Optical switch of a-IGZO TFT and triple junction photovoltaic cell

This study presents an optical switch that integrates an indium gallium zinc oxide (IGZO) thin-film transistor (TFT) and a GaInP/GaAs/Ge triple junction (TJ) photovoltaic (PV) cell. The device is driven by a TJ PV cell, which has a steady output voltage and a voltage regulator, to induce current in it. It should be noted TJ solar cell is broadband sensitive and the open circuit voltages are close under different illumination levels and a voltage regulator can assist it has identical working voltages. Moreover, The switch off (dark) and on (illuminated) current values of the optical switch are around 10−8 and 10−5 A, under different simulated test lights, respectively. The test light source is from solar simulator. The stable characteristics and good on/off ratio levels of this device under different light make it practicable in outdoor and indoor environments. © The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0171709jss] All rights reserved.

Recently, self-powered devices that harvests power from the environment for sensing or electricity generation have attracted a lot of attention. Self-powered devices driven by solar, piezoelectric, thermal, acoustic energy and etc, have already been demonstrated. [1][2][3][4][5][6] In the published reports, Yang and his group fabricated self-powered devices by connecting an nanowire CdSe photodetector and a GaN/ZnO cell in series. 7 Similar design optical driven transistor was also demonstrated. 8 However, those components in their design were easily effected by background light intensities.
To solve the problem, authors introduced a vertical integration design and presented a self-powered optical switch device in this study. Similar study integrated a crystalline-Si interdigitated back contact (IBC) PV cell and a-IGZO TFTs with a direct 3D stacking structure. However, it connected several Si-based PV and cost dimensions. 9 Compared with that, our device integrated an IGZO TFT and a GaInP/GaAs/Ge triple junction TJ PV cell into a chip which saved chip dimensions and got better performance, as shown in Fig. 1. For the TFT, IGZOwas chosen due to its high mobility, superior uniformity, and good transparency to visible light. [10][11][12][13][14][15] A high-k material, Ta 2 O 5 , was used as the gate dielectric underneath IGZO film to control the conduction channel in IGZO. The TJ PV cell was adopted because it has the highest and stable output voltage among PV cells. Moreover, TJ solar cell is broadband sensitive and the open circuit voltages (Voc) are close under different illumination levels and a voltage regulator can assist it has identical working voltages (V m ). The test light source is from solar simulator. The stable characteristics and good on/off ratio levels of this device under different light make it practicable in outdoor and indoor environments.
Which can produce higher current and on/off ratio to resist background light disturbances. 16

Experimental
The TJ PV cell comprises subcells of GaInP (top cell, bandgap energy Eg = 1.9 eV), GaAs (middle cell, Eg = 1.42 eV), and Ge (bottom cell, Eg = 0.67 eV). The structure is grown on Ge substrates using a metal-organic chemical vapor deposition (MOCVD) system at 650 • C and a reactor pressure of 50 mbar. The metal of Ni/Ge/Au alloy was deposited on front side of PV cell by E-beam evaporation. To isolate the IGZO TFT on TJ PV cell, a 2-μm-thick SiO 2 layer was deposited on the rear side. The SiO 2 was patterned and removed. Al electrode was deposited the removed SiO 2 area and taken as electrode

Results and Discussion
The external quantum efficiency (EQE) is defined as the number of photon-generated charge carriers contributing to the photocurrent per incident photon. Fig. 2 shows the EQE plots for the TJ PV cell from 300 to 1800 nm (UV-vis-near-infrared). From the EQE data, it can be seen the device is broadband sensitive, especially the EQE plots are about 80% in the range of 450 to 1500 nm.   Fig. 4 shows the I DS -V DS and I DS -V GS characteristics, respectively. Fig. 4a plots its output I DS -V DS characteristics in a dark environment without connection to the TJ PV cell. The device exhibits typical n-type TFT characteristics with a clear pinch-off and current saturation. With 1.5 V applied gate bias, it was found that the saturation drain current, I DS , was approximately 10 −5 A. Fig. 4b shows I DS -V GS characteristics of the independent TFT device. The values of the field effect mobility (μ FE ) and the threshold voltage (V T ) were determined from the linear I DS − -V GS plots. The values of field effect mobility, μ FE and threshold voltage, V T were 51.5 cm 2 /V-s and 1.25 V, respectively. These values indicate that the performances of the fabricated TFT is good. Moreover, These results indicate that the TJ PV cell can afford to provide enough voltage to drive TFT even under weak illumination by referring to Fig. 3.
To realize the optical device, the conductive paths were connected from the TJ PV cell's electrodes to the gate and source electrodes of the a-IGZO TFT as shown in Fig. 5a. It should be noted that a voltage regulator component was added to keep the V GS at a stable value. Two probes of B1500 analyzer were used to measure the drain and source electrodes.   Fig. 5d. The response is faster with bright light and slower with weak light. That means rising time depends on the velocity of accumulated carriers generated by the illumination. It should be noted the results are not optimized, and we will still work harder to make the responses identical and faster.

Conclusions
This study presents a device that integrates an IGZO TFT and a GaInP/GaAs/Ge TJ PV cell. From the EQE data, TJ PV cell is broadband sensitive and can provide enough voltage values under different illumination levels. The authors also add a voltage regulator to assist the device has identical working voltages and perform identically. The optical switch has a high current on/off ratio about 3 orders to eliminate background light disturbances because it is driven by TJ PV cell but light directly. Under the illumination of 1000, 600, 300, and 100 W/m 2 , the dynamic responses of the device is identical and stable, which means it can be used in outdoor and indoor environments. It is not optimized from rising time chart, but we will work harder to make the responses identical and faster in our next phase.