Vapor phase deposition of formamidinium-rich perovskites is hampered by the decomposition of formamidine in the vapor phase. We use the reversibility of this reaction to establish a dynamic equilibrium that enables the vapor-phase synthesis of formamidinium iodide from the "degradation products" s-triazine, ammonia, and hydrogen iodide and thereby convert lead halide thin films to lead halide perovskites. Finally, we produce the first proof of concept solar cells via this innovative process.

Microscale pyramids of silicon solar cells are often considered incompatible with solution-processed perovskite films. Thus, solution processing has mainly been used with submicron pyramids that are buried under thick perovskite films with flattened front surfaces. Yet, while this modification simplifies the fabrication process, it compromises optical performance compared to conformal perovskite films (e.g., obtained by vapor processing). Here, we show that protrusion-free perovskite films can be formed on random pyramids much higher than the film thickness by tailoring the film thickness to match the pyramids' height profile, notably without modifying the pyramid facets. Accordingly, we demonstrate perovskite cells spin-coated on over 2 μm-high random pyramids with electronic performance comparable to those fabricated on flat substrates, and proof-of-concept perovskite-silicon tandem devices with efficiencies reaching 33%. Finally, we show that the undulated films with enhanced conformality enable optical performance superior to flat surfaces, especially upon encapsulation, most relevant to outdoor applications.

Inverted perovskite solar cells face performance limitations due to non-radiative recombination at the perovskite surfaces in devices, including functional layers. Advanced characterization and density functional theory reveal that phosphonic acids passivate perovskite surface defects, while piperazinium chloride mitigates interface recombination by improving energy level alignment, introducing a field effect, and homogenizing the surface. Together, the quasi-Fermi level splitting of the perovskite is homogeneously increased by ca. 100 mV. This enables two-terminal perovskite-on-silicon tandems to achieve a certified open-circuit voltage of 2 V for a 1 cm² device and high performance in excess of 31%. The scalability of the passivation is furthermore demonstrated with homogeneously passivated devices reaching certified efficiencies of 28.9% for an active area of 60 cm².

The poor intrinsic perovskite absorber stability is arguably a central limitation challenging the prospect of commercialization for photovoltaic (PV) applications. Understanding the nanoscopic structural features that trigger instabilities in perovskite materials is essential to mitigate device degradation. Using nanostructure characterization techniques, we observe the local degradation to be initiated by material loss at stacking faults, forming inherently in the (011)-faceted perovskite domains in different formamidinium lead triiodide perovskite compositions. We introduce Ethylene Thiourea (ETU) as an additive into the perovskite precursor, which manipulates the perovskite crystal growth and results in dominantly in-and out-of-plane (001) oriented perovskite domains. Combining in-depth experimental analysis and density functional theory calculations, we find that ETU lowered the perovskite formation energy, readily enabling crystallization of the perovskite phase at room temperature without the need for an antisolvent quenching step. This facilitated the fabrication of high-quality large area 5 cm by 5 cm blade-coated perovskite films and devices. Encapsulated and unmasked ETU-treated devices, with an active area of 0.2 cm2, retained > 93 % of their initial power conversion efficiency (PCE) for > 2100 hours at room temperature, and additionally, 1 cm2 ETU-treated devices maintained T80 (the duration for the PCE to decay to 80 % of the initial value) for > 600 hours at 65 °C, under continuous 1-sun illumination at the maximum power point in ambient conditions. Our demonstration of scalable and stable perovskite solar cells represents a promising step towards achieving a reliable perovskite PV technology.

Halide-perovskite semiconductors have a high potential for use in single-junction and tandem solar cells. Despite their unprecedented rise in power conversion efficiencies (PCEs) for photovoltaic (PV) applications, it remains unclear whether perovskite solar modules can reach a sufficient operational lifetime. In order to make perovskite solar cells (PSCs) commercially viable, a fundamental understanding of the relationship between their nanostructure, optoelectronic properties, device efficiency, and long-term operational stability/reliability needs to be established. In this review, the phase instabilities in state-of-the-art formamidinium (FA)-rich perovskite absorbers is discussed. Furthermore, the concerted efforts are summarized in this prospect, covering aspects from fundamental research to device engineering. Subsequently, a critical analysis of the dictating impact of the nanoscale landscape of perovskite materials on their resulting intrinsic stability is provided. Finally, the remaining challenges in the field are assessed and future research directions are proposed for improving the operational lifetimes of perovskite devices. It is believed that these approaches, which bridge nanoscale structural properties to working solar cell devices, will be critical to assessing the realization of a bankable PSC product.

Up-scalable coating processes need to be developed to manufacture efficient and stable perovskite-based solar modules. In this work, we combine two Lewis base additives (N,N '-dimethylpropyleneurea and thiourea) to fabricate high-quality Cs(0.15)FA(0.85)PbI(3) perovskite films by blade-coating on large areas. Selected-area electron diffraction patterns reveal a minimization of stacking faults in the alpha-FAPbI(3) phase for this specific cesium-formamidinium composition in both spin-coated and blade-coated perovskite films, demonstrating its scaling potential. The underlying mechanism of the crystallization process and the specific role of thiourea are characterized by Fourier transform infrared spectroscopy and in situ optical absorption, showing clear interaction between thiourea and perovskite precursors and halved film-formation activation energy (from 114 to 49 kJ/mol), which contribute to the obtained specific morphology with the formation of large domain sizes on a short time scale. The blade-coated perovskite solar cells demonstrate a maximum efficiency of approximately 16.9% on an aperture area of 1 cm(2).

As global decarbonisation requires the widespread adoption of solar photovoltaic (PV) electricity, addressing challenges related to land use has become relevant. The conflict between PV installations and other land uses, such as forestry or agriculture, highlights the urgency for alternative solutions. Integrating PV technology into the built environment is a compelling strategy to mitigate these challenges, enabling electricity generation precisely where it is needed. In the context of buildings integrated photovoltaics (BIPV), PV modules serve a dual purpose, functioning both as electricity generators and integral components of the architectural design. Therefore, the architecture requirements — specifically in terms of shape, size, and colour— become relevant for BIPV modules. This paper offers a general overview of the diverse colouring technologies employed for BIPV modules, describing their functioning, challenges, and advantages. An examination of the current landscape of coloured PV products involving considerations of pricing and power output is presented. Additionally, this work addresses the critical topics of reliability and stability in colour solutions, outlining methodologies for quantitative colour characterization. It provides foresight into the potential challenges facing installations in the future and explores the multifaceted social, economic, and environmental implications of this evolving technology.

The impact of the composition of the A-site cations on the intra-domain defect density in cesium-formamidinium perovskite formulations is modulated via alloying.

Combining the two technologies of tandem solar cells and bifacial solar cells has a great potential to maximize energy harvesting while minimizing material and surface usage. Mid-bandgap perovskites (1.50-1.60eV) are important for fulfilling current matching in bifacial perovskite/silicon heterojunction tandem solar cells. Herein, efficient (>20%) and stable planar FAPbI3-based perovskite (1.54eV) solar cells have been fabricated via a hybrid evaporation-spin coating process. X-ray diffraction and electron microscopy data reveal the formation of highly crystalline (001) perovskite domains. The fabricated high-quality perovskite films lead to a more homogenized contact potential difference at the film surface and a reduction in non-radiative losses, resulting in a quasi-Fermi-level splitting (half-cell) of 1.16eV, rendering 120mV non-radiative losses. Transferring these films into tandem devices atop single-side textured silicon heterojunction bottom cells, we obtain an efficiency of >24% under AM1.5G illumination for monofacial devices with an active area of 1.21 cm2. Furthermore, the bifacial devices generate >27mWcm-2 power output with 15% rear illumination fraction.

Perovskite solar cells (PSCs) have made great advances in terms of power conversion efficiency (PCE), yet their subpar stability continues to hinder their commercialization. The interface between the perovskite layer and the charge-carrier transporting layers plays a crucial role in undermining the stability of PSCs. In this work, we propose a strategy to stabilize high-performance PSCs with PCE over 23% by introducing a cesium-doped graphene oxide (GO-Cs) as an interlayer between the perovskite and hole-transporting material. The GO-Cs treated PSCs exhibit excellent operational stability with a projected T80 (the time where the device PCE reduces to 80% of its initial value) of 2143 hours of operation at the maximum powering point under one sun illumination.

Whether illumination influences the ion conductivity in lead-halide perovskite solar cells containing iodide halides has been an ongoing debate. Experiments to elucidate the presence of a photoconductive effect require special devices or measurement techniques and neglect possible influences of the enhanced electronic charge concentrations. Here, we assess the electronic-ionic charge transport using drift-diffusion simulations and show that the well-known increase in capacitance at low frequencies under illumination is caused by electronic currents that are amplified due to the screening of the alternating electric field by the ions. We propose a novel characterization technique to detect a potential photoinduced increase in ionic conductivity based on capacitance measurements on fully integrated devices. The method is applied to a range of perovskite solar cells with different active layer materials. Remarkably, all measured samples show a clear signature of photoenhanced ion conductivity, posing fundamental questions on the underlying nature of the photosensitive mechanism.

Electric vehicles (EVs) currently dominate the sales in the automotive market. A big leap in this market can be made by developing a photovoltaic product that can be integrated to an EV, as it can boost the driving range of the EV while reducing the charging frequency. Such vehicle-integrated photovoltaic (VIPV) products are already successfully demonstrated, but they are usually made with glass as a front sheet – making them bulky and limiting their use to the car roofs due to safety reasons. The contemporary focus of the research in the field of VIPV is on developing a product that is lightweight (LW) and easily integrable into the complex shapes of an EV. Therefore, in this work, we present our initial findings on a novel architecture for LW VIPV modules employing polycarbonate (PC) as a front sheet. The mechanical behaviour of the LW module under bending is successfully simulated using finite elements (FE) modelling to predict the fracture of the solar cells, which can then be used as a predictive tool to check the maximal load on the PV body of an EV before cracking the c-Si solar cells. We demonstrate that a change in the temperature of the PC-based LW modules can modify the interspacing between the cells and thus create stress on the connectors. The dog-bone connectors are found to allow almost unconstrained movement of the cells in the module when subjected to variation of temperature. The cell movements may result in mechanical fatigue of the interconnection, which can ultimately result in disconnection of the cells. Initial performance of the dog-bone connectors is investigated by applying mechanical fatigue experiments, which demonstrate that the special geometry of the dog-bone connector could endure a greater number of thermal cycles than a simple prismatic shape would.

Full-area passivating contacts based on SiOx/poly-Si stacks are key for the new generation of industrial silicon solar cells substituting the passivated emitter and rear cell (PERC) technology. Demonstrating a potential efficiency increase of 1 to 2% compared to PERC, the utilization of n-type wafers with an n-type contact at the back and a p-type diffused boron emitter has become the industry standard in 2024. In this work, variations of this technology are explored, considering p-type passivating contacts on p-type Si wafers formed via a rapid thermal processing (RTP) step. These contacts could be useful in conjunction with n-type contacts for realizing solar cells with passivating contacts on both sides. Here, a particular focus is set on investigating the influence of the applied thermal treatment on the interfacial silicon oxide (SiOx) layer. Thin SiOx layers formed via ultraviolet (UV)-O-3 exposure are compared with layers obtained through a plasma treatment with nitrous oxide (N2O). This process is performed in the same plasma enhanced chemical vapor deposition (PECVD) chamber used to grow the Si-based passivating layer, resulting in a streamlined process flow. For both oxide types, the influence of the RTP thermal budget on passivation quality and contact resistivity is investigated. Whereas the UV-O-3 oxide shows a pronounced degradation when using high thermal budget annealing (T > 860 degrees C), the N2O-plasma oxide exhibits instead an excellent passivation quality under these conditions. Simultaneously, the contact resistivity achieved with the N2O-plasma oxide layer is comparable to that yielded by UV-O-3-grown oxides. To unravel the mechanisms behind the improved performance obtained with the N2O-plasma oxide at high thermal budget, characterization by high-resolution (scanning) transmission electron microscopy (HR-(S)TEM), X-ray reflectometry (XRR) and X-ray photoelectron spectroscopy (XPS) is conducted on layer stacks featuring both N2O and UV-O-3 oxides after RTP. A breakup of the UV-O-3 oxide at high thermal budget is observed, whereas the N2O oxide is found to maintain its structural integrity along the interface. Furthermore, chemical analysis reveals that the N2O oxide is richer in oxygen and contains a higher amount of nitrogen compared to the UV-O-3 oxide. These distinguishing characteristics can be directly linked to the enhanced stability exhibited by the N2O oxide under higher annealing temperatures and extended dwell times.

This work addresses the need for precise control of thin film sputtering processes to enable thin film material tailoring on the example of zinc tin nitride (ZTN) thin films deposited via microwave plasma-assisted high power reactive magnetron sputtering (MAR-HiPIMS). The applied in situ diagnostic techniques (Langmuir probe and energy-resolved time-of-flight mass spectrometry) supported monitoring changes in the deposition environment with respect to microwave (MW) power. During MAR-HiPIMS, the presence of nitride ions in the gas phase (ZnN+, ZnN2+, SnN+, SnN2+) was detected. This indicates that the MW plasma facilitated their production, as opposed to pure R-HiPIMS. Additionally, MW plasma caused post-ionisation of sputtered atoms and reduced the overall energy-per-charge range of incoming charged species. By varying the MW power and substrate biasing, films with comparable chemical compositions (approximately Zn0.92Sn1.08N2) but different structures, ranging from polycrystalline to preferentially textured, were successfully produced. The application of density functional theory (DFT) further enabled the relationship between the lattice parameters and the optical properties of ZTN to be explored, where the material's optical anisotropy nature was determined. It was found that despite considerable differences in crystallinity, the changes induced in the lattice parameters were subangstrom, causing only minor changes in the final optical properties of ZTN.

Tandem solar cells are a key technology to exceed the theoretical efficiency limit of single-junction cells. One of the most promising combinations is the silicon-perovskite tandem cells, considering their potential for high efficiency, large-area fabrication, and low cost. Whereas most research focuses on improvements in each subcell, another key challenge relies on the tunnel junction that connects subcells and affects overall cell characteristics. Here, the first demonstration of tunnel junctions using a stack of p/n polysilicon deposited directly on the passivating tunnel oxide are shown to overcome the drawbacks of conventional metal oxide-based tunnel junctions including low tunneling efficiency and sputter damage. Furthermore, using Random Forest analysis, high implied open circuit voltages over 700 mV in the bottom cell with the polysilicon tunnel junction are achieved. Their contact resistivities are as low as 500 mΩ·cm, suggesting FF losses of less than 1 %abs for the operating conditions of a tandem cell.

This work presents a study of p-type passivating contacts based on SiCx formed via a rapid thermal processing (RTP) step, using conditions compatible with the firing used to sinter screen-printed metallization pastes in industry. The contributions of the two interfaces (wafer/contact and contact/metal) to the contact resistivity are first decorrelated, identifying tunnelling at the wafer interface as the main contribution. We then investigate the influence of the active dopant concentration on the contact resistivity and the SiCx sheet resistance and propose strategies to reduce both resistances by increasing the thermal budget applied during RTP. Lastly, we discuss potentials and limitations of implementing the investigated stacks as rear side contacts of p-type devices with localized metallization. We demonstrate that increasing the thermal budget during RTP can effectively mitigate resistive losses and enhance contact performance and we show that an oxide layer that can withstand high thermal budgets is the key factor for obtaining simultaneously high passivation quality and good electrical properties. We investigate three different oxide types grown by HNO3 immersion, UV-O3 exposure and N2O plasma oxidation. The latter is demonstrated to be a promising candidate for an application in devices fabricated with high RTP thermal budget.

Replacing silver metallization with earth-abundant materials in Si solar cells is a critical step towards the sustainable growth of photovoltaics. In this work, we investigate fire-through processes using Al for the metallization of solar cells with p-type passivating contacts, with the goal to achieve a thin depth of contact, oppositely to what is used for standard back-surface field Al contacts. The interactions taking place during firing between the different elements in our contact stack (Al, SiNx:H, SiCx(p) and tunnel SiOx) are studied. We discuss how Al and SiNx react during firing by performing contact formation through different nitride layers, and show that increasing the N content of the silicon nitride can reduce Al penetration depth. Finally, we also investigate the mechanisms behind metal-induced passivation degradation and identify possible ways of mitigating them by employing an adapted SiOx/SiCx stack to allow for Ag-free metallization of tunnel oxide passivating contacts.

Tandem solar cells are a key technology for exceeding the theoretical efficiency limit of single-junction cells. One of the most promising combinations is the silicon–perovskite tandem cells, considering their potential for high efficiency, fabrication on a large scale, and low cost. While most research focuses on improving each subcell, another key challenge lies in the tunnel junction that connects these subcells, significantly impacting the overall cell characteristics. Here, we demonstrate the first use of tunnel junctions using a stack of p+/n+ polysilicon passivating contacts deposited directly on the tunnel oxide to overcome the drawbacks of conventional metal oxide-based tunnel junctions, including low tunneling efficiency and sputter damage. Using Random Forest analysis, we achieved high implied open circuit voltages over 700 mV and low contact resistivities of 500 mΩ cm2, suggesting fill factor losses of less than 1% abs for the operating conditions of a tandem cell.