Development of reflective back contacts for high-efficiency ultrathin Cu(In,Ga)Se2 solar cells

Gouillart, Louis; Cattoni, Andréa; Goffard, Julie; Donsanti, Frédérique; Patriarche, G.; Jubault, Marie; Naghavi, Negar; Collin, Stéphane

doi:10.1016/j.tsf.2018.12.041

Cited by 25 publications

(30 citation statements)

References 24 publications

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“…Besides, the annealing of the RBC resulted in a significant enhancement of the RBC reflectivity for wavelengths above 600 nm. This improvement may be attributed to a modification of ITO optical indices upon annealing, as determined by ellipsometry in a previous study 30 . The annealed RBC reaches an average reflectance of 92.6% for wavelengths above 500 nm.…”

Section: Resultssupporting

confidence: 53%

“…The optical indices of CIGS were first determined by ellipsometry 34 and subsequently refined close to the bandgap thanks to photocurrent Fourier‐transform spectroscopy measurements, and the thickness of the simulated CIGS layers was fixed at its experimental average value of 510 nm. The optical indices of ITO were also derived from ellipsometry data described in a previous study 30 . More information about this optical model and the optical simulations performed in this study can be found in reference Goffard et al 11 …”

Section: Methodsmentioning

confidence: 99%

“… the formation of a detrimental Ga oxide layer at the CIGS/TCO back interface, 26–30 which is promoted when an external supply of Na is used 31,32 ; the diffusion of metallic elements from the back contact to the absorber 21,24 …”

Section: Introductionmentioning

confidence: 99%

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Interface engineering of ultrathin Cu(In,Ga)Se₂ solar cells on reflective back contacts

Gouillart

Cattoni

Chen

et al. 2020

Progress in Photovoltaics

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Cu(In,Ga)Se2‐based (CIGS) solar cells with ultrathin (≤500 nm) absorber layers suffer from the low reflectivity of conventional Mo back contacts. Here, we design and investigate ohmic and reflective back contacts (RBC) made of multilayer stacks that are compatible with the direct deposition of CIGS at 500°C and above. Diffusion mechanisms and reactions at each interface and in the CIGS layer are carefully analyzed by energy dispersive X‐ray (EDX)/scanning transmission electron microscopy (STEM). It shows that the highly reflective silver mirror is efficiently encapsulated in ZnO:Al layers. The detrimental reaction between CIGS and the top In2O3:Sn (ITO) layer used for ohmic contact can be mitigated by adding a 3 nm thick Al2O3 layer and by decreasing the CIGS coevaporation temperature from 550°C to 500°C. It also improves the compositional grading of Ga toward the CIGS back interface, leading to increased open‐ circuit voltage and fill factor. The best ultrathin CIGS solar cell on RBC exhibits an efficiency of 13.5% (+1.0% as compared to our Mo reference) with a short‐circuit current density of 28.9 mA/cm2 (+2.6 mA/cm2) enabled by double‐pass absorption in the 510 nm thick CIGS absorber. RBC are easy to fabricate and could benefit other photovoltaic devices that require highly reflective and conductive contacts subject to high temperature processes.

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Section: Resultssupporting

confidence: 53%

Section: Methodsmentioning

confidence: 99%

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Interface engineering of ultrathin Cu(In,Ga)Se₂ solar cells on reflective back contacts

Gouillart

Cattoni

Chen

et al. 2020

Progress in Photovoltaics

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“…[ 14 ] Recently, ref. [15] showed an experimental gain in short‐circuit current density of 4.9 mA cm − 2 in a structure Mo/Ag/ITO/0.5 μm CIGSe with respect to the reference structure of Mo/0.5 μm CIGSe. Al/ITO‐based solar cells can be prepared up to at least 600 °C without Al diffusion inside the absorber while providing high reflectivity at long optical wavelengths.…”

Section: Introductionmentioning

confidence: 91%

Ultrathin CIGSe Solar Cells with Integrated Structured Back Reflector

et al. 2020

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Reducing the thickness of thin-film solar cells reduces the total cost of ownership of solar module production. Theoretically, a 500 nm Cu(In,Ga)Se 2 (CIGSe) solar cell allows more than 20% efficiency. [1-4] However, this requires a very low back contact recombination velocity and advanced optical light management. The standard molybdenum back contact is generally attributed with a high recombination velocity [5-7] and has a poor optical reflectivity. [8] In ref. [9], a ZrN layer on top of molybdenum having a reflectivity of 60% was used with the result of increased quantum efficiency at long wavelengths. An even higher reflectivity and higher gain in photocurrent could be achieved by an Au back reflector applied by post-deposition recontacting after removing the Mo layer. [10] In addition to cost aspects, Au is not suitable for direct deposition of CIGSe thin films at high temperatures due to the chemical reactivity with the chalcogen species during CIGSe deposition. The latter is also true for other highly reflective metals such as Ag, Cu, and Al. [11] Instead, these metals have to be encapsulated by a transparent and conductive diffusion barrier layer. Bissig et al. investigated an Al/InZnO back contact for solar cells with an absorber thickness of 2.1 μm and were able to increase the short-circuit density up to 1.4 mA cm À2 compared with a sample with Mo back contact, despite the relatively high absorber thickness. [12] According to former work on transparent back contacts, the transparent conductor indium-tin-oxide (ITO) is a good candidate as a transparent back contact [13] and as a diffusion barrier. [14] Recently, ref. [15] showed an experimental gain in short-circuit current density of 4.9 mA cm À2 in a structure Mo/Ag/ITO/0.5 μm CIGSe with respect to the reference structure of Mo/0.5 μm CIGSe. Al/ITO-based solar cells can be prepared up to at least 600 C without Al diffusion inside the absorber while providing high reflectivity at long optical wavelengths. [16] This raises the question of the electronic properties of the ITO/CIGSe contact. Keller et al. investigated 650 nm CIGSe bifacial solar cells with a hydrogen-doped In 2 O 3 back contact [17] and quantified the back-contact recombination velocity to the range of 10 7 cm s À1. This value dropped by application of an Al 2 O 3 þ NaF precursor layer stack as confirmed by a strongly increased rear-side external quantum efficiency (EQE) at short wavelengths. Unfortunately, the 5 nm Al 2 O 3 conformal layer reduces the fill factor (FF). [17] Although a flat optical reflector enhances the long-wavelength EQE upon front-side illumination to some extent, light scattering is required in addition to approach the Yablonovitch limit. [18] Yin et al. used ITO as transparent back contact and applied both dielectric SiO 2 nanoparticles on top of ITO and an external Ag mirror behind the substrate glass achieving a very high short-circuit current density of 32.4 mA cm À2 with an absorber thickness of only 390 nm. [19] This was an important step toward a structured an...

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“…To minimize this effect thin Ag layer under these openings can be etched away and Mo takes the role of the local contact point. An alternative to prevent Ag diffusion into CIGS is to use TCO layer as Ag cover [18]. TCO layers (e.g.…”

Section: Device Structure and Texturesmentioning

confidence: 99%

Modelling Supported Design of Light Management Structures in Ultra-Thin Cigs Photovoltaic Devices

2019

Informacije MIDEM

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Chalcopyrite solar cells exhibit one of the highest conversion efficiencies among thin-film solar cell technologies (> 23.3%), however a considerably thick absorber ≥1.8 µm is required for an efficient absorption of the long-wavelength light and collection of charge carriers. In order to minimize the material consumption and to accelerate the fabrication process, further thinning down of the absorber layer is important. Using a thin absorber layer results in a highly reduced photocurrent density and to compensate for it an effective light management needs to be introduced. Experimentally supported, advanced optical simulations in a PV module configuration, i.e. solar cell structure including the encapsulation and front glass are employed to design solutions to increase the short current density of devices with ultra-thin (500 nm) absorbers. In particular (i) highly reflective metal back reflector (BR), (ii) internal nano-textures and (iii) external textures by applying a light management (LM) foil are investigated by simulations. Experimental verification of simulation results is presented for the external texture case. In the scope of this contribution we show that any individual aforementioned approach is not sufficient to compensate for the short circuit current drop of the thin CIGS, but only a combination of highly reflective back contact and introduction of textures (internal or external) is able to compensate and also to exceed (by more than 5 % for internal texture) photocurrent density of a thick (1800 nm) CIGS absorber.

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Development of reflective back contacts for high-efficiency ultrathin Cu(In,Ga)Se2 solar cells

Cited by 25 publications

References 24 publications

Interface engineering of ultrathin Cu(In,Ga)Se₂ solar cells on reflective back contacts

Interface engineering of ultrathin Cu(In,Ga)Se₂ solar cells on reflective back contacts

Ultrathin CIGSe Solar Cells with Integrated Structured Back Reflector

Modelling Supported Design of Light Management Structures in Ultra-Thin Cigs Photovoltaic Devices

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Development of reflective back contacts for high-efficiency ultrathin Cu(In,Ga)Se2 solar cells

Cited by 25 publications

References 24 publications

Interface engineering of ultrathin Cu(In,Ga)Se2 solar cells on reflective back contacts

Interface engineering of ultrathin Cu(In,Ga)Se2 solar cells on reflective back contacts

Ultrathin CIGSe Solar Cells with Integrated Structured Back Reflector

Modelling Supported Design of Light Management Structures in Ultra-Thin Cigs Photovoltaic Devices

Contact Info

Product

Resources

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Interface engineering of ultrathin Cu(In,Ga)Se₂ solar cells on reflective back contacts

Interface engineering of ultrathin Cu(In,Ga)Se₂ solar cells on reflective back contacts