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This work was financially supported by Universidad Industrial de Santander-Colciencias, project 110265843664 (VIE 8836). Andres F. Gualdron-Reyes acknowledges Colciencias for a PhD 617-scholarship. The authors are grateful to Jhonatan Rodriguez and Laboratorio de Superficies-SurfLab for spending valuable time in the XPS analysis.

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Hidden energy levels? Carrier transport ability of CdS/CdS1-xSex quantum dot solar cells impacted by Cd-Cd level formation

Publicat a:Nanoscale. 11 (2): 762-774 - 2019-01-14 11(2), DOI: 10.1039/c8nr07073c

Autors: Gualdron-Reyes, Andres F; Melendez, Angel M; Tirado, Juan; Alejandro Mejia-Escobar, Mario; Jaramillo, Franklin; Nino-Gomez, Martha E

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In quantum dot sensitized solar cells (QDSSC), a cascade energy level structure controlled by assembly of cadmium-chalcogenide quantum dots can remarkably improve the sunlight harvesting and charge carrier lifetime. Despite the advantages of using co-sensitizers, energy conversion efficiencies are still low. An increased understanding of the causes of the low photoconversion efficiency (PCE) will contribute to the development of a straightforward approach to improve solar cell performance by exploiting co-sensitization. Herein we discuss how an excess of cadmium causes structural disorder and defect levels impacting the PCE of QDSSC devices. Thus, outer CdS1-xSex/inner CdS QD-co-sensitized B,N,F-co-doped-TiO2 nanotubes (BNF-TNT) were prepared. Chalcogenides were deposited by the SILAR method on BNF-TNT, varying the load of CdS as the inner sensitizer, while for CdS1-xSex, five SILAR cycles were used (5-CdS1-xSex), controlling the nominal S/Se molar ratio of the ternary alloy. Cd defects named as Cd-Cd energy levels were observed during CdS sensitization. Although incorporation of outer CdS1-xSex provides a tunable band gap to achieve good band alignment for carrier separation, Cd-Cd energy levels in the sensitizers act as recombination centers, limiting the overall electron flow at the BNF-TNT/CdS/CdS1-xSex interface. A maximum PCE of 2.58% was reached under standard AM 1.5G solar illumination at 100 mW cm(-2). Additional limitations of SILAR as a deposition strategy of QDs are also found to influence the PCE of QDSSC.

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