J L of 34 and 27 mA/cm 2 , respectively. Corresponding optical conversion efficiencies of solar energy into electron-hole pairs are 32 and 24%. The built-in potential for CdS/Cu 2 SnS 3 and CdS/Cu 4 SnS 4 junctions would be above 0.9 V and above 1.1 V when ZnO replaces CdS as the window layer.

Conversion of chemically deposited photosensitive CdS thin films to n‐type by air annealing and ion exchange reaction

A method is presented for the deposition of CdS thin films of 0.05–0.7 μm thickness from solutions at 50–70 °C containing citratocadmium(II) complex ions and thiourea. The films show an optical band gap Eg≳2.6 eV. Optical transmittance is about 80% for photon energy <Eg. The dark conductivity of the films is of the order of 10−8 Ω−1 cm−1. The photosensitivity of these films is high, 106–107 under illumination with tungsten halogen light of 1 kW m−2. Annealing in air at 400–500 °C for 1 h converts the films to n type. It is possible to obtain sheet resistances of about 150 Ω for a 0.2 μm film (i.e., conductivity of 300 Ω−1 cm−1) by this process. Conversion of the films to n type is possible also by immersing the film in a 0.01 M HgCl2 solution for 15 min followed by air annealing for 1 h at 200 °C. The films show n‐type dark conductivity of ≂0.05 Ω−1 cm−1 and photoconductivity of ≂1 Ω−1 cm−1. X‐ray diffraction and x‐ray photoelectron spectroscopic depth profile studies on the films show that the modificati...

Enhancement of photosensitivity in chemically deposited CdSe thin films by air annealing

Improvement in photosensitivity, (σphoto−σdark)/σdark, of chemically deposited CdSe thin films on annealing in air is discussed. The as‐prepared films of ∼0.5 μm thickness show photosensitivities of <10 under 600 W m−2 illumination. Upon annealing the films in air for 1 h each at various temperatures their photosensitivity increases depending on the temperature of annealing: ∼10 (200 °C), ∼102 (300 °C), ∼103 (400 °C), and ∼107 (450 °C). Air annealing at temperatures beyond 450 °C was found to cause degradation in the photosensitivity. The high photosensitivity is also accompanied by growth in photocurrent while maintaining a fast decay of ∼6 decades in <1 s after shutting off the illumination. Such a short decay time is unusual with chemically deposited photoconductive thin films. The results are explained on the basis of improvement in crystallinity and increase in chemisorption of oxygen upon annealing the films in air. X‐ray‐diffraction data and x‐ray photoelectron spectroscopy depth profiling of the a...

SIMULTANEOUS OBSERVATION OF STRONG AND WEAK QUANTUM CONFINEMENT EFFECT IN CHEMICALLY DEPOSITED CDSE THIN FILMS : A SPECTRO-STRUCTURAL STUDY

CdSe thin films deposited chemically on glass substrates for 4, 8, and 16 h, and subsequently annealed at 400 °C for 1 h, have been studied by a combination of spectroscopic (photoluminescence and Raman scattering) and structure-determining (x-ray diffraction and atomic force microscopy) techniques. Due to a size distribution of constituent grains, photoluminescence spectra of the as-deposited films show weak but broad bands at ∼2.2 eV (strongly confined band) and ∼1.73 eV (weakly confined band). On annealing, intensity of the weakly confined band, at ∼1.7 eV increases as a result of an improvement in the crystalline quality of CdSe nanoclusters. A surface-optic Raman mode at ∼250 cm−1 in as-deposited samples has been observed for the first time. The x-ray diffraction studies of annealed samples show a diffraction peak at 2θ=13° from the (001) plane. The improvement in crystallinity of the films as observed by atomic force microscopy and photoluminescence techniques, the appearance of (001) reflection in ...

Analysis of a Bismuth Sulfide/Silicon Junction for Building Thin Film Solar Cells

Feasibility of combining p-type crystalline Si (c-Si) of 200―8000 nm in thickness with an n-type bismuth sulfide (Bi 2 S 3 ) thin film of 300 nm in thickness for thin film solar cell is analyzed. Theoretical analysis shows that the high optical absorption coefficient (10 5 cm ―1 ) of Bi 2 S 3 results in a light-generated current density (J L ) of >20 mA/cM 2 for a c-Si(200 nm)Bi z S 3 (300 nm) stack at a combined film thickness of 500 nm, and with an open circuit voltage (V oc ) of nearly 600 mV. Proof-of-concept cell structures were prepared on p-type c-Si wafers of electrical resistivity 1 Ω cm. Any oxide layer at the interface significantly deteriorates the cell parameters. In a cell prepared using evaporated n-Bi 2 S 3 on (p) c-Si, J sc is 3 mA/cm 2 ; V oc is 360 mV; and η is 0.5%; which improved to: 7.2 mA/cm 2 , 485 mV and 1.7%, respectively, after heating the cell in forming gas. A cell with an Sb 2 S 3 (40 nm) thin film as an antireflective coating on Bi 2 S 3 , produced: J sc of 10 mA/cm 2 ; V oc of 480 mV; and η of 2.4%. Theoretical simulation suggests that better cell fabrication could lead to: J sc of 26 mA/cm 2 ; V oc of 600 mV; and η of 10%.

PbSe Thin Films in All-Chemically Deposited Solar Cells

We report on PbSe thin films serving as an absorber in solar cell structures, CdS(100 nm)/Sb 2 S 3 (250 nm)/PbSe(100―250 nm). The cells are prepared by sequential chemical deposition of the films on a commercial SnO 2 :F coated sheet glass. These cells show V oc of 690 mV and J sc of 3.5 mA/cm 2 and a conversion efficiency of 0.69% under sunlight. Two distinct routes are taken to deposit PbSe thin films of 100-250 nm thickness: N,N-dimethylselenourea (SU) or sodium selenosulfate (SS) as the source of selenide ions in the bath. These films are p-type with an electrical conductivity of 0.5 (Ω cm) ―1 and optical bandgaps of 0.68 eV (SU) and 0.85 eV (SS) at a film thickness of 150 nm. Without the PbSe absorber, a CdS/Sb 2 S 3 cell has V oc of 595 mV, but its J sc is low, 0.003 mA/cm 2 . In CdS/Sb 2 S 3 /PbSe (SU or SS) cells, J sc is higher by a factor of thousand, and it is consistently higher for a larger PbSe film thickness. Thus, PbSe performs the role of a p + optical absorber in the cell, contributing to the J sc of the cell.