Efficiency-loss analysis of monolithic perovskite/silicon tandem solar cells by identifying the patterns of a dual two-diode model’s current-voltage curves

In this work,we developed a simple and direct circuit model with a dual two-diode model that can be solved by a SPICE numerical simulation to comprehensively describe the monolithic perovskite/crystalline silicon(PVS/c-Si)tandem solar cells.We are able to reveal the effects of different efficiency-loss mechanisms based on the illuminated current density-voltage(J-V),semi-log dark J-V,and local ideality factor(m-V)curves.The effects of the individual efficiency-loss mechanism on the tandem cell’s efficiency are discussed,including the exp(V/VT)and exp(V/2VT)recombination,the whole cell’s and subcell’s shunts,and the Ohmic-contact or Schottky-contact of the intermediate junction.We can also fit a practical J-V curve and find a specific group of parameters by the trial-and-error method.Although the fitted parameters are not a unique solution,they are valuable clues for identifying the efficiency loss with the aid of the cell’s structure and experimental processes.This method can also serve as an open platform for analyzing other tandem solar cells by substituting the corresponding circuit models.In summary,we developed a simple and effective methodology to diagnose the efficiency-loss source of a monolithic PVS/c-Si tandem cell,which is helpful to researchers who wish to adopt the proper approaches to improve their solar cells.

Exploring heteroepitaxial growth and electrical properties of α-Ga_(2)O_(3) films on differently oriented sapphire substrates

This study explores the epitaxial relationship and electrical properties of α-Ga_(2)O_(3) thin films deposited on a-plane, mplane, and r-plane sapphire substrates. We characterize the thin films by X-ray diffraction and Raman spectroscopy, and elucidate thin film epitaxial relationships with the underlying sapphire substrates. The oxygen vacancy concentration of α-Ga_(2)O_(3) thin films on m-plane and r-plane sapphire substrates are higher than α-Ga_(2)O_(3) thin film on a-plane sapphire substrates. All three thin films have a high transmission of over 80% in the visible and near-ultraviolet regions, and their optical bandgaps stay around 5.02–5.16 eV. Hall measurements show that the α-Ga_(2)O_(3) thin film grown on r-plane sapphire has the highest conductivity of 2.71 S/cm, which is at least 90 times higher than the film on a-plane sapphire. A similar orientation-dependence is seen in their activation energy as revealed by temperature-dependent conductivity measurements, with 0.266, 0.079, and 0.075eV for the film on a-, m-, r-plane, respectively. The origin of the distinct transport behavior of films on differently oriented substrates is suggested to relate with the distinct evolution of oxygen vacancies at differently oriented substrates. This study provides insights for the substrate selection when growing α-Ga_(2)O_(3) films with tunable transport properties.

Structural and optical properties of AlN sputtering deposited on sapphire substrates with various orientations

AlN thin films were deposited on c-,a-and r-plane sapphire substrates by the magnetron sputtering technique.The in-fluence of high-temperature thermal annealing(HTTA)on the structural,optical properties as well as surface stoichiometry were comprehensively investigated.The significant narrowing of the(0002)diffraction peak to as low as 68 arcsec of AlN after HTTA implies a reduction of tilt component inside the AlN thin films,and consequently much-reduced dislocation densities.This is also supported by the appearance of E2(high)Raman peak and better Al-N stoichiometry after HTTA.Furthermore,the in-creased absorption edge after HTTA suggests a reduction of point defects acting as the absorption centers.It is concluded that HTTA is a universal post-treatment technique in improving the crystalline quality of sputtered AlN regardless of sapphire orienta-tion.

How to enable highly efficient and large‐area fabrication on specific textures for monolithic perovskite/silicon tandem solar cells?

Perovskite/silicon tandem solar cells(PVSK/Si TSCs)have emerged as a promising photovoltaic technology toward achieving a high power conversion efficiency(PCE)along with cost‐effective manufacturing.The PCE of PVSK/Si TSCs has skyrocketed to a certified 33.9%,surpassing the theoretical limit of any single‐junction solar cell.This achievement is partially attributed to ad-vancements in surface textures for Si bottom cells.In this regard,we present an overview of the recent developments concerning surface textures of Si in monolithic PVSK/Si TSCs,including planar,pyramid texture,and nanotexture.Following,the prevailing perovskite deposition methods on these textures are thoroughly discussed,and the corresponding challenges are evaluated.Addi-tionally,we provide a summary of the advanced morphological,structural,optical,and electrical characterization techniques being utilized for theses textures.Finally,the prospects for further development of PVSK/Si TSCs are outlined,including designing novel textures with industrial compatibility,developing perovskite deposition methods with scalability,and exploring more pertinent characterization techniques for textured PVSK/Si TSCs.

A low-temperature TiO2/SnO2 electron transport layer for high-performance planar perovskite solar cells

Conventional titanium oxide(TiO2) as an electron transport layer(ETL) in hybrid organic-inorganic perovskite solar cells(PSCs) requires a sintering process at a high temperature to crystalize, which is not suitable for flexible PSCs and tandem solar cells with their low-temperatureprocessed bottom cell. Here, we introduce a low-temperature solution method to deposit a TiO2/tin oxide(SnO2) bilayer towards an efficient ETL. From the systematic measurements of optical and electronic properties, we demonstrate that the TiO2/SnO2 ETL has an enhanced charge extraction ability and a suppressed carrier recombination at the ETL/perovskite interface, both of which are beneficial to photo-generated carrier separation and transport. As a result, PSCs with TiO2/SnO2 bilayer ETLs present higher photovoltaic performance of the baseline cells compared with their TiO2 and SnO2 single-layer ETL counterparts. The champion PSC has a power conversion efficiency(PCE) of 19.11% with an open-circuit voltage(Voc)of 1.15 V, a short-circuit current density(Jsc) of 22.77 mA cm^-2,and a fill factor(FF) of 72.38%. Additionally, due to the suitable band alignment of the TiO2/SnO2 ETL in the device, a high Vocof 1.18 V is achieved. It has been proven that the TiO2/SnO2 bilayer is a promising alternative ETL for high efficiency PSCs.

Synergistic effect of TiO_2 hierarchical submicrospheres for high performance dye-sensitized solar cells

The performance of dye-sensitized solar cells(DSCs) could be improved by using rationally designed mesoporous film structure for electron collection, dye adsorption and light scattering. The development of a novel double layer film prepared by TiO_2 hierarchical submicrospheres and nanoparticles was reported in this article. The submicrospheres were composed of rutile nanorods of 10 nm diameter and the length of 150–250 nm, which facilitated fast electron transport, charge collection and light scattering. Using a double layer structure consisting of the 10 wt% film as a dye loading layer and the 50 wt% film as the light scattering layer, C101 sensitizer and liquid electrolyte, DSC yielded power conversion efficiency of 9.68% under 1 sun illumination.

Direct demonstration of carrier distribution and recombination within step-bunched UV-LEDs

AlGaN-based solid state UV emitters have many advantages over conventional UV sources. However, UV-LEDs still suffer from numerous challenges, including low quantum efficiency compared to their blue LED counterparts. One of the inherent reasons is a lack of carrier localization effect inside fully miscible AlGaN alloys. In the pursuit of phase separation and carrier localization inside the active region of AlGaN UV-LED, utilization of highly misoriented substrates proves to be useful, yet the carrier distribution and recombination mechanism in such structures has seldom been reported. In this paper, a UV-LED with step-bunched surface morphology was designed and fabricated, and the internal mechanism of high internal quantum efficiency was studied in detail. The correlation between microscale current distribution and surface morphology was provided, directly demonstrating that current prefers to flow through the step edges of the epitaxial layers. Experimental results were further supported by numerical simulation. It was found that efficient radiative recombination centers were formed in the inclined quantum well regions. A schematic three-dimensional energy band structure of the multiple quantum wells(MQWs) across the step was proposed and helps in further understanding the luminescence behavior of LEDs grown on misoriented substrates. Finally, a general principle to achieve carrier localization was proposed, which is valid for most ternary Ⅲ-Ⅴ semiconductors exhibiting phase separation.

Revealing the surface electronic structures of AIGaN deep-ultraviolet multiple quantum wells with lateral polarity domains

We report on the carrier dynamic and electronic structure investigations on AlGaN-based deep-ultraviolet multiple quantum wells (MQWs)with lateral polarity domains.The localized potential maximum is predicted near the domain boundaries by first-principle calculation,suggesting carrier localization and efficient radiative recombination.More importantly,lateral band diagrams of the MQWs are proposed based on electron affinities and valance band levels calculated from ultraviolet(UV)photoelectron spectroscopy.The proposed lateral band diagram is further demonstrated by surface potential distribution collected by Kelvin probe microscopy and the density-of-state calculation of energy bands.This work illustrates that lateral polarity structures are playing essential roles in the electronic properties of II nitride photonic devices and may provide novel perspective in the realization of high-efficiency UV emitters.

Multi-step in situ interface modification method for emission enhancement in semipolar deep-ultraviolet light emitting diodes

SemipolarⅢ-nitrides have attracted increasing attention in applications of optoelectronic devices due to the much reduced polarization field.A high-quality semipolar AlN template is the building block of semipolar AlGaN-based deep-ultraviolet light emitting diodes(DUV LEDs),and thus deserves special attention.In this work,a multi-step in situ interface modification technique is developed for the first time,to our knowledge,to achieve high-quality semipolar AlN templates.The stacking faults were efficiently blocked due to the modification of atomic configurations at the related interfaces.Coherently regrown AlGaN layers were obtained on the in situ treated AlN template,and stacking faults were eliminated in the post-grown AlGaN layers.The strains between AlGaN layers were relaxed through a dislocation glide in the basal plane and misfit dislocations at the heterointerfaces.In contrast,high-temperature ex situ annealing shows great improvement in defect annihilation,yet suffers from severe lattice distortion with strong compressive strain in the AlN template,which is unfavorable to the post-grown AlGaN layers.The strong enhancement of luminous intensity is achieved in in situ treated AlGaN DUV LEDs.The in situ interface modification technique proposed in this work is proven to be an efficient method for the preparation of high-quality semipolar Al N,showing great potential towards the realization of high-efficiency optoelectronic devices.

Lateral polarity control ofⅢ-nitride thin film and application in GaN Schottky barrier diode

N-polar and Ⅲ-polar GaN and AlN epitaxial thin films grown side by side on single sapphire substrate was reported.Surface morphology,wet etching susceptibility and bi-axial strain conditions were investigated and the polarity control scheme was utilized in the fabrication of Schottky barrier diode where ohmic contact and Schottky contact were deposited on N-polar domains and Ga-polar domains,respectively.The influence of N-polarity on on-state resistivity and Ⅰ–Ⅴ characteristic was discussed,demonstrating that lateral polarity structure of GaN and AlN can be widely used in new designs of optoelectronic and electronic devices.