2.7.1 IV-measurements
In this section the best solar cell of section 2.6 is characterised. For the characterisation the busbars were tabbed by 1.5 mm wide Cu ribbons.
The parameters of the illuminated IV-measurement are given in Table 2-6. A good efficiency of η=16.0% was obtained, which is the highest efficiency for Process I obtained in this work. Concerning the moderate rear surface passivation (SB in the range of 3000-7000 cm/s [Joo98]) of the thin evaporated Al-BSF, the obtained Voc of 601 mV is a reasonable value. The high Jsc of 34.4 mA/cm2 underlines the high quality of mechanical V-texturing for the reduction of reflection losses and is also a consequence of the selective emitter structure and the low grid shadowing losses. For a more detailed analysis, the parameters of the Two-Diode model were determined and are given in Table 2-6. The measured Jsc-Voc and the fitted curve are illustrated in Figure 2.16.
Table 2-6: Results of the illuminated IV-measurements and parameters of the Two-Diode model of the best solar cell for Process I (cell area 156 cm2).
Voc [mV]
Jsc [mA/cm2]
FF [%]
η [%]
J01 [pA/cm2]
J02 [nA/cm2]
Rsh [Ω cm2]
Rs [Ω cm2] 601 34.4 77.5 16.0 1.9 52 12000 0.55
0.30 0.35 0.40 0.45 0.50 0.55 0.60
0.1 1 10
Vmpp n=2 measured Jsc- Voc curve n=1
fitted curve
J sc [mA/cm2 ]
Voc [V]
Figure 2.16: Measured Jsc-Voc characteristics and fitted curve. The parameters J01 and J02 extracted from the fit are given in Table 2-6.
2.7.2 Loss analysis
In this section, the prominent loss mechanisms of resistive, optical and recombination losses are investigated. Based on this analysis, suggestions for a modified processing sequence are made.
Resistive losses: Series resistance and shunt resistance
The series resistance of 0.55 Ωcm2 is a low value for a large area solar cell and therefore underlines the high quality of the developed electroless plating sequence. The main contributions to the series resistance are due to the emitter with Remitter=0.14 Ωcm2 and to the contact fingers with Rfinger=0.29 Ωcm2. The remaining part are contributions of the contact resistance and the busbar. The shunt resistance of 12000 Ωcm2 is sufficiently high and does not significantly affect the FF.
400 500 600 700 800 900 1000 1100
0 20 40 60 80 100
IQE EQE REF
IQE, EQE, REF [%]
λ [nm]
Figure 2.17: Internal Quantum Efficiency (IQE), External Quantum Efficiency (EQE) and reflectivity (REF) of the best mechanically V-textured solar cell within Process I.
Recombination and optical losses
From spectral response and reflectivity measurements, the IQE and EQE were determined as described in Chapter 1 and are shown in Figure 2.17. With these measurements, an electrical and optical loss analysis was carried out [Fis02c]. The result of the analysis is given in Table 2-7.
Table 2-7: Optical and recombination losses of the best solar cell with Process I determined from a spectral analysis. The losses are given in mA/cm2. The incident light corresponds to a total current density of 45.04 mA/cm2 in the wavelength range between 300 and 1180 nm (assuming zero reflectance and IQE=1).
Optical losses Recombination losses
shadowing reflection Emitter Base + rear absorp.
2.03 3.10 0.23 4.92
Discussion
Jsc(SR) determined from the spectral response measurement is 34.7 mA/cm2, which is very close to the measured value of 34.4 mA/cm2 of the illuminated IV-characteristics. The shadowing losses due to the front metallisation follow from a finger width of 30 µm (finger spacing 1.4 mm) and a busbur width of 1.5 mm leading to a metal coverage of 4.5%. This is a very low value compared to screen printed solar cells and a further reduction in the shadowing loss is difficult to realise. The reflection loss on the non-shaded front surface (weighted average reflectance of 5.7%) is in the expected range for mechanically V-textured solar cells applying a texturing wheel. Further reduction of the reflection losses can be achieved by single blade texturing (see section 3 and Chapter 4) as well as by the deposition of a second ARC such as MgF2.
The emitter losses are very low with only 0.23 mA/cm2 proving the quality of the selective emitter structure. A high potential for a further increase in Jsc has the reduction of the base losses. The recombination loss in the base is mainly determined by the effective bulk diffusion length Leff which depends on the rear surface recombination velocity SB and the bulk diffusion length (see equation 2.2). A spectral analysis of the IQE for the determination of Leff can not be applied for macroscopically V-textured cells. Based on experiences in the evaluation of V-textured solar cells and planar reference cells, it can be concluded, that Leff is well above 200 µm and therefore the rudimentary rear surface recombination with SB in the range of several thousand cm/s prevents a higher cell efficiency. Therefore, the base losses can be reduced by improved rear surface passivation.
In order to investigate the homogeneity of the mc-Si with respect to spatial variations in Leff, an LBIC measurement was carried out and the result is shown as EQE-mapping at λ=980 nm in Figure 2.18. Only a small number of grains are present with a low lifetime.
However, in order to obtain higher efficiencies these grains have to be passivated more effectively thereby reducing the base losses. The LBIC scan also reveals differences in the thickness of the Al-BSF. In the red regions, the rear surface passivation is enhanced due to a thicker BSF. This effect is caused by an inhomogenous alloying of Al during the co-diffusion step when the wafers are placed in a vertical position in a quartz boat.
Figure 2.18: Spatially resolved EQE at λ=980 nm obtained from an LBIC-scan for the best solar cell within Process I.
Saturation current density of the second diode
Besides the shunt resistance Rsh and the series resistance Rs other non-ideal behaviour can reduce the FF. At lower voltages the ideality factor is larger than 1 leading to a non-ideal diode behaviour as can be seen in Figure 2.16. In the Two-Diode model this deviation is taken into account by including a second diode. The second diode expresses generation and recombination currents within the space charge region. Generally, the second diode is described by an ideality factor of n2=2, but values up to 4 have been reported. The second diode could be fitted with n2=2 and a rather high value of J02=5.2x10-8 A/cm2. This high J02 leads to a deviation from the ideal case of n=1 at the maximum power point as can be seen in the Jsc-Voc curve. For an estimation of the losses due to a high J02, the IV-curve was simulated for J02=0 and for J02=1x10-8 A/cm2. In the first case Voc increases by
∆Voc=5.3 mV and the FF by ∆FF=2.6%abs. in the second case by ∆Voc=4.3 mV and by
∆FF=2.0%abs. The high J02 leads to a significant decrease in the conversion efficiency.
Different reasons are possible for the increase in current at low voltages leading to the deviation from the ideal case. First of all, it can be caused by SRH-recombination in the space charge region due to metallic impurities or crystal defects. This increase will be inherent to mc-Si and a reduction in J02 can in principle be achieved by passivating the defects in the space charge region. Additionally, the current increase can be caused by the solar cell design, by the process technology or by the solar cell metallisation. Breitenstein et al. [Bre01] observed that shunts in industrial solar cells with thick film metallisation are lying in the edge region and beyond major grid lines. They concluded that the shunts in their study are caused by technological imperfections. Solar cells processed on Cz-Si in Chapter 1 showed a low J02 below 2x10-8 A/cm2 suggesting that the increase is not caused by the design, process technology or metallisation and is therefore due to the material properties of mc-Si.