Quantum Confinement Effects in Organic Lead Tribromide Perovskite Nanoparticles

The optical and electronic properties of nanoparticles/nanocrystals (NC) of methylammonium lead tribromide perovskite (MAPbBr₃) have been studied in detail. We observe the effect of quantum confinement in particles of an average diameter of ∼6 nm and smaller, in the form of an increase in excitonic nature with decrease in particle size. The differences in the photophysical properties in bulk and NC forms of MAPbBr₃ are clearly observed in the temperature dependent measurements, and provide insight into the length scales prevalent in this system. We demonstrate devices consisting of active layers of NC in conjunction with low band gap polymer semiconductors which exhibit the dual functionality of a light emitting diode in the forward bias and a photodetector in the reverse bias.graphic

Following the TRMC Trail: Optimization of Photovoltaic Efficiency and Structure–Property Correlation of Thiophene Oligomers

Semiconducting conjugated oligomers having same end group (<i>N</i>-ethylrhodanine) but different central core (thiophene: <b>OT–T</b>, bithiophene: <b>OT–BT</b>, thienothiophene: <b>OT–TT</b>) connected through thiophene pi-linker (alkylated terthiophene) were synthesized for solution processable bulk-heterojunction solar cells. The effect of the incorporation of an extra thiophene to the central thiophene unit either through C–C bond linkage to form bithiophene or by fusing two thiophenes together to form thienothiophene on the optoelectronic properties and photovoltaic performances of the oligomers were studied in detail. Flash photolysis time-resolved microwave conductivity (FP–TRMC) technique shows <b>OT–TT</b> has significantly higher photoconductivity than <b>OT–T</b> and <b>OT–BT</b> implying that the former can outperform the latter two derivatives by a wide margin under identical conditions in a bulk-heterojunction solar cell device. However, the initial photovoltaic devices fabricated from all three oligomers (with PC₇₁BM as the acceptor) gave power conversion efficiencies (PCEs) of about 0.7%, which was counterintuitive to the TRMC observation. By using TRMC results as a guiding tool, solution engineering was carried out; no remarkable changes were seen in the PCE of <b>OT–T</b> and <b>OT–BT</b>. On the other hand, 5-fold enhancement in the device efficiency was achieved in <b>OT–TT</b> (PCE: 3.52%, <i>V</i>_OC: 0.80 V, <i>J</i>_SC: 8.74 mA cm^–2, FF: 0.50), which was in correlation with the TRMC results. The structure–property correlation and the fundamental reasons for the improvement in device performance upon solvent engineering were deduced through UV–vis absorption, atomic force microscopy, bright-field transmission electron microscopy, photoluminescence quenching analysis and two-dimensional grazing incidence X-ray diffraction studies