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. 2025 Aug 31;129(36):16338–16346. doi: 10.1021/acs.jpcc.5c03520

A Sputtered Gig-Lox TiO2 Sponge Integrated with CsPbI3:EuI2 for Semitransparent Perovskite Solar Cells

C Spampinato †,‡, G Calogero †,*, G Mannino †,*, S Valastro †, E Smecca †, V Arena †, P La Magna †, C Bongiorno †, E Fazio ‡, A Alberti †
PMCID: PMC12434809  PMID: 40959780

Abstract

In this study, we propose a novel approach where in an innovative porous grazing incidence atomic flux coupled with local oxidation (gig-lox) TiO2 electron transport layer (ETL), deposited by sputtering, is integrated with a fully inorganic CsPbI3:EuI2 perovskite. This combination is used as a photoactive layer in semitransparent perovskite solar cells (ST-PSC). The solvent-free, scalable porous oxide sponge-like in structure, is produced via a grazing-incidence Ti flux that undergoes localized progressive oxidation. The sponge offers ≈50% volume porosity that is available for perovskite infiltration. The result of the oxide-perovskite integration is a double-layer struc made of a perovskite-filled sponge covered by a cap of pure perovskite. The material has unique optical properties and structure as attested by spectroscopic ellipsometry and X-ray diffraction analyses. The double-layer structure exhibits semitransparency, and its reduced photoluminescence (PL) intensity relative to that of a single perovskite layer indicates efficient charge carrier injection into the porous TiO2. In perspective, the layer can be used in PSC as demonstrated by simulations.

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Introduction

Perovskite materials have shown remarkable progress in recent years, particularly in photovoltaics and optoelectronics, due to their exceptional light-harvesting capabilities and tunable optoelectronic properties. Within this class, inorganic CsPbI3 has garnered significant attention for ST-PSC. This is attributed to its relatively large bandgap, which favors transmission of light in the visible range, and its thermal compositional stability compared to hybrid organic–inorganic perovskites. , Furthermore, bandgap engineering through compositional modifications enables fine-tuning of the absorption edge, optimizing the trade-off between transparency and power conversion efficiency (PCE).

Despite the intrinsic benefits of CsPbI3, including preparation by solution processing (e.g., spin coating and inkjet printing) and relatively high-power conversion efficiencies exceeding 21%, several challenges impede its large-scale use. These include its photoactive polymorphism instability under ambient conditions, hysteresis effects, and the complexities associated with upscaling. , Strategies to mitigate these drawbacks include encapsulation protocols and additional processes for crystallographic stabilization. −

Among various stabilizing approaches, partial substitution of PbI2 with EuI2 has been shown to enhance both operational stability and device efficiency. However, incorporating Eu limits the practical thickness of the perovskite films, typically resulting in layers of only ∼200 nm. The benefits of adding EuI2 are reported in ref . Although thinner layers benefit from high transparency, their reduced thickness is insufficient to maximize photovoltaic performance in PSC. In contrast, standard CsPbI3 perovskites can readily form thicker layers, typically 500 nm or more, that yield good photovoltaic performance; however, these thicker films significantly compromise transparency, hindering their suitability for ST-PSC applications. , In this work, we address these limitations by integrating a fully inorganic CsPbI3:EuI2 photoactive layer with a nanoporous TiO2 ETL fabricated via a gig-lox process. This solvent-free, upscalable gig-lox methodology allows for the deposition of thick and transparent TiO2 scaffolds, thereby improving mechanical stability and resilience against environmental species action, while preserving the semitransparent character of the perovskite-based device. Specifically, the innovative application of gig-lox TiO2 enables high semitransparency and efficiency, demonstrated by simulations, thanks to its porosity (≈50%) as measured by ellipsometry according to the model in ref .

Experimental Methods

TiO2 Deposition

Spongy gig-lox TiO2 is grown using a technique in which Ti species generated in an Ar-based plasma by sputtering from a metallic source are deposited onto a substrate in grazing incidence undergoing gradual local oxidation during the growth process. This technique was previously described in ref with further angular optimization to enhance the layer’s porosity. This approach offers several advantages. First, a high deposition rate of 4 nm/min is achieved by establishing a metallic plasma at the source side, which prevents charging effects from surface oxidation. Second, it promotes the progressive local oxidation of the deposited atoms at the anode side. Lastly, the tilted (off-axis) Ti flux creates a shadowing from the initial seeds, resulting in additional meso-porosity along the deposited layer, in addition to the nanoporosity achieved by a high-pressure of Ar. Importantly, the overall porosity remains unchanged during postdeposition thermal treatments, and this has been demonstrated for layer thicknesses of up to 1000 nm as discussed in ref . In this paper, we employ a ≈450 nm thick TiO2 film, the typical thickness of chemically deposited mesoporous layers in PSC.

Perovskite Film Fabrication

For the perovskite solution, 1 M PbI2 and 1 M CsI, from Tokyo Chemical Industry were mixed. These compounds were dissolved in a mixed solvent made of DMF and DMSO, in a volumetric ratio of 3:1. Simultaneously, a solution of EuI2 from Sigma-Aldrich was prepared at a concentration of 0.1 M by using the same mixed solvent. Both solutions were subjected to stirring at room temperature for 1 h. For the CsPbI3:EuI2 samples, 1 mL of the PbI2/CsI solution was mixed with 0.5 mL of the EuI2 solution to achieve the desired stoichiometry. These resulting mixtures were additionally stirred for 1 h. Along the entire procedure, ambient air maintained a relative humidity of approximately 35%.

To deposit the perovskite films, spin-coating in a N2 environment was done over thermally pretreated (500 °C 30 min) gig-lox TiO2 substrates. The deposition process consists of two sequential steps: initially spinning at 1000 rpm for 10 s and a subsequent round at 2000 rpm (round per minute) for 25 s

Scanning Electron Microscope

A scanning electron microscope (Zeiss Gemini II FE-SEM) in plan-view configuration operating at an accelerating voltage of 30 kV and at a working distance of 4 mm in transmission mode and at an accelerating voltage of 3 kV for the SEM images was used to investigate the morphology of the gig-lox TiO2 and perovskite layers.

Transmission Electron Microscopy Analysis

To prevent dissolution of the perovskite layer due to the use of water in standard preparation procedures, the gig-lox TiO2 layer was mechanically scratched from the glass substrate directly onto a Transmission electron microscopy (TEM) carbon grid. TEM images were obtained employing a probe-corrected Jeol ARM200 microscope, which was equipped with a JEOL 100 mm2 energy dispersive X-ray (EDX) detector. The imaging process was conducted in scanning mode, utilizing a high-angle annular dark field (HAADF) detector with a 50 mrad aperture. The acquisition of EDX spectra was done in a spectrum image configuration, encompassing the entire film thickness, ranging from the surface down to the lower interface.

X-ray Diffraction Analysis

X-ray diffraction (XRD) patterns were obtained using a SmartLab (Rigaku) diffractometer equipped with a 9 kW rotating anode KαCu source, working at 45 kV and 100 mA. For acquisitions, a HyPix-3000 detector was used. The recording step size for the patterns was set at 0.01°, with an acquisition speed of 0.1° per minute.

Spectroscopic Ellipsometry Analysis

The optical constants were measured using a J.A. Woollam VASE ellipsometer equipped with a rotating compensator, which enhanced measurement accuracy and allowed for the detection of any nonpolarized light components. The sample was placed in a sealed chamber maintained under a slight overpressure of nitrogen (N2) to prevent degradation from exposure to ambient humidity. Spectroscopic ellipsometry was performed over the photon energy range of 0.5–6.5 eV to develop a comprehensive optical model. Since the sample was deposited on a glass substrate, its influence was carefully accounted for, including the potential contribution of back-side reflections. The thickness of the perovskite layer was initially extracted using the Cauchy model in the transparent spectral region and held constant during subsequent fitting. A Kramers–Kronig consistent model was then constructed using a set of multiple critical point parabolic band oscillators to accurately describe the optical response of the layer. −

Photoluminescence Spectroscopy Analysis

Photoluminescence (PL) measurements were conducted using an Arkeo (Cicci Research s.r.l.) instrument. Samples were excited with a green laser (532 nm) at a 45° incident angle, producing a circular spot 1 mm in diameter. After a 1000 ms, the PL signal was collected via a spectrometer.

Device Simulation

Photovoltaic devices were simulated using the 1D electro-optical simulator Setfos (v5.5) by Fluxim, which self-consistently combines the drift-diffusion formalism for charge transport with the transfer-matrix formalism for optical absorption/transmission. The Air Mass 1.5 ASTM G-173–03 (included in the SETFOS database) is adopted as the reference spectrum for 1 sun illumination. The wavelength-dependent complex refractive indices (n(λ) and k(λ)) of the gig-lox TiO2 layer infiltrated and capped with perovskite were extracted from spectroscopic ellipsometry . The n(λ) and k(λ) for PTAA were taken from ref., those for F-doped tin oxide (FTO) and compact TiO2 from ref and those for indium tin oxide (ITO) from the Setfos database. All layers, except glass, are modeled as optically coherent. The drift-diffusion model in steady-state mode is solved by letting Setfos automatically select the appropriate solver (Newton or Gummel) and the corresponding settings for the residuum and damping factors during the computation. The bandgap of the perovskite capping layer and the gig-lox TiO2 infiltrated with perovskite were set to the values extracted by PL analysis (1.75 and 1.78 eV, respectively). The optical properties of gig-lox TiO2 require an optimized model with respect to those of the conventional TiO2, as reported in ref . The remaining electrical parameters for all layers were taken from ref .

Results and Discussion

Film deposition on glass was performed by spinning the CsPbI3:EuI2 solution at 1000 rpm (rounds per minute) as the first step. The spin coating speed (ranging from 1000 to 5500 rpm) at the second step was used as a parameter, and it has been found that the layer thickness is independent of it even using 2000 rpm, as reported in Table S1 and Figure S1 of the Supporting Information file. The thickness measured on glass via spectroscopic ellipsometry is 180 nm. Spin coating was also performed on a 460 nm thick gig-lox TiO2. Figure shows the resulting layer viewed by TEM analysis taken on a scratched portion. The main result is a uniformly infiltrated TiO2 sponge with CsPbI3:EuI2 distributed throughout the whole TiO2 oxide thickness as revealed through by the mass-contrast in the scanning TEM image.

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Transmission electron microscopy analysis: (a) sample portion in mass contrast. (b) EDX spectrum for element recognition. Sample grid produces the gold and carbon peaks.

In the image, the white areas arise from species with higher atomic mass, which indeed reflect the infiltration of the perovskite layer. Additionally, an EDX analysis (Figure b) was conducted to identify the most abundant chemical species. It shows the presence of titanium, oxygen, cesium, lead, and iodine sharing the same thickness, which confirms the in-depth intermixing of the two materials (gold and carbon peaks are typical of the supporting grid).

By the spin-coating process, we achieved the complete infiltration of a CsPbI3:EuI2 solution within the gig-lox TiO2. We deliberately promoted the formation of a perovskite cap layer on top, as described below. The presence of a cap was carefully controlled using a spin-coating process at 2000 rpm, as confirmed by SEM analyses in plan-view.

Figure a shows the CsPbI3:EuI2 surface morphology of the cap. Through some uncovered regions, the surface of the gig-lox TiO2 underneath is visible. As a reference, Figure b displays the morphology of a bare gig-lox TiO2 sample, with its typical grain size, markedly different from that of CsPbI3. Based on this evidence, the layer sequence is schematically depicted in Figure c. This two-layer structure consists of a gig-lox TiO2 infiltrated with CsPbI3:EuI2 and a cap layer. The composition of this two-layer was investigated by X-ray diffraction analyses. In Figure a, the diffractograms collected at various angles of incidence on glass (a) and on gig-lox TiO2 (b) are shown.

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a) SEM plan-view of CsPbI3:EuI2 infiltrated into gig-lox TiO2. (b) SEM plan-view of the reference bare gig-lox TiO2. (c) Schematic of the gig-lox TiO2 structure with infiltrated CsPbI3:EuI2 and the CsPbI3:EuI2 cap. The sponge offers ≈50% volume porosity for perovskite infiltration, as measured by spectroscopic ellipsometry according to the model in ref .

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a) Diffraction patterns at different angles (0.2°- 0.3°- 0.4°- 0.6°- 0.8°) for CsPbI3:EuI2 layer deposited on glass. (b) CsPbI3:EuI2 layer deposited on gig-lox TiO2.

The structure preservation of CsPbI3:EuI2 once infiltrated into gig-lox TiO2 is demonstrated by the presence of the typical crystallographic planes compared to CsPbI3:EuI2 deposited on a glass substrate. In the grazing angle scan at 0.6° incident angle (Figure a), a distinct diffraction peak emerges at 2θ = 37.9°, which corresponds to the crystallographic (004) planes of anatase-TiO2. This peak is not detectable at lower grazing angles. The penetration depth calculated as the distance at which the X-ray beam intensity decreases to 1/e suggests that a capping layer of pure perovskite, ≈150 nm thick, is formed on top of the gig-lox TiO2/CsPbI3:EuI2. From the full width at half maximum (FWHM) of the diffraction peaks and, according to the Debye–Scherrer relation, we measured an average crystal size of 49.2 ± 9.2 nm in the CsPbI3:EuI2 reference on glass and 36.2 ± 9.1 nm in the CsPbI3:EuI2 on gig-lox TiO2. This shrinkage is attributed to the intercalation into the gig-lox TiO2 structure that is highly branched at the nanometer scale. Through Rietveld refinement (data are listed in Table and .), the infiltrated perovskite structure is compared to the reference structure in the film on glass, as listed in Table . For the refinement, an orthorhombic 62:Pnma (χ2 = 1.3) crystal symmetry was considered.

1. Peak Position and Full Width at Half Maximum (FWHM) of the main contributions in the reference CsPbI3:EuI2 on glass substrate and in the CsPbI3:EuI2 infiltrated into gig-lox TiO2 .

CsPbI3:EuI2 on glass 2θ [°] FWHM [°]
(002)/(110) 14.37 0.18
(200)/(112)/(020) 20.67 0.16
(120) 23.09 0.18
(121) 24.16 0.22
(022) 25.23 0.20
(004)/(220) 28.98 0.20
(311)/(130) 32.90 0.17
(132) 36.04 0.23
CsPbI3:EuI2 on gig-lox TiO2 2θ [°] FWHM [°]
(002)/(110) 14.37 0.19
(200)/(112)/(020) 20.64 0.19
(120) 23.12 0.19
(121) 24.52 0.25
(022) 25.28 0.22
(004)/(220) 28.38 0.37
(311)/(130) 32.98 0.22
(132) 36.07 0.26
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2. Lattice parameters of CsPbI3:EuI2 on glass substrate and CsPbI3:EuI2 infiltrated into gig-lox TiO2 from Rietveld refinement.

lattice parameters CsPbI3:EuI2 on glass CsPbI3:EuI2 on gig-lox TiO2
a [Å] 8.842 8.875
b [Å] 12.240 12.578
c [Å] 8.586 8.592
unit cell [Å3] 929 959
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It is interesting to notice that the lattice parameters slightly increased in the perovskite infiltrated into the gig-lox TiO2 structure, resulting in an overall unit cell volume expansion of 3.2%. The results are consistent with the porous nature of the TiO2 substrate, with the fine pores allowing an inner adaptation of the intercalated perovskite, which is indeed free to expand with respect to a more constrained compact film. We investigated the two-layer structure also from the optical point of view by spectroscopic ellipsometry. The experimental data were acquired by placing the samples in a chamber filled with N2 to prevent degradation of the perovskite in humid air. The enclosure limited data collection to an angle of 70°. Figure displays the experimental data and the model for two reference samples (i.e., (a) CsPbI3:EuI2 sample and (b) gig-lox TiO2 sample) and for two-layer structure: CsPbI3:EuI2 cap on the mixed CsPbI3:EuI2/gig-lox TiO2 sample (c). Remarkably, a satisfactory matching between the fitting model and experimental results was achieved across the whole investigated wavelength range.

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Spectroscopic ellipsometry: experimental (symbols) and model (lines) data at 70° angle of incidence for (a) CsPbI3:EuI2, (b) gig-lox TiO2, and (c) the two-layer structure: CsPbI3:EuI2 cap on mixed CsPbI3:EuI2/gig-lox TiO2.

It is noteworthy that the combined layer still retains features coming from the two layers of which it is composed. In particular, in the UV region (>4 eV) where TiO2 is transparent, the optical constants closely resemble those of the perovskite layer, whereas in the IR region (<1.5 eV) those of the gig-lox TiO2.

The absorption coefficient of the two-layer structure: CsPbI3:EuI2 cap on mixed CsPbI3:EuI2/gig-lox TiO2 (Figure a) was then calculated from the dielectric function using the equation in refs , .

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(a) Absorption coefficient of the entire two-layer structure: CsPbI3:EuI2 cap on mixed CsPbI3:EuI2/gig-lox TiO2. (b) PL spectra of reference CsPbI3:EuI2 on glass and on gig-lox TiO2.

A crucial aspect to consider is the region below the bandgap at 1.78 eV, where the material is expected to be transparent. In this region, despite slight absorption, the material maintains a low absorption coefficient, indicating its high optical quality. To investigate the applicability of the two-layer structure in PSC, it is crucial to probe the role that TiO2 plays in extracting charges. To achieve this, we measured the PL of the material in a N2 environment. Figure b shows the emission spectra of both the two-layer struc and the reference sample. The CsPbI3:EuI2 deposited on the gig-lox sample exhibits a significantly reduced PL intensity compared to that on glass, with a remarkable 84% decrease in emission. This substantial reduction in radiative recombination indicates that a high density of carriers is being extracted by the TiO2.

Moreover, in the reference on glass, the peak of PL is centered at 715 nm (1.75 eV), whereas the CsPbI3:EuI2 deposited on the gig-lox TiO2 has a peak at 695 nm (1.78 eV). This variation is attributed to the measured difference in the crystallite sizes, where smaller dimensions correlate with higher bandgaps − and to the volume of the unit cell as discussed before. The findings support an efficient injection of charge carriers from the CsPbI3:EuI2 into the porous TiO2 thanks to branched interfaces, , unlike in the reference layer CsPbI3:EuI2 on glass.

For applicative purposes, it might be of interest to evaluate how this two-layer structure can contribute to the development of PSC. We simulated a complete device using the Setfos simulation software (all computational details are reported in Experimental Methods). Currently, the technological realization of high-performance semitransparent perovskite-based devices remains highly challenging. Through advanced simulations, the aim is to provide a clear and quantifiable target for ongoing research efforts. This approach seeks to identify key parameters and design principles that can guide experimental development, ultimately facilitating progress toward achieving efficient, stable, and scalable semitransparent perovskite devices suitable for real-world applications. The 180 nm thick CsPbI3:EuI2 perovskite layer and the infiltrated CsPbI3:EuI2/gig-lox TiO2 layer are integrated into a simulated ST-PSC architecture similar to that reported in ref as shown in Figure a. It consists of a planar n-i-p architecture on a glass substrate in which the perovskite layer is sandwiched between the 450 nm thick infiltrated CsPbI3:EuI2/gig-lox TiO2, acting as an ETL, and a 25 nm thick poly­[bis­(4-phenyl)­(2,4,6-trimethylphenyl)­amine (PTAA) film, acting as ETL. A 300 nm F-doped tin oxide (FTO) and a 100 nm indium tin oxide (ITO) are chosen as the transparent bottom and top electrodes, respectively. A glass layer is added on the ITO top electrode to consider the cover glass required for PSC encapsulation. The deposition of both ITO and PTAA follows the procedure reported in earlier studies. The thickness of the transport layers and electrodes was set based on values reported in the literature for similar ST-PSCs. The absorption spectra of the individual layers (Figure b) and the transmittance (Figure c) of the device are presented. Reflectance and transmittance data for the individual layers are reported in the supporting file Figure S2. Figure d displays the JV curve of the device, with the electrical details shown as an inset. The fabricated cell achieves an efficiency of 16.91% while maintaining good semitransparency, with an average visible transmittance of 18% across the 390–780 nm visible range. This corresponds to a light utilization efficiency (LUE) of 3%.

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(a) Schematic of the simulated device. (b) Total absorbance and layer-resolved absorbance for single layer. The contribution by c-TiO2 is negligible at all wavelengths. (c) Transmittance of the entire device. (d) J–V curve of the device with the electrical parameters as inset.

In the model, the maximum power point tracking (MPPT) value is constant over time and equals 16.92 mV/cm2. Figure shows the external quantum efficiency (EQE) values of the semitransparent device.

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EQE data of the simulated semitransparent device.

A comparison with a device made with conventional TiO2 ETL is shown in Figure S3 of the supporting file.

Conclusions

We have developed a process that successfully integrates CsPbI3:EuI2 perovskites into gig-lox TiO2, enabling their use as an active layer with built-in electron extraction functionality. The process yields a bilayer structure featuring a thin perovskite cap layer on the top and a perovskite-oxide blend beneath. This configuration preserves semitransparency while ensuring a loading of photoactive material for half of the overall volume. This structure exhibited reduced PL intensity compared to the reference CsPbI3:EuI2 deposited on glass, indicating efficient charge carrier injection into the porous TiO2 via a built-in potential established at the inner interfaces. Additionally, X-ray diffraction analyses revealed a gained adaptability of the perovskite material into gig-lox TiO2 as deduced by the strain relaxation found by measuring the lattice volume. Meanwhile, the confined pore space limits grain growth, which further helps to reduce lattice strain. Consequently , a shift of the bandgap to higher energies is observed, which is beneficial in applications requiring increased semitransparency. Finally, full device simulations based on the measured parameters prospect good device efficiency (better than using a conventional TiO2) while maintaining excellent semitransparency. These outcomes highlight the potential for applications in building integrated photovoltaics and agrivoltaics.

Supplementary Material

jp5c03520_si_001.pdf (369.7KB, pdf)

Acknowledgments

The authors wish to acknowledge the FSE (Fondo Sociale Europeo) and the “Programma Operativo Nazionale” (PON) for Sicily 2014–2020. This work was partially supported by the European Union (NextGeneration EU), through the MUR-PNRR project SAMOTHRACESicilian MicronanoTech Research and Innovation Center (ECS00000022, CUP B63C22000620005) and nuovi Concetti, mAteriali e tecnologie per l’iNtegrazione del fotoVoltAico negli edifici in uno scenario di generazione diffuSa” [CANVAS]/Italian Ministry of the Environment and the Energy Security CUP B53C22005670005. The authors also thank Sebastian Ferranti (CNR-IMM) for technical assistance.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.5c03520.

  • Thickness of samples at different speeds of spin coating; cross-FIB analysis highlighting the perovskite-infiltrated gig-lox TiO2 layer and the pure-perovskite capping layer; and transmittance and reflectance taken from the TiO2/FTO stack (PDF)

The authors declare no competing financial interest.

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