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Thickness and Ga content variations in co-evaporated CIGS solar cells with a flat Ga profile: an electrical characteriyation
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Thin film solar cells)ORCID iD: 0000-0003-0741-5068
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Thin film solar cells)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. (Thin film solar cells)ORCID iD: 0000-0003-4111-4613
2014 (English)In: EU PVSEC 2014 Proceedings Papers, Munchen, 2014, p. 1832-1836Conference paper, Published paper (Other academic)
Abstract [en]

In this work an extensive experimental series has been carried out by co-evaporating CIGS layers with varying thickness (0.5, 1.0, 1.5 µm) and varying Ga content (Ga/(Ga+In)=0.15, 0.30, 0.45 and 0.60). In all CIGS layers the Cu concentration has been held constant at Cu/(In+Ga)=0.85. The cells have been characterized with dark and light current voltage measurements, external quantum efficiency measurements and apparent quantum efficiency measurements at negative bias. In agreement with the literature, we observe a distinctively shorter collection length for high Ga concentrations and voltage dependent photo current collection for all cells. Voltage dependent current collection however cannot alone explain our data and the cells need to be described with an illumination dependent diode current or photo current. The generation dependent diode or photo current increase the slope of the light JV curve at negative bias voltage for all solar cells and dominates the slope in cells with 0.5 µm thin absorbers regardless of Ga content. We propose that this behavior is connected to the recombination at the back contact, as it is smaller in the cells with thick absorber layers and since we do not observe the same behavior in back side passivated cells. Keywords: Cu(InGa)Se2, Modelling, Electrical Characterization, Shunting, Ga content, thin absorbers, superposition principle, shifting approximation

Place, publisher, year, edition, pages
Munchen, 2014. p. 1832-1836
Keywords [en]
CIGS; CIGSe; Thin film solar cells; GGI; Ga content; electrical characterization;
National Category
Condensed Matter Physics
Research subject
Engineering Science with specialization in Electronics
Identifiers
URN: urn:nbn:se:uu:diva-390309OAI: oai:DiVA.org:uu-390309DiVA, id: diva2:1341309
Conference
29th European Photovoltaic Solar Energy Conference and Exhibition,22-26 September, 2014, Amsterdam
Part of project
ARCIGS-MAvailable from: 2019-08-08 Created: 2019-08-08 Last updated: 2019-08-29Bibliographically approved
In thesis
1. In the confines of Cu(In,Ga)Se2 thin film solar cells with rear surface passivating oxide layers
Open this publication in new window or tab >>In the confines of Cu(In,Ga)Se2 thin film solar cells with rear surface passivating oxide layers
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The material supply to build renewable energy conversion systems needs to be considered from both a cost and an energy security perspective. For Cu(In,Ga)Se2 (CIGS) thin film solar cells the use of indium in the absorber layer is most problematic. The material input per service unit can be reduced, if the absorber layers are thinned down without a loss in power conversion efficiency.

Thinning down absorber layers can increase the conversion efficiency. However, for real CIGS solar cells absorption losses and recombination rates at the rear surface between the CIGS absorber and the Mo rear contact as well as shunt-like behavior increase. Thus, both rear surface passivation and optical management are essential for maintaining high power conversion efficiencies.

In this work, thin oxide layers, so-called passivation layers, are introduced between the CIGS absorber layer and the Mo contact. They can passivate the CIGS surface, if the CIGS-oxide interface has a lower defect density than the CIGS-Mo interface and/or if they contain a negative fixed oxide charge, which increases the hole concentration and reduces the electron concentration in the CIGS in the vicinity of the oxide.

As these oxides are insulators, electrical conduction through the passivation layer has to be ensured. In this work, nanopoint contacts were etched into ALD-Al2O3 passivation layers in CIGS solar cells. These solar cells had 0.5 -1.5 µm thin absorber layers with a low In content and a high band gap. Ga grading was not used. Although absorber layers with a high Ga content have a short minority carrier diffusion length, a passivation effect could be discerned with the help of external quantum efficiency measurements and current-voltage measurements under varying temperatures in combination with optical and electrical modeling with a two-diode model. Moreover, the possibility of leaving out the additional fabrication step has been explored for ALD-Al2O3 and HfO2 as passivation layers. The results suggest that the passivation layer does not necessarily need to be opened for electrical conduction in an additional fabrication step, if sodium fluoride (NaF) is deposited onto Al2O3 layers prior to CIGS evaporation. In this case solar cells with 215 nm absorber layers and 6 nm thin passivation layers have a power conversion efficiency of 8.6 %, which is 3 % (absolute) higher than the conversion efficiency on a reference. Shunt-like behavior is additionally reduced. For the HfO2 layers photoluminescence data indicate a good passivation effect, but the layers need to be opened up to ensure conduction.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 118
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1834
Keywords
Alkali, Alumina, Back contact, CIGS, CIGSe, Hafnia, Passivation, Rear contact, Sodium fluoride, Ultra-thin
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-390314 (URN)978-91-513-0713-8 (ISBN)
Public defence
2019-09-30, Room 80101, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:30 (English)
Opponent
Supervisors
Available from: 2019-09-09 Created: 2019-08-08 Last updated: 2019-09-17

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Ledinek, DorotheaVermang, BartEdoff, Marika

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