Structure of the microsystem and its components. Figure 1a shows the schematic illustration of this integrated transparent microsystem. Figure 1b-d exhibit the cross-sectional schematic diagram as well as the SEM and optical images of each essential component. LIB consists of ITO electrode/V2O5 cathode/LiPON electrolyte/IGZO anode/ITO electrode, where V2O5 and LiPON are chose as the cathode and electrolyte respectively mainly due to their high transparency, compatibility with the microelectronic process and good electrochemical properties in the amorphous state.[11–14] TFT consists of ITO gate/HfLaO dielectric/IGZO channel/ITO source and drain, and high-k HfLaO is used as the dielectric mainly because of its high transparency, high crystalline temperature and effectiveness in reducing the operating voltages.[15, 16] PD herein displays a photoresistive structure and consists of ITO electrode/IGZO photosensitive layer/ITO electrode. As demonstrated in Fig. 1e, each component as well as this integrated microsystem shows high transparency (> 76.4% when the wavelength > 700 nm) due to the rational material design. All the devices are fabricated on a single glass substrate for constructing an integrated microsystem by using physical vapor deposition and each layer is patterned by using shadow masks. The fabrication is performed at room temperature to ensure that the prepared films are amorphous, which is desirable to address the nonuniformity issues of a microsystem caused by random grain boundaries in the films.[17] The detailed fabrication processes of each device and this integrated microsystem are shown in Figure S1 and the experimental section of Supplementary Information. According to the SEM images of each device (Fig. 1b-d), all the layers can be clearly distinguished, demonstrating a uniform deposition of each layer.
Preparation and characterization of IGZO thin films. Although no work has been performed to explore the feasibility of IGZO used in LIB, its main compositions (ZnO, In2O3 and Ga2O3) have been demonstrated to be typical conversion-type LIB anode materials with relatively high specific capacities, suggesting great potential of IGZO as the LIB anode.[18–23] The anode electrochemical performance is greatly affected by its electrical and ionic conductivities, which can be modulated by changing the oxygen content in IGZO. Therefore, IGZO films with different oxygen contents are prepared through reactive sputtering by changing the Ar/O2 flowing rate from 15 sccm/0 sccm, 15 sccm/3 sccm to 15 sccm/5 sccm, and the corresponding oxygen partial pressure (PO2) is 0 Pa, 0.04 Pa and 0.15 Pa, respectively. The detailed preparation processes of the IGZO films are shown in the experimental section of Supplementary Information.
Figure 2a shows the Raman spectra of the IGZO films deposited at different PO2. All the films present typical characteristics of IGZO, demonstrating the IGZO formation.[24–26] The film compositions can be quantitatively characterized by using EDX (Fig. 2b) and the relative elemental contents of the films are summarized in Table S2. The oxygen content increases with increasing PO2 and this suggests that the oxygen content in the IGZO film can be effectively modulated simply by changing PO2. Moreover, it is found that each element is evenly distributed for the IGZO films (Figure S2). The electrical resistivity of the films is extracted by using the four-probe measurements. As seen in Fig. 2c, the electrical resistivity of the film increases as PO2 increases. It is known that oxygen vacancy is the main contributor to the electron carriers and electrical conductivity of IGZO.[27] Increasing PO2 enhances the oxygen content (and so suppresses the oxygen-vacancy content) in the IGZO film, thus leading to the high film resistivity. The crystallinity of the films is investigated by XRD (Fig. 2d) and TEM (Fig. 2e and Figure S3), both of which suggest that all the films display an amorphous state. This is quite desirable for improving the uniformity of a microsystem. Figure 2f and Figure S4 show the surface morphologies of the films characterized by using AFM. All the films present a smooth surface with a small root-mean-square (RMS) roughness value (< 2.9 nm). It is also found that the film roughness increases slightly with increasing PO2.
Electrochemical properties of IGZO thin films. As a new anode material, the electrochemical energy-storage characteristics of the IGZO films are first investigated based on the coin half cells. Figure 3a-c show the cyclic voltammetry (CV) curves in a voltage range of 0.01-3.0 V for the samples prepared at various PO2. All the samples display cathodic peaks at 0.6 V and 1.0 V in the initial two cycles, which are mainly derived from the formation of a solid electrolyte interphase (SEI) and Li2O, respectively.[19, 21, 28] In the subsequent cycles, the CV curves are reproducible, suggesting high reversibility of the following reactions. It is further found that the anodic (or cathodic) peaks in the subsequent CV curves are consistent well with the multistep alloying (or dealloying) processes of M (M = In, Ga and Zn metal) with Li+. Because these alloying (or dealloying) processes occur in a similar potential range, thus resulting in relatively broad anodic (or cathodic) peaks in the CV curves. Based on the above results, the electrochemical processes of IGZO with Li+ obey the conversion reaction followed by the alloying reaction and the details can be described by[18–23, 29]
$$\text{M}{\text{O}}_{x}+2xL{i}^{+}+2x{e}^{-}\Rightarrow xL{i}_{2}O+ M (M = In, Ga or Zn) \left(1\right)$$
$$M+yL{i}^{+}+y{e}^{-}\iff {Li}_{y}M (y=3, 3, 2 for In, Ga, Zn) \left(2\right)$$
Although Li2O is an inactive material, it has a high ionic conductivity and thus the Li2O formation can effectively enhance the anode ionic conductivity.[30, 31] In addition, Li2O can also act as a buffer matrix to release the stress induced by the anode volume change during the lithiation/delithiation.[32]
Figure 3d-f depict the galvanostatic charge/discharge (GCD) voltage profiles of the samples between 0.01-3.0 V at a current density of 0.05 A g-1. All the samples display an obvious voltage plateau at around 1.0 V in the 1st discharging process. This is mainly ascribed to the irreversible reaction described by Eq. (1) and thus results in a relatively low initial Coulombic efficiency (CE ~ 72.6%, 70.0%, 70.8% for the anodes prepared at PO2 = 0 Pa, 0.04 Pa and 0.15 Pa, respectively). In addition, the GCD curves of the anodes at PO2 = 0.04 Pa and 0.15 Pa tend to coincide in the following cycles; for comparison, the GCD curves of the anode at PO2 = 0 Pa degrades monotonically. Figure 3g shows the cycling characteristics of the samples at a current density of 0.4 A g-1. The retained reversible capacity after 250 cycles is 361 mAh g-1 (or 241 µAh cm-2 µm-1), 505 mAh g-1 (or 337 µAh cm-2 µm-1) and 497 mAh g-1 (or 331 µAh cm-2 µm-1) for the IGZO anodes prepared at PO2 = 0 Pa, 0.04 Pa and 0.15 Pa, corresponding to capacity retention of 67.1%, 91.5% and 94.8%, respectively. The IGZO anodes prepared at PO2 = 0.04 Pa and 0.15 Pa present better cycling performance than the one at PO2 = 0 Pa mainly due to their more Li2O formation, which effectively acts as a buffer matrix for stress relaxation. Figure S5 presents the SEM images of the IGZO anodes before and after cycling. Different from the one at PO2 = 0 Pa which cracks into pieces, the IGZO anode prepared at PO2 = 0.04 Pa keeps intact after cycling, further demonstrating its stronger mechanical strength against the stress. Moreover, as seen in Figure S6, the anode at PO2 = 0.04 Pa can work well even when the IGZO thickness increases from 400 nm to 800 nm, and the reversible capacity after 250 cycles retains 480 mAh g-1 (or 320 µAh cm-2 µm-1), corresponding to capacity retention of 89.2%.
Figure 3h shows the rate capacities of the samples. For the IGZO anode prepared at PO2 = 0.04 Pa, 36.4% (~ 227 mAh g-1) of its capacity (relative to the initial capacity at 0.2 A g-1) is obtained at a high current rate of 4 A g-1. Also, as the current goes back to 0.2 A g-1, 98.9% (~ 616 mAh g-1) of its capacity can be well recovered. On the contrary, for the anode prepared at PO2 = 0 Pa (or the anode prepared at PO2 = 0.15 Pa), 3.7% (or 25.3%) of its capacity is obtained at 4 A g-1 and only 41.3% (or 53.0%) of its capacity is left as the current returns to 0.2 A g-1. It is obvious that the anode at PO2 = 0.04 Pa displays superior rate performance than the other ones, suggesting its faster reaction kinetics. This can be further confirmed by the EIS spectra shown in Figure S7, which presents that the anode at PO2 = 0.04 Pa shows the smallest charge-transfer resistance among the samples. The faster kinetics of the anode prepared at PO2 = 0.04 Pa should be mainly because of its higher ionic conductivity caused by its sufficient Li2O formation (vs. the anode at PO2 = 0 Pa) and higher electrical conductivity (vs. the anode at PO2 = 0.15 Pa). Based on the above results, The IGZO film prepared at an oxygen partial pressure of 0.04 Pa shows better overall electrochemical performance than the other ones in terms of its higher reversible capacity, longer cycling life and better rate characteristics. Therefore, the film prepared in this condition is used as the functional layers in each essential device and the integrated microsystem in the following.
All-solid-state thin-film transparent LIB with IGZO as anode. After systematically investigating the IGZO electrochemical characteristics, an all-solid-state thin-film transparent LIB with IGZO anode is developed as the on-chip power source. As seen in Fig. 4a, this LIB consists of ITO electrode/V2O5 cathode/LiPON electrolyte/IGZO anode/ITO electrode and each layer can be clearly defined with a sharp and even interface. In addition to the IGZO film, both the electrolyte and cathode films also display an amorphous state (Figure S8), which is beneficial for improving the device and microsystem uniformities. It is known that mechanical stress would be induced by the volume change of the LIB anode during the lithiation/delithiation, and the induced stress increases with increasing the anode thickness, thus a relatively thick IGZO is used in the half cells, which is more effective and suitable to examine its energy-storage characteristics. For comparison, a relatively thin IGZO is used in the thin-film LIB, TFT and PD mainly for improving the fabrication efficiency. Figure 4b shows the charge/discharge voltage profiles of the thin-film LIB between 0.5-3.0 V at a low current density of 1 µA cm-2. The initial CE is about 44.3% and increases obviously in the following cycles. Figure 4c shows the LIB voltage profiles under various current densities. A relatively high reversible capacity of 9.8 µAh cm-2 is achieved at 1 µA cm-2, which is comparable to the values of the reported V2O5-based thin-film LIBs.[33–35] Moreover, a specific capacity of 4.8 µAh cm-2 with capacity degradation of 51.0% can be still obtained even at a high current density of 14 µA cm-2, suggesting its good rate characteristics.
In order to investigate an optimum match between the cathode and anode, thin-film LIBs with different IGZO thicknesses while all the other layers maintaining constant are also prepared. As seen in Figure S9, the discharge capacity of the LIB with 40-nm IGZO anode is 5.7 µAh cm-2 at 1 µA cm-2 and degrades to 2.1 µAh cm-2 at 14 µA cm-2, corresponding to 63.2% degradation. The capacity of the LIB with 120-nm IGZO anode is 9.7 µAh cm-2 at 1 µA cm-2 and reduces to 4.1 µAh cm-2 at 14 µA cm-2, corresponding to 57.7% degradation. It is obvious that the LIB with 80-nm IGZO presents higher reversible capacity and better rate performance than the other ones, suggesting that the anode is well matched with the cathode in this case.
In practical use, devices are usually required to operate at high temperatures. According to the conventions, the devices need to enable work at 85°C in the industrial electronics and at 125°C in the military electronics. Figure 4d shows the LIB voltage profiles at different temperatures. The LIB displays a reliable performance even at 125°C (Fig. 4d). In addition, it can be continually charged and discharged for more than 60 hours at 85°C with only a slight capacity loss of 11.3%, as seen in Figure S10. It is noted that metallic Li is commonly adopted as the anode in the thin-film LIBs. The low melting temperature (~ 180°C) of Li causes that the corresponding LIB is difficult to operate reliably in high temperatures. For comparison, IGZO as the anode not only contributes to the LIB transparency but also improves the LIB operating temperature in this work. Figure 4e exhibits the cycling characteristics of this thin-film LIB. Its capacity has no degradation even over 300 cycles and also a relatively high CE (≥ 96.5%) is retained during cycling, both of which demonstrate good cycling performance of this LIB. Figure 4f shows the self-discharge curve of the LIB. Once the LIB is charged to + 3 V, the self-discharge process versus time is recorded and a quite low decay rate of 6.7 mV h-1 is achieved for this LIB, demonstrating its good energy-storage capability.
TFT with IGZO as the channel layer. TFT is one main kind of electronic devices and its main function in a microsystem includes switching and driving. In this work, TFT with a bottom-gate and top-contact structure is prepared (Fig. 1d) and it consists of ITO gate/HfLaO dielectric/IGZO channel/ITO source and drain. The HfLaO dielectric displays an amorphous state (Figure S11), and thus is beneficial for improving the device and microsystem uniformities. Figure 5a shows the TFT transfer curve and the key electrical parameters, including the saturated carrier mobility (µsat ~ 3.3 cm2 V− 1 s− 1), sub-threshold swing (SS ~ 486 mV dec− 1), turn-on voltage (Vturn−on ~ 2.6 V) and on/off current ratio (Ion/Ioff ~ 103), can be extracted from the curve. Figure 5b presents the TFT output characteristics and a driving output current ID of 3.3 µA is obtained at VDS = 5 V and VGS = 5 V. In addition, its ID maintains constant as VDS increases in the saturation region, demonstrating its excellent saturation characteristics. Moreover, no current-crowding phenomenon occurs in the output curves, indicating good ohmic contacts between the channel layer and source/drain. This TFT exhibits an acceptable performance even prepared at room temperature and without any thermal annealing treatment. Its performance is comparable to that of the reported IGZO-based TFTs prepared in the similar conditions.[36–38]
By shorting the TFT gate to drain terminal, the TFT can work as a rectifier (denoted as TFTR), in which the gate to drain short terminal acts as the rectifier input (Vin) and the source terminal acts as the rectifier output (Vout). This rectifier works in two modes: (i) when Vin < Vturn−on, the device is switched-off; (ii) when Vin ≥ Vturn−on, the device is turned-on and works in the saturation region. As the rectifier is turned-on, Vout ≈ Vin - Vturn−on. Figure 5c shows the rectification characteristic of the device and a rectification ratio of 103 can be obtained. In order to check the effectiveness of this rectifier, AC sinusoidal signals with different frequencies and different amplitudes are applied to the rectifier input by using a signal generator (Fig. 5d). The corresponding rectifier output across a load resistance (~ 50 kΩ) is recorded by using an oscilloscope, as seen in Fig. 5e and f. The output displays half-wave DC signals in all the cases, demonstrating successful rectification. It is further found that the rectifier works well even when the input frequency is up to 1 kHz. Moreover, the rectifier can be also successfully used to process a signal generated by a piezoelectric vibration energy harvester. Figure S12 shows the AC input signal generated by the harvester as well as the corresponding DC output signal processed by the rectifier. This result implies the potential of this TFTR rectifier in constructing a self-powered and autonomous microsystem.
PD with IGZO as the photosensitive layer. As a typical kind of sensors, PD is used in diverse applications such as flame detection, pollution monitoring and medical care.[39] In this work, a transparent PD is prepared as the sensing component in this integrated microsystem. This PD presents a two-terminal photoresistive structure and consists of ITO electrode/IGZO photosensitive layer/ITO electrode (Fig. 1b), in which IGZO has been demonstrated to be a promising photosensitive material due to its suitable bandgap and outstanding electrical properties.[9, 40] Figure S13a and b show the PD current-voltage (I-V) curves under different illumination power intensities and wavelengths as well as in the dark, respectively. The linear I-V relationship indicates ohmic contacts of this PD device, which favors the photogenerated carrier collection. The current increases with increasing the light intensity, indicating good photosensitivity of the IGZO film. In order to further evaluate the photoresponse performance, the relationship between the photocurrent Iphoto (Iphoto = Ip − Idark. Ip represents the PD current under light illumination and Idark means the current in the dark) and the light intensity (P) is also examined and depicted in Fig. 5g. The Iphoto-P relationship can be well described by the following power law[41]
$${I}_{photo}=A{P}^{\theta } \left(3\right)$$
where A is a constant and θ is the empirical value. The θ value is estimated to be 0.79 by fitting, revealing the sublinear Iphoto-P relationship. This commonly appears in the metal-oxide-based photodetectors due to the complex processes of electron-hole generation, trapping and recombination in the metal-oxide semiconductors.[41] As a PD critical parameter, responsivity (R) can be calculated by the following equation
$$R={I}_{photo}/\left(PS\right) \left(4\right)$$
where S is the light illumination area. The responsivity decreases and then tends to saturate with increasing the light intensity and a maximum responsivity of 0.35 A W-1 is obtained at a light intensity of 2.3 mW cm− 2 and a voltage of 5 V. Reproducibility and response speed are another two important parameters of the photodetectors. As shown in Fig. 5h, the PD device presents stable on- and off-state currents under repeatedly chopping the light illumination, confirming its good reproducibility and stability. A high-resolution current-time curve is used to investigate the response speed of the PD device (Fig. 5i). The rise time and decay time are defined as the time interval for the photocurrent changing from 10–90% and vise visa, respectively. Accordingly, the rise time and decay time are measured to be 0.45 s and 2.91 s. This PD performance is comparable to the values of the PDs with a similar structure reported in the literature.[42–44]
Collaborative work of each device. The collaborative capabilities of each device in the microsystem are firstly demonstrated through the thin-film LIB charging process by using the TFTR as the on-chip rectifier. The detailed setup diagram is presented in Fig. 6a, in which AC sinusoidal signals created by a signal generator are applied to the input terminal of the TFTR, and the output of the TFTR is connected to the cathode terminal of the LIB. Figure 6b shows the LIB charging curves under different-frequency sinusoidal signals with constant amplitude of 6 V. The LIB voltage increases and then tends to saturate with charging time, suggesting that the energy is successful charged into the LIB. An interesting phenomenon is that the charging rate increases with increasing the input-signal frequency (12.0 mV s-1 at 1 kHz vs. 11.2 mV s-1 at 300 Hz). After charging for 180 s, the input signal is intentionally shut down, and then the LIB is in the rest state. It is found that the LIB voltage decreases at first and then tends to maintain constant, indicating the low self-discharge of this LIB. The effects of the input-signal amplitude (Vp) on the LIB charging process are also investigated. As seen in Fig. 6c, the LIB charging rate increases with increasing Vp and after 200-s charging, the LIB voltage is approximate to the TFTR output voltage (≈Vp − Vturn-on) in each case, suggesting that a high LIB charging efficiency can be obtained by using the setup shown in Fig. 6a. The LIB voltage can be well maintained after shutting up the input signal mainly because of its low self-discharge rate as mentioned above.
Once the thin-film LIB is charged, it can be used as the on-chip power source to drive the PD, as seen in Fig. 6a. Figure 6d shows the response of the PD powered by the LIB under different-wavelength light illumination. A stable response is observed under a constant light illumination, demonstrating good reproducibility and stability of the PD. Moreover, the current relative variation (defined as (I − I0)/I0⋅100%, where I0 represents the initial current before light illumination. ~ 105.2% at 405-nm light vs. 28.7% at 660-nm light) changes obviously with changing the light wavelength, suggesting high sensitivity of this PD. The good stability and high sensitivity of the PD can be also observed in its response to various light illumination intensities, as seen in Fig. 6e. The above results suggest that the PD can work well by using the LIB as the on-chip power source.