Impact of Nanocrystal Spray Deposition on Inorganic Solar Cells

Solution-synthesized inorganic cadmium telluride nanocrystals (∼4 nm; 1.45 eV band gap) are attractive elements for the fabrication of thin-film-based low-cost photovoltaic (PV) devices. Their encapsulating organic ligand shell enables them to be easily dissolved in organic solvents, and the resulting solutions can be spray-cast onto indium–tin oxide (ITO)-coated glass under ambient conditions to produce photoactive thin films of CdTe. Following annealing at 380 °C in the presence of CdCl_2(s) and evaporation of metal electrode contacts (glass/ITO/CdTe/Ca/Al), Schottky-junction PV devices were tested under simulated 1 sun conditions. An improved PV performance was found to be directly tied to control over the film morphology obtained by the adjustment of spray parameters such as the solution concentration, delivery pressure, substrate distance, and surface temperature. Higher spray pressures produced thinner layers (<60 nm) with lower surface roughness (<200 nm), leading to devices with improved open-circuit voltages (<i>V</i>_oc) due to decreased surface roughness and higher short-circuit current (<i>J</i>_sc) as a result of enhanced annealing conditions. After process optimization, spray-cast Schottky devices rivaled those prepared by conventional spin-coating, showing <i>J</i>_sc = 14.6 ± 2.7 mA cm^–2, <i>V</i>_oc = 428 ± 11 mV, FF = 42.8 ± 1.4%, and Eff. = 2.7 ± 0.5% under 1 sun illumination. This optimized condition of CdTe spray deposition was then applied to heterojunction devices (ITO/CdTe/ZnO/Al) to reach 3.0% efficiency after light soaking under forward bias. The film thickness, surface morphology, and light absorption were examined with scanning electron microscopy, optical profilometry, and UV/vis spectroscopy

Inorganic Photovoltaic Devices Fabricated Using Nanocrystal Spray Deposition

Soluble inorganic nanocrystals offer a potential route to the fabrication of all-inorganic devices using solution deposition techniques. Spray processing offers several advantages over the more common spin- and dip-coating procedures, including reduced material loss during fabrication, higher sample throughput, and deposition over a larger area. The primary difference observed, however, is an overall increase in the film roughness. In an attempt to quantify the impact of this morphology change on the devices, we compare the overall performance of spray-deposited versus spin-coated CdTe-based Schottky junction solar cells and model their dark current–voltage characteristics. Spray deposition of the active layer results in a power conversion efficiency of 2.3 ± 0.3% with a fill factor of 45.7 ± 3.4%, <i>V</i>_oc of 0.39 ± 0.06 V, and <i>J</i>_sc of 13.3 ± 3.0 mA/cm² under one sun illumination