There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Long, balanced electron and hole diffusion lengths greater than 100 nanometers in the polycrystalline organolead trihalide compound CH3NH3PbI3 are critical for highly efficient perovskite solar cells. We found that the diffusion lengths in CH3NH3PbI3 single crystals grown by a solution-growth method can exceed 175 micrometers under 1 sun (100 mW cm(-2)) illumination and exceed 3 millimeters under weak light for both electrons and holes. The internal quantum efficiencies approach 100% in 3-millimeter-thick single-crystal perovskite solar cells under weak light. These long diffusion lengths result from greater carrier mobility, longer lifetime, and much smaller trap densities in the single crystals than in polycrystalline thin films. The long carrier diffusion lengths enabled the use of CH3NH3PbI3 in radiation sensing and energy harvesting through the gammavoltaic effect, with an efficiency of 3.9% measured with an intense cesium-137 source.

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Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals

All of the cations currently used in perovskite solar cells abide by the tolerance factor for incorporation into the lattice. We show that the small and oxidation-stable rubidium cation (Rb + ) can be embedded into a “cation cascade” to create perovskite materials with excellent material properties. We achieved stabilized efficiencies of up to 21.6% (average value, 20.2%) on small areas (and a stabilized 19.0% on a cell 0.5 square centimeters in area) as well as an electroluminescence of 3.8%. The open-circuit voltage of 1.24 volts at a band gap of 1.63 electron volts leads to a loss in potential of 0.39 volts, versus 0.4 volts for commercial silicon cells. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking.

https://science.sciencemag.org/content/sci/early/2016/09/28/science.aah5557.full.pdf

Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance

State-of-the-art photovoltaics use high-purity, large-area, wafer-scale single-crystalline semiconductors grown by sophisticated, high-temperature crystal growth processes. We demonstrate a solution-based hot-casting technique to grow continuous, pinhole-free thin films of organometallic perovskites with millimeter-scale crystalline grains. We fabricated planar solar cells with efficiencies approaching 18%, with little cell-to-cell variability. The devices show hysteresis-free photovoltaic response, which had been a fundamental bottleneck for the stable operation of perovskite devices. Characterization and modeling attribute the improved performance to reduced bulk defects and improved charge carrier mobility in large-grain devices. We anticipate that this technique will lead the field toward synthesis of wafer-scale crystalline perovskites, necessary for the fabrication of high-efficiency solar cells, and will be applicable to several other material systems plagued by polydispersity, defects, and grain boundary recombination in solution-processed thin films.

/pdf/high-efficiency-solution-processed-perovskite-solar-cells-jmd7w75mq9.pdf

High-efficiency solution-processed perovskite solar cells with millimeter-scale grains

Three-dimensional organic-inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties, which have led to power conversion efficiencies exceeding 20 per cent, with the prospect of further improvements towards the Shockley-Queisser limit for a single‐junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions. In contrast to their three-dimensional counterparts, Ruddlesden-Popper phases--layered two-dimensional perovskite films--have shown promising stability, but poor efficiency at only 4.73 per cent. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting inorganic slabs. Here we overcome this issue in layered perovskites by producing thin films of near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. We report a photovoltaic efficiency of 12.52 per cent with no hysteresis, and the devices exhibit greatly improved stability in comparison to their three-dimensional counterparts when subjected to light, humidity and heat stress tests. Unencapsulated two-dimensional perovskite devices retain over 60 per cent of their efficiency for over 2,250 hours under constant, standard (AM1.5G) illumination, and exhibit greater tolerance to 65 per cent relative humidity than do three-dimensional equivalents. When the devices are encapsulated, the layered devices do not show any degradation under constant AM1.5G illumination or humidity. We anticipate that these results will lead to the growth of single-crystalline, solution-processed, layered, hybrid, perovskite thin films, which are essential for high-performance opto-electronic devices with technologically relevant long-term stability.

High-efficiency two-dimensional Ruddlesden–Popper perovskite solar cells

Efficient Perovskite Photovoltaic-Thermoelectric Hybrid Device

Tailoring the Dimensionality of Hybrid Perovskites in Mesoporous Carbon Electrodes for Type-II Band Alignment and Enhanced Performance of Printable Hole-Conductor-Free Perovskite Solar Cells

The Lewis acid SnCl4 is employed as a p-type dopant for 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) for the solution process in solid-state dye sensitized solar cell. The UV–vis absorption spectra and time-resolved photoluminescence (PL) spectra are used to investigate the doping level of spiro-OMeTAD with a p-type dopant, indicating the strong molecular acceptor of SnCl4. X-ray photoelectron spectra (XPS) exhibiting close energy shifts of the Fermi level toward HOMO are observed when adding Li salt or SnCl4. A significant enhancement in fill factor of the photovoltaic devices, corresponding to the power conversion efficiency, is observed when doping with SnCl4. This is attributed to the low charge transport resistance of the hole transport film and high hole injection efficiency from the hole transport material to the counter electrode.

Improvement in Solid-State Dye Sensitized Solar Cells by p-Type Doping with Lewis Acid SnCl4

Efficient triple-mesoscopic perovskite solar mini-modules fabricated with slot-die coating

A novel approach for enhancing the performance of dye-sensitized solar cells is presented. It is based on the analysis of five sensitizers by utilizing triarylamine as donor, thiophene benzothiadiazole as chromophore and substituted thienyl linked with cyanoacrylic acid as the anchoring group (LI-80-LI-84). Accompanied with the increasing steric hindrance of the substituents on the thienyl isolation group, the conformation of the dyes, in particular the angle between the chromophore and the anchoring group, becomes more and more twisted. Surprisingly, sensitizers with poorer conjugation effects (the higher twisted conformation) achieve better photovoltaic performances, showing a contrary trend to the traditional donor-(π-spacer)-acceptor dyes with a better co-planarity. On the basis of the preceding fundamental comprehensions, an empirical method is successfully applied to a new phenyl-based system (LI-85 and LI-86) to improve their performances. The systematical investigation indicates that the twisted structures can contribute to the ECB of the TiO2 film, electron lifetime and resistance at the TiO2/dye/electrolyte interface. Thereby, the efficiency of the initial LI-80-based cell has been dramatically improved to 2.45 times higher for LI-86-based cell, paving a new way for the design of better sensitizers with higher device performances.

Similar or Totally Different: the Adjustment of the Twist Conformation Through Minor Structural Modification, and Dramatically Improved Performance for Dye‐Sensitized Solar Cell

Anyi Mei

Papers

Efficient Perovskite Photovoltaic-Thermoelectric Hybrid Device

Tailoring the Dimensionality of Hybrid Perovskites in Mesoporous Carbon Electrodes for Type-II Band Alignment and Enhanced Performance of Printable Hole-Conductor-Free Perovskite Solar Cells

Improvement in Solid-State Dye Sensitized Solar Cells by p-Type Doping with Lewis Acid SnCl4

Efficient triple-mesoscopic perovskite solar mini-modules fabricated with slot-die coating

Similar or Totally Different: the Adjustment of the Twist Conformation Through Minor Structural Modification, and Dramatically Improved Performance for Dye‐Sensitized Solar Cell