Performance and stability improvement of perovskite solar cells using a nanopillar conductive polymer formed via electropolymerisation.

BF4--doped poly(3-methylthiophene) (P3MT) was formed using electropolymerisation as a hole transport material for inverted perovskite solar cells. The controlled nanopillar morphology of P3MT enables void-less uniform perovskite formation and exhibits conversion efficiency of 11.11%. The P3MT-based cells exhibited superior stability in ambient air to poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid)-based cells.

Effect of Humidity on Crystal Growth of CuSCN for Perovskite Solar Cell Applications.

Copper(I) thiocyanate (CuSCN) is one of the most robust hole-transport materials for perovskite solar cells (PSCs). However, the power conversion efficiency of CuSCN-based PSCs is low due to difficulty in crystallization of CuSCN. In this study, we focused on humidity conditions during the aging process of CuSCN-based PSCs to improve their performance. PSCs aged in humid air, i. e., at a relative humidity of 70 %, exhibited better performance (efficiency; 10.6 %) than those aged in lower humidity (5.9 %) due to improved crystallinity of CuSCN layers. The results of the study provide insights into how to improve fabrication process of CuSCN-based PSCs for higher stability and efficiency.

In Situ Grown Nanocrystalline Si Recombination Junction Layers for Efficient Perovskite-Si Monolithic Tandem Solar Cells: Toward a Simpler Multijunction Architecture.

The perovskite-Si tandem is an attractive avenue to attain greater power conversion efficiency (PCE) than their respective single-junction solar cells. However, such devices generally employ complex stacks with numerous deposition steps, which are rather unattractive from an industrial perspective. Here, we develop a simplified tandem architecture consisting of a perovskite n-i-p stack on a silicon heterojunction structure without applying the typically used indium-tin-oxide (ITO) recombination junction (RJ) layer between the top and bottom cells. It is demonstrated that an n-type hydrogenated nanocrystalline silicon (nc-Si:H) grown in situ on an amorphous silicon hole contact layer of the bottom cell acts as an efficient RJ layer, leading to a high open-circuit voltage (VOC) of >1.8 V and a PCE of 21.4% without optimizing the optical design. Compared to the tandem cell with an ITO RJ layer, the nc-Si:H RJ layer not only improves light management but also achieves a higher VOC due to superior contact properties with an overlying SnO2 electron transport layer of the perovskite top cell. Omitting the costly material and its deposition step offers the opportunity toward realizing industrially feasible high-efficiency tandem solar cells.

Ionic Liquid-Assisted MAPbI3 Nanoparticle-Seeded Growth for Efficient and Stable Perovskite Solar Cells.

With the rapid improvement of perovskite solar cells (PSCs), long-life operational stability has become a major requirement for their commercialization. In this work, we devised a pristine cesium-formamidinium-methylammonium (termed as CsFAMA) triple-cation-based perovskite precursor solution into the ionic liquid (IL)-assisted MAPbI3 nanoparticles (NPs) through a seeded growth approach in which the host IL-assisted MAPbI3 NPs remarkably promote high-quality perovskite films with large single-crystal domains, enhancing the device performance and stability. The power conversion efficiency (PCE) of the MAPbI3 NP-seeded growth of MAPbI3 NPs/CsFAMA-based PSCs is as high as 19.44%, which is superior to those of MAPbI3 NPs and pristine CsFAMA films as the photoactive layer (9.52 and 17.33%, respectively). The long-term light-soaking and moisture stability of IL-aided MAPbI3 NPs/CsFAMA-based devices (non-encapsulated) remain above 90 and 80%, respectively, of their initial output after 2 h of light illumination (1 sun) and 6000 h storage at ambient with a relative humidity range of 30-40%. The use of the IL-assisted MAPbI3 NP-seeded growth for PSCs is a significant step toward developing stable and reliable perovskite photovoltaic devices.

Cesium iodide post-treatment of organic-inorganic perovskite crystals to improve photovoltaic performance and thermal stability.

Organic-inorganic perovskites were treated with a CsI solution to improve the photovoltaic performance and stability. Due to the formation of CsPbI3 and trap filling in Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 perovskites by CsI treatment, the power conversion efficiency was improved to 20.48%. The CsI-treated perovskites exhibited slower degradation under heating at 180 °C than the untreated perovskites.

Electron band tuning of organolead halide perovskite materials by methylammonium and formamidinium halide post-treatment for high-efficiency solar cells.

Perovskite crystals post-treated with methylammonium and formamidinium halide materials were compared. The bandgap energy of perovskites changed upon incorporation of CH5N2+, Br-, and Cl- ions. Perovskites treated with formamidinium iodide yielded the best efficiency of 20.06% due to an increase in photocurrent density by decreased bandgap energy.

Tuning Methylammonium Iodide Amount in Organolead Halide Perovskite Materials by Post-Treatment for High-Efficiency Solar Cells.

In this study, the composition of organic-inorganic perovskite materials is tuned by methylammonium iodide (MAI) post-treatment for high photovoltaic performance. By spin-coating MAI solutions of different concentrations, the amounts of PbI2 and MAI in perovskite layers are tuned. In perovskites, the removal of PbI2 through a reaction with MAI decreases the hysteresis in photocurrent density-voltage curves. Further, by treating perovskites with a high-concentration MAI solution, the excess MAI is incorporated into the perovskites. These perovskites with excess MAI show better power conversion efficiencies (of up to 20.7%) than perovskites with excess PbI2 because of the decrease in trap density. Since the present post-treatment can control perovskite composition without affecting the morphology and crystallinity of the perovskite crystals, this technique would be a useful tool to improve the photovoltaic performance of perovskite solar cells.

Amorphous Metal Oxide Blocking Layers for Highly Efficient Low-Temperature Brookite TiO2-Based Perovskite Solar Cells.

A fully low-temperature-processed perovskite solar cell was fabricated with an ultrathin amorphous TiOx hole-blocking layer in combination with brookite TiO2 prepared at temperature <150 °C. Structured with TiOx/brookite TiO2 bilayer electron collector, the perovskite solar cells exhibit high efficiency up to 21.6% being supported by high open-circuit voltage and fill factor up to 1.18 V and 0.83, respectively. Compared to SnOx hole-blocking layer, TiOx has better electron band alignment with brookite TiO2 and hence, results in higher efficiency.

Controlled Crystal Grain Growth in Mixed Cation-Halide Perovskite by Evaporated Solvent Vapor Recycling Method for High Efficiency Solar Cells.

We developed a new and simple solvent vapor-assisted thermal annealing (VA) procedure which can reduce grain boundaries in a perovskite film for fabricating highly efficient perovskite solar cells (PSCs). By recycling of solvent molecules evaporated from an as-prepared perovskite film as a VA vapor source, named the pot-roast VA (PR-VA) method, finely controlled and reproducible device fabrication was achieved for formamidinium (FA) and methylammonium (MA) mixed cation-halide perovskite (FAPbI3)0.85(MAPbBr3)0.15. The mixed perovskite was crystallized on a low-temperature prepared brookite TiO2 mesoporous scaffold. When exposed to very dilute solvent vapor, small grains in the perovskite film gradually unified into large grains, resulting in grain boundaries which were highly reduced and improvement of photovoltaic performance in PSC. PR-VA-treated large grain perovskite absorbers exhibited stable photocurrent-voltage performance with high fill factor and suppressed hysteresis, achieving the best conversion efficiency of 18.5% for a 5 × 5 mm2 device and 15.2% for a 1.0 × 1.0 cm2 device.

A SnOx-brookite TiO2 bilayer electron collector for hysteresis-less high efficiency plastic perovskite solar cells fabricated at low process temperature.

Thin plastic film-based CH3NH3PbI3-xClx perovskite solar cells were fabricated at low process temperature using a bilayer comprising an amorphous SnOx and mesoporous brookite TiO2 as electron collectors. Void-less high quality heterojunction structures achieve hysteresis-less photovoltaic performance with a power conversion efficiency as high as 13.4% and mechanical stability against cyclic bending.

Photoelectrochemical etching and energy gap control of silver clusters.

An energy gap of Ag clusters is controlled on the basis of photoinduced electron transfer from the clusters to TiO2. With 2.6 eV light irradiation, Ag32 clusters on TiO2 are oxidized and disappear. With <1.6 eV light, the energy gap of Ag32 is changed from ∼1.4 to ∼1.7 eV because of photoetching.

The Interface between FTO and the TiO2 Compact Layer Can Be One of the Origins to Hysteresis in Planar Heterojunction Perovskite Solar Cells.

Organometal halide perovskite solar cells have shown rapid rise in power conversion efficiency, and therefore, they have gained enormous attention in the past few years. However, hysteretic photovoltaic characteristics, found in these solid-state devices, have been a major problem. Although it is being proposed that the ferroelectric property of perovskite causes hysteresis in the device, we observed hysteresis in a device made of nonferroelectric PbI2 as a light absorber. This result evidently supports the fact that ferroelectric property cannot be the sole reason for hysteresis. The present study investigates the roles of some key interfaces in a planar heterojunction perovskite (CH3NH3PbI(3-x)Cl(x)) solar cell that can potentially cause hysteresis. The results confirm that the interface between fluorine doped tin oxide (FTO) substrate and the TiO2 compact layer has a definite contribution to hysteresis. Although this interface is one of the origins to hysteresis, we think that other interfaces, especially the interface of the TiO2 compact layer with perovskite, can also play major roles. Nevertheless, the results indicate that hysteresis in such devices can be reduced/eliminated by changing the interlayer between FTO and perovskite.

Gold cluster-nanoparticle diad systems for plasmonic enhancement of photosensitization.

Quantum-sized gold clusters are deposited on TiO2 both as a photosensitizer and catalyst, and coupled to plasmonic gold nanoparticles as a light harvesting antenna. Photocurrent enhancement was observed for Au25(SG)18 and Au38(SG)24 but not for Au102(SG)44 (SG = glutathione). The maximum enhancement factor of ~9 is reached at 900 nm.

Photoelectrochemical analysis of size-dependent electronic structures of gold clusters supported on TiO2.

Size-dependent electronic structures around the Fermi level of glutathione-protected gold clusters (0.9-1.4 nm in core diameter) were analyzed on the basis of photoinduced charge separation at the interface between the gold cluster and TiO(2). Electron levels such as HOMO and LUMO were estimated from the dependencies of the photocurrents on the irradiation wavelength and the standard electrode potentials of electron donors employed. The potential of the occupied levels involved in the charge separation under visible or near infrared light shifts negatively as the cluster size increases.