Photonic curing for innovative fabrication of flexible metal oxide optoelectronics

Flexible optoelectronics, based on non-planar substrates, hold promise for diverse applications such as wearables, health monitors, and displays due to their cost-effective manufacturing methods. Despite the superior properties of metal oxides, the challenge of processing them at high temperatures incompatible with plastic substrates necessitates innovative annealing approaches. Photonic curing, which delivers microsecond to millisecond broadband (200–1500 nm) light pulses on a sample, emerges as a viable solution. Depending on the optical properties, the targeted film absorbs the radiant energy resulting in rapid heating while the transparent substrate absorbs a minimal amount of light and remains at ambient temperature. The light intensity can be high, but since the light pulse is short, the total energy absorbed by the sample remains low and will not damage the plastic substrate. This perspective explores the innovative application of photonic curing to fabricate flexible metal oxide optoelectronics, including thin-film transistors, metal–insulator–metal devices, solar cells, transparent conductors, and Li batteries, emphasizing the conversion of sol–gel precursors to metal oxides. However, this technique was initially developed for sintering metal nanoparticles to conductive patterns and poses intriguing challenges in explaining its mechanism for metal oxide conversion, especially considering the limited absorption of visible light by most sol–gel precursors. The review delves into UV-induced photochemistry, common flexible metal-oxide optoelectronic components, and non-intuitive distinctions between photonic curing and thermal annealing. By elucidating the distinctive role of photonic curing in overcoming temperature-related challenges and advancing the fabrication of flexible metal oxide optoelectronics, this perspective offers valuable insights that could shape the future of flexible optoelectronics.

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Introduction
Flexible optoelectronics, broadly speaking, include optoelectronic devices that are built on flexible and non-planar substrates.They have a wide range of applications such as wearables, health/wellness monitors, and displays [1].Although their performance is not as good as complementary metal oxide semiconductors (CMOS) used in computing and memories, they can be manufactured inexpensively by using solution printing methods and roll-to-roll fabrication to increase throughput.Using plastic substrates, most commonly polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), further decreases the price of the final products.Additionally, plastic substrates have excellent mechanical flexibility, compared to thin glass substrates, which is a critical requirement for roll-to-roll fabrication [2,3].
Applying solution deposition to fabricate flexible optoelectronics is especially appealing because many such techniques, e.g.slot die, gravure, and flexographic coating, can easily exceed 10 m min −1 and are hence compatible with high-throughput processes such as roll-to-roll manufacturing [2].Many flexible optoelectronics incorporate organic or polymeric materials as the active components, including conductors and semiconductors.This choice is driven by the ease of solution depositing and low-temperature (<150 • C) thermal annealing is sufficient to attain desired properties in organic/polymeric materials.Additionally, their coefficients of thermal expansion (CTEs) are similar to those of plastic substrates, minimizing thermal stress and the risk of mechanical failures.However, specialty chemicals are expensive due to complex synthesis and low yield.Furthermore, organic/polymeric materials generally exhibit inferior material properties compared to metal oxides, e.g.field effect mobility, dielectric strength, and transparency.
Sol-gel chemistry is the most widely practiced method to form metal oxides from solution with metal salts or metalorganic precursors [4].Thermal annealing provides the necessary energy to overcome the reaction barriers in conversion steps, such as solvent and ligand removal, condensation, and densification [4].Sol-gel conversion typically requires temperatures of 300 • C-500 • C for tens of minutes to achieve optimal material properties, e.g.high crystallinity, high dielectric constant, low porosity, or high mobility.However, this temperature range is incompatible with plastic substrates, most of which have low working temperatures well below 200 • C.Moreover, the CTEs of metal oxides are much smaller than those of plastics, leading to mechanical failure during conventional thermal annealing, which heats both the film and the substrate.Therefore, an alternative 'annealing' method is required so that the metal oxide films and the plastic substrates are not heated to high temperatures simultaneously.
Photonic curing, also called intense pulsed light (IPL) or flash lamp annealing, is a novel technique for postdeposition processing without heating.It uses a xenon flash lamp to deliver short (20 µs-100 ms) but high intensity (up to 50 kW cm −2 ) pulses of broadband light (200-1500 nm) to a sample.Only the part of the sample that absorbs the light, typically a thin film on top of a thick, transparent substrate, will be raised to high temperatures.Once the light pulse is off, the film temperature rapidly drops as the thermal energy dissipates away via heat transfer to the substrate or the environment.The high intesity enables localized elevated temperatures, while the short pulses ensure that the total energy imparted on the sample is small.Photonic curing has been successfully applied to sintering printed metal nanoparticle inks into conductive patterns on plastic substrates [5][6][7][8].In this context, the strong light absorption by metal nanoparticle inks results in a rapid rise to high temperatures in the films.Because the rate of all thermal processes depends exponentially on temperature according to the Arrhenius law, forming conductive patterns from particle sintering is significantly accelerated.Surprisingly, despite the wide-bandgap nature of metal oxide precursors, which typically do not absorb much of the processing light, photonic curing has been successfully demonstrated in converting transparent sol-gel precursors into metal oxide films within the millisecond timeframe [9][10][11][12][13].This contradicts the expectation that thermal processes-heating driven by light absorption-are the primary mechanism in sol-gel reactions during photonic processing.Non-thermal processes such as photochemical reactions will need to be considered.
In this Perspective, we focus on optoelectronic devices on flexible substrates that include at least one metal-oxide layer processed by photonic curing.Some examples of novel materials made on rigid substrates using photonic curing are also included to stimulate interest in making them on flexible substrates.Figure 1 illustrates a schematic of a photonic curing process applied to converting sol-gel metal oxide precursor films in a roll-to-roll setup, and figure 2 shows measured temperature vs. time profiles for thin-film metal thermistors subjected to photonic curing.In section 2 we review the photochemistry of applying ultraviolet (UV) light to facilitate metaloxide sol-gel reactions.In section 3, a variety of common flexible optoelectronic components made via photonic curing are reviewed.While the primary focus is on the light-induced conversion of sol-gel precursors to metal oxides, some examples will include using pulsed light to 'activate' or crystalize vapor phase deposited amorphous films.In section 4, we discuss some non-intuitive differences between photonic curing and thermal annealing, before summarizing and providing an outlook on future directions in section 5.

Photochemistry
Metal nitrates have been widely used as metal sources in sol-gel reactions because they can form high-quality metal oxide films and their decomposition temperatures are the lowest among common metal precursors [14].Furthermore, they can enable combustion sol-gel syntheses to lower the decomposition temperature by incorporating a fuel, such as urea or acetylacetone (acac), along with nitrates acting as  the oxidizer [15,16].Additionally, it was found that illuminating precursors with UV light before or during heating can further lower the sol-gel conversion temperatures [17][18][19][20][21][22].It was reported that deep UV light (<300 nm), typically from a low-pressure mercury or a deuterium lamp, is needed.Because the energy of deep UV photons is comparable to that of chemical bonds, the photochemical processes that involve forming or breaking bonds can occur without heat [23].UV light facilitates the decomposition of metal oxide precursors such as nitrates or acetates.Figure 3(a) depicts a schematic of UV interaction with a sol-gel precursor to form a dense metal oxide film.Using a shadow mask to define UV-exposed regions, Daunis et al showed that nitrate is not present in the exposed region and is still present in the non-exposed region (figure 3(b)), confirming that UV photons alone without heat can initiate the precursor decomposition [24].Additionally, UV light produces energetic radicals that contribute to the formation of the metal-oxygen-metal network [22].
These earlier works require UV illumination for many tens of minutes.While the xenon flash lamp used for photonic curing contains UV components, it is not known whether microseconds to milliseconds of exposure time are sufficient to effectively initiate nitrate decomposition.Despite the successful demonstrations reviewed in this perspective, this is an area where further fundamental research is needed.

Flexible metal-oxide optoelectronics
Metal oxides cover the entire range of electrical properties, from insulating, semiconducting, conducting, and even superconducting.Their bandgaps can range from below 1 eV to nearly 5 eV, spanning from infrared to UV.They also can exhibit complex multi-electron phenomena such as ferromagnetism or ferroelectricity.Thus, metal oxides can play a variety of roles in flexible optoelectronic devices.In this section, we will review works where metal-oxide films were made by photonic curing for gate dielectrics and semiconductor channels of thin-film transistors (TFTs), high-k or ferroelectric dielectrics in capacitors, transparent conducting electrodes for solar cells and light-emitting diodes (LEDs), transport layers of emergent and thin-film solar cells, and cathodes of Li batteries.Table 1 details the literature reviewed.As shown, the processing times are milliseconds to seconds, significantly shorter than traditional thermal annealing times.

TFTs
TFTs play a role in flexible electronics similar to CMOS in silicon integrated circuits.Metal oxides are used as both semiconductor channel materials as well as gate dielectrics.The semiconductor materials are typically indium-or zinc-based such as InO x , ZnO, and indium gallium zinc oxide (IGZO).These are n-type semiconductors with electrons generated by oxygen vacancies.The gate oxide materials are typically highk dielectrics such as aluminum oxide, hafnium oxide, or zirconium oxide and will be reviewed in section 3.2.Figure 4(a) illustrates the processing steps to fabricate metal oxide TFTs using photonic curing.
Garlapati et al applied a broadband UV lamp (with maximum intensity at λ = 190 nm) to sinter In 2 O 3 nanoparticles to form the channel for TFTs on PEN substrates.The total UV illumination time was 10 s.The UV-lamp-cured devices exhibited a field-effect mobility (µ FET ) of 8 cm 2 V −1 s -1 , on/off ratio of 2 × 10 6 , and subthreshold swing (SS) of 110 mV/decade.µ FET and SS are 10 and 20 times better than the non-UVilluminated counterparts [25].
Benwadih et al fabricated IGZO TFTs on polyimide (PI) substrates with the conversion of sol-gel films achieved by augmenting thermal annealing with UV pulsed light.They achieved µ FET of 4.5 cm 2 V −1 s -1 but not-so-ideal SS of 1.2 V/decade on polymer gate dielectric.However, there was a pretreatment of the precursor films at 350 • C for 1 min before UV illumination, preventing this processing method from applying to other low-temperature plastic substrates such as PET or PEN [26].
Yarali et al demonstrated photonically cured TFTs on PEN substrates containing bilayer In 2 O 3 /ZnO semiconductor channels and Al 2 O 3 /ZrO 2 high-k gate dielectrics.All oxide layers were formed from sol-gel precursors.Figure 4(b) shows the materials stack and the simulated temperature vs times profiles for each layer.The photonic curing processing time for the entire device was <60 s (33 s for the channel layers) with a maximum substrate temperature <200 • C.They achieved µ FET of 19 cm 2 V −1 s -1 , on/off ratio >10 6 , and SS of 108 mV/decade [27].

Metal-insulator-metal structures
Metal-insulator-metal (MIM) devices can be used as rectifiers, capacitors, mixers, and memories, making them important components in electronics.Metal oxides are the most common insulator materials for MIM devices.Optimal insulators require dense materials with low defect density to minimize leakage current and maximize breakdown strength.
Daunis et al fabricated Al/ZrO 2 /Al MIM structure on PEN substrates with the ZrO 2 photonically cured from zirconium nitrate sol-gel precursors.The photonic curing process only required a single pulse resulting in a processing time of 100 ms.With a single layer of ZrO 2 , the areal capacitance was >300 nF cm −2 and the breakdown field was 4 MV cm −1 .By going to two layers, the breakdown field increased to 8 MV cm −1 but with a lower areal capacitance due to thicker film.Nonetheless, this configuration still exhibited an areal capacitance of >200 nF cm −2 .The leakage current density was <10 −6 A cm −2 at 2 MV cm −1 .They also demonstrated that oxide films can be patterned by using a shadow mask during photonic curing or by using a pulse condition at a lower radiant exposure so that only the oxide precursors above an aluminum contact were converted, forming a self-aligned pattern (figure 5).Such an approach can facilitate the time-saving fabrication of oxide devices.Most significantly, the authors charted a pathway from their laboratory process to a web speed >30 m min −1 , a common web speed targeted by the flexible electronics industry [9].
Hwang demonstrated MIM structures with SrTiO 3 as the insulator on PEN substrates for electro-vibration touch panels.They found the highest dielectric constant was achieved by combining a deep UV lamp (λ = 180-280 nm) with the broadband xenon flash lamp and then subsequently subjecting the sample to a near IR (λ = 800-1500 nm) illumination for 180 s.They reported achieving a dielectric constant of 155 measured at 10 kHz [28].
Ferroelectricity in HfO 2 was discovered in 2011, but the orthorhombic phase associated with ferroelectricity is not the lowest-energy state.Conventional thermal annealing is an equilibrium process and favors the formation of the thermodynamically stable state, hence it is difficult to synthesize these interesting metastable materials.O'Connor et al applied photonic curing as a post-deposition annealing method to TiN/HfZrO 2 /TiN MIM structures on Si substrates in which the HfZrO 2 layer was deposited by atomic layer deposition.The photonic cured sample exhibited the ferroelectric phase (figure 6(a)) with polarization vs electric field hysteresis curve similar to that of the device made by the traditional rapid thermal annealing (RTA) method (figure 6(b)).While the coercive field (∼1.1 MV cm −1 ) was similar for samples made in these two different ways, superior remanent polarization values were obtained for the photonic cured device (21 µC cm −2 ) compared to the RTA device (18 µC cm −2 ) in this study [29].Although this work is not on a flexible substrate, we include it because it showcases how photonic curing can contribute to the novel synthesis of 'hard-to-make' materials under nonequilibrium conditions.

Transparent conductors
Transparent conductors are an indispensable component of photovoltaics, LEDs, and touch screens.Flexible or nonplanar transparent conductors are needed for large area displays and flexible photovoltaics.On rigid substrates, the most common transparent conductor material is indium tin oxide (ITO).However, high-temperature annealing or activation is needed to obtain highly transparent and conductive materials.Due to the low working temperature of plastic substrates, flexible transparent conductors made of ITO display significantly inferior properties than their rigid counterparts, with thin Green areas represent under-converted films, yellow areas represent films formed via self-aligning photonic curing process (sa-PC), blue areas represent films formed via blanket/shadow-mask photonic curing processes (b-PC/sm-PC), and red areas represent conditions that lead to substrate damage.The colored circles represent conditions that were tested experimentally.Reproduced from [9].CC BY 4.0.
ITO films exhibiting undesirably low conductivity and thick ITO films prone to mechanical failure.Thus, much effort has been devoted to exploring silver nanowire (AgNW) networks for flexible transparent conductors.Photonic curing has been shown to lower these AgNW networks' resistance by sintering the joints between overlapping nanowires [41][42][43][44].Here, we focus on using photonic curing to process wide bandgap oxides for flexible transparent conductors.
Piper et al used photonic curing to fabricate flexible hybrid transparent conductors consisting of silver metal bus lines (MBLs) combined with AgNWs overcoated with sol-gel IZO on PET substrates.The silver MBLs were flexographically printed on a roll-to-roll tool, while the AgNW and sol-gel IZO films were solution deposited with spin coating [11] or blade coating [30].All layers were photonic curing in a single processing step (figure 7(a)).The photonic curing process consisted of 9 pulses, each with a pulse fluence of 3.4 J cm −2 , resulting in a processing time of 53 s.For the blade-coated samples, after photonic curing, the hybrid transparent conductors achieved a sheet resistance of 11 Ω sq −1 and an average transmittance from 400 to 700 nm of 81%.Perovskite solar cells (PSCs) were fabricated on top of the hybrid transparent conductors and a champion power conversion efficiency (PCE) of 10% was achieved.This performance was comparable to devices made on glass/patterned-ITO and outperformed PSCs made on PET/ITO from Sigma-Aldrich (figure 7(b)).Finally, they were able to scale up the blade coating and photonic curing processes to fabricate 18 cm × 20 cm area samples with 13% uniformity in sheet resistance and <1% uniformity in average transmittance [30].
Recently, Gerlein et al applied photonic curing to fabricate transparent conductors made from AgNWs overcoated with crystalline TiO 2 .Both layers were deposited via spin-coating and photonic-cured separately.The AgNWs were photonic cured with 9 pulses at a fluence per pulse of 1.15 J cm −2 resulting in a processing time of 4.7 s.The TiO 2 films were photonic cured with 3 pulses at a fluence per pulse of 6 or 7 J cm −2 resulting in a processing time of 20 s.They found that the TiO 2 film could be crystallized into either anatase or rutile phases by adjusting the pulse fluence between 6 and 7 J cm −2 , respectively.Their final transparent conductor films had a transmittance at 550 nm of 84% with a haze value of 7.7% and a sheet resistance of 18 Ω sq −1 [31].Junghähnel et al applied photonic curing as a postdeposition method to annealed ITO and IZO films deposited at room temperature on flexible Willow Glass ® (WG) substrates.Glass substrates thinner than 200 µm are flexible and have several advantages over plastic substrates of the same thickness.Specifically, glass substrates exhibit higher optical transmission, lower roughness, higher chemical inertness, higher thermal stability, and lower oxygen and water vapor permeability.They demonstrated in-line roll-to-roll deposition and photonic curing at 1.2 m min −1 (figure 8).For 150 nm thick ITO films, the resistivity, carrier concentration, and carrier mobility after photonic curing for 2 ms at ∼9 J cm −2 were similar to those of a film annealed at 350 • C for 15 min.They also reported ±5% uniformity across a 30 cm × 10 cm area [32].

Metal oxide transport layer for solar cells
Flexible photovoltaics are envisioned to be useful for mobile, lightweight solar tents, for laminating on automobiles, and for harnessing indoor light to power the Internet of Things.Emergent photovoltaics, including organic, perovskite, and dye-sensitized, rely on carrier transport layers to establish a built-in field to collect electrons and holes at the cathode and anodes respectively.Metal oxides are often used for carrier transport layers.Electron transport layer (ETL) materials, e.g.TiO 2 , ZnO, or SnO 2 , typically have a low work function while hole transport layer (HTL) materials, e.g.MoO 3 , NiO, CuCrO 2 , have a high work function.While the active (lightabsorbing) materials in these solar cells do not need hightemperature processing, the metal oxides often do.There are two approaches to making metal oxide transport layers using photonic curing: (1) sintering pre-formed metal oxide nanoparticles and (2) converting sol-gel precursor to metal oxide films.In the first approach, an extra step is needed to synthesize the metal oxide nanoparticles and disperse them in a suspension that is suitable for solution deposition such as spin coating or blade coating.Because the oxides are already formed, the photonic curing energy required is lower, and therefore less likely to damage plastic substrates.Using this approach, Das et al spin-coated and photonic-sintered TiO 2 nanoparticles on ITO-coated PET substrates to fabricate ETL for PSCs.The PCE of the photonically cured devices was 12.6%, compared to 13.6% for devices with thermally sintered TiO 2 layer on glass substrates.However, thermal sintering of TiO 2 is typically >450 • C, which would not only destroy PET, but it would also damage ITO as well [33].Feleki et al also photonicsintered TiO 2 nanoparticles on ITO-coated PEN substrates as ETL for PSCs.They demonstrated a 100 times reduction in processing time while achieving the same PCEs (16%) (figure 9(a)) [34].Ghahremani et al made PSCs by photonic sintering SnO 2 nanoparticles on ITO-coated PET substrates as ETL followed by depositing halide perovskite precursors on top of it and photonic-curing the perovskite active layer, i.e. thermal annealing is not used at all for fabricating the PSCs.Using 5 pulses of 2.1 kJ, a champion PCEs of 7.6% was achieved for the flexible devices (figure 9(b)) [35].Besides PSCs, Jin et al photonic sintered TiO 2 nanoparticles on ITO-coated PEN substrates as photocathode for dye-sensitized solar cells.They showed that five seconds of 9. (a) Comparison of processing times for forming TiO 2 via thermal annealing, photonic curing, and UV-Ozone.Reprinted from [34], Copyright (2017), with permission from Elsevier.(b) Champion reverse-and forward-scan J-V curves for PSCs made with photonic cured SnO 2 nanoparticle ETLs and perovskite active layers.Reprinted from [35], Copyright (2020), with permission from Elsevier.photonic curing significantly improves the PCE by more than 115% [37].
The second approach is to apply photonic curing to perform chemical reactions, specifically converting sol-gel precursors to metal oxide films.This requires higher energy and is more challenging than sintering readily formed metal oxide nanoparticles.Zhu et al performed photonic curing to convert tin chloride precursor films to SnO 2 ETL for PSCs.Only one pulse with a 20 ms duration was used.Because of the high energy needed, this early work was done on glass/fluorine doped TiO 2 (FTO) substrates [36].Recently, Piper et al photonic cured nickel nitrate precursor films on 100 µm thick flexible ITO-coated WG substrates to form NiO x HTL for PSCs.To fabricate PSCs without any thermal annealing, photonic curing was also used to crystallize the perovskite layer.They achieved comparable device performance for devices regardless of whether HTL and perovskite layers are photonically cured or thermally annealed (figure 10).Furthermore, by using a more powerful tool, they showed that the NiO x HTL can be formed with a single pulse of 450 µs duration, translating to a reduction of HTL processing time by six orders of magnitude and enabling a web speed of 26 m min −1 [10].Note that both examples of converting directly from sol-gel precursors to metal oxides were carried out on glass substrates due to the higher radiant energy needed for chemical reactions than for particle sintering.However, WG is a viable flexible substrate that can be used in roll-to-roll processes [2].

Batteries
Thin-film solid-state batteries are ideal energy storage components for low-power devices such as wearables and radio frequency identification tags.Their cathode is made of Li oxide compounds, such as LiMnO 2 , LiCoO 2 , or LiPO 4 that are crystallized at temperatures >700 • C. Making these batteries on flexible substrates with a non-rigid form factor makes them more easily integrated with the electronic components that they are designed to power.Recently, Chen et al demonstrated photonic curing to make LiMnO 2 cathode from sol-gel precursor films on glass/FTO substrates.Batteries with photonically cured cathodes outperformed UV laser and thermally annealed counterparts with the highest capacity (6 µAh cm −2 ) and ten times shorter processing time (6 vs 60 min.)[38].The same group also demonstrated flexible solid-state batteries on 100 µm thick aluminum foil.The LiCoO 2 cathode was deposited by RF sputtering at room temperature and subsequently photonically annealed.The processing time varied from 50 s to over 100 s depending on the thickness of the film.The flexible batteries performed similarly to those made on rigid Si substrates [39].

Substrate effect in photonic curing
In traditional thermal annealing using ovens or hot plates, the entire materials stack is heated to the same temperature and consideration of the substrate properties is typically ignored.The premise of photonic curing is the transduction of radiant energy from the pulsed light into thermal energy in the film, based on light absorption of the film.Thus, the optical properties of the light-absorbing film determine the temperature it will reach.Figure 2 clearly demonstrates this effect: the Ni thermistor displays a higher peak temperature (177 • C) than the Al thermistor (72 • C) because Ni absorbs more light than Al (45% vs. 5%).Weidling et al fabricated TFTs using Mo instead of Al as the gate metal to assist in the photonic curing of IZO sol-gel precursor to oxides because Mo has higher absorbance than Al (58% vs 5%) [12].
Likewise, the optical properties of the substrates can affect the process results, especially if the film to be photonic cured is thin or not light absorbing, such as in the case of wide bandgap metal oxides discussed here.Piper et al showed that PSCs made on WG/ITO with different transmittance (figure 11(a)) exhibit drastically different performance in a way that is not intuitive.When using thermal annealing to fabricate the NiO x HTL, the most transmissive ('light') substrates produced the PSCs with the highest PCE because more light can reach the perovskite absorber.In contrast, when using photonic curing to fabricate the NiO x HTL, these substrates produced the worst-performing devices (figure 11(b)).XPS results show that nickel nitrate precursor was not fully converted to oxide on these substrates (figure 11(c)).Numerical simulations confirmed that the temperature during photonic curing for these substrates was too low to facilitate sol-gel reactions [40].These results unambiguously demonstrate the substrate's participation in the photonic curing process.
This unique feature of photonic curing can be used to form metal oxide patterns for electronic components.Daunis et al demonstrated that, by judiciously choosing the photonic curing parameters, the ZrO 2 sol-gel films were only converted to oxide on top of the Al bottom electrodes and not on the bare PET substrate [9].Thus, this method can simplify the fabrication of flexible optoelectronics because the metal oxide patterns are automatically self-aligned to the bottom contacts, avoiding performing elaborate lithography that requires extensive handling of flexible substrates.

Summary and outlook
In this Perspective, we reviewed many successful demonstrations of using photonic curing to fabricate flexible metal oxide optoelectronics.Despite the success, the fundamental understanding of photonic curing mechanisms is lacking.As discussed in section 2, blue and UV photons have enough energy to break chemical bonds and facilitate chemical reactions.Numerical simulations of temperature transients during the photonic curing process for sol-gel metal oxide conversion, based on measured optical properties of the precursor films, often produce temperatures that are below the thermal conversion temperature of the precursor.Thus, it is likely that direct photochemical pathways at the ambient temperature might be occurring during photonic curing.However, thermal vs photochemical effects are difficult to untangle because the two simultaneously occur during light illumination.Furthermore, the short timeframe of the flash lamp illumination and the rapid temperature changes can produce non-equilibrium effects.Currently, little is known about the sol-gel reaction mechanisms under millisecond light illumination.Additionally, there are very few experimental techniques that can be applied to accurately study real-time changes in the materials within those short time spans.Recently, a unique photonic curing tool was built to be incorporated into the SLAC beamline for operando, real-time x-ray scattering studies.Using this tool, Karigerasi et al revealed the phase transformation of HfZrO 2 to form the ferroelectric phase during photonic curing [45].More fundamental mechanistic studies similar to this are needed to advance the field of photonic curing.Additionally, the long-term performance of flexible metal-oxide optoelectronics made by photonic curing vs. other annealing methods will need to be investigated.These research results will propel the adoption of photonic curing for future manufacturing of flexible optoelectronics with economic benefits as shown for PSCs [46].

Figure 1 .
Figure 1.Schematic and applications of sol-gel metal oxide photonic curing roll-to-roll process.

Figure 2 .
Figure 2. Measured temperature vs. time measured by an aluminum (black) and a nickel (green) thin film thermistor.

Figure 3 .
Figure 3. (a) Process schematic of UV interaction with sol-gel precursor to create a metal oxide film.[18].John Wiley & Sons.© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.(b) XPS map of N1s signal intensity surrounding a 600 µm diameter UVO exposed circle on an indium nitrate precursor film.Scale bar is 200 µm.Top: N1s spectra for the unexposed precursor area (blue, top) and UVO exposed area (black, bottom).Reproduced from [24], with permission from Springer Nature.

Figure 4 .
Figure 4. (a) Processing schematic for fabricating metal oxide thin film transistors via photonic curing.(b) Simulated temperature vs. time profile of various material stacks (shown above each plot) during the photonic curing process.[27] John Wiley & Sons.© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

2 O 3 /
ZnO TFTs on PEN substrates, Yarali et al used Al 2 O 3 /ZrO 2 bilayers as the high-k gate dielectrics.The processing time for the two dielectric layers is 33 s.Al/Al 2 O 3 /ZrO 2 /Al capacitors exhibit an areal capacitance of 118 nF cm −2 , with leakage current density <10 −5 A cm −2 and a breakdown field in the range of 0.8-1 MV cm −1

Figure 5 .
Figure 5. (a) Schematic of three photonic curing modes to form ZrO 2 on PEN and the optical images of the resulting ZrO 2 on PEN with Al pads.(b) Simulated temperature vs. time profile comparing the photonic curing process with (solid) and without (dashed) an underlying Al contact.The yellow curves represent the top surface of the film, and the grey curves represent the bottom of the substrate.(c) Processing phase diagram depicting the photonic curing outcome as a function of pulse length (horizontal) and radiant exposure (vertical).Green areas represent under-converted films, yellow areas represent films formed via self-aligning photonic curing process (sa-PC), blue areas represent films formed via blanket/shadow-mask photonic curing processes (b-PC/sm-PC), and red areas represent conditions that lead to substrate damage.The colored circles represent conditions that were tested experimentally.Reproduced from[9].CC BY 4.0.

Figure 7 .
Figure 7. (a) SEM images of the hybrid TCEs fabricated by blade-coating and photonic curing.(b) Average reverse-scan current density-voltage curves for PSCs made on blade-coated and photonic cured hybrid TCEs (red) compared to a commercially available PET/ITO substrate (green) and a rigid glass/patterned-ITO substrate (blue) [30].

Figure 8 .
Figure 8.(a) Schematic of roll-to-roll photonic curing process.(b) Resistivity, carrier concentration, and carrier mobility as a function of photonic curing pulse fluence of ITO films processed via photonic curing.[32] John Wiley & Sons.© 2015 The Society for Information Display.

Figure 10 .
Figure 10.(a) Champion reverse-and forward-scan J-V curves comparing PSCs made with thermally annealed or photonic cured NiOx HTLs and thermally annealed or photonic cured perovskite active layers.Inset shows a picture of a flexible PSC.(b) average reverse-scan box plots of J-V parameters for all PSCs.Reproduced from [10].CC BY 4.0.

Figure 11 .
Figure 11.(a) UV-vis transmittance of three different WG/ITO substrates with the average transmittance from 400-800 nm and images of each given in the inset.(b) Champion forward-scan (solid) and reverse-scan (dashed) J-V curves for PSCs made on light (blue), medium (red), and dark (green) WG/ITO substrates.(c) Ni2p 3/2 and N1s XPS peak areas for NiOx films fabricated via photonic curing on top of light, medium, and dark WG/ITO substrates.Peak 1 (solid bars) represents the amount of nickel oxide and peak 2 (hatch bars) represents nickel hydroxide.The solid black circles/line represents the N1s area.© [2022] IEEE.Reprinted, with permission, from [40].

Table 1 .
Summary of photonic curing literature reviewed in this work.