Fengjia Fan

Efficient and stable solution-processed planar perovskite solar cells via contact passivation

Passivating traps in perovskites Low-temperature processing of planar organic-inorganic perovskite solar cells made through solution processing would allow for simpler manufacturing and the use of flexible substrates. However, materials currently in use form interfaces with charge carrier trap states that limit performance. Tan et al. used chlorine-capped TiO2 colloidal nanocrystal films as an electron-selective layer, which limited interface recombination in solution-processed solar cells. Such cells achieved certified efficiencies of 19.5% for active areas of 1.1 cm2. Science, this issue p. 722 Chlorine-capped TiO2 nanocrystal films processed below 150°C effectively passivate detrimental carrier trap states. Planar perovskite solar cells (PSCs) made entirely via solution processing at low temperatures (<150°C) offer promise for simple manufacturing, compatibility with flexible substrates, and perovskite-based tandem devices. However, these PSCs require an electron-selective layer that performs well with similar processing. We report a contact-passivation strategy using chlorine-capped TiO2 colloidal nanocrystal film that mitigates interfacial recombination and improves interface binding in low-temperature planar solar cells. We fabricated solar cells with certified efficiencies of 20.1 and 19.5% for active areas of 0.049 and 1.1 square centimeters, respectively, achieved via low-temperature solution processing. Solar cells with efficiency greater than 20% retained 90% (97% after dark recovery) of their initial performance after 500 hours of continuous room-temperature operation at their maximum power point under 1-sun illumination (where 1 sun is defined as the standard illumination at AM1.5, or 1 kilowatt/square meter).

Homogeneously dispersed, multimetal oxygen-evolving catalysts

Modulating metal oxides The more difficult step in fuel cells and water electrolysis is the oxygen evolution reaction. The search for earth-abundant materials to replace noble metals for this reaction often turns to oxides of three-dimensional metals such as iron. Zhang et al. show that the applied voltages needed to drive this reaction are reduced for iron-cobalt oxides by the addition of tungsten. The addition of tungsten favorably modulates the electronic structure of the oxyhydroxide. A key development is to keep the metals well mixed and avoid the formation of separate phases. Science, this issue p. 333 The addition of tungsten to iron cobalt oxides lowers the overpotential required for the evolution of oxygen from water. Earth-abundant first-row (3d) transition metal–based catalysts have been developed for the oxygen-evolution reaction (OER); however, they operate at overpotentials substantially above thermodynamic requirements. Density functional theory suggested that non-3d high-valency metals such as tungsten can modulate 3d metal oxides, providing near-optimal adsorption energies for OER intermediates. We developed a room-temperature synthesis to produce gelled oxyhydroxides materials with an atomically homogeneous metal distribution. These gelled FeCoW oxyhydroxides exhibit the lowest overpotential (191 millivolts) reported at 10 milliamperes per square centimeter in alkaline electrolyte. The catalyst shows no evidence of degradation after more than 500 hours of operation. X-ray absorption and computational studies reveal a synergistic interplay between tungsten, iron, and cobalt in producing a favorable local coordination environment and electronic structure that enhance the energetics for OER.

Enhanced electrocatalytic CO2 reduction via field-induced reagent concentration

Electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO) is the first step in the synthesis of more complex carbon-based fuels and feedstocks using renewable electricity. Unfortunately, the reaction suffers from slow kinetics owing to the low local concentration of CO2 surrounding typical CO2 reduction reaction catalysts. Alkali metal cations are known to overcome this limitation through non-covalent interactions with adsorbed reagent species, but the effect is restricted by the solubility of relevant salts. Large applied electrode potentials can also enhance CO2 adsorption, but this comes at the cost of increased hydrogen (H2) evolution. Here we report that nanostructured electrodes produce, at low applied overpotentials, local high electric fields that concentrate electrolyte cations, which in turn leads to a high local concentration of CO2 close to the active CO2 reduction reaction surface. Simulations reveal tenfold higher electric fields associated with metallic nanometre-sized tips compared to quasi-planar electrode regions, and measurements using gold nanoneedles confirm a field-induced reagent concentration that enables the CO2 reduction reaction to proceed with a geometric current density for CO of 22 milliamperes per square centimetre at −0.35 volts (overpotential of 0.24 volts). This performance surpasses by an order of magnitude the performance of the best gold nanorods, nanoparticles and oxide-derived noble metal catalysts. Similarly designed palladium nanoneedle electrocatalysts produce formate with a Faradaic efficiency of more than 90 per cent and an unprecedented geometric current density for formate of 10 milliamperes per square centimetre at −0.2 volts, demonstrating the wider applicability of the field-induced reagent concentration concept.

Passivation Using Molecular Halides Increases Quantum Dot Solar Cell Performance.

A solution-based passivation scheme is developed featuring the use of molecular iodine and PbS colloidal quantum dots (CQDs). The improved passivation translates into a longer carrier diffusion length in the solid film. This allows thicker solar-cell devices to be built while preserving efficient charge collection, leading to a certified power conversion efficiency of 9.9%, which is a new record in CQD solar cells.

QUANTUM-DOT-IN-PEROVSKITE SOLIDS

Heteroepitaxy—atomically aligned growth of a crystalline film atop a different crystalline substrate—is the basis of electrically driven lasers, multijunction solar cells, and blue-light-emitting diodes. Crystalline coherence is preserved even when atomic identity is modulated, a fact that is the critical enabler of quantum wells, wires, and dots. The interfacial quality achieved as a result of heteroepitaxial growth allows new combinations of materials with complementary properties, which enables the design and realization of functionalities that are not available in the single-phase constituents. Here we show that organohalide perovskites and preformed colloidal quantum dots, combined in the solution phase, produce epitaxially aligned ‘dots-in-a-matrix’ crystals. Using transmission electron microscopy and electron diffraction, we reveal heterocrystals as large as about 60 nanometres and containing at least 20 mutually aligned dots that inherit the crystalline orientation of the perovskite matrix. The heterocrystals exhibit remarkable optoelectronic properties that are traceable to their atom-scale crystalline coherence: photoelectrons and holes generated in the larger-bandgap perovskites are transferred with 80% efficiency to become excitons in the quantum dot nanocrystals, which exploit the excellent photocarrier diffusion of perovskites to produce bright-light emission from infrared-bandgap quantum-tuned materials. By combining the electrical transport properties of the perovskite matrix with the high radiative efficiency of the quantum dots, we engineer a new platform to advance solution-processed infrared optoelectronics.

Energetic I–III–VI2 and I2–II–IV–VI4 nanocrystals: synthesis, photovoltaic and thermoelectric applications

Due to the promising applications in low-cost and high performance photovoltaic and thermoelectric devices, there has been a booming development of syntheses of colloidal I–III–VI2 and I2–II–IV–VI4 (I = Cu, Ag; II = Zn, Cd; III = Al, Ga, In; IV = Ge, Sn; VI = S, Se, Te) nanocrystals during the past ten years. In this review, we aim to summarize comprehensively recent developments of I–III–VI2, I2–II–IV–VI4 nanocrystals, where we focus more on synthetic procedures towards controlled phases and bandgaps. Combining with already achieved theoretical understanding, we try to find some common features of different synthetic routes, which might be extended to synthesize other I–III–VI2 and I2–II–IV–VI4 nanocrystals with intentioned phases and bandgaps. We also highlight some of the latest advances in applications of these nanocrystals in photovoltaic and thermoelectric devices. Finally, we point out some problems and challenges existing in the fields based on our understanding, e.g., bandgap evaluation, precise cation occupation determination and relatively high cost generated during the synthetic procedures of nanocrystals.

10.6% Certified Colloidal Quantum Dot Solar Cells via Solvent-Polarity-Engineered Halide Passivation

Colloidal quantum dot (CQD) solar cells are solution-processed photovoltaics with broad spectral absorption tunability. Major advances in their efficiency have been made via improved CQD surface passivation and device architectures with enhanced charge carrier collection. Herein, we demonstrate a new strategy to improve further the passivation of CQDs starting from the solution phase. A cosolvent system is employed to tune the solvent polarity in order to achieve the solvation of methylammonium iodide (MAI) and the dispersion of hydrophobic PbS CQDs simultaneously in a homogeneous phase, otherwise not achieved in a single solvent. This process enables MAI to access the CQDs to confer improved passivation. This, in turn, allows for efficient charge extraction from a thicker photoactive layer device, leading to a certified solar cell power conversion efficiency of 10.6%, a new certified record in CQD photovoltaics.

Hybrid organic–inorganic inks flatten the energy landscape in colloidal quantum dot solids

Bandtail states in disordered semiconductor materials result in losses in open-circuit voltage (Voc) and inhibit carrier transport in photovoltaics. For colloidal quantum dot (CQD) films that promise low-cost, large-area, air-stable photovoltaics, bandtails are determined by CQD synthetic polydispersity and inhomogeneous aggregation during the ligand-exchange process. Here we introduce a new method for the synthesis of solution-phase ligand-exchanged CQD inks that enable a flat energy landscape and an advantageously high packing density. In the solid state, these materials exhibit a sharper bandtail and reduced energy funnelling compared with the previous best CQD thin films for photovoltaics. Consequently, we demonstrate solar cells with higher Voc and more efficient charge injection into the electron acceptor, allowing the use of a closer-to-optimum bandgap to absorb more light. These enable the fabrication of CQD solar cells made via a solution-phase ligand exchange, with a certified power conversion efficiency of 11.28%. The devices are stable when stored in air, unencapsulated, for over 1,000 h.

High-Efficiency Colloidal Quantum Dot Photovoltaics via Robust Self-Assembled Monolayers

The optoelectronic tunability offered by colloidal quantum dots (CQDs) is attractive for photovoltaic applications but demands proper band alignment at electrodes for efficient charge extraction at minimal cost to voltage. With this goal in mind, self-assembled monolayers (SAMs) can be used to modify interface energy levels locally. However, to be effective SAMs must be made robust to treatment using the various solvents and ligands required for to fabricate high quality CQD solids. We report robust self-assembled monolayers (R-SAMs) that enable us to increase the efficiency of CQD photovoltaics. Only by developing a process for secure anchoring of aromatic SAMs, aided by deposition of the SAMs in a water-free deposition environment, were we able to provide an interface modification that was robust against the ensuing chemical treatments needed in the fabrication of CQD solids. The energy alignment at the rectifying interface was tailored by tuning the R-SAM for optimal alignment relative to the CQD quantum-confined electron energy levels. This resulted in a CQD PV record power conversion efficiency (PCE) of 10.7% with enhanced reproducibility relative to controls.

Colloidal synthesis of Cu2CdSnSe4 nanocrystals and hot-pressing to enhance the thermoelectric figure-of-merit.

We report a solution-based synthesis of monodispersed Cu(2)CdSnSe(4) nanocrystals and a study on the thermoelectric properties of these wide-band-gap dense materials compacted from nanocrystals for the first time. With the help of copper dopants and selenium vacancies generated during wet-chemistry synthesis, a large increment in the power factor is observed, and the dimensionless figure-of-merit ZT reaches a peak value of 0.65 at 450 °C.