Uncovering the role of the ZnS treatment in the performance of quantum dot sensitized solar cells

Abstract

Among the third-generation photovoltaic devices, much attention is being paid to the so-called Quantum Dot sensitized Solar Cells (QDSCs). The currently poor performance of QDSCs seems to be efficiently patched by the ZnS treatment, increasing the output parameters of the devices, albeit its function remains rather unclear. Here new insights into the role of the ZnS layer on the QDSC performance are provided, revealing simultaneously the most active recombination pathways. Optical and AFM characterization confirms that the ZnS deposit covers, at least partially, both the TiO(2) nanoparticles and the QDs (CdSe). Photoanodes submitted to the ZnS treatment before and/or after the introduction of colloidal CdSe QDs were studied by electrochemical impedance spectroscopy, cyclic voltammetry and photocurrent experiments. The corresponding results prove that the passivation of the CdSe QDs rather than the blockage of the TiO(2) surface is the main factor leading to the efficiency improvement. In addition, a study of the ultrafast carrier dynamics by means of the Lens-Free Heterodyne Detection Transient Grating technique indicates that the ZnS shell also increases the rate of electron transfer. The dual role of the ZnS layer should be kept in mind in the quest for new modifiers for enhancing the performance of QDSCs.

Efficient passivated phthalocyanine-quantum dot solar cells

“…Passivation of these states increases device IPCE and photocurrent. 19 In addition, electrons photoinjected into TiO 2 can also recombine through these QD surface states. This kind of recombination of electrons in the TiO 2 will move down the TiO 2 Fermi level and, consequently, the cell photovoltage.…”

“…QDspresent surface states that may play an important role in the recombination process and, consequently, in the final device performance. 19,20 Photoexcited electrons and holes in QDs can recombine through QD surface states before being injected reducing the expected device photocurrent. 19 On the other hand, electrons photoinjected into the TiO 2 can also recombine before they arrive to the extracting contact.…”

“…19,20 Photoexcited electrons and holes in QDs can recombine through QD surface states before being injected reducing the expected device photocurrent. 19 On the other hand, electrons photoinjected into the TiO 2 can also recombine before they arrive to the extracting contact. Again, surface states in QDs can favor this recombination that reduce the device open circuit potential.…”

“…21 In this sense, efficient passivation of QD surface state is an strategy commonly employed to enhance QDSCs performance. 19,21,22 Phthalocyanines (Pcs) are aromatic macroheterocycles which have been used successfully in DSSCs because they absorb intensely in the red spectral region, are able to inject into titanium oxide and are thermal and photochemicallyverystable compounds. 23 Theirtendency to form aggregates on the TiO 2 surface, which reduces the efficiency of solar cells, can be countered with the introduction ofbulky groups in the peripheral 24 or apical 25 positions going to efficiencies up to 6.5%.…”

Drastic difference between hole and electron injection through the gradient shell of CdxSeyZn1−xS1−y quantum dots

“…They have high extinction coefficient, size-tunable optical band gap, and show multiple exciton generation opening the possibility to break the Shockley-Queisser limit. [1][2][3][4][5][6][7][8] Recently, Sargent and coworkers have achieved ∼11% solar power conversion efficiency using PbS/ZnO QDs via solvent-polarity-engineered halide passivation. 9,10 The long-term photo-stability of the QDs is a critical precondition for their photovoltaic applications.…”