Solution processed thin film photovoltaic devices incorporating organohalide perovskites have progressed rapidly in recent years and achieved energy conversion efficiencies greater than 20%. However, an important issue limiting their commercialization is that device efficiencies often drop within the first few hundred hours of operation. To explore the origin of the device degradation and failure in perovskite solar cells, we investigated the spatial uniformity of current collection at different stages of aging using two-dimensional laser beam induced current (LBIC) mapping. We validated that the local decomposition of the perovskite material is likely due to interactions with moisture in the air by comparing photocurrent collection in perovskite devices that were maintained in different controlled environments. We show that the addition of a poly(methyl methacrylate)/single-wall carbon nanotube (PMMA/SWCNT) encapsulation layer prevents degradation of the device in moist air. This suggests a route toward perovskite solar cells with improved operational stability and moisture resistance.
The effects of electric field on carrier escape in InAs/GaAs quantum dots embedded in a p-i-n solar cell structures have been studied by quantum efficiency. Via band structure simulation, effective barrier height of carriers inside QDs is reduced with increasing local electric field, so tunneling and thermal escape are enhanced. At 300K, when electric field intensity is below 40kV/cm, thermal escape is dominant in all confined states in QDs; when electric field intensity is above 40kV/cm, tunneling is dominant in shallow confined states and thermal escape is dominant in the ground state of QDs.
We describe results of our investigations of the structural, optical, and electronic properties of PbS-QD films fabricated using layer-by-layer dip coating based on 1,2-ethanedithiol as an insolubilizing agent. Our investigations extend to a study of the photovoltaic properties of heterojunction thin film solar cells fabricated by sputter-deposition of a CdS ntype thin film followed by deposition of a PbS-QD thin film. Our CdS/PbS-QD solar cells exhibit open circuit voltage in excess of previously reported PbS-QD solar cells. Under standard simulated AM1.5G illumination, we observe short circuit current density as high as 12 mA cm-2, open circuit voltage as high as 0.65 V, and a maximum efficiency of 3.3%.
We report on the generation of high-power, high-repetition-rate femtosecond pulses tunable in the visible by intracavity frequency-doubling of a KTiOPO4 (KTP) femtosecond optical parametric oscillator (OPO). The KTP OPO is synchronously pumped by a self-modelocked Ti:sapphire laser producing pulses of 225 kW peak power at a repetition rate of 81 MHz (115 fs pulsewidth), and a center wavelength of 790 nm. Using a 1.5 mm thick KTP gain crystal and a 50 micrometers (beta) -BaB2O4 frequency-doubling crystal, we achieve up to 240 mW of sub-100 femtosecond pulses tunable in the range of 580 to 660 nm. The potential tuning range of the frequency-doubled KTP OPO, when pumped at 790 nm, is approximately 500 to 750 nm.
The development of a Ti:sapphire-based femtosecond system capable of producing high repetition rate and high power output that is tunable from the near-ultraviolet to the mid- infrared is reported. This is achieved using the nonlinear optical processes of optical parametric oscillation and second-harmonic generation that results in sub-100 fs pulses and hundreds of mW of power across the full tuning range.
An investigation of hot carrier relaxation in GaAs/AlxGa1-xAs multiple quantum wells and bulk GaAs in the high carrier density limit is presented. Two techniques have been employed: luminescence upconversion with < 80 fs temporal resolution has been used to cover the range from 100 fs to 100 ps, and time-correlated single-photon counting to cover the range from 100 ps to 2 ns. Electron temperatures as a function of time were determined from the slope of the high energy tail of the time-resolved photoluminescence spectra. Our results show that hot electron cooling rates in the quantum wells begin to become significantly slower than that in the bulk when the photogenerated carrier density is above a critical value of approximately 2 X 1018 cm-3; the difference in cooling rates increases rapidly with increasing carrier density. The time constant characterizing the power loss of hot carriers is also determined and discussed. A comparison is made with previous publications to resolve the confusion concerning the difference in cooling rates between quasi-2D and 3D systems.
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