Electrodes For Perovskite Solar Cells Research Articles

Effect of indium ratio in CuInxGa1-xS2/carbon hole collecting electrode for perovskite solar cells

Perovskite solar cells (PSCs) have excellent photovoltaic properties. There are, however, challenges of materials cost and device stability to be solved before commercializing them. Utilizing low-cost inorganic hole transport materials (HTM) as a replacement for spiro-OMeTAD, and replacing the Au electrode with printable carbon could be important steps in this regard. For this purpose, CuInxGa1-xS2 (x = 1, 0.75, 0.5, 0.25, 0) nanoparticle layers are deposited as inorganic HTMs with carbon composite electrode as the back electrode. Photovoltaic properties of PSCs with CuInxGa1-xS2/Carbon hole collecting electrodes are studied by changing the In ratio in the HTM layer. Results from impedance spectroscopy show that by decreasing x value the charge transfer resistance at the interface of hole collector and perovskite layer decrease which matches the downward trend of fill factor of the corresponding cells. The efficiency of PSCs increases during the first week, while, on average, their efficiency drop after one month is less than 5% of the initial values. The best efficiency of 16.45% has been obtained for CuIn0.75Ga0.25S2/carbon hole collectors.

Transparent Ultrathin Metal Electrode with Microcavity Configuration for Highly Efficient TCO-Free Perovskite Solar Cells.

Optical microcavity configuration is one optical strategy to enhance light trapping in devices using planar electrodes. In this work, the potential application of optical microcavity configuration with ultrathin metal electrodes in highly efficient perovskite solar cells (PSCs) was investigated. By comparing with the device with conventional indium-tin-oxide (ITO) electrodes, it is shown that by carefully designing the Ag/dielectric planar electrode, a device with an optical microcavity structure can achieve comparable—or even higher—power conversion efficiency than a conventional device. Moreover, there is a relative high tolerance for the Ag film thickness in the optical microcavity structure. When the thickness of the Ag film is increased from 8 to 12 nm, the device still can attain the performance level of a conventional device. This gives a process tolerance to fabricate devices with an optical microcavity structure and reduces process difficulty. This work indicates the great application potential of optical microcavities with ultrathin metal electrodes in PSCs; more research attention should be paid in this field.

Facile synthesis of “lucky clover” hole-transport material for efficient and stable large-area perovskite solar cells

Hole-transporting materials (HTMs) play a vital role of transporting holes from the perovskite layer to the counter electrode in perovskite solar cells (PSCs). A novel HTM BTPA-8 is feasibly synthesized by incorporating four dimethoxytriphenylamine leaflets and anthracene-based central bridge. BTPA-8 exhibits a suitable band alignment with MAPbI3 (MA = CH3NH3) or FA0.85MA0.15PbI3 (FA=HC(NH2)2), high hole mobility, and high thermal stability. The best FA0.85MA0.15PbI3 device based on BTPA-8 exhibits a power conversion efficiency (PCE) of 17.99% in the reverse scan with an aperture area of 0.09 cm2. Under the same condition, a PCE of 18.92% is achieved by the solar cell based on the standard spiro-OMeTAD (2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene). A comparable PCE of 12.31% is also obtained for BTPA-8 based MAPbI3 device compared with spiro-OMeTAD (13.25%) with an aperture area >1 cm2. BTPA-8 based PSCs exhibit better long-term stability than spiro-OMeTAD due to its high hydrophobicity. A lower synthesis cost of BTPA-8 than that of spiro-OMeTAD along with the elevated long-term stability makes it promising for application in PSCs.

Recycled Utilization of a Nanoporous Au Electrode for Reduced Fabrication Cost of Perovskite Solar Cells.

Perovskite solar cells (PSCs) using metal electrodes have been regarded as promising candidates for next‐generation photovoltaic devices because of their high efficiency, low fabrication temperature, and low cost potential. However, the complicated and rigorous thermal deposition process of metal contact electrodes remains a challenging issue for reducing the energy pay‐back period in commercial PSCs, as the ubiquitous one‐time use of a contact electrode wastes limited resources and pollutes the environment. Here, a nanoporous Au film electrode fabricated by a simple dry transfer process is introduced to replace the thermally evaporated Au electrode in PSCs. A high power conversion efficiency (PCE) of 19.0% is demonstrated in PSCs with the nanoporous Au film electrode. Moreover, the electrode is recycled more than 12 times to realize a further reduced fabrication cost of PSCs and noble metal materials consumption and to prevent environmental pollution. When the nanoporous Au electrode is applied to flexible PSCs, a PCE of 17.3% and superior bending durability of ≈98.5% after 1000 cycles of harsh bending tests are achieved. The nanoscale pores and the capability of the porous structure to impede crack generation and propagation enable the nanoporous Au electrode to be recycled and result in excellent bending durability.

Effect of Surface Treatment by Chemical-Mechanical Polishing for Transparent Electrode of Perovskite Solar Cells

Perovskite solar cells (PSCs) are usually fabricated by using the spin coating method. During the fabrication process, the surface status is very important for energy conversion between layers coated in the substrate. PSCs have multilayer-stacked structures, such as the transparent electrode layer, the perovskite layer, and a metal electrode. The efficiency and uniformity of all layers depend on the surface status of the transparent electrode coated on the glass substrate. Until now, etching methods by chemical processes have been introduced to make the substrate surface smooth and uniform by decreasing surface roughness. However, highly reactive chemical treatments can be harmful to the environment. In this study, we employed an eco-friendly chemical-mechanical polishing (CMP) process to ensure the fluorine-doped tin oxide (FTO) substrate is treated with a smooth surface. Before the perovskite layer and electron transport layer (ETL) are applied, the TiO2 layer is coated with the FTO substrate, and the surface of the FTO substrate is polished using CMP. As a result, the CMP-treated surface of the FTO substrate showed a smooth surface, and the PSCs with CMP treatment did not require conventional TiCl4 treatment.

Low-temperature carbon-based electrodes in perovskite solar cells

This work summarizes recent progress of low-temperature carbon-based electrodes in perovskite photovoltaic devices and their great up-scalability potential.

Effect of Metal Electrodes on Aging-Induced Performance Recovery in Perovskite Solar Cells.

For commercialization of perovskite solar cells (PSCs), it is important to substitute the alternative electrode for Au to decrease the unit cost. From the early stage, Ag exhibits a potential to be a good counter electrode in PSCs; however, there is an abnormal s-shaped J-V curve with the Ag electrode, and it is recovered as time passes. The perception of the aging-induced recovery process and refutation of the raised stability issues are required for commercial application of Ag electrodes. Herein, we compared the aging effect of PSCs with Ag and Au electrodes and found that only devices with Ag electrodes have a dramatical aging-induced recovery process. We observed the change of photoelectronic properties only in the devices with Ag electrodes as time passes, which mainly contributes to recovery of the s-shaped J-V curve. We verified the work function change of an aged Ag electrode and its mechanism by photoelectron spectroscopy analysis. By comparing the light stability under 1 sun intensity illumination, we can assure the practical stability of Ag electrodes in case of being encapsulated. This work suggests the profound understanding of the aging-induced recovery process of PSCs and the possibility of commercial application of Ag electrodes.

Development of Highly Bendable Transparent Window Electrodes Based on MoOx, SnO2, and Au Dielectric/Metal/Dielectric Stacks: Application to Indium Tin Oxide (ITO)-Free Perovskite Solar Cells

Indium-doped tin oxide (ITO) is the transparent conductive material of choice for a wide range of optoelectronic devices such as sensors, light-emitting diodes and solar cells. However, its brittle nature, high cost, scarcity as well as aggressive deposition via sputtering determine the need to find cheap alternatives with high optical transparency, low sheet resistance, and mechanical flexibility. Dielectric/metal/dielectric (D/M/D) electrodes fulfill all these requirements and are deposited via low embodied energy low-temperature processing. We developed D/M/D multilayered electrodes based on thermally evaporated MoOx or solution-processed SnO2 seed layers, a thermally evaporated ultrathin Au film, and a spin-coated SnO2 top layer on rigid glass substrates. We first systematically unraveled the role of each layer on the resistance-transmittance properties of the full D/M/D electrode structure. By optimizing the thickness of the seed, metal and the top layer, we obtained electrodes with transmittance of 72% at 550 nm and a minimum sheet resistance of 9  sq-1. Subsequently, these optimized multilayered stacks were employed as bottom electrodes for perovskite solar cells with glass/D/M/D/mesoporous-TiO2/CH3NH3PbI3/spiro-MeOTAD/Au device architecture, delivering power conversion efficiencies (PCE) of 10.7%. Further, we deposited and characterized D/M/D electrodes on flexible polyethylene terephthalate (PET) films, achieving a maximum PCE of 7.6%. The difference in performance compared to rigid glass devices can be ascribed to the different wetting of the active layer on PET substrates. Flexible D/M/D electrodes displayed excellent mechanical properties compared to commercial PET/ITO, showing completely stable sheet resistance after repeated bending even down to 1.5 mm of curvature radius, whereas PET/ITO showed one order of magnitude increase in sheet resistance in the same mechanical test, due to formation of cracks in the conductive oxide. Our optimized D/M/D stacks on glass and especially on PET or other flexible substrates are therefore excellent alternatives to ITO as transparent window electrodes for low-cost, light-weight and conformal optoelectronics applications.

Enhanced Lifetime and Photostability with Low-Temperature Mesoporous ZnTiO3 /Compact SnO2 Electrodes in Perovskite Solar Cells.

Perovskite solar cells (PSCs) with power conversion efficiencies (PCEs) of 25 % mainly have SnO2 or TiO2 as electron-transporting layers (ETLs). Now, zinc titanate (ZnTiO3 , ZTO) is proposed as mesoporous ETLs owing to its weak photo-effect, excellent carrier extraction, and transfer properties. Uniform mesoporous films were obtained by spinning coating the ZTO ink and annealed below 150 °C. Photovoltaic devices based on Cs0.05 FA0.81 MA0.14 PbI2.55 Br0.45 perovskite sandwiched between SnO2 -mesorporous ZTO electrode and Spiro-OMeTAD layer achieved the PCE of 20.5 %. The PSCs retained more than 95 % of their original efficiency after 100 days lifetime test without being encapsulated. Additionally, the PSCs retained over 95 % of the initial performance when subjected at the maximum power point voltage for 120 h under AM 1.5 G illumination (100 mW cm-2 ), demonstrating superior working stability. The application of ZTO provides a better choice for ETLs of PSCs.

Control of TiO2 electron transport layer properties to enhance perovskite photovoltaics performance and stability

Abstract This study demonstrates the effect of electron collection and transportation for TiO2 electron transport layer (ETL) of the mesoscopic perovskite solar cells (PSCs). The influence of compact TiO2 layer (c-TiO2) with various spray cycles, the particle size effect of mesoporous TiO2 (meso-TiO2) film and post-treatment of TiO2 electrode for perovskite solar cells have been studied systematically. We further optimize the meso-TiO2 thickness to enhance the electron collection and transport efficiency and to reduce the anomalous J-V hysteresis phenomenon of PSCs. After adjusting the fabrication process of TiO2 ETL, the highest performance of small cell PSC shows the power conversion efficiency (PCE) of 19.39% in the reverse scan and 19.12% in the forward scan, respectively. A sub-module PSC within an active area of 11.7 cm2 exhibits impressive PCE of 16.03% under the illumination of 100 mW/cm2 (AM1.5G). Moreover, it also shows an outstanding PCE of 25.49% under the illumination of 6000 lx of T5 indoor light source.

Ethanol vapor phase reduced electrospun CuO NWs networks as transparent electrodes in perovskite solar cells

Abstract Metal nanowires have become an inevitable candidate for the next generation transparent conductive electrodes due to the high conductivity, transparency and flexibility. Here, copper nanowires (Cu NWs) with good uniformity were prepared in sequence by electrospinning technology, calcination and ethanol vapor phase reduction. Through adjusting vapor temperature accurately, the optimized Cu NWs with large aspect ratio of above 105, a low sheet resistance down to 51.4 Ω/口and a high transmittance of 80% in visible spectra were prepared. Inverted planar heterojunction perovskite solar cells with Cu NWs network as transparent electrodes were designed and a power conversion efficiency of 5.32% was achieved when the coverage of Cu NWs was 22%.

Electrodes: High‐Performance Solution‐Processed Double‐Walled Carbon Nanotube Transparent Electrode for Perovskite Solar Cells (Adv. Energy Mater. 27/2019)

Double-walled carbon nanotubes are between single-walled carbon nanotubes and multi-walled carbon nanotubes. They are comparable to single-walled carbon nanotubes with respect to the light optical density, but their mechanical stability and solubility are higher. In article number 1901204, Mansoo Choi, Shigeo Maruyama, Yutaka Matsuo, and co-workers demonstrate a solution-processed double-walled carbon nanotube transparent electrode in perovskite solar cells exhibiting an operating power conversion efficiency of 17.2%.

Ultraflexible and Lightweight Bamboo-Derived Transparent Electrodes for Perovskite Solar Cells.

Wearable devices are mainly based on plastic substrates, such as polyethylene terephthalate and polyethylene naphthalate, which causes environmental pollution after use due to the long decomposition periods. This work reports on the fabrication of a biodegradable and biocompatible transparent conductive electrode derived from bamboo for flexible perovskite solar cells. The conductive bioelectrode exhibits extremely flexible and light-weight properties. After bending 3000 times at a 4 mm curvature radius or even undergoing a crumpling test, it still shows excellent electrical performance and negligible decay. The performance of the bamboo-based bioelectrode perovskite solar cell exhibits a record power conversion efficiency (PCE) of 11.68%, showing the highest efficiency among all reported biomass-based perovskite solar cells. It is remarkable that this flexible device has a highly bendable mechanical stability, maintaining over 70% of its original PCE during 1000 bending cycles at a 4 mm curvature radius. This work paves the way for perovskite solar cells toward comfortable and environmentally friendly wearable devices.

Solution-processed bathocuproine cathode buffer layer towards efficient planar heterojunction perovskite solar cells

Solution process boosts organic and hybrid organic–inorganic electronic devices towards low-cost, scalable and high-throughput production. Facilely solution-processed bathocuproine (sBCP) is innovatively introduced as an effective cathode buffer layer (CBL) sandwiched between the electron transport layer of (6, 6)-phenyl-C61-butyric acid methyl ester (PCBM) and Ag electrode in perovskite solar cells (PSCs) with structure of ‘indium-tin-oxide/poly(3, 4-ethylenedioxythiophene):poly(styrenesulfonate)/CH3NH3PbI3/PCBM/sBCP/Ag’. The effect of sBCP concentration on PSC performance is intensively investigated. Experiments show that sBCP with optimum concentration extremely favors filling voids at the perovskite/PCBM film, which improves surface morphology and ameliorates interface characteristics of PCBM/Ag. It also contributes to the enhancement of power conversion efficiency, owing to the promotion in short circuit current density and fill factor. Impressive power conversion efficiencies of 12.47% average value and 13.24% maximum value with about 60% increment in relative to the reference device without CBL are achieved. Moreover, the incorporation of sBCP effectively blocks diffusion of Ag atoms inside the PSCs, which reasonably accounts for long-term stability of the PSCs. Our results open up an alternative approach for constructing high-performance PSCs with solution process and advancing their applications.

Carbon Nanotubes to Outperform Metal Using Dopant Engineering and Hole-Transporting Layer Control in Perovskite Solar Cells

Lead halide perovskite solar cells (PSCs) are a promising alternative energy source that has received a considerable attention in recent year. Their certified power conversion efficiencies (PCEs) now exceed 20% and only a few challenges, such as long-term stability and cost of fabrication, remain before commercialization. PSCs in general have a structure in which a photo-active material and two charge-selective materials are sandwiched by a transparent bottom electrode and a metal top electrode. Typically, the metal electrodes are thermally evaporated in vacuum, which incur substantial increase in fabrication costs and material costs. Further, using these metal electrodes induces the ion-migration, which is detrimental to the long-term stability of the perovskite layer. Thus, it is crucial that we find an alternative to the metal electrodes to bring PSCs to the next level. Having high conductivity and facile processability, aerosol synthesized single-walled carbon nanotubes (CNTs) have showcased promising potential as the top electrode in PSCs. Using CNTs as the top electrode drastically improves the PSC stability by both removing the ion migration and functioning as an effective moisture barrier. Further, CNTs are mechanically and chemically robust which contribute to the durability of the PSCs. Moreover, the fact that CNTs are made up of earth-abundant carbon atoms only and the aerosol-synthesized CNT films are direct-transferable means the low-cost production. The only shortcoming of employing the CNTs as the top electrode in PSCs, this far, has been the limited PCE. This is because CNTs do not reflect light unlike metals and doping the CNT top electrodes without damaging the layers underneath is extremely difficult. Recently, we reported a vapor-assisted ex-situdoping method using trifluoromethanesulfonic acid (TFMS), to dope the CNT top electrode in PSCs with no damage to the materials underneath and reported the record-high PCE of 17.56%. However, the vapor doping was too weak as a long exposure to the TFMS vapor resulted in a reaction between TFMS and 4-tert-butylpyridine (t-BP). Also, it was difficult to control the exposure time exactly everytime. Therefore, it is necessary to find a more controlled and effective doping technology. The key here is to develop a new doping method that leads to the minimal interaction with t-BP while demonstrating the maximum doping effect. Herein, we report CNT-laminated PSCs in which optimized concentration of TFMS dopant in an apolar solvent was applied by a drop-casting method to increase the conductivity of the CNTs while avoiding the reaction with t-BP and the perovskite layer underneath. The choice of apolar solvent was chlorobenzene which exhibited superior doping effect and stability compared with other solvents. Furthermore, higher concentration of spiro-MeOTAD was used in the CNT-laminated PSCs by exploiting the high mobility of the porous CNT network. The combination of those two led to a PCE of 18.5%, which is comparable to 18.3% of the metal electrode-based PSCs. The CNT-laminated devices demonstrated a stability time of more than 1000 operating hours, which is by far greater than that of the reference devices. Figure 1

(Invited) Lithium-Ion Endohedral Fullerenes on Carbon Nanotube Electrode-Laminated Perovskite Solar Cells As Dopants and Anti-Oxidants

Lead halide perovskite solar cells (PSCs) have become an established photovoltaic technology heading towards commercialisation with exceptionally high conversion efficiency (PCE). Although PSCs show high PCEs, the device stability needs to be substantially improved if PSCs are to be commercialized. There are mainly two factors that are responsible for the low stability of PSCs; they are 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobi-fluorene (spiro-MeOTAD) hole-transporting layer (HTL) and metal electrode. Use of hygroscopic lithium bis(trifluoromethanesulfonyl)imide (Li+TFSI−) in spiro-MeOTAD results in an uncontrolled oxidation of spiro-MeOTAD as well as moisture-driven degradation. Also, using metal electrodes induces a metal ion-driven degradation, in which metal ions on top migrate into the perovskite layer, lowering the PSC stability. As a solution to the Li+TFSI−-driven degradation, we reported lithium-ion-containing [60]fullerene trifluoromethanesulfonylimide salt ([Li+@C60]TFSI−). [Li+@C60]TFSI−induced an instant oxidation of spiro-MeOTAD producing spiro-MeOTAD•+TFSI− and neutral [Li+@C60]•−(= Li@C60), which functioned as an antioxidant, protecting PSCs from intruding oxygen. With the controlled production of spiro-MeOTAD•+TFSI−and anti-oxidation activity of Li+@C60 •−, the stability of PSCs improved by 10-fold compared with the reference devices. However, the PCE of the PSCs was limited by the lack of spiro-MeOTAD•+TFSI−production and aggregated morphology of HTL, both of which arise from the low solubility of [Li+@C60]TFSI−and Li@C60. Moreover, the stability can be improved even further by removing the metal ion migration from the top metal electrode. Replacing the metal electrode by carbon electrode has been reported to be one of the most effective ways to enhance the device stability of PSCs due to no ion migration, and outstanding encapsulation effect. Among the carbon electrodes, the application of free-standing carbon nanotubes (CNTs) has given the highest PCEs with the use of spiro-MeOTAD HTM. However, it is reported that the use of spiro-MeOTAD limits the full potential of the CNT-PSCs in terms of long-terms stability. Combining the two technologies described above can provide a synergic and ultimate solution to the PSC stability. Therefore, we incorporated the mixture of spiro-MeOTAD and [Li+@C60]TFSI−into the CNT top electrode in PSCs. The HTL solution is typically drop-casted onto the CNT network in CNT-PSCs. When the mixture of spiro-MeOTAD and [Li+@C60]TFSI−was drop-casted, a saturated solution seeped through the CNT network while any undissolved [Li+@C60]TFSI−and Li@C60suspensions stayed on the top. This led to more effective hole extraction by avoiding [Li+@C60]TFSI−in the pathway and more effective anti-oxidation activity by placing Li@C60next to air. Since the drop-casting on CNTs separated the dissolved species from the undissolved species, the oxidation reaction of spiro-MeOTAD came to a stop. Thus, the stirring time of the [Li+@C60]TFSI−-containing spiro-MeOTAD solution was directly linked to the photovoltaic performance and stability of the PSCs. From various analyses and investigation, we found that 2 hour stirring for the HTL solution gave the highest PCE of 17% and the longest operating stability of 1000 hours when unencapsulated. The PCE value of the new devices is close to the PCE of gold electrode-based PSCs (18.5%) while the PCE stability is approximately 100 times greater. Such excellent stability is attributed to no ion-migration and antioxidant activity of Li@C60uniformly covering the CNT electrode. Not only did we demonstrate highly stable and efficiency CNT-PSCs but also discovered a new reaction mechanism within the spiro-MeOTAD and [Li+@C60]TFSI−HTL solution. Figure 1

Current progress in interfacial engineering of carbon-based perovskite solar cells

Low cost carbon paste using as the back electrode for perovskite solar cells (PSCs), interfacial engineering plays a crucial role in both bi-interfacial structure and tri-interfacial structure.

Facile fabrication of highly efficient ETL-free perovskite solar cells with 20% efficiency by defect passivation and interface engineering.

Tetramethylammonium hydroxide (TMAH) is employed to modify the surface and electrical properties of fluorine-doped tin oxide (FTO) electrodes in perovskite solar cells. Synchronously, owing to the flow of unbound TMA+ ions into the perovskite, the trap density of the perovskite overlayer is largely reduced. Conductivity of the grain boundaries in the perovskite layer is also greatly increased. A high efficiency of 20.1% along with a reduced J-V hysteresis in our champion perovskite solar cells without electron transport layers is achieved.

Vapor-Assisted Ex-Situ Doping of Carbon Nanotube toward Efficient and Stable Perovskite Solar Cells.

Single-walled carbon nanotubes (CNTs) has been considered as a promising material for a top electrode of perovskite solar cells owing to its hydrophobic nature, earth-abundance, and mechanical robustness. However, its poor conductivity, a shallow work function, and nonreflective nature have limited further enhancement in power conversion efficiency (PCE) of top CNT electrode-based perovskite solar cells. Here, we introduced a simple and scalable method to address these issues by utilizing an ex-situ vapor-assisted doping method. Trifluoromethanesulfonic acid (TFMS) vapor doping of the free-standing CNT sheet enabled tuning of conductivity and work function of the CNT electrode without damaging underneath layers. The sheet resistance of the CNT sheet was decreased by 21.3% with an increase in work function from 4.75 to 4.96 eV upon doping of TFMS. In addition, recently developed 2D perovskite-protected Cs-containing formamidium lead iodide (FACsPbI3) technology was employed to maximize the absorption. Because of the lowered resistance, better energy alignment, and improved absorption, the CNT electrode-based PSCs produced a PCE of 17.6% with a JSC of 24.21 mA/cm2, VOC of 1.005 V, and FF of 0.72. Furthermore, the resulting TFMS-doped CNT-PSCs demonstrated higher thermal and operational stability than bare CNT and metal electrode-based devices.

Effect of annealing temperature on the performance of printable carbon electrodes for perovskite solar cells

Abstract Perovskite solar cells (PSCs) with mesoporous TiO2 electron transport layers have reached >22% efficiency at laboratory scale (