**3. Results and Discussions**

#### **3.1. p-a-Si1-xCx:H**

Initially we deposited, characterized and optimized the p-type layer, where the diborane flow rate with reference that of silane, DBFR = B2H6/SiH4 that was almost always kept at 0.17 % unless otherwise specified. In the following we discuss preparation and properties of bor‐ on doped p-a-Si1-xCx:H films, and then its application to solar cell.

It is known that with increase in methane flow rate the carbon incorporation into the film increases linearly [39]. Fig. 2(a) shows the change in the carbon content of deposited p-a-Si1-x Cx thin film depending on the gas flow ratio y=CH4/(CH4+SiH4). The Fig. 2(a) shows that while the methane flow ratio increases from 0.6 to 0.9, the carbon content x increases almost linearly from 0.1 to 0.3, where the x can be expressed as

Single- and Multiple-Junction p-i-n Type Amorphous Silicon Solar Cells with p-a-Si1-xCx:H and nc-Si:H Films http://dx.doi.org/10.5772/51732 297

$$\mathbf{x} = \begin{array}{c} 0.59\mathbf{y} \ -0.2\mathbf{3} \end{array} \tag{5}$$

for this range of y. The expression for x comes from the linear fit to the data points of Fig. 2(a). It may appear that while methane flow rate is zero the fractional carbon content will become negative. This conclusion is not realistic. The negative intercept of the linear equa‐ tion (or -0.23) may indicate the change in chemical kinetics, that comes into play in the chemical vapor of the RF PECVD system in presence of methane, in comparison to when methane was absent.

**a-Si:H top-cell**

296 Photodiodes - From Fundamentals to Applications

**nc-Si:H middle-cell**

**nc-Si:H bottom-cell**

was 16W for the p-, i-, n- layers. The cell area was 0.36cm2.

**3. Results and Discussions**

**3.1. p-a-Si1-xCx:H**

**p-a-Si1-xCx:H i a-Si:H n nc-Si:H**

**p nc-Si:h i nc-Si:H n nc-Si:H**

**p nc-Si:h i nc-Si:H n a-Si:H**

SiH4(sccm) 6 7 2 H2(sccm) 5 60 150 RF power (W) 6 8 16 Pressure (mTorr) 300 300 500 Thickness 20nm 150nm 20-30nm

SiH4(sccm) 1 5.5 2 H2(sccm) 180 95 150 Pressure (mTorr) 500 300 500 Thickness 20nm 2.0 ηm 20-30nm

SiH4(sccm) 1 5.5/6.0/6.5 2 H2(sccm) 180 95 150 Pressure (mTorr) 500 300 500 Thickness 20nm 3.2 ηm 20-30nm

**Table 4.** Deposition condition for a triple junction solar cell. All the n-layers of the cells were 1% doped with PH3, players of the bottom and middle cells were 0.4% doped, The top cell was deposited with textured front TCO, CH4 flow rate for p-a-Si1-xCx:H as 16 sccm. The p-type layer of the middle and the bottom cells were deposited with 0.4% B2H6 doping, no CH4 flow and the n-layer as 1% doped. With AZO/Ag BR, and less than 10nm thick AZO interlayer as a tunnel junction in between n-type and p-type layers of the cells. For the middle and the bottom cells the RF power

Initially we deposited, characterized and optimized the p-type layer, where the diborane flow rate with reference that of silane, DBFR = B2H6/SiH4 that was almost always kept at 0.17 % unless otherwise specified. In the following we discuss preparation and properties of bor‐

It is known that with increase in methane flow rate the carbon incorporation into the film increases linearly [39]. Fig. 2(a) shows the change in the carbon content of deposited p-a-Si1-x Cx thin film depending on the gas flow ratio y=CH4/(CH4+SiH4). The Fig. 2(a) shows that while the methane flow ratio increases from 0.6 to 0.9, the carbon content x increases almost

on doped p-a-Si1-xCx:H films, and then its application to solar cell.

linearly from 0.1 to 0.3, where the x can be expressed as

Optical gap (Eg) of the films were measured as the photon energy at which absorption coeffi‐ cient is 104 cm-1. Fig. 2(b) shows the change in the Eg of p-a-Si1-x Cx depending on the C con‐ tent. The band gap of p-a-Si1-x Cx deposited ranges within 1.7 ~ 2.3 eV. It can be seen that Eg increases almost linearly with x, where

$$\rm E\_g = \, 140 \, + \, 2.99 \, \text{x} \tag{6}$$

The expression for Eg comes from the linear fit to the data points of Fig. 2(b). It may appear that for a-Si:H samples, where carbon content x=0 the optical gap of the sample will be 1.40 eV. However this not the case, as it is known that Eg of a-Si:H is ~1.7 eV and depends on hydrogen content. The reason may be the role played by carbon in amorphous network. At low carbon content of p-a-Si1-xCx:H samples, the number density of Si-H bonds decrease. In a-Si:H sample it's the Si-H bonds that helps enhancing optical gap. In p-a-Si1-xCx:H samples it has been found that Si-H bond density decreases and C-H bond density increases. Thus while carbon content of the p-a-Si1-xCx:H films were increased role of the Si-Si bonds on optical gap that remain un‐ changed, while number density of Si-H and C-H changes. Thus the role of bonded Si-H, C-H bonds on optical gap may be reflected in the factor 2.99x while the contribution of Si-Si bonds on the optical gap may be contributing to the 1.40 constant of the equation for Eg.

Fig. 2(c) shows effect of C content on the dark conductivity and the related activation ener‐ gy. The σd and activation energy (Ea) were related by Arrhenius relation

$$
\sigma\_d = \sigma\_0 \exp\left[-E\_a / kT\right] \tag{7}
$$

where σ0 is a constant, k-Boltzmann constant, T temperature. Ea is estimated experimentally through slope of a plot of log(σd) vs 1/T in 25o C to 125o C temperature range. We observed that as x increases, the dark conductivity decreases from 10-5 to 10-10 Scm-1 and the activation energy increases from 0.35 to over 0.8 eV. For p-type material the activation energy is the energy differ‐ ence between Fermi level and valence band mobility edge. Furthermore, the relatively higher value of activation energy of the samples may be because of lower doping, which is 0.17%, whereas normal doping used in solar cell is usually 1%. However, the trend in variation of Ea and σd becomes qualitatively obvious as the trend of the traces were nearly complementary to each other; meaning increased activation energy and decreased conductivity were similar in nature with activation energy in linear scale while the conductivity in log scale.

electrical conduction (for 0.1<x<0.15) through Si-Si bonds through Si-rich phase of the mate‐ rial. Thus it appears x= ~0.15 is a suitable alloy composition because the Ea remains low even though optical gap as well as σ<sup>d</sup> is relatively high. A similar trend in intrinsic a-Si1-xCx:H has been observed [9] in which the photo conductivity, carrier mobility and lifetime remains nearly unchanged for x~0.15, and for x>0.15 it reduces faster. Thus it is quiet reasonable to

Single- and Multiple-Junction p-i-n Type Amorphous Silicon Solar Cells with p-a-Si1-xCx:H and nc-Si:H Films

http://dx.doi.org/10.5772/51732

299

Fig. 3(a) shows maximum resistance at TCO/p-type layer interface, the fill factor and open cir‐ cuit voltage of p-i-n-type a-Si:H solar cell, while the activation energy of the p-type layer in‐ creases and fill factor of the cell decreases. So the cell parameters become poorer at higher Ea. At higher x resistivity of the film, activation energy increases and hence the resistance at the TCO/p-interface as well as that of the p-type layer was also higher. Higher activation energy of the p-type layer leads to reduction in energy difference between the Fermi levels of ptype and i-type layers that ultimately leads to reduction in Voc of the cell. The higher Ea is associated to lower conductivity, that in this study, played a role to lower FF, Jsc and Voc.

**Figure 3.** (a) Dependance of TCO/p interface resistance, FF, Voc, on Ea of the p-layer. And variation of (b) I-V character‐

Light induced current voltage (LIV) characteristics is shown in Fig. 3(b). From the LIV charac‐ teristics it appears that at x=0.32 the short circuit current density (Jsc) is higher. This may be be‐ cause of better quantum efficiency of cell as compared to the cell with p-type layer having carbon content x =0.15. Increased Jsc can be observed when larger number of photons enter into the i-type layer and creates increased electron-hole pair. For x=0.32 optical gap of the p-type

From the I-V curve in Fig. 3(b), it can be seen that while the voltage across the cell was <0.5 Volts the current delivered by the cell was higher than the other cell, and the current re‐ mained lower in this cell if the voltage higher than 0.5 Volts, ultimately leading to lower Voc. Such a situation may be possible if localized mid-gap defects at the p-i interface remains high. What may happen is when the cell is short circuited or V< 0.5 Volts, the photo-generat‐

layer is ~2.3 eV and thus a larger number of photons can pass through the p-type layer.

consider x~0.15 is optimum compositional ratio for amorphous silicon carbide alloy.

**3.2. P-type Layer Activation Energy**

istic curve (c) QE with x.

**Figure 2.** p-a-Si1-xCx:H material characteristics. Variation of (a) x with methane flow rate, (b) optical gap with x, (c) Ea with x, (d) dark conductivity with Ea.

Fig. 2(d) shows the data points for samples with respective Ea and σd. The fitting line is an exponential fit, following Arrhenius relation. It shows that the measured dark conductivity and Ea were related by Arrhenius relation. Furthermore it can also be seen from Fig 2(c) that the change in dark conductivity and respective increase in the activation energy are slower for x increasing from 0.10 to ~0.15, whereas Fig. 2(b) shows a nearly linear increase in optical gap with increase in x from 0.10 to 0.32. These materials were p-type, so electrical conduc‐ tion is mostly contributed by movement of holes and activation energy is the energy differ‐ ence between the Fermi level and valence band mobility edge. Usually, when optical gap increases with alloying of Si with other atoms both the valence and conduction band mobili‐ ty edges move apart. The weaker correlation between the increase in optical gap and change in dark conductivity activation energy may indicate that the optical gap enhancement may be controlled by C-H rich phase of p-a-Si1-xCx:H material [17, 40] while electrical conduction is mainly due to silicon rich phase. Thus, although optical gap was enhanced due to C-incor‐ poration into the a-Si network yet the presence of increased C-H bonds does not impede the electrical conduction (for 0.1<x<0.15) through Si-Si bonds through Si-rich phase of the mate‐ rial. Thus it appears x= ~0.15 is a suitable alloy composition because the Ea remains low even though optical gap as well as σ<sup>d</sup> is relatively high. A similar trend in intrinsic a-Si1-xCx:H has been observed [9] in which the photo conductivity, carrier mobility and lifetime remains nearly unchanged for x~0.15, and for x>0.15 it reduces faster. Thus it is quiet reasonable to consider x~0.15 is optimum compositional ratio for amorphous silicon carbide alloy.

#### **3.2. P-type Layer Activation Energy**

each other; meaning increased activation energy and decreased conductivity were similar in

**Figure 2.** p-a-Si1-xCx:H material characteristics. Variation of (a) x with methane flow rate, (b) optical gap with x, (c) Ea

Fig. 2(d) shows the data points for samples with respective Ea and σd. The fitting line is an exponential fit, following Arrhenius relation. It shows that the measured dark conductivity and Ea were related by Arrhenius relation. Furthermore it can also be seen from Fig 2(c) that the change in dark conductivity and respective increase in the activation energy are slower for x increasing from 0.10 to ~0.15, whereas Fig. 2(b) shows a nearly linear increase in optical gap with increase in x from 0.10 to 0.32. These materials were p-type, so electrical conduc‐ tion is mostly contributed by movement of holes and activation energy is the energy differ‐ ence between the Fermi level and valence band mobility edge. Usually, when optical gap increases with alloying of Si with other atoms both the valence and conduction band mobili‐ ty edges move apart. The weaker correlation between the increase in optical gap and change in dark conductivity activation energy may indicate that the optical gap enhancement may be controlled by C-H rich phase of p-a-Si1-xCx:H material [17, 40] while electrical conduction is mainly due to silicon rich phase. Thus, although optical gap was enhanced due to C-incor‐ poration into the a-Si network yet the presence of increased C-H bonds does not impede the

with x, (d) dark conductivity with Ea.

298 Photodiodes - From Fundamentals to Applications

nature with activation energy in linear scale while the conductivity in log scale.

Fig. 3(a) shows maximum resistance at TCO/p-type layer interface, the fill factor and open cir‐ cuit voltage of p-i-n-type a-Si:H solar cell, while the activation energy of the p-type layer in‐ creases and fill factor of the cell decreases. So the cell parameters become poorer at higher Ea.

At higher x resistivity of the film, activation energy increases and hence the resistance at the TCO/p-interface as well as that of the p-type layer was also higher. Higher activation energy of the p-type layer leads to reduction in energy difference between the Fermi levels of ptype and i-type layers that ultimately leads to reduction in Voc of the cell. The higher Ea is associated to lower conductivity, that in this study, played a role to lower FF, Jsc and Voc.

**Figure 3.** (a) Dependance of TCO/p interface resistance, FF, Voc, on Ea of the p-layer. And variation of (b) I-V character‐ istic curve (c) QE with x.

Light induced current voltage (LIV) characteristics is shown in Fig. 3(b). From the LIV charac‐ teristics it appears that at x=0.32 the short circuit current density (Jsc) is higher. This may be be‐ cause of better quantum efficiency of cell as compared to the cell with p-type layer having carbon content x =0.15. Increased Jsc can be observed when larger number of photons enter into the i-type layer and creates increased electron-hole pair. For x=0.32 optical gap of the p-type layer is ~2.3 eV and thus a larger number of photons can pass through the p-type layer.

From the I-V curve in Fig. 3(b), it can be seen that while the voltage across the cell was <0.5 Volts the current delivered by the cell was higher than the other cell, and the current re‐ mained lower in this cell if the voltage higher than 0.5 Volts, ultimately leading to lower Voc. Such a situation may be possible if localized mid-gap defects at the p-i interface remains high. What may happen is when the cell is short circuited or V< 0.5 Volts, the photo-generat‐ ed e-h pairs were rapidly collected by the external circuit and thus most of the e-h pairs pro‐ duce higher photo-generated current from the cell. Whereas, while V>0.5 Volts the average residual time for e-h pairs inside the cell increased, leading to a higher possibility of the charge carriers being trapped at the defect states and lost. Such a model can be true if defect states created by C incorporation is ~0.5 eV above the quasi-Fermi level for holes, which may be ~0.85 eV above the valence band mobility edge. We estimate location of the defect states as 0.85 eV by adding Ea =0.35 eV for x=0.15 and the 0.5 eV. Fig. 2(c) shows that activa‐ tion energy of the p-a-Si1-xCx:H layer for x=0.32, is 0.82 eV, which is very close to the 0.85 and thus the role of midgap defect states become more obvious at higher carbon incorporation. In this situation it leads to lower FF and Voc. So the shunt resistance (Rp) of the cell (with x=0.32) becomes lower than the other cell (with x=0.15).

We have also observed an improved blue response of cell at lower p-type layer thickness. One simple reason is the Beer Lambert's law of absorption IT = I0exp[-αd], where IT is intensity of transmitted light through a material layer of thickness d having absorption coefficient α, I0 is intensity of incident light. Thus with a thicker (higher d) p-a-Si1-xCx:H layer the light penetrat‐ ing though the p-type layer will be lower. We have observed that the maximum available quantum efficiency of the cell also decreases with increase in thickness of the p-type layer.

Single- and Multiple-Junction p-i-n Type Amorphous Silicon Solar Cells with p-a-Si1-xCx:H and nc-Si:H Films

http://dx.doi.org/10.5772/51732

301

Thus, at increased thickness of p-type layer, intensity of transmitted light decreases leading to lower Jsc and lower quantum efficiency. Increased thickness also indicates higher electrical resistance across the p-type layer and thus reduced FF as well as efficiency. Whereas at re‐ duced thickness (<8nm) of the p-type layer the formation of p-type layer remains insufficient

This result is similar to that observed by Myong et. al., [8] and Lee et al [44] that at lower ptype layer thickness the quantum efficiency of the cell at shorter wavelength improves while efficiency, short circuit current density FF etc improves at the beginning but decreases at

We have observed that the higher hydrogen dilution for the p-type layer deposition over the TCO, leads to defective TCO/p interface, mostly due to the chemical reduction of the top

Use of hydrogen dilution during deposition is an important step for defect reduction of de‐ posited films. However, while the same technique is used for solar cell fabrication, specially for deposition of p-type layer, unless caution is maintained it leads to cells with poorer per‐ formance. During the p-layer deposition, if higher H-dilution was used the Voc and Jsc re‐ duces. At higher hydrogen dilution (R), the H-radicals in the plasma might have eroded top surface of the TCO by chemically reducing it through removal of part of bonded oxygen.

It is known that at higher hydrogen dilution the characteristics of intrinsic a-Si1-xCx:H films im‐ proves, like its conduction band Urback energy decreases and carrier life time increases [45] yet during device fabrication with TCO, the situation changes and one faces limitation in using higher hydrogen dilution that risks deteriorating top surface of the TCO. Our results are simi‐ lar to that of Tawada et al. [46] however the Voc in our sample is higher may be because of im‐

Although optimization of cell performance has been carried out, yet there is possibility to

As the ratio of SiH4 gas RSi, increases from 3.5% to 8.0%, the deposition rate increases from 0.9Å/sec to 1.7 Å/sec. At a higher hydrogen flow rate, the film deposition rate reduces due to selective etching of amorphous silicon phase by energetic hydrogen radicals, and thus, crys‐

proved band gap matching at p-type and i-type layers and lower interface defects.

further improve the Jsc, cell efficiency etc. by optimization of the cell structure.

**3.5. Nano crystalline Silicon and application in Solar cell**

and enough built-in field is not generated.

higher p-type layer thickness.

surface of the TCO.

**3.4. Effect of Hydrogen Dilution**

Thus a model of increased interface defects for x~0.32 may also be supported to some extent from the quantum efficiency measurements. As it is well known that absorption coefficient of amorphous silicon alloy films increase with reduction in wavelength of incident photon. Thus shorter wavelength photons will have a smaller penetration depth from the surface of incidence. In QE measurements, Fig. 3(c), when wavelength of incident radiation was gradu‐ ally reduced the electron-hole pair generation takes place closer to the p-i interface. It can be seen from Fig. 3(c) that 360 nm wavelength the EQE of the cell with the x~0.18 was same to that with cell with x~0.32. It can be assumed that at this wavelength the photo generated e-h pairs were created at the edge of p-i interface. So when the incident photon energy was low‐ er than the above critical wavelength, the EQE became lower, as the e-h pairs were generat‐ ed at the defective interface region.

Thus, it seems that at higher carbon incorporation the p-i interface as well as TCO/p-type layer interface the defects increased that lead to poorer performance of the cell.

Although there are several disadvantages for samples with higher x, yet its advantage is bet‐ ter transmission of higher energy photons into the i-type layer of the cell and hence generat‐ ing electron-hole pairs with shorter wavelength light, as shown in Fig. 3(c). It shows better external quantum nearly at all wavelength except below 360 nm.

At higher level of boron doping the activation energy falls significantly [41]. However, it is known that with the increased boron doping optical gap of the material falls [42] and this is a regular feature.

#### **3.3. P-type Layer Thickness**

We have observed that with a thin p-type layer, the cell performs better. We have observed that the open circuit voltage decreases as the thickness of p-a-Si1-xCx:H decreases especially when it is below 10nm. We also observed that Voc does not change much for p-type layer thickness more than 12nm. So optimized thickness of the p-type layer can be considered as 10 nm. It is also obvious that at higher sample thickness optical transmission through the film lowers. Urbach energy of a-Si1-xCx:H films increase from ~50 meV with increased optical gap and is expected to saturate around 90 meV at optical gap higher than 2.1 eV [43]. The FF continuously decreases with increased thickness of the p-type layer.

We have also observed an improved blue response of cell at lower p-type layer thickness. One simple reason is the Beer Lambert's law of absorption IT = I0exp[-αd], where IT is intensity of transmitted light through a material layer of thickness d having absorption coefficient α, I0 is intensity of incident light. Thus with a thicker (higher d) p-a-Si1-xCx:H layer the light penetrat‐ ing though the p-type layer will be lower. We have observed that the maximum available quantum efficiency of the cell also decreases with increase in thickness of the p-type layer.

Thus, at increased thickness of p-type layer, intensity of transmitted light decreases leading to lower Jsc and lower quantum efficiency. Increased thickness also indicates higher electrical resistance across the p-type layer and thus reduced FF as well as efficiency. Whereas at re‐ duced thickness (<8nm) of the p-type layer the formation of p-type layer remains insufficient and enough built-in field is not generated.

This result is similar to that observed by Myong et. al., [8] and Lee et al [44] that at lower ptype layer thickness the quantum efficiency of the cell at shorter wavelength improves while efficiency, short circuit current density FF etc improves at the beginning but decreases at higher p-type layer thickness.

#### **3.4. Effect of Hydrogen Dilution**

ed e-h pairs were rapidly collected by the external circuit and thus most of the e-h pairs pro‐ duce higher photo-generated current from the cell. Whereas, while V>0.5 Volts the average residual time for e-h pairs inside the cell increased, leading to a higher possibility of the charge carriers being trapped at the defect states and lost. Such a model can be true if defect states created by C incorporation is ~0.5 eV above the quasi-Fermi level for holes, which may be ~0.85 eV above the valence band mobility edge. We estimate location of the defect states as 0.85 eV by adding Ea =0.35 eV for x=0.15 and the 0.5 eV. Fig. 2(c) shows that activa‐ tion energy of the p-a-Si1-xCx:H layer for x=0.32, is 0.82 eV, which is very close to the 0.85 and thus the role of midgap defect states become more obvious at higher carbon incorporation. In this situation it leads to lower FF and Voc. So the shunt resistance (Rp) of the cell (with

Thus a model of increased interface defects for x~0.32 may also be supported to some extent from the quantum efficiency measurements. As it is well known that absorption coefficient of amorphous silicon alloy films increase with reduction in wavelength of incident photon. Thus shorter wavelength photons will have a smaller penetration depth from the surface of incidence. In QE measurements, Fig. 3(c), when wavelength of incident radiation was gradu‐ ally reduced the electron-hole pair generation takes place closer to the p-i interface. It can be seen from Fig. 3(c) that 360 nm wavelength the EQE of the cell with the x~0.18 was same to that with cell with x~0.32. It can be assumed that at this wavelength the photo generated e-h pairs were created at the edge of p-i interface. So when the incident photon energy was low‐ er than the above critical wavelength, the EQE became lower, as the e-h pairs were generat‐

Thus, it seems that at higher carbon incorporation the p-i interface as well as TCO/p-type

Although there are several disadvantages for samples with higher x, yet its advantage is bet‐ ter transmission of higher energy photons into the i-type layer of the cell and hence generat‐ ing electron-hole pairs with shorter wavelength light, as shown in Fig. 3(c). It shows better

At higher level of boron doping the activation energy falls significantly [41]. However, it is known that with the increased boron doping optical gap of the material falls [42] and this is

We have observed that with a thin p-type layer, the cell performs better. We have observed that the open circuit voltage decreases as the thickness of p-a-Si1-xCx:H decreases especially when it is below 10nm. We also observed that Voc does not change much for p-type layer thickness more than 12nm. So optimized thickness of the p-type layer can be considered as 10 nm. It is also obvious that at higher sample thickness optical transmission through the film lowers. Urbach energy of a-Si1-xCx:H films increase from ~50 meV with increased optical gap and is expected to saturate around 90 meV at optical gap higher than 2.1 eV [43]. The FF

layer interface the defects increased that lead to poorer performance of the cell.

external quantum nearly at all wavelength except below 360 nm.

continuously decreases with increased thickness of the p-type layer.

x=0.32) becomes lower than the other cell (with x=0.15).

ed at the defective interface region.

300 Photodiodes - From Fundamentals to Applications

a regular feature.

**3.3. P-type Layer Thickness**

We have observed that the higher hydrogen dilution for the p-type layer deposition over the TCO, leads to defective TCO/p interface, mostly due to the chemical reduction of the top surface of the TCO.

Use of hydrogen dilution during deposition is an important step for defect reduction of de‐ posited films. However, while the same technique is used for solar cell fabrication, specially for deposition of p-type layer, unless caution is maintained it leads to cells with poorer per‐ formance. During the p-layer deposition, if higher H-dilution was used the Voc and Jsc re‐ duces. At higher hydrogen dilution (R), the H-radicals in the plasma might have eroded top surface of the TCO by chemically reducing it through removal of part of bonded oxygen.

It is known that at higher hydrogen dilution the characteristics of intrinsic a-Si1-xCx:H films im‐ proves, like its conduction band Urback energy decreases and carrier life time increases [45] yet during device fabrication with TCO, the situation changes and one faces limitation in using higher hydrogen dilution that risks deteriorating top surface of the TCO. Our results are simi‐ lar to that of Tawada et al. [46] however the Voc in our sample is higher may be because of im‐ proved band gap matching at p-type and i-type layers and lower interface defects.

Although optimization of cell performance has been carried out, yet there is possibility to further improve the Jsc, cell efficiency etc. by optimization of the cell structure.

#### **3.5. Nano crystalline Silicon and application in Solar cell**

As the ratio of SiH4 gas RSi, increases from 3.5% to 8.0%, the deposition rate increases from 0.9Å/sec to 1.7 Å/sec. At a higher hydrogen flow rate, the film deposition rate reduces due to selective etching of amorphous silicon phase by energetic hydrogen radicals, and thus, crys‐ tallinity of the film increases. At a higher hydrogen flow rate, the hydrogen atoms may act as etchant to remove the amorphous silicon phase [47, 48].

250℃. At a higher Ts, a decrease in crystalline volume fraction can be observed, may be be‐

Single- and Multiple-Junction p-i-n Type Amorphous Silicon Solar Cells with p-a-Si1-xCx:H and nc-Si:H Films

http://dx.doi.org/10.5772/51732

303

Fig. 5(b) shows the electrical properties of the films, deposited at different substrate temper‐ atures (Ts). When the Ts were 100℃ and 140℃, the dark conductivity of the films remained around 10-10S/cm indicating an amorphous film. Raman spectra of the films also indicate similar things. When the temperature was over 200℃, the dark conductivity increased rap‐ idly. Here also the photo conductivity did not change much in the temperature range of 100

**Figure 5.** (a) Raman spectra and (b) conductivities of the nc-Si:H films deposited at various substrate temperatures in a

The quantum efficiency (QE) is the current generated per unit incident photons, or solar cell current/number of incident photons at a particular wavelength. The spectral response (SR) is the solar cell current (A) per unit incident energy (W) of incident light. The QE and SR is

where h is Planck's constant, c is the speed and λ is the wavelength of the light. The solar

Fig. 6 shows the QE spectra of a-Si:H/nc-Si:H MJ solar cells with various thickness of a-Si:H i-layer of the top cell. As the i-layer thickness was increased from 100 to 300nm, the top cell

For the QE measurement of a multiple-junction solar cell, a bias light was used to saturate all but one of the cell under investigation. Whereas the light (AM1.5) induced current-volt‐

l

) (8)

QE SR hc / = ´(

cause of higher H-etching at the film surfaces, by the reactive H radicals.

~ 250℃ although there was a small increase at 260℃ temperature.

HW CVD.

thus related as

**3.7. Analysis of solar cells**

cells were characterized by the QE spectra.

QE increased while that of the bottom one reduced.

Fig. 4(a) shows the Raman characteristics of the above described films. These films, that were deposited with RSi in between 3.6% and 4.5%, were nc-Si:H films. The transverse optic (TO) mode was observed around 520cm-1 indicating the presence of nano crystallites in the film. When the R was greater than 4.5%, a broad peak around 480cm-1 was observed indicat‐ ing the presence of an amorphous phase. The crystalline volume fraction of thin films de‐ posited with the ratio of 3.6% and 4.5% is found to be 70% and 52% respectively, the other films show amorphous character.

**Figure 4.** (a) Raman spectra and (b) electrical conductivities of the films prepared at various silane flow rates RSi.

Fig. 4(b) shows the electrical characteristics of nc-Si:H thin film deposited with varying RSi. The σd of the film deposited with the ratio of 3.6% is 10-6S/cm. When the flow ratio was in‐ creased more than that, the dark conductivity rapidly reduced to 10-10S/cm. Generally, an amorphous silicon thin film lower defect density and hence a lower dark conductivity. However, when transition from amorphous to nanocrystallinity occurs, crystalline grains were formed and the amorphous phase exists in defective grain boundary. Thus, the amor‐ phous silicon films show lower dark conductivity in comparison to nanocrystalline silicon.

Unlike the dark conductivity (σd), the photo-conductivity (σph) did not change much with nano-crystallinity, and remains around 1×10-4S/cm, Fig. 4(b). Thus the photo-sensitivity was high for the a-Si:H films in comparison to that of the nc-Si:H films.

#### **3.6. Analysis of nc-Si:H thin film dependence on substrate temperature**

Another important parameter that affect the nano-crystallinity of the nc-Si:H thin film is the substrate temperature [49]. Fig. 5(a) shows the Raman spectrum of the films as the substrate temperature was varied. The films formed at 100℃ and 140℃ temperature, were amor‐ phous. Nano-crystalline films were observed when the Ts was above 200℃. The crystalline volume fraction increased from 55% to 65% as the temperature increased from 200℃ to 250℃. At a higher Ts, a decrease in crystalline volume fraction can be observed, may be be‐ cause of higher H-etching at the film surfaces, by the reactive H radicals.

Fig. 5(b) shows the electrical properties of the films, deposited at different substrate temper‐ atures (Ts). When the Ts were 100℃ and 140℃, the dark conductivity of the films remained around 10-10S/cm indicating an amorphous film. Raman spectra of the films also indicate similar things. When the temperature was over 200℃, the dark conductivity increased rap‐ idly. Here also the photo conductivity did not change much in the temperature range of 100 ~ 250℃ although there was a small increase at 260℃ temperature.

**Figure 5.** (a) Raman spectra and (b) conductivities of the nc-Si:H films deposited at various substrate temperatures in a HW CVD.

#### **3.7. Analysis of solar cells**

tallinity of the film increases. At a higher hydrogen flow rate, the hydrogen atoms may act

Fig. 4(a) shows the Raman characteristics of the above described films. These films, that were deposited with RSi in between 3.6% and 4.5%, were nc-Si:H films. The transverse optic (TO) mode was observed around 520cm-1 indicating the presence of nano crystallites in the film. When the R was greater than 4.5%, a broad peak around 480cm-1 was observed indicat‐ ing the presence of an amorphous phase. The crystalline volume fraction of thin films de‐ posited with the ratio of 3.6% and 4.5% is found to be 70% and 52% respectively, the other

**Figure 4.** (a) Raman spectra and (b) electrical conductivities of the films prepared at various silane flow rates RSi.

Fig. 4(b) shows the electrical characteristics of nc-Si:H thin film deposited with varying RSi. The σd of the film deposited with the ratio of 3.6% is 10-6S/cm. When the flow ratio was in‐ creased more than that, the dark conductivity rapidly reduced to 10-10S/cm. Generally, an amorphous silicon thin film lower defect density and hence a lower dark conductivity. However, when transition from amorphous to nanocrystallinity occurs, crystalline grains were formed and the amorphous phase exists in defective grain boundary. Thus, the amor‐ phous silicon films show lower dark conductivity in comparison to nanocrystalline silicon.

Unlike the dark conductivity (σd), the photo-conductivity (σph) did not change much with nano-crystallinity, and remains around 1×10-4S/cm, Fig. 4(b). Thus the photo-sensitivity was

Another important parameter that affect the nano-crystallinity of the nc-Si:H thin film is the substrate temperature [49]. Fig. 5(a) shows the Raman spectrum of the films as the substrate temperature was varied. The films formed at 100℃ and 140℃ temperature, were amor‐ phous. Nano-crystalline films were observed when the Ts was above 200℃. The crystalline volume fraction increased from 55% to 65% as the temperature increased from 200℃ to

high for the a-Si:H films in comparison to that of the nc-Si:H films.

**3.6. Analysis of nc-Si:H thin film dependence on substrate temperature**

as etchant to remove the amorphous silicon phase [47, 48].

films show amorphous character.

302 Photodiodes - From Fundamentals to Applications

The quantum efficiency (QE) is the current generated per unit incident photons, or solar cell current/number of incident photons at a particular wavelength. The spectral response (SR) is the solar cell current (A) per unit incident energy (W) of incident light. The QE and SR is thus related as

$$\text{QE} = \text{SR} \times \text{(hc/}\text{\AA}\text{)}\tag{8}$$

where h is Planck's constant, c is the speed and λ is the wavelength of the light. The solar cells were characterized by the QE spectra.

Fig. 6 shows the QE spectra of a-Si:H/nc-Si:H MJ solar cells with various thickness of a-Si:H i-layer of the top cell. As the i-layer thickness was increased from 100 to 300nm, the top cell QE increased while that of the bottom one reduced.

For the QE measurement of a multiple-junction solar cell, a bias light was used to saturate all but one of the cell under investigation. Whereas the light (AM1.5) induced current-volt‐ age characteristics (LIV) can give the output Jsc, Voc, FF and η of the MJ cell as a whole. From the QE characteristic spectra, one can estimate approximate Jsc of the component cells of a MJ cell, and thus it becomes evident that the Jsc of a MJ solar cell is limited by the Jsc of the top cell. Generally, the limitation comes from any of the component cells, that has the lowest Jsc, that acts as the current limiting component of the MJ cell.

thickness, with Jsc of 9.5 ~ 10.0mA/cm2

cell of tandem solar cell was about 1 ~ 2mA/cm2

greatly dependent on the reflectivity of back reflector electrode.

and top cell of a double junction cell with change in top cell i-layer thickness.

junction cell.

which is much less than 13mA/cm2

Single- and Multiple-Junction p-i-n Type Amorphous Silicon Solar Cells with p-a-Si1-xCx:H and nc-Si:H Films

junction solar cell to have an efficiency of above 13%. Unlike the single p-i-n type solar cell, there is no internal reflection effect for light at back electrode (Ag) for a-Si:H/nc-Si:H tandem solar cell so It is important to maximize the collection efficiency around the wavelength range of 500 ~ 700nm. Fig. 7(b) shows Jsc of single p-i-n type a-Si:H solar cell and Jsc of the top cell of tandem solar cell with different thickness of absorption layers. As mentioned before, Jsc of top

a single p-i-n type a-Si:H solar cell, the response in the wavelength range of 500 ~ 700nm is

**Figure 7.** Changes in (a) Voc and (b) Jsc of a double junction cell during current matching. (c) Changes in Jsc of a single

**Figure 8.** (a) LIV characteristics of a double and triple junction solar cell, (b) QE of top and bottom cells of the triple

Fig.8(a) shows the illuminated I-V curves of a-Si:H/nc-Si:H/nc-Si:H triple junction solar cell and a-Si:H/nc-Si:H double junction solar cell. Jsc of triple junction solar cell decreased and Voc increased from 1.42 to 1.83V compared with double junction solar cell. Fig.8(b) shows the QE of top and bottom cells of the triple junction solar cell. Jsc of the a-Si:H top cell is 7.54 mA/cm2

response in the range of 550 ~ 700nm decreases because of the thin layer of 150nm. The Jsc of

bottom cell with 3.2μm thick nc-Si:H absorption layer is 5.44mA/cm2

required for multiple

305

http://dx.doi.org/10.5772/51732

. The

which was close to the

lower than that of single p-i-n type solar cell. In

**Figure 6.** Variation in QE spectra of the top and bottom cells of a double junction cell during current matching experiment.

#### **3.8. Multiple- junction cell, The thickness of top cell**

It is important to achieve a current matching among the component cells of a MJ cell. In a best condition, each cell should be designed to have equal Jsc having highest possible current densi‐ ty. A few nm thick p μc-Si:H thin film was inserted as a buffer layer between AZO and p-a-Si1 xCx:H window layer of the top cell in order to improve the interface. The thickness of pure a-Si:H absorption layer of the top cell was varied in the range of 100 ~ 300nm by controlling the deposition time and the i-layer of the nc-Si:H bottom cell was kept fixed to 1.7μm. To improve the tunnel junction between the top and bottom cells, less than a few nm thick AZO was depos‐ ited by RF magnetron sputtering. The area of the solar cell was 0.36cm2 .

The QE of the top cell increased with increase in i-layer thickness (di-layer) in the wavelength range of 450 ~ 700nm due to increased absorption of incident light. With a thicker i-layer, the Jsc of the top cell increased from 6.5 to 9.5mA/cm2 but Jsc of nc-Si:H bottom cell decreased from 13.0 to 10.0mA/cm2 . This decrease in the Jsc of the bottom cell was due to the reduction in QE of the bottom cell in the wavelength longer than 450nm. The Voc of the MJ cell decreased from 1.435 to 1.405 V with the increase in thickness of i-layer of the top cell. The Jsc of the MJ solar cell was limited by Jsc of the top cell regardless of the top cell thickness. With the increase in top cell thickness, the efficiency increased from 6 to 10%. Fig.7(b,c) shows the change in short circuit current densities of top and bottom cells as top cell thickness was varied.

With increase of thickness of the i-layer, the current of the top cell increased while that of the bottom cell decreased. It seems that a current matching occurs at around 350nm top cell i-layer thickness, with Jsc of 9.5 ~ 10.0mA/cm2 which is much less than 13mA/cm2 required for multiple junction solar cell to have an efficiency of above 13%. Unlike the single p-i-n type solar cell, there is no internal reflection effect for light at back electrode (Ag) for a-Si:H/nc-Si:H tandem solar cell so It is important to maximize the collection efficiency around the wavelength range of 500 ~ 700nm. Fig. 7(b) shows Jsc of single p-i-n type a-Si:H solar cell and Jsc of the top cell of tandem solar cell with different thickness of absorption layers. As mentioned before, Jsc of top cell of tandem solar cell was about 1 ~ 2mA/cm2 lower than that of single p-i-n type solar cell. In a single p-i-n type a-Si:H solar cell, the response in the wavelength range of 500 ~ 700nm is greatly dependent on the reflectivity of back reflector electrode.

age characteristics (LIV) can give the output Jsc, Voc, FF and η of the MJ cell as a whole. From the QE characteristic spectra, one can estimate approximate Jsc of the component cells of a MJ cell, and thus it becomes evident that the Jsc of a MJ solar cell is limited by the Jsc of the top cell. Generally, the limitation comes from any of the component cells, that has the lowest

**Figure 6.** Variation in QE spectra of the top and bottom cells of a double junction cell during current matching experiment.

It is important to achieve a current matching among the component cells of a MJ cell. In a best condition, each cell should be designed to have equal Jsc having highest possible current densi‐ ty. A few nm thick p μc-Si:H thin film was inserted as a buffer layer between AZO and p-a-Si1 xCx:H window layer of the top cell in order to improve the interface. The thickness of pure a-Si:H absorption layer of the top cell was varied in the range of 100 ~ 300nm by controlling the deposition time and the i-layer of the nc-Si:H bottom cell was kept fixed to 1.7μm. To improve the tunnel junction between the top and bottom cells, less than a few nm thick AZO was depos‐

The QE of the top cell increased with increase in i-layer thickness (di-layer) in the wavelength range of 450 ~ 700nm due to increased absorption of incident light. With a thicker i-layer, the Jsc

bottom cell in the wavelength longer than 450nm. The Voc of the MJ cell decreased from 1.435 to 1.405 V with the increase in thickness of i-layer of the top cell. The Jsc of the MJ solar cell was limited by Jsc of the top cell regardless of the top cell thickness. With the increase in top cell thickness, the efficiency increased from 6 to 10%. Fig.7(b,c) shows the change in short circuit

With increase of thickness of the i-layer, the current of the top cell increased while that of the bottom cell decreased. It seems that a current matching occurs at around 350nm top cell i-layer

. This decrease in the Jsc of the bottom cell was due to the reduction in QE of the

.

but Jsc of nc-Si:H bottom cell decreased from 13.0

Jsc, that acts as the current limiting component of the MJ cell.

304 Photodiodes - From Fundamentals to Applications

**3.8. Multiple- junction cell, The thickness of top cell**

of the top cell increased from 6.5 to 9.5mA/cm2

to 10.0mA/cm2

ited by RF magnetron sputtering. The area of the solar cell was 0.36cm2

current densities of top and bottom cells as top cell thickness was varied.

**Figure 7.** Changes in (a) Voc and (b) Jsc of a double junction cell during current matching. (c) Changes in Jsc of a single and top cell of a double junction cell with change in top cell i-layer thickness.

**Figure 8.** (a) LIV characteristics of a double and triple junction solar cell, (b) QE of top and bottom cells of the triple junction cell.

Fig.8(a) shows the illuminated I-V curves of a-Si:H/nc-Si:H/nc-Si:H triple junction solar cell and a-Si:H/nc-Si:H double junction solar cell. Jsc of triple junction solar cell decreased and Voc increased from 1.42 to 1.83V compared with double junction solar cell. Fig.8(b) shows the QE of top and bottom cells of the triple junction solar cell. Jsc of the a-Si:H top cell is 7.54 mA/cm2 . The response in the range of 550 ~ 700nm decreases because of the thin layer of 150nm. The Jsc of bottom cell with 3.2μm thick nc-Si:H absorption layer is 5.44mA/cm2 which was close to the 5.72mA/cm2 measured with solar simulator. It can be said that Jsc of triple junction solar cell is limited by the bottom cell and Jsc of the nc-Si:H middle cell sould be at least 5.4mA/cm2 .

ciency of the top cell, the intrinsic layer thickness should be as thin as possible and improve‐ ment in Jsc should be carried out through improvement in the short wavelength response. Since the middle and bottom cells absorb light in the relatively long wavelength range, the electrical and optical properties and the thickness of the intrinsic layer need to be optimized. Higher Jsc could be obtained by using nc-SiGe:H thin film since its absorption coefficients are

Single- and Multiple-Junction p-i-n Type Amorphous Silicon Solar Cells with p-a-Si1-xCx:H and nc-Si:H Films

, Juyeon Jang1

1 College of Information and Communication Engineering, Sungkyunkwan University, Re‐

2 Korea Institute of Energy Research, Gajeong-ro, Yuseong-gu, Daejeon, Republic of Korea

[1] Tawada, Y., Tsuge, K., Kondo, M., Okamoto, H., & Hamakawa, Y. (1982). Properties and structure of a-SiC:H for high-efficiency a-Si solar cell. *Journal of Applied Physics*,

[2] Konenkamp, R., Muramatsu, S., Matsubara, S., & Shimada, T. (1992). Space-charge distribution and trapping kinetics in amorphous silicon solar cells. *Applied Physics.*

[3] Reichman, J. (1981). Collection efficiency of low-mobility solar cells. *Applied Physics*

[4] Boer & K. W. (1981). Influence of the electric field on collection efficiencies of solar

[5] D'Aiello, R. V., Robinson, P. H., & Kressel, H. (1976). Epitaxial silicon solar cells. *Ap‐*

[6] Sinencio, F. S., & Williams, R. (1983). Barrier at the interface between amorphous sili‐ con and transparent conducting oxides and its influence on solar cell performance.

3 Department of Energy Science, Sungkyunkwan University, Republic of Korea

, Yeun-Jung Lee1

and Junsin Yi1,3

http://dx.doi.org/10.5772/51732

307

, Jieun Lee2

\*Address all correspondence to: smiftiquar@gmail.com

higher than those of nc-Si:H thin film.

S. M. Iftiquar1\*, Jeong Chul Lee2

**Author details**

public of Korea

**References**

53, 5273-5281.

*Letters*, 60(9), 1120-1122.

*Letters*, 38(4), 251-253.

cells. *Applied Physics Letters*, 38(7), 537-539.

*Journal of Applied Physics*, 54(5), 2757-2760.

*plied Physics Letters*, 28(4), 231-234.

Even though the bottom cell was composed of 3.2μm thick intrinsic layer, the Jsc was low. One of the reasons can be the absorption of incident light by the middle cell. Low haze ratio of the TCO and increased recombination of generated electron hole pair in nc-Si:H absorp‐ tion layer can also be one of the reasons. In this study, the observed Voc was 1.83V. The rea‐ son for the lower Voc is presumed to be the decrease in Voc of the microcrystalline bottom cell. In order to have initial efficiency of over 14%, Jsc of unit cells should be at least 9mA/ cm2 . For the top cell, it can be done by raising the thickness of the intrinsic a-Si:H layer to ~ 250nm or by increasing the short wavelength QE. However, for the middle and bottom cells, more efforts are needed such as light trapping, improved property of nc-Si:H and develop‐ ment of new intrinsic layer such as nc-SiGe:H [50-52] as well as improvement in the TRJ.

#### **4. Summary**

a-Si:H/nc-Si double and a-Si:H/nc-Si:H/nc-Si:H triple junction solar cells have been made and the effects of the top cell thickness and interlayer on the current matching and solar cell characteristics have been investigated. There is a significant impact of the multijunction cell performance on the current matching of the component cells as well as the tunnel junction in between them. When the Si:H top cell thickness was varied from 100 to 300nm, Jsc of the top cell increased from 6.5 to 9.5 mA/cm2 . For the bottom cell, Jsc decreased from 13.0 to 10.0mA/cm2 and current matching of the multiple junction solar cell occurred around 330nm resulting in lower Jsc. For the top cell of DJ solar cells, unlike the single p-i-n type solar cell, there is no back reflector electrodes (Ag or AZO) present, because of the presence of the bot‐ tom cell. So it was difficult to raise the Jsc of this cell without increasing its i-layer thickness. The AZO inter-layer was inserted between the top and bottom cells to make the junction like a TRJ. This AZO layer may also work as a partial reflector of unabsorbed light. With a 150nm thick inter-layer, the current gain of the top cell was +1.3mA/cm2 . However, for the nc-Si:H bottom cell, the current loss of -1.77mA/cm2 occurred due to the reflection and ab‐ sorption of AZO. By using textured AZO front layer electrode with high haze ratio, it was possible to develop a a-Si:H/nc-Si:H double junction solar cell with Voc of 1.424V, Jsc of 12.09mA/cm2 , FF of 72.84%. To develop multiple junction solar cells with initial efficiency over 13%, further studies for improvements on inter-layer property, light trapping and high‐ er response of bottom cell in long wavelength range should be carried out.

For an a-Si/nc-Si/nc-Si triple junction solar cell, 3 units of solar cells were connected in series electrically and optically. Thus, the current matching between each unit was im‐ portant to get higher efficiency. The a-Si(150nm)/nc-Si(2.0μm)/nc-Si(3.2μm) triple junction solar cell fabricated in this study, showed the Voc of 1.832V, Jsc of.773mA/cm2 , FF of 71.41%, and efficiency of 7.49%.

It may be possible to raise the Voc of the MJ cell to 1.95V by optimizing the top cell and the tunnel junction. With this, the FF is also expected to increase. To increase the stabilized effi‐ ciency of the top cell, the intrinsic layer thickness should be as thin as possible and improve‐ ment in Jsc should be carried out through improvement in the short wavelength response. Since the middle and bottom cells absorb light in the relatively long wavelength range, the electrical and optical properties and the thickness of the intrinsic layer need to be optimized. Higher Jsc could be obtained by using nc-SiGe:H thin film since its absorption coefficients are higher than those of nc-Si:H thin film.
