Impact Analysis of Metallization Design and Recombination Losses on Performance of Crystalline Silicon Solar Cells

Abstract

Using Griddler software, this study aims to select the optimal metallization design by analyzing the impact of the number and sizes of busbars and fingers on a solar cell’s performance. There is interest in the PV industry to reduce the finger size toward 25 μm in upcoming years. It is shown that an increase in the number and size of busbars and fingers causes an increase in the fill factor; however, with regards to the cell’s efficiency, the shading factor should be considered in addition to the size and number of metal contacts. The results of this study indicate that solar cells’ efficiency could be increased by 0.33–0.84% when using five busbars and a finger width of 35 μm. Moreover, this increase is achieved by reducing the emitter resistance to less than 60 ohm/sq and considering a recombination rate of about 165 fA/cm².

1. Introduction

As an alternative to fossil fuels, renewable energy sources, including wind and solar energy, biomass, and hydropower, offer sustainable options for meeting global energy needs [1,2,3]. In recent decades, photovoltaics (PV) have undergone significant growth. According to the latest report by the International Energy Agency [4], the global PV installed capacity reached 942 GW by the end of 2021, with an average annual growth rate of more than 82% from 2011 to 2021. In addition, according to Fraunhofer’s newest report [5], the price of PV modules (or solar panels) has decreased by 26% for every doubling of worldwide module production during the past four decades. This cost reduction is due to economies of scale and technical advancements in PV module components. Particularly, metallization contacts are considered one of the most expensive processes in solar cell fabrication [6].

Numerous studies were conducted to improve the front surface of solar cells and minimize losses, especially metallization contacts [7,8,9]. Recent silicon solar cell technologies focus on lowering the thickness of the cells for a more efficient surface passivation and focus on reducing the carrier recombination. In the case of thinner silicon solar cells, the front surface is textured to increase light absorption via multiple reflection. It was reported that solar cells that have an n-type emitter thickness of less than 1 μm feature a better surface quality compared to a p-type emitter in terms of absorbing the maximum light [10,11]. In addition, the front metallization design of solar cells is very important in order to obtain the maximum output power from the cells. This affects the performance of the solar cell both optically and electrically [12,13]. For example, shading losses increased with the increase of the number and size of metal contacts, which will reduce the amount of sunlight reaching the cell’s surface [14]. Ebong et al. [15] reviewed the optical and electrical effects of metallization on solar cells’ performance. They showed that the width of the metal lines can cause shading losses, affecting the short-circuit current value. Additionally, the increase in the series resistance of the fingers and busbars can strongly reduce the fill factor values.

Screen printing and plating are the most known methods for the metallization of crystalline silicon (c-Si) solar cells, where the first method is implemented more due to its simplicity, low cost, and high throughput in the production line of solar cells [16,17]. Enhanced silver paste chemistries that are used in metallization allow for much narrower silver finger widths, lowering optical shadowing and metal contact recombination losses [18]. Besides the improvements of pastes, selecting the proper front contact design leads to the achievement of a higher solar cell efficiency with a lower cost. The advanced metallization pastes can result in a high aspect ratio, which is the ratio of the finger height to the finger width. However, with the rapid advances of metallization technology and optimization of the geometry of fingers and busbars, it is mandatory to consider the reliability issues that may develop [19]. Fingers’ geometry, particularly the width, should be carefully designed to avoid additional losses or failures, such as electromigration, which rises due to high current density stress (Jsc) [20]. Metal fingers of c-Si solar cells have seen huge progress: in 2010, the finger width was about 100 μm compared to 35 μm in 2019 [18].

Metallization designs have also been studied, using simulation methodologies to evaluate the solar cell performance. Siraj et al. [11] conducted simulation research using Griddler software to assess the influence of the front and rear metallization pattern of crystalline silicon solar cells. They compared the efficiency, fill factor (FF), short-circuit current (Isc), and open-circuit voltage (Voc) parameters for 80–120 metal fingers with a width of 60 and 1–5 metal busbars. The results showed that a metallization design of 115 fingers, four busbars, and a layer sheet resistance of 60 ohm/sq was the optimal choice, resulting in a 19.49% efficiency and 81.36% fill factor. However, this study did not consider the recombination losses that may further degrade the performance of c-Si solar cells. Another numerical study was conducted by Sangi et al. [21] to investigate the variation of fingers and busbars on the efficiency and fill factor of c-Si solar cells. Besides analyzing the size and number of metal contact, they also included soldering/probe points with a diameter of 0.55 mm. They found that the increase of soldering points positively influenced both the efficiency and fill factor due to an increase in the current collection from the solar cell’s surface. The optimization of metal contact sizes and soldering/probe points resulted in an efficiency and fill factor above 20% and 80%, respectively [21].

This work will contribute to a better understanding of the optimization of solar cell performance by focusing on metal contact enhancement and recombination losses. Simulation studies, as previously mentioned, only focused on changing the number of fingers and busbars. Other loss factors, such as resistive and recombination losses, are also not adequately explained. Therefore, regarding c-Si solar cell fabrication and industrial solar cell design parameters, we will study the effect of metallization geometry and recombination loss on PV cell performance and select the optimal design for c-Si solar cells.

2. Simulation Methods

In this work, Griddler 2.5, which is a 2-D modeling program, was utilized to adequately evaluate the impact of metal contacts’ design on solar cell efficiency. Griddler is a finite element solver simulation software developed by Solar Energy Research Institute of Singapore [22]. It provides the user with many features, such as designing different types of solar cells, measuring their efficiencies, and anticipating improvements from various design perspectives. Griddler is an easily used and rigorous simulation program that provides an understanding of solar cells’ manufacturing environment [23].

Parameters of the Simulation Design

The main point of this study is to understand the effect of front metal contacts’ design on different crystalline silicon solar cell (c-Si) parameters, including the efficiency, fill factor, short-circuit current, and open-circuit voltage. For this purpose, different numbers and geometries of fingers and busbars have been studied. Metal fingers of 86, 96, 106, and 116 were used in the design of a solar cell with 3–6 busbars.

Figure 1a shows the first page interface of the simulation, which contains the sizing and geometry of the solar cell and its metallization. The solar cell size was set to be similar to the industrial standard size of 15.6 cm² and ingot diameter of 20 cm. The design parameters of the busbars included 15 solder points with a straight style and end joining pairs as well as a 0.5 mm edge gap for both front and rear sides of the solar cell. The width of the busbars was fixed to be 0.75–1.5 mm in the frontal side and 1.7 mm in the rear side of the solar cell throughout the simulation. These values are quite similar to the busbar’s width for commercial c-Si solar cells available on the market. Furthermore, the number of fingers ranged from 86–116 and the width from 50 μm to 35 μm when selecting the optimum solar cell design.

In our optimization scheme, we focused on a finger width as small as 35 μm, as this value was already implemented in the c-Si solar cell production [18]. In addition, the continuous improvement of front silver pastes has been one of the most important contributions to increasing c-Si solar cell conversion efficiencies over the last decades [24]. This improvement helped in reducing the recombination rate to less than 200 fA/cm² [24]. The recombination rate parameter in the Griddler simulation is J01, as shown in Figure 1b. Therefore, we are going to evaluate the finger width and recombination rate (J01) parameters to check their influence on the performance of the solar cell.

After completing the sizing of the solar cell and metallization contacts, we moved on to setting the electrical properties of the solar cell, including the resistive and recombination properties to control their losses. Figure 1b depicts the second page interface of the simulation, which contains various options for choosing front and rear metallization parameters. Most of these parameters were fixed throughout the simulation, such as the rear metallization side, since they did not contribute much to the total output power. The resistive and passivation parameters of the metal and semiconductor were taken from [25,26]. With the default series and shunt resistance magnitudes, the operating temperature was set to 25 °C, and the illumination was set to one sun.

Table 1. Parameters’ input used in Griddler simulator.

Parameter	Value
Cell size	15.6 cm2
Ingot diameter	20 cm
Number of busbars	3–6
Busbar width	1 mm front
	1.7 mm rear
Busbar style	Straight
Number of fingers	86–116
Finger width	50 μm
Finger sheet resistance	3
Front finger contact resistance	0
Front layer sheet resistance	80
Rear finger sheet resistance	3
Rear finger contact resistance	0
Rear layer sheet resistance	80
Front ${\mathrm{J}}_{01} \mathrm{passivated}$	200
Front ${\mathrm{J}}_{01} \mathrm{metal}$	600
Front ${\mathrm{J}}_{02} \mathrm{passivated}$	10
Front ${\mathrm{J}}_{02} \mathrm{metal}$	50
Rear ${\mathrm{J}}_{01} \mathrm{passivated}$	0
Rear ${\mathrm{J}}_{01} \mathrm{metal}$	0

summarizes the parameters, with their values as used in the simulation in this study.

3. Results

3.1. Impact of Busbars’ Geometry on Solar Cell Performance

We compared the designs of solar cell metallization for different numbers of busbars and fingers with reference to industrial c-Si solar cells. The front metallization design can be modified in three ways to reduce series resistance effects. These include raising the number of gridlines, lowering contact resistance, and adding more busbars. Therefore, the results show that 86 fingers on the front side of the solar cell is the optimal design, as it reduced the shading losses and kept the pitch of the fingers lower than 2 mm. Figure 2 illustrates the result of comparing different numbers of busbars with a fixed width and a similar number of fingers.

The results of comparing the performance of solar cells with a fixed busbar width show that efficiency decreased as the number of busbars increased from three to six. This is due to the increased metal coverage on the solar cell’s surface, which mainly increased the shading losses. For three busbars, the total shading was only 4.62% compared to six busbars, for which it was 6.47%. On the other hand, the fill factor experienced a slight increase as the number of busbars increased from three to six. By increasing the number of busbars, more currents are being collected from the active area of the solar cell. In addition, the maximum output voltage value contributed to the fill factor parameter, as it increased from 561 mV to 566 mV for three and six busbar designs, respectively.

Furthermore, the results in Figure 3 are based on the input parameters listed in Table 1. The total metallization coverage is essential to prevent shading effects and a reduction in the short-circuit current. Thus, the busbar width was changed as the number of busbars increased, while the total metal coverage was the same in order to keep the shading area similar for all designs. As a result, the efficiency and the FF were increased as the number of busbars increased. Moreover, the efficiency was increased from 19.79% to 20.02% when using six busbars instead of three busbars, although the shading coverage was the same in both designs. This occurred because the collected carriers by the finger metals take a shorter path when being transported to the busbars. In the case of three busbars, the distance between the center of the finger and the edge of the busbar is greater than for six busbars. In addition, the short transporting path in the case of six busbars reduces resistances that the current may encounter during travel from the finger to busbar.

3.2. Impact of Finger Pitch on Solar Cells Performance

The spacing between the metal fingers of a solar cell can contribute highly to total power loss. Figure 4 illustrates the effect of the metal finger pitch on the parameters of the solar cell, such as efficiency, FF, Voc, and Jsc. It is observed that the efficiency of the solar cell increases as the space between fingers increases. However, the solar cell’s efficiency started to drop as the pitch value exceeded 2 mm because of the shading effects. On the other hand, the fill factor showed an inverse relationship with the finger pitch, where the fill factor value decreased as the finger pitch increased. In addition, the current density exhibited a gradual increase as the space between fingers got larger due to the decrease in shading losses.

Figure 5 illustrates the schematic of front finger geometry in solar cells. The current trend of metal fingers is to reduce the width to improve the solar cell’s performance and to minimize the amount of silver metal usage in the metallization process. Therefore, the finger geometry must be designed with more considerations to maintain its reliability. To avoid reliability issues when decreasing the metal fingers width, the spacing between fingers should be reduced. By implementing small spacing between fingers, the current that passes through the finger will be reduced, preventing the possibility of a high current density that may lead to hot spot issues. The amount of current density that passes through the finger can be calculated using Equations (1) and (2) [27]:

$$I_f=X\times J_{mp}\times S_f$$

(1)

$$J_f=\frac{I_f}{A_f}$$

(2)

where $I_f$ is the current passing through the finger, $X$ is the length of the finger from end to busbar, $J_{mp}$ is the current density at the maximum power point, $S_f$ is the spacing between fingers, $J_f$ is the current density passing through the finger, and $A_f$ is the area of the finger.

3.3. Optimization of Solar Cell’s Metal Fingers

Metal fingers optimization is currently trending in the latest c-Si solar cell technologies, aiming to reduce the finger width to less than 50 μm. In addition, the improvement in contact pastes will result in lower recombination rates. Therefore, a finger width of 35 μm and passivated recombination of 165 fA/cm² are used as optimized values to check their influence on solar cell performance. Moreover, we optimized the emitter sheet resistance to be 60 ohm/sq instead of 80 ohm/sq. As a result, Figure 6a,b below shows the I-V curve before and after optimizing the electrical parameters and geometry of metal fingers. In addition,

Table 2. Difference between parameters of optimized and non-optimized solar cells.

Parameter	Non-Optimized Solar Cell	Optimized Solar Cell
Efficiency (%)	19.91	20.31
FF (%)	81.05	81.32
Voc (mV)	659	664
Jsc (mA)	37.28	37.6
Vmp (mV)	565	571
Imp (A)	8.57	8.66

illustrates the difference between the optimized and non-optimized solar cell. The efficiency of the solar cell was increased by an absolute of 0.33% compared to the non-optimized solar cell with five busbars. Furthermore, these results show that incorporating more fingers with a smaller width reduces the effective emitter resistance because of a reduced finger spacing and shorter distance for current transfer. Therefore, higher values of FF and efficiency were attained, as well as a lower series resistance.

The Griddler simulator provides an accurate and simple overview of the power loss of the designed solar cell. Figure 7 shows the power output and the contribution of various losses to the total efficiency (20.31%) of the solar cell. These losses include the resistive, recombination, and shading of the front and rear sides of the solar cell. Front resistive losses such as contact resistance, finger resistance, and sheet resistance contributed to about 1% of the output power. Apart from that, recombination losses of the front side contributed to about 1.5% of the total output power of the solar cell. Lastly, shading loss was calculated as lowering the output power by about 0.5%, which was less than resistive and recombination losses thanks to the optimization of the finger width.

4. Conclusions

In conclusion, this simulation study has contributed to understanding the effect of metal contacts on the performance of c-Si PV cells by enhancing the metal finger size and lowering recombination losses. In our metallization design, we found that:

• A total of 86 fingers constitute the optimal choice for a c-Si solar cell, since they provide minimal shading losses and keep the finger spacing to less than 2 mm.
• Furthermore, increasing the number of busbars will help in reducing the transport distance of carriers, hence lowering the layer sheet resistance to 60 ohm/sq for solar cells.
• Reducing recombination at the emitter and front metal contact is also considered to ensure a high performance.

Therefore, the results showed that when using metal fingers of 35 μm and an emitter recombination rate of 165 fA/cm², the efficiency could be increased to 20.31% and the fill factor to 81.32%. Our study can help improve c-Si solar cells’ performance by focusing more on minimizing the front metallization losses that affect the output power. In future work, an investigation could be conducted to set a threshold for finger geometry, especially width size, in order to ensure a more reliable metallization and a high solar cell performance.