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

Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

/pdf/dye-sensitized-solar-cells-50b0w2bhsr.pdf

Dye-Sensitized Solar Cells

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.

/pdf/electron-hole-diffusion-lengths-175-mm-in-solution-grown-1xynkkb87f.pdf

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

We fabricated a perovskite solar cell that uses a double layer of mesoporous TiO2 and ZrO2 as a scaffold infiltrated with perovskite and does not require a hole-conducting layer. The perovskite was produced by drop-casting a solution of PbI2, methylammonium (MA) iodide, and 5-ammoniumvaleric acid (5-AVA) iodide through a porous carbon film. The 5-AVA templating created mixed-cation perovskite (5-AVA)x(MA)1- xPbI3 crystals with lower defect concentration and better pore filling as well as more complete contact with the TiO2 scaffold, resulting in a longer exciton lifetime and a higher quantum yield for photoinduced charge separation as compared to MAPbI3. The cell achieved a certified power conversion efficiency of 12.8% and was stable for >1000 hours in ambient air under full sunlight.

https://science.sciencemag.org/content/sci/345/6194/295.full.pdf

A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability

Formamidinium lead trihalide perovskite (FAPbI3) was successfully introduced into hole-conductor-free, fully printable mesoscopic perovskite solar cells with a carbon counter electrode. With the sequential deposition method, a FAPbI3 based solar cell yielded an efficiency of 11.9%, superior to the methylammonium lead trihalide perovskite (MAPbI3) solar cell efficiency of 11.4%, which is due to broadening of the light to 840 nm. By optimizing the mixing ratio of the formamidimium and methylammonium cations to 3 : 2, a power conversion efficiency of 12.9% was achieved with this low-cost, fully printable mesoscopic solar cell, which indicated a promising prospect for low-cost photovoltaic technology.

Efficient hole-conductor-free, fully printable mesoscopic perovskite solar cells with a broad light harvester NH2CHNH2PbI3

Mesoporous graphite/carbon black counter electrodes (CEs) using flaky graphite with different sizes were applied in hole-conductor-free mesoscopic perovskite solar cells by a screen-printing technique. Conductivity measurements, current–voltage characteristics, and impedance spectroscopy measurements were carried out to study the influence of CEs on the photovoltaic performance of devices. The results indicated that graphite, which acted as the conductor in carbon counter electrodes (CCEs), could significantly affect the square resistance of CCEs, thus resulting in differences in fill factor and power conversion efficiency (PCE) of the devices. Based on the optimized CCEs with a thickness of 9 μm, PCEs exceeding 11% could be achieved for the fully printable hole-conductor-free mesoscopic perovskite solar cells due to the low square resistance and large pore size of graphite based CCEs. The abundant availability, low cost and excellent properties of such carbon material based CEs offer a wide prospect for their further applications in perovskite solar cells.

The effect of carbon counter electrodes on fully printable mesoscopic perovskite solar cells

The size effect of the TiO2 photoanode has been investigated on the hole-conductor-free fully printable mesoscopic perovskite solar cells based on the carbon counter electrode and (5-AVA)x(MA)1−xPbI3 perovskite. With TiO2 nanoparticles with an optimized diameter of 25 nm, a champion device exhibits an efficiency of 13.41%.

Min Hu

Papers

A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability

Efficient hole-conductor-free, fully printable mesoscopic perovskite solar cells with a broad light harvester NH2CHNH2PbI3

The effect of carbon counter electrodes on fully printable mesoscopic perovskite solar cells

The size effect of TiO2 nanoparticles on a printable mesoscopic perovskite solar cell

Critical parameters in TiO 2 /ZrO 2 /Carbon-based mesoscopic perovskite solar cell