Highly Efficient Flexible Quantum Dot Solar Cells with Improved Electron Extraction Using MgZnO Nanocrystals Xiaoliang Zhang,† Pralay Kanti Santra,‡ Lei Tian,† Malin B. Johansson,† Håkan Rensmo,‡ and Erik M. J. Johansson*,† †
Department of Chemistry-Ångström, Physical Chemistry and ‡Department of Physics and Astronomy, Molecular and Condensed Matter Physics, Uppsala University, 75120 Uppsala, Sweden S Supporting Information *
ABSTRACT: Colloidal quantum dot (CQD) solar cells have high potential for realizing an efficient and lightweight energy supply for flexible or wearable electronic devices. To achieve highly efficient and flexible CQD solar cells, the electron transport layer (ETL), extracting electrons from the CQD solid layer, needs to be processed at a low-temperature and should also suppress interfacial recombination. Herein, a highly stable MgZnO nanocrystal (MZO-NC) layer is reported for efficient flexible PbS CQD solar cells. Solar cells fabricated with MZONC ETL give a high power conversion efficiency (PCE) of 10.4% and 9.4%, on glass and flexible plastic substrates, respectively. The reported flexible CQD solar cell has the record efficiency to date of flexible CQD solar cells. Detailed theoretical simulations and extensive characterizations reveal that the MZO-NCs significantly enhance charge extraction from CQD solids and diminish the charge accumulation at the ETL/CQD interface, suppressing charge interfacial recombination. These important results suggest that the low-temperature processed MZO-NCs are very promising for use in efficient flexible solar cells or other flexible optoelectronic devices. KEYWORDS: colloidal quantum dot, flexible solar cell, PbS, interfacial recombination, charge transport, charge extraction metal (Au) contact electrode.3,19−21 ZnO nanocrystals (ZnONCs) are generally applied as an ETL in the CQD solar cells. However, during the solar cell operation, the charge carrier collection intensively competes with charge recombination occurring within the CQD solid and at the ETL/CQD interface, greatly affecting the device performance.22,23 Interfacial recombination at the ETL/CQD interface occurs with faster kinetics than the charge recombination within the CQD solid film, especially when the photoelectron drifts or diffuses to/near that interface, limiting the charge extraction and the quasi-Fermi level separation.22 MgZnO (MZO) thin film with tunable energy levels (using different Mg content) was proven to be an effective strategy to diminish the interfacial recombination and therefore improve device photovoltaic performance.24 However, the MZO thin film is generally prepared using a sol−gel method under a high temperature (300−400 °C) to covert the precursors to oxides, which is incompatible with many lightweight flexible substrates used in, for example, a roll-to-roll fabrication approach.24−27
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olloidal quantum dot (CQD) solar cells are believed to be one of the most potential renewable energy technologies for next generation solar cells due to their high efficiency, low-cost, high stability, solution processability, and flexible substrate compatibility.1−8 Rapid progress in improving the power conversion efficiency (PCE) of CQD solar cells was achieved in the past few years by controlling CQD surface chemistry, solar cell architecture, and device physics,9−14 and a PbS CQD solar cell with a PCE over 11% was reported.15 Flexible solar cells are of increasing interest for many potential applications, such as portable or wearable lightweight energy supplying devices. Meanwhile, flexible solar cells can be fabricated with a continuous fabrication approach, for example, a roll-to-roll fabrication method, lowering device cost.16−18 However, so far flexible CQD solar cells show inferior photovoltaic performance compared to traditional CQD solar cells fabricated on glass substrates, and one reason is that it is difficult to find an efficient electron transport layer (ETL) that can be processed at low temperatures (
