Enhanced Up‑Conversion TiO₂ Nanomaterial Boosts Perovskite Solar Cell Efficiency to 16.3 %

Abstract

A sol‑gel synthesized Ho3+‑Yb3+‑Mg2+ tri‑doped TiO2 (UC‑Mg‑TiO2) demonstrates markedly enhanced up‑conversion fluorescence when Mg2+ is incorporated. The material was applied as a thin interlayer on the electron‑transport layer (ETL) of perovskite solar cells (PSCs). Devices incorporating UC‑Mg‑TiO2 achieved a power‑conversion efficiency (PCE) of 16.3 % compared with 15.2 % for reference cells, confirming that the nanomaterial effectively converts near‑infrared (NIR) photons into visible light that the perovskite absorbs.

Background

Perovskite solar cells (PSCs) have rapidly advanced, with PCEs exceeding 22 % in recent years. However, their absorption spectrum is limited to wavelengths below 800 nm, leaving more than half of the solar spectrum—including the NIR region—untapped. Up‑conversion nanomaterials can bridge this gap by converting NIR photons to visible photons that PSCs can harvest. While β‑NaYF4‑based up‑conversion hosts are common, they are insulating and unsuitable for electron‑transport layers (ETLs). Titanium dioxide (TiO2) in its anatase phase is a widely used ETL due to its appropriate band alignment, cost efficiency, and stability, but its wide bandgap (3.2 eV) restricts visible‑NIR absorption. Doping TiO2 with rare‑earth ions or transition metals can extend its spectral response and improve charge‑transport properties. Prior studies have shown that Ho3+‑Yb3+ co‑doped TiO2 can up‑convert NIR light for dye‑sensitized solar cells, and Mg2+ doping can shift the Fermi level to enhance PSC performance. In this work, we combine Ho3+, Yb3+, and Mg2+ into TiO2 to create a new tri‑doped material (UC‑Mg‑TiO2) and investigate its impact on PSC efficiency.

Methods

Materials

Formamidinium iodide (FAI), methylammonium bromide (MABr), lead diiodide (PbI2), 2,2′,7,7′‑tetrakis‑(N,N‑di‑p‑methoxyphenylamine)-9,9′‑spirobifluorene (Spiro‑OMeTAD), and lead dibromide (PbBr2) were sourced from Xi’an Polymer Light Technology Corp. The SnO2 colloid solution was purchased from Alfa Aesar. Dimethyl sulfoxide (DMSO), N,N‑dimethylformamide (DMF), 4‑tert‑butylpyridine (TBP), and lithium bis(trifluoromethanesulfonyl)imide (Li‑TFSI) were obtained from Shanghai Aladdin Bio‑Chem Technology Co., LTD.

Synthesis of Ho3+‑Yb3+‑Mg2+ Tri‑doped TiO2

The UC‑Mg‑TiO2 was prepared via a modified sol‑gel route. Titanium tetrabutanolate was first reacted with acetylacetone (AcAc) and then diluted with isopropyl alcohol (IPA). A mixed solution of IPA, nitric acid, and water was slowly added, and after 6 h stirring a light‑yellow TiO2 sol formed. Ho(NO3)3·5H2O, Yb(NO3)3·5H2O, and Mg(NO3)2·6H2O were introduced in molar ratios of Ho:Yb:Mg:Ti = 1:4:x:100 (x = 0, 1, 1.5, 2, 2.5). The solvent was removed at 100 °C for 10 h, and the resulting powders were calcined at 500 °C for 30 min.

Preparation of PSCs

Fluorine‑doped tin oxide (FTO) substrates were cleaned and treated with UV‑O3. A blocking layer of TiO2 (30NR‑D) was spin‑coated and annealed at 500 °C. The ETL consisted of TiO2 diluted in ethanol (1:6, mass ratio) and annealed sequentially at 100 °C and 450 °C. For the UC‑Mg‑TiO2 modified cells, a mixture of UC‑Mg‑TiO2 sol and TiO2 sol (x : 100 – x, v/v) was spin‑coated (x = 0, 20, 40, 60, 80, 100) and annealed at 500 °C. The perovskite precursor solution (FAI, PbI2, MABr, PbBr2, and CsI) was spin‑coated with a chlorobenzene anti‑solvent drop, followed by a Spiro‑OMeTAD hole‑transport layer. Finally, an Au electrode was thermally evaporated.

Characterization

Photoluminescence (PL) spectra were recorded with a FLS 980 E fluorometer. X‑ray diffraction (XRD) used a DX‑2700 diffractometer. X‑ray photoelectron spectroscopy (XPS) employed a THS‑103 spectrometer. UV‑Vis absorption was measured with a Varian Cary 5000. Scanning electron microscopy (SEM) used a JSM‑7001F. Current–voltage (I‑V) curves under AM 1.5 illumination were taken with a Keithley 2440 Sourcemeter. Electrochemical impedance spectroscopy (EIS) was performed with a CHI660e workstation, and incident photon‑to‑current efficiency (IPCE) spectra were obtained using a Crowntech Qtest Station 500ADX.

Results and Discussion

Optimizing the Ho3+ and Yb3+ ratios revealed that a 1:4 Ho:Yb ratio produced the strongest up‑conversion emission at 547 nm and 663 nm when excited at 980 nm (Fig. 1a). Introducing Mg2+ further amplified the fluorescence; the maximum emission occurred at Ho:Yb:Mg = 1:4:2 (Fig. 1b). XRD confirmed that all samples maintained the anatase TiO2 phase (Fig. 2). XPS analysis verified the incorporation of Ho, Yb, and Mg into the TiO2 lattice (Fig. 3). UV‑Vis spectra showed that UC‑Mg‑TiO2 absorbed additional visible and NIR light, reducing its bandgap to 3.09 eV from 3.18 eV for undoped TiO2 (Fig. 4).

SEM images revealed similar nanoparticle sizes (~25–28 nm) and uniform film morphology for both TiO2 and UC‑Mg‑TiO2 (Fig. 5).

Device testing showed that the PCE increased with UC‑Mg‑TiO2 content, peaking at 60 % UC‑Mg‑TiO2 in the ETL (Fig. 6a). The best cell achieved Voc = 1.05 V, Jsc = 22.6 mA cm‑2, and PCE = 16.3 % versus 1.03 V, 21.2 mA cm‑2, and 15.2 % for the reference (Table 1). Faster PL decay and lower recombination resistance in UC‑Mg‑TiO2 devices (Fig. 8, Fig. 9) indicate more efficient charge extraction.

Under 980 nm NIR illumination, cells with UC‑Mg‑TiO2 displayed a higher Jsc, confirming the up‑conversion contribution (Fig. 10). IPCE spectra showed enhanced response between 400 nm and 650 nm for the modified cells (Fig. 10b).

Conclusions

We successfully synthesized Ho3+‑Yb3+‑Mg2+ tri‑doped TiO2 (UC‑Mg‑TiO2) that exhibits strong up‑conversion fluorescence enhanced by Mg2+ doping. When applied as an interlayer on the ETL, UC‑Mg‑TiO2 improved the PSC performance, raising V_oc and J_sc to 1.05 V and 22.6 mA cm‑2, respectively, and boosting the PCE from 15.2 % to 16.3 %. These results demonstrate that UC‑Mg‑TiO2 effectively converts NIR light into usable visible photons, offering a viable route to further enhance perovskite solar cell efficiency.

Abbreviations

EIS

Electrochemical impedance spectroscopy

NIR

Near‑infrared

PCE

Power conversion efficiency

PL

Photoluminescence

PSCs

Perovskite solar cells

TRPL

Time‑resolved photoluminescence