Single Molecular Precursor Solution for CuIn(S,Se)₂ Thin Films Photovoltaic Cells: Structure and Device Characteristics

A single molecular precursor solution is described for the deposition of CuIn­(S,Se)₂ (CIS) film onto Mo-coated glass substrates by spin coating, followed by annealing in Se atmosphere. Characterization of the films by X-ray diffraction, Raman spectroscopy and scanning electron microscopy demonstrates the formation of a highly homogeneous and compact 1.1 μm thick CIS layer, with a MoSe₂ under-layer. Atomic force microscopy reveals the presence of spherical grains between 400 and 450 nm, featuring surface corrugation in the range of 30 nm. Film composition is found to be in close agreement with that of the precursor solution. Diffuse reflectance spectroscopy shows a direct band gap (<i>E</i>_g) of 1.36 eV. Intensity and temperature dependence photoluminescence spectra show characteristic features associated with a donor–acceptor pair recombination mechanism, featuring activation energy of 34 meV. Over 85 solar cell devices with the configuration Mo/CIS/CdS/i-ZnO/Al:ZnO/Ni–Al and an total area of 0.5 cm² were fabricated and tested. The champion cell shows a power efficiency of 3.4% with an open circuit voltage of 521 mV and short circuit current of 14 mA/cm² under AM 1.5 illumination and an external quantum efficiency above 60%. Overall variation in each of solar cell parameters remains below 10% of the average value, demonstrating the remarkable homogeneity of this solution processing method. To understand the limitation of devices, the dependence of the open-circuit voltage and impedance spectra upon temperature were analyzed. The data reveal that the CuIn­(S,Se)₂/CdS interface is the main recombination pathway with an activation energy of 0.79 eV as well as the presence of two “bulk” defect states with activation energies of 37 and 122 meV. We also estimated that the MoSe₂ under-layer generates back contact barrier of 195 meV

Self-Assembled, Stabilizer-Free ZnS Nanodot Films Using Spray-Based Approaches

A direct self-assembly of high-quality, uncoated ZnS nanodots on a given substrate was obtained using two techniques: the sequential and cyclic spray ion layer gas reaction (spray-ILGAR) as well as the simultaneous and continuous spray chemical vapor deposition (spray-CVD). The spray-ILGAR nanodots are homogeneous in size (3–6 nm), regular in shape, and uniform in composition, while the spray-CVD nanodots are larger and irregular in shape with inclusions of ZnO. By employing these two spray-based techniques, the synthesis of nanodots directly assembled on the substrate surface can be realized in a controlled manner, covering a certain range of compositions, tunable sizes, and controllable interparticle distances. <i>In situ</i> mass spectrometry was implemented in the real-time process in order to achieve better understanding of the intrinsic chemistry involved. We systematically study the influence of the process parameters on the formation of the nanodots and compare the morphology, composition, and property of the obtained nanodots. Based on these investigations, the underlying mechanism that controls the special growth of the nanodots in spray-ILGAR and spray-CVD processes is proposed. It can account for the similarities and differences of these two kinds of nanodots. A passivation/point contact bilayer, composed of the spray-based ZnS nanodots covered by a homogeneous ILGAR In₂S₃ layer, is used as the buffer in the chalcopyrite solar cells, resulting in the cell performance improvement compared to the pure ILGAR In₂S₃ buffer