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Frontiers in Energy

ISSN 2095-1701 (Print)
ISSN 2095-1698 (Online)
Formerly Known as Frontiers of Energy and Power Engineering in China

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, Volume 11 Issue 1 Previous Issue    Next Issue
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EDITORIAL
RESEARCH ARTICLE
Statistical analysis of recombination properties of the boron-oxygen defect in p-type Czochralski silicon
Nitin NAMPALLI,Tsun Hang FUNG,Stuart WENHAM,Brett HALLAM,Malcolm ABBOTT
Front. Energy. 2017, 11 (1): 4-22.   DOI: 10.1007/s11708-016-0442-6
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This paper presents the application of lifetime spectroscopy to the study of carrier-induced degradation ascribed to the boron-oxygen (BO) defect. Specifically, a large data set of p-type silicon samples is used to investigate two important aspects of carrier lifetime analysis: ① the methods used to extract the recombination lifetime associated with the defect and ② the underlying assumption that carrier injection does not affect lifetime components unrelated to the defect. The results demonstrate that the capture cross section ratio associated with the donor level of the BO defect (k1) vary widely depending on the specific method used to extract the defect-specific recombination lifetime. For the data set studied here, it was also found that illumination used to form the defect caused minor, but statistically significant changes in the surface passivation used. This violation of the fundamental assumption could be accounted for by applying appropriate curve fitting methods, resulting in an improved estimate of k1 (11.90±0.45) for the fully formed BO defect when modeled using the donor level alone. Illumination also appeared to cause a minor, apparently injection-independent change in lifetime that could not be attributed to the donor level of the BO defect alone and is likely related to the acceptor level of the BO defect. While specific to the BO defect, this study has implications for the use of lifetime spectroscopy to study other carrier induced defects. Finally, we demonstrate the use of a unit-less regression goodness-of-fit metric for lifetime data that is easy to interpret and accounts for repeatability error.

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Laser enhanced gettering of silicon substrates
Daniel CHEN,Matthew EDWARDS,Stuart WENHAM,Malcolm ABBOTT,Brett HALLAM
Front. Energy. 2017, 11 (1): 23-31.   DOI: 10.1007/s11708-016-0441-7
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One challenge to the use of lightly-doped, high efficiency emitters on multicrystalline silicon wafers is the poor gettering efficiency of the diffusion processes used to fabricate them. With the photovoltaic industry highly reliant on heavily doped phosphorus diffusions as a source of gettering, the transition to selective emitter structures would require new alternative methods of impurity extraction. In this paper, a novel laser based method for gettering is investigated for its impact on commercially available silicon wafers used in the manufacturing of solar cells. Direct comparisons between laser enhanced gettering (LasEG) and lightly-doped emitter diffusion gettering demonstrate a 45% absolute improvement in bulk minority carrier lifetime when using the laser process. Although grain boundaries can be effective gettering sites in multicrystalline wafers, laser processing can substantially improve the performance of both grain boundary sites and intra-grain regions. This improvement is correlated with a factor of 6 further decrease in interstitial iron concentrations. The removal of such impurities from multicrystalline wafers using the laser process can result in intra-grain enhancements in implied open-circuit voltage of up to 40 mV. In instances where specific dopant profiles are required for a diffusion on one surface of a solar cell, and the diffusion process does not enable effective gettering, LasEG may enable improved gettering during the diffusion process.

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Impact of thermal processes on multi-crystalline silicon
Moonyong KIM,Phillip HAMER,Hongzhao LI,David PAYNE,Stuart WENHAM,Malcolm ABBOTT,Brett HALLAM
Front. Energy. 2017, 11 (1): 32-41.   DOI: 10.1007/s11708-016-0427-5
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Fabrication of modern multi-crystalline silicon solar cells involves multiple processes that are thermally intensive. These include emitter diffusion, thermal oxidation and firing of the metal contacts. This paper illustrates the variation and potential effects upon recombination in the wafers due to these thermal processes. The use of light emitter diffusions more compatible with selective emitter designs had a more detrimental effect on the bulk lifetime of the silicon than that of heavier diffusions compatible with a homogenous emitter design and screen-printed contacts. This was primarily due to a reduced effectiveness of gettering for the light emitter. This reduction in lifetime could be mitigated through the use of a dedicated gettering process applied before emitter diffusion. Thermal oxidations could greatly improve surface passivation in the intra-grain regions, with the higher temperatures yielding the highest quality surface passivation. However, the higher temperatures also led to an increase in bulk recombination either due to a reduced effectiveness of gettering, or due to the presence of a thicker oxide layer, which may interrupt hydrogen passivation. The effects of fast firing were separated into thermal effects and hydrogenation effects. While hydrogen can passivate defects hence improving the performance, thermal effects during fast firing can dissolve precipitating impurities such as iron or de-getter impurities hence lower the performance, leading to a poisoning of the intra-grain regions.

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POCl3 diffusion for industrial Si solar cell emitter formation
Hongzhao LI,Kyung KIM,Brett HALLAM,Bram HOEX,Stuart WENHAM,Malcolm ABBOTT
Front. Energy. 2017, 11 (1): 42-51.   DOI: 10.1007/s11708-016-0433-7
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POCl3 diffusion is currently the de facto standard method for industrial n-type emitter fabrication. In this study, we present the impact of the following processing parameters on emitter formation and electrical performance: deposition gas flow ratio, drive-in temperature and duration, drive-in O2 flow rate, and thermal oxidation temperature. By showing their influence on the emitter doping profile and recombination activity, we provide an overall strategy for improving industrial POCl3 tube diffused emitters.

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SiO2 passivation layer grown by liquid phase deposition for silicon solar cell application
Yanlin CHEN,Sihua ZHONG,Miao TAN,Wenzhong SHEN
Front. Energy. 2017, 11 (1): 52-59.   DOI: 10.1007/s11708-016-0429-3
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Surface passivation is one of the primary requirements for high efficient silicon solar cells. Though the current existed passivation techniques are effective, expensive equipments are required. In this paper, a comprehensive understanding of the SiO2 passivation layer grown by liquid phase deposition (LPD) was presented, which was cost-effective and very simple. It was found that the post-annealing process could significantly enhance the passivation effect of the LPD SiO2 film. Besides, it was revealed that both chemical passivation and field-effect passivation mechanisms played important roles in outstanding passivation effect of the LPD SiO2 film through analyzing the minority carrier lifetime and the surface recombination velocity of n-type and p-type silicon wafers. Although the deposition parameters had little influence on the passivation effect, they affected the deposition rate. Therefore, appropriate deposition parameters should be carefully chosen based on the compromise of the deposition rate and fabrication cost. By utilizing the LPD SiO2 film as surface passivation layer, a 19.5%-efficient silicon solar cell on a large-scale wafer (156 mm × 156 mm) was fabricated.

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Defect passivation on cast-mono crystalline screen-printed cells
Alison WENHAM,Lihui SONG,Malcolm ABBOTT,Iskra ZAFIROVSKA,Sisi WANG,Brett HALLAM,Catherine CHAN,Allen BARNETT,Stuart WENHAM
Front. Energy. 2017, 11 (1): 60-66.   DOI: 10.1007/s11708-016-0443-5
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Cast-mono crystalline silicon wafers contain crystallographic defects, which can severely impact the electrical performance of solar cells. This paper demonstrates that applying hydrogenation processes at moderate temperatures to finished screen print cells can passivate dislocation clusters within the cast-mono crystalline silicon wafers far better than the hydrogenation received during standard commercial firing conditions. Efficiency enhancements of up to 2% absolute are demonstrated on wafers with high dislocation densities. The impact of illumination to manipulate the charge state of hydrogen during annealing is investigated and found to not be significant on the wafers used in this study. This finding is contrary to a previous study on similar wafers that concluded increased H or H0 from laser illumination was responsible for the further passivation of positively charged dangling bonds within the dislocation clusters.

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An industrial solution to light-induced degradation of crystalline silicon solar cells
Meng XIE,Changrui REN,Liming FU,Xiaodong QIU,Xuegong YU,Deren YANG
Front. Energy. 2017, 11 (1): 67-71.   DOI: 10.1007/s11708-016-0430-x
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Boron-oxygen defects can cause serious light-induced degradation (LID) of commercial solar cells based on the boron-doped crystalline silicon (c-Si), which are formed under the injection of excess carriers induced either by illumination or applying forward bias. In this contribution, we have demonstrated that the passivation process of boron-oxygen defects can be induced by applying forward bias for a large quantity of solar cells, which is much more economic than light illumination. We have used this strategy to trigger the passivation process of batches of aluminum back surface field (Al-BSF) solar cells and passivated emitter and rear contact (PERC) solar cells. Both kinds of the treated solar cells show high stability in efficiency and suffer from very little LID under further illumination at room temperature. This technology is of significance for the suppression of LID of c-Si solar cells for the industrial manufacture.

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Plated contacts for solar cells with superior adhesion strength to screen printed solar cells
R. CHEN,S. WANG,A. WENHAM,Z. SHI,T. YOUNG,J. JI,M. EDWARDS,A. SUGIANTO,L. MAI,S. WENHAM,C. CHONG
Front. Energy. 2017, 11 (1): 72-77.   DOI: 10.1007/s11708-016-0428-4
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The improvement of adhesion strength and durability of plated contacts is required for cell manufacturers to gain confidence for large-scale manufacturing. To overcome weak adhesion at the metal/Si interface, new approaches were developed. These involve the formation of laser-ablated anchor points, or grooves in the extreme case of overlapping anchor points, in the heavily doped silicon surface. When plated, these features greatly strengthen the mechanical adhesion strength of the metal. A stylus-based adhesion tester was developed specifically for evaluating the effectiveness of plated contacts to smooth silicon surfaces. The use of such a tester was also extended in this work to textured and roughened surfaces to allow evaluation of different metal contacting approaches. The adhesion strengths for various metal contacting schemes were evaluated, including screen-printed silver contacts, nickel/copper (Ni/Cu) light-induced plated (LIP) contacts for laser-doped selective emitter (LDSE) cells, buried-contact solar cells (BCSCs), and Ni/Cu LIP contacts formed with laser-ablated anchoring points in selective emitter (LAASE) cells. The latter has superior adhesion strength. The standard “peel test” of the industry was compared to the stylus-based adhesion testing, with the latter shown value for testing metal contacts on smooth surfaces but with caution needed for use with textured or roughened surfaces.

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High-quality industrial n-type silicon wafers with an efficiency of over 23% for Si heterojunction solar cells
Fanying MENG,Jinning LIU,Leilei SHEN,Jianhua SHI,Anjun HAN,Liping ZHANG,Yucheng LIU,Jian YU,Junkai ZHANG,Rui ZHOU,Zhengxin LIU
Front. Energy. 2017, 11 (1): 78-84.   DOI: 10.1007/s11708-016-0435-5
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n-type CZ-Si wafers featuring longer minority carrier lifetime and higher tolerance of certain metal contamination can offer one of the best Si-based solar cells. In this study, Si heterojuction (SHJ) solar cells which was fabricated with different wafers in the top, middle and tail positions of the ingot, exhibited a stable high efficiency of>22% in spite of the various profiles of the resistivity and lifetime, which demonstrated the high material utilization of n-type ingot. In addition, for effectively converting the sunlight into electrical power, the pyramid size, pyramid density and roughness of surface of the Cz-Si wafer were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). Furthermore, the dependence of SHJ solar cell open-circuit voltage on the surface topography was discussed, which indicated that the uniformity of surface pyramid helps to improve the open-circuit voltage and conversion efficiency. Moreover, the simulation revealed that the highest efficiency of the SHJ solar cell could be achieved by the wafer with a thickness of 100 µm. Fortunately, over 23% of the conversion efficiency of the SHJ solar cell with a wafer thickness of 100 µm was obtained based on the systematic optimization of cell fabrication process in the pilot production line. Evidently, the large availability of both n-type ingot and thinner wafer strongly supported the lower cost fabrication of high efficiency SHJ solar cell.

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Plasma enhanced chemical vapor deposition of excellent a-Si:H passivation layers for a-Si:H/c-Si heterojunction solar cells at high pressure and high power
Lei ZHAO,Wenbin ZHANG,Jingwei CHEN,Hongwei DIAO,Qi WANG,Wenjing WANG
Front. Energy. 2017, 11 (1): 85-91.   DOI: 10.1007/s11708-016-0437-3
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The intrinsic a-Si:H passivation layer inserted between the doped a-Si:H layer and the c-Si substrate is very crucial for improving the performance of the a-Si:H/c-Si heterojunction (SHJ) solar cell. The passivation performance of the a-Si:H layer is strongly dependent on its microstructure. Usually, the compact a-Si:H deposited near the transition from the amorphous phase to the nanocrystalline phase by plasma enhanced chemical vapor deposition (PECVD) can provide excellent passivation. However, at the low deposition pressure and low deposition power, such an a-Si:H layer can be only prepared in a narrow region. The deposition condition must be controlled very carefully. In this paper, intrinsic a-Si:H layers were prepared on n-type Cz c-Si substrates by 27.12 MHz PECVD at a high deposition pressure and high deposition power. The corresponding passivation performance on c-Si was investigated by minority carrier lifetime measurement. It was found that an excellent a-Si:H passivation layer could be obtained in a very wide deposition pressure and power region. Such wide process window would be very beneficial for improving the uniformity and the yield for the solar cell fabrication. The a-Si:H layer microstructure was further investigated by Raman and Fourier transform infrared (FTIR) spectroscopy characterization. The correlation between the microstructure and the passivation performance was revealed. According to the above findings, the a-Si:H passivation performance was optimized more elaborately. Finally, a large-area SHJ solar cell with an efficiency of 22.25% was fabricated on the commercial 156 mm pseudo-square n-type Cz c-Si substrate with the open-circuit voltage (Voc) of up to 0.732 V.

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Analysis of the double-layer α-Si:H emitter with different doping concentrations for α-Si:H/c-Si heterojunction solar cells
Haibin HUANG,Gangyu TIAN,Tao WANG,Chao GAO,Jiren YUAN,Zhihao YUE,Lang ZHOU
Front. Energy. 2017, 11 (1): 92-95.   DOI: 10.1007/s11708-016-0432-8
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Double-layer emitters with different doping concentrations (DLE) have been designed and prepared for amorphous silicon/crystalline silicon (α-Si:H/c-Si) heterojunction solar cells. Compared with the traditional single layer emitter, both the experiment and the simulation (AFORS-HET, http://www.paper.edu.cn/html/releasepaper/2014/04/282/) prove that the double-layer emitter increases the short circuit current of the cells significantly. Based on the quantum efficiency (QE) results and the current-voltage-temperature analysis, the mechanism for the experimental results above has been investigated. The possible reasons for the increased current include the enhancement of the QE in the short wavelength range, the increase of the tunneling probability of the current transport and the decrease of the activation energy of the emitter layers.

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Influence of using amorphous silicon stack as front heterojunction structure on performance of interdigitated back contact-heterojunction solar cell (IBC-HJ)
Rui JIA,Ke TAO,Qiang LI,Xiaowan DAI,Hengchao SUN,Yun SUN,Zhi JIN,Xinyu LIU
Front. Energy. 2017, 11 (1): 96-104.   DOI: 10.1007/s11708-016-0434-6
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Interdigitated back contact-heterojunction (IBC-HJ) solar cells can have a conversion efficiency of over 25%. However, the front surface passivation and structure have a great influence on the properties of the IBC-HJ solar cell. In this paper, detailed numerical simulations have been performed to investigate the potential of front surface field (FSF) offered by stack of n-type doped and intrinsic amorphous silicon (a-Si) layers on the front surface of IBC-HJ solar cells. Simulations results clearly indicate that the electric field of FSF should be strong enough to repel minority carries and cumulate major carriers near the front surface. However, the over-strong electric field tends to drive electrons into a-Si layer, leading to severe recombination loss. The n-type doped amorphous silicon (n-a-Si) layer has been optimized in terms of doping level and thickness. The optimized intrinsic amorphous silicon (i-a-Si) layer should be as thin as possible with an energy band gap (Eg) larger than 1.4 eV. In addition, the simulations concerning interface defects strongly suggest that FSF is essential when the front surface is not passivated perfectly. Without FSF, the IBC-HJ solar cells may become more sensitive to interface defect density.

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