High Minority Carrier Lifetime Research Articles

Enhancement of NiO/Si solar cell by adding porous silicon and a silicon dioxide SiO2 carrier selective layer

In this work, we fabricated a novel heterojunction, Silicon-based solar cell. We used nickel oxide NiO thin films on porous silicon PS separated by a thin silicon dioxide SiO2 carrier-selective layer p-NiO/(SiO2) PS/(n+) Si (Silicon). For comparison, we also manufactured a p-NiO/(n+) Si device. The Atomic Force microscopy AFM and scanning electron microscopy SEM images highlighted the nanoporous pyramidal like structures for p-NiO/(SiO2) PS/(n+) Si devices that decreased the optical reflectivity to 2 %. From the electrochemical study, low resistance and high minority carrier lifetime (1.44 ms) were estimated for p-NiO/(SiO2) PS/n+-Si. The results are promising and agree with a high power conversion efficiency of 19.52 % for p-NiO/(SiO2) PS/n+-Si versus 3.8 % for p-NiO/Si.

Effect of plasma-assisted N2O/Ar oxidation on TOPCon solar cells

N2O/Ar gas mixture are used to deposit ultra-thin silicon oxide (SiOx) layer by plasma enhanced chemical vapor deposition (PECVD), which is used as the tunneling oxide layer in the TOPCon (tunnel oxide passivated contacts) structure. It is found that the N2O/Ar flow ratios strongly affect the properties of SiOx layer, such as deposition rate, the resistance of high temperature, passivation quality, etc., thus significantly affect the properties of polysilicon passivated contact structures and the efficiency of TOPCon solar cells. The deposition rate of SiOx layer and contact resistivity of SiOx/poly-Si(n+)/SiNx:H structure decrease with the increases of Ar flow. However, there is an optimal N2O/Ar flow ratio to obtain the best passivation quality and the champion efficiency. The optimal flow ratio of N2O/Ar= 5:2 is found, and obtain the optimum thickness of SiOx and uniform polysilicon passivated contact. The highest minority carrier lifetime (τ) of 5751 μs, highest implied open circuit voltage (iVoc) of 741.3 mV, lowest single side recombination saturation current density(J0) of 4.79 fA/cm2, contact resistivity(ρcontact) of 2.23 mΩ/cm2 and champion efficiency of 25.06% are obtained at the flow ratio of N2O/Ar= 5:2.

Effect of metal impurities concentration on electrical properties in N-type Recharged-Czochralski silicon

The electrical properties of N-type Recharged-Czochralski (RCz) silicon solar cells are significantly affected by the concentration of metal impurities in the silicon materials. In this work, several N-type RCz silicon ingots were grown using feedstock with different impurities concentration, and the distribution of minority carrier lifetime was tested. It was discovered that a broader range of feedstock impurity concentrations resulted in high and stable minority carrier lifetime of the ingots. Solar cells made from these ingots had an average efficiency of over 24.57%. Meanwhile, the results of impurities segregation calculation and solar cells electroluminescence(EL) test were given. The results indicated that ring defects were the primary cause of differences in electrical properties among silicon ingots produced from feedstock with varying impurity concentrations. Further, the process window to produce high efficiency N-type Czochralski(Cz) silicon solar cells was broadened, and the production cost could be reduced.

Effects of Silicon Wafer’s Resistivity on Passivation and Devices Performances of Solar Cell

In the manufacture of solar cells, the resistivity of silicon wafers has a crucial impact on their performance. This study investigated the effects of different resistivities on p-TOPCon solar cells. The results indicate that lower resistivity wafers have a higher implied open-circuit voltage (iVoc) value, but higher carrier mobility due to the low resistivity leads to an increase in saturation current density (J0). Conversely, solar cells made on higher resistivity silicon wafers have a lower carrier mobility, leading to slower electron-hole recombination and lower bulk recombination, resulting in the advantage of lower saturation current density and higher minority carrier lifetime. At the same time, simulation shows that as the resistivity increases, the Voc and efficiency increase. However, cost considerations need to be taken into account as higher resistivity silicon wafers are more expensive. Therefore, resistivity between 2 - 3 Ω·cm2 is considered the preferred substrate for solar cells as it offers a better balance between cost and achieving high cell efficiency.

Defect tolerant Cs2SnI6 double perovskite thin films with ultrahigh carrier lifetime for high efficiency solar cells

Phase pure, device quality inorganic Cs2SnI6 double perovskite thin films are grown using thermal evaporation method. Both sequential and co-evaporation methods are attempted and preliminary analysis confirmed that the samples prepared by co-evaporation technique are more stable, demonstrating better properties. Effect of variation in the CsI/SnI2 ratio on the properties of Cs2SnI6 samples prepared using co-evaporation method are studied. CsI impurity phase is found to be suppressed when the ratio is reduced to 1:1. The films possess a direct bandgap of ∼1.53 eV and a polygon morphology with grains of size ∼0.4 μm. Depth-wise elemental analysis using XPS substantiated the uniform distribution of all the elements throughout the material without any additional phases. The sample has exceptionally high minority carrier lifetime of the order of 9.6 μs as determined from the time-resolved photoluminescence. The work function and valence band maximum of Cs2SnI6 film are found to be located at 4.9 eV and 6.31 eV respectively. These findings highlight the superior properties of the co-evaporated Cs2SnI6 thin film for the application as an absorber layer in solar cells.

Numerical and experimental investigation of the removal of Fe impurity during different DS process under axial magnetic field

The effects of axial magnetic field on the different directional solidification (DS) process were studied by using a transient numerical model of heat and mass transfer and corresponding experiments. The shape of melt-crystal (m-c) interface, melt flow morphology, thermal stress and Fe impurity concentration in the crystal for different DS process have been studied. The simulation results indicate that axial magnetic field provided a slight convex m-c interface shape, better flow pattern, lower thermal stress as well as lower Fe impurity concentration throughout the entire DS process. To further verify the simulation results, multi-crystalline silicon (mc-Si) ingots were prepared and testing were carried out. The experimental results are in good agreement with the simulation results. The experimental results show that a slight convex m-c interface shape was obtained, and the Fe impurity concentration in the middle area of the ingot was lower and higher minority carrier lifetime could be achieved by using the axial magnetic field.

Optimization of Silicon Heterojunction Interface Passivation on p‐ and n‐Type Wafers Using Optical Emission Spectroscopy

To increase the efficiency in p‐type wafer‐based silicon heterojunction (SHJ) technology, one of the most crucial challenges is the achievement of excellent surface passivation. Herein, chemical passivation techniques known for n‐type technology are successfully applied on p‐type float–zone (FZ) wafers, and wafer surface passivation quality is correlated with parameters from plasma diagnostics, namely crystallization rate and electron temperature indices. It is shown that plasma ignition at higher powers than deposition powers enhances effective minority carrier lifetimes τeff fourfold for p‐ (0.6–2.1 ms) and sixfold for n‐type (0.6–3.2 ms) wafers while giving opportunity to process under lower electron temperature indices during the nucleation phase. A subsequent hydrogen plasma treatment has a further beneficial effect on chemical passivation, leading to high effective minority carrier lifetimes of 4.5 and 3.1 ms, and implies open‐circuit voltages, i‐VOC, of 735 and 720 mV for p‐ and n‐type wafers, respectively. In particular, cell precursors built on p‐type wafers demonstrate excellent surface passivation with τeff and i‐VOC (4.1 ms and 745 mV). Using these process optimizations, SHJ cells on both p‐ and n‐type wafers are fabricated with efficiencies exceeding 21%.

Surface Passivation of Crystalline Silicon Wafer Using H2S Gas

We report the effects of H2S passivation on the effective minority carrier lifetime of crystalline silicon (c-Si) wafers. c-Si wafers were thermally annealed under an H2S atmosphere at various temperatures. The initial minority carrier lifetime (6.97 μs) of a c-Si wafer without any passivation treatments was also measured for comparison. The highest minority carrier lifetime gain of 2030% was observed at an annealing temperature of 600 °C. The X-ray photoelectron spectroscopy analysis revealed that S atoms were bonded to Si atoms after H2S annealing treatment. This indicates that the increase in minority carrier lifetime originating from the effect of sulfur passivation on the silicon wafer surface involves dangling bonds.

A critical study on different hydrogen plasma treatment methods of a-Si: H/c-Si interface for enhanced defect passivation

Defect engineering of amorphous-crystalline silicon interface is a critical aspect for high efficiency silicon heterojunction solar cells. In this work, we have done a comparative study of different hydrogen plasma treatment (HPT) methods namely Pre, Post and Intermediate (Inter) HPT using radio frequency plasma-enhanced chemical vapor deposition (rf-PECVD) to understand the passivation mechanism to obtain state of the art passivation. It was observed that Inter and Post HPT can give high minority carrier lifetime on both p and n type wafers and has lesser interface defect states. The analysis from Fourier Transform Infrared Spectroscopy and high resolution Transmission Electron Microscopy shows that these HPT methods results in higher bulk disorder yet atomically sharp and superior interface due to hydrogen diffusion which saturates the interface dangling bonds. While a Pre-HPT results in epitaxial growth leading to poorer performance inspite of lowest dihydrides in the bulk which further highlights the importance of the interface. We have demonstrated Inter and Post HPT of ultrathin (5 nm) a-Si:H films which could give surface recombination velocity as low as 13 cm/s. The degradation due to hydrogen effusion in these HPT a-Si:H films has also been studied.

Investigating physical origin of dominant hysteresis phenomenon in perovskite solar cell

The progress of perovskite solar cell (PSC) technology is held back due to the presence of anomalous hysteresis in its current–voltage (J–V) characteristics. Understanding the physical origin of J–V hysteresis is crucial for the development of hysteresis-free solar cell. We computationally explore the relative contribution of dominant physical phenomenon that could cause hysteresis in PSC. We explore that accumulation of mobile ions at the interfaces of the cell inside perovskite produces a space charge which in combination with charge trapping/detrapping in deep traps results in hysteresis which is often characterized by an S shaped behavior of J–V curve. We further explore that shallow traps at the interfaces of carrier-extraction layers alone exhibit the combined effects of ion accumulation and deep charge traps, and lead to J–V hysteresis in PSCs. Moreover, we investigate the effect of energy level of deep/shallow traps and show that the energy level of deep/shallow traps plays an important role in J–V characteristics of the cell. Lastly, we explore that high minority carrier lifetime in the bulk of the perovskite mitigates the effects of ion accumulation and/or charge trapping/detrapping and leads to hysteresis-free cell. The results presented in this work are beneficial for the development of hysteresis-free PSCs.

Heteroleptic Tin-Antimony Sulfoiodide for Stable and Lead-free Solar Cells

Summary The quaternary chalcogenide halides of group IV and V elements have attracted much attention due to their interesting semiconducting properties as well as a suitable band gap for solar cells. Here, for the first time, we report on solar cells using tin-antimony sulfoiodide (Sn2SbS2I3). Sn2SbS2I3 solar cells were fabricated using a chemical single-step deposition process with a solution containing a SbCl3-thiourea complex and SnI2 with the configuration of TiO2 and poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-enzothiadiazole)] as the electron- and hole-transporting layers, respectively. The best-performing cell exhibits a power conversion efficiency of 4.04% under the illumination of standard AM 1.5G conditions (100 mW cm−2). These unencapsulated cells exhibited good stabilities at 80% relative humidity, 85°C in air, and under illumination, respectively. These results provide guidelines for fabrication of lead-free heteroleptic perovskite solar cells by hosting divalent or combinations of monovalent and trivalent metal cations.

Passivation of textured crystalline silicon with small pyramids by silicon nitride films formed by catalytic chemical vapor deposition and phosphorus catalytic impurity doping

Abstracts Silicon nitride (SiNx) films formed by catalytic chemical vapor deposition (Cat-CVD) and phosphorus (P) catalytic impurity doping (Cat-doping) are applied on textured crystalline silicon (c-Si) wafers with a pyramid size of 1–2 µm to reduce the surface recombination of minority carriers. SiNx single layer passivation realizes a surface recombination velocity (SRV) of less than 10 cm/s. The addition of a P Cat-doped layer results in a reduction in a SRV to ~7 cm/s owing to field-effect passivation. These values are comparable to those obtained in our previous study for textured c-Si surface with larger-sized pyramids, indicating the high passivation ability of Cat-CVD SiNx films and P Cat-doping independent of the size of pyramids. In addition to the double-side textured wafers, we also prepare a single-side textured wafer using Cat-CVD SiNx as an etching barrier. We find that Cat-CVD SiNx films can be utilized as an etching barrier against alkali solution, and a high effective minority carrier lifetime (τeff) of 2.3 ms has been obtained by the passivation with Cat-CVD SiNx films. These achievements will contribute to an improvement in the performance of back-contact c-Si solar cells.

Air Stable, High‐Efficiency, Pt‐Based Halide Perovskite Solar Cells with Long Carrier Lifetimes

The search for Pb‐free perovskite materials continues with limited success to find a suitable replacement for Pb with outstanding optoelectronic properties. Herein, Pb‐free inorganic halide perovskite Cs2PtI6 with excellent absorption coefficient, long minority carrier lifetime, and optical bandgap of 1.4 eV is reported. Atmospheric precursor‐based solution processing results in high‐quality Cs2PtI6 with absorption coefficient of for photon energies of >1.5 eV and high minority carrier lifetimes of >2 μs indicating low defect density in the material. Superstrate n–i–p solar cells processed with the structure F:SnO2/CdS/Cs2PtI6/carbon/Cu show promising device efficiency of 13.88%. These planar devices processed under atmospheric conditions show low Voc deficit (<0.3 V) without any hysteresis in forward and reverse scans indicating low trap densities. Pt offers an excellent model system for the replacement of Pb due to high atomic number, oxidation resistance, and stability. Cs2PtI6 is an atmospherically stable phase under AM1.5G at 65 °C for 1000 h.

Extrinsic dopants in quasi-one-dimensional photovoltaic semiconductor Sb2S3: A first-principles study

Extrinsic elements such as C, Zn, Sn, Bi, and Ti had been doped into Sb2S3 in order to increase the electrical conductivity and thus the photovoltaic efficiency. However, the influences of these dopants are so far unclear. Using the first-principles calculations, we show that (i) Zn and Sn doping can slightly increase the p-type conductivity of Sb2S3 through forming ZnSb2 and SnSb2 acceptors, explaining the observed increase in photocurrent and carrier concentration; (ii) in contrast, the formation energies of C dopants on different sites are high, which means C doping cannot increase the conductivity of Sb2S3 obviously, so the highly reduced resistivity of C-doped Sb2S3 in experiments cannot be explained and the effects of C doping should be revisited; (iii) Bii acts as the carrier recombination center, so the photocurrent of the Bi-doped Sb2S3 solar cells decreases; and (iv) the formation energies (concentration) of Tii and TiSb1 donors are extremely low (high), so the photocurrent of the Ti doped Sb2S3 solar cells increases significantly. Considering the influences on both electrical conductivity and carrier non-radiative recombination, we propose that Pb and Cl are relatively benign p-type and n-type dopants, respectively. Cl doping can make Sb2S3 show high n-type conductivity and long minority carrier lifetime, thus offering a promising method for overcoming the current efficiency bottleneck of Sb2S3 solar cells.

Influence of impurities from SiC and TiC crucible cover on directionally solidified silicon

To enhance the efficiency of solar cells, it is important to fully understand and weigh the contributions of various factors on the contamination of multi-crystalline silicon during directional solidification. We assessed the influence of hot zone geometry, crucible cover material, purging gas flow rate and gas composition on the concentration of oxygen, carbon and 20 metallic impurities in phosphorus-doped (n-type) multi-crystalline silicon ingot of size G1 (14 kg), as well as on its minority carrier lifetime and resistivity. Studies were performed both experimentally and by 3D CFD simulations. It was found that the crucible cover made of massive titanium carbide ceramic and high flow rates of purging gas (pure argon) are the most beneficial factors for high minority carrier lifetime and reduced red zone.

Suppression of Parasitic Epitaxy Growth and Realization of High-Quality Wafer Surface Passivation of Silicon Heterojunction Solar Cells

Intrinsic hydrogenated amorphous silicon (i-a-Si:H) films lead to excellent surface passivation of crystalline silicon wafers. However, a-Si:H deposition on a crystalline silicon wafer often results in undesired epitaxy growth, which deteriorates the passivation property, In this paper, we studied the influence of varying plasma enhanced chemical vapor deposition (PECVD) parameters, such as the product of the gas pressure (P) and the electrode distance (Di) and the hydrogen dilution ratio (P = SiH4/(H2 + SiH4) × 100), on the passivation quality and the properties of silicon heterojunction (SHJ) solar cells. Measurements showed that proper combinations of high P × Di and large P values can yield high minority carrier lifetimes (MCLTs) of passivated silicon wafers. Also, the tendency of MCLTs measured from passivated wafers is the same as that for open circuit voltages (Voc) of fabricated SHJ solar cells. A high Voc is obtained from SHJ solar cells when unwanted epitaxial growth is minimized at the wafer surface.

Surface structure chemical transfer method to fabricate low reflectance multi-crystalline Si wafers produced with fixed abrasive machining technology

Conventional acidic etching cannot form low reflectance surfaces from multi-crystalline silicon (multi-Si) wafers sawn by fixed abrasive (FXA) machining technology, which makes it difficult to replace the time- and cost-consuming free abrasive (FRA) machining method. In the present work, a nanocrystalline Si (nc-Si) layer is formed by use of the surface structure chemical transfer (SSCT) method, and the layer is used as a starting point of alkaline etching to fabricate low reflectance submicron texture on FXA multi-Si wafers. Although the nc-Si layer cannot be passivated by deposition of a silicon nitride (SiN) layer, the submicron textured surface formed from the nc-Si layer by alkaline etching can effectively be passivated by the SiN layer. Using the developed method, the SiN passivated submicron textured FXA multi-Si wafers possess both high minority carrier lifetime and lower reflectance than that of acidic textured FRA multi-Si wafers. The excellent passivation effect of the SiN layer on the low reflectance textured surface is attributed to the low interface state density of 1.2×1011 cm-2eV-1.

Silicon Nitride Barrier Layers Mitigate Minority-Carrier Lifetime Degradation in Silicon Wafers During Simulated MBE Growth of III–V Layers

We observe a degradation of the minority-carrier lifetime in silicon substrates after a temperature cycle in a molecular beam epitaxy (MBE) chamber that is representative of the growth of III–V materials. This decrease in the lifetime is from milliseconds to microseconds, and in some cases into the sub-microsecond range. The degradation appears to be caused by thermally activated diffusion of metals from the back side of the substrate, occurring at temperatures above 500 °C. This impacts the ability to achieve high-performance monolithic III–V/Si multi-junction solar cells, since the epitaxial growth usually requires high-temperature steps (over 700 °C) for surface de-oxidation or surface reconstruction and temperatures of 400–600 °C during the epitaxial growth. We show that, through phosphorous diffusion gettering, the lifetimes of degraded wafers can be recovered to the millisecond range. Further, we demonstrate that a silicon nitride coating functions both as a diffusion barrier and as an interfacial gettering or hydrogenation agent, enabling high minority-carrier lifetimes directly out of the MBE chamber. This approach allows for the silicon minority-carrier lifetime to be maintained in the millisecond range without the need for post-growth recovery, providing a path to achieve high-efficiency III–V/Si solar cells.

Passivation optimization and performance improvement of planar a-Si:H/c-Si heterojunction cells in perovskite/silicon tandem solar cells

Recently, the monolithic spin-coating perovskite/planar silicon heterojunction tandem solar cells with high performance have attracted attention mainly due to simple fabrication, low preparation cost and high efficiency, especially compared with fully textured multi-junction perovskite/silicon tandem device. As is well known, the excellent passivation of a-Si:H/c-Si interface is the key to achieving a high-efficiency planar silicon heterojunction solar cell, which further improves the performance of the corresponding tandem cell. Therefore, we investigate the elements affecting a-Si:H/c-Si interface passivation, including the c-Si surface treatment technique, a-Si:H passivation layer and P-type emitter layer and so on. In these experiments, we adjust the immersed time of diluent hydrofluoric acid and pre-deposited hydrogen plasma with different gas mixture flows. Also, the suitable deposition parameters of intrinsic a-Si:H passivation layer are regulated by varying hydrogen dilution and time, and variously slight silane content is embedded into i-a-Si:H /P-type (I/P) emitter interface by hydrogen-rich plasma treating which is for acquiring optimal experimental processing conditions to promote the chemical passivation. In addition, the p-a-Si:H and p-nc-Si:H are comparatively studied as buffer layers to further improve the I/P interface passivation by varying the hydrogen dilution in the gas mixture during deposition. It can be found that p-nc-Si:H buffer layer with high conductivity and wide bandgap can not only reduce the defect density at the I/P interface, but also increase the conductivity of P-type emitter, which further improves the field passivation effect. By the above- mentioned optimization, the highest minority carrier lifetime and implied open-circuit voltage (<i>iV</i><sub>oc</sub>) of the structure of P-type emitter/a-Si:H(i)/c-Si/a-Si:H(i)/N-type layer (inip) sample can respectively reach 2855 μs and 709 mV, which demonstrates authentically outstanding passivation performance. An efficiency of 18.76% can be obtained for the planar a-Si/c-Si heterojunction solar cell with a <i>V</i><sub>oc</sub> of 681.5 mV, which is 34.3 mV higher than that of the reference device. Regarding the optimized planar a-Si:H/c-Si heterojunction solar cell as the bottom cell, we also obtain an efficiency of 21.24% for perovskite/silicon heterojunction tandem solar cell with an open-circuit voltage of 1780 mV, which proves that the above strategies are very effective for improving the passivation optimization and performance of bottom cell in the tandem device.

Moving Beyond p-Type mc-Si: Quantified Measurements of Iron Content and Lifetime of Iron-Rich Precipitates in n-Type Silicon

N-type multicrystalline silicon (mc-Si) is a promising alternative to the dominant p-type mc-Si for solar cells because it combines the cost advantages of mc-Si while benefiting from higher tolerance to transition metal contamination. A detailed understanding of the relative roles of point defect and precipitated transition metals has enabled advanced processing and high minority carrier lifetimes in p-type mc-Si. This contribution extends that fundamental understanding to Fe contamination in n-type mc-Si, helping enable processing of this material into an economical and high-performance photovoltaic device. By directly correlating micro-photoluminescence-based minority carrier lifetime mapping and synchrotron-based micro-X-ray fluorescence mapping of Fe-rich precipitates, we develop a quantitative, physical understanding of the recombination activity of Fe-rich precipitates in n - type mc-Si.