Boosting Perovskite Solar Cell Efficiency Using Er3+-Yb3+-Li+ Tri‑Doped TiO₂ Up‑Conversion Layer

Background

Organolead halide perovskite solar cells have attracted significant attention due to their rapid efficiency gains, low cost, and straightforward fabrication. While PSCs now reach efficiencies above 22%, their narrow bandgap limits absorption mainly to the UV–visible spectrum, leaving a large portion of NIR light unutilized.

Up‑conversion materials that convert NIR photons into visible light can bridge this gap. Previous work has employed β‑NaYF₄ doped with Er³⁺/Yb³⁺ ions; however, such systems can impede electron transport in the TiO₂ layer. Recent studies have shown that tri‑doping TiO₂ with Er³⁺, Yb³⁺, and Li⁺ enhances up‑conversion emission and improves dye‑sensitized solar cell performance. Moreover, Li‑doping has been reported to boost PSC efficiency relative to undoped TiO₂. These findings motivated the present investigation of Er³⁺-Yb³⁺-Li⁺ tri‑doped TiO₂ in PSCs.

In this study, we synthesized UC‑TiO₂ by adding Li⁺ to Er³⁺-Yb³⁺ co‑doped TiO₂, confirmed its enhanced up‑conversion emission, and incorporated it into mesoporous TiO₂ layers for perovskite solar cells. The resulting devices exhibited a 19% increase in PCE.

Methods

Synthesis of Er³⁺-Yb³⁺-Li⁺ Tri‑Doped TiO₂

TiO₂ nanocrystals were prepared by a modified sol‑gel route. Titanium(IV) n‑butoxide (Ti(OBu)₄) was first complexed with acetylacetone (AcAc) at room temperature for 1 h under agitation. The mixture was then hydrolyzed by adding isopropanol, deionized water, and concentrated nitric acid, yielding a light‑yellow TiO₂ sol after 6 h of stirring. Er(NO₃)₃·5H₂O, Yb(NO₃)₃·5H₂O, and LiNO₃ were subsequently introduced to achieve a molar ratio of Er:Yb:Li:Ti = 0.5:10:x:100 (x = 0, 10, 15, 20, 25). The solvent was removed by drying at 100 °C for 8 h, and the resulting powder was calcined at 500 °C for 30 min.

Fabrication of Perovskite Solar Cells

Fluorine‑doped tin oxide (FTO) glass was sequentially cleaned with acetone, isopropanol, and ethanol, followed by 15 min of UV‑O₃ treatment. A compact TiO₂ layer was spin‑coated from a 0.1 M titanium diisopropoxide bis(acetylacetonate) solution in 1‑butanol and annealed at 500 °C for 30 min. A mesoporous TiO₂ film was then deposited at 4000 rpm for 30 s, followed by annealing at 100 °C (10 min) and 500 °C (30 min). The mesoporous layer was either pure commercial TiO₂ (30NR‑D) or a mixture of UC‑TiO₂ sol and diluted TiO₂ paste (UC‑TiO₂:TiO₂ = x:100 v/v, x = 10–40).

A perovskite precursor comprising PbI₂ (1.1 M), FA_I (1 M), PbBr₂ (0.22 M), and MA_Br (0.2 M) in a DMF/DMSO (4:1 v/v) mixture was spin‑coated in two steps (1000 rpm, 10 s; 4000 rpm, 30 s) with 200 µL chlorobenzene added during the second step. The film was then annealed at 100 °C for 1 h. A CsI stock solution (1.5 M in DMSO) was added to the perovskite precursor to improve thermal stability. The hole‑transport layer was formed by spin‑coating spiro‑MeOTAD (4000 rpm, 30 s), and a 80 nm gold electrode was evaporated on top.

Characterization

Up‑conversion photoluminescence was measured with a FLS 980 E fluorometer. Structural analysis employed XRD (DX‑2700) and XPS (THS‑103, Al Ka). UV‑vis‑NIR absorption was recorded on a Varian Cary 5000 spectrophotometer. Morphology was examined by SEM (JEM‑7001F). Photovoltaic performance was evaluated by J‑V curves under AM 1.5 G illumination using a Keithley 2440 sourcemeter. Electrochemical impedance spectroscopy (EIS) was performed with a CHI660e workstation.

Results and Discussion

Figure 1a shows that UC‑TiO₂ with Li content x = 20 exhibits the strongest up‑conversion emission under 980‑nm excitation, with green (525/545 nm) and red (658 nm) bands attributable to Er³⁺ transitions. Increasing Li beyond this level reduces emission intensity.

a Up‑conversion spectra of Er³⁺-Yb³⁺-Li⁺ tri‑doped TiO₂ (x = 0–25). b Energy‑transfer scheme of Er³⁺.

XRD patterns confirm that UC‑TiO₂ retains the anatase phase (JCPDS 21‑1272). XPS spectra (Figure 3) detect Ti 2p, Er 4d, Yb 4d, and Li 1s peaks, verifying successful doping.

XPS spectra of UC‑TiO₂: a Ti 2p, b Er 4d, c Yb 4d, d Li 1s.

UV‑vis‑NIR absorption (Figure 4a) reveals a pronounced tail extending to 1000 nm for UC‑TiO₂, and Tauc analysis shows a reduced bandgap of 3.10 eV versus 3.20 eV for commercial TiO₂. SEM images (Figure 5) indicate that the particle size (~30 nm) and morphology are unchanged by Li incorporation.

SEM images of the mesoporous layers: a without UC‑TiO₂, b with UC‑TiO₂.

Device performance (Figure 6a) shows that adding UC‑TiO₂ at 20 % v/v maximizes PCE, rising from 14.0 % (control) to 16.5 % (UC‑TiO₂)—a 19 % boost. Table 1 lists the key photovoltaic parameters. Enhanced short‑circuit current (I_sc) and open‑circuit voltage (V_oc) drive the efficiency gain.

a PCE versus UC‑TiO₂ content. b Representative J‑V curves.

Steady‑state PL (Figure 7a) shows reduced intensity for perovskite on UC‑TiO₂, indicating more efficient electron extraction. Time‑resolved PL (Figure 7b) reveals a shorter fast decay component (2.8 ns) and a higher fraction (98.2 %) for UC‑TiO₂ devices, confirming faster charge transfer.

a PL and b TRPL of perovskite on 30NR‑D versus UC‑TiO₂‑doped 30NR‑D.

EIS Nyquist plots (Figure 8a) show a reduced series resistance and increased recombination resistance for UC‑TiO₂ devices, indicating improved charge transport and suppressed recombination.

a Nyquist plots. b Equivalent circuit model.

Under 980‑nm filtered illumination, UC‑TiO₂ devices produce higher photocurrent (Figure S3), confirming that the up‑conversion layer effectively converts NIR photons into usable visible light.

Band‑diagram analysis (Figure 9) shows that UC‑TiO₂ has a lower conduction‑band edge than pure TiO₂, creating a larger offset with the perovskite absorber. This barrier enhances V_oc by reducing back‑tunneling of carriers.

Schematic energy‑band alignment of UC‑TiO₂, TiO₂, perovskite, and Spiro‑OMeTAD.

In summary, incorporating UC‑TiO₂ into the mesoporous scaffold expands the spectral response into the NIR, reduces recombination, and accelerates charge extraction, together yielding a 19 % PCE increase.

Conclusions

Tri‑doping TiO₂ with Er³⁺, Yb³⁺, and Li⁺ creates UC‑TiO₂ that emits strong up‑conversion light. When applied to PSCs, UC‑TiO₂ improves I_sc, V_oc, and fill factor, raising PCE from 14.0 % to 16.5 % (19 % enhancement). The performance gains arise from extended NIR absorption, enhanced charge transport, and a favorable band‑offset.

Abbreviations

EIS

Electrochemical impedance spectroscopy

NIR

Near‑infrared

PCE

Power conversion efficiency

PL

Photoluminescence

PSCs

Perovskite solar cells

TRPL

Time‑resolved photoluminescence