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HOIP as a sensitizer were about 3.13% for bromide compound (CH 3 NH 3 PbBr 3 ) and 3.81% for iodide compound (CH 3 NH 3 PbI 3 ). In 2011, Im et al [2] reported the efficiency of ~4.7% for a photovoltaic cell containing HOIP as quantumdot-sensitizer. In 2012, Lee et al used HOIP materials as the active layer material for the first time; it was one of the most important events in the HOIP photovoltaics field. Lee et al reported that the efficiency of their HOIP photovoltaic cell was 10.9% in a single junction device [3]. Numerous research results of HOIP photovoltaic cells have been reported since then. In 2016, a much higher efficiency of 22.

Hybrid lead halide perovskite possesses attractive properties and outstanding performance. The electronic structure of hybrid lead halide perovskite single crystal can be used to comprehend its intrinsic properties. In this study, the crystal structure of CH 3 NH 3 PbBr 3 perovskite crystal was elucidated by transmission electron microscopy. Phonon modes and CH 3 NH 3 + rocking modes were analyzed. Band gaps of the crystal were obtained by temperature-dependent Raman scattering spectroscopy and photoluminescence measurement. Surface potential measurement for a (001) plane exhibited a work function of the material and other phases of PbBr 2 and CH 3 NH 3 Br. From experimentally determined values, an electronic structure of CH 3 NH 3 PbBr 3 was built. It showed a p-type characteristic.

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We investigated the influence of CuO amount (0.5-3.0 mol%), sintering temperature (900°C-1000°C), and sintering time (2-6 h) on the low-temperature sintering behavior of CuOadded Bi 0.5 (Na 0.78 K 0.22 ) 0.5 TiO 3 (BNKT22) ceramics. Normalized strain (S max /E max ), piezoelectric coefficient (d 33 ), and remanent polarization (P r ) of 1.0 mol% CuO-added BNKT22 ceramics sintered at 950°C for 4 h was 280 pm/V, 180 pC/N, and 28 lC/cm 2 , respectively. These values are similar to those of pure BNKT22 ceramics sintered at 1150°C. In addition, we investigated the performance of multilayer ceramic actuators made from CuO-added BNKT22 in acoustic sound speaker devices. A prototype sound speaker device showed similar output sound pressure levels as a Pb(Zr,Ti)O 3 -based device in the frequency range 0.66-20 kHz. This result highlights the feasibility of using low-cost multilayer ceramic devices made of lead-free BNKT-based piezoelectric materials in sound speaker devices.