ZiQi Sun

A hole-transport-material-free planar solar cell of cesium lead mixed halide perovskite (CsPbIBr₂) is deposited by dual source thermal evaporation for the first time, achieving an efficiency of 4.7%. The addition of iodine into the bromide lowers the bandgap resulting in wider solar spectrum absorption. Compared to the hybrid halide perovskites, CsPbIBr₂ demonstrates better thermal stability.

Perovskite solar cells fabricated from organometal halide light harvesters have captured significant attention due to their tremendously low device costs as well as unprecedented rapid progress on power conversion efficiency (PCE). A certified PCE of 20.1% was achieved in late 2014 following the first study of long-term stable all-solid-state perovskite solar cell with a PCE of 9.7% in 2012, showing their promising potential towards future cost-effective and high performance solar cells. Here, notable achievements of primary device configuration involving perovskite layer, hole-transporting materials (HTMs) and electron-transporting materials (ETMs) are reviewed. Numerous strategies for enhancing photovoltaic parameters of perovskite solar cells, including morphology and crystallization control of perovskite layer, HTMs design and ETMs modifications are discussed in detail. In addition, perovskite solar cells outside of HTMs and ETMs are mentioned as well, providing guidelines for further simplification of device processing and hence cost reduction.

Device architectures of perovskite solar cells, including perovskite light absorbers, hole-transporting materials (HTMs), and electron-transporting materials (ETMs) are reviewed. Controllable morphology and crystallization of perovskite absorber, organic and inorganic HTMs, nanostructured ETMs and ETMs modification, along with HTM and ETM-free architectures are promising in future device optimization. Recent developments in fabrication of perovskite solar cells are discussed to highlight progress towards achieving high performance devices.

Block-copolymer templated chemical solution deposition is used to prepare mesoporous Nd-doped TiO₂ electrodes for perovskite-based solar cells. X-ray diffraction and photothermal deflection spectroscopy show substitutional incorporation into the TiO₂ crystal lattice for low Nd concentration, and increasing interstitial doping for higher concentrations. Substitutional Nd-doping leads to an increase in stability and performance of perovskite solar cells by eliminating defects and thus increasing electron transport and reducing charge recombination in the mesoporous TiO₂. The optimized doping concentration of 0.3% Nd enables the preparation of perovskite solar cells with stabilized power conversion efficiency of >18%.

Neodymium doping of TiO₂ is shown to be an effective way to increase perovskite solar cell performance. Efficiency is enhanced by the passivation of deep trap states, leading to reduced recombination and increased transport. Stability is simultaneously enhanced by the elimination of oxygen defects.

Highly efficient transparent conductive oxide (TCO)-free perovskite (CH₃NH₃PbI₃) solar cells are demonstrated by using a graphene transparent anode and organic carrier transport materials. By adding a few nanometer-thick MoO₃ layer, wettability and work function of the graphene electrode are enhanced to enable a 17.1% power conversion efficiency, which is so far the highest efficiency for TCO-free solar cells.

PEDOT:PSS doped with PEO (poly­ethylene oxide) is demonstrated to be an efficient hole extraction layer due to its high electrical conductivity and extremely smooth surface. This material is able to boost the efficiency of planar heterojunction perovskite hybrid solar cells.

Beneficial effects are demonstrated by PbI₂ incorporated into perovskite materials as a light absorber in solar cells. The PbI₂ distributed into the perovskite layers leads to reduced hysteresis and ionic migration, and enables the fabrication of remarkably improved solar cells with a certified power conversion efficiency of 19.75% under air-mass 1.5 global (AM 1.5G) illumination of 100 mW cm⁻² intensity.

A novel polymer-solar-cell architecture using the conjugated polymer PFS as both the anode and cathode interlayers is constructed, and a high power conversion efficiency of 9.48% is achieved using the corresponding photovoltaic device.

Conjugated small-molecule hole-transport materials (HTMs) with tunable energy levels are designed and synthesized for efficient perovskite solar cells. A champion device with efficiency of 16.2% is demonstrated using a dopant-free DERDTS-TBDT HTM, while the DORDTS-DFBT-HTM-based device shows an inferior performance of 6.2% due to its low hole mobility and unmatched HOMO level with the valence band of perovskite film.

In this work, alcohol-vapor solvent annealing treatment on CH₃NH₃PbI₃ thin films is reported, aiming to improve the crystal growth and increase the grain size of the CH₃NH₃PbI₃ crystal, thus boosting the performance of perovskite photovoltaics. By selectively controlling the CH₃NH₃I precursor, larger-grain size, higher crystallinity, and pinhole-free CH₃NH₃PbI₃ thin films are realized, which result in enhanced charge carrier diffusion length, decreased charge carrier recombination, and suppressed dark currents. As a result, over 43% enhanced efficiency along with high reproducibility and eliminated photocurrent hysteresis behavior are observed from perovskite hybrid solar cells (pero-HSCs) where the CH₃NH₃PbI₃ thin films are treated by methanol vapor as compared with that of pristine pero-HSCs where the CH₃NH₃PbI₃ thin films are without any alcohol vapor treatment. In addition, the dramatically restrained dark currents and raised photocurrents give rise to over ten times enhanced detectivities for perovskite hybrid photodetectors, reaching over 10¹³ cm Hz^1/2 W⁻¹ (Jones) from 375 to 800 nm. These results demonstrate that the method provides a simple and facile way to boost the device performance of perovskite photovoltaics.

High performance of perovskite photovoltaics (perovskite solar cells and perovskite photodetectors) is realized by alcohol-vapor solvent annealing treatment on CH₃NH₃PbI₃ thin films to enhance the crystal growth and the grain size of the CH₃NH₃PbI₃ crystals.

Degradation mechanisms of CH₃NH₃PbI₃-based planar perovskite solar cells (PSCs) are investigated using a thermally stimulated current technique. Hole traps lying above the valence-band edge of the CH₃NH₃PbI₃ are detected in PSCs degraded by continuous simulated solar illumination. One source of the hole traps is the photodegradation of CH₃NH₃PbI₃ in the presence of water.

The electronic structure of a large sample set of CH₃NH₃PbI₃-based perovskites is studied. Combined investigations by UV/X-ray photoelectron spectroscopy and X-ray diffraction reveal that interstitials present in the film lead to changes in the occupied density of states close to the valence band, which in turn influences the performance of solar cells. Changes in elemental composition tune the ionization energy of the perovskite film by almost 1 eV without introducing significant amounts of gap states.

Interface engineering is critical for achieving efficient solar cells, yet a comprehensive understanding of the interface between a metal electrode and electron transport layer (ETL) is lacking. Here, a significant power conversion efficiency (PCE) improvement of fullerene/perovskite planar heterojunction solar cells from 7.5% to 15.5% is shown by inserting a fulleropyrrolidine interlayer between the silver electrode and ETL. The interface between the metal electrode and ETL is carefully examined using a variety of electrical and surface potential techniques. Electrochemical impedance spectroscopy (EIS) measurements demonstrate that the interlayer enhances recombination resistance, increases electron extraction rate, and prolongs free carrier lifetime. Kelvin probe force microscopy (KPFM) is used to map the surface potential of the metal electrode and it indicates a uniform and continuous work function decrease in the presence of the fulleropyrrolidine interlayer. Additionally, the planar heterojunction fullerene/perovskite solar cells are shown to have good stability under ambient conditions.

Inverted planar heterojunction perovskite solar cells are optimized to achieve a maximum efficiency of 15.5% by inserting fulleropyrrolidine as an interface modification layer. The interface between silver electrode and electron transport layer is carefully examined using a variety of electrical and surface potential techniques. Interface engineering is critical for achieving high-performance perovskite solar cells.

The electronic structure and physical mechanisms of carrier generation and transport in the organic bulk heterojunction are reviewed. The electronic structure describes the bands and band-tail states, the band alignment at the bulk-heterojunction interface, and the overall density-of-states model. The different electronic character of excitons and mobile charge is discussed, the former being highly molecular and the latter more delocalized. Dissociation of the exciton via the charge-transfer (CT) states is attributed to weak binding of the CT state arising from charge delocalization. Carrier transport and charge collection is strongly influenced by the presence of localized band-tail states. Recombination is attributed primarily to transitions from mobile carriers to band-tail or deep trap states.

The bulk heterojunction (BHJ) structure has enabled the creation of organic solar cells. The electronic structure of the BHJ is characterized by the phase morphology, molecular structure, and disorder. Exciton dissociation, carrier transport, and recombination each reflect the electronic structure of the BHJ interface. A broad physical understanding is emerging but important details remain unresolved.