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- Platzer Björkman, Charlotte, 1976-, et al.
(författare)
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Ultrathin wide band gap kesterites
- 2022
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Ingår i: Faraday discussions. - : Royal Society of Chemistry. - 1359-6640 .- 1364-5498. ; 239, s. 38-50
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Tidskriftsartikel (refereegranskat)abstract
- Kesterite Cu2ZnSnS4 (CZTS), used for thin film solar cells, has a band gap energy around 1.5–1.6 eV with possibilities for further increase through alloying. In some applications for wide band gap solar cells, reduced absorber thickness can be beneficial, to allow partial light transmission. Reduced thickness can also be beneficial to reduce bulk recombination, and so called ultrathin solar cells (<700 nm thick) have been studied for several materials systems. Here, we report performance for CZTS devices down to 250 nm thickness and show that performance loss from thickness reduction is relatively small, partly due to short minority carrier diffusion length. Insertion of thin passivation layers (Al2O3, SiO2 or HfO2) at the Mo/CZTS interface gives improved performance of ultrathin devices, from 4.7% to 5.6% efficiency for best performing cells having 250 nm thick CZTS with Mo as compared to Mo/Al2O3 back contact. The approach of NaF post deposition for making isolating passivation layers conductive is tested for the first time for CZTS and is shown to work. For fabrication of CZTS devices on transparent ITO back contact, the insertion of passivation layers can reduce diffusion of indium into CZTS, but device performance is lower than on Mo back contacts.
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| 2. |
- Platzer Björkman, Charlotte, et al.
(författare)
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Zn(O,S) buffer layers by atomic layer deposition in Cu(In,Ga)Se-2 based thin film solar cells : Band alignment and sulfur gradient
- 2006
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 100:4, s. 044506-
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Tidskriftsartikel (refereegranskat)abstract
- Thin film solar cells with the structure soda lime glass/Mo/Cu(In,Ga)Se-2/Zn(O,S)/ZnO/ZnO:Al are studied for varying thickness and sulfur content of the Zn(O,S) buffer layer. These Zn(O,S) layers were deposited by atomic layer deposition (ALD) at 120 degrees C. Devices with no or small concentrations of sulfur in the buffer layer show low open-circuit voltages. This is explained by the cliff, or negative conduction-band offset (CBO), of -0.2 eV measured by photoelectron spectroscopy (PES) and optical methods for the Cu(In,Ga)Se-2 (CIGS)/ZnO interface. Devices with ZnS buffer layers exhibit very low photocurrent. This is expected from the large positive CBO (spike) of 1.2 eV measured for the CIGS/ZnS interface. For devices with Zn(O,S) buffer layers, two different deposition recipes were found to yield devices with efficiencies equal to or above reference devices in which standard CdS buffer layers were used; ultrathin Zn(O,S) layers with S/Zn ratios of 0.8-0.9, and Zn(O,S) layers of around 30 nm with average S/Zn ratios of 0.3. The sulfur concentration increases towards the CIGS interface as revealed by transmission electron microscopy and in vacuo PES measurements. The occurrence of this sulfur gradient in ALD-Zn(O,S) is explained by longer incubation time for ZnO growth compared to ZnS growth. For the Zn(O,S) film with high sulfur content, the CBO is large which causes blocking of the photocurrent unless the film is ultrathin. For the Zn(O,S) film with lower sulfur content, a CBO of 0.2 eV is obtained which is close to ideal, according to simulations. Efficiencies of up to 16.4% are obtained for devices with this buffer layer.
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