High Minority Carrier Lifetime Research Articles

From simulation to experiment: Understanding BO-regeneration kinetics

Regeneration of boron–oxygen related defects is investigated in differently compensated silicon wafers. It is shown for the first time that boron–oxygen defects can be transformed into a stable regenerated state also in compensated n-type silicon.The coupling between regeneration rate and the completeness of the regeneration reaction is simulated based on the 3-state-model of BO defects. Maximum regeneration temperatures that can be applied are determined for differently regenerating samples. The results are used to develop a high-speed process that can accelerate regeneration by two orders of magnitude without compromising neither the completeness of the regeneration process nor the stability of the resulting high minority carrier lifetime values.

Identifying defect-tolerant semiconductors with high minority-carrier lifetimes: beyond hybrid lead halide perovskites

Abstract

Minority carrier lifetime and efficiency improvement of multicrystalline silicon solar cells by two-step process

Impurities and defects are of significant interest in multicrystalline silicon, due to the detrimental effect they can have on carrier lifetimes and electrical properties. In view of that, it is important to incorporate certain processing steps to decrease the recombination activities. In this study, a novel experiment was applied as a beneficial approach to improve the electronic quality of low-resistivity mc-Si substrates via a two-step process. Initially, the first step involves gettering multicrystalline substrates using sacrificial porous silicon layer on both sides, which was introduced as a simple sequence for efficient extrinsic gettering schemes. The gettering experiment was performed at 600–900 °C, and optimum results were obtained at 900 °C. Then, the second step involves coating the front surface of gettered mc-Si at 900 °C with vanadium oxide that serves as an excellent antireflection layer and leads to improve furthermore the electrical properties. Significant improvements were obtained after the deposition of vanadium oxide antireflection coating, in view of the fact that gettered mc-Si substrate at 900 °C provides the highest minority carrier lifetime and the lowest effective surface recombination velocity. An overall increase of the electrical properties was obtained after the described two-step process. The conversion efficiency increases from 6% (reference) and reached 13.7%.

On the surface passivation of c-silicon by RF sputtered Al2O3 for solar cell application

In this contribution we investigated the surface passivation properties of Al2O3 deposited by RF sputtering on c-silicon substrate. MIS capacitors with Al2O3 as dielectric were fabricated and characterized. Post deposition anneal in N2 ambient for 20 min was carried out to activate the passivation. Fixed charge, interface defect density and minority carrier lifetimes were evaluated from the C-V, G-V and C-t characteristics of the MIS capacitor respectively to study the level of surface passivation. It has been demonstrated that by optimizing various sputtering process parameters such as power and base pressure etc. high level of passivation can be achieved for RF sputtered alumina. High level passivation of Al2O3 is attributed to high minority carrier lifetimes. Negative fixed charge density (Qf) as high as 5 × 1012 cm−2 and interface defect density (Dit) as low as 1.85 × 1011 eV−1 cm−2 have been achieved. Minority carrier lifetime was observed to vary from 70 to 290 μs for various sputter deposition and annealing processing condition. The obtained carrier lifetimes are comparable to the value achieved for atomic layer deposited Al2O3 which is generally considered to be favorable process for passivation of solar cells.

Ultra high amorphous silicon passivation quality of crystalline silicon surface using in-situ post-deposition treatments

A parametric study of post-deposition hydrogen plasma treatment of intrinsic a:Si:H films is performed. We demonstrate a significant improvement in passivation of c-Si(100) promoting epitaxy after an in-situ hydrogen plasma treatment depending mainly on the pressure and slightly on the power. Plasma diagnostic indicates an increase of Hα* signal with high power and low pressure. However, our analysis reveals a better hydrogen incorporation with high pressure and a slight increase in monohydride with high power. Longer H2 plasma duration up to 50 s shows no detrimental effect on the passivation quality. Optimizing the in-situ H2 plasma treatment, high minority carrier lifetime over 15 ms was achieved after short thermal annealing. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

결정질 실리콘 태양전지의 패시베이션 적용을 위한 Al2O3/SiON 적층구조의 열적 안정성에 대한 연구

We investigated the influence of blistering on Al₂O₃/SiON stacks and Al₂O₃/SiNx:H stacks passivation layers. Al₂O₃ film provides outstanding Si surface passivation quality. Al₂O₃ film as the rear passivation layer of a p-type Si solar cell is usually stacked with a capping layer, such as SiO₂, SiNx, and SiON films. These capping layers protect the thin Al₂O₃ layer from an Al electrode during the annealing process. We compared Al₂O₃/SiON stacks and Al₂O₃/SiNx:H stacks through surface morphology and minority carrier lifetime after annealing processes at 450℃ and 850℃. As a result, the Al₂O₃/SiON stacks were observed to produce less blister phenomenon than Al₂O₃/SiNx:H stacks. This can be explained by the differences in the H species content. In the process of depositing SiNx film, the rich H species in NH₃ source are diffused to the Al₂O₃ film. On the other hand, less hydrogen diffusion occurs in SiON film as it contains less H species than SiNx film. This blister phenomenon leads to an increase insurface defect density. Consequently, the Al₂O₃/SiON stacks had a higher minority carrier lifetime than the Al₂O₃/SiNx:H stacks.

Improving the Quality of Epitaxial Foils Produced Using a Porous Silicon-based Layer Transfer Process for High-Efficiency Thin-Film Crystalline Silicon Solar Cells

A porous silicon-based layer transfer process to produce thin (30-50 μm) kerfless epitaxial foils (epifoils) is a promising approach toward high-efficiency solar cells. For high efficiencies, the epifoil must have high minority carrier lifetimes. The epifoil quality depends on the properties of the porous layer since it is the template for epitaxy. It is shown that by reducing the thickness of this layer and/or its porosity in the near-surface region, the near-surface void size is reduced to <;65 nm and in certain cases achieve a 100 nm-thick void-free zone below the surface. Together with better void alignment, this allows for a smoother growth surface with a roughness of <;35 Å and reduced stress in the porous silicon. These improvements translate into significantly diminished epifoil crystal defect densities as low as ~420 defects/cm 2. Although epifoils on very thin porous silicon were not detachable, a significant improvement in the lifetime (diffusion length) of safely detachable n-type epifoils from ~85 (~300 μm) to ~195 μs (~470 μm) at the injection level of 10 15/cm 3 is achieved by tuning the porous silicon template. Lifetimes exceeding ~350 μs have been achieved in the reference lithography-based epifoils, showing the potential for improvement in porous silicon-based epifoils.

Hydrogen plasma treatments for passivation of amorphous-crystalline silicon-heterojunctions on surfaces promoting epitaxy

The impact of post-deposition hydrogen plasma treatment (HPT) on passivation in amorphous/crystalline silicon (a-Si:H/c-Si) interfaces is investigated. Combining low temperature a-Si:H deposition and successive HPT, a high minority carrier lifetime &amp;gt;8 ms is achieved on c-Si 〈100〉, which is otherwise prone to epitaxial growth and thus inferior passivation. It is shown that the passivation improvement stems from diffusion of hydrogen atoms to the heterointerface and subsequent dangling bond passivation. Concomitantly, the a-Si:H hydrogen density increases, leading to band gap widening and void formation, while the film disorder is not increased. Thus, HPT allows for a-Si:H band gap and a-Si:H/c-Si band offset engineering.

Ultra-high frequency photoconductivity decay in GaAs/Ge/GaAs double heterostructure grown by molecular beam epitaxy

GaAs/Ge/GaAs double heterostructures (DHs) were grown in-situ using two separate molecular beam epitaxy chambers. High-resolution x-ray rocking curve demonstrates a high-quality GaAs/Ge/GaAs heterostructure by observing Pendellösung oscillations. The kinetics of the carrier recombination in Ge/GaAs DHs were investigated using photoconductivity decay measurements by the incidence excitation from the front and back side of 15 nm GaAs/100 nm Ge/0.5 μm GaAs/(100)GaAs substrate structure. High-minority carrier lifetimes of 1.06-1.17 μs were measured when excited from the front or from the back of the Ge epitaxial layer, suggests equivalent interface quality of GaAs/Ge and Ge/GaAs. Wavelength-dependent minority carrier recombination properties are explained by the wavelength-dependent absorption coefficient of Ge.

High Efficiency Silicon Solar Cells

Silicon remains the material of choice for photovoltaics because of its abundance, non-toxicity, high and stable cell efficiencies, the maturity of production infrastructure and the deep and widespread level of skill available in relation to silicon devices. Rapidly decreasing module prices mean that area-related balance of systems costs are an increasing proportion of photovoltaic systems price. This places a premium on efficient cells. In recent years there have been large improvements in mass production of high quality wafers, the ability to handle thin wafers, maintenance of high minority carrier lifetimes, surface passivation, minimisation of optical losses, device characterisation and in other areas. Many of these improvements are viable in mass production. The upper limit of silicon solar cell efficiency is 29%, which is substantially higher than the best laboratory (25%) [1] and large-area commercial (24%) [2,3] cells. Cell efficiencies above 25% appear to be feasible in both a laboratory and commercial environment. Such a cell will have minimal bulk recombination due to a combination of a thin substrate with a very high minority carrier lifetime; superb surface passivation; small-area electrical contacts consistent with low contact recombination, free carrier absorption and contact resistance; excellent optical control through the use of texturing, antireflection coatings and rear surface reflectors; low edge recombination assisted by the use of thinner wafers, larger cells and edge passivation; and sufficient metal coverage to minimise resistive losses. This paper will survey current work in high- performance silicon solar cell design and fabrication, and discuss approaches to efficiency improvements.

High quality amorphous–crystalline silicon heterostructure prepared by grid-biased remote radio-frequency plasma enhanced chemical vapor deposition

Very high effective minority carrier lifetime (6.3ms) and very low surface recombination velocity (2.6cm/s) have been demonstrated on float-zone 1–2Ωcm crystalline silicon (c-Si) wafers by depositing a-Si:H films using grid-biased remote radio-frequency plasma enhanced chemical vapor deposition (RF PECVD). This method employs a semi-transparent DC biased-grid within the conventional RF PECVD configuration. The DC-biased grid is positioned between the RF electrodes in order to develop a remote plasma and thus allow control of the flux and type of precursors involved in the growth of hydrogenated amorphous silicon (a-Si:H) film. It is shown that compared to conventional RF diode, the grid-biased remote RF PECVD method produces a-Si:H films with superior passivating properties as well as significantly lower concentrations of void and SiH2 bonding and a lower overall hydrogen content, all of which contribute to a higher quality a-Si:H–c-Si heterostructure.

Electrical, optical and structural investigation of plasma-enhanced chemical-vapor-deposited amorphous silicon oxynitride films for solar cell applications

Hydrogenated amorphous silicon oxynitride (a-SiOxNy:H) films, which are deposited by the plasma decomposition of silane and nitrous oxide at low temperatures (Tdep<300°C), are investigated in order to evaluate the potential of these films for photovoltaic applications. In this work both, intrinsic and doped a-SiOxNy:H films are investigated in terms of their electrical, optical and structural properties using Fourier-transform infra-red (FTIR) and secondary-ion mass-spectroscopy (SIMS), as well as high-resolution transmission electron microscopy (HRTEM), photo-conductance decay (PCD), spectral ellipsometry and temperature-dependent conductivity measurements. The plasma deposition parameters are optimized in terms of effective minority carrier lifetime, dark conductivity and low absorbance (i.e. high optical band gap). The optical band gap of the a-SiOxNy:H films can be widened up to 2.2eV compared to a-Si:H due to the incorporation of oxygen and nitrogen into the amorphous network. Not only the optical band gap but also the passivation quality and the dark conductivity of the films are well correlated with the oxygen and nitrogen concentration, which are monitored by means of SIMS measurements. When applying an a-SiOxNy:H film with an optical band gap of 2.0eV, a very high effective minority carrier lifetime of 2.5ms is measured. In case of doped films, conductivities up to σdark=4.5×10−3S/cm for the n-type doping and σdark=3.9×10−4S/cm for the p-type doping are achieved. Combining the intrinsic and doped a-SiOxNy:H films to heteroemitter stacks on a crystalline Si base, a very high implied open circuit voltage of up to 733mV is demonstrated. FTIR and HRTEM measurements reveal a homogenous distribution of Si–Si and Si–O–Si bonds in the a-SiOxNy:H films.

Seed-assisted cast quasi-single crystalline silicon for photovoltaic application: Towards high efficiency and low cost silicon solar cells

We have demonstrated the seed-assisted cast quasi-single crystalline (QSC) silicon technique to achieve high efficiency solar cells with low cost. Compared to multicrystalline (mc) silicon, the QSC silicon has better material properties, having higher minority carrier lifetime and fewer grain boundaries and dislocations. Furthermore, the 〈100〉 oriented QSC silicon can achieve a lower surface reflectance using alkaline texturing. Based on these two factors, the efficiency of the QSC silicon solar cells with the industrial size has been improved by up to 1% absolutely from the mc-Si counterparts. Compared to the Czochralski (CZ) silicon solar cells, the QSC cells have slightly lower efficiency but high productivity and negligible light-induced degradation. These results suggest a great potential of the QSC silicon applied in photovoltaic industry as the next generation substrate. To make the QSC silicon more competitive in industry, further efforts should be focused on the recycling of seed crystals, the coverage of mono-region and the control of structural defects.

High-efficiency core–shell solar cell array from Si wafer

High-efficiency micro core–shell solar cell (μ-CSSC) arrays are fabricated from Si wafer by using traditional lithography and phosphorus diffusion. The p–n junction depth is around 450 nm, indicating that it forms core–shell structure in micropillars by using phosphorus diffusion. The μ-CSSC arrays have high minority carrier lifetime of 23 μs and long diffusion length of ∼150 μm. The best power efficiency of the μ-CSSC reaches to 9.2%. It is a convenient method for fabricating Si μ-CSSC arrays in wafer scale for future applications.

Low band gap liquid-processed CZTSe solar cell with 10.1% efficiency

A low band gap liquid-processed Cu2ZnSn(Se1−xSx)4 (CZTSSe) kesterite solar cell with x ≈ 0.03 is prepared from earth abundant metals, yielding 10.1% power conversion efficiency. This champion cell shows a band gap of 1.04 eV, higher minority-carrier lifetime, lower series resistance and lower Voc deficit compared to our previously reported higher band gap (Eg = 1.15 eV; x ≈ 0.4) cell with similar record efficiency. The ability to vary the CZTSSe band gap using sulfur content (i.e., varying x) facilitates the examination of factors limiting performance in the current generation of CZTSSe devices, as part of the thrust to achieve operational parity with CdTe and Cu(In,Ga)(S,Se)2 (CIGSSe) analogs.

The path towards a high-performance solution-processed kesterite solar cell

Despite the promise of thin-film Cu(In,Ga)(S,Se) 2 (CIGSSe) chalcopyrite and CdTe photovoltaic technologies with respect to reducing cost per watt of solar energy conversion, these approaches rely on elements that are either costly and/or rare in the earth's crust (e.g., In, Ga, Te) or that present toxicity issues (e.g., Cd), thereby potentially limiting these technologies in terms of future cost reduction and production growth. In order to develop a photovoltaic technology that is truly compatible with terawatt deployment, it is desirable to consider material systems that employ less toxic and lower cost elements, while maintaining the advantages of the chalcopyrite and CdTe materials with respect to appropriate direct band gap tunability over the solar spectrum, high device performance (e.g., >10% power conversion efficiency) and compatibility with low-cost manufacturing. In this review, the development of kesterite-based Cu 2ZnSn(S,Se) 4 (CZTSSe) thin-film solar cells, in which the indium and gallium from CIGSSe are replaced by the readily available elements zinc and tin, will be reviewed. While vacuum-deposited devices have enabled optimization within the compositional phase space and yielded selenium-free CZTS device efficiencies of as high as 6.8%, more recently a liquid-based approach has been described that has enabled deposition of CZTSSe devices with power conversion efficiency of 9.7%, bringing the kesterite-based technology into a range of potential commercial interest. Electrical characterization studies on these high-performance CZTSSe cells reveal some of the key loss mechanisms (e.g., dominant interface recombination, high series resistance and low minority carrier lifetime) that limit the cell performance. Further elucidation of these mechanisms, as well as building an understanding of long-term device stability, are required to help propel this relatively new technology forward.

Desenvolvimento de células solares em silício tipo n com emissor formado por Boro

O Sol é fonte de energia renovável e o seu uso para produzir energia elétrica é uma das alternativas promissoras para enfrentar os desafios energéticos e ambientais do novo milênio. O silício é o segundo elemento mais abundante da Terra. Este material semicondutor é largamente usado na indústria de células solares e dispositivos de microeletrônica e permite a fabricação de dispositivos de alta durabilidade. O silício cristalino tipo n vem despertando o interesse mundial por apresentar menor degradação e maior tempo de vida dos portadores minoritários quando comparado com silício tipo p. O objetivo deste trabalho está centrado no desenvolvimento de um processo de fabricação industrial de células solares p+nn+, pseudoquadradas de 80 mm x 80 mm, em silício crescido por fusão zonal flutuante (Si-FZ) tipo n, com metalização por serigrafia. A região p+ foi produzida a partir de dopante com boro depositado por spin-on e difusão a alta temperatura em forno convencional com tubo de quartzo. A dopagem com boro foi otimizada considerando as características elétricas das células solares. O método consistiu em variar a temperatura de difusão de 900 ºC a 1020 ºC e tempo de difusão do processo de 10 min a 40 min. O processo de passivação de superfície foi avaliado utilizando SiO2 o que demonstrou não ser eficaz para o tipo de célula solar deste trabalho. Os melhores dispositivos foram fabricados com difusão de boro a 1000 ºC por 30 min, sem passivação de superfície, atingindo-se a eficiência de 14,6 %.

Silicon PV devices based on a single step for doping, anti-reflection and surface passivation

A simplified process for photovoltaic device fabrication is explored. The simplified process incorporates a single step wherein a thin film of phosphorus doped spin on dopant (SOD) serves as dopant source, anti-reflection coating (ARC) and a surface passivation layer. Detailed fabrication and photovoltaic device characterization results are presented. Device results for solar cells fabricated on low minority carrier lifetime Cz-substrates and high minority carrier lifetime Fz (polished and chemically textured substrates) are presented. Using this simplified process, photovoltaic devices with conversion efficiency of over 15% on c-Si wafers have been demonstrated. Further improvements in efficiency are possible by reduction in series resistance, control of surface dopant concentration and a rear passivated electrode configuration.

Ga segregation during Czochralski-Si crystal growth with Ge codoping

The segregation of Ga during the growth of Czochralski-Si crystals with Ge codoping was investigated. The effective segregation coefficient of Ga in Ga/Ge-codoped Si crystal growth was nearly constant over a wide Ge concentration range, even at high Ge concentrations of about 10 21 cm −3. In contrast, the effective segregation coefficient increased at high B concentrations in Ga/B-codoped CZ-Si crystal growth. The segregation behavior of Ga in Ga/Ge- and Ga/B-codoped CZ-Si crystal growth was theoretically compared. The difference in the segregation coefficients of Ga as a function of the codoped impurity (Ge or B) between the two Si crystals was attributed to a difference in the excess enthalpy due to impurity incorporation into the Si crystal between Ga–Ge pairs and Ga–B pairs The effect of Ge codoping on the minority carrier lifetime in Ga/Ge-codoped CZ-Si crystals was also investigated. The minority carrier lifetime increased with increasing Ge concentration. The higher minority carrier lifetime was associated with a decrease in interstitial oxygen related to D-defects in the Si crystal.

High effective minority carrier lifetime on silicon substrates using quinhydrone-methanol passivation

Iodine-methanol (I2/ME), a chemical passivation method, is extensively used in silicon (Si) solar cell fabrication for measuring minority carrier lifetime in bulk regions. We demonstrate that quinhydrone-methanol (QHY/ME) provides higher lifetimes than I2/ME. For 0.01 mol/dm−3 QHY/ME on float-zone (FZ) wafers at 1×1015 cm−3 injection level, a high lifetime of 3.3 ms and surface recombination velocity of 7 cm/sec on n-type (100 Ω cm) and 1.1 ms on p-type (2 Ω cm) is reported. The surface recombination velocity is also measured out of solution for several days. Chemical characterization results indicate increasing surface oxidation with decreasing passivation, consistent with proposed bonding mechanism.