Chun Gong

Screen-Printed Aluminum-Alloyed

We demonstrate the use of industrial-orientated screen-printed aluminum-alloyed emitter for high-efficiency n-type interdigitated back-contact silicon solar cells. Different cell designs with various pitch sizes and emitter fractions have been studied. With an improved front surface field (FSF), short-circuit current densities up to 40 can be obtained. By combining the best cell design and the improved FSF, a high conversion efficiency of 19.1% with Czochralski n-type material has been achieved.

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Low frequency direct plasma-enhanced chemical vapor deposited Si–SiNx interface properties with and without NH3 plasma pretreatment, with and without rapid thermal annealing (RTA) have been investigated with deep-level transient spectroscopy (DLTS) on both n- and p-type monocrystalline silicon samples. It is shown that four different defect states are identified at the Si–SiNx interface. Energy-dependent electron and hole capture cross sections were also measured by small-pulse DLTS. Samples with plasma NH3 pretreatment and RTA show the lowest DLTS signals, which suggest the lowest overall interface states density. Moreover, SiNx with RTA passivates interface states more efficiently in n-type Si compared with p-type Si; also the deep-level parameters change in n-type Si but not in p-type Si. The combination of plasma NH3 pretreatment and RTA is suggested for application in the solar cell fabrication.

Emitter on High-Efficiency N-Type Interdigitated Back-Contact Silicon Solar Cells

Deep Level Transient Spectroscopy (DLTS) has been applied to Metal-Oxide-Semiconductor (MOS) capacitors fabricated on crystalline silicon n- and p-type substrates, with a SiO 2 or a SiO 2 /SiN x passivation stack, covered by an Al gate. It is shown that similar interface state distributions are obtained in both cases, from which it is concluded that the SiN x deposition does not degrade the interface. It is also shown that a rather large density of dangling bond defects is present at the Si/SiO 2 interface, which is related to the absence of a post metallization forming gas annealing.

A deep-level transient spectroscopy study of silicon interface states using different silicon nitride surface passivation schemes

Thin layers of intrinsic and doped amorphous silicon have been used as the emitter in a processing scheme to form heterojunction interdigitated back contact (HJ-IBC) solar cells. Such processing involved depositing the emitter across the wafer, and subsequent patterning to define the base and emitter regions. Average cell efficiencies of 14.6% have been achieved (with a best cell efficiency of 15.2%), with average short circuit current densities as high as 37.8 mA/cm2 (maximum 39.1 mA/cm2), demonstrating the potential of such a solar cell structure. However, the open circuit voltage and fill factor values are lower than could be expected. The reasons behind this are discussed, as are proposed solutions to further enhance the cell performance.

A DLTS Study of SiO2 and SiO2/SiNx Surface Passivation of Silicon

Deep-level transient spectroscopy (DLTS) has been applied to metal–insulator–semiconductor (MIS) capacitors fabricated on planar (1 0 0), planar (1 1 1) orientations and textured n-type silicon wafers. Low frequency direct plasma-enhanced chemical vapour deposition Si–SiNx interface properties with and without plasma NH3 pre-treatment, with and without rapid thermal annealing (RTA) have been investigated. It is shown that three different kinds of defect states are identified at the Si–SiNx interface. For the planar (1 0 0) surface, samples with plasma NH3 pre-treatment plus RTA show the lowest DLTS signals, which suggests the lowest overall interface states density. For planar (1 1 1) Si surfaces, plasma NH3 pre-treatment and RTA yield a small improvement. With the textured surface, the RTA step improves the surface passivation quality further but no obvious impact is found with plasma NH3 pre-treatment. Energy-dependent electron capture cross sections were also measured by small-pulse DLTS. The capture cross sections depend strongly on the energy level and decrease towards the conduction band edge.

Interdigitated rear contact solar cells with amorphous silicon heterojunction emitter

N-type interdigitated back contact (IBC) silicon solar cells have been successfully applied industrially with high-efficiency of 23.4% by Sunpower and are being investigated by several research groups. However, the formation of p+ emitter is still an issue. A traditional method is boron diffusion which needs high temperature processes to form the emitter and to remove the silicate glass boron skin. In recent years, people have shown excellent results on n-type front contact rear junction cells with a screen-printed Al-alloyed emitter. In this work, we demonstrate the use of such emitters on n-type IBC silicon solar cells.

Study of silicon-silicon nitride interface properties on planar (1 0 0), planar (1 1 1) and textured surfaces using deep-level transient spectroscopy

Silicon nitride (SiNx) films deposited by direct plasma-enhanced chemical vapor deposition (PECVD) are widely used in silicon solar cell fabrication as passivation layers, yielding very low surface recombination velocities on crystalline Si (c-Si) material. So far, there have been some reports on deep-level transient spectroscopy (DLTS) of as-deposited SiNx layers on Si, but the impact of rapid thermal anneal (RTA) processing step and the textured surface has not been investigated yet. In this paper, low frequency direct PECVD Si-SiNx interface properties with and without plasma NH3 pre-treatment, with and without RTA on both flat (100) and (111) orientations and textured n-type silicon samples have been investigated with DLTS.

High efficient N-type interdigitated back contact silicon solar cells with screen-printed al-alloyed emitter

Interdigitated Back Junction (IBJ) solar cells are suited to be realized on thin wafers, since they need a combination of high diffusion lengths and thin substrates. Here we present an industrial type passivation stack at the rear surface of an IBJ solar cell, consisting of a low quality SiO2 and a SiNx layer, which has proven its value in the i-PERC concept developed at IMEC. Quasi-Steady-State Photo Conductance (QSSPC) lifetime measurements indicate an improvement of lifetime from 10µs to 180µs after application and rapid thermal processing (RTP) of the industrial passivation stack. For the IBJ solar cell concept without back surface field (BSF); this results in an absolute increase in energy conversion efficiency of 2% absolute, going from 14% to 16% and with a BSF up to 18%. Out of the presented cell data we can conclude the excellent passivation quality of the applied industrial passivation stack.

Study of silicon-silicon nitride interface properties on flat and textured surfaces by deep level transient spectroscopy

In this paper heterojunction emitters formed by intrinsic and doped amorphous silicon layers on top of the crystalline silicon are presented as a promising option for rear junction n-type silicon solar cells which are suited for the indoor application. In a preliminary test, n-type solar cells with a rear-junction heterojunction emitter show higher open circuit voltages compared to rear-junction cells with a screen-printed Al-alloyed emitter. 15.6% efficiency heterojunction emitter cells have been obtained without any optimization. From a characterization of the emitter saturation current density, we conclude that optimization of the amorphous silicon deposition conditions can improve the HIT emitter quality and that the implied open circuit voltage can be as high as 698 mV, which is very encouraging especially also for low light intensity applications.