Oxygen Vacancies Mediated Co‐Sputtered Ti‐Doped BiVO4 Thin Films‐Based Transparent Photovoltaic Device

BiVO4, a narrow bandgap material (2.5 eV), has been widely explored for photocatalytic applications, but its applications in the optoelectronic field are unexplored. This work explores BiVO4 for photovoltaic devices using the oxygen vacancies mediated co‐sputtered Ti‐doped BiVO4 (Ti:BiVO4) that exhibits on‐site power production by photovoltaics and see‐through features. The structural, chemical, and optical properties of Ti:BiVO4 are investigated for heterojunction formation with p‐type NiO film. The sputtering power of Ti plays a significant role in improving the light absorption capability by increasing oxygen vacancy concentration, enhancing the device performance. The devices show an open‐circuit voltage value of 676 mV and a short‐circuit current density value of 4.83 mA cm−2 with a maximum power production value of 122.2 µW under UV illumination of intensity 53.1 mW cm−2. The obtained device performances correlate with the Ti dopant's deposition power that tunes the structural and optical properties of BiVO4 films. Moreover, in self‐powered mode, the fabricated devices show a fast photoresponse speed of 0.8 ms with a high detectivity value of 2.22 × 1012 Jones. This work establishes the suitability of co‐sputtered Ti:BiVO4 for the next generation of transparent self‐powered optoelectronic devices.


Introduction
[6] Photovoltaic devices are generally opaque, whereas TPV devices produce electric power from the incident lights without compromising visible transparency.12][13][14][15] In order to develop an effective TPV device, it is essential to comprise an active layer with substantial absorption of ultraviolet (UV) and visible edge lights.18][19][20][21] Although BiVO 4 shows photoresponsivity under visible light illuminations, poor carrier separation, and fast recombination rate limits its applications in photovoltaics and photodetectors.In this scenario, defect engineering is an effective strategy to tune the properties of BiVO 4 . [22][25] Various methods have been deployed, such as doping, surface modification, and deposition of various cocatalysts on its surface to control the oxygen vacancies in BiVO 4 . [24,26,27]Among the methods mentioned above, doping is more effective for controlling the oxygen vacancy concentrations in BiVO 4 , which will improve its conductivity and prolong the charge lifetime by creating intermediate levels between bandgap for better performance. [28]o 6+ , [29] W 6+ , [30] and Nb 5+ [31] have been explored as dopants in BiVO 4 to improve photocatalytic performance.Such doping is anticipated to enhance the electron concentration of BiVO 4 by providing additional free electrons to its conduction band, which optimizes electron transport. [28,32]But there is a probability of forming donor-acceptor complexes in the host material when it is doped with a higher valence dopant (5+ or 6+) due to the strong Coulombic interaction between the donor dopant and metal vacancy defects, which reduces the dopant efficiency. [33]In this regard, tetra valence Ti 4+ would be an effective dopant to improve the photoresponse of the BiVO 4 , [34] which is also very effective in generating oxygen vacancies in the BiVO 4 films.Peng et al. [23] exhibited that the Ti dopant endows BiVO 4 with a less recombination rate and high oxygen vacancy concentration, effectively improving the photocatalytic activity.Whereas, Wang et al. [20] have reported enhanced photocatalytic performance for Ti-doped BiVO 4 films due to accelerated polaron hopping.There are several works have been performed on BiVO 4 /semiconductor heterojunction-based photocatalytic activity, such as BiVO 4 /WO 3 , [35] BiVO 4 /ZnO, [36] BiVO 4 /BiFeO 3 . [37]o our knowledge, the study on Ti-doped BiVO 4 (Ti:BiVO 4 ) films and their application to transparent optoelectronics has remained unexplored.This led to an effective study on developing Ti:BiVO 4 thin films-based TPV devices for on-site power production.
To fabricate Ti:BiVO 4 -based TPV devices, forming a good quality p-n junction by the built-in electric field is important.This work has chosen p-type NiO for p-type semiconductors to fabricate Ti:BiVO 4 -based TPV devices due to its suitable band alignment, high bandgap value (3.8 eV), transparency, and mobility. [38]he reactive magnetron co-sputtering has been used to deposit the Ti:BiVO 4 and NiO layers due to better doping control, fast growth rate, and large-scale production. [16,39]The sputtering power of Ti has been varied from 25 to 50 W during the co-sputtering of Ti:BiVO 4 to control its structural, optical, and doping concentration by extrinsic doping effect.Fabricated devices of Ti:BiVO 4 , specifically for Ti power of 50 W, showed an open-circuit voltage (V OC ) value of 676 mV and a shortcircuit current density (J SC ) value of 4.83 mA cm −2 along with a maximum power of 122.2 μW under UV (385 nm, 53.1 mW cm −2 ) illuminations.Moreover, the fabricated devices obtained the highest photo-to-dark current ratio value of 6.0 × 10 5 with a responsivity value of 112.4 mA W −1 and a detectivity value of 2.22 × 10 12 Jones at zero bias.All the TPV devices exhibit a fast photoresponse speed of 0.8 to 2.2 ms in self-powered mode.Therefore, this work indicates the potential of Ti:BiVO 4 as a photovoltaic material for next-generation photovoltaic applications.
The full width at half maximum (FWHM) values of the peak (121) are 0.74°, 0.62°, and 0.60°for Ti25, Ti35, and Ti50 thin films (Table S1, Supporting Information), respectively, whereas this value is 0.83°for undoped BiVO 4 film.The decreasing trend in the FWHM values with an increase in Ti deposition power indicates that the crystallinity of Ti:BiVO 4 thin films improves with Ti deposition power. [41]Moreover, the reduction in the FWHM values indicates the decrease in the dislocation-type defects or planar imperfections, such as grain boundaries for the higher Ti deposition power Ti:BiVO 4 thin films, reflected in the device performances. [42]The crystallite size (D) for these films is estimated using the Debye-Scherrer formula: [43] where K is the dimensionless shape factor with a value of 0.94;  is the wavelength of the X-ray radiation (1.5406 Å);  is the FWHM value.The estimated D values based on the (121) peak are tabulated in Table S1 (Supporting Information), confirming the crystallite size increase from 11.5 to 14.3 nm with the increase in Ti deposition power from 25 to 50 W, whereas the crystallite size is 10.3 nm for undoped BiVO 4 film.Moreover, the peak intensity ratios of the (040) and (121) (i.e., the intensity of (040)/intensity of (121)) planes increased from 0.33 to 0.40 for the Ti50 film with respect to the Ti25 film.This enhancement is known for its better photocatalytic performance, [44][45][46] which is also applicable to photovoltaic performance, and this will be discussed later.Another interesting observation from the XRD pattern is that the (200) plane disappeared for the higher Ti deposition power, which has some positive effects on the Ti:BiVO 4 -based TPV device performance which will also be discussed later.The lattice parameters of undoped BiVO 4 and Ti:BiVO 4 thin films were estimated from the XRD patterns (Table S1, Supporting Information), and these values are a good match with the BiVO 4 monoclinic structure. [42]attice parameter c decreased with increasing the Ti deposition power (Table S1, Supporting Information) due to the generation of strain in higher Ti deposition power Ti:BiVO 4 thin films. [47]he surface microstructure of Ti:BiVO 4 thin films was studied by the field-emission scanning electron microscope (FESEM) images, as shown in Figure 1b,c, indicating the formation of crack-free and compact films.Moreover, with increased Ti deposition power, the films become slightly more compact due to better crystallinity, as confirmed by the XRD. Figure S1a,c (Supporting Information) shows the uniform distribution of Ti, Bi, V, and O elements on the surface of Ti:BiVO 4 thin films.Elemental analysis of Ti:BiVO 4 thin films was carried out using energy-dispersive X-ray spectroscopy (EDS) (Figure S1b,d, Supporting Information), which confirms that Ti contents strongly depend on the Ti deposition power during the co-sputtering and it can be easily tuned to obtain better device performance.Ti content increased from 0.46 at% to 1.02 at% with an increase in the Ti deposition power from 25 W to 50 W.Additionally, oxygen amount is reduced from 71.6 at% to 70.84 at% with increasing Ti doping concentration from 0.46 at% to 1.02 at%, as shown in Figure S1 (Supporting Information), indicating generation of oxygen vacancies with increasing Ti deposition power.Figure S1e (Supporting Information) shows the elemental mapping of the Ti:BiVO 4 -based TPV device cross-sectional view, which confirms the presence of the BiVO 4 and NiO layers.

Valence State of the Elements
The valence state of the Ti, Bi, and V in Ti:BiVO 4 thin films was investigated from the X-ray photoelectron spectroscopy (XPS) analysis (Figure 1d-g).Figure S2 (Supporting Information) shows the survey scan spectra for Ti25 and Ti50 films, confirming the presence of Ti, Bi, V, and O.The XPS spectrum of Ti 2p shows two peaks at the binding energy of 457.2 and 464.8 eV corresponding to Ti 2p 3/2 and Ti 2p 1/2 (Figure 1d), respectively. [23]urther to confirm the valence state of the Ti in the BiVO 4 , we have deconvoluted the Ti 2p 1/2 peak into two peaks for Ti25 and Ti50 films as shown in Figure 1d.The deconvoluted Ti 2p peaks are positioned at ≈463.5 and 465.1 eV respectively, which indicates the presence of Ti 3+ and Ti 4+ both valence states of Ti in the Ti:BiVO 4 films. [48]Moreover, the intensity of Ti 2p 3/2 peaks is enhanced for Ti50 films, indicating more concentration of Ti for higher Ti deposition power, which is at par FESEM EDS results (Figure S1, Supporting Information).The Bi 4f spectrum shows (Figure 1e) two peaks at the binding energy of 158 and 163.4 eV with a peak splitting value of 5.4 eV for both Ti25 and Ti50, indicating the valence state of Bi is 3+. [25]Figure 1f shows two peaks at the binding energy of 515.4 and 522.8 eV of the V 2p spectrum with a peak splitting value of 7.4 eV for both Ti25 and Ti50, indicating the valence state of V is 5+. [25]Figure 1g shows the deconvoluted O 1s spectrum for Ti25 and Ti50 films.The O 1s peaks are deconvoluted into two peaks at 529.1 eV (529.22 eV) and 531.0 eV (531.1 eV) for Ti25 (Ti50) film, which correspond to the lattice oxygen and oxygen deficiencies in the BiVO 4 lattice, respectively. [23,25]Interestingly, the relative oxygen vacancy (oxygen vacancy/(oxygen vacancy + lattice oxygen)) amount increases from 9% to 13% for Ti50 film as shown in Table S2 (Supporting Information), indicating the generation of more oxygen vacancies in Ti:BiVO 4 thin films for higher Ti deposition power.This can be explained by the charge-neutrality principle, where one/two Ti (Ti 3+ /Ti 4+ ) atoms would substitute one/two V (V 5+ ) atoms in the BiVO 4 lattice by introducing an oxygen vacancy. [23]ow, at higher Ti deposition power, the Ti dopant concentration will be high in BiVO 4 , which leads to the formation of more oxygen vacancies in the Ti:BiVO 4 films.In addition, a slight shift of ≈0.12 eV in Bi 4f spectra of Ti50 can also be observed compared to Ti25 film (Figure 1e), which is attributable to the different chemical environments caused by the oxygen vacancies. [20]

Optical Properties
Figure S3 (Supporting Information) and Figure 2a shows the transmittance spectra of glass/undoped BiVO 4 and glass/Ti:BiVO 4 with different Ti deposition power.The transparency values in the visible region are decreased with an increase in Ti deposition power, and the corresponding absorption coefficient () values are increased, as shown in Figure 2b, which is at par the absorbance spectra (Figure S4a, Supporting Information).The  values for the Ti:BiVO 4 thin films have been estimated using the following equation: where d, R, and T are the thickness, reflectance, and transmittance values respectively.This enhancement in the  values with Ti deposition power indicates the ability of light absorption is improved with Ti deposition power, which is required for better photovoltaic performance.Figure 2c shows the optical bandgap values of the Ti:BiVO 4 thin films are ≈2.5 eV, whereas this value is ≈2.77eV for undoped BiVO 4 thin film (inset in Figure S3, Supporting Information).The optical bandgap values were estimated from the Tauc plots, and this value decreased from 2.51 eV to 2.44 eV with increasing Ti deposition power from 25 W to 50 W due to the generation of more oxygen vacancies. [42,49]Interestingly, the inset in Figure S4a (Supporting Information) shows the tail absorbance band in the range of 700 -850 nm for Ti:BiVO 4 films, indicating the formation of oxygen vacancies in those films. [50]The tail absorbance value increases with Ti deposition power, indicating the formation of more oxygen vacancies for higher Ti deposition power, which is at par with the XPS results.Generally, the oxygen-deficient BiVO 4 thin films exhibit better photocatalytic performance due to the low recombination rates. [42]Similarly, better TPV device performances have been obtained for higher power Ti deposited Ti:BiVO 4 thin films-based devices.Therefore, by controlling the Ti deposition power during the co-sputtering, one can easily tune the optical properties of Ti:BiVO 4 thin films, which is a simple and effective way to control the device's performance.Ti:BiVO 4 thin films show <30% reflectance in the UV and visible edge region, as shown in Figure S4b (Supporting Information), which is effective for the photoelectric effect.The refractive index (n) and extinction coefficient (k) are the important optical constants that depend on the wavelength of the electromagnetic wave through the dispersion relation.An electromagnetic wave propagating through a medium also experiences attenuation due to free carrier absorption, lattice vibration, photogeneration, and scattering. [51]Figure 2d,e shows the Ti deposition power-dependent variation in n and k values of the Ti:BiVO 4 thin films as a function of wavelength in the range 380 to 800 nm. Figure 2d,e shows that the n and k values decrease with increasing wavelength.
The n values are 2.57, 2.61, and 2.62 at wavelength 460 nm for Ti25, Ti35, and Ti50 films, respectively, indicating higher attenuation can be obtained for higher Ti deposition power (i.e., Ti50).The optical conductivity ( op ) is another important optical parameter of a material, which implies the electrical conductivity in the presence of an alternating electric field.The value of the  op can be estimated using the following equation: [52]  op = nc 4 where c is the speed of light.Figure 2f shows the variation of  op values as a function of photon energy (h) and higher  op values obtained at higher photon energy due to the higher  and n values.Figure 2f clearly shows that the  op values increase with the Ti deposition power, indicating that the contribution of electron transitions between the valence and conduction bands increases with the Ti deposition power of the Ti:BiVO 4 thin films.The highest  op values were obtained for Ti50 film in the range of 4.17 × 10 13 to 3.14 × 10 14 s −1 .

TPV Device Structure and Performances
To investigate the effect of Ti deposition power on the photoelectric performance of the Ti:BiVO 4 /NiO heterojunction, I-V characteristics were carried out under two different light sources ( = 385 and 460 nm). Figure 3a shows the schematic of the Ti:BiVO 4 device (FTO/Ti:BiVO 4 /NiO/AgNWs) with illumination direction.
Figure 3b shows the cross-sectional FESEM image of the device structure, which confirms the thickness of the Ti:BiVO 4 layer is ≈410 nm for the Ti50 device.The transmission spectra of the devices are shown in Figure 3c, which confirms the visible transparency of the FTO/Ti:BiVO 4 /NiO/AgNWs devices.The average visible transparency (AVT) in the visible region (400 -825 nm) was estimated using the following equation: [53] AVT = ∫ 825 400 S () P () T () d ∫ 825 400 S () P () d Figure S5 (Supporting Information) shows the semi-log I-V plots of undoped BiVO 4 -based devices under dark and illumination conditions, confirming the no photovoltaic effect for the undoped BiVO 4 thin film-based device.In contrast, the I-V characteristics of the Ti:BiVO 4 film-based devices exhibit prominent photovoltaic effects under the 385 nm and 460 nm illuminations, as shown in Figure 3d-f.The dark current values for these Ti:BiVO 4 -based devices are in the order of nA, which confirms the formation of good quality Ti:BiVO 4 /NiO heterojunction.The lowest dark current value of ≈2 nA was obtained for the Ti50 device, indicating the formation of a superior junction compared to the others, which may be due to the better crystalline quality, as confirmed by the XRD pattern.In order to verify the photoresponses of the devices, the photo-to-dark current ratio (I photo /I dark ) values are estimated, and the highest I photo /I dark value of 6.0 × 10 5 was obtained for the Ti50 device at zero bias under the 385 nm illuminations (53.1 mW cm −2 ) as shown in Figure 3f.This value is 7.3 × 10 3 and 1.5 × 10 4 for the Ti25 and Ti35 devices, respectively.Interestingly, all the devices show a V OC value under both 385 nm and 460 nm illuminations, and this V OC value is increased with Ti deposition power (Figure 3d-f), indicating more generation of electron-hole pairs due to higher light absorption ability in the Ti:BiVO 4 thin films for a higher Ti deposition power.The obtained V OC values are 432, 583, and 676 mV for Ti25, Ti35, and Ti50 devices, respectively, under the 385 nm illuminations (53.1 mW cm −2 ), whereas these values are 303, 385, and 406 mV under the 460 nm illuminations (103.9 mW cm −2 ), as shown in Figure 3d-f and Table 1.
Figure 4a,b shows the I-V plots of Ti:BiVO 4 -based TPV devices in a linear scale under 385 nm (53.1 mW cm −2 ) and 460 nm (103.9 mW cm −2 ) illuminations.All the TPV devices exhibited the fourth quadrant photo-induced I-V under light illuminations, as shown in Figure 4a,b, indicating the ability of the on-site power generation.The photovoltaic effect varied with the Ti deposition power.The power conversion efficiency (PCE) values improved with the Ti deposition power under both 385 and 460 nm illuminations, and the highest PCE value of 0.92% was obtained for the Ti50 device under 385 nm illuminations.In contrast, the highest fill factor (FF) value of 23.02% is obtained for Ti35 de- vices under 460 nm illuminations (Table 1).Figure 4c,d shows that each TPV device generates electric power under 385 nm (53.1 mW cm −2 ) and 460 nm (103.9 mW cm −2 ) illuminations.
The fabricated TPV devices produce a power of 33.5, 89.7, and 122.2 μW for Ti25, Ti35, and Ti50 devices, respectively, under the 385 illuminations (53.1 mW cm −2 ).While under the 460 nm illuminations (103.9 mW cm −2 ) the power production value is 18.03, 26.3, and 31.3 μW for Ti25, Ti35, and Ti50 devices, respectively.The obtained results of power production fulfill the concept of on-site power generation.Figure 4e-h shows the V OC and J SC values of the Ti:BiVO 4based TPV devices in different light intensities under 385 nm and 460 nm illuminations.Semi-log I-V plots for different light intensities are shown in Figure S6 (Supporting Information).Figure 4e,g shows an enhancement in the V OC and J SC values of Ti50 device under 385 nm illuminations from 412 to 676 mV and 0.24 to 4.83 mA cm −2 for the intensity range of 2.1 to 53.1 mW cm −2 .Similarly, Figure 4f,h shows an enhancement in the V OC and J SC values of the Ti50 device under 460 nm illuminations from 251 to 406 mV and 0.074 to 1.51 mA cm −2 for the intensity range of 4.1 to 103.9 mW cm −2 .On the other hand, the variation in V OC (J SC ) values for Ti25 and Ti35 devices are 230 to 432 mV (0.085 to 1.57 mA cm −2 ) and 348 to 583 mV (0.14 to 3.29 mA cm −2 ) under 385 nm illuminations (2.1 to 53.1 mW cm −2 ) and 107 to 303 mV (0.058 to 1.03 mA cm −2 ) and 222 to 385 mV (0.064 to 1.19 mA cm −2 ) under 460 nm illuminations (4.1 to 103.9 mW cm −2 ).The enhancement in the J SC values with light intensities indicates more incident photons interact with the material implying more generation of charge carriers in the conduction band.The enhancement in the V OC values with light intensities indicates the formation of a strong electric field at a high illuminating light intensity, which efficiently collects the photogenerated carriers.J sc versus light intensity plots show a linear relationship under both illuminations for all TPV devices, while V OC versus light intensity plots show that V OC values tend to saturation with increasing the illuminations light intensity.Figure S7 (Supporting Information) shows the I-V characteristics of the Ti:BiVO 4 -based TPV devices for the thicknesses ranging from 200 to 500 nm of Ti:BiVO 4 layer under the UV illuminations (365 nm, 60 mW cm −2 ).During deposition of the Ti:BiVO 4 layer the Ti deposition power was kept at 50 W.The V OC value of the Ti50 device under 365 nm illuminations (60 mW cm −2 ) was increased from 380 to 756 mV for the thickness range of 200 to 500 nm.On the other hand, the J SC values decreased from 2.9  Table 2 shows the comparison of crucial parameters of previously reported TPV devices with the present Ti:BiVO 4 -based TPV devices.Table 2 shows that the Ti:BiVO 4 -based TPV devices exhibit better J SC and V OC values compared to the previously reported oxide-based TPV.Though organic and perovskite-based TPV devices exhibit better performance, these devices have stability issues.In contrast Ti:BiVO 4 -based TPV device exhibit better stability performance which will be discussed later.The fabricated Ti:BiVO 4 -based TPV devices are high-sensitive under light illuminations at zero bias, indicating their suitability as a self-powered TPD.To examine this, responsivity (R) and detectivity (D * ) values are estimated using the following formulas: [64] where P Light is the intensity of illumination light, A is the device area, and q is the elementary charge.Figure 5a-d shows the R and D * values for the TPV devices in different light intensities under 385 and 460 nm illuminations at zero bias.The R values increase from 29.4 to 39.9 mA W −1 , 61.5 to 66.6 mA W −1 , and 90.6 to 112.4 mA W −1 with decreases in the light intensity from 53.1 to 2.1 mW cm −2 of 385 nm illuminations for Ti25, Ti35, and Ti50 TPV devices, respectively (Figure 5a).Similarly, The D * val-ues increase from 1.14 × 10 11 to 1.54 × 10 11 Jones, 2.34 × 10 11 to 2.54 × 10 11 Jones, and 1.79 × 10 12 to 2.22 × 10 12 Jones with decreases in the light intensity from 53.1 to 2.1 mW cm −2 of 385 nm illuminations for Ti25, Ti35, and Ti50 TPV devices, respectively (Figure 5c).Ti50 TPV devices exhibit the highest R and D * values of 112.4 mA W −1 and 2.22 × 10 12 Jones under 385 nm illuminations.
Figure 5b,d shows the R and D * values for the TPV devices in different light intensities under 460 nm illuminations at zero bias.The R values increase from 9.8 to 13.6 mA W −1 , 11.4 to 16.5 mA W −1 , and 14.5 to 21.5 mA W −1 with decreases in the light intensity from 103.9 to 4.1 mW cm −2 of 460 nm illuminations for Ti25, Ti35, and Ti50 TPV devices, respectively (Figure 5b).Similarly, The D * values increase from 3.81 × 10 10 to 5.27 × 10 10 Jones, 4.33 × 10 10 to 6.31 × 10 10 Jones, and 2.87 × 10 11 to 4.24 × 10 11 Jones with decreases in light intensity from 103.9 to 4.1 mW cm −2 of 460 nm illuminations for Ti25, Ti35, and Ti50 TPV devices, respectively (Figure 5d).All the TPV devices exhibit higher R and D * values at a lower intensity under UV and visible illuminations.
The optical detection phenomena of these TPV devices were further studied.Figure 5e,f show the current-time (I-t) spectra of the Ti50 TPV device in different light intensities at zero bias under pulsed 385 and 460 nm illuminations.Figure S9 (Supporting Information) shows the I-t spectra of Ti25 and Ti35 TPV devices in different light intensities at zero bias under pulsed 385 nm and 460 nm illuminations.The I photo values for all the devices decrease with a decrease in both UV and visible light intensities.The response speed for the TPV devices is estimated from these I-t spectra.The rise ( rise ) and fall times ( fall ) for the TPV devices have been estimated for 10% to 90% growth and 90% to 10% decay of the maximum I photo values from the I-t spectra, as shown in the insets of Figure 5e,f and Figure S9 (Supporting Information). [65]Figure S10 (Supporting Information) shows the  rise and  fall of TPV devices under 385 nm and 460 nm illuminations at different Ti deposition powers.All the TPV devices exhibit a fast  rise and  fall in the range of 0.8 -2.2 ms, which can be attributed to less trap states-based charge carrier transport in Ti:BiVO 4 -based TPV devices.Figure S10a,b (Supporting Information) shows that the  rise and  fall values were slightly reduced with an increase in the Ti deposition power.The fastest  rise and  fall values of 0.8 and 1.1 ms have been obtained for the Ti50 device under 385 nm illuminations.Whereas these values are 2.2 and 2.2 ms, and 0.9 ms and 1.2 ms under 385 nm illuminations for Ti25 and Ti35 devices, respectively.This slight improvement in the  rise and  fall values with Ti deposition power may be due to better charge separation or crystallinity for higher Ti deposited Ti:BiVO 4 thin films-based TPV devices.To confirm this, log-log J SC versus light intensity plots (Figure 4g,h) were fitted by the power law (J SC ∝ P  Light , where  is the corresponding power law coefficient, Figure S11, Supporting Information). [66]his  value indicates the charge separation, recombination or trapping process of the photogenerated electron-hole pair in the Ti:BiVO 4 thin films and can be estimated from the slope of the log-log J SC versus light intensity plots, as shown in Figure S11 (Supporting Information).The estimated  values are 0.90, 0.92, and 0.96 for Ti25, Ti35, and Ti50 devices, respectively, under the 385 nm illuminations (Figure S11a, Supporting Information).These values are 0.88, 0.92, and 0.95 for Ti25, Ti35, and Ti50 devices, respectively, under the 460 nm illuminations (Figure S11b, Supporting Information).The  values tend to be closer to 1 with an increase in the Ti deposition power, indicating comparatively better charge separation and minimum recombination along to the increased Ti deposition power. [67]The stability performance of the Ti50 device was carried out under pulsed 365 nm illuminations (1.2 mW cm −2 with 10 Hz), as shown in Figure S12 (Supporting Information).The photocurrent values reduced from 15 to 13 μA after 5 days of measurements (Figure S12, Supporting Information), confirming the high stability of the device.

TPV Device Mechanism
To verify the photovoltaic mechanism of Ti:BiVO 4 /NiO TPV devices, the energy diagrams are presented in Figure 6 for both undoped and Ti-doped BiVO 4 -based TPV devices.According to the TPV device structure, Ti:BiVO 4 /NiO heterojunction formed on the FTO, where FTO works as a transparent conducting layer, Ti:BiVO 4 layer works as a photon absorbing layer, and NiO layer creates a heterojunction to the Ti:BiVO 4 and also works as a hole transport layer.Finally, AgNWs were deposited as a top electrode to complete the TPV device.The corresponding bandgap values for Ti:BiVO 4 (BiVO 4 ) and NiO films are 2.5 eV (2.8 eV) and 3.8 eV, respectively, which were obtained from the Tauc plots.For the Ti:BiVO 4 /NiO heterostructure, the Fermi level splitting (∆E f ) enhances with Ti deposition power due to more generation of electron-hole (e − -h + ) pairs under the light illuminations.The V OC values of the Ti:BiVO 4 /NiO heterostructure depend on the value of ∆E f .For higher ∆E f values, the corresponding V OC values will also be high.Moreover, oxygen vacancies and Ti 3+ states in the Ti:BiVO 4 layer create some intermediate defect states (E defects ), as shown in Figure 6b, which is at par with the XPS results.The formation of E defects states in Ti:BiVO 4 assists in more light absorption, endowing the device's performance with reduced recombination rates. [50,68]Wang et al. [50] have reported that this oxygen vacancies in BiVO 4 films act as shallow donors.On the other hand, the doping scheme influences the minority carrier lifetime to improve the photovoltaic performance. [69]The photovoltaic performance enhancement by the Ti-doped BiVO 4 TPV device can be clarified by the increased minority carrier lifetime.In this context, minority carrier lifetime was estimated using the transient photovoltage spectra measurements.BiVO 4 and Ti-doped BiVO 4 TPV devices were measured for photovoltage profiles under the pulsed illumination of 365 nm with a frequency of 10 Hz and intensity of 2.7 mW cm −2 , as shown in

Conclusion
We demonstrate a large photovoltaic effect in oxygen vacancies mediated co-sputtered Ti:BiVO 4 thin films-based TPV devices, where Ti deposition power was varied from 25 to 50 W to tune the structural and optical properties of the films for better device performance.The fabricated TPV devices exhibit V OC and J SC

Experimental Section
Device Fabrication: A large-scale magnetron sputtering system (Solarlight, Korea) was used to fabricate undoped and Ti:BiVO 4 thin films-based TPV devices.The basic device structure is FTO/Ti:BiVO 4 /NiO/AgNWs.Before the deposition of Ti:BiVO 4 layer, FTO (735159, Aldrich; sheet resistance = 7 Ω □ −1 ) substrate was cleaned with a sequence of acetone, methanol, and deionized water under ultrasonication for 10 min each and then dried under N 2 gas flow.Ti:BiVO 4 layer was deposited by reactive co-sputtering methods using a ceramic BiVO 4 target (99.99%,iTASCO) and a metal Ti target (99.99%,iTASCO).The details of the deposition parameters and nomenclature of the samples are summarized in Table 3.According to the deposition condition of Ti in BiVO 4 films, samples were nominated as Ti25 (Ti sputtering at 25 W), Ti35 (Ti sputtering at 35 W), and Ti50 (Ti sputtering at 50 W), respectively.To obtain the monoclinic crystalline phase of the BiVO 4 , a post-growth rapid thermal annealing (RTA) was carried out at 450 °C in air ambient for 10 min.For heterojunction formation, the NiO layer was deposited at room temperature (RT) using a Ni metal (99.99%, iTASCO) target with an Ar/O 2 flow rate of 20/5 sccm at 50 W DC power and a working pressure of 3 mTorr for 25 min.Before sputtering gas injection, the sputtering chamber was evacuated to a base pressure value of 4 × 10 −6 Torr and the substrate was rotated at 5 rpm to obtain a uniform film.Silver nanowires (AgNWs, diameter ≈20 nm and length ≈20 μm, Nanopyrix) were spin-coated over the NiO layer to complete the TPV device fabrication.After the AgNWs coating, a 10 nm ZnO layer was deposited for reliable electric contacts. [70,71]haracterizations: The crystal structure and phase of undoped and Ti:BiVO 4 thin films were studied using XRD (Rigaku, SmartLab) with Cu K radiation ( K = 1.5406Å).The chemical state of Ti:BiVO 4 thin films was studied using XPSwith Al K (h = 1486.6eV) monochromatic radiation.The FESEM (JEOL, model: JSM_7800F) was used to measure the surface morphology and elemental analysis of Ti:BiVO 4 thin films.The thickness of the Ti:BiVO 4 layer was confirmed from the cross-sectional FESEM image.The UV-vis-NIR diffused reflectance spectrophotometer (Shimadzu, model: UV-2600) was used to record the transmittance, absorbance, and reflection spectra of the Ti:BiVO 4 thin films and devices.The optical con-stants of Ti:BiVO 4 thin films were obtained using ellipsometry (ALPHA-SE, ellipsometer).
Linear sweep voltammetry (LSV) measurements were carried out to measure the current-voltage (I-V) and transient (I-t) characteristics of the devices under dark and illumination conditions using a potentiostat/galvanostat (PGStat, WonA Tech, ZIVE SP1) with a scan rate of 100 mV s −1 .During I-V and I-t measurements, the bottom electrode (i.e., FTO) was connected with the negative bias, while the top electrode (i.e., AgNWs) was connected with the positive bias of a potentiostat/galvanostat.As light sources, a UV light-emitting diode (LED) ( = 385 nm) and a blue LED ( = 460 nm) were used.A function generator (Topward, model: 8150) was used for pulsed illumination during I-t measurements.A solar simulator and PV power meter (McScience, K300, K101, Korea) were used to measure the performance of the devices, where light intensity was tuned to 100 mW cm −2 by controlling the current source to the xenon arc lamp.All the measurements were carried out at RT.The active area of the devices is ≈25 mm 2 , which is defined by the AgNWs' top electrode size.

Figure 1 .
Figure 1.a) XRD pattern of undoped and Ti-doped BiVO 4 films.Top-view FESEM images of the b) Ti25 and c) Ti50 films.Binding energy spectra of d) Ti 2p, e) Bi 4f, f) V 2p, and g) O 1s for Ti25 and Ti50 films.

Figure 2 .
Figure 2. Optical properties of Ti:BiVO 4 thin films.a) Transmittance spectra, b) absorption coefficient plots, and c) Tauc plots show bandgap values.Variation of d) refractive index and e) extinction coefficient of Ti:BiVO 4 thin films with Ti deposition power as a function of wavelength.f) Optical conductivity plot of Ti:BiVO 4 thin films as a function of photon energy.

( 4 )
where S, P, and T are the solar photon flux, photopic response, and transmittance values, respectively.The AVT values in the 400 -825 nm range are 38%, 31%, and 27% for Ti25, Ti35, and Ti50 devices, respectively.The decrease in the AVT values with an increase in the Ti deposition power indicates the enhancement in the oxygen vacancies in the Ti:BiVO 4 thin Ti deposition power as mentioned earlier.The inset of Figure3cshows the digital photograph of the fabricated Ti:BiVO 4 thin film-based TPV devices.

Figure 5 .
Figure 5. Responsivity versus light intensity plots under a) 385 nm and b) 460 nm illuminations.Detectivity versus light intensity plots under c) 385 nm and d) 460 nm illuminations.Responsivity and detectivity values are estimated at zero bias.Transient photocurrent spectra of the Ti50 device in different intensities under e) 385 nm and f) 460 nm illuminations.Corresponding insets show the rise and fall time of the devices at zero bias.

Figure 6c .
The obtained minority lifetime time values are 0.8 and 10.6 ms for BiVO 4 and Ti:BiVO 4 -based TPV devices, respectively, indicating the formation of intermediate defect states in the Ti:BiVO 4 layer.

Table 2 .
Comparison of crucial parameters of previously reported TPV with the present Ti:BiVO 4 -based TPV devices.for the thickness range of 200 to 400 nm.The highest PCE value of 0.50% was obtained for the thickness of 400 nm with an FF value of 22.5%.With further increase in the thickness of the Ti:BiVO 4 layer (such as 500 nm), the FF value reduced from 22.5% to 15.9%, indicating the optimized thickness is ≈400 nm for the Ti:BiVO 4 layer for the thickness range 200 to 500 nm.FigureS8(Supporting Information) shows the undoped BiVO 4 and Ti:BiVO 4 -based TPV device performance under the standard solar spectrum (AM1.5G).The obtained V OC and J SC values are 730 mV and 0.17 mA cm −2 , respectively, for Ti:BiVO 4based TPV devices, respectively, whereas these values are 60 mV and 0.1 mA cm −2 for undoped BiVO 4 -based TPV device.

Table 3 .
Parameters for Ti:BiVO 4 reactive co-sputtering and nomenclature of the samples.BiVO 4 -based TPV devices enable the electrical power generation (122.2 μW) from UV light, while it passes the visible light for high transparency (38%).Moreover, the fabricated Ti:BiVO 4 devices show a fast photoresponse in self-powered mode with a rise and fall time of 0.8 ms and 1.1 ms with a detectivity value of 2.22 × 10 12 Jones, indicating the suitability of Ti:BiVO 4 for self-reliant photoelectric applications.Overall, this work exhibited the potential of Ti:BiVO 4 for the see-through onsite power generation.