Double-side 2D/3D heterojunctions for inverted perovskite solar cells.

Defects at the top and bottom interfaces of three-dimensional (3D) perovskite photoabsorbers diminish the performance and operational stability of perovskite solar cells owing to charge recombination, ion migration and electric-field inhomogeneities1-5. Here we demonstrate that long alkyl amine ligands can generate near-phase-pure 2D perovskites at the top and bottom 3D perovskite interfaces and effectively resolve these issues. At the rear-contact side, we find that the alkyl amine ligand strengthens the interactions with the substrate through acid-base reactions with the phosphonic acid group from the organic hole-transporting self-assembled monolayer molecule, thus regulating the 2D perovskite formation. With this, inverted perovskite solar cells with double-side 2D/3D heterojunctions achieved a power conversion efficiency of 25.6% (certified 25.0%), retaining 95% of their initial power conversion efficiency after 1,000 h of 1-sun illumination at 85 °C in air.

Capillary Manganese Halide Needle-Like Array Scintillator with Isolated Light Crosstalk for Micro-X-Ray Imaging.

The exacerbation of inherent light scattering with increasing scintillator thickness poses a major challenge for balancing the thickness-dependent spatial resolution and scintillation brightness in X-ray imaging scintillators. Herein, a thick pixelated needle-like array scintillator capable of micrometer resolution is fabricated via waveguide structure engineering. Specifically, this involves integrating a straightforward low-temperature melting process of manganese halide with an aluminum-clad capillary template. In this waveguide structure, the oriented scintillation photons propagate along the well-aligned scintillator and are confined within individual pixels by the aluminum reflective cladding, as substantiated from the comprehensive analysis including laser diffraction experiments. Consequently, thanks to isolated light-crosstalk channels and robust light output due to increased thickness, ultrahigh spatial resolutions of 60.8 and 51.7 lp mm-1 at a modulation transfer function (MTF) of 0.2 are achieved on 0.5 mm and even 1 mm thick scintillators, respectively, which both exceed the pore diameter of the capillary arrays' template (Φ = 10 µm). As far as it is known, these micrometer resolutions are among the highest reported metal halide scintillators and are never demonstrated on such thick scintillators. Here an avenue is presented to the demand for thick scintillators in high-resolution X-ray imaging across diverse scientific and practical fields.

Sublimed C60 for efficient and repeatable perovskite-based solar cells.

Thermally evaporated C60 is a near-ubiquitous electron transport layer in state-of-the-art p-i-n perovskite-based solar cells. As perovskite photovoltaic technologies are moving toward industrialization, batch-to-batch reproducibility of device performances becomes crucial. Here, we show that commercial as-received (99.75% pure) C60 source materials may coalesce during repeated thermal evaporation processes, jeopardizing such reproducibility. We find that the coalescence is due to oxygen present in the initial source powder and leads to the formation of deep states within the perovskite bandgap, resulting in a systematic decrease in solar cell performance. However, further purification (through sublimation) of the C60 to 99.95% before evaporation is found to hinder coalescence, with the associated solar cell performances being fully reproducible after repeated processing. We verify the universality of this behavior on perovskite/silicon tandem solar cells by demonstrating their open-circuit voltages and fill factors to remain at 1950 mV and 81% respectively, over eight repeated processes using the same sublimed C60 source material. Notably, one of these cells achieved a certified power conversion efficiency of 30.9%. These findings provide insights crucial for the advancement of perovskite photovoltaic technologies towards scaled production with high process yield.

Pathways toward commercial perovskite/silicon tandem photovoltaics.

Perovskite/silicon tandem solar cells offer a promising route to increase the power conversion efficiency of crystalline silicon (c-Si) solar cells beyond the theoretical single-junction limitations at an affordable cost. In the past decade, progress has been made toward the fabrication of highly efficient laboratory-scale tandems through a range of vacuum- and solution-based perovskite processing technologies onto various types of c-Si bottom cells. However, to become a commercial reality, the transition from laboratory to industrial fabrication will require appropriate, scalable input materials and manufacturing processes. In addition, perovskite/silicon tandem research needs to increasingly focus on stability, reliability, throughput of cell production and characterization, cell-to-module integration, and accurate field-performance prediction and evaluation. This Review discusses these aspects in view of contemporary solar cell manufacturing, offers insights into the possible pathways toward commercial perovskite/silicon tandem photovoltaics, and highlights research opportunities to realize this goal.

Enhanced optoelectronic coupling for perovskite/silicon tandem solar cells.

Monolithic perovskite/silicon tandem solar cells are of great appeal as they promise high power conversion efficiencies (PCEs) at affordable cost. In state-of-the-art tandems, the perovskite top cell is electrically coupled to a silicon heterojunction bottom cell by means of a self-assembled monolayer (SAM), anchored on a transparent conductive oxide (TCO), which enables efficient charge transfer between the subcells1-3. Yet reproducible, high-performance tandem solar cells require energetically homogeneous SAM coverage, which remains challenging, especially on textured silicon bottom cells. Here, we resolve this issue by using ultrathin (5-nm) amorphous indium zinc oxide (IZO) as the interconnecting TCO, exploiting its high surface-potential homogeneity resulting from the absence of crystal grains and higher density of SAM anchoring sites when compared with commonly used crystalline TCOs. Combined with optical enhancements through equally thin IZO rear electrodes and improved front contact stacks, an independently certified PCE of 32.5% was obtained, which ranks among the highest for perovskite/silicon tandems. Our ultrathin transparent contact approach reduces indium consumption by approximately 80%, which is of importance to sustainable photovoltaics manufacturing4.

Suppressed phase segregation for triple-junction perovskite solar cells.

The tunable bandgaps and facile fabrication of perovskites make them attractive for multi-junction photovoltaics1,2. However, light-induced phase segregation limits their efficiency and stability3-5: this occurs in wide-bandgap (>1.65 electron volts) iodide/bromide mixed perovskite absorbers, and becomes even more acute in the top cells of triple-junction solar photovoltaics that require a fully 2.0-electron-volt bandgap absorber2,6. Here we report that lattice distortion in iodide/bromide mixed perovskites is correlated with the suppression of phase segregation, generating an increased ion-migration energy barrier arising from the decreased average interatomic distance between the A-site cation and iodide. Using an approximately 2.0-electron-volt rubidium/caesium mixed-cation inorganic perovskite with large lattice distortion in the top subcell, we fabricated all-perovskite triple-junction solar cells and achieved an efficiency of 24.3 per cent (23.3 per cent certified quasi-steady-state efficiency) with an open-circuit voltage of 3.21 volts. This is, to our knowledge, the first reported certified efficiency for perovskite-based triple-junction solar cells. The triple-junction devices retain 80 per cent of their initial efficiency following 420 hours of operation at the maximum power point.

Regulating surface potential maximizes voltage in all-perovskite tandems.

The open-circuit voltage (VOC) deficit in perovskite solar cells is greater in wide-bandgap (over 1.7 eV) cells than in perovskites of roughly 1.5 eV (refs. 1,2). Quasi-Fermi-level-splitting measurements show VOC-limiting recombination at the electron-transport-layer contact3-5. This, we find, stems from inhomogeneous surface potential and poor perovskite-electron transport layer energetic alignment. Common monoammonium surface treatments fail to address this; as an alternative, we introduce diammonium molecules to modify perovskite surface states and achieve a more uniform spatial distribution of surface potential. Using 1,3-propane diammonium, quasi-Fermi-level splitting increases by 90 meV, enabling 1.79 eV perovskite solar cells with a certified 1.33 V VOC and over 19% power conversion efficiency (PCE). Incorporating this layer into a monolithic all-perovskite tandem, we report a record VOC of 2.19 V (89% of the detailed balance VOC limit) and over 27% PCE (26.3% certified quasi-steady state). These tandems retained more than 86% of their initial PCE after 500 h of operation.

Life on the Urbach Edge.

The Urbach energy is an expression of the static and dynamic disorder in a semiconductor and is directly accessible via optical characterization techniques. The strength of this metric is that it elegantly captures the optoelectronic performance potential of a semiconductor in a single number. For solar cells, the Urbach energy is found to be predictive of a material's minimal open-circuit-voltage deficit. Performance calculations considering the Urbach energy give more realistic power conversion efficiency limits than from classical Shockley-Queisser considerations. The Urbach energy is often also found to correlate well with the Stokes shift and (inversely) with the carrier mobility of a semiconductor. Here, we discuss key features, underlying physics, measurement techniques, and implications for device fabrication, underlining the utility of this metric.

Efficient and stable perovskite-silicon tandem solar cells through contact displacement by MgFx.

The performance of perovskite solar cells with inverted polarity (p-i-n) is still limited by recombination at their electron extraction interface, which also lowers the power conversion efficiency (PCE) of p-i-n perovskite-silicon tandem solar cells. A MgFx interlayer with thickness of ~1 nanometer at the perovskite/C60 interface favorably adjusts the surface energy of the perovskite layer through thermal evaporation, which facilitates efficient electron extraction and displaces C60 from the perovskite surface to mitigate nonradiative recombination. These effects enable a champion open-circuit voltage of 1.92 volts, an improved fill factor of 80.7%, and an independently certified stabilized PCE of 29.3% for a monolithic perovskite-silicon tandem solar cell ~1 square centimeter in area. The tandem retained ~95% of its initial performance after damp-heat testing (85°C at 85% relative humidity) for >1000 hours.

Damp heat-stable perovskite solar cells with tailored-dimensionality 2D/3D heterojunctions.

If perovskite solar cells (PSCs) with high power conversion efficiencies (PCEs) are to be commercialized, they must achieve long-term stability, which is usually assessed with accelerated degradation tests. One of the persistent obstacles for PSCs has been successfully passing the damp-heat test (85°C and 85% relative humidity), which is the standard for verifying the stability of commercial photovoltaic (PV) modules. We fabricated damp heat-stable PSCs by tailoring the dimensional fragments of two-dimensional perovskite layers formed at room temperature with oleylammonium iodide molecules; these layers passivate the perovskite surface at the electron-selective contact. The resulting inverted PSCs deliver a 24.3% PCE and retain >95% of their initial value after >1000 hours at damp-heat test conditions, thereby meeting one of the critical industrial stability standards for PV modules.

Tin Oxide Electron-Selective Layers for Efficient, Stable, and Scalable Perovskite Solar Cells.

Perovskite solar cells (PSCs) have become a promising photovoltaic (PV) technology, where the evolution of the electron-selective layers (ESLs), an integral part of any PV device, has played a distinctive role to their progress. To date, the mesoporous titanium dioxide (TiO2 )/compact TiO2 stack has been among the most used ESLs in state-of-the-art PSCs. However, this material requires high-temperature sintering and may induce hysteresis under operational conditions, raising concerns about its use toward commercialization. Recently, tin oxide (SnO2 ) has emerged as an attractive alternative ESL, thanks to its wide bandgap, high optical transmission, high carrier mobility, suitable band alignment with perovskites, and decent chemical stability. Additionally, its low-temperature processability enables compatibility with temperature-sensitive substrates, and thus flexible devices and tandem solar cells. Here, the notable developments of SnO2 as a perovskite-relevant ESL are reviewed with emphasis placed on the various fabrication methods and interfacial passivation routes toward champion solar cells with high stability. Further, a techno-economic analysis of SnO2 materials for large-scale deployment, together with a processing-toxicology assessment, is presented. Finally, a perspective on how SnO2 materials can be instrumental in successful large-scale module and perovskite-based tandem solar cell manufacturing is provided.

Carrier Extraction from Perovskite to Polymeric Charge Transport Layers Probed by Ultrafast Transient Absorption Spectroscopy.

The efficiency of state-of-the-art perovskite solar cells is limited by carrier recombination at defects and interfaces. Thus, understanding these losses and how to reduce them is the way forward toward the Shockley-Queisser limit. Here, we demonstrate that ultrafast transient absorption spectroscopy can directly probe hole extraction and recombination dynamics at perovskite/hole transport layer (HTL) interfaces. To illustrate this, we employed PDPP-3T as HTL because its ground-state absorption is at lower energy than the perovskite's photobleach, enabling direct monitoring of interfacial hole extraction and recombination. Moreover, by fitting the carrier dynamics using a diffusion model, we determined the carrier mobility. Afterwards, by varying the perovskite thickness, we distinguished between carrier diffusion and carrier extraction at the interface. Lastly, we prepared device-like structures, TiO2/perovskite/PDPP-3T stacks, and observed reduced carrier recombination in the perovskite. From PDPP-3T carrier dynamics, we deduced that hole extraction is one order faster than recombination of holes at the interface.