Reduction Of Defect Density Research Articles

Epitaxial growth of high-quality GaAs on Si(001) using ultrathin buffer layers

The direct growth of III–V semiconductors on silicon holds tremendous potential for photonics applications. However, the inherent differences in their properties lead to defects in the epitaxial layer, including threading dislocations (TDs), antiphase boundaries (APBs), and thermal cracks, significantly impacting device performance. Current processes struggle to suppress these defects simultaneously, necessitating the development of methods to inhibit TDs and APBs in a thin buffer on silicon. This study introduces a GaSb buffer layer during GaAs epitaxy on a silicon (001) substrate. This approach successfully suppresses defect formation by promoting the formation of interfacial misfit dislocation arrays at both the AlSb/Si and GaAs/GaSb interfaces. The resulting GaAs layer exhibits a step-flow surface with a rough mean square of ∼3.8 nm and a full width at half maximum of 158 arcsec. Remarkably, the growth is achieved without any observable interfacial intermixing. Building on this platform, InAs/GaAs quantum dots are grown with a density of 3.8 × 1010 cm−2, emitting at a wavelength of 1288 nm. This breakthrough holds immense promise for developing high-quality GaAs films with reduced defect densities on silicon for O band lasers, laying the foundation for the mass production of silicon-based integrated circuits.

Improving device performance of sputtered CZTSe based solar cells by Manganese doping

Despite many efforts, Cu2ZnSnSe4 (CZTSe) based solar cells have remained relatively low efficiency compared to other thin film-based devices. One of the prevalent strategies used to improve cell performance in CZTSe is cation substitution. In this contribution, the effects of substituting Mn for Zn on CZTSe solar cell performance were systematically examined. Samples with various Mn content were grown by a two-stage method involving high temperature annealing of precursor layers comprising Cu, Zn, Sn, Mn and Se. All of the samples were found to have Cu-poor and (Zn + Mn)-rich or nearly stoichometric compositions. They crystalized in kesterite phase irrespective of the Mn content as confirmed by X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) measurements. The highest efficiency solar cell (5.56%) was obtained employing the 5% Mn doped CZTSe absorber. This film was found to have improved microstructure and reduced defect density. This is the first report on over 5% efficient kesterite thin film solar cell fabricated on a Cu2Mn0.05Zn0.95SnSe4absorber layer.

Synchronous Optimization of Light Outcoupling and Carrier Recombination for Efficient and Spectral Stable Blue Perovskite Light‐Emitting Diodes

AbstractDespite the significant progress of blue perovskite light‐emitting diodes (PeLEDs) in recent years, huge challenges still remain in improving their efficiency and spectral stability. In this work, a new strategy is reported to realize efficient and spectral stable blue PeLEDs through synchronously optimizing light outcoupling and carrier recombination of perovskite films. By introducing additives in both precursor solution and antisolvent, the crystallization of the perovskite films is regulated, leading to a spontaneous formation of undulant morphology, which favorably enhances the light outcoupling efficiency. Meanwhile, the formation of low‐dimensional perovskite phases is suppressed, in accompany with a reduced defect density and accelerated energy transfer, leading to a rapid concentration of excitons in the target perovskite phases and a significant enhancement of radiative carrier recombination. Resultantly, the optimized blue perovskite films show a high photoluminescent quantum yield approaching 80%, and the corresponding optimized PeLEDs show a high external quantum efficiency (EQE) of 11.88%, which significantly outperforms the control device with only 1.96%. Importantly, the optimized perovskite films and PeLEDs also exhibit improved spectral stability. This work provides a new pathway toward realizing efficient and spectral‐stable PeLEDs via simultaneously tackling the issues of light outcoupling and carrier recombination in the devices.

Regulating the Quantum Dots Integration to Improve the Performance of Tin–Lead Perovskite Solar Cells

AbstractDespite their advantageous attributes, such as a narrow bandgap and reduced toxicity, tin–lead halide perovskites (TLHPs) have received limited attention due to their lower power conversion efficiency (PCE) relative to lead‐only variants. In this study, a transformative approach is introduced that leverages perovskite quantum dots (PQDs) to optimize TLHP solar cells. While conventional oleyl‐capped PQDs enhance the open circuit voltage (VOC), the long‐chain ligands hinder charge transport. To overcome this limitation, a post‐treatment with isopropyl alcohol effectively dissociates these ligands and PQD crystals, resulting in reduced defect density, improved charge transfer, and elevated quasi‐Fermi level splitting in the TLHP device. Consequently, the PCE of the device is notably increased from 19.0% to 23.74% and elevated the VOC from 0.78 to 0.87 V, without compromising the photocurrent or fill factor. The findings highlight PQD modification as a compelling avenue for TLHP solar cell enhancement, particularly in boosting VOC.

Effect of phase distribution and defect passivation on amplified spontaneous emission of quasi-2D Dion–Jacobson perovskite

Quasi-2D Dion–Jacobson (DJ) halide perovskites with large exciton binding energy, self-assembled quantum wells, and high quantum yield attract growing attention in light-emitting diodes and solar devices. DJ-layered perovskites have the eliminated van der Waals gap and show improved photophysical features. However, there are a variety of defects and complex phase distributions produced during the solution procedure and the fast crystal development. In this paper, we reveal the effect of phase distribution and defect passivation on amplified spontaneous emission of quasi-2D perovskite (PDMA)(MA)2Pb3Br10 thin films through solvent engineering and additive methods, by establishing the correlation between the precursor compositions and the photophysical performance of the layered DJ perovskites. The energy transfer rate and the photoluminescence quantum yield increase due to the spatially homogeneous domain distribution and the reduced defect density after the addition of KBr in the DMSO:DMF. More importantly, we realize green amplified spontaneous emission and single-mode vertical-cavity surface-emitting lasing with low thresholds of 8.8 and 10.5 μJ cm−2, respectively. This work provides a guideline to achieve low-threshold multicolor lasers based on DJ perovskites.

Improved Photovoltaic Performance of Inverted Two-Dimensional Perovskite Solar Cells via a Simple Molecular Bridge on Buried Interface.

NiOx-based two-dimensional perovskite solar cells (2D-PSCs) have the advantages of low fabrication temperature, suitable energy level matching, suppressed hysteresis, and superior stability, while the poor interfacial contacts between NiOx and perovskite layers limit the perovskite film growth and charge transfer. Herein, a simple molecule, urea, was used as a molecular modifier to form bifacial passivation on the buried interface of NiOx/perovskite, resulting in better interfacial contact and efficient bifacial passivation. We demonstrated that efficient bifacial passivation mainly comes from strong interactions between urea and NiOx or perovskite, which make urea a molecular bridge for smoother charge transfer. Moreover, urea can regulate the ratio of Ni3+/Ni2+, therefore boosting the conductivity of NiOx, and adjust the morphology of the NiOx film for better 2D-perovskite crystal growth. Besides, urea also passivates the bifacial defect states of both NiOx and perovskite film, yielding reduced defect density of the perovskite film and superior charge transfer on the buried interface. Consequently, inverted 2D-PSCs with urea modification proved significant improvements in short-circuit current density and fill factor, resulting in improved power conversion efficiency from 14.64 to 16.84% with better stability in air.

Strengthened Buried Interface via Metal Sulfide Passivation Toward High‐Performance CsPbBr3 Perovskite Solar Cells

Although SnO2 has been widely used as the electron transport material (ETM) of the perovskite solar cells (PSCs), the energy level mismatch at the SnO2/CsPbBr3 buried interface is as high as 1 eV, which is disastrous for the CsPbBr3‐based PSCs. Herein, a buffer layer of metal sulfide (CdS, ZnS) is introduced to solve this problem. The power conversion efficiency (PCE) of CsPbBr3 PSCs has been increased from 8.16% to 9.48% for ZnS‐treated SnO2 (ZnS‐SnO2), and a champion efficiency of 10.61% has been achieved in CdS‐treated SnO2 (CdS‐SnO2) devices. Aside from the reduced energy loss, the mobility of the SnO2 ETM has been greatly enhanced after the metal sulfide treatment. The CdS‐SnO2 devices also enjoy the benefits of reduced defect density and speeded carrier extraction, contributing to an almost 30% performance enhancement. This 10.61% PCE is among the highly efficient CsPbBr3‐based PSCs reported to date. Finally, CdS‐SnO2 devices survive a harsh damp heat test (120 °C with a relative humidity of 50%) for a month with less than 15% efficiency loss, demonstrating the superior stability of our CsPbBr3 PSCs.

Aqueous synthesis of perovskite precursors for highly efficient perovskite solar cells.

High-purity precursor materials are vital for high-efficiency perovskite solar cells (PSCs) to reduce defect density caused by impurities in perovskite. In this study, we present aqueous synthesized perovskite microcrystals as precursor materials for PSCs. Our approach enables kilogram-scale mass production and synthesizes formamidinium lead iodide (FAPbI3) microcrystals with up to 99.996% purity, with an average value of 99.994 ± 0.0015%, from inexpensive, low-purity raw materials. The reduction in calcium ions, which made up the largest impurity in the aqueous solution, led to the greatest reduction in carrier trap states, and its deliberate introduction was shown to decrease device performance. With these purified precursors, we achieved a power conversion efficiency (PCE) of 25.6% (25.3% certified) in inverted PSCs and retained 94% of the initial PCE after 1000 hours of continuous simulated solar illumination at 50°C.

Grain boundary passivation by alkylammonium salt for highly stable perovskite solar cells

Perovskite solar cells (PSCs) are a strong candidate for next-generation photovoltaics, but their long-term stability poses challenges for use as a future renewable energy source. In particular, ambient moisture causes detrimental effects on not only the stability but also performance of PSCs. Herein, alkylammonium salts with perfluorinated long alkyl chains are introduced to passivate the grain boundaries and form hydrophobic surfaces. The perfluorinated alkyl chain moiety introduced into the perovskite layer reduces defect density and suppresses nonradiative recombination, thereby enhancing carrier transport capability and achieving a power conversion efficiency (PCE) of 20.31 %. Given the hydrophobic effects of fluorinated compounds on PSCs, about 90 % of the initial PCE can be retained after 1,000 h of continuous operation under a relative humidity of ∼ 40 % without encapsulation. The proposed additive strategy thus enables long-term operational stability of PSCs under humid conditions.

Elimination of buried interfacial voids for efficient perovskite solar cells

Establishing optimal interfacial contact is crucial for enhancing the efficiency of perovskite solar cells (PSCs). However, formation of voids at buried interface of perovskite films often hinders this critical objective. Our investigation reveals that these buried interfacial voids predominantly manifest at sites characterized by large-angle pits on rough substrate, due to their elevated nucleation barriers compared to their small-angle counterparts. To address this challenge and promote uniform nucleation and growth across all sites, regardless of their angles, we develop an innovative precursor regulation strategy to reduce the nucleation barrier discrepancy between different sites by introducing nonstoichiometric homologous ions or by facilitating the formation of intermediate phases. Notably, the incorporation of FACl additive into the FAPbI3 precursor combines the dual benefits of this approach, yielding high-quality perovskite film with intimate contact and reduced defect density. Consequently, this leads to a significant enhancement in the photovoltaic performance of PSCs.

Potassium doping for grain boundary passivation and defect suppression enables highly-efficient kesterite solar cells

The complicated and diverse deep defects, voids, and grain boundary in the CZTSSe absorber are the main reasons for carrier recombination and efficiency degradation. The further improvement of the open-circuit voltage and fill factor so as to increase the efficiency of CZTSSe device is urgent. In this work, we obtained K-doped CZTSSe absorber by a simple solution method. The medium-sized K atoms, which combine the advantages of light and heavy alkali metals, are able to enter the grain interior as well as segregate at grain boundary. The K-Se liquid phase can improve the absorber crystallinity. We find that the accumulation of the wide bandgap compound K2Sn2S5 at grain boundary can increase the contact potential difference of grain boundary, form more effective hole barriers, and enhance the charge separation ability. At the same time, K doping passivates the interface as well as bulk defects and suppresses the non-radiative recombination. The improved crystallinity, enhanced charge transport capability and reduced defect density due to K doping result in a significant enhancement of the carrier lifetime, leading to 13.04 % device efficiency. This study provides a new idea for simultaneous realization of grain boundary passivation and defect suppression in inorganic kesterite solar cells.

Spatially confined growth of VO2(M) nanoparticles with improved thermochromic property

In the pursuit of VO2-based energy efficient smart window, high-temperature annealing is a common way to prepare well-crystallized VO2 nanoparticles (NPs) with reduced defect density. However, it is usually accompanied by the formation of oversized VO2 NPs due to Ostwald ripening, making unsatisfactory optical performances of VO2-based films. In this work, we reported a spatial confinement route via incorporating commercial fumed silica (FS) as space barrier during the process of annealing ultrafine γ-VOOH NPs, and the size of the obtained VO2 NPs could be reduced to ∼50 nm. Moreover, VO2 films based on the multilevel spatial confinement strategy exhibit outstanding optical performances with balanced solar regulation efficiency (ΔTsol ∼ 17.82 %) and luminous transmittance (Tlum ∼ 51.74 %). A simple dipping process could be carried out to enable a hydrophobic self-cleaning surface of the obtained VO2 films, which was meaningful for smart window applications. VO2@SiO2 core-shell structure, referring to a strong spatial confinement strategy, was also prepared via coating γ-VOOH NPs with SiO2 followed by annealing treatment. The top-level optical performances with ΔTsol of 20.46 % and Tlum of 44.26 % of VO2@SiO2/PVP films were achieved successfully. The spatial confinement strategy may inspire more facile and efficient processes for development of high-performance VO2 films for various applications.

Unlocking the plasma advantage in elevating the performance of silicon nanowires/ poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonate) hybrid solar cell

Silicon Nanowires (SiNWs) can be easily synthesized by following the two-step electroless metal-assisted chemical etching method. These SiNWs are well known for their low reflectivity due to their excellent light-harvesting characteristics. In this paper, we demonstrated the effect of plasma treatment time on the performance of SiNWs solar cells. The 1 min plasma-treated SiNWs/Poly 3,4-[ethylene dioxy thiophene:Polystyrene sulfonate] (PEDOT׃PSS) solar cell showed increased power conversion efficiency than the untreated SiNWs/PEDOT׃PSS solar cell. The photo Current density-Voltage (J-V) analysis showed that the open circuit voltage (VOC) of the plasma treated solar cell for 1 min is higher (270 mV) than that of the untreated solar cell (120 mV). The increase in the value of VOC comes from different factors׃ one is due to the formation of a good interface between the SiNWs and PEDOT׃PSS layer; other is the reduction of defect densities in SiNWs/PEDOT׃PSS heterojunction, consequently the reduction in the recombination processes. Hence from this research, we found that by controlling the plasma treatment on SiNWs, it can improve the SiNWs properties which can further improve the efficiency of the solar cell.

Suppressing surface and interface recombination to afford efficient and stable inverted perovskite solar cells.

The top surface of the perovskite layer and the interface with the electron transporting layer play a key role in influencing the performance and operational stability of inverted perovskite solar cells (PSCs). A deficient or ineffective surface passivation strategy at the perovskite/electron transport layer interface can significantly impact the efficiency and scalability of PSCs. This study introduces phenyl dimethylammonium iodide (PDMAI2) as a passivation ligand that exhibits improved chemical and field-effect passivation at the perovskite/C60 interface. It was found that PDMAI2 not only passivates surface defects and suppresses recombination through robust coordination but also repels minority carriers and reduces contact-induced interface recombination. The approach leads to a twofold reduction in defect densities and photoluminescence quantum yield loss. This approach enabled high power conversion efficiencies (PCEs) of 25.3% for small-area (0.1 cm2) and 23.8% for large-area (1 cm2) inverted PSCs. Additionally, PDMAI2 passivation enabled PSCs to demonstrate steady operation at 65 °C for >1200 hours in an ambient environment.

Oxidation-induced modulation of photoresponsivity in monolayer MoS2 with sulfur vacancies.

Two-dimensional transition metal dichalcogenides (TMDs), such as MoS2, hold great promise for next-generation electronics and optoelectronics due to their unique properties. However, the ultrathin nature of these materials renders them vulnerable to structural defects and environmental factors, which significantly impact their performance. Sulfur vacancies (VS) are the most common intrinsic defects in MoS2, and their impact on device performance in oxidising environments remains understudied. This study investigates the impact of VS defects on the photoresponsivity of CVD-grown monolayer MoS2 devices, when exposed to oxidising environments at high temperatures. Our findings reveal a dynamic process of defect generation and healing through oxygen passivation, leading to a significant difference in photocurrent between environments. Temperature-dependent analysis shows defect healing and a notable reduction in defect density upon cooling. This study provides crucial insights into the stability and performance of 2D materials-based devices under varying environmental conditions, essential for designing and controlling the performance of TMD-based devices. Our results pave the way for the development of robust and reliable 2D materials-based electronics and optoelectronics.

Structural and optoelectronic properties of electrodeposited CdSe thin films: Effect of Cu-dopant

Cadmium selenide (CdSe) is a promising semiconductor extensively used in manufacturing optoelectronic devices. Compared to the undoped CdSe (Cu 1), the crystallite size of Cu 2 (1 % at. Cu-doped CdSe) increases from 27 to 29 nm , further Cu-doping into CdSe reduced crystallite size to 5 nm. The absorbance behavior exhibited improvement after Cu-doping. Additionally, the reflectance spectrum of Cu 2 displayed enhancement compared to the Cu 1 attributed to increase in crystallite size. The band gap (Eg) of undoped CdSe was measured at 1.78 eV, showing an increase to 1.89 eV after Cu-doping. Optical conductivity of CdSe samples exhibited an increase following Cu-doping. Consequently, Cu 2 exhibited optimal characteristics for photodetection among all samples, attributed to its higher crystalline quality, lower lattice strain, reduced defect density, and higher carrier concentration. This study reveals the influence of Cu-doping on CdSe, highlighting essential characteristics to enhance its application in optoelectronics.

Heteroepitaxial Growth to Construct Hexagonal/Hexagonal β-NaYF4:Yb,Tm/Cs4PbBr6 Multi-Code Emitting Core/Shell Nanocrystals.

Synthesis of upconversion nanoparticles (UCNPs)-metal halide perovskites (MHPs) heterostructure is garnered immense attentions due to their unparalleled photophysical properties. However, the obvious difference in their structural forms makes it a huge challenge. Herein, hexagonal β-NaYF4 and hexagonal Cs4PbBr6 are filtrated to construct the UCNP/MHP heterostructural luminescent material. The similarity in their crystal structures facilitate the heteroepitaxial growth of Cs4PbBr6 on the surface of β-NaYF4 NPs, leading to the formation of high-quality β-NaYF4:Yb,Tm/Cs4PbBr6 core/shell nanocrystals (NCs). Interestingly, this heterostructure endows the core/shell NCs with typically narrow-band green emission centered at 524nm under 980nm excitation, which should be attributed to the Förster resonance energy transfer (FRET) from Tm3+ to Cs4PbBr6. It is noteworthy that the FRET efficiency of β-NaYF4:Yb,Tm/Cs4PbBr6 core/shell NCs (58.33%) is much higher than that of the physically mixed sample (1.84%). In addition, the reduced defect density, lattice anchoring effect, as well as diluted ionic bonding proportion induced by the core/shell structure further increase the excellent water-resistance and thermal cycling stability of Cs4PbBr6. These findings open up a new way to construct UCNP/MHP heterostructure with better multi-code luminescence performance and stability and promote its wide optoelectronic applications.

(Invited) Two-Dimensional Low-Toxicity Absorbers for Efficient and Air-Stable Indoor Photovoltaics

The toxicity and bioavailability of lead (Pb) in halide perovskites motivates the search of Pb-free perovskite-inspired materials (PIMs), which would replicate the great optoelectronic properties of the traditional Pb-based counterparts. Among them, pnictohalides employing antimony or bismuth are especially attractive due to their low toxicity and inherent air stability. Their wide bandgaps (⁓2 eV) can broaden the application scenario beyond single-junction solar cells, being specifically suitable for e.g., silicon-perovskite tandems, indoor photovoltaics (IPVs), and photocatalysis.Despite outstanding indoor power conversion efficiencies up to 60% have been theoretically predicted for PIM-based IPVs, this research is still in a very early stage. In this talk, I will highlight some of our key findings on two-dimensional Sb- and/or Bi- based two-dimensional pnictohalides for sustainable indoor light harvesting. The considerable presence of both intrinsic and surface defects in wide-bandgap PIMs is mostly responsible for the modest indoor power conversion efficiency (PCE(i)) reported so far up to ⁓5% at 1000 lux white LED illumination. I will discuss the effective defect mitigation strategies relying on compositional engineering that we proposed to significantly improve the PCE(i) up to nearly 10% (9.53%). Our extensive characterizations at both material and device level evidence the successful suppression of trap-assisted and interfacial charge recombination as the result of a reduced defect density within the PIM domains and/or at their surface.The importance of our findings for the energy research community (within and beyond halide perovskite) lies in promoting future compositional design effort to reduce the defects in the next-generation wide-bandgap absorbers for eco-friendly energy generation and consumption.

Seed‐Mediated Growth for High‐Efficiency Perovskite Solar Cells: The Important Role of Seed Surface

AbstractAdditive engineering has emerged as one of the most promising strategies to improve the performance of perovskite solar cells (PSCs). Among additives, perovskite nanocrystals (NCs) have a similar chemical composition and matched lattice structure with the perovskite matrix, which can effectively enhance the efficiency and stability of PSCs. However, relevant studies remain limited, and most of them focus on bromide‐involved perovskite NCs, which may undergo dissolution and ion exchange within the FAPbI3 host, potentially resulting in an enlarged band gap. In this work, we employ butylamine‐capped CsPbI3 NCs (BPNCs) as additives in PSCs, which can be well maintained and serve as seeds for regulating the crystallization and growth of perovskite films. The resultant perovskite film exhibits larger domain sizes and fewer grain boundaries without compromising the band gap. Moreover, BPNCs can alleviate lattice strain and reduce defect densities within the active layer. The PSCs incorporating BPNCs show a champion power conversion efficiency (PCE) of up to 25.41 %, well over both Control of 22.09 % and oleic acid/oleylamine capped CsPbI3 NC (PNC)‐based devices of 23.11 %. This work illustrates the key role of nanosized seed surfaces in achieving high‐performance photovoltaic devices.

Seed-Mediated Growth for High-Efficiency Perovskite Solar Cells: The Important Role of Seed Surface.

Additive engineering has emerged as one of the most promising strategies to improve the performance of perovskite solar cells (PSCs). Among additives, perovskite nanocrystals (NCs) have a similar chemical composition and matched lattice structure with the perovskite matrix, which can effectively enhance the efficiency and stability of PSCs. However, relevant studies remain limited, and most of them focus on bromide-involved perovskite NCs, which may undergo dissolution and ion exchange within the FAPbI3 host, potentially resulting in an enlarged band gap. In this work, we employ butylamine-capped CsPbI3 NCs (BPNCs) as additives in PSCs, which can be well maintained and serve as seeds for regulating the crystallization and growth of perovskite films. The resultant perovskite film exhibits larger domain sizes and fewer grain boundaries without compromising the band gap. Moreover, BPNCs can alleviate lattice strain and reduce defect densities within the active layer. The PSCs incorporating BPNCs show a champion power conversion efficiency (PCE) of up to 25.41 %, well over both Control of 22.09 % and oleic acid/oleylamine capped CsPbI3 NC (PNC)-based devices of 23.11 %. This work illustrates the key role of nanosized seed surfaces in achieving high-performance photovoltaic devices.