Advancing perovskite solar cell technologies toward their theoretical power conversion efficiency (PCE) requires delicate control over the carrier dynamics throughout the entire device. By controlling the formation of the perovskite layer and careful choices of other materials, we suppressed carrier recombination in the absorber, facilitated carrier injection into the carrier transport layers, and maintained good carrier extraction at the electrodes. When measured via reverse bias scan, cell PCE is typically boosted to 16.6% on average, with the highest efficiency of ~19.3% in a planar geometry without antireflective coating. The fabrication of our perovskite solar cells was conducted in air and from solution at low temperatures, which should simplify manufacturing of large-area perovskite devices that are inexpensive and perform at high levels.

http://science.sciencemag.org/content/sci/345/6196/542.full.pdf

Interface engineering of highly efficient perovskite solar cells

The performance of solar cells based on organic–inorganic perovskites strongly depends on the device architecture and processing conditions. It is now shown that solvent engineering enables the deposition of very dense perovskite layers on mesoporous titania, leading to photovoltaic devices with a high light-conversion efficiency and no hysteresis.

Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells

The large photocurrent hysteresis observed in many organometal trihalide perovskite solar cells has become a major hindrance impairing the ultimate performance and stability of these devices, while its origin was unknown. Here we demonstrate the trap states on the surface and grain boundaries of the perovskite materials to be the origin of photocurrent hysteresis and that the fullerene layers deposited on perovskites can effectively passivate these charge trap states and eliminate the notorious photocurrent hysteresis. Fullerenes deposited on the top of the perovskites reduce the trap density by two orders of magnitude and double the power conversion efficiency of CH(3)NH(3)PbI(3) solar cells. The elucidation of the origin of photocurrent hysteresis and its elimination by trap passivation in perovskite solar cells provides important directions for future enhancements to device efficiency.

/pdf/origin-and-elimination-of-photocurrent-hysteresis-by-3c3hhy02s3.pdf

Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells

Hybrid organic/inorganic perovskites (e.g., CH3NH3PbI3) as light absorbers are promising players in the field of third-generation photovoltaics. Here we demonstrate a low-temperature vapor-assisted solution process to construct polycrystalline perovskite thin films with full surface coverage, small surface roughness, and grain size up to microscale. Solar cells based on the as-prepared films achieve high power conversion efficiency of 12.1%, so far the highest efficiency based on CH3NH3PbI3 with the planar heterojunction configuration. This method provides a simple approach to perovskite film preparation and paves the way for high reproducibility of films and devices. The underlying kinetic and thermodynamic parameters regarding the perovskite film growth are discussed as well.

/pdf/planar-heterojunction-perovskite-solar-cells-via-vapor-bj4oej5a2t.pdf

Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process

Solar cells based on organometallic halide perovskite absorber layers are emerging as a high-performance photovoltaic technology. Using highly sensitive photothermal deflection and photocurrent spectroscopy, we measure the absorption spectrum of CH3NH3PbI3 perovskite thin films at room temperature. We find a high absorption coefficient with particularly sharp onset. Below the bandgap, the absorption is exponential over more than four decades with an Urbach energy as small as 15 meV, which suggests a well-ordered microstructure. No deep states are found down to the detection limit of ∼1 cm–1. These results confirm the excellent electronic properties of perovskite thin films, enabling the very high open-circuit voltages reported for perovskite solar cells. Following intentional moisture ingress, we find that the absorption at photon energies below 2.4 eV is strongly reduced, pointing to a compositional change of the material.

Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance

All-solid-state donor/acceptor planar-heterojunction (PHJ) hybrid solar cells are constructed and their excellent performance measured. The deposition of a thin C60 fullerene or fullerene-derivative (acceptor) layer in vacuum on a CH3 NH3 PbI3 perovskite (donor) layer creates a hybrid PHJ that displays the photovoltaic effect. Such heterojunctions are shown to be suitable for the development of newly structured, hybrid, efficient solar cells.

CH3NH3PbI3 Perovskite/Fullerene Planar‐Heterojunction Hybrid Solar Cells

A low temperature (<100 °C), flexible solar cell based on an organic–inorganic hybrid CH3NH3PbI3 perovskite–fullerene planar heterojunction (PHJ) is successfully demonstrated. In this manuscript, we study the effects of energy level offset between a solar absorber (organic–inorganic hybrid CH3NH3PbI3 perovskite) and the selective contact materials on the photovoltaic behaviors of the planar organometallic perovskite–fullerene heterojunction solar cells. We find that the difference between the highest occupied molecular orbital (HOMO) level of CH3NH3PbI3 perovskite and the Fermi level of indium-tin-oxide (ITO) dominates the voltage output of the device. ITO films on glass or on the polyethylene terephthalate (PET) flexible substrate with different work functions are investigated to illustrate this phenomenon. The higher work function of the PET/ITO substrate decreases the energy loss of hole transfer from the HOMO of perovskite to ITO and minimizes the energy redundancy of the photovoltage output. The devices using the high work function ITO substrate as contact material show significant open-circuit voltage enhancement (920 mV), with the power conversion efficiency of 4.54%, and these types of extra-thin planar bilayer heterojunction solar cells have the potential advantages of low-cost and lightweight.

High voltage and efficient bilayer heterojunction solar cells based on an organic-inorganic hybrid perovskite absorber with a low-cost flexible substrate.

Shin Rung Peng

Papers

CH3NH3PbI3 Perovskite/Fullerene Planar‐Heterojunction Hybrid Solar Cells

High voltage and efficient bilayer heterojunction solar cells based on an organic-inorganic hybrid perovskite absorber with a low-cost flexible substrate.