Chapter 6: Influence of a-Si:H(i) Layer Thickness on the Performance of c-Si/a-Si:H
6.2 Results and discussion
6.2.4 Current-Voltage measurement
Fig. 6.14 shows the dark current density-voltage (J-V) characteristics of the nip0, nip2, nip3 and nip4 cells. The reverse saturation current density (𝐽0) for nip0 cell is more than one order of magnitude higher than that of nip3 and nip4 cells. For nip2 cell, 𝐽0 is more than that of nip4 cell, though it is smaller compared to nip0 cell. The lowest 𝐽0 is obtained for nip3 cell. The forward to reverse bias current density ratio is also higher for nip3 and nip4 cells as compared to that for nip0 and nip2 cells. The high injection region started around 700 mV for nip3 and nip4 cells.
Influence of a-Si:H(i) Layer Thickness on c-Si/a-Si:H Heterojunction Solar Cells
Fig. 6.14. Dark J-V characteristics of nip0, nip2, nip3 and nip4 cells (open symbols represent reverse bias, whereas filled symbols represent forward bias).
Fig. 6.15 show the J-V characteristics of c-Si/a-Si:H based hetero-junction nip1-nip4 solar cells under AM1.5 illumination. The dark J-V characteristic of nip3 and nip4 cells is also shown in Fig. 6.15. For comparison, the J-V characteristics under illumination of nip0 cell, which was not subjected to any hydrogen plasma and having 10 nm thick a-Si:H(i) layer, is also included in this figure. The values of 𝐽𝑠𝑐, 𝑉𝑜𝑐, 𝐹𝐹 (Eq. 2.29) and efficiency (𝜂) (Eq. 2.30) for all the cells are estimated and listed in Table 6.5. A systematic increase in these values are observed from nip0 to nip4 cells. This improvement in 𝐽𝑠𝑐 and 𝑉𝑜𝑐 values is due to the systematic decrease in thickness of a-Si:H(i) layer, reduction in void fractions of n/i and i/p interface layers and overall improvement in microstructure of a-Si:H layers, which is reflected as a shift in position of peak < 𝜀2 > in SE spectra [22]. A crossover of J-V characteristic in dark and under illumination is observed around 700 mV for the nip3 and nip4 cells. For these cells, the dark current density near 𝑉𝑜𝑐 is slightly low compared to 𝐽𝑠𝑐 value for corresponding cells.
The open circuit voltage for nip3 and nip 4 cells are 711 and 703 mV respectively, which are best among the values (628-635 mV) reported so far for single sided hetero-junction solar cells on n type c-Si [23–26]. To the best of our knowledge, the highest 𝑉𝑜𝑐 of 652 mV has been reported for single side c-Si/a-Si:H on p-type c-Si(FZ) by Wang et.al [27,28]. Though 𝑉𝑜𝑐 as high as 750 mV has been reported on double side heterojunction cells on thin n-type c-Si substrates [29]. For nip2 cell, the 𝑉𝑜𝑐 is 645 mV, which is close to the values reported for such cells. For this cell (nip2), though 𝐽𝑠𝑐 is not very different than that of nip3 and nip4 cell, the 𝑉𝑜𝑐 is low due to relatively higher value of 𝐽0.
Fig. 6.15. J-V characteristics of c-Si/a-Si:H heterojunction solar cells.
It is very challenging to achieve such high open circuit voltage with one side c-Si/a-Si:H solar cells. However, in our case, it could be achieved since 10 min H2 plasma treatment on c-Si(n) absorber layer has passivated most of the surface dangling bonds and reduced the defect density at n/i interface. This less defective interface layer further helped the growth of a-Si:H layer having an improved microstructure [30,31]. The 2 min H2 plasma treatment on a-Si:H(i) layer has also improved the n/i interface when a-Si:H(i) layer thickness was low (nip3 and nip4 cells) and atomic hydrogen could diffuse to the interface layer and removed the weak Si-Si bonds.
The hydrogen plasma treatment of a-Si:H(i) layer also improved the i/p interface by removing the weak Si-Si bonds and surface defects before the a-Si:H(p) layer was deposited [31,32]. It can be seen from Table 6.2, that for nip3 and nip4 cells, the void fraction of all the layers is significantly low compared to that for nip1 and nip2 cells. This has resulted in reduced recombination losses of photo generated carriers for nip3 and nip4 cells having good interfaces and transport properties [33]. The nip1 and nip2 cells have low 𝑉𝑜𝑐 values, though these values are close to those reported for single side SHJ solar cells, due to higher density of defects at interface and also in bulk of the layers as the thickness of a-Si:H layer is still high. The short circuit current density value has increased with decrease in thickness of a-Si:H(i) layer due to lower absorption of photons in the a-Si:H(i) layer and also smaller path length of photo generated carriers to reach metal contacts [23,34]. Our best 𝐹𝐹 values are 0.63 and 0.66 corresponding to the nip3 and nip4 cells respectively. The 𝐹𝐹 mainly depends on shunt and series resistance and void fraction in the different layers and also on micro voids and defects in band tail states [35–37]. The interface and a-Si:H layers of nip3 and nip4 cells have less void fraction as estimated from SE. In case of nip1 and nip2, the thickness of a-Si:H(i) layer as well as void fraction are more, which has increased the series resistance of the device and a lower
Influence of a-Si:H(i) Layer Thickness on c-Si/a-Si:H Heterojunction Solar Cells
value of 𝐹𝐹 is observed. The 𝐽𝑠𝑐, 𝑉𝑜𝑐, 𝐹𝐹 and 𝜂 values of nip0 cell are very low compared to plasma treated cells due to unpassivated surface, defective interface and a-Si:H(i) layers. It is found that the efficiency and 𝐹𝐹 of cells are directly related to the amplitude of < 𝜀2 > spectra of cells (Fig. 6.16) which is in agreement with the reports in literature [37]. The shunt (𝑅𝑠ℎ) and series (𝑅𝑠) resistance values are estimated from the J-V curve of the cells using Eq. 2.31 and Eq. 2.32 and listed in the Table 6.5. The series resistance has decreased, whereas shunt resistance has increased from nip0 to nip4 cells due to reduction in thickness of a-Si:H(i) layer and micro voids in the cells which has created path to shunting of device.
Fig. 6.16. The maximum of < 𝜀2> spectra, fill factor and efficiency of nip1-nip4 cells.
Table 6.5. The estimated solar cell parameters for nip1-nip4 c-Si/a-Si:H heterojunction solar cells.
cell
J-V measurement EQE
measurement 𝐽𝑠𝑐
(mA/cm2) 𝑉𝑜𝑐 (mV)
𝐽𝑚𝑎𝑥 (mA/cm2)
𝑉𝑚𝑎𝑥 (mV)
𝐹𝐹 𝜂
(%) 𝑅𝑠ℎ (Ωcm2)
𝑅𝑠 (Ωcm2)
𝐽𝑠𝑐 (mA/cm2)
𝜂
(%)
nip4 21.53 703 17.71 563 0.66 9.97 235 4.4 20.87 9.68
nip3 20.64 711 16.72 553 0.63 9.25 233 5.7 19.07 8.55
nip2 20.0 645 15.20 504 0.59 7.66 223 6.5 18 6.86
nip1 15.15 614 11.33 456 0.56 5.15 187 10.3 14 4.81
nip0 6.27 436 3.70 305 0.42 1.14 115 26.3 6.94 1.27