Semi-transparent solar cells have attracted massive industry interest as they offer tri-functionality by forming the skin of the building, enabling natural light illumination, and generating power. Cu(In,Ga)Se₂ (CIGSe) photovoltaics are a promising technology option for such applications. There are two different approaches to achieve transparent CIGSe solar cells. One involves the fabrication of ultra-thin CIGSe absorbers on see-through transparent back contacts. This method, however, is limited by the formation of GaO_x parasitic layers at the back contact interface, which leads to efficiency losses. An alternative approach involves spatially segmented full-thickness solar cells with transparent parts separating them to give the semi-transparent characteristic.

As proof of concept, we present a top-down micro-structuring approach, where an opaque CIGSe solar cell is spatially segmented into micro-sized line-shaped solar cells. By varying the lines’ width and spacing, it is possible to control the window’s average visual transparency (AVT), making it suitable for different applications. The fabrication process begins with a photolithography step, where the selected pattern of micro-lines is designed on top of a complete CIGSe solar cell stack. An aqueous bromine solution etches the developed solar cell areas, while the photoresist protects the solar cell lines [1]. The bromine solution results in a slight over-etch of the window layer by about 20-30 µm. This over-etch reduces the active device area, leading to a performance loss, which is particularly pronounced in narrower lines (narrower than 200 µm). After the bromine etching, the sample was dipped in sodium hypochlorite (commercial bleach) to remove the exposed molybdenum, thereby making the areas in between the solar cell lines fully transparent. To ensure proper charge carrier collection, we performed simulations to design a metallic front contact grid that minimizes shadowing. Narrow, tapered aluminum electrical contacts were evaporated on top of the full length of the line solar cells, with a width between 30 mm and 1 mm. The top-down, materials inefficient process resulted in a semi-transparent CIGSe solar cell with a total area efficiency of 6.1 % and an AVT of 49 %.

Developing a more sustainable and materials-efficient fabrication approach, we use sputter-deposition and lift-off processes to achieve selective deposition of the CIGSe micro-striped solar cells. Here, Cu-In-Ga is deposited onto a patterned photoresist, followed by a lift-off process. The Cu-In-Ga lines are subsequently transformed into CIGSe lines by a selenization process. Solar cell devices are then completed by depositing a buffer layer in a chemical bath and by sputtering a i-ZnO/ZnO:Al window layer. With this approach, we have achieved a power conversion efficiency of approximately 6% for individual lines.

Transparent Photovoltaic (TPV) technologies represent a promising branch within photovoltaics, seeking to expand their applications by overcoming challenges related to on-site integration, especially within architectural elements related to Building Integrated PV (BIPV), and more recently, also in the areas of Indoor PV (IPV), IoT and Agrivoltaics (APV). Unlike conventional approaches solely focused on efficiency, TPV introduces two additional dimensions: transparency and aesthetics, which pose added challenges to the device architecture. Moreover, for TPV technologies to be translated into competitive products it is critical to work on low-cost, sustainable and stable materials and fabrication processes that at least meet the stringent requirements for any PV technologies in conjunction with the transparency and aesthetic values that allow for seamless integration.

The goal for TPV is to have a device that absorbs in the visible range as little as possible while it absorbs the remainder part of the spectrum (i.e. UV/IR) in order to be visible transparent to the human eye, and as so, it can be split into two main categories or approaches, namely wavelength-selective and non-wavelength-selective. These two approaches are being actively investigated using different materials, such as organic materials and perovskites. However, inorganic-based structures constitute a very attractive prospect as they can be integrated as different functional layers in the solar cell architecture (i.e. as absorber, Charge Transport Layer and transparent electrical contacts). Additionally, many inorganic materials present high bandgap, tuneable conductivity, low deposition temperatures and can be deposited by a plethora of techniques that are possible to upscale for industrial purposes. Another key aspect is that these materials are stable, have low thermal budgets and are CRM-free. Given these aspects the challenge is on how to combine them in advanced device architectures to develop a final device that is efficient, transparent and aesthetically pleasing that can be integrated in architectural components (windows, canopies, façades) and/or on devices that present low power draws such as smartphones, wearables and IoT devices and sensors.

Herein, we will discuss the basic principles and figures of merit in TPV, as well as the state of the art for inorganic-based strategies. The talk will also focus into two main approaches that we are developing: ZnO1-xSx UV-selective absorbers and on the optimisation of oxide-based architectures integrating nanometric a-Si:H layers as a non-wavelength-selective approach. Main challenges and late results attained with both strategies will be reviewed, including the achievement of record devices with Light Utilisation Efficiency (LUE) up to 2.3%, transparency in the range between 30% and 70% and photoconversion efficiencies up to 5%.

Development of new photovoltaic (PV) technologies based on novel green materials with higher efficiency and lower cost options could create a new industry with independent supply chains, foster IoT market diversification, and attenuate market volatility. Sb-chalcogenide compounds have recently gained increasing attention as defect tolerant, non-toxic and highly stable materials for earth abundant thin film polycrystalline PV technology.  Despite of their recent addition to the thin film device value chain, efficiency values of these materials have climbed rapidly and are now approaching 11%. Yet despite of this rapid progress, there is still headroom to increase performance significantly by addressing both, fundamental material challenges and innovation in device designing and development. This presentation will review the most relevant progress achieved for Sb-chalcogenides thin film PV, with the emphasis on key processing strategies to optimize absorber material and thin film solar cells properties (doping and alloying), understanding of buried interfaces and push the boundaries of understanding and performance. Based on the state-of-the art progress in performance of emerging Sb-chalcogenide PV materials, a roadmap of customized PV application variability, including semitransparent and tandem PV devices for BIPV, PIPV and IoT markets will be presented.

Sb₂(S,Se)₃ has attracted increasing attention as a photovoltaic absorber due to its high absorption coefficient, excellent stability and Restriction of Hazardous Substances (RoHS)-compliant composition. However, current high-efficiency Sb₂(S,Se)₃ solar cells are typically opaque and mono-facial configurations, creating a technological gap to their tandem application with silicon solar cells. Here, we demonstrate a single-junction bifacial and semitransparent Sb₂(S,Se)₃ solar cell, achieved by employing an indium tin oxide (ITO) back electrode capping the MnS hole-transporting layer (HTL). The ultrathin and fully depleted absorber layer, fabricated by the hydrothermal method, allows carriers to drift towards the respective functional layers, thereby greatly increasing the bipolar transport and bifacial absorption. Under AM1.5 illumination, the device achieves PCEs of 7.41% (front) and 6.36% (rear), indicating an impressive bifaciality of 0.86. This architecture exhibits high transmittance in the long-wavelength region, enabling the integration of Sb₂(S,Se)₃ as the top cell in a tandem solar cell. A preliminary Sb₂(S,Se)₃/Si tandem solar cell achieves a PCE of 11.66%. Subsequently, we further optimized the Sb₂(S,Se)₃ absorber layer by introducing an appropriate additive into the precursor solution. This strategy promotes more favourable band alignment and suppresses the formation of deep-level point defects, thereby significantly improving the material quality and device performance. In parallel, the relationship among transparency, bandgap and efficiency is systematically investigated to further enhance the performance of Sb₂(S,Se)₃/Si tandem device. These exciting findings imply that bifacial and semitransparent Sb₂(S,Se)₃ solar cells possess tremendous potential in tandem and practical applications.

The development of TPV devices has gained significant attention in recent years due to their potential for seamless integration into Building-Integrated Photovoltaics (BIPV). BIPV has been identified as a key enabling technology for the development of "Near Zero Energy Buildings" (NZEB), achieved through the integration of a new generation of PV modules capable of replacing traditional architectural elements. TPV devices are also of interest in other applications, such as Product-Integrated Photovoltaics (PiPV), and any scenarios where aesthetics are important. TPV devices have the potential to revolutionize photovoltaic technology by enabling on-site generation while minimizing visual impact.

One of the most promising approaches to achieve TPV is to utilize the ultraviolet region of the solar spectra, while avoiding absorption in the visible range and more specifically on the photopic response range of the human eye, thus attaining complete apparent transparency. One promising approach to achieve high transparency on solar cells is by using wide bandgap materials with bandgap energy (E_g) above 2.7 eV. However, the choice of absorber material is critical, due to the lower number of photons in this spectral region, making it a must to have a good spectral match with the bandgap and the UV onset to maximize photon absorption. In this way, ZnO_1-xS_x has been proposed as an ideal UV absorber thanks to a composition-tunable bandgap that can shift from 3.2 eV (pure ZnO) down to 2.7 eV (x=0.5) by anionic substitution of oxygen by sulfur. ZnO_1-xS_x is a material composed of earth-abundant raw elements, compatible with scalable fabrication processes and can be synthesized at low temperatures, which potentially allows minimizing the carbon footprint, economic costs and energy expenditure associated with device manufacturing.

In this work we present a fully oxide based solar cells with structure SGL/FTO/MoO_x/ZnO_1-xS_x/ZnO/AZO. This device is a full ALD-compatible process, which enables high process reproducibility and conformality for the coverage of 3D structures. We explore the different compositional ranges from sulfur rich to sulfur poor and how it affects to performance, also crystal formation and phase segregation is explored at different synthesis temperatures. In a second level, two samples with different HTL are prepared and studied, one with MoO_x and another with NiO. Based on these prelimienary results, we explore an aproach to significantly increase the Voc of the ZnO_1-xS_x TPV solar cells by the fine tuning of the MoO_x HTL thickness, preparing two 5x5 cm² samples with a gradient of the MoO_x HTL layer. The complete optimized structure demostrated an Average Photopic Transmittance of 73%, showing indeed excelent transparency, while achieving colore rendering Index (CRI) of 97, coriming its high color neutrality and achieving record-high Voc values close to 800 mV. 

Selenium (Se) holds historical significance as the first material used in a solar cell, in 1883, marking the initial exploration of materials capable of harnessing solar energy. However, it took over a century before being seriously considered for photovoltaics, achieving at this time an efficiency of 5% with the following Au/Se/TiO2/FTO architecture. In 2017, IBM improved the device architecture with more suitable selective contacts (Au/MoOx/Se/ZnMgO/FTO) and an impressive efficiency of 6.5% was achieved. This breakthrough reignited interest in selenium for photovoltaics, resulting in many recent research publications, with a particular focus on its potential use as a top cell in tandem configurations, and much more recently as indoor devices. During the last year, two new world-record efficiencies have been reported: 7.2% and 8.1% under AM1.5 illumination, and more than 20% under indoor conditions. However, all these results were achieved for thick absorber layer (higher than 1 um) except for the IBM device, where the impressive 6.5% efficiency was achieved with a thickness of just 100 nm.

Selenium benefits from a low temperature processing (melting point around 200 ºC) and a direct bandgap of approximatively 1.95 eV. Although this bandgap may initially not seem ideal for semi-transparent applications, recent findings suggest that very thin (less than 50 nm) amorphous silicon devices can achieve high visible transparency and impressive efficiencies, with promising light utilization efficiencies (LUE) of 1%. An initial optical simulation of a complete device architecture using a 100 nm thick selenium indicated an average visible transmittance (AVT) of 51% across the visible spectrum. This could potentially lead to LUE values higher than 2.5%, which is an outstanding result, suggesting that c-Se is inherently compatible for semi-transparent applications.

To check the viability of Se for semi-transparent PV, different strategies for the synthesis of Se layers have been established. A baseline processing technology allowing the fabrication of state-of-the-art Se devices has been developed and optimized. Subsequently, strategies to get transparency have been implemented and developed. All the results of this work will be presented at the Matsus Fall 25 conference where the viability of selenium for semi-transparent photovoltaics will be discussed.

Transparent photovoltaics (TPV) is a disruptive approach in which the solar cells can selectively transmit the visible light to human eyes harvesting UV and/or NIR photons.[1] TPV is attractive as it widens the deployment of PV into new sectors, like building integrated photovoltaics (BIPV), greenhouses, car windows and sunglasses, thus providing an immense potential to generate solar electricity beyond the conventional rooftops and solar power plants. One possible approach to TPV is based on wavelength-selective absorbers where the chromophore requires an absorption far from the photopic response of the human eye.

DSSCs offer a sustainable solution for transparent and colorless photovoltaic cells, thanks to the exceptional tunability of both dyes and electrolytes. However limited classes of dyes possess energetic levels that can ensure an efficient injection while having a bandgap sufficiently narrow to selectively absorb the NIR region.[2] Among these classes, polymethine dyes (cyanines and squaraines) are promising for their high molar extinction coefficient and easily tunable properties through modification of central core or lateral units. Cyanines in particular have already been investigated for dye sensitized solar cell (DSSC) devices with promising results in terms of transparency and performance.[3]

In recent years, our research group has developed a series of polymethine-based NIR dyes for wavelength-selective DSSCs, as well as different colorless redox shuttle. Most of these molecules were synthesized using microwave-assisted procedures aligned with green chemistry principles, enabling efficient and cost-effective production. This work presents the design, synthesis, and device integration of NIR dyes that achieve high transparency coupled with acceptable performances. Additionally, we report on the development of compatible colorless redox mediators and transparent electrodes. A comprehensive discussion is provided on the interplay of materials and the current technological challenges in realizing fully transparent and colorless NIR-DSSCs for BIPV applications.

These new molecules have been deeply characterized in terms of their optical, photophysical and electrochemical properties, showing interesting structure/property relationships. Finally, photovoltaic performances have been evaluated in lab-scale DSSCs and optimized by different anode modifications and electrolyte formulations.

Among the diverse array of photovoltaic technologies, organic and perovskite photovoltaics stand out as particularly promising candidates for transparent solar applications, each offering unique advantages. Organic photovoltaics (OPVs), based on an excitonic semiconductor, inherently provide true transparency while simultaneously enabling the harvesting of near-infrared (NIR) light, making them ideal for integration into windows and building facades. Perovskite solar cells, characterized by their direct bandgap and exceptionally high open-circuit voltages (Voc), stand out in capturing high-energy photons in the blue region of the spectrum with very low Voc losses, positioning them most suitable for tandem configurations.

The combination of perovskites with organic PVs into tandem architectures emerges as a natural approach, leveraging the complementary spectral absorption and electronic properties of both materials. Such tandem systems not only improve overall efficiency but also offer tunability of transparency and color rendering index (CRI) through bandgap engineering of the individual layers. This flexibility enables the customization of optical appearance to meet aesthetic and functional requirements.

In this contribution, we outline the fundamental design principles for developing highly efficient, transparent and fully solution processed organic PV modules. Additionally, first fully solution processed transparent wide bandgap perovskite modules will be shown. We further introduce an innovative voltage-matching concept tailored to reconcile the differing voltages of wide-bandgap perovskite and low-bandgap organic cells, addressing a key challenge in tandem device fabrication. Finally, we present the first demonstrators of transparent perovskite-organic tandem modules developed within the European project Citysolar, showcasing their potential for sustainable, aesthetically pleasing, and high-performance building-integrated photovoltaics.

The solar cell technology is experiencing tremendous growth globally, as well as the building integrated photovoltaics (BIPV) field [1-4]. The latter is growing extremely fast because the integration of photovoltaics (PV) into building roofs and façades provides cost-effective energy solutions, as modules can substitute building envelopes, such as roofing or glass windows. Windows make up a large percentage of modern building real estate, therefore transforming them into PV devices they would drastically increase the available area for on-site electricity generation.

However, the adoption of this technology depends on its ability to transmit light in terms of average visual transmittance (AVT) coupled with a reasonably high photo-conversion efficiency (PCE) [1-4].

Currently, most commercially available devices consist of patterned crystalline silicon (c-Si) wafer technology [1]. Although Si-based PV offers both high PCE and AVT, it results in an unpleasant and impeded view. On the other hand, amorphous silicon (a-Si) can enable a homogeneous appearance for semi-transparent photovoltaic (STPV) by decreasing the thickness of the light-absorbing material such that semi-transparency is achieved. However, the latter result in windows with an inherent low color rendering index (CRI).

Perovskite PV technology has taken giant steps from fundamental science to device engineering, achieving up to 26% photo-conversion efficiency [5-7] in almost a decade time. The possibility to exploit this technology on glass substrates gives an unbeatable power to weight ratio in comparison to similar photovoltaic systems, thus opening new possibilities and new integration concepts in BIPV.

Also, perovskite solar cells (PSCs) hold an advantage over traditional silicon solar cells in the simplicity of their manufacturing processing [6-7]. While silicon cells require expensive, multi-step processes, at high temperature and under high vacuum in cleanroom facilities, PSC materials can be realized in lab environment using a variety of inexpensive, simpler, and low-temperature solution processing and deposition techniques with the potential to be scaled up for large-area device fabrication [7].

Moreover, there is an enormous effort to push PSC from research and development at the lab-scale level to a  large-scale industrial level, making PSC an outstanding contender for STPV. So far, top-performing STPV using thin perovskites have reached 12.6% PCE with 21.5% AVT, and 2.7% LUE [6].

Here, we developed a STPV technology with a relatively high AVT (43%), PCE (6%), LUE (3%), and CRI (89%) based on laser patterning of thin-film PSC. Our design philosophy is based on micro-striped solar cells separated by a fully transparent gap, providing high transparency. The device architecture is a thin-film, planar, n-i-p configuration with a commercial transparent conductive oxide-coated glass bottom contact electrode, an inorganic electron-transport layer, a hybrid halide perovskite absorbing layer, an organic hole-transport layer, and a thermally evaporated or sputtered top contact electrode. The material layers are fabricated by inexpensive solution processing methods and deposited onto substrates at low temperature by scalable and rapid printing techniques, such as spincoating.

Furthermore, all developments target material-efficient and sustainable fabrication approaches for technology transfer to industry.

With the world's growing population, the demand for food and electricity is increasing rapidly. To meet these demands, the agricultural and energy sectors need to work together to achieve sustainable development. Agrivoltaics is one of the promising approaches that combines the production of crops and renewable energy generation in the same land footprint. However, the use of conventional opaque solar panels in agrivoltaic systems creates shading effects, being unable to share the solar spectrum, given that plants absorb only >1.7 eV photons whilst silicon (or other inorganic) solar cells absorb all photon energies above the near IR. To overcome this issue, low-cost semi-transparent photovoltaic modules produced using scalable manufacturing offer an exciting alternative. Such cells enable wavelengths between 400 and 700 nm, which is referred to as photosynthetically active radiation (PAR) to pass through to the crop. Thus, Organic Photovoltaics (OPV) have gained significant attention due to their ability to harness photons in the near-infrared and ultraviolet spectra while allowing visible light to pass through, providing high transparency.

The current state-of-the art power conversion efficiency (PCE) of OPVs exceeds 20% for a single junction OPV¹and reaches 11% for semitransparent based OPV devices², following rapid improvement in recently developed non-fullerene small-molecules acceptors (NFAs) to replace fullerene-counterpart. Moreover, high potential PCEs are offered by a ternary system where either the third component acts as an additional donor or additional acceptors that serve to extend the range of absorption and can also tune material properties. Interestingly, the incorporation of the third counterpart can regulate the accumulation and orientation of the molecule, as well as the phase separation of donor and acceptor, providing high crystallinity and ordered molecular stacking that can improve the charge transport and inhibit the bimolecular recombination through well optimized phase separation³.

Furthermore, most research on ternary strategies is based on the bulk hetero junction (BHJ) system, which is sensitive to material properties and processing conditions. This makes it difficult to control other important morphological parameters, such as molecular orientation and domain purity as it further complicates the morphological regulation, especially the D:A orientation in the vertical direction of blend films which is mainly related to the charge transport and collection⁴. Thus, to tailor vertical phase distribution efficiently, the sequential deposition or named layer by layer (LBL) deposition approach of the D and A materials is considered as a promising alternative to the BHJ⁵.

Hence, I have developed a novel OPV structure, introducing a perovskite quantum dots (PQDs) interlayer sandwiched between the organic semiconductor donor and the NFA layers using LBL deposition approach, resulting in enhancement in the performance of OPV devices by 11% ( from PCE=16.6% for the pristine binary OPV to PCE= 18.8% for the QDs-based ternary OPV) along with 99 % performance retention after 3 months of storage compared to only 30% for the bilayer devices without PQDs. This unique structure allows for the LBL preferred vertical phase separation and well-controlled D/QDs/A interface film morphology, exhibiting efficient charge transport and extraction properties. Interestiingly, from the perspective of the agrivoltaic application, the PQD interlayer (third component) does not absorb much light in the visible region, allowing it to be transmitted to the plants without influencing the crop yields.

In summary, the semitransparency and promising PCE along with the cost-effective, eco-friendly processing, are promising selling points of this OPV technology for the agrivoltaic system relative to traditional PV technologies.