Automatic light-adjusting electrochromic device powered by perovskite solar cell

Rising cooling demands in buildings and automobiles increase energy consumption, motivating technologies that modulate light and heat flow between indoors and outdoors. Electrochromic devices (ECDs) can tune transmittance under a small bias, offering energy savings, but reliance on external power leads to response lag and poor responsiveness to changing sunlight. Photovoltaic (PV)-powered ECDs have emerged to address this, yet many prior configurations are insensitive to decreasing light intensity and require reverse bias to bleach, complicating real-time balance between visibility and light-heat regulation. Additionally, limited PV efficiency, slow response, and insufficient ECD stability hinder practical applications. All-in-one gel-type ECDs simplify architecture and self-bleach without reverse voltage, but viologen-based systems suffer from radical cation aggregation and stability issues. This work targets long-term-stable, dynamically tunable, multi-color smart ECDs by synthesizing mono- and di-alkynyl-substituted viologens (MPV and DPV) to enhance radical stabilization, integrating them with efficient perovskite solar cells (PSCs) to realize self-powered, light-sensitive, automatically adjusting electrochromic smart windows.

Prior PV-powered ECDs include vertically integrated solar-powered electrochromic windows combining PV cells and polymer ECDs (Dyer et al.), semi-transparent PV with WO3-based ECDs that tune from neutral semi-transparent to dark blue (Cannavale et al.), and PSC-charged EC batteries using NiO/WO3 electrodes (Xia et al.). These typically employ sandwich structures with long coloration retention but often require reverse bias to return to transparency, limiting sensitivity to weakening light. Reported challenges include low PV PCE, long response times, and poor ECD stability. All-in-one gel-based ECDs are simpler and self-bleaching, with viologens widely used due to favorable electrochemistry, yet radical cation dimerization causes aggregation and hinders bleaching, harming stability. The present study addresses these gaps by introducing alkynyl-substituted viologens intended to stabilize radical species and integrating with high-PCE inverted PSCs to enhance light sensitivity and self-powered operation.

Materials and synthesis: Two viologens were prepared. MPV (1-(pent-4-yn-1-yl)-[4,4'-bipyridin]-1-ium chloride) was synthesized by reacting 4,4'-bipyridine (1.9 mmol) with 5-chloro-1-pentylene (1.7 mmol) in DMF at 90 °C (16 h), workup and silica gel purification afforded MPV (20% yield). DPV (1,1'-di(pent-4-yn-1-yl)-[4,4'-bipyridine]-1,1'-diium dichloride) was synthesized from 4,4'-bipyridine (1.9 mmol) and 5-chloro-1-pentylene (4.8 mmol) in DMF at 110 °C (48 h), filtration and washing yielded DPV (66% yield). NMR and HRMS confirmed structures. Electrochromic gel and device fabrication: EC gels contained 0.1 mmol viologen (MPV or DPV), 150 mg propylene carbonate, 23 mg ferrocene (complementary redox couple), 160 mg LiTFSI electrolyte, and 300 mg PVB in 1.5 mL dry methanol. After N2 stirring (3 h) and degassing, the gel was injected into liquid crystal cells between ITO electrodes with 70 µm spacing (parafilm spacers); circular active area ~2 cm² (radius ~0.8 cm). Perovskite solar cell (PSC) fabrication: On cleaned ITO (DI water/acetone/IPA; UVO 20 min), PTAA (2 mg/mL in chlorobenzene) was spin-coated (5000 rpm, 25 s) and annealed (100 °C, 10 min). A two-step perovskite process deposited PbI2 (600 mg/mL in DMF:DMSO 95:5; 1500 rpm) followed by FAI:MABr:MACl (60:6:6 mg in 1 mL IPA; 1300 rpm, 30 s), then annealed at 150 °C for 15 min (20–30% RH). Electron transport and electrode layers C60 (40 nm)/BCP (8 nm)/Ag (100 nm) were thermally evaporated (<1e-4 Pa). Inverted architecture with PTAA HTL was chosen to avoid TiO2 and doped Spiro-OMeTAD stability issues. Characterization and testing: Electrochemistry by cyclic voltammetry, chronoabsorptometry, and current–time response. Coloration efficiency (η) from ΔOD vs charge density. Energy storage assessed by galvanostatic charge–discharge (3–10 mA/cm²) to compute areal capacitance C = IΔt/(SΔV). Optical transmittance via UV–vis; colorimetry via Lab* measurements under varying light intensity simulated by changing PSC–light distance. PSC J–V under AM 1.5G, 100 mW/cm² (masked area 0.0576 cm²), forward and reverse scans. PSC-powered ECD operation assessed under strong/weak/no light, stability tests over 2000 s cycling in ambient, and demonstration of powering a red LED from charged ECDs.

• Electrochemistry: DPV shows two redox couples (DPV2+ ↔ DPV+· ↔ DPV0) with peaks at −0.57/−1.01 V (first) and 1.24/−1.50 V (second). MPV shows one redox pair (MPV+ ↔ MPV0) with Epa/Epc −1.00/−1.87 V.
• Coloration efficiency: DPV η = 89.4 cm²/C at 605 nm; MPV η = 37.2 cm²/C at 550 nm. DPV consumes less charge for the same optical density change and outperforms several reported viologen gels.
• Stability: DPV-based gel ECD retains high optical contrast over long cycling. Initial ΔT = 74.3% (1.6/−0.3 V, 10/10 s). ΔT loss only 3.2% after 20,000 cycles and 6.1% after 50,000 cycles; colored-state transmittance increased minimally (7.3%→7.7%). After 70,000 cycles, ΔT remains 60.9%. MPV shows lower stability (ΔT 62.6% initially, decreasing to 43.2% after 10,000 cycles) due to radical aggregation.
• Response time and current: DPV coloring/bleaching times to 90% ΔT: 2.4 s / 2.8 s; MPV: 5.6 s / 5.2 s. Steady colored-state current density at the same bias: DPV 2.78 mA/cm² vs MPV 2.16 mA/cm². Discharge times: DPV 8.1 s; MPV 9.1 s.
• Energy storage (ECS behavior): Areal capacitance for DPV-based ECS: 6.7, 4.9, 5.0, 2.2 mF/cm² at 3, 4, 5, 10 mA/cm², respectively. MPV-based ECS: 7.1, 6.2, 5.5, 3.5 mF/cm² at the same currents. Faster diffusion in MPV accounts for higher C at equal current densities. Both exhibit rapid charge/discharge enabling fast color changes. Charged ECDs can power a red LED.
• PSC performance: Inverted PSC with PTAA HTL achieved PCE 18.3% (forward scan), Voc 1.02 V, Jsc 22.8 mA/cm², FF 78.4%; reverse scan PCE 17.7%, Voc 1.00 V, Jsc 22.1 mA/cm², FF 80.2%.
• PSC-powered operation and light sensitivity: Under decreasing illumination, transmittance across the visible increases automatically, returning to high transparency with no light. DPV-based devices vary from deep blue (high light) to light blue to transparent; MPV-based devices vary from purple/blue to magenta/transparent, enabling visual indication of light intensity and stored charge. In ambient moisture conditions, PSC-powered ECDs showed stable switching over 2000 s with ΔT of ~71% (DPV) and ~62% (MPV) and response times under 30 s. Colorimetry (Lab*) confirms real-time tunability with increasing L* and reduced chroma as light intensity drops.

The study demonstrates that integrating efficient inverted PSCs with all-in-one viologen-based gel ECDs solves key limitations of traditional self-powered ECDs: sluggish bleaching and poor sensitivity to weakening light. Alkynyl substitution, particularly di-alkynyl DPV, stabilizes viologen radical cations, suppresses dimerization, and enables fast, reversible coloration/bleaching with exceptional cycling stability (up to 70,000 cycles). The PSC provides both energy harvesting and real-time light sensing: higher sunlight increases PSC output to drive deeper coloration, while reduced light lowers current so the gel ECD self-bleaches rapidly without reverse bias. This automatic transmittance adjustment maintains indoor visual comfort by attenuating strong sunlight and admitting more light when ambient light is weak. The ECDs also act as electrochromic supercapacitors, temporarily storing harvested energy and powering small loads (e.g., a red LED), illustrating multifunctional operation combining sensing, modulation, and storage suitable for intelligent window applications.

This work introduces mono- and di-alkynyl viologen-based, all-in-one gel ECDs powered by high-efficiency inverted PSCs to realize automatic, self-powered light adjustment. DPV-based ECDs deliver high coloration efficiency, rapid response, and outstanding long-term stability up to 70,000 cycles, while MPV-based ECDs offer multicolor tunability. The integrated PSC–ECD systems exhibit excellent light sensitivity, autonomous switching between colored and bleached states with changing illumination, and the capacity to store energy to power a LED. These results highlight strong potential for all-day intelligent windows and advanced displays that jointly manage solar radiation and energy. Future work should address practical deployment considerations such as robust encapsulation, large-area scaling, and material safety to enable use in buildings and automobiles.

The authors note latent toxicity concerns for the viologen materials (DPV and MPV), necessitating strict sealing/encapsulation for real-world applications in buildings and automobiles. While not a limitation in lab tests, ensuring long-term environmental safety and containment is essential for deployment.