Thermal Annealing Effects of Plasmonic Cu_1.8S Nanocrystal Films and Their Photovoltaic Properties

Abstract

Colloidal Cu 2−x S nanocrystals are potential abundant, low-cost, and environment-friendly candidates for photovoltaic and photothermal applications. The fabrication of high-quality nanocrystal films through a solution process is a key step toward the exploration of their applications. In this work, we fabricated high-quality Cu 1.8 S nanocrystal films, characterized their phase transformation under thermal annealing treatments, and investigated the evolution of the corresponding optical and electrical properties. It was demonstrated that the Cu 1.8 S nanocrystal films undergo a phase transformation from metastable rhombohedral phase to stable tetragonal phase (Cu 2 S) after annealing at a temperature higher than 240 °C, which is much lower than that of the bulk materials (544 °C). Along with the transformation, both optical and conductivity properties exhibit well-defined evolution from nonstoichiometric semiconductor to stoichiometric semiconductor, which can be interpreted through a combined electronic structure analysis and theoretical modeling. The correlations between the crystal structure, composition, optical and electrical properties enable us to gain further insights into the structure−property relationship in Cu 2−x S nanocrystals. More importantly, a highly conductive Cu 2−x S nanocrystal film with electrical conductivity up to 6.7 S/cm was obtained, implying the potential to be used as conductive electrodes. We further integrated the annealed Cu 2−x S nanocrystal films into a photovoltaic device by adopting a FTO/TiO 2 /Cu 2−x S:CdS/MoO 3 /Au structure, and a preliminary power conversion efficiency of 0.24% was achieved.

Phase Transformations of Copper Sulfide Nanocrystals: Towards Highly Efficient Quantum‐Dot‐Sensitized Solar Cells

“…Previously reported results have also shown that Cu 1.8 SN Cs transform from the rhombohedral Cu 1.8 Sp haset ot he tetragonal Cu 2 Sp hase at 240 8C. [41] It is also noted that the phase transformationi nh exagonal CuS NCs is similar to that in their bulk phase. [44,46,47] Figure 2b shows the evolution of the XRD pattern of the monoclinic Cu 1.75 Sp hase upon heating from room temperature to 500 8C.…”

“…Details aboutt he evolution of the crystal structure and composition of rhombohedral Cu 1.8 S NCs after thermal annealing can be found in our previousr eports. [41] The above findings suggest that the crystal structures of the Cu 2Àx SN Cs change from low symmetry to high symmetry upon heating, and that their compositions change towards higherC u/S molar ratios. In detail, the crystal structure changed from ah exagonal toatetragonal framework, andt he Cu content increased, owing to al oss of sulfur.T hermodynamics plays an important role in determining the phase-transformationprocess.…”

“…our previous work. [41] Here, we mainly focus on the transformation of hexagonal CuS and monoclinic Cu 1.75 SNCs. Figure 1a,b shows the TEMi mages of the as-synthesized CuS and Cu 1.75 SN Cs.…”

Plasmonic doped semiconductor nanocrystals: Properties, fabrication, applications and perspectives

“…[294] Copper chalcogenide NCs were also used as high conductive electrodes and in solar cells to improve the charge transport. [138,138,[295][296][297][298][299][300][301][302][303] In a recent work, a layer of CuS NCs was used as an inorganic hole-selective layer in inverted planar perovskite solar cells. [304] In the inverted device usually two planar charge transport layers selectively pass electrons and holes created in the perovskite light- Matsui et al [308] showed that the three dimensional assembly of surface-modified ITO NCs leads to plasmon coupling induced high reflectance performance in the near-and mid-IR range due to interparticle plasmon coupling on large-size flexible substrates.…”

Incorporation of High-Mobility and Room-Temperature-Deposited CuxS as a Hole Transport Layer for Efficient and Stable Organo-Lead Halide Perovskite Solar Cells

“…Various mineral forms of Cu x S (1 ≤ x ≤2) exist, in which the Cu atoms change as a function of x . The copper sulfide family is composed of various solid‐phase materials, such as chalcocite Cu 2 S, djurleite Cu 1.96 S, digenite Cu 1.8 S, anilite Cu 1.75 S, covellite CuS, and others . All of these phases are p‐type semiconductors owing to Cu vacancies within the lattice.…”