Chen Weijie

Publication date: January 2022

Source: Nano Energy, Volume 91

Author(s): Xin Song, Po Sun, Dawei Sun, Yongchuan Xu, Yu Liu, Weiguo Zhu

A quite thick c-Si bottom-cell has to be used to fully absorb near-infrared photons, which greatly improves the cost of the perovskite/c-Si tandem solar cells (TSCs). The bifacial two-terminal TSCs can not only improve the energy output, but also significantly reduce the Si thickness while maintaining high efficiency.

Many studies have confirmed that the perovskite/crystalline silicon (c-Si) tandem solar cells (TSCs) can achieve excellent photovoltaic performance far exceeding that of single-junction solar cells. However, the quite thick c-Si bottom-cells have to be used to fully absorb near-infrared photons, which greatly improves the cost of the TSCs. The bifacial two-terminal TSCs not only can improve the energy output by introducing rear incident light, but also significantly reduce the Si thickness while maintaining high efficiency. Herein, the photovoltaic performance of bifacial perovskite/c-Si TSCs under different Si thicknesses, pyramid heights, and albedos have been calculated. It is found that the thickness of the c-Si sub-cells can be reduced from the current 250 to 25 μm, and only the albedos need to be increased by 18.6% to cover the absorption loss in the near-infrared. It is recommended that 100-μm thick c-Si is a suitable candidate and optimized the size of the Si pyramids (1.0 μm) to obtain excellent light trapping performance. This work can serve as a guidance for experimental preparation of low-cost and high-efficiency bifacial perovskite/c-Si TSCs.

4-Chlorobenzamidine hydrochloride is developed as spacer to form orientationally crystallized nanorod-like 1D perovskite on the top surface of 3D perovskite for surface passivation of FAPbI₃ perovskite. The coexistence of 1D–3D hybrid perovskite regulates the crystallization and morphology effectively and assists in promoting charge extraction, and suppressing charge recombination and exhibits a boosted power conversion efficiency of 21.95%.

Abstract

The regulation of perovskite crystallization and nanostructure have revolutionized the development of high-performance perovskite solar cells (PSCs) in recent years. Yet the problem of stably passivating perovskite surface defects remains perplexing. The 1D perovskites possess superior physical properties compared with bulk crystals, such as excellent moisture stability, self-healing property, and surface defects passivation. Here, 4-chlorobenzamidine hydrochloride (CBAH) is developed as spacer to form orientationally crystallized nanorod-like 1D perovskite on the top surface of 3D perovskite for surface passivation of FAPbI₃ perovskite. Further structure characterizations indicate the coexistence of 1D–3D hybrid perovskite lattices in nanorod-like perovskite passivation layer, which regulates the crystallization and morphology effectively and assists in promoting charge extraction, and suppressing charge recombination. As a result, the CBAH treated FAPbI₃-based PSCs exhibit a boosted power conversion efficiency of 21.95%. More importantly, the resultant unencapsulated devices display improved thermal, moisture, and illumination stability, and high reproducibility in terms of device performance. These results indicate the potential of organic halide salts for regulation of perovskite crystallization, offering a promising route of utilizing 1D perovskites nanorods in photovoltaic fields.

A multifunctional molecule of polypropylene glycol bis (2-aminopropyl ether) is introduced to reconstruct and in situ form the quasi-2D perovskite layer on 3D perovskite bulk, which tunes the energy array of functional layers, passivates defects and mitigates carrier recombination, and improves the stability of the device. Consequently, the 2D/3D perovskite device exhibits an improved efficiency of 22.24% with a distinguished open-circuit voltage of 1.21 V.

Abstract

The 2D/3D composite structure possesses both the excellent stability of 2D perovskite and the excellent performance of 3D perovskite, which recently have attracted special attention. Different from the popular isopropanol, a novel additive solvent—polypropylene glycol bis (2-aminopropyl ether) (A-PPG) is introduced here to dissolve excess PbI₂ and perovskite, and then reconstruct and in situ form the quasi-2D perovskite layer on 3D perovskite bulk. The lone electron pairs of the ether-oxygen and amino in A-PPG can form coordination bonds with Pb²⁺. The introduction of A-PPG tunes the energy array of functional layers, passivates defects, and mitigates carrier nonradiative recombination. Consequently, the 2D/3D perovskite device exhibits a championship efficiency of 22.24% with a distinguished open-circuit voltage of 1.21 V (the thermodynamic limit of 1.30 V). Moreover, the 2D/3D device still maintains 90% of the original efficiency in the ambient atmosphere with a relative humidity of 30 ± 10% after 50 days.

A vacuum deposited NdCl₃ interface layer (NdCl₃-IL) is used at the rear interface to suppress iodide ion migration. Compared to the control device, the NdCl₃-IL-based perovskite solar cells (PvSCs) show improved efficiency and negligible hysteresis, along with enhanced device stability. Combined with designed surface passivation, a PCE over 22% (certified value of 21.68%) can be achieved on large-area (1 cm²) PvSCs.

Abstract

Accurate interface engineering can effectively inhibit iodide ion migration, thereby improving the stability and photovoltaic performance of perovskite solar cells (PvSCs). The time-of-flight secondary-ion mass spectrometry reveals that in an aged n–i–p-type PvSC, the iodide ions will move toward the rear side and enter the FTO cathode. In this regard, the authors describe a simple thermal evaporation strategy for introducing an NdCl₃ interface layer (NdCl₃-IL) at the rear interface of perovskites to interdict the iodine ion migration pathway, leading to reduced trap densities throughout the whole perovskite region. As a result, a boosted open-circuit voltage (V _OC) is achieved, resulting in power conversion efficiency (PCE) up to 22.16% with negligible hysteresis. The NdCl₃-IL also enhances the device stability, maintaining 83% of initial PCE after the maximum-power-point tracking test for 100 h. More encouragingly, a certified PCE of 21.68% is demonstrated on a large-area (1 cm²) device with combined 2D/3D passivation strategies.

Residual PbI₂ at the bottom of perovskites can damage the efficiency and stability of fully-textured perovskite/silicon tandem solar cells. Here, a thermal-evaporated CsBr layer is introduced between the perovskite and hole transport layers to interact with residual PbI₂ and construct a gradient perovskite absorber for optimized energy level alignment. Tandem device efficiency of 27.48% and stability in nitrogen over 10 000 h are obtained.

Abstract

The perovskite/silicon tandem solar cell (PK/c-Si TSC) is a reasonable choice that can break through the efficiency limitations of silicon cells. Here, the p-i-n perovskite solar cell is conformally grown by the evaporation–solution combination technique on fully-textured silicon heterojunction cells to realize two-terminal PK/c-Si TSCs. Due to the adverse effect of the residual PbI₂ at the bottom of the perovskite bulk on device performance, a thermal-evaporated CsBr thin layer is introduced between the perovskite layer and the hole transport layer to construct a gradient perovskite absorber for optimized energy level alignment, so as to improve the open-circuit voltage and fill factor of the device. Finally, the PK/c-Si tandem cell achieves an efficiency of 27.48% and is stable in nitrogen over 10 000 h.

A high power conversion efficiency (PCE) of 15.2% is achieved by via a halogen-free, polymer donor in TPD-3:Y6-based organic solar cells, which is far higher than that of its fluorined counterpart, TPD-3F (11.4%). Comparative characterization including, transmission electron microscopy, grazing incidence wide-angle X-ray scattering, transient absorption, miscibility, measurements explain this result. Additionally, a PCE of 9.31% is achieved by TPD-3:Y6-based 20.4 cm² modules.

Abstract

Fluorination of the donor and/or acceptor blocks of photoactive semiconducting polymers is a leading strategy to enhance organic solar cell (OSC) performance. Here, the effects are investigated in OSCs using fluorine-free (TPD-3) and fluorinated (TPD-3F) donor polymers, paired with the nonfullerene acceptor Y6. Interestingly and unexpectedly, fluorination negatively affects performance, and fluorine-free TPD-3:Y6 OSCs exhibit a far higher power conversion efficiency (PCE = 14.5%) than in the fluorine-containing TPD-3F:Y6 blends (PCE = 11.5%). Transmission electron microscopy (TEM) analysis indicates that the TPD-3F:Y6 blends have larger phase domain sizes than TPD-3:Y6, which reduces exciton dissociation efficiency to 81% for TPD-3F:Y6 versus 93% for TPD-3:Y6. Additionally, grazing incidence wide-angle X-ray scattering (GIWAXS) reveals that the TPD-3F:Y6 blends are less textured than those of TPD-3:Y6, while space-charge limited currents reveal lower and unbalanced hole/electron mobility in TPD-3F:Y6 versus TPD-3:Y6 blends. Charge recombination dynamic, transient absorption, and donor–acceptor miscibility assays additionally support this picture. Furthermore, conventional architecture TPD-3:Y6 OSCs deliver a PCE of 15.2%, among the highest to date for halogen-free polymer donor OSCs. Finally, a large-area (20.4 cm²) TPD-3:Y6 blend module exhibits an outstanding PCE of 9.31%, one of the highest to date for modules of area >20 cm².

A dynamic strategy is proposed to fabricate high-quality perovskite films through resonance modulation, leading to a V _oc of 1.16 V and high power conversion efficiencies approaching 22.0% and 19.5% for small-area (0.09 cm²) and large-area (1.02 cm²) inverted perovskite solar cells (PSCs), respectively. More importantly, the unencapsulated PSCs exhibit excellent long-term stability under various environmental conditions (moisture, light, and heat).

Abstract

Manipulating perovskite crystallization to prepare high-quality perovskite films is the key to achieving highly efficient and stable perovskite solar cells (PSCs). Here, a dynamic strategy is proposed to modulate perovskite crystallization using a resonance hole-transporting material (HTM) capable of fast self-adaptive tautomerization between multiple electronic states with neutral and charged resonance forms for mediating perovskite crystal growth and defect passivation in situ. This approach, based on resonance variation with self-adaptive molecular interactions between the HTM and the perovskite, produces high-quality perovskite films with smooth surface, oriented crystallization, and low charge recombination, leading to high-performance inverted PSCs with power conversion efficiencies approaching 22% for small-area devices (0.09 cm²) and up to 19.5% for large-area devices (1.02 cm²). Also, remarkably high stability of the PSCs is observed, retaining over 90%, 88%, or 83% of the initial efficiencies in air with relative humidity of 40–50%, under continuous one-sun illumination, or at 75 °C annealing for 1000 h without encapsulation.

Perovskite quantum dots (QDs) display the quantum confinement effect yet maintain the characteristics of bulk materials. In this Review the advantages and disadvantages of perovskite QDs and significant strategies (exchange chemistry, passivation engineering, and structure engineering) for the advancement of perovskite solar cells with perovskite QDs as an absorber are discussed.

Abstract

Perovskite quantum dots (QDs) preserve the attractive properties of perovskite bulk materials and present additional advantages, owing to their quantum confinement effect, leading to their suitability as an absorber in perovskite solar cells. In this Review, the issues and advantages of perovskite QDs are analyzed in the context of purification, device fabrication with perovskite QDs, light absorption, charge transport, and stability. In addition, promising strategies to enhance perovskite QDs and QD-based solar cells are elucidated based on exchange chemistry (ion and ligand exchange), passivation engineering (ion and ligand passivation), and structure engineering (conventional/inverted, planar/mesoscopic and dimensionally graded structures). These discussions will give a clue to the further development of perovskite QDs and thus the advancement of QD-based solar cells.

Doped p−n junctions represent the prototype of solar cells in text books, while p−i−n junctions support charge carrier collection for low-quality absorbers. For halide-perovskite cells the design via doping is challenging and substantial permittivity variations occur. The concept of the dielectric junction is introduced and designs electrostatics, recombination, and performance of solar cells by selecting materials with certain permittivities.

Conventional solar cells typically use doping of the involved semiconducting layers and work function differences between highly conductive contacts for the electrostatic design and the charge selectivity of the junction. In some halide perovskite solar cells, however, substantial variations in the permittivity of different organic and inorganic semiconducting layers strongly affect the electrostatic potential and thereby indirectly also the carrier concentrations, recombination rates, and eventually efficiencies of the device. Here, numerical simulations are used to study the implications of electrostatics on device performance for classical p−n junctions and p−i−n junctions, and for device geometries as observed in perovskite photovoltaics, where high-permittivity absorber layers are surrounded by low-permittivity and often also low-conductivity charge transport layers. The key principle of device design in materials with sufficiently high mobilities that are still dominated by defect-assisted recombination is the minimization of volume with similar densities of electrons and holes. In classical solar cells this is achieved by doping. For perovskites, the concept of a dielectric junction is proposed by the selection of charge transport layers with adapted permittivity if doping is not sufficient.

The hydrophobic Er@C₈₂ is a bifunctional additive to the 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD) hole transport layer that can enhance the photovoltaic performance and the stability of perovskite solar cells (PSCs) simultaneously.

Perovskite solar cells (PSCs) based on 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene Spiro-OMeTAD hole transport layer (HTL) have achieved a huge success in power conversion efficiency (PCE), but the required lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) dopant in Spiro-OMeTAD HTL is hygroscopic, not only impairing the charge transport but also inducing the instability of PSCs. Herein, Er@C₈₂, which consists of a hydrophobic fullerene cage encapsulating an Er³⁺ ion, is first introduced as a novel additive to modify the Li-TFSI-based Spiro-OMeTAD HTL. By adding a tiny amount of Er@C₈₂ (0.09 mg mL⁻¹) in the Spiro-OMeTAD HTL, the PSC exhibits an efficiency promotion from 17.53% to 19.22%. The PCE enhancement is mainly attributed to the improved film quality of HTL after adding Er@C₈₂, which promotes the oxidation of Spiro-OMeTAD, resulting in faster hole transport and less charge recombination. Simultaneously, the hydrophobic Er@C₈₂ and the improved film quality of HTL lead to a dramatically enhanced stability of PSCs. Accordingly, the Er@C₈₂-modified devices can maintain over 70% and 80% of the initial efficiencies after exposure in air for 400 h and in an Ar atmosphere for 2000 h, respectively. Therefore, this bifunctional Er@C₈₂ additive provides a promising pathway for fabricating highly efficient and stable PSCs.

By using the optimized n-propylamine–zinc oxide electron transporting layer, an organic solar cell with high power conversion efficiency and outstanding stability is achieved.

Abstract

As a representative electron transporting layer in organic solar cells, zinc oxide (ZnO) can be fabricated by the meniscus-guided coating with the promotion of sol–gel technology. In order to fabricate stable and flexible organic solar cells (OSCs) based on the printable ZnO layers, here, a new method for simultaneously manipulating fluidics of the sol–gel ZnO precursor and optimizing processability of the ZnO layer for flexible OSCs is developed. It is found that the Marangoni recirculation in meniscus and the annealing temperature of the sol–gel ZnO precursor can be effectively modulated by changing the Lewis base. With the use of propylamine, the high-quality ZnO layer that is suitable for flexible OSCs can be fabricated through blade coating. Under such a condition, the formation of polar facet in ZnO layer is well restrained, which favors the photostability of the cells. As a result, the best 1.00 cm² flexible cell outputs a power conversion efficiency of 16.71%, which is the best value till now.

Publication date: Available online 2 November 2021

Source: Chem

Author(s): Yingping Zou, Long Ye

The development of large-area fabrication of perovskite solar cells is essential to their commercial applications. In this review, the recent progress of this field is first summarized. Then, the crystallization mechanism of perovskite films is addressed, followed by detailed descriptions on the deposition methods and optimization strategies for large-area perovskite films. Finally, an outlook is provided for further improvement.

The development of large-area fabrication of perovskite solar cells is essential to their commercial applications. In this review, the recent progress of this field is first summarized. Then, the crystallization mechanism of perovskite films is addressed, followed by detailed descriptions on the deposition methods and optimization strategies for large-area perovskite films. Finally, an outlook is provided for further improvement.

Recently, Förster resonance energy transfer (FRET)-based strategy has been successfully applied to improve the efficiencies of organic solar cells. However, the role of FRET has not been deconvolved unambiguously due to the complex excited state photophysics. Herein, a comprehensive view of the recent progress on FRET strategy is presented through analysis of those published examples including our works.

Recently, Förster resonance energy transfer (FRET)-based strategy has been successfully applied to promote the efficiencies of ternary blend organic solar cells (TOSCs). However, the intrinsic mechanism of FRET in the observed enhancement of efficiency has not been deconvolved unambiguously due to the complex photophysics mechanism. In this review, by deeply analyzing recent examples of FRET-incorporated TOSCs, diverse framework structures of FRET pairs are summarized, then the theory, prerequisites, and the confirmation methods for FRET are discussed. In particular, the role of FRET theory in the photoconversion process is discussed in detail, including exciton harvesting, exciton diffusion, and charge generation. Finally, the existing challenges and future research directions of FRET applications in TOSCs are proposed.

Herein, three iodized diammonium spacers are selected to study the effects of chain length and heteroatom incorporation on the related interfacial properties of 2D/3D perovskite heterostructures. The structure tailoring and concentration control of organic spacers contribute to the well-controlled phase purity, improved quantum well orientation, and energetic band alignment at 2D/3D interfaces, and thus enhanced device efficiency.

Abstract

Perovskite solar cells (PSCs) based on 2D/3D heterostructures show great potential to combine the advantages of the high efficiency of 3D perovskites and the high stability of 2D perovskites. However, an in-depth understanding of the organic-spacer effects on the 2D quantum well (QW) structures and electronic properties at the 2D/3D interfaces is yet to be fully achieved, especially in the case of 2D perovskites based on diammonium spacers/ligands. Here, a series of diammonium spacers is considered for the construct ion 2D/3D perovskite heterostructures. It is found that the chemical structure and concentration of the spacers can dramatically affect the characteristics of the 2D capping layers, including their phase purity and orientation. Density functional theory calculations indicate that the spacer modifications can induce shifts in the energy-level alignments at the 2D/3D interfaces and therefore influence the charge-transfer characteristics. The strong intermolecular interactions between the 2,2-(ethylenedioxy)bis(ethylammonium) (EDBE) cations and inorganic [PbI₆]⁴⁻ slabs facilitate a controlled deposition of a phase-pure QW structure (n = 1) with a horizontal orientation, which leads to better surface passivation and carrier extraction. These benefits endow the EDBE-based 2D/3D devices with a high power conversion efficiency of 22.6% and remarkable environmental stability, highlighting the promise of spacer-chemistry design for high-performance 2D/3D PSCs.

An inverted perovskite solar cell employing a thermally-induced phase-change VO₂ electron extraction layer shows high efficiency of over 23% at high temperature and superior thermal stability simultaneously, which is mainly attributed to the dramatic change in the electrical properties and better electron extraction caused by the metal-to-insulator transition of VO₂ beyond its critical phase-change temperature.

Abstract

Reducing carrier recombination and facilitating charge extraction at the interface is of great significance to improve the device performance of perovskite solar cells (PSCs) towards commercial use. However, there has been little work done concerning transportation and recombination mechanism at the interface of the metal electrode and the electron transport layer in inverted PSCs. Herein, a new strategy of interface modification is reported that leverages the unique metal-to-insulator transition (MIT) characteristics of vanadium dioxide which is inserted as the electron extraction layer (EEL) in p-i-n planar PSCs. Benefiting from the suitable intermediate energy level of VO₂, the optimized device shows a power conversion efficiency (PCE) up to 22.11% with negligible hysteresis, as compared to the 20.96% benchmark at room temperature. Interestingly, the PCE of VO₂-based PSC increases to over 23% at 85 °C, which can be attributed to the dramatic change in the electrical properties and better electron extraction caused by the MIT of VO₂ beyond its critical phase-change temperature. In addition, the encapsulated VO₂-PSC shows superior thermal stability for 1000 h at 85 °C under 1 Sun illumination, maintaining over 90% of initial PCE. This work initiates the state-of-art concept of inserting thermally-induced phase-transition material as an EEL to achieve efficient and durable perovskite photovoltaics.

Nature Energy, Published online: 01 November 2021; doi:10.1038/s41560-021-00923-5