Jiro Nishinaga's research while affiliated with National Institute of Advanced Industrial Science and Technology and other places

Silicon is commonly used as a sensor material in a wide variety of imaging application. In recent high-energy and intensity beam experiments, high radiation tolerance is required, and new semiconductor detector consisting of radiation-hard materials have been investigated. The Cu(In,Ga)Se2 (CIGS) semiconductor is expected to possess high radiation tolerance, with the ability to recover from radiation damage through the compensation of defects by ions. The CIGS has originally developed for a solar cell and its radiation tolerance was investigated for the usage in space. The CIGS, featuring a recovery capability, would shed new light to particle detecror in high radiation environments. CIGS detectors (2 and 5 μm thick) were tested by Xe ion (400 MeV/u, ¹³² Xe ⁵⁴⁺ ) at HIMAC, successfully detecting single Xe ion with a fast response. The output charge is understandable through estimation with the GEANT4 simulation. With 0.6 MGy irradiation by Xe ions, the CIGS output degraded to 50%, but it was recovered to 97% after the heat treatment under 130°C for 2 hours. This marks a significant step in confirming that CIGS semiconductors can serve as particle detectors with recovery features for radiation damage.

We investigated a material stability by real-time measurements of a vertical GaN Schottky barrier diode (SBD) during proton irradiation. The current of the GaN SBD reduced by 18% for the proton irradiation with displacement-damage doses ( D d ) =10 ¹² MeV/g. For the further high proton fluence, the reverse current gradually decreased with increasing the proton fluence. We also report on signal degradation of a vertical GaN-on-GaN p-n diode (PND) during xenon-ion irradiation. The signal gradually decreased with increasing the xenon-ion fluence. The corrected charges for the GaN PND and a Si detector for the xenon-ion irradiation with D d =10 ¹² MeV/g reduced by 11% and 40%, respectively, indicating the GaN PND have the more radiation tolerance than the Si detector.

The optoelectronic reciprocity theorem has been proposed as a theorem relating electroluminescence (EL) and photovoltaic external quantum efficiency in solar cells. The theorem is vital for a fundamental understanding of solar cell operation and its application to device evaluation. Furthermore, with this theorem, it is also possible to estimate the open-circuit voltage (VOC) of solar cells using the external radiative efficiency (ηext) obtained from the absolute value of EL emission intensity, even for solar cells for which we cannot directly measure VOC, such as subcells of multijunction solar cells or individual Si cells in modules and arrays. However, it is not a priori obvious that the optoelectronic reciprocity theorem holds for the various solar cells that exist. In this study, we report the results of qualitative and quantitative confirmation of the validity of the optoelectronic reciprocity theorem for high-efficiency CuIn1−xGaxSe2 (CIGS) solar cells fabricated by us. The results confirm that our CIGS solar cells qualitatively and quantitatively satisfy the optoelectronic reciprocity theorem within the limits of measurement uncertainty. Also, we experimentally confirm that the value of the diode ideality factor for the applied voltage dependence of the EL emission intensity is exactly unity when the emission mechanism is the band-edge emission due to the direct recombination of electron-hole pairs. Finally, the importance of the ηext for improving solar cell efficiency is explained.

The detrimental effect of tail states on the radiative and non‐radiative voltage loss has been demonstrated to be a limiting factor for the open circuit voltage ( V OC ) in Cu(In,Ga)Se 2 solar cells. A strategy that has proven effective in reducing tail states is the addition of alkali metals, the effect of which has been associated with the passivation of charged defects at grain boundaries. Herein, tail states in Cu(In,Ga)Se 2 are revisited by studying the effect of compositional variations and alkali incorporation into single‐crystal films. The results demonstrate that sodium and potassium decrease the density of tail states despite the absence of grain boundaries, suggesting that there is more to alkalis than just grain boundary effects. Moreover, an increase in doping as a result of sodium or potassium incorporation is shown to contribute to the reduced tail states, which are demonstrated to arise largely from electrostatic potential fluctuations and to be determined by grain interior properties. By analyzing the voltage loss in high‐efficiency polycrystalline and single crystalline devices, this work presents a model that explains the entirety of the voltage loss in Cu(In,Ga)Se 2 based on the combined effect of doping on tail states and V OC .

Radiation tolerance of Cu(In,Ga)Se 2 (CIGS) solar cells has been investigated using high-fluence proton beam irradiation for application to devices in extremely-high-radiation environment. CIGS solar cells deteriorated after high-energy proton irradiation with non-ionizing energy loss of 1 × 10 ¹⁶ MeV n eq /cm ² , however, the CIGS solar cells could generate power after high-fluence irradiation. The ideality factors increased from 1.3 to 2.0, and series resistance increased, indicating that the concentration of recombination centers increased in CIGS layers. After heat-light annealing, the conversion efficiencies gradually recovered, and the recombination centers were confirmed to be partly passivated by annealing at 90 °C. The short-circuit currents for 10-μm-thick CIGS solar cells were recovered by dark annealing in the same manner as 2-μm-thick CIGS solar cells. Dark annealing on irradiated CIGS solar cells has beneficial effects to passivate the recombination centers even using thicker CIGS layers.

Cu(In,Ga)Se2-based solar cells exceed power conversion efficiencies of 23%. However, the fill factor of these solar cells, with best values around 80%, is relatively low (Si reaches 84.9%) mostly due to diode factors greater than 1. Recently, we proposed metastable defects, a general feature of the Cu(In,Ga)Se2 alloy, to be the origin of the increased diode factor even in low injection. Here, we measure the diode factor of the bare absorber layers using excitation-dependent photoluminescence. The increased diode factor above 1 can be well described by the model of metastable defects, as well as a slight excitation dependence within the experimentally accessible range of excitation intensities. We discuss how the excitation dependence of the diode factor depends on the parameters of the metastable defects. Within the same model, we can additionally describe the experimental diode factors of n- and p-type epitaxial Cu(In,Ga)Se2 films. We find that the diode factors measured optically by photoluminescence impose a lower limit for the diode factor measured electrically on a finished solar cell. Interestingly, the lowest diode factor (optical and electrical) and consequently highest fill factor of 81.0% is obtained by Ag alloying, i.e., an (Ag,Cu)(In,Ga)Se2 absorber. This finding hints to a pathway to increase fill factors and thus efficiencies for Cu(In,Ga)Se2-based solar cells.

The response property and stability of diamond Schottky barrier photodiodes (SBPDs) were investigated for the monitor applications of deep ultraviolet (DUV) light and high-energy radiation particles. The SBPDs were fabricated on the unintentionally doped insulating diamond epilayer grown on a heavily boron-doped p⁺-diamond (100) conductive substrate by microwave plasma chemical vapor deposition. The vertical-type SBPDs were constructed of semitransparent tungsten carbide (WC) Schottky contact on the top of the device and a WC/titanium ohmic contact on the bottom. The SBPDs were operated to detect the DUV light and protons in zero-bias photovoltaic mode. The spectral response of the SBPDs showed that the peak wavelength was at 182 nm with a sensitivity of 46 ± 1 mA/W. The response speed was shorter than 1 sec, with a negligible charge-up effect and persistent photoconductivity. The SBPDs showed a stable response upon the irradiation by 172-nm xenon excimer lamp with 70 mW/cm² for 200 hrs and 70 MeV protons for the dose of 10 MGy, corresponding to a non-ionizing energy loss of 1.4 × 10¹⁶ MeV neq/cm².

The detrimental effect of tail states on the radiative and non-radiative voltage loss has been demonstrated to be a limiting factor for the open circuit voltage in Cu(In,Ga)Se2 solar cells. A strategy that has proven effective in reducing tail states is the addition of alkali metals, the effect of which has been associated with the passivation of charged defects at grain boundaries. Herein, tail states in Cu(In,Ga)Se2 are revisited by studying the effect of compositional variations and alkali incorporation into single crystals. The results demonstrate that alkalis decrease the density of tail states despite the absence of grain boundaries, suggesting that there is more to alkalis than just grain boundary effects. Moreover, an increase in doping as a result of alkali incorporation is shown to contribute to the reduced tail states, which are demonstrated to arise largely from electrostatic potential fluctuations and to be determined by grain interior properties. By analyzing the voltage loss in high-efficiency polycrystalline and single crystalline devices, this work presents a model that explains the entirety of the voltage loss in Cu(In,Ga)Se2 based on the combined effect of doping on tail states and VOC.