Effects of the Boron-Doped p⁺ Emitter on the Efficiency of the n-Type Silicon Solar Cell

Abstract

The optimum structure of the p⁺ emitter for the n-type silicon solar cell was determined with the simulation of the boron doping concentration. The boron concentration (𝑁_B) in the p⁺ emitter was varied in the range of 1×10¹⁷ and 2×10²² atoms/cm³ while maintaining the base doping concentration at 2×10¹⁶ atoms/cm³. With the increase of the boron concentration, the open circuit voltage (𝑉_OC) of the cell increased up to 0.525 V and then was nearly saturated at 𝑁_B>5×10¹⁸ atoms/cm³. On the other hand, the short circuit current density (𝐽_SC) began to decrease at 𝑁_B>1×10¹⁹ atoms/cm³ due to the increase of the surface recombination loss, and without considering the variation of the contact resistance along the emitter doping level, the maximum efficiency of the cell was obtained at around 𝑁_B=5×10¹⁸ atoms/cm³. While the contact resistance of the electrode decreases with the increase of the doping concentration in the p⁺ emitter, and with consideration of the variation of the contact resistance, the optimum value of 𝑁_B for maximum efficiency shifted to the higher doping level.

1. Introduction

Currently, the p-type silicon solar cell comprises a large portion of the industrial solar cells. On the other hand, the n-type silicon solar cell has been known to have many advantages and has subsequently received a great deal of attention and has become one of the main development topics in PV industries. Crystalline silicon solar cell using the n-type wafer showed the highest efficiency record among commercial silicon solar cells [1]. The n-type wafer has a longer diffusion length than the p-type wafer as a result of a higher tolerance to common transition metal impurities [2, 3]. Also, it does not contain any boron-oxide pairs, which are considered as the origin of light-induced degradation (LID) in the p-type Si wafer [4]. Thus, the performance of the n-type silicon solar cell can almost be maintained without degradation during the illumination of light. Furthermore, since the doping profile of boron is more similar to the theoretical model than that of phosphorous, the application of simulation to the real emitter diffusion process is more suitable for the n-type Si wafer.

In spite of the definite advantages of n-type silicon, p-type silicon comprises 85% of industrial silicon solar cells. Historically, research and process infrastructure have been mainly developed for p-type silicon solar cells, which are essential to make economical and high-efficient commercial solar cells.

In order to realize a p⁺ emitter on n-type silicon wafer, three kinds of methods are usually applied: (i) boron-diffused emitter, (ii) Al-alloyed emitter, and (iii) heterojunction using p-type a-Si [5]. Among these methods, the boron-diffused emitter is most similar to the phosphorus-diffused emitter of the conventional p⁺ silicon solar cell. But, in the case of the diffusion of boron, a noticeable gettering effect was not shown compared with the diffusion of phosphorus. And BSG (boron silicate glass), which is formed during boron diffusion [6, 7], is not completely removed by chemical solution (usually HF), while it is easy to remove PSG (phosphorus silicate glass) using HF solution.

In this study, the structure of the p⁺ emitter doped with boron was optimized for the n-type silicon solar cell. The performance of the solar cell was simulated varying the contact resistance (𝑅_𝐶) of the electrode and the doping concentration of boron.

2. Simulation Scheme

The Silvaco TCAD tool, which is commonly used for the simulation of semiconductor devices, was used to simulate and optimize the emitter structure for the n-type silicon solar cell. During the simulation, an n-type crystalline silicon wafer with an orientation of (100) was chosen and the base doping concentration of phosphorus was set to 1×10¹⁶ atoms/cm³. The p⁺ emitter was formed diffusing the boron into the silicon at 900°C for 10 min varying the boron concentration [8], and silver was used for electrode materials on the front and rear surfaces. Figure 1 shows the structure of the n-type silicon solar cell used in this simulation.

Figure 1 

Schematic cross-section of boron-doped p+ emitter for n-type silicon solar cell.

The width and the height of the simulation region in Figure 1 were 120 and 1200 μm, respectively, and the default depth of the simulation region was 1 μm. The centers of the front electrodes with widths of 100 μm were spaced out 600 μm apart.

The diffusion models describe how dopants and defects in the silicon redistribute themselves during thermal treatment, due to concentration gradients and internal electric fields. When modeling the actual diffusion process, there are additional effects to consider such as impurity clustering, activation, and how interfaces are treated [9]. In this study, the diffusion of the boron was simulated using the advanced diffusion model developed at CNRS-Phase (Strasbourg, France), CEA-LETI (Grenoble, France), and Silvaco [10]. Basically, the dopants and the point defects move in the silicon along the following diffusion equation [10]: 𝜕𝐶_tot𝜕𝑡=−∇𝐽_𝐴+𝑆, (1) where 𝐶_tot is the total impurity concentration, 𝑆 is the source and sink terms, and 𝐽_𝐴 is the flux of mobile particle as 𝐽_𝐴=−𝐷_𝐴∇𝐶_𝐴+𝐶_𝐴𝜇𝐸. (2) In the above equation, 𝐶_𝐴 is the mobile impurity concentration, 𝐷_𝐴 is the diffusivity of the impurity, 𝜇 is the mobility, and 𝐸 is the electric field.

Generally, the shallow emitter shows low surface recombination and good blue response at the short wavelength region, but the metal contact on the front surface is ready to show high resistance. To decrease the contact resistance, the emitter should be doped heavily with the doping materials [11]. Firstly, assuming the contact resistance, 𝑅_𝐶, as zero, the boron concentration, 𝑁_B, in the p⁺ emitter was varied in the range of 1×10¹⁷--2×10²²atoms/cm³. In the second simulation, the contact resistance was varied in the range of 0–0.5 Ωcm², fixing the concentration of boron at 5×10¹⁸ atoms/cm³. Finally, two parameters, 𝑁_B and 𝑅_𝐶, were simultaneously considered, and the output parameters of the cell were compared with the former results.

3. Results and Discussion

3.1. Dependence of the Cell Parameters on N_B

Figure 2 shows the dependence of the output parameters of the cell on the boron concentration, 𝑁_B. The 𝑁_B in the p⁺ emitter was varied in the range of 1×10¹⁷--2×10²² atoms/cm³, and the contact resistance was assumed to be zero. With the increase of 𝑁_B, the open circuit voltage, 𝑉_OC, of the cell increased as well and saturated at 𝑁_B>5×10¹⁸ atoms/cm³. Meanwhile, the short circuit current density, 𝐽_SC, began to decrease at 𝑁_B>1×10¹⁹ atoms/cm³. The increase of the doping concentration in the p⁺ emitter may cause an increase in the carrier density and a decrease of the quasi-fermi level of the hole, which resulted in the increase of the 𝑉_OC. However, since heavy doping can also bring about an increase in recombination loss in the emitter region, 𝑉_OC was saturated at 𝑁_B>5×10¹⁸ atoms/cm³. Also, the increase of recombination loss of the light-induced carrier meant the decrease of 𝐽_SC at 𝑁_B>1×10¹⁹ atoms/cm³. Consequently, the efficiency of the cell showed a maximum value of 𝑁_B=5×10¹⁸--1×10¹⁹ atoms/cm³.

(a)

(b)

(c)

(d)

Figure 2 

Dependence of the output parameters: (a) short circuit current density, (b) open circuit voltage, (c) fill factor, and (d) efficiency of the cell, on the boron concentration.

In order to confirm the recombination loss in the emitter region for the high doping concentration of boron, internal quantum efficiency (IQE) of the cell was calculated along the 𝑁_B as shown in Figure 3. In the short wavelength range, IQE rapidly decreased at 𝑁_B>1×10¹⁹ atoms/cm³, while there was no significant difference in the long wavelength range. This means that the recombination loss occurred at the front emitter region and is closely related to the decrease of the 𝐽_SC, as mentioned above.

Figure 3 

Variation of the internal quantum efficiency of the cell with different boron concentration.

3.2. Effect of the Contact Resistance

Though the surface recombination loss increases with the increase of 𝑁_B, conventional cell generally employs a heavily doped emitter that provides low contact resistance of the front electrode. On the other hand, the shallow emitter shows a low surface recombination loss and a good blue response of the cell at the short wavelength region, but a high contact resistance.

Figure 4 shows the output parameters of the cell along the contact resistance of the front electrode fixing the boron concentration at 5×10¹⁸atoms/cm³ in the emitter region. The fill factor of the cell was most affected by the contact resistance, as expected. 𝐽_SC and 𝑉_OC were slightly dependent on the contact resistance in such a way that 𝐽_SC decreased but 𝑉_OC increased with the increase of the contact resistance. The behavior of 𝑉_OC could be understood from the decrease of the remanent carrier concentration in the emitter region for low contact resistance. Finally, the efficiency of the cell increased with the decrease of the contact resistance.

(a)

(b)

(c)

(d)

Figure 4 

Dependence of the output parameters: (a) short circuit current density, (b) open circuit voltage, (c) fill factor, and (d) efficiency of the cell, on the contact resistance. The doping concentration of boron was set as 5×1018 atoms/cm3.

3.3. Dependence of Cell Parameters on N_B Including Contact Resistance

There are several factors determining the contact resistance of the crystalline silicon solar cell, such as the area and the quality of the metal electrode, the sheet resistance of the front surface of the cell, and the interface between the electrode and the silicon. Only the sheet resistance was considered to vary the contact resistance in this study. From the simulation, the sheet resistance of the n-type crystalline silicon wafer was identified along the doping concentration of boron, as shown in Figure 5. The contact resistance used in this simulation is shown in Table 1 [12].

Table 1 

Contact resistance at each doping concentration of boron.

Figure 5 

Dependence of the sheet resistance on the doping concentration of boron.

Figure 6 shows the output parameters of the cell along the doping concentration of boron in the case of considering the effects of the contact resistance. The overall behaviors of 𝐽_SC and 𝑉_OC were nearly similar to the results of Figure 2. However, the fill factor of the cell, which did not vary in the doping concentration of 𝑁_B>5×10¹⁷ atoms/cm³ for zero contact resistance, slightly increased in that concentration range due to the decrease of contact resistance. This increase in the fill factor, with the increased doping concentration, affected the tendency in the final efficiency of solar cell. While the efficiency decreased at 𝑁_B=1×10¹⁹ atoms/cm³ for zero contact resistance, the lower contact resistance due to the higher doping concentration prevented the efficiency of the cell from decreasing up to 𝑁_B=1×10²⁰ atoms/cm³. Finally, it was observed that, considering the contact resistance, the optimum doping concentration of boron for the n-type crystalline silicon solar cell was in the range of 1×10¹⁹--1×10²⁰ atoms/cm³.

(a)

(b)

(c)

(d)

Figure 6 

Dependence of the output parameters: (a) short circuit current density, (b) open circuit voltage, (c) fill factor, and (d) efficiency of the cell, on the boron concentration, including the effects of the contact resistance.

4. Conclusion

In order to discover the optimized p⁺ emitter for the n-type crystalline silicon solar cell, the boron-doped emitter was formed by the simulation method. The doping concentration of boron in the p⁺ emitter was varied in the range of 1×10¹⁷--2×10²² atoms/cm³ to optimize the emitter structure. In the case that the contact resistance of the front metal electrode was assumed to be zero, the efficiency of the cell showed the maximum value at 𝑁_B=5×10¹⁸--1×10¹⁹ atoms/cm³. On the other hand, the nonzero contact resistance affected the performance of the solar cell. And with the increase of the contact resistance, which is closely related to the concentration of boron, the fill factor of the cell slightly decreased. Finally, the optimized doping concentration of boron was obtained in the range of 1×10¹⁹--1×10²⁰ atoms/cm³.

Conflict of Interests

None of the authors have any conflict of interests with respect to the material contained in this paper.

Acknowledgments

This work was supported by the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea Government Ministry of Knowledge Economy (no. 20113010010140).