Organolead Halide Perovskite Solar Cells Research Articles

(Invited, Digital Presentation) Carbon-Based Multi-Porous-Layered-Electrodes for Highly-Stable Perovskite Solar Cells for Space Application

Organo-lead halide perovskite solar cells (PSCs) have attracted the attention due to the high efficiency and the cost-effective manufacturing by printing system. However, PSCs have been unstable due to water from outside and iodide (I-) reacts with the back contact metal (Au or Ag) inside. To solve these two issues, porous-carbon-electrodes have been introduced in PSCs by a group of Hongwei Han. The MPLE-PSCs show very high stability: thermal stability at 100 ֩C over 4500 h in dark (by Univ. of Hyogo, Japan); damp-heat stability at 85 ֩C-85%RH over 3,000 h in dark (by Univ. of Hyogo and Kishu Giken Kogyo, Japan); light stability at 55 ֩C over 10,000 h (by EPFL, Switzerland). In this conference, we will show a challenging work of high thermal stability at 130 ֩C, which is required for outer space application.

(Invited, Digital Presentation) Carbon-Based Multi-Porous-Layered-Electrodes for Highly-Stable Perovskite Solar Cells for Space Application

Organo-lead halide perovskite solar cells (PSCs) have attracted the attention due to the high efficiency and the cost-effective manufacturing by printing system. However, PSCs have been unstable due to water from outside and iodide (I-) reacts with the back contact metal (Au or Ag) inside. To solve these two issues, porous-carbon-electrodes have been introduced in PSCs. We are showing a challenging work of high thermal stability at 100-140 ֩C in vacuum, which is required for outer space application.

(Invited) Tailoring Crystal Morphology and Interfacial Charge Transfer in Perovskite Solar Cells

The past decade has witnessed tremendous advances in organometal halide perovskite solar cells (PSCs) due largely to the stellar set of optoelectronic properties of perovskites such as high absorption coefficient, long carrier diffusion length, and low electron-hole recombination. Notably, organolead halide PSC has reached a certified champion power conversion efficiency (PCE) of 25.5%. In this talk, first, I will discuss a meniscus-assisted solution printing (MASP) strategy to yield large-grained dense perovskite film with good crystallization and preferred orientation. The growth kinetics of perovskite crystals is scrutinized by in-situ optical microscopy tracking to understand the crystallization mechanism. The perovskite film produced by MASP exhibits excellent optoelectronic properties for high-efficiency PSCs. We also MASP cross-aligned conductive nanowires for biodegradable flexible PSCs. Second, I will present a simple yet robust acid-treatment strategy to judiciously create an amorphous TiO2 buffer layer intimately situated on the anatase TiO2 surface as electron transport layer (ETL) for efficient electron transport. The facile acid treatment is capable of weakening the bonding of zigzag octahedral chains in anatase TiO2, thereby shortening staggered octahedron chains to form an amorphous buffer layer on the anatase TiO2 surface. Such amorphous TiO2-coated ETL possesses an increased electron density due to the presence of oxygen vacancies, leading to efficient electron transfer from perovskite to TiO2. Compared to pristine TiO2-based devices, the perovskite solar cells (PSCs) with acid-treated TiO2 ETL exhibit enhanced short circuit current and PCE. Notably, we also tailor carrier dynamics in PSCs via precise dimension and architecture control and interfacial positioning of plasmonic nanoparticles in TiO2 ETL. Third, I will demonstrate synergistic cascade carrier extraction via dual interfacial positioning of ambipolar black phosphorene (BP) for high-efficiency PSCs. Concurrently enhanced carrier extraction at both electron transport layer/perovskite and perovskite/hole transport layer interfaces is achieved through judicious design and position of BP with tailored thickness as dual-functional nanomaterials. Fourth, I will introduce a robust route that simultaneously allows defect passivation and reduced energy difference between perovskite and hole transport layer (HTL) via the judicious placement of polar chlorine-terminated silane molecules at the interface. An integrated experimental and density functional theory study demonstrates that the dipole layer formed by the silane molecules decreases the perovskite work function, imparting an Ohmic character to the perovskite/HTL contact. The corresponding PSCs manifest a nearly 20% increase in PCE over pristine devices and a markedly enhanced device stability. Finally, I will show the spatial poling-initiated perovskite self-healing for defects passivation. The spatial poling significantly enhances the ion migration recovery after removing electric field. Particularly, the ion recovery tends to form a perfect lattice structure, leading to decreased defects in the perovskite layer with enhanced stability.

(Invited) Rational Strategies for Efficient Perovskite Solar Cells

The past decade has witnessed tremendous advances in organometal halide perovskite solar cells (PSCs) due largely to the stellar set of optoelectronic properties of perovskites such as high absorption coefficient, long carrier diffusion length, and low electron–hole recombination rate constant. Notably, the organolead halide PSC has reached a certified champion power conversion efficiency (PCE) of 25.2%. Rapid progress in PCE has been realized by structure design, materials chemistry, process engineering, and device physics. In this talk, I will first introduce a meniscus-assisted solution printing method (MASP) towards large-grained perovskite films for morphology and crystallinity control. Intriguingly, the outward convective flow triggered by fast solvent evaporation at the edge of the meniscus ink imparted the diffusion of perovskite solutes, thus facilitating the growth of micrometer-scale perovskite grains, leading to perovskite film with good crystallization and preferred orientation. This robust MASP strategy may in principle be easily extended to craft other solution-printed perovskite-based optoelectronics. Secondly, I will demonstrate an interesting interface engineering approach with 2D ambipolar black phosphorene (BP) for high-efficiency and stable PSCs. BP carries a stellar set of physical properties such as conveniently tunable bandgap and extremely high ambipolar carrier mobility. Concurrently enhanced carrier extraction at both electron transport layer/perovskite and perovskite/hole transport layer interfaces was achieved through judicious design and position of BP with tailored thickness as dual-functional nanomaterials. Thirdly, a spatial poling approach will be introduced for perovskite recrystallization towards preferred orientation, crystallinity, and morphology. Finally, the challenges facing the future development and commercialization of PSCs will be summarized and provided.

The Characteristics of Perovskite Solar Cells Fabricated Using DMF and DMSO/GBL Solvents

Organic solar cells are used in low-cost solar energy applications. Recently, organo-lead halide perovskite solar cells have attracted considerable interest. Low-temperature spin-coating is the simplest method to fabricate the low-cost solar cells; however, forming a continuous perovskite film by spin coating a precursor solution of lead iodide (PbI2) and methylammonium iodide (CH3NH3I) is challenging. This work investigated the characteristics of perovskite thin film, prepared using dimethylformamide (DMF) and dimethyl sulfoxide (DMSO)/γ-butyrolactone (GBL) solvents in the direct mixing of PbI2 and CH3NH3I. Perovskite films prepared using the DMSO/GBL solvent had greater crystallinity and compactness than those using the DMF solvent. Cell performance was improved by a perovskite absorber using the DMSO/GBL solvent to a power conversion efficiency of 8.87%, an open-circuit voltage of 0.82 V, a current density of 17.13 mA/cm2, and a fill factor of 63.01%.

Electronic Structure of Nonionic Surfactant-Modified PEDOT:PSS and Its Application in Perovskite Solar Cells with Reduced Interface Recombination.

The interfacial properties of organolead halide perovskite solar cells (PSCs) affect the exciton and charge-transport dynamics significantly. Thus, proper modification of the interfaces between perovskite and charge-transport layers is an efficient method to increase the power conversion efficiency (PCE) of PSCs. In this work, we explore the effect of a nonionic surfactant, that is, Triton X-100 (TX) additive, in the poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) hole-transport layer. The electronic structure of TX-modified PEDOT:PSS is investigated with ultraviolet/X-ray photoelectron spectroscopy and X-ray absorption spectroscopy with various TX concentrations. The surface of the TX-modified PEDOT:PSS layer showed high TX content, and thus the semimetallic properties of PEDOT:PSS were suppressed conspicuously by its insulating nature. With the TX-modified PEDOT:PSS, the PCE of methylammonium lead iodide (MAPbI3) PSCs increased significantly. To elucidate the origin of the improved device performance, the electrical properties and photoluminescence were investigated comprehensively. Consequently, it was found that the TX additive inhibits interface recombination between PEDOT:PSS and MAPbI3, which is caused by the suppression of semimetallic properties of the PEDOT:PSS surface. Hence, we fabricated flexible PSCs successfully using a graphene electrode and TX-modified PEDOT:PSS.

In situ investigation of light soaking in organolead halide perovskite films

Organolead halide perovskite solar cells (PSCs) have generated extensive attention recently with power conversion efficiency (PCE) exceeding 23%. However, these PSCs exhibit photoinduced instability in the course of their current-voltage measurements. In this work, we study the light-induced behavior in CH3NH3PbI3−xClx films in situ, by employing wide-field photoluminescence (PL) microscopy to obtain both the spatially and temporally resolved PL images simultaneously. Along with the increase in the PL intensity under continuous illumination, some areas render PL inactive. By characterizing the excitation energy dependent long-time PL decay behavior, we suggest that the PL quenching can be ascribed to a localized accumulation of iodide ions driven by the optical field. This ion localization leads to an enhancement of non-radiative recombination. The appearance of the PL inactive areas in the perovskite film impedes its photovoltaic device performance approaching the theoretical maximum PCE. Therefore, the herein presented real-time investigation of the light soaking of perovskite films is a versatile and adaptable method providing more details to improve the performance of PSCs.

Multi Band Gap Electronic Structure in CH3NH3PbI3

Organo-lead halide perovskite solar cells represent a revolutionary shift in solar photovoltaics, introducing relatively soft defect containing semiconductors as materials with excellent charge collection for both electrons and holes. Although they are based on the nominally simple cubic perovskite structure, these compounds are in fact very complex. For example, in (CH3NH3)PbI3 the dynamics and ensuing structural fluctuations associated with the (CH3NH3)+ ions and the interplay with the electronic properties are still not fully understood, despite extensive study. Here, using ab-initio calculations, we show that at room and higher temperature, the rotation of CH3NH3 molecules can be viewed as effectively giving local structures that are cubic and tetragonal like from the point of view of the PbI3 framework, though in fact having lower symmetry. Both of these structures are locally polar, with sizable polarization, ~10 μC/cm2 due to the dipoles on the organic. They become energetically degenerate in the volume range, V ~ 250 Å3/f.u–265 Å3/f.u. We also find very significant dependence of the band gap on the local structure. This type of transition is analogous to a transition between two ferroelectric structures, where in-spite of strong electron phonon coupling, there is strong screening of charged defects which can lead to enhanced mobility and charge collection. The results provide insights into the enhanced light absorption near the band edge and good charge collection in this material.

Performance enhancement of perovskite solar cells using NH4I additive in a solution processing method

A two-step sequential deposition has been widely adopted to fabricate a perovskite film of planar heterojunction organo-lead halide perovskite solar cells. However, in this method complete conversion of Lead iodide (PbI2) into CH3NH3PbI3 is limited by the diffusion of CH3NH3I into PbI2 film, thus limits current density. Here, we report a facile method for enhancing perovskite film morphology by introducing a small amount of ammonium iodide (NH4I) as a halogen additive into PbI2 solution, which can react with PbI2 and forms nanostructured platelet structure that helps to diffusion of CH3NH3I, for further improving CH3NH3PbI3 film morphology. Under the optimized NH4I additive concentration of 5 wt%, the best power conversion efficiency (PCE) is 17.4% achieved, which is an improvement of 14% compared to that of the original device without the additive. SEM studies reveal that addition of NH4I into PbI2 films improves the porosity of the films, which helps to improve the perovskite film morphology by increasing the grain size. The enhancement of CH3NH3PbI3 film morphology is useful for increasing the optical absorption of perovskite film thereby increasing short-circuit current density, whence an improvement in the efficiency of the perovskite solar cell devices. The perovskite solar cells fabricated by this method preserved high crystallinity, negligible hysteresis, reliable reproducibility and improved stability.

Unraveling the Charge Extraction Mechanism of Perovskite Solar Cells Fabricated with Two-Step Spin Coating: Interfacial Energetics between Methylammonium Lead Iodide and C60.

In organolead halide perovskite solar cells (PSCs), interfacial properties between the perovskite and charge transport layers are the critical factors governing charge extraction efficiency. In this study, the effect of interfacial energetics between two-step spin-coated methylammonium lead iodide (MAPbI3) with different methylammonium iodide (MAI) concentrations and C60 on the charge extraction efficiency is investigated. The electronic structures of perovskite films are significantly varied by the MAI concentrations due to the changes in the residual precursor and MA+ defect content. As compared to the optimum PSCs with 25 mg mL-1 MAI, PSCs with other MAI concentrations show significantly lower power conversion efficiencies and severe hysteresis. The energy level alignment at the C60/MAPbI3 interface determined by ultraviolet and inverse photoelectron spectroscopy measurements reveals the origin of distinct differences in device performances. The conduction band offset at the C60/MAPbI3 interface plays a crucial role in efficient charge extraction in PSCs.

Improving Efficiency and Stability of Perovskite Solar Cells by Modifying Mesoporous TiO2–Perovskite Interfaces with Both Aminocaproic and Caproic acids

AbstractIn the field of photovoltaic industry, organolead halide perovskite solar cells (PSCs) as cleaner and cheaper photo‐voltaic devices have shown a bright prospect with power conversion efficiency (PCE) rapidly climbing from 3.8% to certified 22.1% in a few years. Interface engineering has been proven to be highly effective to solve the instability‐to‐moisture issue and enhance the performance of PSCs. Here, this work develops a simple and easy‐proceeding strategy that depositing both aminocaproic acid [H2NCH2(CH2)4COOH, abbreviated as AmCA] and caproic acid [CH3(CH2)4COOH, CA] at the mesoporous TiO2/perovskite interface leads to significant enhancement in both the efficiency and stability of PSCs. These two organic modifiers work synergistically to enhance the overall performance of PSCs by promoting electron transfer through the interaction between amino groups on AmCA and perovskite layer, and resisting moisture with alkyl chains from CA. The champion efficiency of modified cells reaches 18.2%, with an average PCE of 17.5% in reliable reproducibility, with significant improvement in stability under 50 ± 5% relative humidity in air. The developed simple interfacial modification approach should be effective to enhance both efficiency and stability of PSCs with other architectures as well. More importantly, this scenario may provide insight into the commercialization of perovskite solar cells in the not‐too‐distant future.

Upconversion luminescence co-enhanced by Li+ ions doping and localized surface plasmon resonance for perovskite solar cells

The minimization of non-absorption loss of solar photons is highly desirable for organolead halide perovskite solar cells by extending the spectral absorption from visible into near-infrared (NIR) range. NaYF4:Yb,Er/Li-Ag@SiO2 composites were created by employing Li+ ions doping and the localized surface plasmon resonance for co-enhancing upconversion luminescence intensity. NaYF4:Yb,Er/Li-Ag@SiO2 composites were doped into spiro-OMeTAD-based hole-transfer layer (HTM) of perovskite solar cells to enhance NIR absorption. The results show that the short circuit current density is apparently enhanced by doping NaYF4:Yb,Er/Li-Ag@SiO2 into the HTM while maintaining the open circuit voltage and fill factor, leading to an increase in power conversion efficiency from 7.83% to 9.34%.

Highly π-extended copolymer as additive-free hole-transport material for perovskite solar cells

Organolead halide perovskite solar cells have achieved a certified power-conversion efficiency (PCE) of 22.1% and are thus among the most promising candidates for next-generation photovoltaic devices. To date, most high-efficiency perovskite solar cells have employed arylamine-based hole-transport materials (HTMs), which are expensive and have a low mobility. The complicated doping procedures and the potentially stability-adverse dopants used in these HTMs are among the major bottlenecks for the commercialization of perovskite solar cells (PSCs). Herein, we present a polythiophene-based copolymer (PDVT-10) with a hole mobility up to 8.2 cm2·V−1·s−1 and a highest occupied molecular orbital level of −5.28 eV as a hole-transport layer (HTL) for a PSC. A device based on this new HTM exhibited a high PCE of 13.4% under 100 mW·cm−2 illumination, which is one of the highest PCEs reported for the dopant-free polymer-based HTLs. Moreover, PDVT-10 exhibited good solution processability, decent air stability, and thermal stability, making it a promising candidate as an HTM for PSCs.

Temperature effect of the compact TiO2 layer in planar perovskite solar cells: An interfacial electrical, optical and carrier mobility study

The power conversion efficiency of TiO2 based organo-lead halide perovskite solar cells has already surpassed 20% by utilizing low temperature solution processing techniques. However, the electronic structures and exciton/electron transport mechanism at the TiO2/perovskite interface are not fully understood. In this report, simple planar TiO2/perovskite heterojunctions are fabricated with different TiO2 compact layers followed by studies of their effect on the charge extraction/transport properties of the deposited heterojunctions. Efficient perovskite solar cells based on these TiO2/perovskite heterojunctions [with CH3NH3PbI3 and FA0.81MA0.15Pb(I0.836Br0.15)3 films] have been fabricated and the devices exhibit reproducible efficiency values and good stability. The impact of interfacial potential barrier and TiO2 surface properties on photogenerated carrier transport mechanisms and overall power conversion efficiency of perovskite solar cells is investigated by using comprehensive electrical and optical characterization methods such as SEM, XRD, XPS, Raman spectroscopy, TRPL, TAS and TPD. Obtained results indicate that efficient device can be fabricated by optimizing of TiO2 annealing conditions.

Highly Efficient and Stable Perovskite Solar Cells by Interfacial Engineering Using Solution-Processed Polymer Layer

Solution-processed organo-lead halide perovskite solar cells with deep pinholes in the perovskite layer lead to shunt-current leakage in devices. Herein, we report a facile method for improving the performance of perovskite solar cells by inserting a solution-processed polymer layer between the perovskite layer and the hole-transporting layer. The photovoltaic conversion efficiency of the perovskite solar cell increased to 18.1% and the stability decreased by only about 5% during 20 days of exposure in moisture ambient conditions through the incorporation of a poly(methyl methacrylate) (PMMA) polymer layer. The improved photovoltaic performance of devices with a PMMA layer is attributed to the reduction of carrier recombination loss from pinholes, boundaries, and surface states of perovskite layer. The significant gain generated by this simple procedure supports the use of this strategy in further applications of thin-film optoelectronic devices.

Hysteresis Analysis of Organolead Halide Perovskite Solar Cells by Transient Current Measurement

Hysteresis Analysis of Organolead Halide Perovskite Solar Cells by Transient Current Measurement

Improving the performance and reliability of inverted planar perovskite solar cells with a carbon nanotubes/PEDOT:PSS hybrid hole collector.

Adopting an efficient charge transport layer is crucial to improve the photovoltaic (PV) performances of organo-lead halide perovskite (PRV) solar cells. In this study, we suggest a novel hybrid hole-transport layer (HTL) consisting of single-walled carbon nanotubes (SWNTs) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) for inverted-planar PRV devices. The SWNTs were drop-cast on ITO/glass substrates, and they were partly grown perpendicular to the substrates. Then, we coated PEDOT:PSS to cover the SWNTs for complete electron-blocking. A PRV light-harvester was spin-cast on the hybrid HTL, and the vertical SWNTs protruded into the PRV through penetrating the PEDOT:PSS. Steady-state photoluminescence spectroscopy evidenced that the SWNTs/PEDOT:PSS hybrid HTL showed enhanced charge-carrier quenching properties. The hybrid HTL also revealed negligible parasitic absorption loss checked by UV-Vis spectroscopy. These contributed to improve the average power conversion efficiency from 9.4% to 11.0% (up to 12.5% for the best cell) based on fabricated 90 devices. Furthermore, significant suppression of current-voltage hysteresis was attained by employing the hybrid HTL. This study not only manifests unprecedented utilization of the SWNTs for the HTL in inverted planar PRV cells but also paves the way for the development of high-performance and reliable PRV solar cells compatible with flexible processing at low temperature (<150 °C).

From unstable CsSnI3 to air-stable Cs2SnI6: A lead-free perovskite solar cell light absorber with bandgap of 1.48 eV and high absorption coefficient

All-inorganic and lead-free cesium tin halides (CsSnX3, X=Cl, Br, I) are highly desirable for substituting the organolead halide perovskite solar cells. However, the poor stability of CsSnX3 perovskites has so far prevented the fabrication of devices that can withstand sustained operation under normal conditions. In this paper, a two-step sequential deposition method is developed to grow high-quality B-γ-CsSnI3 thin films and their unique phase change in atmosphere is explored in detail. We find the spontaneous oxidative conversion from unstable B-γ-CsSnI3 to air-stable Cs2SnI6 in air. Allowing the phase conversion of the CsSnI3 film to evolve in ambient air it gives the semiconducting perovskite Cs2SnI6 with a bandgap of 1.48eV and high absorption coefficient (over 105cm−1 from 1.7eV). More importantly, the Cs2SnI6 film, for the first time, is adopted as a light absorber layer for a lead-free perovskite solar cell and a preliminary estimate of the power conversion efficiency (PCE) about 1% with open-circuit voltage of 0.51V and short-circuit current of 5.41mA/cm2 is realized by optimizing the perovskite absorber thickness. According to the bandgap and the Shockley-Queisser limit, such inorganic perovskite solar cell with higher efficiency and pronounced stability can be expected by material quality improvement and device engineering.

Tin oxide nanosheets as efficient electron transporting materials for perovskite solar cells

Organolead halide perovskite solar cells with mesoporous metal oxide films as support layers have made great achievements in the field of solar cells in recent years. Here, tin oxide (SnO2) nanosheets are prepared by hydrothermal method. The as-synthesized SnO2 nanosheet layer is used as electron-transporting material to replace traditional TiO2 mesoporous layer in perovskite solar cells. The crystallizing time of CH3NH3PbI3 on SnO2 is investigated and optimized as 10min. The surface of SnO2 nanosheets is treated with TiCl4 aqueous solution, which improves the photovoltaic performance of the perovskite solar cells. The SnO2 nanosheet-based perovskite solar cell achieves excellent photovoltaic parameters: short-circuit current density of 17.58mAcm−2, open-circuit voltage of 1.01V, and power conversion efficiency of 7.81%.

(Invited) Real Time Analysis of the Crystallization Dynamics of Organilead Halide Perovskite

Organolead-halide perovskite solar cells have been attracting much attention because of the rapid improvement of its power conversion efficiency. Simple fabrication process can be advantageous for the cost reduction, where just mixing lead halide and amine halide results in the perovskite crystal. Although there have been published numbers of method for the fabrication of perovskite solar cells, reproducibility of the device characteristics is great issue. The fundamentals of crystallization process is important to improve the device performance as well as reproducibility, however, such studies are still underway. We have been conducted in-situ analysis of the crystallization process of organolead-halide perovskite by means of grazing incident wide angle X-ray scattering (GIWAXS) [1]. The measurement was performed at BL46XU in SPring-8 and 2 dimensional X-ray detector (PILATUS 300K) was used, which enables the real time GIWAXS measurement. Perovskite film was synthesized on the measurement stage of the beamline to observe the crystallization dynamics. A PbI2 thin film was placed and CH3NH3I solution was dropped onto the PbI2. The reduction in the amount of PbI2 raw material over time was observed as well as the formation process of the perovskite crystal. Analysis of the rate of progression of this reaction revealed that reaction progression did not follow the normal diffusion phenomenon but instead followed an anomalous diffusion process. This can be originated from the anomalous diffusion of CH3NH3I into the PbI2thin film medium, which shows the heterogeneous structure. Next we focus on the angle dependent X-ray diffraction patterns. In the early stage of the reaction the formed crystals were oriented in two specific directions, but with the progress of time they changed to a random orientation. This finding suggests that the crystals changed continuously during the process of formation of the perovskite crystal. The fluctuating characteristics of the crystallization process of perovskites, such as fractal penetration and orientational transformation, should be controlled to allow the fabrication of high-quality perovskite crystals. The characteristic reaction dynamics observed in this study should assist in establishing reproducible fabrication processes for perovskite solar cells. [1] T. Miyadera et al., Nano Lett., 2015, 15 (8), pp 5630–5634. This work is financially supported by the new energy and industrial technology development organization (NEDO) and Japan science and technology agency (JST) through its funding program for Precursory Research for Embryonic Science and Technology (PRESTO). Figure 1