Doping level of the n+ emitter region is an essential parameter that controls the performance of the n+ pp+ poly-silicon solar cells. Also, most poly-silicon n+ pp+ solar cell manufacturers apply hydrogenation from the phosphorus emitter n+ side to improve photovoltaic efficiency. Although hydrogen can passivate defects as well as it changes initial phosphorus doping level through phosphorus-hydrogen complex formation. Consequently, phosphorus deactivation can have a harmful effect on photovoltaic efficiency. In this context, the primary purpose of this work is to investigate the phosphorus deactivation in n+ emitter region and its effect on defects passivation of hydrogenated n+ pp+ poly-silicon solar cells. To do this, hydrogenation is performed by microwave plasma discharge involving an electron cyclotron resonance system. Besides, hydrogen passivates defects in poly-silicon, at the same time it deactivates phosphorus. For this reason, we have chosen to separate these simultaneous effects. So, we performed phosphorus deactivation on Schottky diodes-based mono-silicon, while defect passivation was operated in n+ pp+ poly-silicon solar cells. Our results reveal that hydrogen effectively deactivates phosphorus dopant. This effect is deeper in Schottky diodes with low initial phosphorus doping level where hydrogen diffuses easily in the bulk. This behavior is clearly revealed in open circuit-voltage values (Voc) measured on n+ pp+ samples. In fact, solar cells with low phosphorus concentration in n+ region revealed 319 mV compared to 230 mV for high doping level. Also, all n+ pp+ poly-silicon solar cells show a saturation of Voc at high microwave plasma power. Reasons for such case were explained and discussed in detail.
The main objective of this work is to investigate the effect of thermal annealing in forming gas atmosphere on the mechanism of deactivation and reactivation of phosphorus in silicon-based Schottky diodes. Firstly, the microwave plasma power, initial phosphorus concentration in the samples and hydrogen flux were fixed as 650 W, 1015 cm–3, and 30 sccm, respectively, to investigate the behavior of different working parameters of diodes, specifically the duration and temperature of hydrogenation. Secondly, few samples hydrogenated at 400 °C for 1 h were annealed under the forming gas (10% H2 + 90% N2) within the temperature range from 100 to 700 °C for 1 h. The profiles of active phosphorus concentration were monitored by evaluating the change in concentration of phosphorus after hydrogenation or thermal annealing in a forming gas environment through capacitance-voltage measurements. The obtained results depict the temperature and duration of hydrogenation, which ultimately reveals the complex behavior of phosphorous and hydrogen in silicon. However, the phosphorus passivation rate is homogeneous over all the depths measured at 400 °C. The thermal annealing in a forming gas indicates the increase in passivation rate of phosphorus as a function of annealing temperature, till the passivation rate attains saturation in the sample annealed at 400 °C. At higher temperatures, a decrease in the concentration of phosphorous-hydrogen complexes is observed due to the dissociation of these complexes and reactivation of phosphorus under thermal effect.
A significant cost reduction in photovoltaic cells could be achieved if they could be made from thin polycrystalline silicon (poly-Si) films. Despite hydrogenation treatments of poly-Si films are necessary to obtain high energy conversion, the role of the n+ emitter on defects passivation via hydrogen diffusion in n+pp+ polysilicon solar cells is not yet understood thoroughly. In this connection, influence of hydrogenation temperature and doping level of the n+ emitter on open-circuit voltage (VOC) were analyzed. It was found that VOC greatly improved by a factor of 2.9 and reached up to 430 mV at a microwave plasma power and hydrogenation temperature of 650 W and 400°C, respectively for a duration of 60 min. Moreover, slow cooling is more advantageous for high VOC compared to the rapid cooling. However, etching of the emitter region was observed, and this degradation is similar for both cooling methods. Furthermore, annealing of the hydrogenated cells in inert gas for 30 min revealed a slight increase in VOC, which reached 40-80 mV, depending on the annealing temperature. These results were explained by hydrogen atoms diffusing into the bulk of the material from subsurface defects that are generated during plasma hydrogenation process. Also, our findings show clearly that VOC values are much higher for a less doped phosphorus emitter compared to that of heavily doped. The origin of these behaviors was clarified in detail.
Here we have investigated the hydrogenation process of thin film polycrystalline n+pp+ silicon cells using MW-ECR plasma in a standard PECVD system. Influence of various process parameters such as MW-ECR power, hydrogenation temperature and hydrogen flow on the sheet resistance of the n+ emitter region and on the open-circuit voltage of the structure were investigated. The n+- type emitter regions were obtained by phosphorous diffusion using a spin-on dopant P507 or P509 solutions from Filmtronics. For both levels of emitter doping, the open-circuit voltage of poly-Si mesa cells increases with increasing the MW-ECR plasma power from 180-220 mV without hydrogenation up to 368 mV with plasma hydrogenation at 650 W. The sheet resistances of the n+ emitter region measured by the four-point probe technique show an increase upon hydrogenation and quantitatively depend on the initial doping level. In a further study hydrogenated and non-hydrogenated samples were annealed under a forming or neutral gas. Post-hydrogenation in FGA reveals an increase of Voc that can reach 20-80 mV depending on annealing temperature.
C-V measurement is an efficient method to determine the active doping concentration in silicon. In this study, we use it in the aim to understand the mechanisms which govern the phosphorus deactivation by hydrogen. To do this, The hydrogenation experiments were carried out in hydrogen plasma generated in an electron cyclotron resonance system (MW-ECR) using microwave power (PMW) for a fixed parameters like hydrogen flux, process time and hydrogenation temperature. The hydrogenation revealed a dopant deactivation due to the formation of phosphorushydrogen (PH) bonding as evident from the changes in the doping level after hydrogenation of schottky diode made using FZ-single crystalline silicon. It was also that deactivation of phosphorus was more pronounced at low microwave plasma power and for samples with low initial phosphorus concentration. On the other hand, the formation of molecular hydrogen below the silicon surface called platelets increase with increasing the initial phosphorus concentration. Therefore, the increase of the average platelets size decreases the density of atomic hydrogen and in tourn lowers the effective hydrogen diffusivity.
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