Ievgen Levchuk

A generic interface to reduce the efficiency-stability-cost gap of perovskite solar cells

Minimizing losses at interfaces Among the issues facing the practical use of hybrid organohalide lead perovskite solar cells is the loss of charge carriers at interfaces. Hou et al. show that tantalum-doped tungsten oxide forms almost ohmic contacts with inexpensive conjugated polymer multilayers to create a hole-transporting material with a small interface barrier. This approach eliminates the use of ionic dopants that compromise device stability. Solar cells made with these contacts achieved maximum efficiencies of 21.2% and operated stably for more than 1000 hours. Science, this issue p. 1192 Tantalum-doped tungsten oxide forms nearly ohmic contacts with conjugated polymers to create efficient hole transporters. A major bottleneck delaying the further commercialization of thin-film solar cells based on hybrid organohalide lead perovskites is interface loss in state-of-the-art devices. We present a generic interface architecture that combines solution-processed, reliable, and cost-efficient hole-transporting materials without compromising efficiency, stability, or scalability of perovskite solar cells. Tantalum-doped tungsten oxide (Ta-WOx)/conjugated polymer multilayers offer a surprisingly small interface barrier and form quasi-ohmic contacts universally with various scalable conjugated polymers. In a simple device with regular planar architecture and a self-assembled monolayer, Ta-WOx–doped interface–based perovskite solar cells achieve maximum efficiencies of 21.2% and offer more than 1000 hours of light stability. By eliminating additional ionic dopants, these findings open up the entire class of organics as scalable hole-transporting materials for perovskite solar cells.

Brightly Luminescent and Color-Tunable Formamidinium Lead Halide Perovskite FAPbX3 (X = Cl, Br, I) Colloidal Nanocrystals

In the past few years, hybrid organic-inorganic and all-inorganic metal halide perovskite nanocrystals have become one of the most interesting materials for optoelectronic applications. Here, we report a facile and rapid room temperature synthesis of 15-25 nm formamidinium CH(NH2)2PbX3 (X = Cl, Br, I, or mixed Cl/Br and Br/I) colloidal nanocrystals by ligand-assisted reprecipitation (LARP). The cubic and platelet-like nanocrystals with their emission in the range of 415-740 nm, full width at half-maximum (fwhm) of 20-44 nm, and radiative lifetimes of 5-166 ns enable band gap tuning by halide composition as well as by their thickness tailoring; they have a high photoluminescence quantum yield (up to 85%), colloidal and thermodynamic stability. Combined with surface modification that prevents degradation by water, this nanocrystalline material is an ideal candidate for optoelectronic devices and applications. In addition, optoelectronic measurements verify that the photodetector based on FAPbI3 nanocrystals paves the way for perovskite quantum dot photovoltaics.

Pushing efficiency limits for semitransparent perovskite solar cells

While perovskite-based semitransparent solar cells deliver competitive levels of transparency and efficiency to be envisioned for urban infrastructures, the complexity and sensitivity of their processing conditions remain challenging. Here, we introduce two robust protocols for the processing of sub-100 nm perovskite films, allowing fine-tuning of the active layer without compromising the crystallinity and quality of the semiconductor. Specifically, we demonstrate that a method based on solvent-induced crystallization with a rapid drying step affords perovskite solar cells with 37% average visible transmittance (AVT) and 7.8% PCE. This process enhances crystallization with a preferential phase orientation presumably at the interface, yielding a high fill factor of 72.3%. The second method is based on a solvent–solvent extraction protocol, enabling active layer films as thin as 40 nm and featuring room-temperature crystallization in an ambient environment on a few second time span. As a result, we demonstrate a maximum AVT of 46% with an efficiency of 3.6%, which is the highest combination of efficiency and transparency for a full device stack to date. By combining the two methods presented here we cover a broad range of thicknesses vs. transparency values and confirm that solvent-induced crystallization represents a powerful processing strategy toward high-efficiency semitransparent solar cells. Optical simulations support our experimental findings and provide a global perspective of the opportunities and limitations of semitransparent perovskite photovoltaic devices.

Coloring Semitransparent Perovskite Solar Cells via Dielectric Mirrors

While perovskite-based semitransparent solar cells for window applications show competitive levels of transparency and efficiency compared to organic photovoltaics, the color perception of the perovskite films is highly restricted because band gap engineering results in losses in power conversion efficiencies. To overcome the limitation in visual aesthetics, we combined semitransparent perovskite solar cells with dielectric mirrors. This approach enables one to tailor the device appearance to almost any desired color and simultaneously offers additional light harvesting for the solar cell. In the present work, opto-electrical effects are investigated through quantum efficiency and UV-to-visible spectroscopic measurements. Likewise, a detailed chromaticity analysis, featuring the transmissive and reflective color perception of the device including the mirror, from both sides and in different illumination conditions, is presented and analyzed. Photocurrent density enhancement of up to 21% along with overall device transparency values of up to 31% (4.2% efficiency) is demonstrated for cells showing a colored aesthetic appeal. Finally, a series of simulations emulating the device chromaticity, transparency, and increased photocurrent density as a function of the photoactive layer thickness and the design wavelength of the dielectric mirror are presented. Our simulations and their experimental validation enabled us to establish the design rules that consider the color efficiency/transparency interplay for real applications.

Extending the environmental lifetime of unpackaged perovskite solar cells through interfacial design

Solution-processed oxo-functionalized graphene (oxo-G1) is employed to substitute hydrophilic PEDOT:PSS as an anode interfacial layer for perovskite solar cells. The resulting devices exhibit a reasonably high power conversion efficiency (PCE) of 15.2% in the planar inverted architecture with oxo-G1 as a hole transporting material (HTM), and most importantly, deploy the full open-circuit voltage (Voc) of up to 1.1 V. Moreover, oxo-G1 effectively slows down the ingress of water vapor into the device stack resulting in significantly enhanced environmental stability of unpackaged cells under illumination with 80% of the initial PCE being reached after 500 h. Without encapsulation, ∼60% of the initial PCE is retained after ∼1000 h of light soaking under 0.5 sun and ambient conditions maintaining the temperature beneath 30 °C. Moreover, the unsealed perovskite device retains 92% of its initial PCE after about 1900 h under ambient conditions and in the dark. Our results underpin that controlling water diffusion into perovskite cells through advanced interface engineering is a crucial step towards prolonged environmental stability.

Photoinduced degradation of methylammonium lead triiodide perovskite semiconductors

Photoinduced degradation is a critical obstacle for the real application of novel semiconductors for photovoltaic applications. In this paper, the photoinduced degradation of CH3NH3PbI3 in a vacuum and air (relative humidity 40%) is analyzed by ex situ and advanced in situ technologies. Without light illumination, CH3NH3PbI3 films slowly degrade under vacuum and air within 24 hours. However, we find that CH3NH3PbI3 converts to metallic lead (Pb0) when exposed to vacuum and light illumination. Further, a series of lead salts (e.g. PbO, Pb(OH)2 and PbCO3) are formed when CH3NH3PbI3 is degraded under environmental conditions, i.e. under the combination of light, oxygen and moisture. Photoinduced degradation is found to be determined by the environmental atmosphere as CH3NH3PbI3 films remain very stable under nitrogen conditions. The results from vacuum conditions underpin that the high volatility of the organic component (CH3NH3I) is in conflict with reaching excellent intrinsic stability due to its role in creating ion vacancies. The degradation in air suggests that both oxygen and water contribute to the fast photodecomposition of CH3NH3PbI3 into lead salts rather than water alone. Given these basic yet fundamental understandings, the design of hydrophobic capping layers becomes one prerequisite towards long-term stable perovskite-based devices.

Ligand-assisted thickness tailoring of highly luminescent colloidal CH3NH3PbX3 (X = Br and I) perovskite nanoplatelets

Quantum size-confined CH3NH3PbX3 (X = Br and I) perovskite nanoplatelets with remarkably high photoluminescence quantum yield (up to 90%) were synthesized by ligand-assisted re-precipitation. Thickness-tunability was realized by varying the oleylamine and oleic acid ligand ratio. This method allows tailoring the nanoplatelet thickness by adjusting the number of unit cell monolayers. Broadly tunable emission wavelengths (450-730 nm) are achieved via the pronounced quantum size effect without anion-halide mixing.

Industrially scalable and cost-effective Mn2+ doped ZnxCd1−xS/ZnS nanocrystals with 70% photoluminescence quantum yield, as efficient down-shifting materials in photovoltaics

We present colloidally stable and highly luminescent ZnxCd1−xS:Mn/ZnS core–shell nanocrystals (NCs) synthesized via a simple non-injection one-pot, two-step synthetic route, which can be easily upscaled. A systematic variation of the reaction component, parameters and thickness of the ZnS shell yielded doped nanocrystals with a very high photoluminescence quantum yield (Φpl) of 70%, which is the highest value yet reported for these Mn-doped sulfide-semiconductor NCs. These materials can be synthesized with high reproducibility in large quantities of the same high quality, i.e., the same Φpl using accordingly optimized reaction conditions. The application of these zero-reabsorption high quality NCs in the light conversion layers, deposited on top of a commercial monocrystalline silicon (mono-Si) solar cell, led to a significant enhancement of the external quantum efficiency (EQE) of this device in the ultraviolet spectral region between 300 and 400 nm up to ca. 12%. EQE enhancement is reflected by an increase in the power conversion efficiency (PCE) by nearly 0.5 percentage points and approached the theoretical limit (0.6%) expected from down-shifting for this Si solar cell. The resulting PCE may result in a BoM (bill of materials) cost reduction of app. 3% for mono-Si photovoltaic modules. Such small but distinct improvements are expected to pave the road for an industrial application of doped semiconductor NCs as cost-effective light converters for silicon photovoltaic (PV) and other optoelectronic applications.

Epitaxial growth of single crystalline film phosphors based on the Ce3+-doped Ca2YMgScSi3O12 garnet

In this work, we present for the first time the results on crystallization and investigation of the luminescence properties of new prospective phosphors based on single crystalline films (SCFs) of Ce3+-doped Ca2YMgScSi3O12 silicate garnet, grown by the liquid phase epitaxy (LPE) method onto Y3Al5O12 (YAG) substrates. The luminescence properties of Ca2YMgScSi3O12:Ce SCFs were compared with the properties of the reference YAG:Ce SCF sample. The influence of thermal annealing at 1000–1300 °C in a N2/H2 reducing atmosphere on the optical properties of Ca2YMgScSi3O12 SCFs was investigated as well. The results of this research can be suitable for the development of a new generation of white LED converters and scintillators based on the epitaxial structures of Ca2+–Si4+ containing garnets, grown by the LPE method onto undoped or doped substrates of garnet compounds.

Structural fluctuations cause spin-split states in tetragonal (CH3NH3)PbI3 as evidenced by the circular photogalvanic effect

Significance Lead halide perovskites are successfully used in thin-film solar cells, with efficiencies on the laboratory scale exceeding 22%. The electronic structure underlying their exceptional phototransport properties is complex because of the organic–inorganic character of the materials, their mechanical softness, and the strong spin–orbit coupling induced by the constituting heavy elements. Calculations predict that a dynamical Rashba effect could enhance the lifetimes and diffusion lengths of photocarriers in perovskite solar cells. The dynamical Rashba effect is characterized by spin splittings in the band structure at elevated temperatures, induced by local structural disorder. The mechanism should be general to structurally flexible materials composed of heavy elements, making those potentially attractive for both optoelectronics and spintronics. Lead halide perovskites are used in thin-film solar cells, which owe their high efficiency to the long lifetimes of photocarriers. Various calculations find that a dynamical Rashba effect could significantly contribute to these long lifetimes. This effect is predicted to cause a spin splitting of the electronic bands of inversion-symmetric crystalline materials at finite temperatures, resulting in a slightly indirect band gap. Direct experimental evidence of the existence or the strength of the spin splitting is lacking. Here, we resonantly excite photocurrents in single crystalline (CH3NH3)PbI3 with circularly polarized light to clarify the existence of spin splittings in the band structure. We observe a circular photogalvanic effect, i.e., the photocurrent depends on the light helicity, in both orthorhombic and tetragonal (CH3NH3)PbI3. At room temperature, the effect peaks for excitation photon energies ΔE=110 meV below the direct optical band gap. Temperature-dependent measurements reveal a sign change of the effect at the orthorhombic–tetragonal phase transition, indicating different microscopic origins in the two phases. Within the tetragonal phase, both ΔE and the amplitude of the circular photogalvanic effect increase with temperature. Our findings support a dynamical Rashba effect in this phase, i.e., a spin splitting caused by thermally induced structural fluctuations which break inversion symmetry.Long carrier lifetimes and diffusion lengths form the basis for the successful application of the organic-inorganic perovskite (CH3NH3)PbI3 in solar cells and lasers. The mechanism behind the long carrier lifetimes is still not completely understood. Spin-split bands and a resulting indirect band gap have been proposed by theory. Using near band-gap left-handed and righthanded circularly polarized light we induce photocurrents of opposite directions in a single-crystal (CH3NH3)PbI3 device at low temperature (4 K). The phenomenom is known as the circular photogalvanic effect and gives direct evidence for phototransport in spin-split bands. Simultaneous photoluminecence measurements show that the onset of the photocurrent is below the optical band gap. The results prove that an indirect band gap exists in (CH3NH3)PbI3 with broken inversion symmetry as a result of spin-splittings in the band structure. This information is essential for understanding the photophysical properties of organic-inorganic perovskites and finding lead-free alternatives. Furthermore, the optically driven spin currents in (CH3NH3)PbI3 make it a candidate material for spintronics applications.