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• Jian, Jingxin; Kumar, Raj & Sun, Jianwu (2020). Cu2O/ZnO p–n Junction Decorated with NiOx as a Protective Layer and Cocatalyst for Enhanced Photoelectrochemical Water Splitting. ACS Applied Energy Materials. 3(11), p. 10408–10414. doi: 10.1021/acsaem.0c01198. Full text in Research Archive
• Kumar, Raj; Bergum, Kristin; Riise, Heine Nygard; Monakhov, Eduard; Galeckas, Augustinas & Svensson, Bengt Gunnar (2020). Impact of post annealing and hydrogen implantation on functional properties of Cu2O thin films for photovoltaic applications. Journal of Alloys and Compounds. ISSN 0925-8388. 825(5), p. 1–8. doi: 10.1016/j.jallcom.2020.153982. Full text in Research Archive Show summary

  Polycrystalline cuprous oxide (Cu2O) thin films were sputtered, annealed (900 °C rapid thermal annealing) and subsequently implanted with various hydrogen ion (H+) doses from 5E13 to 2E15 cm−2 with a low acceleration energy of 36 keV at room temperature to tailor the functional properties of the thin films for solar cell application. The annealed and H+ implanted Cu2O thin films were post annealed at low temperatures from 100 °C to 600 °C in an inert atmosphere to promote hydrogen passivation of prevalent intrinsic acceptors and tune the carrier concentration for optimum performance as an absorption layer in a heterojunction solar cell. The H+ incorporation and post annealing tuned the structural, optical and electrical properties of annealed polycrystalline Cu2O thin films. The results show an enhancement of the excitonic feature around ∼2.0 eV with H+ dose. The normalized photoluminescence (PL) area around ∼1.7 eV was drastically enhanced with increasing H+ doses compared to excitonic and copper vacancy related area. The normalized total PL quantum efficiency shows an enhancement in yield with elevated H+ doses by two orders of magnitude. The hole concentration decreases down to ∼1013 cm−3, while hole mobility and resistivity increase to ∼27 cm2/V and ∼2.4 kΩcm, respectively, as the H+ implantation goes from lower to higher doses. In addition, the post annealing and H+ incorporation lead to a change in the energy level of the major acceptor from 0.21 eV to 0.27 eV above the valence band maximum. By following the qualitative (PL analysis) and quantitative (Hall data) outcomes, we can conclude that H+ implantation and post annealing likely indicates the passivation of both acceptor defects and compensating donor defects.

• Fara, Laurentiu; Chilibon, Irinela; Nordseth, Ørnulf; Craciunescu, Dan; Savastru, Dan & Vasiliu, Cristina [Show all 11 contributors for this article] (2020). Complex investigation of high efficiency and reliable heterojunction solar cell based on an improved Cu2O absorber layer. Energies. 13(18), p. 1–19. doi: 10.3390/en13184667. Full text in Research Archive Show summary

  This study is aimed at increasing the performance and reliability of silicon-based heterojunction solar cells with advanced methods. This is achieved by a numerical electro-optical modeling and reliability analysis for such solar cells correlated with experimental analysis of the Cu2O absorber layer. It yields the optimization of a silicon tandem heterojunction solar cell based on a ZnO/Cu2O subcell and a c-Si bottom subcell using electro-optical numerical modeling. The buffer layer affinity and mobility together with a low conduction band offset for the heterojunction are discussed, as well as spectral properties of the device model. Experimental research of N-doped Cu2O thin films was dedicated to two main activities: (1) fabrication of specific samples by DC magnetron sputtering and (2) detailed characterization of the analyzed samples. This last investigation was based on advanced techniques: morphological (scanning electron microscopy—SEM and atomic force microscopy—AFM), structural (X-ray diffraction—XRD), and optical (spectroscopic ellipsometry—SE and Fourier-transform infrared spectroscopy—FTIR). This approach qualified the heterojunction solar cell based on cuprous oxide with nitrogen as an attractive candidate for high-performance solar devices. A reliability analysis based on Weibull statistical distribution establishes the degradation degree and failure rate of the studied solar cells under stress and under standard conditions.

• Nordseth, Ørnulf; Kumar, Raj; Bergum, Kristin; Chilibon, Irinela; Foss, Sean Erik & Monakhov, Eduard (2019). Nitrogen-Doped Cu2O Thin Films for Photovoltaic Applications. Materials. 12(18). doi: 10.3390/ma12183038. Full text in Research Archive Show summary

  Cuprous oxide (Cu2O) is a p-type semiconductor with high optical absorption and a direct bandgap of about 2.1 eV, making it an attractive material for photovoltaic applications. For a high-performance photovoltaic device, the formation of low-resistivity contacts on Cu2O thin films is a prerequisite, which can be achieved by, for instance, nitrogen doping of Cu2O in order to increase the carrier concentration. In this work, nitrogen-doped p-type Cu2O thin films were prepared on quartz substrates by magnetron sputter deposition. By adding N2 gas during the deposition process, a nitrogen concentration of up to 2.3 × 1021 atoms/cm3 in the Cu2O thin films was achieved, as determined from secondary ion mass spectroscopy measurements. The effect of nitrogen doping on the structural, optical, and electrical properties of the Cu2O thin films was investigated. X-ray diffraction measurements suggest a preservation of the Cu2O phase for the nitrogen doped thin films, whereas spectrophotometric measurements show that the optical properties were not significantly altered by incorporation of nitrogen into the Cu2O matrix. A significant conductivity enhancement was achieved for the nitrogen-doped Cu2O thin films, based on Hall effect measurements, i.e., the hole concentration was increased from 4 × 1015 to 3 × 1019 cm−3 and the resistivity was reduced from 190 to 1.9 Ω⋅cm by adding nitrogen to the Cu2O thin films.

• Kumar, Raj; Nordseth, Ørnulf; Vasquez, Geraldo Cristian; Foss, Sean Erik; Monakhov, Eduard & Svensson, Bengt Gunnar (2019). Investigation of n-Al:ZnO/p-Cu2O heterojunction for c-Si tandem heterojunction solar cell applications. AIP Conference Proceedings. ISSN 0094-243X. 2147(1). doi: 10.1063/1.5123885. Full text in Research Archive Show summary

  In this work, an in-situ growth approach has been employed to fabricate Al:ZnO/Cu2O/Cu and Al:ZnO/ZnO/Cu2O/Cu heterojunctions using direct current (DC) and radio frequency (RF) magnetron sputtering technique in a controlled atmospheric condition. The effect of ZnO buffer layer thickness (30 and 50 nm) as well as in-situ Cu2O annealing at 600 °C in low vacuum (~ 10−6 Torr) prior to Al:ZnO deposition were studied. The carrier density of Al:ZnO was ~2 × 1020 cm3 with mobility ~8 cm2/V·s and resistivity ~1 × 10−3. Ω·cm, while the carrier density of Cu2O was 1 × 1015 cm3 with mobility ~19 cm2/V·s and resistivity ~200 .Ω·cm. The heterojunctions were investigated by depth resolved Cathodoluminescence (CL) spectroscopy at 80 K to analyze the influence of defects at the interface. The two emissions at 1.51 eV (Vo+) and 1.69 eV (Vo2+) dominate in all CL spectra related to oxygen vacancy defects in Cu2O. The relative intensity of the defect luminescence band Vo+ respect to Vo2+ is greater at the interface compared to bulk Cu2O, while the incorporation of the ZnO layer reduces significantly both radiative recombination and the Vo2+ related emission at the interface.

• Dumitru, Constantin; Fara, Laurentiu; Nordseth, Ørnulf; Chilibon, Irinela; Kumar, Raj & Svensson, Bengt Gunnar [Show all 10 contributors for this article] (2018). Electro-optical analysis and numerical modeling of Cu <inf>2</inf> O as The absorber layer in advanced solar cells, 2018 International Conference on Photovoltaic Science and Technologies (PVCon) 4-6 July 2018. IEEE (Institute of Electrical and Electronics Engineers). ISSN 9781538675380. doi: 10.1109/PVCon.2018.8524000. Full text in Research Archive
• Nordseth, Ørnulf; Kumar, Raj; Bergum, Kristin; Fara, Laurentiu; Dumitru, Constantin & Craciunescu, Dan [Show all 11 contributors for this article] (2018). Metal oxide thin-film heterojunctions for photovoltaic applications. Materials. 11(12), p. 1–10. doi: 10.3390/ma11122593. Full text in Research Archive Show summary

  Silicon-based tandem solar cells incorporating low-cost, abundant, and non-toxic metal oxide materials can increase the conversion efficiency of silicon solar cells beyond their conventional limitations with obvious economic and environmental benefits. In this work, the electrical characteristics of a metal oxide thin-film heterojunction solar cell based on a cuprous oxide (Cu2O) absorber layer were investigated. Highly Al-doped n-type ZnO (AZO) and undoped p-type Cu2O thin films were prepared on quartz substrates by magnetron sputter deposition. The electrical and optical properties of these thin films were determined from Hall effect measurements and spectroscopic ellipsometry. After annealing the Cu2O film at 900 °C, the majority carrier (hole) mobility and the resistivity were measured at 50 cm2/V·s and 200 Ω·cm, respectively. Numerical modeling was carried out to investigate the effect of band alignment and interface defects on the electrical characteristics of the AZO/Cu2O heterojunction. The analysis suggests that the incorporation of a buffer layer can enhance the performance of the heterojunction solar cell as a result of reduced conduction band offset.

• Brun, Henrik; Bugge, Robin Antony Birkeland; Suther, Lene Kathrine R; Birkeland, Sigurd; Kumar, Raj & Pelanis, Egidijus [Show all 7 contributors for this article] (2018). Mixed reality holograms for heart surgery planning: first user experience in congenital heart disease. European Heart Journal-Cardiovascular Imaging. ISSN 2047-2404. 0, p. 1–6. doi: 10.1093/ehjci/jey184. Full text in Research Archive Show summary

  Aims: Proof of concept and feasibility study for preoperative diagnostic use of mixed reality (MR) holograms of individual 3D heart models from standard cardiac computed tomography angiograms (CTA) images. Optimal repair for complex congenital heart disease poses high demands on 3D anatomical imagination. Three-dimensional printed heart models are increasingly used for improved morphological understanding during surgical and interventional planning. Holograms are a dynamic and interactive alternative, probably with wider applications. Methods and results: A 3D heart model was segmented from CTA images in a patient with double outlet right ventricle and transposition of the great arteries (DORV-TGA). The hologram was visualized in the wearable MR platform HoloLens® for 36 paediatric heart team members who filled out a diagnostic and quality rating questionnaire. Morphological and diagnostic output from the hologram was assessed and the 3D experience was evaluated. Locally developed app tools such as hologram rotation, scaling, and cutting were rated. Anatomy identification and diagnostic output was high as well as rating of 3D experience. Younger and female users rated the app tools higher. Conclusion: This preliminary study demonstrates that MR holograms as surgical planning tool for congenital heart disease may have a high diagnostic value and contribute to understanding complex morphology. The first users experience of the hologram presentation was found to be very positive, with a preference among the female and the younger users. There is potential for improvement of the hologram manipulation tools.

• Chilibon, Irinela; Fara, Laurentiu; Nordseth, Ørnulf; Kumar, Raj; Svensson, Bengt Gunnar & Dumitru, Constantin [Show all 10 contributors for this article] (2018). Characterization of Cu2O thin films used in solar cells by fluorescence and FTIR spectroscopy. Annals of the Academy of Romanian Scientists Series on Science and Technology of Information. ISSN 2066-8562. 11(1), p. 61–70. Full text in Research Archive
• Chilibon, Irinela; Fara, Laurentiu; Nordseth, Ørnulf; Kumar, Raj; Svensson, Bengt Gunnar & Dumitru, Constantin [Show all 10 contributors for this article] (2018). Structural and electrical analyses of Cu2O layers for solar cells applications. Annals of the Academy of Romanian Scientists Series on Science and Technology of Information. ISSN 2066-8562. 11(1), p. 53–60. Full text in Research Archive
• Nordseth, Ørnulf; Kumar, Raj; Bergum, Kristin; Fara, Laurentiu; Foss, Sean Erik & Haug, Halvard [Show all 15 contributors for this article] (2017). Optical Analysis of a ZnO/Cu2O Subcell in a Silicon-Based Tandem Heterojunction Solar Cell. Green and Sustainable Chemistry. ISSN 2160-6951. 7(1). doi: 10.4236/gsc.2017.71005. Full text in Research Archive

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• Kumar, Raj; Nordseth, Ørnulf; Foss, Sean Erik; Vasquez, Geraldo Cristian; Monakhov, Eduard & Svensson, Bengt Gunnar (2019). Investigation of n-Al:ZnO/p-Cu2O Heterojunction for c-Si Tandem Heterojunction Solar Cell Applications. Full text in Research Archive
• Nordseth, Ørnulf; Kumar, Raj; Bergum, Kristin; Fara, Laurentiu; Chilibon, I.la & Foss, S. E. [Show all 8 contributors for this article] (2017). Silicon-Based Tandem Solar Cells Boosting the Performance Using Non-Toxic Metal Oxides. GIT Laboratory Journal Europe. ISSN 1611-6038. Full text in Research Archive
• Nordseth, Ørnulf; Fara, Laurentiu; Kumar, Raj; Foss, S. E.; Dumitru, C. & Muscurel, V.-F. [Show all 17 contributors for this article] (2017). Electro-Optical Modeling of a ZnO/Cu2O Subcell in a Silicon-Based Tandem Heterojunction Solar Cell. Full text in Research Archive
• Nordseth, Ørnulf; Foss, S. E.; Chilibon, I.la; Vasiliu, C.; Iordanescu, R. & Baschir, L. [Show all 13 contributors for this article] (2017). Cu2O Photosensitive Thin Films for Solar Cell Application. Full text in Research Archive
• Kumar, Raj; Bergum, Kristin; Riise, Heine Nygard; Azarov, Alexander; Monakhov, Edouard & Svensson, Bengt Gunnar (2017). Impact of post deposition annealing on electrical properties of Cu2O thin films. Full text in Research Archive
• Kumar, Raj; Bergum, Kristin; Riise, Heine Nygard; Monakhov, Edouard & Svensson, Bengt Gunnar (2017). Electrical and optical characterization of p type Cu2O thin film for solar cell applications. Full text in Research Archive
• Kumar, Raj; Bergum, Kristin; Riise, Heine Nygard; Monakhov, Edouard & Svensson, Bengt Gunnar (2017). Impact of post annealing on hydrogen implanted Cu2O thin film for solar cell applications. Full text in Research Archive

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