S.Q. Xiao

High-Efficiency Silicon Solar Cells—Materials and Devices Physics

High-efficiency Si solar cells have attracted great attention from researchers, scientists, engineers of photovoltaic (PV) industry for the past few decades. Many researchers, scientists, and engineers in both academia and industry seek solutions to improve the cell efficiency and reduce the cost. This desire has drawn stronger support from major funding agencies and industry and stimulated a growing number of major research and research infrastructure programs, and a rapidly increasing number of publications in this filed. This article reviews materials, devices, and physics of high-efficiency Si solar cells developed over the last 20 years and presents representative examples of superior performances and competitive advantages. In this paper there is a fair number of topics, not only from the material viewpoint, introducing various materials that are required for high-efficiency Si solar cells, such as base materials (FZ-Si, CZ-Si, MCZ-Si, and multi-Si), emitter materials (diffused emitter and deposited emitter), passivation materials (Al-BSF, high-low junction, SiO2, SiOx, SiNx, Al2O3 and a-Si:H), and other functional materials (antireflective layer, transparent conductive oxide and metal electrode), but also from the device and physics point of view, elaborating on physics, cell concept, development, and status of most types of high-efficiency Si solar cells, including passivated emitter and rear contact (PERC), passivated emitter and rear locally diffused (PERL), passivated emitter and rear totally-diffused (PERT), Pluto, PANDA, interdigitated back-contacted (IBC), emitter-wrap-through (EWT), metallization-wrap-through (MWT), heterojunction with intrinsic thin-layer (HIT), and so on. Finally, the technical data of these high-efficiency Si solar cells has been tabulated.

Si surface passivation by SiOx?:?H films deposited by a low-frequency ICP for solar cell applications

Hydrogenated silicon suboxide (SiOxu2009:u2009H) thin films are fabricated by a low-frequency inductively coupled plasma of hydrogen-diluted SiH4xa0+xa0CO2 at a low temperature (100xa0°C). Introduction of a small amount of oxygen into the film results in a predominantly amorphous structure, wider optical bandgap, increased H content, lower conductivity and higher activation energy. The minority carrier lifetime in the SiOxu2009:u2009H-passivated p-type Si substrate is up to 428xa0µs with a reduced incubation layer at the interface. The associated surface recombination velocity is as low as 70xa0cmxa0s−1. The passivation behaviour dominantly originates from the H-related chemical passivation. The passivation effect is also demonstrated by the excellent photovoltaic performance of the heterojunction solar cell with the SiOxu2009:u2009H-based passivation and emitter layers.

Plasma-aided fabrication in Si-based photovoltaic applications: an overview

Plasma-aided fabrication has been largely employed in the photovoltaic industry and widely reported in the literature for the growth of Si-based solar cells and the dry etching of Si substrates. This paper reviews the current status of plasma technologies for the synthesis of Si-based thin films (including silicon nitride: SiN) and solar cells, removal of phosphorus silicate glass or parasitic emitters, wafer cleaning, masked or mask-free surface texturization and the direct formation of a p–n junction by means of p-to-n type conductivity conversion. The plasma physics and chemistry involved in these processes and their fundamental mechanisms are briefly discussed. Some examples of superior performance and competitive advantages of plasma processes and techniques are selected to represent a range of applications for solar cells. Finally, an outlook in the field of plasma-aided fabrication for photovoltaic applications is given.

Crystalline silicon surface passivation by intrinsic silicon thin films deposited by low-frequency inductively coupled plasma

Amorphous and microcrystal hydrogenated intrinsic silicon (a-Si:H/μc-Si:H) thin films with good silicon surface passivation effect were deposited using a precursor gases of silane and hydrogen, which were discharged by low frequency inductively coupled high density plasma source. With regard to silicon surface passivation, the effect of discharge power on thin films properties, including the optical band gap, the crystal fraction, and bond configuration, as well as the deposition rate were thoroughly investigated. It was found that the best passivation effect was obtained at the region near the transition regime from a-Si:H to μc-Si:H with a minimized incubation layer between the passivation layer and substrate. Cz-silicon wafer passivated by as-deposited μc-Si:H thin films without any post-deposition thermal annealing possesses minority carrier lifetime of about 234u2009μs. This is attributed to the chemical annealing from the high-density hydrogen plasma during the deposition process. Subsequent thermal ann...

Amorphous/crystalline silicon heterojunction solar cells via remote inductively coupled plasma processing.

Low-frequency inductively coupled plasma (ICP) has been widely used to deposit amorphous or microcrystalline Si thin films, but the intrinsic drawback namely ion bombardment effect limits its application in Si heterojunction solar cells. In this letter, we redesigned typical ICP and realized a remote plasma deposition with suppressed ion bombardment effect. This remote ICP system enables the synthesis of high quality amorphous Si layers with a compact network and a high hydrogen content (10.5%). By using this remote ICP system, we achieved amorphous/crystalline silicon heterojunction solar cells with an efficiency of 14.1% without any back surface field or textures.

Low-temperature deposition of µc-Si : H thin films by a low-frequency inductively coupled plasma for photovoltaic applications

Low-temperature depositions of Si films from hydrogenated amorphous silicon (a-Si : H) to highly crystallized hydrogenated microcrystalline silicon (µc-Si : H) were realized by the low-frequency inductively coupled plasma (LF-ICP) technique, with low hydrogen dilution (50%) and without any intentional substrate heating. µc-Si : H films with a thin incubation layer ( 0.8). Low-temperature growth of µc-Si : H is attributed to high atomic H flux and suppression of high-energy ion bombardment due to the high density of low-temperature electrons in the plasma. A µc-Si : H solar cell with a less dense intrinsic layer (on a SnO2 : F glass substrate) exhibits a high Voc (584 mV), showing great potential for photovoltaic applications.

Effect of silane/hydrogen ratio on microcrystalline silicon thin films by remote inductively coupled plasma

Hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared by remote low frequency inductively coupled plasma (ICP) chemical vapor deposition system, and the effect of silane/hydrogen ratio on the microstructure and electrical properties of μc-Si:H films was systematically investigated. As silane/hydrogen ratio increases, the crystalline volume fraction Fc decreases and the ratio of the intensity of (220) peak to that of (111) peak drops as silane flow rate is increased. The FTIR result indicates that the μc-Si:H films prepared by remote ICP have a high optical response with a low hydrogen content, which is in favor of reducing light-induced degradation effect. Furthermore, the processing window of the phase transition region for remote ICP is much wider than that for typical ICP. The photosensitivity of μc-Si:H films can exceed 100 at the transition region and this ensures the possibility of the fabrication of microcrystalline silicon thin film solar cells with a open-circuit voltage of abo...

Status and Progress of High-efficiency Silicon Solar Cells

High-efficiency Si solar cells have attracted more and more attention from researchers, scientists, engineers of photovoltaic (PV) industry for the past few decades. Many high-quality researchers and engineers in both academia and industry seek solutions to improve the cell efficiency and reduce the cost. This desire has stimulated a growing number of major research and research infrastructure programmes, and a rapidly increasing number of publications in this filed. This chapter reviews materials, devices and physics of high-efficiency Si solar cells developed over the last 20 years. In this chapter there is a fair number of topics, not only from the material viewpoint, introducing various materials that are required for high-efficiency Si solar cells, such as base materials (FZ-Si, CZ-Si, MCZ-Si and multi-Si), emitter materials (diffused emitter and deposited emitter), passivation materials (Al-back surface field, high–low junction, SiO2, SiO x , SiN x , Al2O3 and a-Si:H), and other functional materials (antireflective layer, TCO and metal electrode), but also from the device and physics point of view, elaborating on physics, cell concept, development and status of all kinds of high-efficiency Si solar cells, such as passivated emitter and rear contact (PERC), passivated emitter and rear locally diffused (PERL), passivated emitter and rear totally diffused (PERT), Pluto, interdigitated back-contacted (IBC), emitter-wrap-through (EWT), metallization-wrap-through (MWT), Heterojunction with intrinsic thin-layer (HIT) and so on. Some representative examples of high-efficiency Si solar cell materials and devices with excellent performance and competitive advantages are presented.

Silicon homojunction solar cells via a hydrogen plasma etching process

We report on the one-step formation of an efficient Si homojunction solar cell produced by a simple exposure of p-type Si wafers to low-temperature inductively coupled hydrogen plasma. The formation of oxygen thermal donors during hydrogen plasma treatment is responsible for the conductivity type conversion and the final formation of Si homojunction. The hydrogen plasma etching with suppressed heavy ion bombardment results in a relatively flat surface, which is favourable for deposition of passivation layers such as silicon nitride. The integrated Si homojunction solar cell consisting of Al/p-c-Si/n-c-Si/SiN/Al-grid has demonstrated a maximum photovoltaic conversion efficiency of 13.6%.

Phase evolution and room-temperature photoluminescence in amorphous SiC alloy

Amorphous SiC thin films with varying phases and compositions have been synthesized using a low frequency inductively coupled high density plasma source in a hydrogen diluted methane (CH4) and silane (SiH4) mixture. The optical and electrical properties along with the microstructures of the thin films are systematically investigated. The feedstock gas ratio of CH4/SiH4 leads to the fluctuations of the optical bandgap, the carbon content, and the transition of Si–Si bonding structure from crystalline to intermediate phase and finally to amorphous phase. Room temperature photoluminescence (PL) with nearly fixed emission energy has been observed in the thin films. The underlying PL mechanism is explained in the framework of quantum confinement-luminescence center model. The photoexcitation process occurs in the nc-Si quantum dots embedded in the host SiC matrix, whereas the photoemission process occurs in the luminescence centers in the surrounding SiC or at SiC-Si interfaces. The PL evolution with the chemi...