Modern buildings and automobiles have increasing cooling demands, leading to higher energy consumption. Smart electrochromic (EC) windows offer a promising solution by modulating light and heat flow. However, traditional EC devices rely on external power, causing response lag. Self-powered EC devices, using photovoltaic (PV) cells, have emerged to address this issue. Previous studies have demonstrated solar-powered EC windows with vertically integrated configurations or combined semi-transparent PV and WO3-based ECDs. However, limitations remain: insensitivity to changing light intensity (particularly weakening light), long response times, low PV cell efficiency, and poor ECD stability. This study aims to overcome these limitations by integrating all-in-one gel-type ECDs with high-efficiency perovskite solar cells (PSCs). All-in-one gel-type ECDs, simpler than layer-by-layer structures, offer faster self-bleaching without reverse voltage. Viologens, known for their electrochemical properties, are investigated as electrochromic materials, but their tendency to aggregate and cause instability needs addressing. This work introduces two alkynyl-substituted viologen derivatives (MPV and DPV) to mitigate aggregation and improve stability. The study integrates these viologen-based ECDs with PSCs to create a self-powered, light-adjusting system, aiming to achieve real-time dynamic light tuning and multi-color capabilities.

The existing literature highlights the need for self-powered electrochromic devices to improve energy efficiency in buildings and vehicles. While previous research explored various approaches, including vertically integrated solar-powered windows and combined PV-ECD systems, limitations such as slow response times, low photovoltaic cell efficiency, and unsatisfactory electrochromic device stability persist. Studies using perovskite solar cells to power electrochromic batteries have shown promise but still lack the sensitivity and stability needed for practical applications. The all-in-one gel-type electrochromic device configuration, with its simpler structure, offers advantages over layer-by-layer designs; however, challenges associated with the stability of viologen-based electrochromic materials remain. This work builds upon these prior investigations by addressing the stability issues of viologen-based ECDs through the introduction of alkynyl groups and the utilization of high-efficiency perovskite solar cells to power the system.

Two alkynyl-containing viologen derivatives, 1-(pent-4-yn-1-yl)-[4,4'-bipyridin]-1-ium chloride (MPV) and 1,1'-di(pent-4-yn-1-yl)-[4,4'-bipyridine]-1,1'-diium dichloride (DPV), were synthesized via a Sonogashira coupling reaction. The synthesis involved a one-step reaction of 4,4'-bipyridine with 5-chloro-1-pentylene in DMF at different temperatures. The resulting compounds were characterized using NMR and HRMS. Electrochromic devices (ECDs) were fabricated using an all-in-one gel configuration. The gel comprised the viologen derivative, propylene carbonate (PC), ferrocene, lithium bis-(trifluoromethane)sulfonimide, and polyvinyl butyral (PVB) dissolved in methanol. This gel was injected into liquid crystal cells with a 70 µm gap between ITO electrodes. Perovskite solar cells (PSCs) were fabricated using an inverted structure with PTAA as the hole transporting material (HTM) and C60/BCP/Ag as the electron transporting layer (ETL). The perovskite layer was deposited using a two-step spin-coating method. The PSC performance was evaluated using J-V curves under AM 1.5G one-sun illumination. The performance of the ECDs was characterized by cyclic voltammetry, chronoabsorptometry, galvanostatic charge-discharge curves, and UV-Vis spectroscopy. The stability of the ECDs was assessed by measuring transmittance changes over numerous switching cycles. Light sensitivity was evaluated under various light intensities and in natural sunlight. Colorimetric measurements were used to characterize the color changes of the ECDs. Finally, the ability of the charged ECDs to power a red LED was demonstrated.

Cyclic voltammetry revealed different redox behaviors for MPV and DPV. DPV exhibited two redox couples, while MPV showed only one. Coloration efficiency (η) was higher for DPV (89.4 cm²/C at 605 nm) than MPV (37.2 cm²/C at 550 nm). DPV-based ECDs showed exceptional long-term stability, retaining 60.9% of their initial average transmittance (AT) after 70,000 cycles, significantly outperforming most reported viologen-based ECDs. MPV-based ECDs exhibited lower stability. Response times for color change were faster for DPV than MPV. Galvanostatic charge-discharge curves demonstrated that both DPV and MPV-based ECDs could function as electrochromic supercapacitors (ECSs). The perovskite solar cells (PSCs) achieved a power conversion efficiency (PCE) of 18.3%. The integrated PSC-powered ECDs demonstrated automatic light adjustment capabilities. Under varying light intensities, the DPV-based ECD switched between transparent and deep blue states, while the MPV-based ECD exhibited a transparent-blue-magenta color range. Both devices showed excellent light sensitivity, rapid response times (<30s), and good switching stability under various light conditions and even in humid environments. The charged ECDs were able to power a red LED. Colorimetric analysis (L*, a*, b*) confirmed the real-time color changes in response to varying light intensities.

The outstanding stability of the DPV-based ECD, attributed to interactions between viologen radical species and alkynyl groups, addresses a significant challenge in viologen-based electrochromic devices. The successful integration of all-in-one gel-type ECDs with high-efficiency perovskite solar cells demonstrates a practical approach for self-powered, light-adjusting smart windows. The observed multi-color capabilities and real-time response to changing light intensity highlight the potential for advanced applications. The ability of the charged ECDs to power an LED further showcases their dual functionality as both light regulators and energy storage devices. This work contributes to the development of energy-efficient buildings and vehicles by offering a dynamic and responsive solution for controlling solar radiation.

This research successfully demonstrated a self-powered, light-adjusting electrochromic device by integrating high-performance perovskite solar cells with novel alkynyl-substituted viologen-based electrochromic devices. The exceptional stability, rapid response time, and multi-color capabilities of the resulting device hold significant promise for applications in smart windows and other advanced displays. Future research could focus on exploring other viologen derivatives, optimizing the device architecture for improved efficiency, and investigating the long-term reliability of these devices under real-world conditions. Addressing the potential toxicity of the viologen derivatives through encapsulation would be crucial for commercial applications.

While the synthesized viologens showed improved stability compared to conventional viologens, potential long-term degradation mechanisms in the real-world environment, especially under UV irradiation and extreme temperatures, need further investigation. The efficiency of the perovskite solar cells, while high, could be further optimized for better energy harvesting. The study focused on a specific set of viologen derivatives; exploration of a broader range of molecules might uncover even superior electrochromic materials. The potential toxicity of the viologen derivatives is a factor that needs to be addressed for commercial applications, potentially through the use of environmentally benign encapsulants.