Takuya Izuishi

Mn doped quantum dot sensitized solar cells with power conversion efficiency exceeding 9

Transition metal ion (especially Mn2+) doping has been proven to be an effective approach to modify the intrinsic photo-electronic properties of semiconductor quantum dots (QDs). However, previous works to directly grow Mn doped QDs on TiO2 film electrodes at room temperature resulted in the potential of the Mn dopant not being fully demonstrated in quantum dot sensitized solar cells (QDSCs). Herein, Mn doped CdSe0.65Te0.35 QDs (simplified as Mn : QD) were pre-synthesized via a “growth doping” strategy at high temperature. A QD-sensitized photoanode with the configuration TiO2/Mn : QD/Mn : ZnS/SiO2 was prepared and corresponding cell devices were constructed using Cu2S/brass counter electrodes and polysulfide electrolyte, together with reference cells with the photoanode configurations TiO2/Mn : QD/ZnS/SiO2, TiO2/QD/Mn : ZnS/SiO2, and TiO2/QD/ZnS/SiO2. The photovoltaic performance results indicate that TiO2/Mn : QD/Mn : ZnS/SiO2 cells exhibit the best photovoltaic performance among all the studied cell devices with a power conversion efficiency (PCE) for the champion cell of 9.40% (Jsc = 20.87 mA cm−2, Voc = 0.688 V, FF = 0.655) under AM 1.5 G one full sun illumination, which is among the best results for QDSCs. The open circuit voltage decay (OCVD), impedance spectroscopy (IS) and transient absorption (TA) measurements confirm that the Mn2+ dopant can suppress charge recombination and improve the photovoltage and PCE of the resulting cells.

Slow hot carrier cooling in cesium lead iodide perovskites

Lead halide perovskites are attracting a great deal of interest for optoelectronic applications such as solar cells, LEDs, and lasers because of their unique properties. In solar cells, heat dissipation by hot carriers results in a major energy loss channel responsible for the Shockley–Queisser efficiency limit. Hot carrier solar cells offer the possibility to overcome this limit and achieve energy conversion efficiency as high as 66% by extracting hot carriers. Therefore, fundamental studies on hot carrier relaxation dynamics in lead halide perovskites are important. Here, we elucidated the hot carrier cooling dynamics in all-inorganic cesium lead iodide (CsPbI3) perovskite using transient absorption spectroscopy. We observe that the hot carrier cooling rate in CsPbI3 decreases as the fluence of the pump light increases and the cooling is as slow as a few 10 ps when the photoexcited carrier density is 7 × 1018 cm−3, which is attributed to phonon bottleneck for high photoexcited carrier densities. Our findings suggest that CsPbI3 has a potential for hot carrier solar cell applications.

Femtosecond dynamics of optical nonlinearities in nanocomposite films highly dispersed with semiconductor CdSe quantum dots

Photonic nanocomposite materials, consisting of one or several nanometer-size guest materials dispersed in a transparent host, have been paid much attention owing to their improved performance and new functionalities as well as to their flexibility in materials design [1]. They are generally fabricated by deposition/sputtering methods and take advantage of the local field effect and the nanostructuring properties to control the linear optical, laser gain and nonlinear optical properties [2]. Since 2002 we have developed a new class of inorganic-organic nanocomposite materials, the so-called photopolymerizable nanoparticle-polymer composites (NPCs) [3] in which multi-dimensional photonic lattice structures can be formed by holographic assembly of nanoparticles [4]. We have studied the nonlinear optical properties of NPCs dispersed with semiconductor CdSe quantum dots (QDs) acting as nonlinear nanoparticles [5]. We showed [6] that the dispersion of CdSe QDs as high as 6.8 vol.% to NPCs gave the cascaded fifth-order optical nonlinearity [7] as well as the saturable absorption. In this work we report on femtosecond pump-probe measurements of absorptive and refractive nonlinearities induced in NPC films highly dispersed with CdSe QDs by means of femtosecond pump-probe and transient grating methods.

High-order optical nonlinearities in nanocomposite films dispersed with semiconductor quantum dots at high concentrations

We describe the nonlinear optical properties of inorganic-organic nanocomposite films in which semiconductor CdSe quantum dots as high as 6.8 vol.% are dispersed. Open/closed Z-scan measurements, degenerate multi-wave mixing and femtosecond pump-probe/transient grating measurements are conducted. It is shown that the observed fifth-order optical nonlinearity has the cascaded third-order contribution that becomes prominent at high concentrations of CdSe QDs. It is also shown that there are picosecond-scale intensity-dependent and nanosecond-scale intensity-independent decay components in absorptive and refractive nonlinearities. The former is caused by the Auger process, while the latter comes from the electron-hole recombination process.