Ideal Band Gap Research Articles

Novel Tl2SnX6 (X=Cl,Br) Double Perovskites for Photovoltaic Applications: A DFT Insight

New double perovskites Tl2SnCl6 and Tl2SnBr6 have been computationally investigated for the first time to determine their structural, mechanical, and optoelectronic properties. The calculations were carried out using Density Functional Theory (DFT), implemented in the Quantum Espresso and Thermo_pw codes. DFT is a theoretical framework used to accurately predict the ground state properties of quantum many-body systems ranging from atoms to condensed matter systems. The results indicated that Tl2SnCl6 and Tl2SnBr6 possess equilibrium lattice parameters of 10.28 Å and 10.77 Å, respectively. Their negative formation energy ensures chemical stability while their positive elastic tensors imply mechanical stability, which is also confirmed by the fulfillment of the Born-Huang stability criteria. Analysis of their Poisson’s and Pugh’s ratio suggested that the materials are brittle but Tl2SnCl6 was estimated to be ductile in some directions. The materials were found to possess semiconducting behavior with bandgap of 1.19 eV (1.48 eV) and 2.48 eV (2.84 eV) for Tl2SnBr6 and Tl2SnCl6, respectively, using the PBE (SCAN) functionals. The calculated optical characteristics indicated that both Tl2SnCl6 and Tl2SnBr6 exhibit remarkable optical properties including significantly high absorption coefficient and low reflectivity, rendering their potential as light absorbers. In particular, with an ideal band gap of 1.48 eV and strong absorption in the visible region of the solar spectra, Tl2SnBr6 is predicted to be an excellent absorber for perovskite solar cells.

Mechanically Exfoliated InP Thin Films for Solar Energy Conversion Devices

III‐V semiconductors are favoured photo absorber materials for solar energy conversion due to their ideal bandgap, yet their high‐cost hinders widespread adoption. Utilizing thin films of these semiconductors presents a viable way to address the cost‐related challenges. Here, a novel mechanical exfoliation technique is demonstrated, also known as controlled spalling, as a cost‐effective and facile way to obtain thin films of III‐V semiconductors. As a proof of concept, 15 μm thick InP films are successfully exfoliated from their original wafers. Thorough characterization using cathodoluminescence and photoluminescence spectroscopy confirms that the opto‐electronic properties of the exfoliated InP films remain unaffected. Utilizing these InP thin films, InP thin‐film heterojunction solar cells with efficiencies exceeding 13% are demonstrated. Additionally, InP photoanodes are fabricated by integrating NiFeOOH catalyst onto these InP thin‐film solar cells, achieving an impressive photocurrent density of 19.3 mA cm−2 at 1.23 V versus reversible hydrogen electrode, along with an applied bias photon‐to‐current efficiency of ≈4%. Overall, this study showcases the efficacy of controlled spalling in advancing economically viable and efficient III‐V semiconductor‐based solar energy conversion devices.

A-Site Engineering with Thiophene-Based Ammonium for High-Efficiency 2D/3D Tin Halide Perovskite Solar Cells.

TTin halide perovskites are the most promising candidate materials for lead-free perovskite solar cells (PSCs) thanks to their low toxicity and ideal bandgap energies. The introduction of 2D/3D mixed perovskite phases in tin-based PSCs (TPSCs) has proven to be the most effective approach to improving device efficiency and stability. However, a 2D perovskite phase normally shows relatively low carrier mobility, which will be unfavorable for carrier transfer in the devices. In this work, we used a thiophene-based cation 2-(thiophen-3-yl)ethan-1-aminium (3-TEA) as a spacer to form a novel 2D perovskite phase in TPSCs, which shows the most promising effect on the performance enhancement in comparison with other cations like 2-(thiophen-2-yl)ethan-1-aminium (2-TEA) and benzene-based 2-phenylethan-1-aminium (PEA). Theoretical calculations reveal that 3-TEA enables the most compact crystal packing of [SnI6]4- octahedral layers, resulting in the lowest hole effective mass and formation energy in the 2D phase. This effect significantly enhances device efficiency and stability by facilitating more efficient carrier transfer within the 2D phase. These findings indicate that thiophene-based 2D perovskites are well-suited for high-performance TPSCs.

A‐Site Engineering with Thiophene‐Based Ammonium for High‐Efficiency 2D/3D Tin Halide Perovskite Solar Cells

TTin halide perovskites are the most promising candidate materials for lead‐free perovskite solar cells (PSCs) thanks to their low toxicity and ideal bandgap energies. The introduction of 2D/3D mixed perovskite phases in tin‐based PSCs (TPSCs) has proven to be the most effective approach to improving device efficiency and stability. However, a 2D perovskite phase normally shows relatively low carrier mobility, which will be unfavorable for carrier transfer in the devices. In this work, we used a thiophene‐based cation 2‐(thiophen‐3‐yl)ethan‐1‐aminium (3‐TEA) as a spacer to form a novel 2D perovskite phase in TPSCs, which shows the most promising effect on the performance enhancement in comparison with other cations like 2‐(thiophen‐2‐yl)ethan‐1‐aminium (2‐TEA) and benzene‐based 2‐phenylethan‐1‐aminium (PEA). Theoretical calculations reveal that 3‐TEA enables the most compact crystal packing of [SnI6]4‐ octahedral layers, resulting in the lowest hole effective mass and formation energy in the 2D phase. This effect significantly enhances device efficiency and stability by facilitating more efficient carrier transfer within the 2D phase. These findings indicate that thiophene‐based 2D perovskites are well‐suited for high‐performance TPSCs.

Wall-applicable metamaterial: mitigating resonance dip for enhanced sound transmission loss

In recent years, there has been a significant advancement in thermal insulation technology, allowing for the implementation of energy-efficient solutions in existing buildings while preserving their architectural and cultural significance. This recognition has led to the widespread adoption of External Thermal Insulation Composite Systems (ETICS). However, this brings the common challenge in acoustic engineering, the resonance dip, into the spotlight. When the frequency of an incident sound wave coincides with a mass-spring-mass resonance of the thermally insulated wall, the sound insulation drop occurs, possibly resulting in a reduction in transmission losses. This study introduces a wall-applicable design of Plate-type Acoustic Metamaterial (PAM) to effectively enhance sound transmission loss. We investigated in development a transfer matrix method combined with the effective mass density of PAM to model the ideal metamaterial wall and further compared it with the results based on mass-law methods. Design strategies to construct the ideal bandgap were developed by analyzing dispersion curves.

Synergetic effect of non-invasive posttreatment agents enables highly efficient and stable all-inorganic Sn-Pb perovskite solar cells

All-inorganic tin–lead perovskites present a promising avenue for achieving an ideal bandgap, approaching the Shockley-Queisser limit, while circumventing the use of volatile organic cations. However, challenges such as relatively low power conversion efficiency (PCE) and rapid degradation dynamics—stemming from complex film crystallization mechanisms and Sn oxidation—hinder their advancement. In this study, we employed sequential interface engineering on the CsPb0.6Sn0.4I3 system, utilizing non-invasive carbazole propylamine bromide and ethylenediamine diiodide as post-treatment agents. This approach significantly enhances both the bulk structure and surface morphology, thereby promoting charge transport, suppressing non-radiative recombination, and improving structural stability. The champion device achieved a PCE of 16.62 % and demonstrated over 2200 h of T80 stability in dry N2. Remarkably, stability in high-humidity environments was validated through device aging tests and in situ GIWAXS analysis. These results underscore the substantial potential of sub-1.4 eV bandgap all-inorganic Sn-Pb perovskite solar cells.

Comprehensive evaluation of recombination confined performance of CuGaO2 for solar cell application

In the quest to find an outstanding solar energy capturing system that meets requirements like affordability, widespread availability, eco-friendliness, remarkable efficiency, and enduring stability, thorough investigations have been carried out to explore the possibilities presented by ‘Delafossite’ copper gallium oxide (CuGaO2). β-CuGaO2 has an ideal bandgap of 1.5 eV, along with a high absorption coefficient and excellent carrier mobility, making it well-suited for high-efficiency solar cell applications. Theoretical modelling, utilizing the optical and electrical attributes of the CuGaO2 (CGO) material, is employed to analyze its photovoltaic performance when used as an absorber. The detailed balance analysis showed 56.9% of the incident power is wasted in spectrum loss (as thermalisation and non-absorption loss), 10.1% is wasted in intrinsic losses (such as radiative recombination, radiation dilution, entropy generation etc,), extrinsic recombination (originating from electrical losses, parasitic resistance, finite mobility, surface recombination velocity (SRV), non-ohmic contacts etc), eats up another 9.5% and the resultant 23.6% is available as net useful efficiency. Through the careful selection of a suitable buffer counterpart and optimization of material parameters, absorber thickness, defect density, contacts, and SRV, the CGO device dem onstrates an efficiency of 23.6%.

Preferentially coordinating tin ions to suppress composition segregation for high-performance tin-lead mixed perovskite solar cells

Tin-lead mixed perovskites (TLPs) with a tunable and ideal bandgap exhibit great potential in approaching the Shockley–Queisser limit of power conversion efficiency (PCE). However, two critical issues are necessary to be addressed, including the oxidation of Sn2+ and negligible composition and phase segregation. The latter derives from the unbalanced crystallization rate between Sn- and Pb-based perovskites. Here, we report a strategy to address the above critical issues by introducing 3,4-Dihydroxybenzylamine hydrobromide (DHBABr) in the TLP precursor solution. DHBABr was revealed to promote the crystallization of FAPbI3 perovskite by suppressing the formation of crystalline DMSO-FA-Pb-I intermediates and retard the crystallization rate of FASnI3 by preferentially forming a steady amorphous DHBA-FA-Sn-I intermediate. This, therefore, balances the crystallization rate between Sn- and Pb-based perovskites. As a result, the spatial distribution of Sn/Pb ratio is much more uniform across the whole TLP film, which benefits the upscaling of the manufacturing process. Relying on this doping strategy accompanied by the surface passivation with DHBABr, which reduces the defect density of TLP, inhibits the oxidation of Sn2+, and optimizes the band alignment of the device, we have achieved a PCE of 22.44 % with Voc of 0.853 V and FF of 80.0 %, along with an enhanced long-term stability of T80 = 476 h under continuously light illumination in the champion device.

Tin-Lead Perovskite Solar Cells with Preferred Crystal Orientation by Buried Interface Approach.

Mixed tin-lead perovskite solar cells (PSCs) have garnered much attention for their ideal bandgap and high environmental research value. However, poly (3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS), widely used as a hole transport layer (HTL) for Sn-Pb PSCs, results in unsatisfactory power conversion efficiency (PCE) and long-term stability of PSCs due to its acidity and moisture absorption. A synergistic strategy by incorporating histidine (HIS) into the PEDOT: PSS HTL is applied to simultaneously regulate the nucleation and crystallization of perovskite (PVK). HIS neutralizes the acidity of PEDOT: PSS and enhances conductivity. Especially, the coordination of the C═N and -COO- functional groups in the HIS molecule with Sn2+ and Pb2+ induces vertical growth of PVK film, resulting in the release of residual surface stress. Additionally, this strategy also optimizes the energy level alignment between the perovskite layer and the HTL, which improves charge extraction and transport. With these cooperative effects, the PCE of Sn-Pb PSCs reaches 21.46% (1 sun, AM1.5), maintaining excellent stability under a nitrogen atmosphere. Hence, the buried interface approach exhibits the potential for achieving high-performance and stable Sn-Pb PSCs.

Tuning the optoelectronic properties of graphene monoxide by electric field, strain and heterostructure with Cd[formula omitted] ([formula omitted]=S, Se or Te)

Hydrogen production via photocatalytic water splitting has become a promising technique for developing clean and sustainable energy system. Low-dimensional carbon-based materials are under focus to develop high efficiency visible-light photocatalysts. Among the various 2D graphene-based materials, graphene oxides (GO) attracted attention in recent years. Recently identified GO such as graphene monoxide (GMO) shows promising electronic structure characteristics such as direct band gap behavior, high optical absorption, and well-dispersed band edges to use them in higher efficiency optoelectronic applications. Nevertheless, its valence band edge is distant from the water oxidation potential, and band gap value exceeds the ideal band gap required to effectively harvest solar energy, which minimizes its practical application in photocatalytic water splitting. Here, using hybrid density functional theory calculations, we are exploring the effect of applied electric field/strain on the photocatalytic water splitting and photovoltaic properties of GMO. We found that even after applying an electric field or strain, GMO retains the desirable direct band gap behavior, well-dispersed bands at the band edges, and high optical absorption in the visible range. Further, the band alignment analysis suggests that the application of compressive strain enhances the efficiency of photocatalytic water splitting by straddling the band edges with water redox potentials. Additionally, we explored the optoelectronic properties of GMO by designing its heterostructure with CdS, CdSe, and CdTe monolayers. Remarkably, the studied heterostructures demonstrated superior optical absorption in the visible spectrum and a low recombination rate of charge carriers by separating them across different layers with type-II band alignment; suggesting that these heterostructures will be efficient candidates for visible energy photocatalytic water splitting and higher-efficiency solar cells.

High-Performance Ideal Bandgap Sn-Pb Mixed Perovskite Solar Cells Achieved by MXene Passivation.

Ideal bandgap (1.3-1.4eV) Sn-Pb mixed perovskite solar cells (PSC) hold the maximum theoretical efficiency given by the Shockley-Queisser limit. However, achieving high efficiency and stable Sn-Pb mixed PSCs remains challenging. Here, piperazine-1,4-diium tetrafluoroborate (PDT) is introduced as spacer for bottom interface modification of ideal bandgap Sn-Pb mixed perovskite. This spacer enhances the quality of the upper perovskite layer and forms better energy band alignment, leading to enhanced charge extraction at the hole transport layer (HTL)/perovskite interface. Then, 2D Ti3C2Tx MXene is incorporated for surface treatment of perovskite, resulting in reduced surface trap density and enhanced interfacial electron transfer. The combinations of double-sided treatment afford the ideal bandgap PSC with a high efficiency of 20.45% along with improved environment stability. This work provides a feasible guideline to prepare high-performance and stable ideal-bandgap PSCs.

Green-solvent-processed lead-free perovskite solar cells

Tin-based perovskite has been considered as one of the most potential candidates for lead-based perovskite. The solution proceed method was widely utilized in fabricating tin perovskite solar cells. So far, all fabrication processes for tin perovskite solar cells involved toxic organic solvents, which is contrary to the development of environmentally friendly perovskite solar cells. In this study, we report for the first time, by using a mixed green solvent N-diethyl formamide and green 1,3-dimethyl-3,4,5,6-Tetrahydro-2 (1H)-pyrimidinone as precursor solvent, and a green solvent dibutyl ether as antisolvent, a high-quality FA0.75MA0.25SnI3 film was achieved. The optical band gap of the prepared perovskite layer was 1.36 eV, which was close to the ideal band gap. The green-solution-proceed perovskite films showed reduced defect density. As a consequence, the champion green-solution-proceed photovoltaic device achieved a power conversion efficiency of 4.4%. Moreover, it still maintains 80% of the initial efficiency after 600 h of storage in a nitrogen atmosphere. This work would promote the perovskite solar cells from a ‘new’ technique to a ‘new and green’ technique.

Synthesis of ZnO/CZTS Hetero-Structure Junction by Sol–Gel Spin Coating Technique

Pertaining to the optimal band alignment of ZnO and CZTS, the heterostructure was fabricated with CZTS as the absorber layer and ZnO being the buffer layer coated on the FTO glass for photovoltaic application. Both the layers were deposited by using sol–gel spin coating technique. The results revealed the polycrystalline nature of both ZnO and CZTS films with distinct phase formations, with ZnO exhibiting prominent diffraction peaks at 31.77, 34.31, 36.20, 47.47, and 56.59° corresponding to (100), (002), (101), (102), and (110) planes, while CZTS showed diffraction peaks at (112), (220), and (312) planes. ZnO thin films demonstrated high optical transmittance (∼90%) and exhibited a wide band gap of 3.24 eV, while CZTS demonstrated a band gap of 1.44 eV. AFM images showed smooth surfaces with favorable topography for both films. Hall effect measurements indicated differences in electrical properties of the two films. Raman spectroscopy revealed vibrational energy patterns in both films. Additionally, current–voltage (I-V) characteristics revealed the electrical behavior of the ZnO/CZTS heterojunction, shedding light on its diode characteristics under both dark and illuminated conditions. With CZTS’s ideal band gap of 1.44 eV, sunlight is easily absorbed, and ZnO’s high electron mobility makes it an effective layer for electron transport. As a result, power conversion efficiency can be improved.

Effects of palladium doping on structural, mechanical，and opto-electronic behavior of Cs2Pt1-xPdxBr6 double perovskites

Stability in structure and mechanical properties, along with an appropriate band structure, plays a pivotal role in enhancing the performance of the absorber layer materials in perovskite solar cells. In this study, we have employed first-principles calculations to explore the effects of palladium (Pd) doping on the structural, mechanical, and optoelectronic properties of Cs2PtBr6 perovskite. The aim is to explore and identify absorber layer materials for solar cells with superior physical characteristics. Results demonstrated that pristine and Pd4+ doped Cs2PtBr6 are mechanically and dynamically stable, and they all belong to ductile materials. Electronic structure calculations show that Pd4+doping can induce the transition of Cs2PtBr6 from an indirect bandgap to a direct bandgap. Concurrently, with the increase of Pd4+doping concentration, the bandgap decreases, which is beneficial for light absorption. Optical property analyses have also confirmed that with the increase in Pd4+ doping concentration, there is a significant improvement in the optical absorption of Cs2PtBr6 within the visible light range. Particularly, double perovskite Cs2Pt0.25Pd0.75Br6 and Cs2PdBr6 stand out as the best candidates for the solar cell absorption layer due to their close proximity to the ideal bandgap (∼1.5 eV) and strong light absorption coefficients (exceeding 105 cm−1). The findings contribute to the development of light absorption layer materials in perovskite solar cells.

Advances in Mixed Tin-Lead Narrow-Bandgap Perovskites for Single-Junction and All-Perovskite Tandem Solar Cells.

Organic-inorganic metal-halide perovskites have received great attention for photovoltaic (PV) applications owing to their superior optoelectronic properties and the unprecedented performance development. For single-junction PV devices, although lead (Pb)-based perovskite solar cells have achieved 26.1% efficiency, the mixed tin-lead (Sn-Pb) perovskites offer more ideal bandgap tuning capability to enable an even higher performance. The Sn-Pb perovskite (with a bandgap tuned to ≈1.2eV) is also attractive as the bottom subcell for a tandem configuration to further surpass the Shockley-Queisser radiative limit for the single-junction devices. The performance of the all-perovskite tandem solar cells has gained rapid development and achieved a certified efficiency up to 29.1%. In this article, the properties and recent development of state-of-the-art mixed Sn-Pb perovskites and their application in single-junction and all-perovskite tandem solar cells are reviewed. Recent advances in various approaches covering additives, solvents, interfaces, and perovskite growth are highlighted. The authors also provide the perspective and outlook on the challenges and strategies for further development of mixed Sn-Pb perovskites in both efficiency and stability for PV applications.

Depolarization Field-Induced Photovoltaic Effect in Graphene/α-In2Se3/Graphene Heterostructures.

The ferroelectric photovoltaic effect (FPVE) enables alternate pathways for energy conversion that are not allowed in centrosymmetric materials. Understanding the dominant mechanism of the FPVE at the ultrathin limit is important for defining the ultimate efficiency. In contrast to the wide band gap conventional thin-film ferroelectrics, 2D α-In2Se3 has an ideal band gap of 1.3 eV and enables the fabrication of ultrathin and stable heterostructures, providing the perfect platform to explore FPVE in the nanoscale limit. Here, we study the ferroelectric layer thickness-dependent FPVE in vertical few-layer graphene/α-In2Se3/graphene heterostructures. We find that the short-circuit photocurrent is antiparallel to the ferroelectric polarization and increases exponentially with decreasing thickness. We show that the observed behavior is predicted by the depolarization field model, originating from the unscreened bound charges due to the finite density of states in semimetal few-layer graphene. As a result, the heterostructures show enhancement of the power conversion efficiency, reaching 2.56 × 10-3% under 100 W/cm2 in 18 nm thick α-In2Se3, approximately 275 times more than the 50 nm thick α-In2Se3. These results demonstrate the importance of the depolarization field at the nanoscale and define design principles for the potential of harnessing FPVE at reduced dimension.

The Phononic Properties and Optimization of 2D Multi-Ligament Honeycombs.

Honeycomb structures have attracted much attention for their excellent characteristics of reducing vibration and noise in recent years. In this study, through band analysis of different ligament structures, we aim to optimize the design of a steel structure that can isolate most of the noise in the 1500-5000 Hz range. The present study examines several different chiral structures. We calculate the band gaps of chiral structures under different geometric configurations and identify the variations in band gaps with geometric layouts. It is found that compared to other chiral structures, the triligaments chiral structure exhibits excellent band gap characteristics. The calculation results demonstrate that enhancing axial symmetry while filling central nodes can effectively enhance the structure's band gap properties. Frequency-response functions of different lattice structures are computed, and the results align with the calculations of band structures. This study then analyzes the influence of the number of periods on the magnitude of vibration attenuation, revealing that under the same number of periods, the wider the band gap of the structure, the greater the vibration attenuation. Both the triligaments chiral structure and the vertical triligaments structure possess ideal band gap widths, effectively suppressing wave propagation. Subsequently, harmonic response analyses and transient wave calculations further validate the accuracy of the band structure and frequency-response curve calculations. Our study results provide a new way to design a sound insulation structure that can isolate noise signals within the frequency range from 1500 to 5000 Hz in engineering.

Thermal, optoelectronic and thermoelectric properties of inorganic double perovskites semiconductors Cs2(Sn, Pt, Te)I6 for application as intermediate-band solar cells

First-principle calculations using the Wien2k code and the GGA-mBJ exchange potential were used in the study of the thermodynamic, dynamic, chemical and elastic stability, as well as the electronic, optical and thermoelectric properties of Cs2(Sn, Pt, Te)I6. The presence of an intermediate band in Cs2(Sn, Pt, Te)I6 semiconductors confirmed by absorption peaks appeared at photon energy corresponding to the band gap enhances the efficiency of solar cells. The ideal band gap, high dielectric constants and optimal absorption make the double perovskites under study perform well in solar cells. The calculated minimum formation energy, Helmholtz free energy and phonon modes through the first Brillouin zone for the investigated Cs2(Sn, Pt, Te)I6 family confirm their thermal, thermodynamic and dynamic stability. The acoustic phonon contribution modes come from the Cs-6s and I-5p electrons, while the Pt-6s, Sn-5p, Te-5p and I-5p electrons participate in the optical phonon modes. The narrowness of the upper valence band and the band gap in the visible region for Cs2PtI6 advantage this double perovskite in energy harvesting. The geometric Goldschmidt tolerance factor value between 0.8 and 1.0 and octahedral factor 0.41 indicate that Cs2SnI6 double perovskite is more stable.

Stable lead free double perovskites Cs2XAu(Br/I)6 (X = Y, Sc) as an emerging aspirant for solar cells and thermoelectric applications

In present article, the electronic, optical, mechanical, and thermoelectric characteristics of Cs2XAu(Br/I)6, where X = Y, Sc are addressed comprehensively. The Born stability criteria ensure the mechanical stability, whereas the structural, thermodynamic, and lattice stabilities are calculated from tolerance factor, formation energy, and phonons dispersion band structures. The ductility, hardness, lattice thermal conductivity, melting, and Debye temperatures are discussed in terms of elastic constants. The direct band gaps 1.90/1.35 eV Cs2ScAu(Br/I)6, and 2.23/1.74 eV for Cs2YAu(Br/I)6 fall the absorption of light energy in visible to near ultraviolet regions. The Cs2ScAuI6 has ideal band gap (1.35 eV) for solar cells because broad absorption band in visible region. Finally, thermoelectric behavior has been studied by Seebeck coefficient, electrical conductivity, power factor, and thermal conductivity in the temperature range 200 K–600 K. The highest figure of merit (1.0 at 300 K) for Cs2YAuBr6 increases demand for thermoelectric generators.

Narrow Bandgap Metal Halide Perovskites for All-Perovskite Tandem Photovoltaics.

All-perovskite tandem solar cells are attracting considerable interest in photovoltaics research, owing to their potential to surpass the theoretical efficiency limit of single-junction cells, in a cost-effective sustainable manner. Thanks to the bandgap-bowing effect, mixed tin-lead (Sn-Pb) perovskites possess a close to ideal narrow bandgap for constructing tandem cells, matched with wide-bandgap neat lead-based counterparts. The performance of all-perovskite tandems, however, has yet to reach its efficiency potential. One of the main obstacles that need to be overcome is the─oftentimes─low quality of the mixed Sn-Pb perovskite films, largely caused by the facile oxidation of Sn(II) to Sn(IV), as well as the difficult-to-control film crystallization dynamics. Additional detrimental imperfections are introduced in the perovskite thin film, particularly at its vulnerable surfaces, including the top and bottom interfaces as well as the grain boundaries. Due to these issues, the resultant device performance is distinctly far lower than their theoretically achievable maximum efficiency. Robust modifications and improvements to the surfaces of mixed Sn-Pb perovskite films are therefore critical for the advancement of the field. This Review describes the origins of imperfections in thin films and covers efforts made so far toward reaching a better understanding of mixed Sn-Pb perovskites, in particular with respect to surface modifications that improved the efficiency and stability of the narrow bandgap solar cells. In addition, we also outline the important issues of integrating the narrow bandgap subcells for achieving reliable and efficient all-perovskite double- and multi-junction tandems. Future work should focus on the characterization and visualization of the specific surface defects, as well as tracking their evolution under different external stimuli, guiding in turn the processing for efficient and stable single-junction and tandem solar cell devices.