In view that enhancing near-infrared response of photocathodes is critical to the detection performance, we propose two technical approaches by changing the structure of buffer-layer underneath the active-layer, wherein one is to produce a graded band gap using the graded-composition structure, and the other is to produce a distributed Bragg reflector using the AlAs/GaAs supperlattice structure. Three types of reflection-mode GaAs photocathode samples grown by molecular beam epitaxy were prepared under the same condition. By comparison of activation photocurrent and spectral response among the three different samples, it is found that compared with the conventional sample, the samples with graded-composition and distributed Bragg reflector can obtain higher photocurrent and better response. The measured results of spectral response indicate that the samples without a distributed Bragg reflector exhibit a typical smooth spectral behavior, while the spectral response of the sample with a distributed Bragg reflector structure has a different resonance feature. The sample with the distributed Bragg reflector structure can obtain higher response than those without distributed Bragg reflector at some near-infrared wavelength positions. The peak positions of spectral response curve agree quite well with the dip positions in the reflectivity spectrum. This agreement demonstrates that the response enhancements are ascribed to the resonant absorption effect.
Recently solid state hybrid organometal halide perovskite solar cells have become the research hotspot. We fabricated perovskite solar cells by a simple two-step method completely in the regular ambient condition without a glove box, and all materials were just stored in ambient air. The cross-section SEM image of the perovskite solar cells exhibits a well-defined layer-by-layer structure with clear interfaces. XRD pattern shows main diffraction peaks centred at 14.2° (110) and 28.5° (220), which can be assigned to the CH3NH3PbI3 phase, suggesting a crystal structure, and the peak centred at 12.7° is attributed to PbI2. Our best Jsc is 19.2 mA/cm2, the best Voc is 1.0 V, the best FF is 0.65, and the best PCE is 10.2%.
The stability for reflection-mode GaN photocathode has been investigated by monitoring the photocurrent and the
spectral response at room temperature. We watch that the photocurrent of the cathode decays with time in the vacuum
system, and compare the spectral response curves after activation and after degradation. The photocurrent decay
mechanism for reflection-mode NEA GaN photocathode was studied by the surface model [GaN (Mg) :Cs]:O-Cs. The
reduction of the effective dipole quantity, which is caused by harmful gases, is the key factor of the photocurrent
reduction.
For the characteristics such as wide bandgap, low dielectric constant, ability to bear high temperature, ability to resist radiation etc., GaN material can be used for UV solar blind detection in very rigorous environments. But for a long time, the preparation technology for GaN material has been still keeping it from being used extensively. GaN photocathode with good future is developed slowly in the field of UV detection. The key method of obtaining effective photoemission is to reduce the vacuum energy level of GaN emission surface, make it lower than bulk conduction band minimum. Negative electron affinity (NEA) GaN photocathode can convert the light under 365 nm to the photoelectrons that can be sent to the free space. The surface potential of NEA GaN photocathode is made up of two straight line sections with different slope. As the first dipole layer, [GaN(Mg):Cs] dipole brings 3.0 eV decline of the vacuum energy level, make the GaN photocathode surface obtain about -1.0 eV effective negative electron affinity. The second dipole O-Cs makes effective electron affinity reduce further to - 1.2 eV. The results show: near 37% quantum efficiency can be gotten at the wavelength 200 nm for reflection-mode GaN photocathode, and the quantum efficiency reaches up to 13% at 290 nm in transmission mode. The large quantum efficiency and high stability are very good properties for UV detection devices employing GaN photoemitter.
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