Dissertation/Thesis Abstract

Preparation of Ultra-Thin Cuin1-xGax Se2 Solar Cells and Their Light Absorption Enhancement
by Yin, Guanchao, Ph.D., Technische Universitaet Berlin (Germany), 2015, 140; 27609889
Abstract (Summary)

CuIn1-xGaxSe2 (CIGSe) solar cells are the promising thin-film candidates to compete with the dominant crystalline Si solar cells in the photovoltaic market. One of the major concerns in mass production is the consumption of the rare element Indium and the resultant high manufacturing cost. To achieve the goal of reduced consumption of Indium, one approach is to reduce the thickness of CIGSe absorbers from typical 2-3 μm to below 500 nm. However, the ultra-thin (CIGSe thickness less than 500 nm thick) CIGSe solar cells have failed to maintain their high performance compared to their thick counterparts. Back recombination and incomplete absorption are assumed to be the main reasons for this reduced performance. Therefore, the work in this thesis centers on improving the performance of the ultra-thin CIGSe solar cells by restraining back recombination and improving light absorption.

A reduction in back recombination is achieved using a high back Ga/[Ga+In] ([Ga]/[III]) grading. To create a high back [Ga]/[III] grading, a low substrate temperature (440 °C) is employed for the CIGSe absorber deposition instead of the typical high temperatures above 500 °C. It is discovered that the low substrate temperature 440 °C can reduce the inter-diffusion of Ga-In and thus create a higher back [Ga]/[III] grading compared to the high substrate temperature of 610 °C. This higher back [Ga]/[III] grading is evidenced to both electrically and optically contribute to the efficiency enhancement (an increase of 17.8%) in contrast to the lower back [Ga]/[III] grading at 610 °C for the solar cells with a 460-nm-thick CIGSe layer.

To overcome the incomplete absorption arising from the CIGSe thickness reduction, the implementation of light-trapping structures is indispensable. The effectiveness of these structures is simulated prior to the implementation in order to reduce the experiment effort. Towards this, optical constants of the layers in the solar cells are firstly required. For obtaining accurate optical constants (n, k) or complex refractive index , Transfer-Matrix (TM) method is applied to calculate the optical constants of the individual layer with a focus on CIGSe layers since they determine the optoelectronic properties of solar cells to a great extent. The influence of surface roughness and substrate temperature are particularly investigated. In this work, the TM method is modified to include scalar scattering theory for considering the scattering arising from surface roughness. It is shown that the modified Transfer-Matrix method improves the accuracy of n values in the short wavelength range. Regarding the effect of substrate temperature on the optical constants, it is shown that the temperature has little influence in CGSe. For CIGSe (x = 0.4), the refractive index n for the sample at low temperature (440 °C) stayed relatively unchanged, although the grain size was reduced and the [Ga]/[III] profile was altered compared to that at high temperature (610 °C). In contrast, the extinction coefficient (k) values at 440 °C show higher absorption at long wavelengths due to a lower minimum bandgap (Eg,min) originating from the reduced inter-diffusion of Ga-In. Finally, using TM method, a database of optical constants of CIGSe and other layers in the solar cells in the experiments is established and ready for the optical simulations.

To enhance the absorption of ultra-thin CIGSe solar cells, metallic Ag nanoparticles under Sn:In2O3 (ITO) back contact, closely-packed SiO2 sphere arrays on the surface and SiO2 nanostructures at the interface of Mo/CIGSe, are investigated as the light-trapping structures. It is found that the ITO layer failed to block the diffusion of Ag during CIGSe deposition even at the low substrate temperature (440 °C). A 50-nm-thick Atomic layer deposited (ALD) prepared Al2O3 film is used to passivate the thermal diffusion of Ag nanoparticles. Theoretical optical simulations prove the concept that the Ag nanoparticles are able to greatly enhance the effective absorption in the solar cells. Regarding the closely-packed SiO2 sphere arrays on the surface, it is theoretically demonstrated that large spheres dominate the light absorption in terms of whispering gallery modes and small spheres by forming an effective anti-reflection layer. Due to the anti-reflection effect being more broadband than whispering gallery modes, the maximum absorption enhancement is achieved for the small sphere at a diameter size of 110 nm. Experimentally, the solar cells with a 460-nm-thick absorber gain a photocurrent density enhancement of 2.17 mA/cm2 after coating a 120-nm-diameter SiO2 sphere array, which agrees quite well with the theoretical simulations. SiO2 nanostructures (205 nm in radius, 210 nm in height and 513 nm in pitch) at the interface of CIGSe/Mo are able to scatter the unabsorbed light back into the CIGSe layer via Mie resonances. Simulations confirm that this leads to a significant absorption improvement in the CIGSe layer by reducing the parasitic absorption in Mo, which is considered to be the main parasitic absorption source in ultra-thin CIGSe solar cells. Experiments are in accordance with the simulations, the efficiency increase from 11.0% to 12.4% is mainly due to the photocurrent enhancement from 28.6 mA/cm2 to 30.6 mA/cm2 after incorporating SiO2 nanostructures for the solar cells with a CIGSe thickness of 470 nm. Further with the 120-nm-diameter SiO2 sphere array on the surface, R is restraint and the photocurrent density is further improved to 31.6 mA/cm2 and efficiency to 13.1%. This is the first time that the photocurrent current density is reported to exceed 30 mA/cm2 for ultra-thin CIGSe solar cells.

Indexing (document details)
Advisor: Schmid, Walter
School: Technische Universitaet Berlin (Germany)
School Location: Germany
Source: DAI-C 81/4(E), Dissertation Abstracts International
Subjects: Applied physics
Keywords: Absorption enhancement , Ultra thin
Publication Number: 27609889
ISBN: 9781392884843
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