Transparent solar cells represent a promising advancement in photovoltaic technology, enabling the integration of energy harvesting capabilities into windows, displays, and other see-through surfaces without compromising aesthetics or functionality. Unlike conventional opaque solar cells, which absorb visible light, transparent variants selectively capture ultraviolet (UV) and near-infrared (NIR) wavelengths while allowing visible light to pass through. This article explores the materials, mechanisms, and applications of transparent solar cells, focusing on organic semiconductors, non-fullerene acceptors, and UV-selective absorbers, while addressing the critical balance between transparency and power conversion efficiency.

Organic semiconductors have emerged as a key material class for transparent solar cells due to their tunable optical properties and compatibility with flexible substrates. These materials consist of conjugated polymers or small molecules that can be engineered to absorb specific wavelengths. For instance, donor-acceptor polymer blends can be designed to minimize visible light absorption while maintaining strong UV or NIR activity. The bandgap of these organic materials is carefully adjusted to achieve high transparency in the visible spectrum, typically above 70%, while still generating electricity from non-visible photons. Recent developments in molecular design have enabled organic transparent solar cells with average visible transmittance (AVT) values exceeding 80%, though this often comes at the expense of reduced power conversion efficiency (PCE), which typically ranges between 5% and 10% for such high-transparency devices.

Non-fullerene acceptors (NFAs) have revolutionized the performance of organic solar cells, including transparent variants. Unlike traditional fullerene-based acceptors, NFAs offer broader absorption tunability, higher open-circuit voltages, and improved stability. In transparent solar cells, NFAs are paired with low-bandgap donors that absorb primarily in the UV or NIR regions. For example, ITIC and its derivatives are widely used NFAs that can be optimized for transparency by reducing their overlap with the visible spectrum. The combination of NFAs with carefully selected donors has led to devices with PCEs approaching 8% while maintaining AVT values around 60%. The trade-off between transparency and efficiency remains a central challenge, as increasing the active layer thickness or broadening the absorption spectrum can enhance PCE but reduce transparency.

UV-selective absorbers are another critical component in transparent solar cells, particularly for applications where visible light transmission is paramount. Materials such as zinc oxide (ZnO) and titanium dioxide (TiO2) nanoparticles can be incorporated into thin-film architectures to selectively harvest UV light. These wide-bandgap semiconductors absorb photons with energies above 3 eV, corresponding to wavelengths below 400 nm, while remaining transparent to visible light. UV-selective solar cells often achieve AVT values above 85%, but their PCE is limited by the relatively small fraction of solar energy in the UV spectrum, typically resulting in efficiencies below 3%. To mitigate this, researchers are exploring hybrid designs that combine UV absorbers with NIR-active organic materials to harvest a broader range of invisible wavelengths without sacrificing transparency.

The primary application of transparent solar cells is in building-integrated photovoltaics (BIPV), where they can be deployed as energy-generating windows or skylights. These installations offer the dual benefit of reducing a building's energy consumption while maintaining natural daylighting. For instance, a window with 70% AVT and 6% PCE could generate approximately 30 W/m2 under standard solar illumination, contributing to the building's power needs without obstructing the view. The actual energy yield depends on factors such as geographic location, window orientation, and local climate, but simulations suggest that widespread adoption could offset a significant portion of urban energy demand.

Another promising application is in electronic displays, where transparent solar cells can be integrated into screens for smartphones, tablets, or wearable devices. In this context, the solar cell must not only be highly transparent but also compatible with the display's optical stack and manufacturing process. Organic semiconductors are particularly well-suited for this purpose due to their solution-processability and flexibility. For example, a transparent solar cell integrated into a smartphone screen could harvest ambient light to extend battery life, though the achievable power output is modest, typically in the milliwatt range. The challenge lies in optimizing the cell's performance under indoor lighting conditions, which have a different spectral composition compared to sunlight.

The trade-off between transparency and efficiency is a fundamental consideration in transparent solar cell design. Increasing the active layer thickness or broadening the absorption spectrum improves PCE but reduces AVT, while highly transparent devices often suffer from low current generation. To address this, researchers employ strategies such as tandem architectures, where two or more transparent cells with complementary absorption profiles are stacked to harvest a wider range of wavelengths without blocking visible light. Another approach involves using transparent conductive electrodes, such as silver nanowires or ultrathin metal films, to minimize optical losses while maintaining efficient charge collection.

Stability and longevity are also critical factors for real-world applications. Organic semiconductors are susceptible to degradation from moisture, oxygen, and UV exposure, necessitating robust encapsulation techniques. Accelerated aging tests indicate that state-of-the-art transparent organic solar cells can retain over 80% of their initial PCE after 1,000 hours of continuous illumination, but further improvements are needed to meet the 20-year lifespan expected for building materials. In contrast, inorganic UV-selective absorbers like ZnO and TiO2 exhibit excellent environmental stability but face limitations in efficiency and manufacturing scalability.

The environmental impact of transparent solar cells is another consideration, particularly for large-scale deployment. Organic semiconductors often rely on synthetic processes that involve hazardous solvents or rare materials, though recent advances in green chemistry are reducing these concerns. Lifecycle assessments suggest that the energy payback time for transparent solar windows could be as short as two years, depending on the materials and manufacturing methods used. This makes them a sustainable option for reducing the carbon footprint of buildings and electronic devices.

Future research directions include the development of new materials with higher selectivity for UV and NIR wavelengths, as well as improved charge transport properties to boost efficiency without compromising transparency. Machine learning and computational screening are being employed to identify promising candidates from vast chemical spaces. Additionally, advances in fabrication techniques, such as roll-to-roll printing or spray coating, could enable cost-effective mass production of transparent solar cells.

In summary, transparent solar cells based on organic semiconductors, non-fullerene acceptors, and UV-selective absorbers offer a versatile solution for integrating photovoltaics into windows and displays. While the trade-off between transparency and efficiency remains a challenge, ongoing innovations in materials science and device engineering are steadily improving their performance and viability. As these technologies mature, they have the potential to transform urban landscapes and consumer electronics by seamlessly combining energy generation with everyday transparency.