Enhanced Efficiency of Air-Stable CsPbBr3 Perovskite Solar Cells by Defect Dual Passivation and Grain Size Enlargement with a Multifunctional Additive

Moisture-Resilient Perovskite Solar Cells for Enhanced Stability

With the rapid rise in device performance of perovskite solar cells (PSCs), overcoming instabilities under outdoor operating conditions has become the most crucial obstacle toward their commercialization. Among stressors such as light, heat, voltage bias, and moisture, the latter is arguably the most critical, as it can decompose metal-halide perovskite (MHP) photoactive absorbers instantly through its hygroscopic components (organic cations and metal halides). In addition, most charge transport layers (CTLs) commonly employed in PSCs also degrade in the presence of water. Furthermore, photovoltaic module fabrication encompasses several steps, such as laser processing, subcell interconnection, and encapsulation, during which the device layers are exposed to the ambient atmosphere. Therefore, as a first step toward long-term stable perovskite photovoltaics, it is vital to engineer device materials toward maximizing moisture resilience, which can be accomplished by passivating the bulk of the MHP film, introducing passivation interlayers at the top contact, exploiting hydrophobic CTLs, and encapsulating finished devices with hydrophobic barrier layers, without jeopardizing device performance. Here, existing strategies for enhancing the performance stability of PSCs are reviewed and pathways toward moisture-resilient commercial perovskite devices are formulated.

Strengthened Interficial Adhesive Fracture Energy by Young's Modulus Matching Degree Strategy in Carbon-Based HTM Free MAPbI3 Perovskite Solar Cell with Enhanced Mechanical Compatibility

Carbon-based hole transport layer-free perovskite solar cells (PSCs) based on methylammonium lead triiodide (MAPbI3 ) have become one of the research focus due to low cost, easy preparation, and good optoelectronic properties. However, instability of perovskite under vacancy defects and stress-strain makes it difficult to achieve high-efficiency and stable power output. Here, a soft-structured long-chain 2D pentanamine iodide (abbreviated as "PI") is used to improve perovskite quality and interfacial mechanical compatibility. PI containing CH3 (CH2 )4 NH3 + and I- ions not only passivate defects at grain boundaries, but also effectively alleviate residual stress during high temperature annealing via decreasing Young's modulus of perovskite film. Most importantly, PI effectively increases matching degree of Young's modulus between MAPbI3 (47.1 GPa) and carbon (6.7 GPa), and strengthens adhesive fracture energy (Gc ) between perovskite and carbon, which is helpful for outward release of nascent interfacial stress generated under service conditions. Consequently, photoelectric conversion efficiency (PCE) of optimal device is enhanced from 10.85% to 13.76% and operational stability is also significantly improved. 83.1% output is maintained after aging for 720 h at room temperature and 25-60% relative humidity (RH). This strategy of regulation from chemistry and physics provides a strategy for efficient and stable carbon-based PSCs.

The Electrical Behaviors of Grain Boundaries in Polycrystalline Optoelectronic Materials

Polycrystalline optoelectronic materials are widely used for photoelectric signal conversion and energy harvesting and play an irreplaceable role in the semiconductor field. As an important factor in determining the optoelectronic properties of polycrystalline materials, grain boundaries (GBs) are the focus of research. Particular emphases are placed on the generation and height of GB barriers, how carriers move at GBs, whether GBs act as carrier transport channels or recombination sites, and how to change the device performance by altering the electrical behaviors of GBs. This review introduces the evolution of GB theory and experimental observation history, classifies GB electrical behaviors from the perspective of carrier dynamics, and summarizes carrier transport state under external conditions such as bias and illumination and the related band bending. Then the carrier scattering at GBs and the electrical differences between GBs and twin boundaries are discussed. Last, the review describes how the electrical behaviors of GBs can be influenced and modified by treatments such as passivation or by consciously adjusting the distribution of grain boundary elements. By studying the carrier dynamics and the relevant electrical behaviors of GBs in polycrystalline materials, researchers can develop optoelectronics with higher performance.

Harnessing the potential of CsPbBr3-based perovskite solar cells using efficient charge transport materials and global optimization

Perovskite solar cells (PSCs) have become a possible alternative to traditional photovoltaic devices for their high performance, low cost, and ease of fabrication. Here in this study, the SCAPS-1D simulator numerically simulates and optimizes CsPbBr₃-based PSCs under the optimum illumination situation. We explore the impact of different back metal contacts (BMCs), including Cu, Ag, Fe, C, Au, W, Pt, Se, Ni, and Pd combined with the TiO₂ electron transport layer (ETL) and CFTS hole transport layer (HTL), on the performance of the devices. After optimization, the ITO/TiO₂/CsPbBr₃/CFTS/Ni structure showed a maximum power conversion efficiency (PCE or η) of 13.86%, with Ni as a more cost-effective alternative to Au. After the optimization of the BMC the rest of the investigation is conducted both with and without HTL mode. We investigate the impact of changing the thickness and the comparison with acceptor and defect densities (with and without HTL) of the CsPbBr₃ perovskite absorber layer on the PSC performance. Finally, we optimized the thickness, charge carrier densities, and defect densities of the absorber, ETL, and HTL, along with the interfacial defect densities at HTL/absorber and absorber/ETL interfaces to improve the PCE of the device; and the effect of variation of these parameters is also investigated both with and without HTL connected. The final optimized configuration achieved a V_OC of 0.87 V, J_SC of 27.57 mA cm^-2, FF of 85.93%, and PCE of 20.73%. To further investigate the performance of the optimized device, we explore the impact of the temperature, shunt resistance, series resistance, capacitance, generation rate, recombination rate, Mott-Schottky, JV, and QE features of both with and without HTL connected. The optimized device offers the best thermal stability at a temperature of 300 K. Our study highlights the potential of CsPbBr₃-based PSCs and provides valuable insights for their optimization and future development.

Taurine as a powerful passivator of perovskite layer for efficient and stable perovskite solar cells

Due to the ionic lattice property and the solution manufacture process of the perovskite light absorbing layer, there are several intrinsic defects (such as vacancies and low coordination Pb²⁺ and I^-) in perovskite films, which cause undesired photon-generated carrier recombination in the perovskite solar cells (PSCs) and seriously affect the power conversion efficiency (PCE) of devices. Defect passivation strategy is one of the most effective ways to eliminate the defects in perovskite films. Herein, a multifunctional Taurine molecule was introduced into CH₃NH₃PbI₃ (MAPbI₃) perovskite precursor solution to passivate the defects. It was found that Taurine with sulfonic acid (-SOOOH) and amino (-NH₂) groups can bind with uncoordinated Pb²⁺ and I^- ions, respectively, which can significantly reduce the defect density and suppress the carrier non-radiative recombination. Under atmospheric environment, non-hole transport layer FTO/TiO₂/perovskite/carbon structure PSCs were prepared. The device with Taurine showed a PCE of 13.19%, which is 17.14% higher than that of the control device (11.26%). With the suppressed defects, the Taurine passivated devices also showed enhanced device stability. The unencapsulated Taurine passivated device stored in ambient air after 720 h (temp. ∼25 °C and RH ∼25%) maintained 58.74% original PCE, while that of the control device was only about 33.98%.

Biomolecules incorporated in halide perovskite nanocrystals: synthesis, optical properties, and applications

Halide perovskite nanocrystals (HPNCs) have emerged at the forefront of nanomaterials research over the past two decades. The physicochemical and optoelectronic properties of these inorganic semiconductor nanoparticles can be modulated through the introduction of various ligands. The use of biomolecules as ligands has been demonstrated to improve the stability, luminescence, conductivity and biocompatibility of HPNCs. The rapid advancement of this field relies on a strong understanding of how the structure and properties of biomolecules influences their interactions with HPNCs, as well as their potential to extend applications of HPNCs towards biological applications. This review addresses the role of several classes of biomolecules (amino acids, proteins, carbohydrates, nucleotides, etc.) that have shown promise for improving the performance of HPNCs and their potential applications. Specifically, we have reviewed the recent advances on incorporating biomolecules with HP nanomaterials on the formation, physicochemical properties, and stability of HP compounds. We have also shed light on the potential for using HPs in biological and environmental applications by compiling some recent of proof-of-concept demonstrations. Overall, this review aims to guide the field towards incorporating biomolecules into the next-generation of high-performance HPNCs for biological and environmental applications.

Facilitating the Carrier Transport Kinetics at the CsPbBr3/Carbon Interface through SbX3 (X = Cl, Br, I) Passivation

The nonradiative carrier recombination at the perovskite/carrier selective layer (CSL) interface was accounted for the inferior power conversion efficiency (PCE) of perovskite solar cells (PSCs), especially rigid all-inorganic perovskite (CsPbI₃ and CsPbBr₃). In this study, targeting the poor interface, we introduce SbX₃ (X = Cl, Br, I) surface passivation at the CsPbBr₃/carbon interface. Smoothed compressive strain, reduced defect density, and enhanced energy-level alignment were achieved simultaneously, facilitating carrier extraction at the selective interface. With the simple aqueous solution-based two-step process, the PCE of our SbI₃ passivated carbon-based CsPbBr₃ PSCs has increased from 7.81% (without passivation) to 9.69%, a ∼25% enhancement. Specifically, V_oc (1.657 V) of the SbI₃-passivated cells was much higher than that of the control ones (1.488 V), confirming the ameliorated interface. Finally, our unencapsulated SbI₃ passivated devices maintain 90% of their initial PCEs while left in the air for 30 days with a relative humidity of 60%. To conclude, we present an interfacial carrier extraction-enhanced strategy for preparing high-performance and stable CsPbBr₃-based PSCs.

Polymerizable Surfactant Ligand for Stabilization and Film Formation of CsPbBr3 Nanocrystals

Surfactant ligands are important in the synthesis of inorganic perovskite nanocrystals (NCs), not only for stabilizing NCs but also for surface defect passivation. A new polymerizable surfactant ligand with a multidentate l-cysteine head, a long oleoyl tail, and a polymerizable styrenyl group (NOSVC) is designed for the post-synthesis treatment and stabilization of colloidal CsPbBr₃ NCs in this work. ¹H nuclear magnetic resonance and X-ray photoelectron spectroscopy analysis show that the l-cysteine head has strong interactions with the NCs. The absolute photoluminescence quantum yields of the colloidal NCs are increased from 45.1% of the pristine NCs stabilized with oleic acid/oleyl amine to 91.8% after NOSVC treatment. NOSVC-stabilized CsPbBr₃ colloidal NCs show enhanced stabilities when exposed in polar solvents. The NOSVC-stabilized CsPbBr₃ NCs in a solid film state allow for a photopolymerization to be carried out with the assistance of a photoinitiator. The polymerized films of NOSVC-treated NCs exhibit significantly enhanced stability against thermal radiation, ultraviolet irradiation, and humidity. We also fabricated self-healing polymer films incorporating NOSVC-treated CsPbBr₃ NCs as a green filter for a white light-emitting diode device. The green light-emitting films are very stable in humid environments, revealing the great application potential of NOSVC-treated CsPbBr₃ NCs in flexible display and lighting devices.

Improved Perovskite/Carbon Interface through Hot-Pressing: A Case Study for CsPbBr3-Based Perovskite Solar Cells

Due to the low cost and printable nature of the carbon paste, carbon-based perovskite solar cells (PSCs) are attractive for real application. However, the poor contact at the perovskite/carbon interface obviously hinders the achievable fill factor of the carbon-based PSCs. In this work, we introduce a pressure-assisted method to improve the contact at the perovskite/carbon interface. Via modulating the applied pressure, the power conversion efficiency of CsPbBr₃ PSCs (small area) can be improved from the initial 7.40% to 7.95% (pressing) and 8.34% (hot-pressing). A more remarkable feature is that the hot-pressing process boosted the performance from 5.1% (normal) to 6.9% (hot-pressing assisted) of large-scale (0.5 cm²) devices, a more than 30% enhancement. Finally, the hot-pressing method introduced in this work shows great prospects for improving the efficiency of carbon-based PSCs, especially large-scale PSCs.

Rational Design on Chemical Regulation of Interfacial Microstress Engineering by Matching Young's Modulus in a CsPbBr3 Perovskite Film with Mechanical Compatibility toward Enhanced Photoelectric Conversion Efficiency

Thermodynamically induced tensile stress in the perovskite film will lead to the formation of atomic vacancies, seriously destroying the photovoltaic efficiency stability of the perovskite solar cells (PSCs). Among them, cations and halide anions vacancies are unavoidable; these point vacancies are considered to be a major source of the ionic migration and perovskite degradation at the crystal boundary and surface of the perovskite films. Here, we use choline bromide to modify the perovskite film by occupying the atomic defects in the CsPbBr₃ perovskite film. The results show that the zwitterion quaternary ammonium ions and bromide ions in choline bromide can simultaneously occupy the Cs⁺ cation and Br^- anions vacancies in the perovskite film by the ionic bonding effect, for which the defect-state density on the surface of the perovskite film can be significantly reduced, leading to the effective enhancement of carrier lifetime. In addition, the residual stress at the crystal boundary can be effectively reduced by lowering the Young's modulus in the CsPbBr₃ perovskite film. As a result, the optimized device achieves a photoelectric conversion efficiency (PCE) of 9.06% with an increase of 41.1% compared to the control device with a PCE of 6.42%. Most importantly, the newborn thermal stress due to thermal expansion during heat working conditions can be transferred from the polycrystalline perovskite to the carbon layer by the matched Young's modulus, thus resulting in improved stability perovskite film under environmental conditions. The work provides new insights for preparing high-quality perovskite films with low defect-state density and residual stress.

Impacts of the Electron Transport Layer Surface Reconstruction on the Buried Interface in Perovskite Optoelectronic Devices

Using density functional theory combined with ab initio molecular dynamics, we comprehensively investigated the performance enhancement mechanism of the device after surface reconstruction by passivating different halogen groups (i.e., F or Cl) at the ETL/perovskite interface. We demonstrated that the halogen group at the ETL layer could stabilize the geometric structure of the perovskite surface by balancing the interfacial interaction, ionic migration, and lead iodide framework. Even though halogen passivation decreased and increased the interface charge transfer at the O- and SnO-terminated MAPbI₃/SnO₂ interfaces, respectively, halogen passivation optimized surface reconstruction and could theoretically relieve the interface carrier recombination according to the changes in conduction band offsets generated by halogen passivation. Furthermore, the interfacial carrier recombination of the MAPbI₃/SnO₂ interface was also connected to the interfacial gap states, which were smaller for O-terminated MAPbI₃/SnO₂ interfaces with halogen passivation-induced surface reconstruction but larger for the SnO-terminated cases. Hence, our findings have implications for the design of buried interface optimization in perovskite optoelectronic devices.

Defect Passivation via Additive Engineering to Improve Photodetection Performance in CsPbI2Br Perovskite Photodetectors

Solution-processable all-inorganic lead halide perovskites are under intensive attention due to their potential applications in low-cost high-performance optoelectronic devices such as photodetectors. However, solution processing usually generates structural and chemical defects which are detrimental to the photodetection performance of photodetectors. Here, a polymer additive of polyethylene glycol (PEG) was employed to passivate the localized defects in CsPbI₂Br films through the Lewis acid-base interaction. The interfacial defects were passivated efficiently by introducing a trace amount of a PEG additive with a concentration of 0.4 mg mL^-1 into the CsPbI₂Br precursor solution, as suggested by the significantly reduced trap density of state, which was revealed using thermal admittance spectroscopy. Fourier transform infrared spectrum characterization showed that rather than Cs⁺ or I^-, a Lewis acid-base interaction was established between Pb²⁺ and PEG to passivate the defects in the CsPbI₂Br perovskite, which leads to large suppression of noise current. Both specific detectivity and linear dynamic range improved from 4.1 × 10⁹ Jones and 73 dB to 2.2 × 10¹¹ Jones and 116 dB, respectively. Our work demonstrates the feasibility of employing an environmentally stable polymeric additive PEG to passivate defects for high photodetection performance in all-inorganic perovskite photodetectors.

High-performance perovskite photodetectors based on CsPbBr3 microwire arrays

All inorganic perovskite materials have drawn extensive attention, owing to their outstanding performance, facile solution-processed method, and potential applications in optoelectronic devices. However, uncontrollable morphology, high defect density, and instability of perovskites prepared via solution-processed method are the main challenges for their large-scale production and commercialization. Herein, we prepared large-scale CsPbBr₃ microwire arrays with highly ordered morphology and high crystalline quality by a template-assisted method. The photodetectors based on CsPbBr₃ microwire arrays exhibited remarkable on/off photocurrent ratio of 9.02×10³, high detectivity of 1.59×10¹³ Jones, high responsivity of 4.55 A/W, and fast response speed of 4.9/3 ms. More importantly, the photocurrent of the photodetectors hardly changed in air after being stored for two months, indicating remarkable stability. This study demonstrates that CsPbBr₃ microwire arrays provide the possibility for preparing large-scale and high-performance optoelectronic devices.

Evolutionary exploration of polytypism in lead halide perovskites

The regular ABX₃ cubic perovskite structure is composed of close-packed AX₃ layers stacked along the 〈111〉 axis. An equivalent hexagonal close-packed network can also be formed, in addition to a series of intermediate polytype sequences. Internally, these correspond to combinations of face- and corner-sharing octahedral chains that can dramatically alter the physical properties of the material. Here, we assess the thermodynamics of polytypism in CsPbI₃ and CsPbBr₃. The total energies obtained from density functional theory are used to paramaterize an axial Ising-type model Hamiltonian that includes linear and cubic correlation terms of the pseudo-spin. A genetic algorithm is built to explore the polytype phase space that grows exponentially with the number of layers. The ground-state structures of CsPbX₃ polytypes are analysed to identify features of polytypism such as the distinct arrangements of layers and symmetry forbidden sequences. A number of polytypes with low ordering energies (around thermal energy at room temperature) are predicted, which could form distinct phases or appear as stacking faults within perovskite grains.

Beyond the regular perovskite structure based on cubic-close packing exists a range of possible polytypes that we explore using computational chemistry.

Near-Unity-Efficiency Energy Transfer from Perovskite to Monolayer Semiconductor through Long-Range Migration and Asymmetric Interfacial Transfer

van der Waals heterostructures combining perovskites of strong light absorption with atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDs) hold great potential for light-harvesting and optoelectronic applications. However, current research studies integrating TMDs with low-dimensional perovskite nanomaterials generally suffer from poor carrier/energy transport and harnessing, stemming from poor interfacial interaction due to the nanostructured nature and ligands on surface/interface. To overcome the limitations, here, we report prototypical three-dimensional (3D)/2D perovskite/TMD heterostructures by combing highly smooth and ligand-free CsPbBr₃ film with a WSe₂ monolayer. We show that the energy transfer at interface occurs through asymmetric two-step charge-transfer process, with ultrafast hole transfer in ∼200 fs and subsequent electron transfer in ∼10 ps, driven by the asymmetric type I band alignment. The energy migration and transfer from CsPbBr₃ film to WSe₂ can be well described by a one-dimensional diffusion model with a carrier diffusion length of ∼500 nm in CsPbBr₃ film. Thanks to the long-range carrier migration and ultrafast interfacial transfer, highly efficient (>90%) energy transfer to WSe₂ can be achieved with CsPbBr₃ film as thick as ∼180 nm, which can capture most of the light above its band gap. The efficient light and energy harvesting in perovskite/TMD 3D/2D heterostructures suggest great promise in optoelectronic and photonic devices.