We use electroabsorption (EA) spectroscopy to measure the exciton binding energy (E B), electron–hole reduced effective mass (μ), and one-electron band gap (E g) at the fundamental absorption edge of the hybrid organic–inorganic perovskite CH3NH3PbI3 in its tetragonal phase at 300 K. By studying the second-harmonic EA spectra at the fundamental absorption edge we establish that the room-temperature EA response in CH3NH3PbI3 follows the low-field Franz–Keldysh–Aspnes (FKA) effect. Following FKA analysis we find that μ = 0.12 ± 0.03m 0, E B = 7.4 meV, and E g = 1.633 eV. Our results provide direct experimental evidence that at room temperature primary transitions occurring in CH3NH3PbI3 can essentially be described in terms of free carrier generation.

We study how tailoring noncovalent interactions through the regiochemistry of common donor− acceptor polymers can alter their excited-state electronic properties. Specifically, we compare two regioregular analogues (referred to as P1 and P2) of the widely used donor− acceptor copolymer poly [4,8-bis(5-(2-ethylhexyl)which differ only in the orientation of a fluorine atom on the thieno[3,4-b]thiophene acceptor units with respect to the benzodithiophene unit. We show that in thin films, this subtle change along the polymer backbone results in a transformation from typical H-like to an unusual HJ-like aggregate behavior, indicating significant differences in intrachain exciton coupling along the polymer backbone and interchain coupling between polymer chains. We also use electroabsorption spectroscopy to relate the dominant coupling mechanisms to differences in the excess polarization volume of the excitons in P1 and P2. Overall, our results show that seemingly small changes in noncovalent interactions along a polymer backbone can have profound effects on the exciton electronic structure and dominant electronic coupling mechanisms in the solid state (intrachain vs interchain). Furthermore, these results provide evidence that interchain and intrachain coupling interactions may be correlated with electrostatic properties of the excitons in terms of their excess polarization volume.

We study the effects of bismuth doping on the crystal structure and phase transitions of single crystals of the perovskite semiconductor methylammonium lead tribromide, MAPbBr 3 . By measuring the temperature-dependent specific heat capacity (C p ) we find that, as the Bi doping level increases, the temperatures for the bismuth structural phase transitions shift, and the phase boundary assigned to the transition from the cubic to tetragonal phase decreases in temperature. Furthermore, after doping we only observe one phase transition between 135 and 155 K, in contrast to two transitions observed in the undoped single crystal. These results appear strikingly similar to the previously reported effects of mechanical pressure on the perovskite structure. Using X-ray diffraction, we show that, as more Bi is incorporated into the crystal, the lattice constant decreases, as predicted by density functional theory (DFT). Based on the lattice contraction and DFT, we propose that bismuth substitutional doping on the lead site is dominant, resulting in Bi Pb + centers which induce compressive chemical strain that alters the crystalline phase transitions. TOC GRAPHICSKEYWORDS Bismuth doping, MAPbBr 3 perovskite, specific heat capacity, lattice contraction, X-ray diffraction, Halide perovskites have emerged as promising semiconductor materials for applications including solar cells, light-emitting diodes, photodetectors, and lasers. [1][2][3][4] They exhibit unique and tunable optoelectronic properties via facile tailoring of the chemical composition of the structure.In the archetypal perovskite ABX3 crystal structure, A represents a monovalent cation species (A = Cs + , CH3NH3 + (MA + ), or (NH2)2CH3 + (FA + )), B represents a divalent cation (B = Pb +2 , Sn +2 ), and X represents a halide (X = Cl -, Br -, I -). Diverse electronic and structural motifs are thereby accessible by modification of the chemical composition and the dimensionality of the material. 5