Analysis of Losses in Open Circuit Voltage for an 18-μm Silicon Solar Cell

Abstract

An 18 μm thin crystalline silicon solar cell was demonstrated, and its best open circuit voltage is 642.3 mV. However, this value is far from the cell’s theoretical upper limit in an ideal case. This paper explores the open circuit voltage losses of the thin silicon solar cell, starting from the ideal case, through first principle calculation and experiments. The open circuit voltage losses come from the introduced recombination due to the non-ideal surface passivation and contacts integration on front and rear surfaces, and edge isolation. This paper presents a roadmap of the open circuit voltage reduction from an ideal case of 767.0 mV to the best measured value of 642.3 mV.

1. Introduction

The theoretical upper limit performance of both thick silicon solar cells [1] and thin silicon solar cells [2,3] have been well studied. There is a significant performance gap between the ideal and the practical cells, which is caused by optical losses, bulk recombination, surface recombination, contact recombination, and resistance losses in a manner similar to that observed by Swanson [4].

In a previous paper, an 18 μm thin crystalline silicon solar cell on steel has achieved a best efficiency of 16.8% and a best open circuit voltage (V_oc) of 642.3 mV [5]. The structure of the ultrathin silicon (UTSi) solar cell is shown in Figure 1. The ultrathin silicon is 20 μm thick, which is attached onto 125 μm steel substrate, and includes three layers: top front surface field (FSF) layer which is n⁺ and 1 μm, middle base layer, which is n type and 18 μm, and bottom emitter which is p⁺ 1 μm. Both surfaces of this thin silicon are well passivated and optically designed.

Figure 1. Diagram of the UTSi solar cell on steel substrate [5].

Figure 1. Diagram of the UTSi solar cell on steel substrate [5].

However, according to our analysis the theoretical maximum V_oc of this thin silicon solar cell can be as high as 767 mV [5]. The V_oc difference between an ideal case and a practical case is due to the introduced recombination. This paper discusses these recombination sources that include non-ideal passivated front and rear surfaces, laser-introduced damage on the front surface, contacted regions on both front and rear surfaces and edge recombination. Based on our previous analysis, bulk recombination is negligible for such thin crystalline silicon solar cells [5]; thus, it is not discussed in this paper.

This paper explores the V_oc losses through the first principle calculation and the measurement results on test samples and the UTSi solar cells. The V_oc loss at each step is figured out and a roadmap of V_oc reduction from the no loss (or modeled) case of 767 mV to the best measured value of 642.3 mV is established.

2. Fabrication

The detailed fabrication of this UTSi solar cell has been described [5,6]. All semiconductor layers including n⁺ front surface field, n base and p⁺ emitter are epitaxially grown on a porous silicon layer on a p⁺ wafer. After the thin silicon is grown on its host p⁺ wafer, the rear surface undergoes passivation and metallization. Then, the epitaxial thin silicon layer is exfoliated and transferred onto the steel substrate. After exfoliation, the porous silicon surface becomes the front side, and this surface is shallow textured in an alkaline solvent. This textured front surface is then passivated by PECVD deposited SiO_xN_y (silicon oxynitride). Front contacts are formed by combining selective laser doping and self-aligned Ni/Cu light induced plating. Fabrication is completed by edge isolation, a combination of laser cut and chemical etching processes, because well isolated edges lead to reduced FF and V_oc losses.

3. V_oc Losses Analysis

3.1. V_oc in Ideal Case

In an ideal case, the UTSi solar cell has a high V_oc and efficiency potential. PC1D [7,8] is used to calculate the upper limit efficiency and V_oc of a 20 μm silicon solar cell. Lifetimes of the n⁺ front surface field (FSF) layer, p⁺ emitter and n base are assumed to be 10 μs, 10 μs and 1000 μs, respectively. The modeled upper values of V_oc and efficiency are 767 mV and 25.4%, respectively [5]. To separate the surface recombination from bulk recombination, we assume the surface recombination velocity (SRV) of both surfaces to be 0 cm/s, so that there is only bulk recombination. Table 1 shows the reverse saturation current densities and the corresponding V_oc values.

Table 1. J₀ components, values and their corresponding V_oc values when SRV is 0 cm/s. J_0e is the reverse saturation current density in the emitter, J_0b is the reverse saturation current density in the base and FSF layer.

Steps	J0 Components	J0 Value (A/cm2)	Voc (mV)
--	J0b	4.73 × 10−15	--
--	J0e	3.03 × 10−17	--
1	J0b + J0e	4.76 × 10−15	767.0

Table 1. J₀ components, values and their corresponding V_oc values when SRV is 0 cm/s. J_0e is the reverse saturation current density in the emitter, J_0b is the reverse saturation current density in the base and FSF layer.

Steps	J0 Components	J0 Value (A/cm2)	Voc (mV)
--	J0b	4.73 × 10−15	--
--	J0e	3.03 × 10−17	--
1	J0b + J0e	4.76 × 10−15	767.0

In addition to the PC1D model, first principle calculations are used to calculate V_oc. We first separate the surface recombination from bulk recombination by assuming SRV(S_n, S_p) to be 0 cm/s so that there is only bulk recombination in the base (J_0b) and emitter (J_0e). The J₀ equation becomes $J_0=J_{0b}+J_{0e}=\frac{q{D}_n{n}_1^2}{L_n{N}_A}\times \tanh (\frac{W_p}{L_n})+\frac{q{D}_p{n}_1^2}{L_p{N}_D}\times \tanh (\frac{W_n}{L_p})$, where N_A and N_D are doping densities; L_n and L_p are minority carriers diffusion lengths; W_p and W_n are the thickness; S_n and S_p are minority carriers surface recombination velocities; D_n and D_p are minority carriers diffusivity at p and n silicon, respectively; and n_i is the intrinsic concentration. J₀ is determined by $\frac{W_p}{L_n}$ and $\frac{W_n}{L_p}$, the ratio of thickness over diffusion length. The maximum V_oc is 758 mV for a 20-μm cell, with an n base with a thickness of 18-μm, a doping density N_D of 5 × 10¹⁵ cm⁻³ and a lifetime of 1000 μs, a p⁺ emitter 1 μm, 5 × 10¹⁷ cm⁻³ and lifetime of 10 μs. No n⁺ FSF layer is considered in this calculation.

3.2. V_oc Loss Due to Surface Recombination

3.2.1 SRV on a Passivated n type Surface

There is a 1–2 μm 5 × 10¹⁷ cm⁻³ or 1 × 10¹⁸ cm⁻³ n type FSF layer in this UTSi solar cell, and this layer is mostly etched away during the shallow texturing process [5]. The front surface of the textured UTSi solar cell is mainly an n (5 × 10¹⁵ cm⁻³) surface with a partially remaining n⁺ (5 × 10¹⁷ cm⁻³ or 1 × 10¹⁸ cm⁻³) surface. The front surface is passivated by silicon oxynitride (SiO_xN_y) [9]. This section tests the surface recombination in the SiO_xN_y passivated n type surfaces. Two groups of test samples were designed, as shown in Figure 2: group A1 with n (5 × 10¹⁵ cm⁻³) epitaxial surfaces and group B1 with n⁺ (5 × 10¹⁷ cm⁻³) surfaces. Float-zone (FZ) substrates were chosen because they have negligible bulk recombination and provide an ideal substrate for epitaxial growth. In this way, their lifetime or implied V_oc values are not limited by bulk but by surfaces. These FZ substrates were from the same wafer and these samples were processed along with control samples to eliminate extraneous variables. After SiO_xN_y deposition, these samples were annealed at 400 °C for 10 min to activate surface passivation [9]. Their implied V_oc (iV_oc) and lifetime were measured by a Sinton lifetime tester [10]. An iV_oc of 718 mV and effective lifetime of 1085 μs were obtained in the sample with 18 μm 5 × 10¹⁵ cm⁻³ n type silicon epitaxial layers, while an iV_oc of 696 mV and lifetime of 556 μs were measured in the sample with 1 μm heavily doped 1 × 10¹⁸ cm⁻³ n⁺ epitaxial layers. To calculate the SRV of both surfaces, the equation $\frac{1}{{\tau }_{eff}}=\frac{1}{{\tau }_{bulk}}+\frac{2\times S}{W}$ is used, where τ_eff is the effective lifetime measured, S is the SRV, W is the thickness of silicon, τ_bulk is the bulk lifetime of the FZ wafer. In this calculation, the intrinsic bulk lifetime of the FZ wafer, 1.8 × 10⁴ μs, was estimated using an online program PV Lighthouse [11]. Intrinsic bulk lifetime eliminates the effect of Shockley-Read-Hall recombination and leads to conservative values for the SRV. SRV values for the 5 × 10¹⁵ cm⁻³ surface and 1 × 10¹⁸ cm⁻³ surface are 13 cm/s and 26 cm/s, respectively.

Figure 2. Control sample and two groups of test samples for n and n⁺ epitaxial layers and their passivation.

Figure 2. Control sample and two groups of test samples for n and n⁺ epitaxial layers and their passivation.

Table 2. iV_oc and J₀ components for samples in Figure 2.

ID	iVoc (mV)	Lifetime (μs)	SRV (cm/s)
Group A1	718	1085	13
Group B1	696	556	26

Table 2. iV_oc and J₀ components for samples in Figure 2.

ID	iVoc (mV)	Lifetime (μs)	SRV (cm/s)
Group A1	718	1085	13
Group B1	696	556	26

3.2.2. SRV on a Passivated p⁺ Surface

SRV depends strongly on the processing methods, including oxidation conditions, annealing conditions, surface roughness and contamination. King reported a surface recombination velocity of 1640 cm/s on a SiO₂ passivated p⁺ silicon surface, which is independent of injection level [12]. Thermal oxide is used to passivate the heavily doped p⁺ epitaxial emitter of the UTSi solar cell. This section tests the SRV of the thermal oxide passivated heavily doped p⁺ type surface. Control samples were used at each step to eliminate contamination and extraneous variables. A doping density of 5 × 10¹⁷ cm⁻³ for the rear emitter was selected in this experiment, and these test samples’ structures are plotted in Figure 3. Group A2 has 1 μm, 5 × 10¹⁷ cm⁻³ epitaxial p⁺ layers on both surfaces, which are passivated by SiO_xN_y [9]; Group B2 has 1 μm, 5 × 10¹⁷ cm⁻³ epitaxial p⁺ silicon layers on both surfaces, which are passivated by SiO₂. These samples were from the same wafer and were processed along with the control ones.

Figure 3. Test structures for p⁺ epitaxial layers and their passivation.

Figure 3. Test structures for p⁺ epitaxial layers and their passivation.

SRV (S_p) values are extracted in Table 3, using the relation $J_0=qs{n}_{ie}^2/N_{A,surf}$ [12], where J₀ is extracted from the above lifetime measurements, which is determined by the surface when the bulk recombination is negligibleis; N_A,surf is the doping density of surface; q is the electronic charge; n_ie is the intrinsic carrier concentration in the emitter, and equals to $n_{i0}^2{e}^{\frac{\Delta E}{kT}}$, where n_i0 is the intrinsic carrier concentration in the absence of bandgap narrowing and ΔE is the bandgap narrowing. ΔE = 0.03 eV for 5 × 10¹⁷ cm⁻³ and ΔE = 0.01 eV for 2 × 10¹⁷ cm⁻³ [12,13]. According to the calculated results, the SRV values of the SiO₂ passivated p⁺ emitter surface are 1551–1677 cm/s, as shown in the table, which is consistent with the value of 1640 cm/s reported by King [12].

Table 3. Lifetime, iV_oc, J_0e and SRV of Group A2 and B2 test samples from Figure 3.

Group	Lifetime (μs)	iVoc (mV)	J0e (A/cm2)	SRV (cm/s)
A2	161–221	644–658	--	--
B2	139–157	637–642	1.85 × 10−13 ~ 2.0 × 10−13	1551–1677

Table 3. Lifetime, iV_oc, J_0e and SRV of Group A2 and B2 test samples from Figure 3.

Group	Lifetime (μs)	iVoc (mV)	J0e (A/cm2)	SRV (cm/s)
A2	161–221	644–658	--	--
B2	139–157	637–642	1.85 × 10−13 ~ 2.0 × 10−13	1551–1677

3.3. V_oc Losses Calculation

The theoretical maximum open circuit voltage achievable in a 20 μm silicon solar cell is 767 mV, when both surface recombination velocities are 0 cm/s. However, the front n type surface and the rear p⁺ emitter surface of the UTSi solar cell are passivated by SiO_xN_y and SiO₂ respectively, and neither SiO_xN_y nor SiO₂ are the perfect passivation layers; thus, there is surface recombination on each surface. This section calculates the J₀ of the UTSi solar cell after introducing the surface recombination. According to Table 2, the SRV value of the SiO_xN_y passivated n front surface is less than 13 cm/s; According to Table 3, the SRV value of the SiO₂ passivated p⁺ rear surface is 1551–1677 cm/s, and 1640 cm/s is chosen to calculate the new J₀ for rear emitter (J_0es) because it is very close to the mean value and also consistent with the value reported by King. The rest parameters in this calculation are the same as those in the calculation of Table 1. The J₀ values for each layer and the corresponding V_oc are shown in Table 4. The V_oc decrease from 767.0 mV to 710.1 mV is caused by the recombination on the SiO_xN_y passivated surface. Since a maximum SRV is used in this calculation, 710.1 mV is a minimum value; in other words, a higher V_oc at this step is possible, such as when the front SRV is 5 cm/s, the corresponding V_oc is 725 mV. V_oc decrease from 710.1 mV to 687.0 mV is caused by recombination on the SiO₂ passivated p⁺ surface.

Table 4. J₀ and iV_oc of the UTSi solar cell when passivated by SiO_xN_y and SiO₂. J_0b + J_0e is from Table 1, and J_0bs is the reverse saturation current density of the base and J_0es is the reverse saturation current density of the emitter when taking SRV into consideration.

Steps	Component	J0 (A/cm2)	Voc (mV)	Steps
1	J0b + J0e	4.76 × 10−15	767.0	Maximum
2	J0bs	4.23 × 10−14	>710.1	Front SiOxNy
--	J0es	6.12 × 10−14	--	--
3	J0bs + J0es	1.04 × 10−13	687.0	Front SiOxNy + Rear SiO2

Table 4. J₀ and iV_oc of the UTSi solar cell when passivated by SiO_xN_y and SiO₂. J_0b + J_0e is from Table 1, and J_0bs is the reverse saturation current density of the base and J_0es is the reverse saturation current density of the emitter when taking SRV into consideration.

Steps	Component	J0 (A/cm2)	Voc (mV)	Steps
1	J0b + J0e	4.76 × 10−15	767.0	Maximum
2	J0bs	4.23 × 10−14	>710.1	Front SiOxNy
--	J0es	6.12 × 10−14	--	--
3	J0bs + J0es	1.04 × 10−13	687.0	Front SiOxNy + Rear SiO2

3.4. Measured V_oc on Test Samples

To verify the above calculated V_oc, test samples based on epi on FZ wafers were designed. These test samples had a similar structure to that of the UTSi solar cell, excluding the fact that the ultrathin silicon in the UTSi solar cell was supported by a steel substrate and the epitaxial layers in the test samples were grown directly on FZ thus no foreign support was needed. Figure 4 shows the structure of these test samples. Again, in the FZ substrate, bulk recombination is negligible and surface recombination dominates the final iV_oc. These test samples were processed together with the UTSi solar cells and control samples to eliminate extraneous variables during fabrication. Table 5 lists the lifetime and iV_oc values of the test samples measured using a Sinton lifetime tester. These measured iV_oc values are between 682–703 mV, and their mean value is 691.7 mV, consistent with the calculated value of 687.0 mV in Table 4.

Figure 4. Structure of the test samples for the UTSi solar cells based on epi on FZ wafer.

Figure 4. Structure of the test samples for the UTSi solar cells based on epi on FZ wafer.

Table 5. Lifetime and iV_oc of epi on FZ wafer test samples.

ID	Lifetime (μs)	iVoc (mV)
Test samples	383–674	682–703

Table 5. Lifetime and iV_oc of epi on FZ wafer test samples.

ID	Lifetime (μs)	iVoc (mV)
Test samples	383–674	682–703

4. V_oc Loss Due to Rear Surface Metallization

4.1. V_oc Losses Calculation

The reverse saturation current density (J_0e) of the emitter is composed of recombination both at the passivated surface (J_0es) and at the point contact region (J_0e-pc). Therefore, $J_{0e}=J_{0es}(1-f)+J_{0e-pc}$ where $J_{0pc}=J_{100}f(1+\frac{4\ln 2\times W}{\pi r}+\frac{W^2}{8r^2})$ was introduced when the passivated emitter and rear cell (PERC ) solar cell was invented [14], J₁₀₀ is the saturation current density with 100% rear contact coverage, f is the contact ratio 0.56%, r is the radius of the contact points and W is the cell thickness. The SRV of the point contact region is 10⁷ cm/s [15]. Table 6 summarizes the calculated J₀ and V_oc values.

Table 6. J₀ and V_oc values of the thin silicon solar cell after rear surface metallization. J_0e-pc is the reverse saturation current density introduced by the point contact on the rear surface.

Steps	Component	J0 (A/cm2)	Voc (mV)	Steps
3	J0bs + J0es	1.04 × 10−13	687.0	Rear SiO2
--	J0e-pc	9.76 × 10−14	--	--
4	J0bs + J0es + J0e-pc	2.02 × 10−13	670.0	Rear contact

Table 6. J₀ and V_oc values of the thin silicon solar cell after rear surface metallization. J_0e-pc is the reverse saturation current density introduced by the point contact on the rear surface.

Steps	Component	J0 (A/cm2)	Voc (mV)	Steps
3	J0bs + J0es	1.04 × 10−13	687.0	Rear SiO2
--	J0e-pc	9.76 × 10−14	--	--
4	J0bs + J0es + J0e-pc	2.02 × 10−13	670.0	Rear contact

4.2. Measured V_oc on Test Samples

To confirm the calculated results in Table 6 experimentally, test samples based on epitaxial on FZ wafers were made to examine the recombination introduced by the rear pattern and contacts. The test structures are shown in Figure 5, where the left image is the structure after passivation, the right image is the structure after point contact patterning and metallization. The minority carrier lifetime, and iV_oc and V_oc of these samples were measured at each step as shown in Table 7. The rear point contact patterning and metallization led to a slight decrease in lifetime, iV_oc and V_oc. The best V_oc measured after aluminium metallization was as high as 673 mV. The final V_oc values were in the range of 665–673 mV, which are consistent with the calculated value of 670 mV in Table 5. This result demonstrates that the calculation is valid.

Figure 5. Test sample before rear contact patterning (left), PERC rear contact with Al metallization (right).

Figure 5. Test sample before rear contact patterning (left), PERC rear contact with Al metallization (right).

Table 7. Lifetime, iV_oc and V_oc change of test structures before and after rear surface metallization.

Steps	Front SiOxNy/Rear SiO2		Rear metallization
Values	Lifetime (μs)	iVoc (mV)	Voc (mV)
	445.2	685.4	665–673

Table 7. Lifetime, iV_oc and V_oc change of test structures before and after rear surface metallization.

Steps	Front SiOxNy/Rear SiO2		Rear metallization
Values	Lifetime (μs)	iVoc (mV)	Voc (mV)
	445.2	685.4	665–673

5. V_oc Loss Due to Front Surface Metallization

5.1. Laser Doping

The n type front surface is patterned by laser doping, which introduces locally heavy doping within openings. However, laser doping inevitably introduces damage to the surface and bulk of the silicon solar cell. Laser damage can be partially healed by subsequent annealing. The V_oc loss depends on the power, speed and wavelength of the laser, the structure of the solar cell and the subsequent annealing temperature. A minimum V_oc loss of 6.1 mV has been reported [16]. Figure 6 illustrates this laser doping process on a test sample made of epi on FZ wafer, whose front surface was shallowly textured and passivated by SiO_xN_y. Based on our experiments, the V_oc loss at this laser doping step can be 10–20 mV, and Figure 7 shows the iV_oc changes on a test sample in this process.

Figure 6. Laser doping on a textured surface, where the front surface is passivated by SiO_xN_y (left) and patterned by laser doping (right).

Figure 6. Laser doping on a textured surface, where the front surface is passivated by SiO_xN_y (left) and patterned by laser doping (right).

Figure 7. iV_oc changes in the laser doping process on a test sample epi layer on FZ wafer.

Figure 7. iV_oc changes in the laser doping process on a test sample epi layer on FZ wafer.

5.2. Metallization

Laser doping is followed by self-aligned metallization (Ni/Cu plating). According to the experimental results, the metallization caused V_oc losses are no more than 5 mV. V_oc increases after metallization were even observed. This is because the V_oc measured before metallization is a local value within the laser opening, while the measurement on the grid represents an average V_oc of the whole cell.

According to the measurements, the average V_oc loss due to laser doping and metallization is 20 mV. So, after these two steps, V_oc decreased from the previous 670.0 mV to 650.0 mV. 650.0 mV is a reasonable estimation, since a maximum V_oc of 653 mV was measured in the UTSi solar cells at this processing stage. Table 8 lists the V_oc and J₀ changes due to front surface processing.

Table 8. J₀ and V_oc of the thin silicon solar cell after the front surface laser doping and metallization. J_0b-ld is the reverse saturation current density introduced by the laser doping and metallization on the front surface.

Steps	Component	J0 (A/cm2)	Voc (mV)	Steps
4	J0bs + J0es + J0e-pc	2.02 × 10−13	670.0	Rear contact
--	J0b-ld	2.14 × 10−13	--	--
5	J0bs + J0es + J0e-pc + J0b-ld	4.16 × 10−13	650.0	Laser doping/metallization

Table 8. J₀ and V_oc of the thin silicon solar cell after the front surface laser doping and metallization. J_0b-ld is the reverse saturation current density introduced by the laser doping and metallization on the front surface.

Steps	Component	J0 (A/cm2)	Voc (mV)	Steps
4	J0bs + J0es + J0e-pc	2.02 × 10−13	670.0	Rear contact
--	J0b-ld	2.14 × 10−13	--	--
5	J0bs + J0es + J0e-pc + J0b-ld	4.16 × 10−13	650.0	Laser doping/metallization

6. V_oc Loss Due to Edge Isolation

Edge isolation is an important step for either removing shunts on old edges or redefining active areas of a solar cell. Edge isolation approaches include laser grooving, sawing, grinding with sandpaper and plasma etching [17]. V_oc increases have been observed after removing shunts on old edges [18]. In our experiment, the major purpose of edge isolation is to redefine the active areas of a solar cell, and is achieved by a combined process of laser cutting and chemical edge cleaning. A 3 × 3 cm² raw sample was edge isolated into a 2 × 2 cm² cell. Based the measurements before and after isolation, a V_oc loss as low as 10 mV can be achieved on both the UTSi solar cells on steel and the epitaxial layer on FZ samples. Table 9 lists J₀ and V_oc values by assuming 10 mV loss during this step. The final V_oc is 640.0 mV, which is very close to the best V_oc of 642.3 mV measured in the finished UTSi cells on steel.

Table 9. J₀ and V_oc of the UTSi solar cell after edge isolation. J_0-edge is the reverse saturation current density introduced by edge isolation.

Steps	Component	J0 (mA/cm2)	Voc (mV)	Steps
5	J0bs + J0es + J0e-pc + J0b-ld	4.16 × 10−13	650.0	Laser doping/metallization
--	J0-edge	1.93 × 10−13	--	--
6	J0bs + J0es + J0e-pc + J0b-ld + J0-edge	6.09 × 10−13	640.0	Edge isolation

Table 9. J₀ and V_oc of the UTSi solar cell after edge isolation. J_0-edge is the reverse saturation current density introduced by edge isolation.

Steps	Component	J0 (mA/cm2)	Voc (mV)	Steps
5	J0bs + J0es + J0e-pc + J0b-ld	4.16 × 10−13	650.0	Laser doping/metallization
--	J0-edge	1.93 × 10−13	--	--
6	J0bs + J0es + J0e-pc + J0b-ld + J0-edge	6.09 × 10−13	640.0	Edge isolation

7. Summary of V_oc Losses

Figure 8 summarizes the V_oc at each step. This solar cell has a very high V_oc potential when assuming an SRV of 0 cm/s. Each subsequent processing step, such as non-ideal passivation of the front and rear surfaces, laser doping on the front surface, metallization on both front and rear surfaces and edge isolation, introduces an increase of recombination, and thus an increase in J₀. Calculation results and measurement results on either FZ test structures or the UTSi solar cells on steel are coincident with each other. The best achieved V_oc was of 642.3 mV. The primary V_oc loss is due to the recombination at the SiO₂ passivated p⁺ surface. Thus the greatest opportunity to improve V_oc lies in the improvement of the rear surface passivation. The second dominant V_oc loss that can be further reduced is the 10 mV loss due to edge isolation. Once the cell size increases from 2 × 2 cm² to 10 × 10 cm², edge recombination is less critical and zero V_oc loss is possible. The third largest V_oc loss is caused by the relatively high front SRV. If the SRV decreases from 13 to 5 cm/s, there will be another 15 mV increase. Improvements in these three aspects can lead to a V_oc increase of more than 40 mV, hence a V_oc of more than 680 mV can be achieved in the UTSi solar cells.

Figure 8. Summary of V_oc variation during the fabrication process. The first data point was from the calculation, while the remaining data points were based on the measured results, thus with error bars.

Figure 8. Summary of V_oc variation during the fabrication process. The first data point was from the calculation, while the remaining data points were based on the measured results, thus with error bars.