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Glancing angle deposited ITO films for efficiency enhancement of a-Si:H/μc-Si:H tandem thin film solar cells

Jung Woo Leem and Jae Su Yu

Open Access Open Access

Abstract

Indium tin oxide (ITO) thin films with relatively high transparency and low absorption are prepared by glancing angle deposition (GLAD) method and their effect on the device performance of a-Si:H/μc-Si:H tandem thin film solar cells is theoretically investigated by applying the experimentally measured physical data of the fabricated films to the simulation parameters. The GLAD of ITO produces inclined porous columnar nanostructures due to the atomic shadowing effect. With increasing the incident flux angle, the columns are increasingly inclined, thus resulting in the improved transmission property as well as the decrease of the refractive index and extinction coefficient because of enhanced porosity within the film. Furthermore, the antireflection characteristics are improved over a wide wavelength range of 300-1100 nm. For a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the 0° ITO/80° ITO bi-layer structure, the conversion efficiency (η) of 13.6% is obtained from simulation under AM1.5g illumination, indicating an efficiency improvement compared to the device with the 0° ITO/0° ITO bi-layer structure (i.e. η = 12.58%).

©2011 Optical Society of America

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Figures (8)
Fig. 1
Fig. 1 Schematic diagram for depositing the ITO films fabricated by GLAD method.

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Fig. 2
Fig. 2 XRD patterns of the ITO films deposited on glass substrate for different incident flux angles.

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Fig. 3
Fig. 3 Top-view and cross-sectional SEM images of the deposited ITO films on Si substrate at incident flux angles of (i) θα = 0°, (ii) θα = 40°, (iii) θα = 60°, and (iv) θα = 80°, respectively.

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Fig. 4
Fig. 4 Measured (a) refractive index and (b) extinction coefficient of the GLAD ITO films on Si substrate in the wavelength range of 350-1100 nm. The inset of (a) shows the measured refractive index and the estimated relative porosity of the GLAD ITO films versus incident flux angle at 633 nm. The inset of (b) shows the (αhν)2 versus hν plots of GLAD ITO films.

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Fig. 5
Fig. 5 Optical transmittance spectra of the GLAD ITO films on glass substrate for different incident flux angles. The insets show the simulation model (upper) and calculated transmittance spectra (lower) of corresponding film structures.

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Fig. 6
Fig. 6 (a) Resistivity and sheet resistance and (b) measured reflectance spectra of the 0° ITO (200 nm)/GLAD ITO bi-layer structures as a function of incident flux angle. The insets of (a) show the SEM images (upper) of the 0° ITO/0° ITO and 0° ITO/80° ITO bi-layer structures, respectively, and the carrier concentration and Hall mobility (lower) of the corresponding films at different incident flux angles. The insets of (b) show the simulation model (left) and calculated reflectance spectra (right) of the corresponding film structures.

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Fig. 7
Fig. 7 (a) Jsc versus the intrinsic layer thickness in the top a-Si:H cell under AM1.5g illumination and (b) schematic diagram of optimized a-Si:H/μc-Si:H tandem thin film solar cell structure with the 0° ITO/GLAD ITO bi-layer structures as a TCO layer used in this simulation.

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Fig. 8
Fig. 8 (a) J-V characteristics and (b) EQE spectra of the optimized a-Si:H/μc-Si:H tandem thin film solar cells with the 0° ITO/GLAD ITO bi-layer structures as a TCO layer under AM1.5g illumination. The inset of (a) shows the photogeneration rate of optimized a-Si:H/μc-Si:H tandem thin film solar cells with the 0° ITO/80° ITO bi-layer structure for incident light of 400 nm and 800 nm wavelengths under AM1.5g illumination.

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Tables (2)

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Table 1 Physical Parameters of Main Materials Used for this Simulation

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Table 2 Parameters of a-Si:H/μc-Si:H Tandem Thin Film Solar Cells with 0° ITO/GLAD ITO Bi-Layer Structures at Different Incident Flux Angles under AM1.5g Illumination

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