Ferrocene Derivatives for Improving the Efficiency and Stability of MA-Free Perovskite Solar Cells from the Perspective of Inhibiting Ion Migration and Releasing Film Stress

Further improvement of the performance and stability of inverted perovskite solar cells (PSCs) is necessary for commercialization. Here, ferrocene derivative dibenzoylferrocene (DBzFe) is used as an additive to enhance the performance and stability of MA- and Br- free PSCs. The results show that the introduction of DBzFe not only passivates the defects in the film but also inhibits the ion migration in the film. The final device achieves a power conversion efficiency (PCE) of 23.53%, which is one of the highest efficiencies currently based on self-assembled monolayers (SAMs). Moreover, it maintains more than 96.4% of the original efficiency when running continuously for 400 h at the maximum power point.

Efficiency Enhancement of Wide Bandgap Lead Perovskite Solar Cells with PTAA Surface-Passivated with Monomolecular Layer from the Viewpoint of PTAA Band Bending

This report is on the efficiency enhancement of wide bandgap lead halide perovskite solar cells (WBG Pb-PVK PSCs) consisting of FA0.8Cs0.2PbI1.8Br1.2 as the light-harvesting layer. WGB Pb-PVK PSCs have attracted attention as the top layer of all perovskite-tandem solar cells. Poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA), a conductive polymer, is always used as the hole transporting layer (HTL) for Pb-PVK PSCs. Nevertheless, the hydrophobic surface of the PTAA sometimes destroys the growth of the FA0.8Cs0.2PbI1.8Br1.2 film. On the other hand, the Fermi level of PTAA is not well matched with that of perovskite film. Thus, the PCE of the WBG Pb-based PSCs with PTAA as the HTL was not very high. In this report, the efficiency of the FA0.8Cs0.2PbI1.8Br1.2 is improved by passivating the surface of the PTAA with a monomolecular layer, where the surface becomes hydrophilic, and the band bending of the PTAA layer is improved to cause swift hole collection. Finally, WBG Pb-PVK PSCs (1.77 eV) with 16.52% efficiency are reported.

14.31% Power Conversion Efficiency of Sn-Based Perovskite Solar Cells via Efficient Reduction of Sn 4

The photoelectric properties of nontoxic Sn-based perovskite make it a promising alternative to toxic Pb-based perovskite. It has superior photovoltaic performance in comparison to other Pb-free counterparts. The facile oxidation of Sn2+ to Sn4+ presents a notable obstacle in the advancement of perovskite solar cells that utilize Sn, as it adversely affects their stability and performance. The study revealed the presence of a Sn4+ concentration on both the upper and lower surfaces of the perovskite layer. This discovery led to the adoption of a bi-interface optimization approach. A thin layer of Sn metal was inserted at the two surfaces of the perovskite layer. The implementation of this intervention yielded a significant decrease in the levels of Sn4+ and trap densities. The power conversion efficiency of the device was achieved at 14.31% through the optimization of carrier transportation. The device exhibited operational and long-term stability.

Indent-Free Vapor-Assisted Surface Passivation Strategy toward Tin Halide Perovskite Solar Cells

Sn halide perovskite solar cells (PKSCs) are the most promising competitors to conventional lead PKSCs. Nevertheless, defects at the surfaces and grain boundaries hinder the improvement of the PKSCs' performance. Liquid surface passivation on the perovskite layer is commonly used to decrease these defects. In the case of tin perovskite solar cells, the liquid passivation improved the open-circuit voltage (V_oc). However, this decreased the short-circuit current density (J_sc). We found that this J_sc loss is brought about by the thickness loss after the liquid passivation because tin perovskite layers are partially soluble in common solvents, and the calculated impact pressure was up to 155.4 kPa. Here, we introduce new vapor passivation including solvent and passivation molecules and report efficiency enhancement without decreasing J_sc. The vapor-passivated film showed longer time-resolved photoluminescence decay, smoother morphology, and lower defect densities. Most importantly, the vapor passivation method significantly enhanced the efficiency from 9.41 to 11.29% with J_sc increasing from 22.82 to 24.05 mA·cm^-2. On the contrary, the corresponding liquid passivation method gave an efficiency of 10.90% with a decreased J_sc from 22.82 to 22.38 mA·cm^-2. A commonly used and simple indent-free surface passivation strategy is proposed to enhance the efficiency and stability of PKSCs.

VOC Over 1.4 V for Amorphous Tin-Oxide-Based Dopant-Free CsPbI2Br Perovskite Solar Cells

CsPbI₂Br perovskite solar cells have attracted much attention because of the rapid development in their efficiency and their great potential as a top cell of tandem solar cells. However, the V_OC outputs observed so far in most cases are far from that desired for a top cell. Up to now, with various kinds of treatments, the reported champion V_OC is only 1.32 V, with a V_OC deficit of 0.60 V. In this work, we found that aging of the SnCl₂ precursor solution for the electron-transporting layer can promote the V_OC of CsPbI₂Br solar cells by employing a dopant-free-polymer hole transport material (HTM) over 1.40 V and efficiency over 15.5% with high reproducibility. With the champion V_OC of 1.43 V, the V_OC deficit was reduced to <0.50 V, which is achieved for the first time. This simple technique of SnCl₂ solution aging forms a uniform and smooth amorphous SnO_x film with pure Sn⁴⁺, elevates the conduction band of SnO_x, and reduces the interfacial gaps and the trap state density of the device, resulting in enhancement in average V_OC from ∼1.2 V in the nonaged case to ∼1.4 V in the aged case. Furthermore, the device using an aged SnCl₂ solution also exhibits a much better long-term stability than that made of the fresh solution. These achievements in dopant/additive-free CsPbI₂Br solar cells can be useful for future research on CsPbI₂Br and tandem solar cells.

Low-Temperature Synthesized Nb-Doped TiO2 Electron Transport Layer Enabling High-Efficiency Perovskite Solar Cells by Band Alignment Tuning

An Nb-doped TiO₂ (Nb-TiO₂) film comprising a double structure stacked with a bottom compact layer and top mesoporous layers was synthesized by treating a Ti precursor-coated substrate using a one-step low-temperature steam-annealing (SA) method. The SA-based Nb-TiO₂ films possess high crystallinity and conductivity, and that allows better control over the conduction band (CB) of TiO₂ for the electron transport layer (ETL) of the perovskite solar cells by the Nb doping level. Optimization of power conversion efficiency (PCE) for the Nb-TiO₂-based ETL was combined with the CB level tuning of the mixed-halide perovskite by changing the Br/I ratio. This band offset management enabled to establish the most suitable energy levels between the ETL and the perovskites. This method was applied to reduce the band gap of perovskites to enhance the photocurrent density while maintaining a high open-circuit voltage. As a result, the optimal combination of 5 mol % Nb-TiO₂ ETL and 10 mol % Br in the mixed-halide perovskite exhibited high photovoltaic performance for low-temperature device fabrication, achieving a high-yield PCE of 21.3%.

Charge transfer dynamics in chlorophyll-based biosolar cells

We fabricated a chlorophyll (Chl)-based biosolar cell with H₂Chl-sensitized TiO₂ as an acceptor and (ZnChl)_n as a donor. This solar cell gives a relatively high quantum yield from the absorption spectral contribution from both the donor and acceptor species. We employed subpicosecond time-resolved absorption spectroscopy (TAS) to study the excited state dynamics at the Chl interface. A charge transfer (CT) state between TiO₂-H₂Chl and (ZnChl)_n was observed at 640 nm after excitation at the Q_y peaks, 680 nm and 720 nm. This CT state is entirely different from the CT states observed for either TiO₂-H₂Chl (TiO₂-H₂Chl/spiro-OMeTAD) or TiO₂-(ZnChl)_n systems. Due to the slower charge transfer process from H₂Chl⁺ to TiO₂ as compared to that from (ZnChl)_n⁺ to H₂Chl, the CT lifetimes of H₂Chl^--(ZnChl)_n⁺ (τ₁ = 0.1 ps, τ₂ = 1.4 ps) excited at 720 nm are slightly shorter than that excited at 680 nm (τ₁ = 0.2 ps, τ₂ = 5.6 ps). The TAS results suggest that the interface of TiO₂-H₂Chl and (ZnChl)_n not only transfers holes as spiro-OMeTAD does, but also provides a built-in field for charge dissociation between the two Chl species.

Thiocyanate Containing Two-Dimensional Cesium Lead Iodide Perovskite, Cs2PbI2(SCN)2: Characterization, Photovoltaic Application, and Degradation Mechanism

We explored thiocyanate (SCN)-based two-dimensional (2D) organometal lead halide perovskite families toward photovoltaic applications. Using an SCN axial ligand and various cation species, we examined AA'PbI₂(SCN)₂-type 2D perovskite by replacing the cation species (AA') between methylammonium (MA), formamidinium (FA), and cesium. Among various cation compositions, only all-inorganic cesium-based SCN perovskite, Cs₂PbI₂(SCN)₂, film showed high thermal stability compared to known 2D perovskites. Perovskite solar cells (PSCs) using the Cs₂PbI₂(SCN)₂ absorber yielded approximately 2% conversion efficiency on the mesoscopic device. Relatively low efficiency is attributed, in addition to optical properties (large band gap (2.05 eV) and exciton absorption), to the orientation of perovskite layer parallel to the layered structure, preventing carrier extraction from the light-absorber perovskite. In device stability, the Cs-based 2D perovskite was stable against oxygen (oxidation), whereas it was found to be unstable against humidity. X-ray diffraction and X-ray photoelectron spectroscopy measurements showed that, unlike long alkylammonium-based 2D perovskite families such as BA₂PbI₄ (BA = butylammonium), the Cs-based 2D perovskite can undergo hydrolysis due to the hydrophilic Cs cations.

Spontaneous Synthesis of Highly Crystalline TiO2 Compact/Mesoporous Stacked Films by a Low-Temperature Steam-Annealing Method for Efficient Perovskite Solar Cells

Highly crystalline TiO₂ nanostructured films were synthesized by a simple steam treatment of a TiCl₄ precursor film under a saturated water vapor atmosphere at 125 °C, here referred to as the steam-annealing method. In a single TiO₂ film preparation step, a bilayer structure comprising a compact bottom layer and a mesoporous surface layer was formed. The mesoporous layer was occupied by bipyramidal nanoparticles, with a composite phase of anatase and brookite crystals. Despite the low-temperature treatment process, the crystallinity of the TiO₂ film was high, comparable with that of the TiO₂ film sintered at 500 °C. The compact double-layered TiO₂ film was applied to perovskite solar cells (PSCs) as an electron-collecting layer. The PSC exhibited a maximum power conversion efficiency (PCE) of 18.9% with an open-circuit voltage ( V_OC) of 1.15 V. The PCE and V_OC were higher than those of PSCs using a TiO₂ film formed by 500 °C sintering.

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 TiO_x hole-blocking layer in combination with brookite TiO₂ prepared at temperature <150 °C. Structured with TiO_x/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 SnO_x hole-blocking layer, TiO_x has better electron band alignment with brookite TiO₂ and hence, results in higher efficiency.

Lead-free perovskite solar cells using Sb and Bi-based A3B2X9 and A3BX6 crystals with normal and inverse cell structures

Research of CH3NH3PbI3 perovskite solar cells had significant attention as the candidate of new future energy. Due to the toxicity, however, lead (Pb) free photon harvesting layer should be discovered to replace the present CH3NH3PbI3 perovskite. In place of lead, we have tried antimony (Sb) and bismuth (Bi) with organic and metal monovalent cations (CH3NH3+, Ag+ and Cu+). Therefore, in this work, lead-free photo-absorber layers of (CH3NH3)3Bi2I9, (CH3NH3)3Sb2I9, (CH3NH3)3SbBiI9, Ag3BiI6, Ag3BiI3(SCN)3 and Cu3BiI6 were processed by solution deposition way to be solar cells. About the structure of solar cells, we have compared the normal (n-i-p: TiO2-perovskite-spiro OMeTAD) and inverted (p-i-n: NiO-perovskite-PCBM) structures. The normal (n-i-p)-structured solar cells performed better conversion efficiencies, basically. But, these environmental friendly photon absorber layers showed the uneven surface morphology with a particular grow pattern depend on the substrate (TiO2 or NiO). We have considered that the unevenness of surface morphology can deteriorate the photovoltaic performance and can hinder future prospect of these lead-free photon harvesting layers. However, we found new interesting finding about the progress of devices by the interface of NiO/Sb3+ and TiO2/Cu3BiI6, which should be addressed in the future study.

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.

Impacts of Heterogeneous TiO2 and Al2O3 Composite Mesoporous Scaffold on Formamidinium Lead Trihalide Perovskite Solar Cells

Heterogeneous TiO2 and Al2O3 composites were employed as a mesoporous scaffold in formamidinium lead trihalide (FAPbI3-xClx)-based perovskite solar cells to modify surface properties of a mesoporous layer. It was found that the quality and morphology of the perovskite film were strongly affected by the TiO2/Al2O3 ratio in the mesoporous film. The conversion efficiency of the perovskite solar cell was improved by using a composite of TiO2 and Al2O3 in comparison with TiO2- and Al2O3-based cells, yielding 11.0% for a cell with a 7:3 TiO2/Al2O3 composite. Our investigation shows a change of electron transport path depending on a composition ratio of insulating Al2O3 to n-type semiconducting TiO2 in a mesoporous layer.

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.

A Switchable High-Sensitivity Photodetecting and Photovoltaic Device with Perovskite Absorber

Amplified photocurrent gain has been obtained by photodiodes of inorganic semiconductors such as GaAs and Si. The avalanche photodiode, developed for high-sensitivity photodetectors, requires an expensive vapor-phase epitaxy manufacture process and high driving voltage (50-150 V). Here, we show that a low-cost solution-processed device using a planar-structured ferroelectric organo-lead triiodide perovskite enables light detection in a large dynamic range of incident power (10(-7)-10(-1) W cm(-2)) by switching with small voltage (-0.9 to +0.5 V). The device achieves significantly high external quantum conversion efficiency (EQE) up to 2.4 × 10(5)% (gain value of 2400) under weak monochromatic light. On a single dual-functional device, incident small power (0.2-100 μW cm(-2)) and medium to large power (>0.1 mW cm(-2)) are captured by reverse bias and forward bias modes, respectively, with linear responsivity of current. For weak light detection, the device works with a high responsivity value up to 620 A W(-1).