Next-generation Photovoltaic Technology Research Articles

Crown Ethers with Different Cavity Diameters Inhibit Ion Migration and Passivate Defects toward Efficient and Stable Lead Halide Perovskite Solar Cells.

Organic-inorganic hybrid metal halide perovskite solar cells have been considered as one of the most promising next-generation photovoltaic technologies. Nevertheless, perovskite defects and Li+ ionic migration will seriously affect the power conversion efficiency and stability of the formal device. Herein, we designed two crown ether derivatives (PC12 and PC15) with different cavity diameters, which selectively bind to different metal cations. It is found that PC15 in perovskite precursor solution can actively regulate the nucleation and crystallization processes and passivate the uncoordinated Pb2+ ions, while PC12 at the interface between the perovskite layer and hole-transporting layer can effectively inhibit the migration of Li+ ions and reduce nonradiative recombination losses. Therefore, PC12 and PC15 can act as "lubricant" and defect passivators, as well as inhibitors of ion migration, when they are synergistically applied at the surface and bulk of perovskite layer. Consequently, the optimized device achieved a champion efficiency of 24.8% with significantly improved humidity, thermal, and light stability.

Tetracyclic aromatic lactam-substituted tin-free donors for high-efficiency ternary organic solar cells via regulation of pre-aggregation and crystallization kinetics

Organic solar cells (OSCs) are rapidly developed for next-generation photovoltaic technologies due to low-cost and solution-processability etc. However, so far high-efficiency organic photovoltaic materials are typically prepared by multi-step Stille reactions, which are neither cost-efficient nor atom-economic, even raise safety concerns and incompatible with large-scale process. Large-scale industrial production of photovoltaic materials through simple, cost-effective, and green chemistry remains to be addressed. In this work, we present a case where two isomeric small-molecule donors based on polycyclic aromatic lactams (PAL) unit, named oPh2F and pPh2F with wide-bandgap and high-crystallinity, were designed and synthesized by atom-economic direct C-H activation strategy, modulates pre-aggregation and crystallization kinetics of host system PM6:Y6, induce distinct morphology evolution and effectively boost photovoltaic performance. The best power conversion efficiency (PCE) of 18.40 % for pPh2F-based ternary devices are found to be a fruit of improved light-harvesting ability, favorable vertical phase separation with enhanced crystallinity, the separated crystallization process of host materials and the higher and more balanced charge transport. To our knowledge, this study achieved the highest PCE among PM6:Y6-based ternary OSCs containing tin-free small-molecule donor, which not only highlights the great potential of PAL-based photovoltaic materials, but also promotes the integration of renewable energy with green chemistry.

Beyond Planar Structure: Curved π-Conjugated Molecules for High-Performing and Stable Perovskite Solar Cells.

Perovskite solar cell (PSC) shows a great potential to become the next-generation photovoltaic technology, which has stimulated researchers to engineer materials and to innovate device architectures for promoting device performance and stability. As the power conversion efficiency (PCE) keeps advancing, the importance of exploring multifunctional materials for the PSCs has been increasingly recognized. Considerable attention has been directed to the design and synthesis of novel organic π-conjugated molecules, particularly the emerging curved ones, which can perform various unmatched functions for PSCs. In this review, the characteristics of three representative such curved π-conjugated molecules (fullerene, corannulene and helicene) and the recent progress concerning the application of these molecules in state-of-the-art PSCs are summarized and discussed holistically. With this discussion, we hope to provide a fresh perspective on the structure-property relation of these unique materials toward high-performance and high-stability PSCs.

Green Anisole as Antisolvent in Planar Triple-Cation Perovskite Solar Cells with Varying Cesium Concentrations.

The feasibility of replacing toxic chlorobenzene antisolvents with environmentally friendly anisole in the fabrication of planar triple-cation perovskite solar cells was explored here. The successful integration of anisole not only ensures comparable device performance but also contributes to the development of more sustainable and green fabrication processes for next-generation photovoltaic technologies. Nevertheless, to ensure the possibility of achieving well-functioning unencapsulated devices whose working operation depends on outdoor atmospheric conditions, we found that adjusting the cesium concentrations in the perovskite layers enabled the electrical characterization of efficient devices even under high relative humidity conditions (more than 40%). We found that 10% of CsI in the precursor solution will make devices with low hysteresis indexes and sustained performance stability over a 90-day period both with cholorobenzene and anisole antisolvent. These results further confirm that green anisole can replace chlorobenzene as an antisolvent.

Approaching Full-Scale Passivation in Perovskite Solar Cells via Valent-Variable Carbazole Cations

Hybrid halide perovskite solar cells (PSCs) have emerged as the next-generation photovoltaic technology. Compared to steady silicon solar cells, PSCs are facilely processable but easily generate defects/traps during the thin-film fabrication from the solution. To passivate these defects, which have been considered as the origin of PSC instability, numerous large-sized organic cations (LSOCs) were applied via post-treatment methods. Unfortunately, along with the passivation on defects, these LSOCs could also react with regular perovskite phases and convert them into layered perovskite phases with poorer optoelectronic performances. Herein, we have designed carbazole ethylammonium iodide (CzEAI), a LSOC salt which exhibits a variation from monovalent to divalent state. Importantly, unlike traditional LSOC passivators in monovalent states mostly consumed by a regular perovskite phase and merely affecting the upper domain in thin films, CzEA in monovalent state could penetrate through the whole domain in perovskite films and then accurately convert into a divalent state at defect sites and thus realize a full-scale passivation in PSCs. Both simulation and experimental results proved that a CzEA passivator could overcome the formation of poor optoelectronic layered perovskite phases. As a result, the CzEA passivated PSC demonstrated an optimized photon-to-electron conversion efficiency (PCE) of 24.14% together with a significantly improved long-term stability over 5000 h.

Enhanced near-infrared photoresponse for efficient organic solar cells using hybrid plasmonic nanostructures

Nonfullerene organic solar cells (OSCs) have recently made remarkable progress as one of the most promising next-generation photovoltaic technologies. Still, it is highly desirable to enhance the light harvesting for the sake of photoactive layer with a limited thickness. In this work, we developed efficient nonfullerene OSCs through incorporating hybrid plasmonic metal nanostructures (MNS) consisted of gold nanobipyramids (AuNBPs) and gold nanospheres (AuNSs). The synergistic localized surface plasmon resonance (LSPR) of hybrid MNS has been predesigned to perfectly match with the absorption range of nonfullerene photoactive layer especially in the near-infrared (NIR) region, subsequently delivering enhanced light harvesting and photo-response. The plasmonic enhancement mechanisms were systematically investigated by theoretical simulations and various experimental measurements, showing that hybrid MNS exhibited significant broadband near field enhancement and scattering effect as well as the advantage in electrical aspect to facilitate the charge extraction and transport. As a result, the power conversion efficiency (PCE) was improved from 15.46% to 16.62% for OSCs based on PM6:Y6 due to the synergistic plasmonic effect of hybrid MNS. Overall, this work paves the way for using plasmonic MNS with tunable optical properties as an alternative approach for the development of high-performance OSCs.

D 6 h Symmetric Radical Donor–Acceptor Nanographene Modulated Interfacial Carrier Transfer for High-Performance Perovskite Solar Cells

<i>D</i> ^{6 <i>h</i>} Symmetric Radical Donor–Acceptor Nanographene Modulated Interfacial Carrier Transfer for High-Performance Perovskite Solar Cells

Nonhalogenated Dual-Slot-Die Processing Enables High-Efficiency Organic Solar Cells.

Organic solar cells (OSCs) are promising candidates for next-generation photovoltaic technologies, with their power conversion efficiencies (PCEs) reaching 19%. However, the typically used spin-coating method, toxic halogenated processing solvents, and the conventional bulk-heterojunction (BHJ), which causes excessive charge recombination, hamper the commercialization and further efficiency promotion of OSCs. Here, a simple but effective dual-slot-die sequential processing (DSDS) strategy is proposed to address the above issues by achieving a continuous solution supply, avoiding the solubility limit of the nonhalogen solvents, and creating a graded-BHJ morphology. As a result, an excellent PCE of 17.07% is obtained with the device processed with o-xylene in an open-air environment with no post-treatment required, while a PCE of over 14% is preserved in a wide range of active-layer thickness. The unique film-formation mechanism is further identified during the DSDS processing, which suggests the formation of the graded-BHJ morphology by the mutual diffusion between the donor and acceptor and the subsequent progressive aggregation. The graded-BHJ structure leads to improved charge transport, inhibited charge recombination, and thus an excellent PCE. Therefore, the newly developed DSDS approach can effectively contribute to the realm of high-efficiency and eco-friendly OSCs, which can also possibly be generalized to other organic photoelectric devices.

Developments in Perovskite Materials Based Solar Cells: In Pursuit of Hysteresis Effect, Stability Issues and Lead-Free Based Perovskite Materials

Abstract: Over the past few years, significant advances in science and technology have occurred in the field of perovskite-based solar cells (PSC), which has sparked significant interest in next-generation photovoltaic technologies. Perovskite solar cells, which have a current certified power conversion efficiency of 25.5 %, are the first solution processed photovoltaic to outperform silicon-based photovoltaic technologies. Perovskite solar cells are comparable to Silicon-based solar cells due to their low-cost fabrication techniques and high efficiency. Nevertheless, the research community is still concerning about future design optimization, series degradation issues, stability, and practical efficiency restrictions. As a result, comprehensive knowledge of the perovskite solar cell's operating mechanism and operating principles is more important than ever before applying these technologies in the real world for future optimization. Recent research findings in the material science of innovative halide perovskites, as well as numerous architectures based on alternative materials for lead-free perovskites, band-gap engineering, impact of materials on various electron transport layers (ETL) and hole transport layers (HTL), the device instability and J-V hysteresis issues of perovskite solar cells are the focus of this study. In order to better understand the potential of perovskite solar cell, factors such as hysteresis-inducing factors, interface engineering, device stability, and a variety of recombination processes are being investigated. For future optimization of perovskite solar cells, the following review findings provide a clear focus for current research needs and future research directions to address issues and understand the working potential of the perovskite solar cell.

Device simulation of quasi-two-dimensional perovskite/silicon tandem solar cells towards 30%-efficiency

Perovskite/silicon (Si) tandem solar cells have been recognized as the next-generation photovoltaic technology with efficiency over 30% and low cost. However, the intrinsic instability of traditional three-dimensional (3D) hybrid perovskite seriously hinders the lifetimes of tandem devices. In this work, the quasi-two-dimensional (2D) (BA)2(MA) n – 1Pb n I3n + 1 (n = 1, 2, 3, 4, 5) (where MA denotes methylammonium and BA represents butylammonium), with senior stability and wider bandgap, are first used as an absorber of semitransparent top perovskite solar cells (PSCs) to construct a four-terminal (4T) tandem devices with a bottom Si-heterojunction cell. The device model is established by Silvaco Atlas based on experimental parameters. Simulation results show that in the optimized tandem device, the top cell (n = 4) obtains a power conversion efficiency (PCE) of 17.39% and the Si bottom cell shows a PCE of 11.44%, thus an overall PCE of 28.83%. Furthermore, by introducing a 90-nm lithium fluoride (LiF) anti-reflection layer to reduce the surface reflection loss, the current density (J sc) of the top cell is enhanced from 15.56 mA/cm2 to 17.09 mA/cm2, the corresponding PCE reaches 19.05%, and the tandem PCE increases to 30.58%. Simultaneously, in the cases of n = 3, 4, and 5, all the tandem PCEs exceed the limiting theoretical efficiency of Si cells. Therefore, the 4T quasi-2D perovskite/Si devices provide a more cost-effective tandem strategy and long-term stability solutions.

Avoiding Structural Collapse to Reduce Lead Leakage in Perovskite Photovoltaics

Perovskite solar cells (PSCs) have become a promising candidate for the next-generation photovoltaic technologies. As an essential element for high-efficiency PSCs however, the heavy metal Pb is soluble in water, causing a serious threat to the environment and human health. Due to the weak ionic bonding in three-dimensional (3D) perovskites, drastic structure decomposition occurs when immersing the perovskite film in water, which accelerates the Pb leakage. By introducing the chemically stable Dion-Jacobson (DJ) 2D perovskite at the 3D perovskite surface, the film dissolution is significantly slowed down, which retards lead leakage. As a result, the Pb contamination is dramatically reduced under various extreme conditions. In addition, the PSCs device deliver a power conversion efficiency (PCE) of 23.6 % and retain over 95 % of their initial PCE after the maximum power point tracking for over 1100 h.

Sub-nanometer atomic layer deposited Al2O3 barrier layer for improving stability of nonfullerene organic solar cells

Organic solar cells (OSCs) is a promising next-generation photovoltaic technology, however, the device stability remains to be the main barrier for its future commercialization. Herein, we reported the application of a sub-nanometer Al2O3 barrier layer in nonfullerene OSCs via atomic layer deposition (ALD), for the purpose of preventing metal ion diffusion from indium tin oxide (ITO) into the polymer layer caused by the corrosion of Poly(3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS). The thickness of the ALD-Al2O3 barrier layer was precisely optimized by controlling the number of ALD cycles (n) to achieve simultaneously good photoelectric properties and conformal coverage. An average power conversion efficiency (PCE) of 15.02% was demonstrated for the optimal OSCs with ALD-Al2O3 barrier layer. The above mentioned suppression of metal ion diffusion was experimentally confirmed by the cross sectional observations of transmission electron microscopy (TEM) and chemical mapping from energy-dispersive X-ray spectroscopy (EDX), resulting in a significantly improved operational stability with a device lifetime 3-times longer than that without ALD-Al2O3 barrier layer. Such ALD-assisted interface modification provides an effective approach to realize high-performance and stable OSCs.

Methylammonium- and bromide-free perovskites enable efficient and stable photovoltaics

Hybrid perovskite solar cell (PSC) has attracted extensive research interest due to its rapid increase in efficiency, regarding as one of the most promising candidates for the next-generation photovoltaic technology. The certified power conversion efficiency of the devices based on formamidinium lead iodide (FAPbI3) perovskite has reached 25.5%, approaching the record of monocrystalline silicon solar cells. Unfortunately, the black α-phase FAPbI3 materials can spontaneously transform to non-optically active δ-phase at room temperature, which greatly hinder their photovoltaic application. In order to overcome this problem, various strategies, especially introducing methylammonium (MA+), caesium (Cs+) and bromide (Br-) ions into the materials, have been widely adopted. However, MA+ can largely reduce the thermal stability of the materials. Furthermore, the introduction of Br- can enlarge the materials’ bandgap, resulting in a reduced theoretical efficiency. Keeping these in mind, developing the strategies which without using MA+ and Br- is the inevitable trend. Here, we focus on the recent progresses of stabilizing α FAPbI3 without employing MA+ and Br-, and discuss the advantages of inorganic ions doping and dimensionality engineering to stabilized α FAPbI3. Meanwhile, in order to deeply understand the relationship between the semiconducting properties and device performance of the corresponding materials, we then summarize several significant strategies to suppress the non-radiation recombination, such as interface modification and trap passivation. Finally, we propose to develop more effective ‘A-site’ alternatives to stabilize α FAPbI3, which is expected to achieve high-efficient PSCs with long-term stability, facilitating its commercialization process.

Roles and Impacts of Ancillary Materials for Multi-Component Blend Organic Photovoltaics towards High Efficiency and Stability.

Organic photovoltaics (OPVs) are a promising next-generation photovoltaic technology with great potential for wearable and transparent device applications. Over the past decades, remarkable advances in device efficiency close to 20 % have been made for bulk heterojunction (BHJ)-based OPV devices with long-term stability, and room for further improvements still exists. In recent years, ancillary components have been demonstrated as effective in improving the photovoltaic performance of OPVs by controlling the optoelectronic and morphological properties of BHJ blends. Herein, an updated understanding of polymer-based blend OPVs is provided, and the role and impact of ancillary components in various blend systems are categorized and discussed. Lastly, a strategic perspective on the ancillary components of blend-based OPVs for commercialization is provided.

Narrow-Bandgap Semiconductors of Perovskite Rare-Earth Orthoferrites (REFeO3)

Background: Great achievements have been made in improving the power conversion efficiencies of solar cells. However, the price of photovoltaic (PV)-generated electricity still cannot compete with that of conventionally generated power. Thus, novel solar cells made from non-toxic, earth-abundant, and chemically stable materials are desirable to decrease the costs of PV electricity generation. Objective: All-oxide solar cells are a promising next-generation PV technology, fulfilling the requirement of low-cost manufacturing under ambient conditions. This work aims to search for nontoxic, earth-abundant, and chemically stable narrow-bandgap semiconductors for energy applications like all-oxide solar cells. Methods: This work examines a family of perovskite rare-earth (i.e., Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Yb) orthoferrites with XRD analysis, SEM, photoresponse, absorption spectra, Xray photoelectron spectroscopy, fluorescence spectra measured with the laser Raman spectrometer, valence band spectra, the Kelvin probe, the Hall Effect experiment, and theoretical calculation on band structure and the density of states, to screen for the narrow-bandgap semiconductors. Results: The novel Pb-free perovskite narrow-bandgap absorbers, CeFeO3, PrFeO3, TbFeO3, DyFeO3, HoFeO3, and YbFeO3, which are earth-abundant and non-toxic, were screened out potentially for use in all-oxide solar cells or other photovoltaic and optoelectronic applications. Among them, YbFeO3 is approved, having an indirect bandgap of approximately 1.0 eV with a maximum Shockley-Queisser efficiency of 31% for single p-n junction solar cells. Conclusion: The chemically stable, non-toxic, earth-abundant, and narrow-bandgap semiconductors of rare-earth orthoferrites promisingly serve as absorbers, photocatalysts, photoelectrodes, photodetectors, and photoelectronic materials. This work breaks new ground in the search for narrow bandgap all-oxide semiconductors potential for energy applications.

Room-temperature carbon electrodes with ethanol solvent interlacing process for efficient and stable planar hybrid perovskite solar cells

Among the next-generation photovoltaic technologies, perovskite solar cells (PSCs) have attracted significant attention and interest. However, PSCs typically use high-vacuum processed metal electrodes, which are considerably more expensive. Furthermore, this process using noble metals, results in dramatic performance degradation after halide ion migration and thermal stress. Therefore, replacing metal electrodes with carbon electrodes is a promising way to address the stability issues of PSCs. This is due to carbon materials possessing unique qualities, such as low cost, high stability, good conductivity, and inherent water resistance. In contrast, some solvents in commercial carbon paste cause damage to the structure of perovskite films. Herein, room-temperature carbon films were successfully prepared utilizing an ethanol solvent interlacing process with time optimization. The resulting carbon films exhibited good flexibility inducing tight adhesion onto the hole transporting layer as well as excellent electrical properties. The carbon films were then applied as working electrodes in PSCs structured as FTO/TiO2/Cs0.17FA0.83Pb(I0.83Br0.17)3/spiro-OMeTAD/carbon. By optimizing time, in the ethanol solvent interlacing process, an excellent power conversion efficiency (PCE) was achieved. The devices improved up to 12.2% using the soaked carbon films at 2 h. Moreover, the device exhibits excellent long-term stability of 80% over 1000 h in air environment without encapsulation. Finally, this simple and effective technique proposes an alternate choice for developing efficient carbon-based PSCs.

Low-frequency carrier kinetics in triple cation perovskite solar cells probed by impedance and modulus spectroscopy

Organometallic halide perovskites-based solar cells have emerged as next-generation photovoltaic technology. However, many of its intriguing optoelectronic properties at low frequency are highly debated. Here, we investigate the low-frequency carrier kinetics of the state-of-the-art triple cation perovskite Cs0.06FA0.79MA0.15Pb(I0.85Br0.15)3 solar cells using bias-dependent impedance and modulus spectroscopy under dark and illumination conditions. We observe a strong dependence of dielectric permittivity on frequency in 1 Hz to 1 MHz region with a dielectric relaxation, which is observed to follow the Maxwell-Wagner type interfacial polarization possibly originating from the grain boundary/ionic defects. Furthermore, correlating the impedance and modulus spectra reveals the localized charge carrier relaxation in this triple cation device from which we obtain a phenomenological picture of the hopping process and developing an understanding of the charge carrier kinetics in these high-efficiency perovskite photovoltaics.

Pseudo-bilayer architecture enables high-performance organic solar cells with enhanced exciton diffusion length

Solution-processed organic solar cells (OSCs) are a promising candidate for next-generation photovoltaic technologies. However, the short exciton diffusion length of the bulk heterojunction active layer in OSCs strongly hampers the full potential to be realized in these bulk heterojunction OSCs. Herein, we report high-performance OSCs with a pseudo-bilayer architecture, which possesses longer exciton diffusion length benefited from higher film crystallinity. This feature ensures the synergistic advantages of efficient exciton dissociation and charge transport in OSCs with pseudo-bilayer architecture, enabling a higher power conversion efficiency (17.42%) to be achieved compared to those with bulk heterojunction architecture (16.44%) due to higher short-circuit current density and fill factor. A certified efficiency of 16.31% is also achieved for the ternary OSC with a pseudo-bilayer active layer. Our results demonstrate the excellent potential for pseudo-bilayer architecture to be used for future OSC applications.

Efficient and stable perovskite solar cells via surface passivation of an ultrathin hydrophobic organic molecular layer

Organic and inorganic metal halide perovskite solar cells (PSCs) with an outstanding performance of low-cost manufacturing and low-temperature solution processing, have been regarded as the next-generation photovoltaic technology. However, the solution processed perovskite films usually have abundant defects, especially at the grain boundaries and surfaces, causing carrier recombination, hysteresis, and degradation. Herein, 4-(trifluoromethyl)benzylamine (TFMBA) is deposited on the surface of perovskites to form an ultrathin hydrophobic organic molecular layer, passivating the interfacial imperfections. By optimizing the post-treatment conditions, a remarkable power conversion efficiency (PCE) of 20.56% is realized based on Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 perovskite. Impressively, the passivated ultrathin organic molecular layer effectively prohibits the penetration of moisture, resulting in unsealed PSCs retaining 84% of the original PCE after aged 1300 h at 65%−75% relative humidity (RH). SEM and XRD confirm that TFMBA can effectively passivate defects on the surface and grain boundaries, thereby prevent the perovskites from the irreversible decomposition with the immersion of moisture. This suggests that the TFMBA post-treatment has great potential in obtaining high-quality and stable perovskite films.

Cationic polyelectrolytes as convenient electron extraction layers in perovskite solar cells

The potential of hybrid organic-inorganic perovskite solar cells (PSCs) as a next-generation photovoltaic technology has attracted enormous research interest due to their remarkably low cost and outstanding photo-physical properties. Increasing their power conversion efficiency (PCE) using materials which are similarly low-cost as the perovskite layer remains a critical issue for their commercialization; interlayers play important roles in elevating the PCEs of PSCs and the commonly used interlayers used in PSCs are typically orders of magnitude more costly than the perovskite layer itself. Herein, we report the effect of polyelectrolytes on the energy band structure and device characteristics when they are incorporated as electron extraction layers (EELs) of photovoltaics with both N–I–P and P–I–N geometries. To tune and improve device performance, we used cationic nonconjugated polyelectrolytes (NPEs) based on the poly(ethyleneimine) (PEI) backbone with different anions including bromide (Br−), iodide (I−), and tetrakis (imidazole) borate (BIm4-). This series of NPEs formed electric dipoles at the NPE/metal electrode interfaces; consequently allowing tuning of the energy levels, and work functions (WFs) of the electrodes. Notably, the PCE could be improved from 8.11 to 14.71% in the case of the NIP geometry, or from 7.40 to 13.79% in the case of the P–I–N geometry using a PEIH+BIm4− interlayer. Ultraviolet photoelectron spectroscope (UPS) results reveal that all of the polyelectrolytes, and particularly PEIH+BIm4− effectively decreased the WFs of the metal electrodes. Interestingly, a tunable dipole was achieved on the conducting surfaces (e.g. both cathode and anode electrodes) modified with electrolytes by simply varying the identity of counterions, as verified by the significantly reduced effective WF. These n-Type electrolytes created Ohmic contacts between the electrodes and the perovskite layer. These findings demonstrate that NPEs are effective EELs for high efficiency PSCs.