Chen Weijie

Publication date: 18 January 2023

Source: Joule, Volume 7, Issue 1

Author(s): Min Ju Jeong, Chan Su Moon, Seungmin Lee, Jeong Min Im, Mun Young Woo, Jun Hyeok Lee, Hyeonah Cho, Soo Woong Jeon, Jun Hong Noh

A new method allows calculating the efficiency of four-terminal perovskite/crystalline-Si tandem cells using the detailed-balance efficiency fraction of the corresponding single-junction solar cells. The method is experimentally validated with 28.0% efficient tandem cells and literature data. Based on present-day records for perovskite and crystalline-Si cells, the model predicts a practical efficiency limit for four-terminal tandem cells of 36%.

Recently perovskite/crystalline Si (cSi) tandem cells draw considerable attention because their high efficiency can reduce the levelized cost of electricity and increase the power density of photovoltaics (PVs) to accelerate the energy transition. While the theoretical limits for tandem cells are well known, the practical limits are less clear. Herein, a new method is presented to calculate the efficiency of a four-terminal (4T) tandem based on the performance of single-junction perovskite and cSi cells, using their detailed-balance efficiency fraction. This calculation method is validated with experiments on 4T perovskite/cSi tandem cells that provide a maximum efficiency of 28.0% and with the literature data available for similar configurations. A maximum efficiency of about 36% is estimated for 4T perovskite/cSi tandem cells that would use present record perovskite and cSi PV cells. This can be regarded as the practical efficiency limit for 4T perovskite/cSi tandem devices.

Hexakis(hexyloxy)triphenylene (HAT6) discotic liquid crystal material is first employed as a transparent hole-transporting material (HTM) for perovskite solar cells (PSCs), and the applicability of liquid crystal for highly stable PSCs fabrication is demonstrated.

Hexakis(hexyloxy)triphenylene (HAT6) discotic liquid crystal is employed as a transparent hole-transporting material (HTM) for perovskite solar cells (PSCs) and a power conversion efficiency (PCE) of 15.7% is obtained, which is the highest using HAT6 type of HTMs. Despite lower PCE than spiro-OMeTAD-based devices (20.3%), the PSCs based on HAT6 exhibit much higher ambient and thermal stability. A fused polyaromatic core with six alkyl chains leads to high hydrophobicity, and the π-stacked molecular columns of the HAT6 shield the bis(trifluoromethane)sulfonamide lithium salt dopant migration into the perovskite absorber. The PSCs (under N₂ condition) exhibit superior stability compared to the devices employing spiro-OMeTAD, retaining nearly 92% of their initial efficiency after 1200 h operation. Under ambient conditions, HAT6-based hole-transportation layer devices retain 93% of the initial efficiency for 690 h. Under continuous thermal stress of 85 °C, the devices based on HAT6 retain 95% of the initial PCE. The results demonstrate the time applicability of liquid crystal for stable PSCs fabrication.

The CsF interlayer between SnO₂ and perovskite can significantly reduce the surface hydroxyl and release the interfacial strain. Meanwhile, with the insertion of CsF, the work function of SnO₂ and the crystallization process of perovskite can be effectively tailored. Finally, the efficiency of the perovskite photovoltaic devices reaches up to 23.13% and demonstrates enhanced stability.

Defects passivation strategy for the atop perovskite films is widely investigated, while the buried interface between the tin oxide electron transport layer and the perovskite active layer should gain more attention since the interfacial strains and surface hydroxyl are inevitable during the fabrication process which would affect the efficiency and stability of the fabricated perovskite devices. Herein, the CsF interlayer between SnO₂ and perovskite film is adopted to release the interfacial strain and decrease the surface hydroxyl through the atomic interaction and chemical doping. Furthermore, the CsF can tailor the energy level of SnO₂ for a more favorable alignment to reduce the energy loss and improve charge extraction. Correspondingly, the perovskite photovoltaic devices with the efficiency of 23.13% are achieved. Moreover, CsF-doped devices demonstrate enhanced stability, which could maintain 87% of its initial efficiency after 1000 h under environmental storage. The developed CsF interfacial engineering provides a promising strategy for the interfacial modulation.

l-Phenylalanine (l-Phe) as a passivation layer at NiO _x /perovskite interface can improve the morphology and conductivity of the NiO _x film, promote carrier transport at the interface, and enhance the performance of perovskite films. Furthermore, the passivated device effectively slows down the lead leakage process in water through strong interaction with perovskite.

Although perovskite solar cells have achieved great breakthroughs in photoelectric conversion efficiency (PCE), some challenges still need to be addressed before commercialization. Lead leakage is harmful to the environment and many methods are developed to prevent lead leakage; among them, chemical adsorption has proved to be an effective way. Herein, a simple and low-cost strategy that can enhance the device performance and mitigate the lead leakage by applying l-phenylalanine in the interface of NiO _x /perovskite is reported. The results show that this strategy can improve the morphology and conductivity of the NiO _x film, optimize the NiO _x /perovskite interface energy level, resulting in an efficient and stable device with a PCE of 19.0%. Furthermore, the interface modification improves the stability of the perovskite film through strong interaction with the perovskite, inhibits the decomposition of the film in water, slows down the process of lead leakage, and protects the environment from lead pollution. The devices maintain 86% initial efficiency for 200 h maximum power point measurement and 94% for 2100 h under nitrogen.

The role of the Cs/Br ratio in crystallization, phase homogeneity, and stability for wide-bandgap perovskites is systematically studied. It is found that the Cs-rich perovskites undergo a critical transformation of 2D intermediate perovskite into 3D perovskite films, giving rise to much enhanced crystallinity, better phase homogeneity, and stability. Solar cells made from Cs_0.3FA_0.7PbI_2.7Br_0.3 deliver highest efficiency of 20.17%.

Wide-bandgap (WBG) perovskites are promising candidates for front cells in tandem devices. Taking advantage of composition engineering, most WBG perovskites are successfully obtained by heavy Br or Cs doping. However, the role of Cs/Br ratio in crystallization, phase homogeneity, and stability are fuzzy. Herein, three perovskites with a bandgap of 1.68 eV via tailoring the Cs/Br ratio are systematically studied, that are Cs_0.15FA_0.85PbI_2.5Br_0.5, Cs_0.3FA_0.7PbI_2.7Br_0.3, and Cs_0.4FA_0.6PbI_2.8Br_0.2. It is found that the Br-rich precursor film undergoes ultrafast crystallization, forming a 3D structure with random crystal orientation, while Cs-rich systems demonstrate a 2D-dominated intermediate phase. After annealing, all the precursor films transform into (100)-oriented perovskite films, and Cs_0.3FA_0.7PbI_2.7Br_0.3 perovskite presents the highest crystallinity, lowest microstrain, and best-phase homogeneity. As the Cs/Br ratio changes, both Cs and Br ions show the ability of arising phase segregation in the perovskite films. Increasing Cs content significantly improves the thermal stability, but heavy Cs content also sacrifices the air stability. Among the three systems, Cs_0.3FA_0.7PbI_2.7Br_0.3 solar cells offer the highest efficiency of 20.17% with superior air, light, and thermal stability. The findings highlight the importance of rational composition design to achieve high-quality WBG perovskite films for tandem applications.

In this work, a double-layer modification engineering strategy is designed that vitamin C and vitamin D2 are added to SnO₂ and perovskite respectively to achieve charge-carrier transport balance. The optimized perovskite solar cells (PSCs) achieve a power conversion efficiency of 24.20% with remarkable fill factor (81.01%). This work demonstrates that modifying perovskites with natural molecules is an effective approach to achieve highly efficient PSCs.

Abstract

The defect passivation and interface energetics-modification between perovskite and transport layers are significant for the further improvement of efficiency and stability of perovskite solar cells (PSCs). Here, a double-layer modification engineering strategy is employed by different functionalized natural vitamins into the electron transport layer and perovskite, respectively. Considering the different role of each functional layer in PSCs, the vitamin C (VC) with high conductivity is introduced into SnO₂, showing electron mobility enhancement, an interface energy-levels offsets reduction, and enhanced interfacial charge transfer. Meanwhile, antioxidant vitamin D2 (VD2) with multiple passivating functional groups is introduced into the perovskite bulk to moderately tailor its intrinsic characteristics. The surface energetics of perovskite are changed from n-type to p-type, the thickness of the p-type perovskite is 80 nm, thus the spontaneous n–p homojunction is formed in perovskite caused by VD2, which increases the built-in electric field and the efficiency of perovskite hole extraction. The synergistic effect of VC and VD2 better heightens the charge extraction efficiency and achieves charge-carrier transport balance in PSCs. The optimum device achieves a power conversion efficiency of 24.20% and a fill factor of 81.01% with negligible hysteresis. This efficiency is among the best PSCs employing natural molecules reported so far.

4-iodo-2,3,5,6-Tetrafluorobenzoic acid (I-TFBA) with strong halogen bonding is introduced at the interface of NiO/perovskite. I-TFBA is proven to effectively passivate the undercoordinated halide anions (I⁻) traps to suppress I₂ generation and induce oriented growth of the perovskite. As a result, the encapsulated champion device obtains a power conversion efficiency of 22.02% and maintains 91.86% of its initial value under 1000 h light soaking.

Abstract

The inverted perovskite solar cell has made great progress in recent years and the quality of the heterojunction has played a key role. Here, a series of halide-substituted benzoic acid molecules are investigated as the bridge between nickel oxide and the perovskite, constructing a stable and efficient buried heterojunction via halogen bonding. The designed molecules are anchored at the surface of NiO by the coordination between the carboxyl and hydroxyl groups. On the opposite site of the molecules, strong halogen bonding is formed by binding the undercoordinated I⁻ at the buried surface of the perovskite, which inhibits the generation of I₂ under continuous light soaking and thereby suppresses the formation of voids. Moreover, the highly directional halogen bonding is beneficial for the oriented growth of perovskite crystals, which accelerate the carrier transport. As a result, the champion device yields a power conversion efficiency (PCE) of 22.02% and the encapsulated device maintains 91.86% of the initial PCE under continuous 1-sun illumination at 55 °C for 1000 h.

Fibrillization of non-fullerene acceptor L8-BO is realized by employing a conjugated fused-ring solvent additive 1-fluoronaphtalene that acts as the molecular bridge, which contributes to realize a power conversion efficiency of 19% in the pseudo-bulk heterojunction D18/L8-BO binary organic solar cell, featuring a high fill factor of 80% with improved charge transport.

Abstract

The structural order and aggregation of non-fullerene acceptors (NFA) are critical toward light absorption, phase separation, and charge transport properties of their photovoltaic blends with electron donors, and determine the power conversion efficiency (PCE) of the corresponding organic solar cells (OSCs). In this work, the fibrillization of small molecular NFA L8-BO with the assistance of fused-ring solvent additive 1-fluoronaphthalene (FN) to substantially improve device PCE is demonstrated. Molecular dynamics simulations show that FN attaches to the backbone of L8-BO as the molecular bridge to enhance the intermolecular packing , inducing 1D self-assembly of L8-BO into fine fibrils with a compact polycrystal structure. The L8-BO fibrils are incorporated into a pseudo-bulk heterojunction (P-BHJ) active layer with D18 as a donor, and show enhanced light absorption, charge transport, and collection properties, leading to enhanced PCE from 16.0% to an unprecedented 19.0% in the D18/L8-BO binary P-BHJ OSC, featuring a high fill factor of 80%. This work demonstrates a strategy for fibrillating NFAs toward the enhanced performance of OSCs.

A versatile and low-cost 4-chlorothiazole-based polymer donor PBTTz3Cl is designed and synthesized. PBTTz3Cl not only exhibits excellent photovoltaic performances with various small molecule acceptors (e.g., BTP-Ec9 and L8-BO), but also possesses a decent power conversion efficiency of 19.12% in ternary devices when blended with BTP-Ec9:L8-BO mixture.

Abstract

Benefiting from the emergence of narrow-band-gap small-molecule acceptors (SMAs), especially “Y” series, the power conversion efficiency (PCE) of polymer solar cells (PSCs) is rapidly improved. However, polymer donors with high efficiency, easy synthesis, and good universality are relatively scarce except PBDB-TF and D18. Herein, two polymer donors are designed and synthesized based on 4-chlorothiazole derivatives with simple structures, namely PTz3Cl and PBTTz3Cl. The OSCs based on PBTTz3Cl with slightly weaker intermolecular forces in comparison to PTz3Cl exhibits a decent PCE of 18.38% in blending with SMA L8-BO, owing to its strong donor/acceptor interaction with L8-BO, which shapes suitable phase separation morphology. Further research finds that PBTTz3Cl can exhibit excellent photovoltaic performances with various SMA materials, highlighting its universality. Based on this, ternary PSCs are designed where BTP-eC9 is introduced as a guest into the PBTTz3Cl:L8-BO host system. Thanks to further optimal blend morphology and more balanced charge transport, the PCE is improved up to 19.12%, which is among the highest values for PSCs. This work provides a new design of low-cost electron-deficient units for constructing highly versatile, high-performance polymer donors.

The use of the transparent ZnO nanoparticles/Ag nanowire/ZnO nanoparticles electrode in an inverted organic solar cell improves the optical properties and shortens the lifetime of the polymer excitons. Improved performances compared to the indium tin oxide-based reference are recorded, including a 20% increase in short-circuit current. Despite the use of high-absorbing thick active layer, the plasmonic electrodes are efficient.

Transparent electrodes are a key component in the manufacturing of optoelectronic devices such as light-emitting diodes, touch screens, and solar cells. The transparent electrode commonly used in this field is based on indium tin oxide (ITO). It contains indium, which is an expensive, rare, difficult to extract, and depleting element, hence the need to find an alternative. Silver nanowire (AgNW) electrodes are one of the best alternatives due to their excellent electrical, optical, and mechanical properties. Herein, it is shown that embedding AgNWs between two layers of ZnO nanoparticles (ZnONPs) leads to superior optical and electrical performance. The validation of the ZnONPs/AgNWs/ZnONPs electrode using the reference PF2:PC70BM organic active layer, known to present a long carrier lifetime, in inverted solar cell architectures shows an increased absorption of the active layer. This enhancement is due to the electrical field resulting from plasmonic resonance because the absorption is proportional to the square of the electric field amplitude. Through photoluminescence spectroscopy, a shorter exciton lifetime in PF2 in the presence of AgNWs is also observed. All these processes lead to improved photovoltaic performance compared to ITO-based reference, with approximately 20% increase in photocurrent and overall power conversion efficiency.

A Cu paste is used to metallize interdigitated back contact solar cells. Efficiency and reliability are tested. With only 4.5 mg W⁻¹ of Ag usage, Cu paste printed cells demonstrate a 23% average efficiency. Under damp heat (85 °C, 85% relative humidity) and thermal stress (200 °C) for 1000 h, solar cells remain stable.

The high usage of silver in industrial solar cells may limit the growth of the solar industry. One solution is to replace Ag with copper. A screen printable Cu paste is used herein to metallize industrial interdigitated back contact (IBC) solar cells. A novel metallization structure is proposed for making solar cells. Cu paste is applied to replace the majority of the Ag used in IBC cells as busbars and fingers. Cu paste is evaluated for use as fingers, and solar cells are made to test conversion efficiency and reliability. The Cu paste achieves comparably low resistivity, and Cu paste printed cells demonstrate similar efficiency to Ag paste printed cells, with an average efficiency of 23%, and only 4.5 mg W⁻¹ of Ag usage. Also, the solar cells are stable and no Cu in-diffusion is observed under damp heat (85 °C, 85% relative humidity) and thermal stress (200 °C) for 1000 h, respectively. All processes used in this study can be carried out with industrial equipment. These findings reveal a new application for Cu pastes and point to a new direction for reducing Ag utilization and cost.

Two small molecule donors BT-CN/BT-ER with different end groups are synthesized and introduced into the blend film of PM6:Y6. The result ternary OSCs with 20 wt.% BT-CN/ BT-ER achieves a PCE of 16.8%/17.2%, receptively. When replacing Y6 with L8-BO, ternary devices incorporating these two SM donors both reach PCE of over 18%, indicating the universality of these two SM donors as the third component for highly efficient PM6-based ternary OSCs.

Abstract

Ternary architecture has been widely demonstrated as a facile and efficient strategy to boost the performance of organic solar cells (OSCs). However, the rational design of the third component with suitable core and end-group modification is still a challenge. Herein, two new small-molecule (SM) donors BT-CN and BT-ER, featuring the identical conjugated backbone with distinct end group, have been designed, synthesized, and introduced into the PM6:Y6 binary system as the second donor. Both molecules exhibit complementary absorption and good miscibility with PM6, contributing to the nanofibrous phases and strong face-on molecular packing. Importantly, the incorporation of BT-CN/BT-ER has significantly facilitated charge collection and transportation with remarkable suppression of carrier recombination. As a result, ternary OSCs with 20 wt% BT-CN/BT-ER achieved a PCE of 16.8%/17.22% with synchronously increased open-circuit voltage (V _OC), short-circuit current density (J _SC) and fill factor (FF). Moreover, replacing Y6 with L8-BO further improves the PCE to 18.05%/18.11%, indicating the universality of both molecules as the third component. This work demonstrates not only two efficient SM donors with 4,8-bis(4-chloro-5-(tripropylsilyl)thiophen-2-yl) benzo[1,2-b:4,5-b′]dithiophene (BDTT-SiCl) as the core but also end group modification strategy to fine-tune the absorption spectrum, molecular packing, and energy levels of SM donors to construct high-performance ternary OSCs.

Here, a new versatile zwitterion ion is developed, which can passivate both electron transporting layer (ETL) and perovskite, and improve the band alignment at interface, thus the high power conversion efficiencies of 20.67% and 24.62% are achieved in CsPbI₃ and FA_0.9Cs_0.1PbI₃ solar cells, respectively. This work will provide important guidance for the design of new interface passivators.

Abstract

All-inorganic CsPbI₃ perovskite solar cells (PSCs) have been extensively studied due to their high thermal stability and unprecedented rise in power conversion efficiency (PCE). Recently, the champion PCE of CsPbI₃ PSCs has reached up to 21%; however, it is still much lower than that of organic–inorganic hybrid PSCs. Interface modification to passivate surface defects and minimize charge recombination and trapping is important to further improve the efficiency of CsPbI₃ PSCs. Herein, a new zwitterion ion is deposited at the interface between electron transporting layer (ETL) and perovskite layer to passivate the defects therein. The zwitterion ions can not only passivate oxygen vacancy (V_O) and iodine vacancy (V_I) defects, but also improve the band alignment at the ETL-perovskite interface. After the interface treatment, the PCE of CsPbI₃ device reaches up to 20.67%, which is among the highest values of CsPbI₃ PSCs so far. Due to the defect passivation and hydrophobicity improvement, the PCE of optimized device remains 94% of its original value after 800 h storing under ambient condition. These results provide an efficient way to improve the quality of ETL-perovskite interface by zwitterion ions for achieving high performance inorganic CsPbI₃ PSCs.

A direct implied-V _OC imaging method via the novel use of a single bandpass filter (s-BPF) is developed for large-area photovoltaic solar cells and solar cell precursors with multiple advantages over the existing calibration methods for camera-based spectrally-integrated imaging tools. This method images the luminescence emission using a narrow BPF with centre energy in the high-energy tail of the luminescence emission.

Abstract

A novel, camera-based method for direct implied open-circuit voltage (iV _OC) imaging via the use of a single bandpass filter (s-BPF) is developed for large-area photovoltaic solar cells and precursors. The photoluminescence (PL) emission is imaged using a narrow BPF with centre energy inside the high-energy tail of the PL emission, utilising the close-to-unity and nearly constant absorptivity of typical photovoltaic devices in this energy range. As a result, the exact value of the sample's absorptivity within the BPF transmission band is not required. The use of an s-BPF enables a fully contactless approach to calibrate the absolute PL photon flux for spectrally integrated detectors, including cameras. The method eliminates the need for knowledge of the imaging system spectral response. Through an appropriate choice of the BPF centre energy, a range of absorber compositions or a single absorber with different surface morphologies, such as planar and textured, can be imaged, all without the need for additional detection optics. The feasibility of this s-BPF method is first validated. The relative error in iV _OC is determined to be ≤1.5%. The method is then demonstrated on device stacks with two different perovskite compositions commonly used in single-junction and monolithic tandem solar cells.

A deformation-free ultrathin (≈10 µm) indium-tin-oxide (ITO)-based transparent conducting electrode is fabricated through the incorporation of an AlO _x layer into a substrate. Flexible perovskite solar cells (19.16%) and modules (13.26%) using an ultrathin ITO conductor outperforms the devices using commercially available high-performing ITO/polyethylenenaphthalate electrode, while exhibiting foldable flexibility maintaining 100% and 92% of their initial efficiency at a radius of curvature of 0.5 mm, respectively.

Abstract

Flexible transparent conducting electrodes (TCEs) play a critical role when achieving highly flexible perovskite solar cells (PSCs) for potential applications such as wearable, portable, and aerospace power sources. Despite extensive exploration of electrode materials and substrate engineering, there have been few reports on flexible PSCs with both satisfactory performance and flexibility. Here, highly conductive indium-tin-oxide (ITO) based ultrathin TCEs are developed for highly efficient, and foldable perovskite solar cells and modules. By introducing an additional aluminum oxide (AlO _x ) layer to the substrate, deformation-free ITO-based ultrathin (≈10 µm) TCEs are successfully fabricated. A champion flexible perovskite solar cell and module using ultrathin TCEs achieve efficiencies of 19.16% and 13.26% (aperture areas of 0.078 and 16 cm²), respectively, outperforming reference devices using commercial high-performing flexible TCEs. The modules maintain 100% and 92% of their initial performance after 10 000 bending cycles with a radius of 1 and 0.5 mm, respectively, which is unprecedented on module scale.

Highly spectrally reproducible multimode random lasing is observed in thin polycrystalline films of tin iodide perovskite that cannot be explained by a strong mode localization. The observed spectral reproducibility can be due to the strong inhomogeneous broadening of photoluminescence spectra measured in tin-based perovskites on the basis of a comparison with an analogous Pb-based counterpart.

Abstract

An unusual spectrally reproducible near-IR random lasing (RL) with no fluctuation of lasing peak wavelength is disclosed in polycrystalline films of formamidinium tin triiodide perovskite, which have been chemically stabilized against Sn²⁺ to Sn⁴⁺ oxidation. Remarkably, a quality Q-factor as high as ≈10⁴ with an amplified spontaneous emission (ASE) threshold as low as 2 µJ cm⁻² (both at 20 K) are achieved. The observed spectral reproducibility is unprecedented for semiconductor thin film RL systems and cannot be explained by the strong spatial localization of lasing modes. Instead, it is suggested that the spectral stability is a result of such an unique property of Sn-based perovskites as a large inhomogeneous broadening of the emitting centers, which is a consequence of an intrinsic structural inhomogeneity of the material. Due to this, lasing can occur simultaneously in modes that are spatially strongly overlapped, as long as the spectral separation between the modes is larger than the homogeneous linewidth of the emitting centers. The discovered mechanism of RL spectral stability in semiconductor materials, possessing inhomogeneous broadening, opens up prospects for their practical use as cheap sources of narrow laser lines.

The thermodynamic relaxation of small-molecule acceptors (SMAs) in its blend with polymer donor raises concerns related to the long-term operational stability of polymer solar cells. With flexible spacers to restrict the motion of individual SMAs, the tethered SMAs show higher efficiency, and, most important, large glass transition temperatures to suppress the thermodynamic relaxation in mixed domains.

Abstract

For polymer solar cells (PSCs), the mixture of polymer donors and small-molecule acceptors (SMAs) is fine-tuned to realize a favorable kinetically trapped morphology and thus a commercially viable device efficiency. However, the thermodynamic relaxation of the mixed domains within the blend raises concerns related to the long-term operational stability of the devices, especially in the record-holding Y-series SMAs. Here, a new class of dimeric Y6-based SMAs tethered with differential flexible spacers is reported to regulate their aggregation and relaxation behavior. In their polymer blends with PM6, it is found that they favor an improved structural order relative to that of Y6 counterpart. Most importantly, the tethered SMAs show large glass transition temperatures to suppress the thermodynamic relaxation in mixed domains. For the high-performing dimeric blend, an unprecedented open circuit voltage of 0.87 V is realized with a conversion efficiency of 17.85%, while those of regular Y6-base devices only reach 0.84 V and 16.93%, respectively. Most importantly, the dimer-based device possesses substantially reduced burn-in efficiency loss, retaining more than 80% of the initial efficiency after operating at the maximum power point under continuous illumination for 700 h. The tethering approach provides a new direction to develop PSCs with high efficiency and excellent operating stability.

Nature Communications, Published online: 18 November 2022; doi:10.1038/s41467-022-34786-5