Temperature Dependence of the Band Gap of CH₃NH₃PbI₃ Stabilized with PMMA: A Modulated Surface Photovoltage Study

CH₃NH₃PbI₃ layers chemically stabilized with poly­(methyl methacrylate) (PMMA), relevant for photovoltaic applications, have been investigated by modulated surface photovoltage (SPV) spectroscopy at temperatures (<i>T</i>) between −182 and 60 °C. SPV is sensitive only to the PMMA/CH₃NH₃PbI₃ interface region where photogeneration and charge separation take place. The <i>T</i> dependencies of the Tauc gap (<i>E</i>_g‑Tauc, equivalent to absorption measurements) and of the gap determined from the maximum slope (<i>E</i>_g‑tp, almost at increased absorption) were analyzed on the basis of the in-phase SPV spectra. At 32 °C, the values of <i>E</i>_g‑Tauc and <i>E</i>_g‑tp were 1.540 and 1.560 eV, respectively. A jump of <i>E</i>_g‑Tauc and <i>E</i>_g‑tp by 10 meV at 40 °C was interpreted as the transition from the tetragonal to the cubic phase at <i>T</i> lower than values known from literature. <i>E</i>_g‑Tauc and <i>E</i>_g‑tp of the cubic phase decreased with increasing <i>T</i>. In contrast, <i>E</i>_g‑Tauc and <i>E</i>_g‑tp of the tetragonal phase decreased moderately with decreasing <i>T</i> to 1.528 and 1.546 eV at −182 °C, respectively. No signature has been observed in <i>E</i>_g‑Tauc and <i>E</i>_g‑tp for the transition from the tetragonal to the orthorhombic phase. Structural interactions at PMMA/CH₃NH₃PbI₃ interfaces seem important for phase transitions in CH₃NH₃PbI₃ layers

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