**4. Modeling of HIT solar cells on P-type wafer**

 

### **4.1 Simulation of experimental results of P-type HIT cells**

We have studied both front and double "HIT" structure solar cells on P-type c-Si wafers. These have the structure: N-a-Si:H emitter/ P-c-Si/ aluminum diffused BSF (front HIT) and N-a-Si:H emitter/ P-c-Si/ P+-a-Si:H BSF (double HIT). The experimental data were obtained from the Laboratoire de Physique des Interfaces et des Couches Minces (LPICM), Ecole Polytechnique, Palaiseau, France. Table 2 compares our modeling results to the measured output parameters for front and double HIT structures. Two thicknesses of the N-a-Si:H layer are employed for the front HIT structures, while results are given for two types of

Computer Modeling of Heterojunction

**IQE**

  **QE**

**0 0.2 0.4 0.6 0.8 1**

**(a)**

RS of the c-Si wafer.

with Intrinsic Thin Layer "HIT" Solar Cells: Sensitivity Issues and Insights Gained 281

where *R(λ)* is the reflectivity of the HIT cell and *ITOabs(λ)* is the fraction of the light that is absorbed in the transparent conducting oxide, that is indium tin oxide (ITO) in this case.

Fig. 3. Comparison of the experimentally measured external and internal QE curves of (a) a double heterojunction cell (case B2 of Table 2); and (b) of a front and the above double HIT solar cell, to modeling results, indicating a higher long wavelength IQE for the double HIT case, both experimentally and in the modeling calculations. The ITO layer is different for the two cases resulting in the difference in the short wave length QE. The lines represent the

**0 0.2 0.4 0.6 0.8 1**

**(b)**

**Internal QE**

**0.2 0.4 0.6 0.8 1 1.2**

**Wavelength (microns)**

**Double HIT\_model Front HIT\_model Double HIT\_expt Front HIT\_expt**

We have used the above simulations to extract the parameters that characterize different layers of the double HIT cells A and B on P-type wafers. These are given in Table 3, together with the extracted parameters of double HIT cells on N-type wafers. The experimental results used to extract the latter and comments thereon, will be discussed in section 5.1. The data in Table 3 includes some measured data: the thickness and doping density of each layer/ wafer, the band gaps of the layers and the electron and hole mobility in the c-Si wafer (Sze, 1981). We also found that a higher value of Nss (as indicated in Case B2 in Table 2 and Table 3) was necessary at the RS to simulate the experimental results. No layer was

Since the 4-nm P+-a-SiC:H layer on the RS of the c-Si wafer (part of the highly doped thin film BSF layer) produces a small but reproducible improvement in the overall device performance, we have tried to understand the basic reasons for this improvement. To realize the role of the thin P-a-SiC:H layer on the RS in case B2 we have made the P+-a-SiC:H layer thicker than in case B2 and adjusted the thickness of following P+-a-Si:H layer to yield a total BSF thickness of 23 nm. We found an all-round deterioration of the solar cell output for the thicker P+-a-SiC:H layers, including a striking fall in the fill factor. We have thus concluded that the introduction of the thin carbide layer as such is not responsible for the observed improvement in cell efficiency of case B2 relative to case A (Table 2). Rather, it appears likely that this wider band gap material helps in passivating the defects on the RS of the c-Si wafer (for which a very thin layer is sufficient) and thereby improves cell performance. In the next section we will discuss how solar cell performance is affected by the defects on the FS and

calculated results, experimental measurements are shown as symbols.

**0.2 0.4 0.6 0.8 1 1.2**

**Wavelength (microns)**

**EQE**

**EQE\_model IQE\_model EQE\_expt IQE\_expt**

intentionally deposited to passivate these defects in cells A and B.

double HIT cells having the following structures: (A) 8 nm N-a-Si:H/ 3 nm pm-Si:H intrinsic layer/ P-c-Si wafer/ 23 nm P+-a-Si:H/ 1.5 m Al, and (B) the above structure, but with a 4 nm P+-a-SiC:H layer sandwiched between the P-c-Si wafer and a 19 nm P+-a-Si:H layer. The pm-Si:H intrinsic layer on the front surface (FS) of the c-Si wafer is there to passivate the defects on this surface. However, no such passivating layer has been deposited on the rear surface (RS) of the c-Si wafer. The defect density on FS was deduced by modeling to be 1011 cm-2. Cell B, which has a 4 nm P-type a-SiC:H layer on the rear c-Si wafer surface, has a slightly higher Voc but a lower FF relative to case A, leading to a better efficiency. However, we could not replicate these results in our modeling calculations by the introduction of a P+ a-SiC:H layer of the given properties alone (case B1 in Table 2). In fact, the defect density on the rear wafer surface had to be slightly reduced (case B2, Table 2) to match the experimental results.

Table 2 indicates good agreement between experiments and modeling, except that our modeling results appear to overestimate the FF and hence the efficiency of front HIT cells. In reality this is because screen-printed contacts with low temperature silver paint was used for these cells; resulting in high series resistance and low FF experimentally, which cannot be accounted for by modeling. For double HIT structures, developed later, improved contact formation resulted in very low series resistance and high fill factors experimentally, which agree well with model calculations (Table 2).


Table 2. Comparison between measured (E) and modeled (M) solar cell output parameters of front and double P-c-Si HIT cells with a flat ITO front contact. DL refers to the defective layer on the wafer surface.

In Fig. 3 (a), we compare the experimentally measured external and internal quantum efficiency (EQE and IQE respectively) curves of the solar cell B to modeling results, while in Fig. 3 (b) we compare the measured IQE curves of a front HIT and the above-mentioned double HIT solar cells, both deposited in the same reactor and under approximately the same conditions of RF power and pressure as solar cells A and B above. The IQE is obtained from the EQE using the formula:

$$\text{IQE}(\mathcal{\lambda}) = \text{EQE}(\mathcal{\lambda}) \left/ \left( \mathbf{1} - \mathbf{R}(\mathcal{\lambda}) - \text{ITOabs}(\mathcal{\lambda}) \right) \right. \tag{2}$$

double HIT cells having the following structures: (A) 8 nm N-a-Si:H/ 3 nm pm-Si:H intrinsic layer/ P-c-Si wafer/ 23 nm P+-a-Si:H/ 1.5 m Al, and (B) the above structure, but with a 4 nm P+-a-SiC:H layer sandwiched between the P-c-Si wafer and a 19 nm P+-a-Si:H layer. The pm-Si:H intrinsic layer on the front surface (FS) of the c-Si wafer is there to passivate the defects on this surface. However, no such passivating layer has been deposited on the rear surface (RS) of the c-Si wafer. The defect density on FS was deduced by modeling to be 1011 cm-2. Cell B, which has a 4 nm P-type a-SiC:H layer on the rear c-Si wafer surface, has a slightly higher Voc but a lower FF relative to case A, leading to a better efficiency. However, we could not replicate these results in our modeling calculations by the introduction of a P+ a-SiC:H layer of the given properties alone (case B1 in Table 2). In fact, the defect density on the rear wafer surface had to be slightly reduced (case B2, Table 2) to match the

Table 2 indicates good agreement between experiments and modeling, except that our modeling results appear to overestimate the FF and hence the efficiency of front HIT cells. In reality this is because screen-printed contacts with low temperature silver paint was used for these cells; resulting in high series resistance and low FF experimentally, which cannot be accounted for by modeling. For double HIT structures, developed later, improved contact formation resulted in very low series resistance and high fill factors experimentally, which

> Nss on the DL (cm-2)

Voc (mV)

X1 (E) 12 634 31.90 0.711 14.38 X1 (M) 12 FS- 4x1011 636 31.85 0.823 16.67 X2 (E) 8 640 32.54 0.730 15.20 X2 (M) 8 FS- 4x1011 640 32.57 0.824 17.18

A (E) 8 650 32.90 0.790 16.90

B (E) 8 664 33.10 0.779 17.12

Table 2. Comparison between measured (E) and modeled (M) solar cell output parameters of front and double P-c-Si HIT cells with a flat ITO front contact. DL refers to the defective

In Fig. 3 (a), we compare the experimentally measured external and internal quantum efficiency (EQE and IQE respectively) curves of the solar cell B to modeling results, while in Fig. 3 (b) we compare the measured IQE curves of a front HIT and the above-mentioned double HIT solar cells, both deposited in the same reactor and under approximately the same conditions of RF power and pressure as solar cells A and B above. The IQE is obtained

*IQE EQE R ITOabs* ( ) ( ) /(1 ( ) ( ))

Jsc

660 32.84 0.781 16.93 RS-8x1011

653 33.17 0.749 16.24 RS-8x1011

667 33.21 0.773 17.12 RS- 3x1011

 

, (2)

(mA cm-2 ) FF (%)

experimental results.

HIT

Front

Double

layer on the wafer surface.

from the EQE using the formula:

agree well with model calculations (Table 2).

(nm)

A (M) 8 FS-1011

B1(M) FS-1011

B2 (M) 8 FS-1011

type Sample N-a-Si:H

where *R(λ)* is the reflectivity of the HIT cell and *ITOabs(λ)* is the fraction of the light that is absorbed in the transparent conducting oxide, that is indium tin oxide (ITO) in this case.

Fig. 3. Comparison of the experimentally measured external and internal QE curves of (a) a double heterojunction cell (case B2 of Table 2); and (b) of a front and the above double HIT solar cell, to modeling results, indicating a higher long wavelength IQE for the double HIT case, both experimentally and in the modeling calculations. The ITO layer is different for the two cases resulting in the difference in the short wave length QE. The lines represent the calculated results, experimental measurements are shown as symbols.

We have used the above simulations to extract the parameters that characterize different layers of the double HIT cells A and B on P-type wafers. These are given in Table 3, together with the extracted parameters of double HIT cells on N-type wafers. The experimental results used to extract the latter and comments thereon, will be discussed in section 5.1. The data in Table 3 includes some measured data: the thickness and doping density of each layer/ wafer, the band gaps of the layers and the electron and hole mobility in the c-Si wafer (Sze, 1981). We also found that a higher value of Nss (as indicated in Case B2 in Table 2 and Table 3) was necessary at the RS to simulate the experimental results. No layer was intentionally deposited to passivate these defects in cells A and B.

Since the 4-nm P+-a-SiC:H layer on the RS of the c-Si wafer (part of the highly doped thin film BSF layer) produces a small but reproducible improvement in the overall device performance, we have tried to understand the basic reasons for this improvement. To realize the role of the thin P-a-SiC:H layer on the RS in case B2 we have made the P+-a-SiC:H layer thicker than in case B2 and adjusted the thickness of following P+-a-Si:H layer to yield a total BSF thickness of 23 nm. We found an all-round deterioration of the solar cell output for the thicker P+-a-SiC:H layers, including a striking fall in the fill factor. We have thus concluded that the introduction of the thin carbide layer as such is not responsible for the observed improvement in cell efficiency of case B2 relative to case A (Table 2). Rather, it appears likely that this wider band gap material helps in passivating the defects on the RS of the c-Si wafer (for which a very thin layer is sufficient) and thereby improves cell performance. In the next section we will discuss how solar cell performance is affected by the defects on the FS and RS of the c-Si wafer.

Computer Modeling of Heterojunction

  **Field on holes (volt cm-1)**

**0**

**(a)**

**10<sup>11</sup> 2x10<sup>12</sup> 3x10<sup>13</sup>**

**2x106**

**4x106**

Nss on FS (cm-2)

at the rear surface of the c-Si wafer is 1011 cm-2.

band diagram over the front part of the device.

**0.001 0.01 0.1 1**

**0**

**0.01 0.1 1**

**2x10<sup>4</sup> 4x10<sup>4</sup>**

**Position (microns)**

with Intrinsic Thin Layer "HIT" Solar Cells: Sensitivity Issues and Insights Gained 283

1011 37.50 672 0.770 19.40 2x1012 38.33 586 0.658 14.79 3x1013 38.14 463 0.545 9.65 Table 4. Calculated values of the solar cell output parameters Jsc, Voc, FF and , for different values of the defect density (Nss) on that (front) surface of the crystalline silicon wafer through which light enters, indicating high sensitivity to the Voc and FF. The defect density

Fig. 4. Effect of changing the defect density (shown in units of cm-2) on the front surface of the c-Si wafer under 100 mW cm-2 of AM1.5 light and 0 volts, on (a) the electric field (the inset shows the electric field on an expanded scale over the depletion region) and (b) the

**-2 -1.5 -1 -0.5 0 0.5 1 1.5**

**Energy (eV)**

**0.001 0.01 0.1 1**

**Position (microns)**

**(b)**

and a collapse of the field over the adjacent depletion region of the c-Si wafer (Fig. 4a inset) for the case with Nss = 3x1013 cm-2. This results in the flattening of the energy bands in the totality of the P-type crystalline silicon wafer (Fig. 4b, dashed lines), and a consequent fall in Voc and FF (Table 4). For the case of low Nss, the space charge region on the P-c-Si wafer is not localized and more field exists up to the neutral zone of the c-Si wafer (Fig. 4a inset and

Table 5 gives the calculated solar cell output parameters Jsc, Voc, FF and efficiency for different values of the defect density (Nss) on the rear surface of the c-Si wafer (away from the side where light enters). We have again varied Nss between 1011 cm-2 and 3x1013 cm-2, but this time the largest effect is on the fill factor and the short-circuit current density, as seen from Table 5 and Fig. 5 (a). In order to understand why, we have traced the band diagrams for different Nss on the RS, with the Nss at the FS held at 1011 cm-2 (Fig. 5b). We find that the band bending over the depletion region has completely disappeared for the highest value of Nss (3x1013 cm-2) at RS. From our modeling calculations we also note that up to a defect density of ~1012 cm-2 at RS, the solar cell output parameters do not deteriorate appreciably. For higher values of Nss the decrease in Jsc and FF in particular, is extremely rapid, the sensitivity to Voc being relatively small. Experimentally also it has been found that whether or not an intrinsic passivating layer is deposited on the rear face of the P-type c-Si wafer, the

band diagram in Fig. 5b, solid lines); resulting in higher Voc and FF (Table 4).

**4.3 Influence of the defect density on the rear surface of the c-Si wafer** 

Voc

(mV) FF (%)

Jsc (mA cm-2)


Table 3. Input parameters, extracted by modeling, that characterize the above HIT cells. The defect density of 3.3x1017 cm-3 on the front wafer surface corresponds to a defect density of 1011 cm-2 (FS) and 3.5x1018 cm-3 to 8x1011 cm-2 on the rear surface (RS). The P+-a-SiC:H BSF layer in P-type HIT cells has a larger band gap (1.84 eV), and broader band taills: ED=0.7 eV, EA=0.5 eV

#### **4.2 Influence of the defect density on the front surface of the c-Si wafer:**

The effect on the solar cell output parameters of varying the defect density, Nss, on front surface of the P-type c-Si wafer (that which faces the incoming light) is shown in Table 4, using as the base case the double HIT cell B2, but with an assumed textured wafer to reproduce state-of-the-art currents obtainable in HIT cells. The defect density on the RS is held at 1011 cm-2 for all cases. The results indicate a sharp fall in Voc, and FF.

To understand the sensitivity, we turn to Fig. 4. We note that the electric field is higher at the amorphous - crystalline interface, when Nss = 3x1013 cm-2 than when Nss = 1011 cm-2 (Fig. 4a). This is because when the N-a-Si:H layer is joined to a P-c-Si wafer, with a high defect density on its surface, most of the electrons that flow from the N-side to the P-side during junction formation, to bring the thermodynamic equilibrium Fermi levels on either side to the same level, are trapped in these states. The space charge region on the P-c-Si wafer side is therefore localized near the surface and does not extend appreciably into the c-Si wafer. We therefore have a huge density of trapped electrons, a very high interface field (Fig. 4a),

Electron affinity (eV) 4 3.95 4 4.22 4.22 4 Mobility gap (eV) 1.80 1.96 1.80 1.12 1.12 1.78/1.80

I-a-Si:H buffer

> 2x1020 2x1020


tail (ED) (eV) 0.05 0.05 0.07 ― 0.05

tail (EA) (eV) 0.03 0.03 0.04 — — 0.03

(cm2/V-s) 20/25 30 25 1000/1500 1000/1500 20

(cm2/V-s) 6/5 12 5 450/500 450/500 6/4

Table 3. Input parameters, extracted by modeling, that characterize the above HIT cells. The defect density of 3.3x1017 cm-3 on the front wafer surface corresponds to a defect density of 1011 cm-2 (FS) and 3.5x1018 cm-3 to 8x1011 cm-2 on the rear surface (RS). The P+-a-SiC:H BSF layer in P-type HIT cells has a larger band gap (1.84 eV), and broader band taills: ED=0.7 eV,

The effect on the solar cell output parameters of varying the defect density, Nss, on front surface of the P-type c-Si wafer (that which faces the incoming light) is shown in Table 4, using as the base case the double HIT cell B2, but with an assumed textured wafer to reproduce state-of-the-art currents obtainable in HIT cells. The defect density on the RS is

To understand the sensitivity, we turn to Fig. 4. We note that the electric field is higher at the amorphous - crystalline interface, when Nss = 3x1013 cm-2 than when Nss = 1011 cm-2 (Fig. 4a). This is because when the N-a-Si:H layer is joined to a P-c-Si wafer, with a high defect density on its surface, most of the electrons that flow from the N-side to the P-side during junction formation, to bring the thermodynamic equilibrium Fermi levels on either side to the same level, are trapped in these states. The space charge region on the P-c-Si wafer side is therefore localized near the surface and does not extend appreciably into the c-Si wafer. We therefore have a huge density of trapped electrons, a very high interface field (Fig. 4a),

DL on P-c-Si/ N-c-Si on emitter side

0.0065 0.003 0.003 0.003 300/220 0.019

1.41x1019 0 0 9x1014 9x1014 1.4x1019/

2.8x1019 1.04x1019 P-c-Si/N-csi wafer

> 2.8x1019 1.04x1019

4.5x1018 <sup>1012</sup>8x1018/

P+-a-Si :H / N+-a-Si : H BSF

1.45x1019

2x1020 2x1020

 

9x1018

I-pm Si:H buffer

2x1020 2x1020

Parameters

Don (accep)doping (cm-3)

Eff. DOS in CB (cm-3) Eff. DOS in VB (cm-3)

Exp.tail prefact.

Charac.energy – VB

Charac.energy – CB

Elec.mobility

Hole mobility

Gaussian

EA=0.5 eV

Layer thickness (m) 0.008/

*N-a-Si:H/Pa-Si:H*  emitter

1019/

2x1020 2x1020

defect density (cm-3) 9x1018 7x1014 9x1016 2.6x1018/

**4.2 Influence of the defect density on the front surface of the c-Si wafer:** 

held at 1011 cm-2 for all cases. The results indicate a sharp fall in Voc, and FF.


Table 4. Calculated values of the solar cell output parameters Jsc, Voc, FF and , for different values of the defect density (Nss) on that (front) surface of the crystalline silicon wafer through which light enters, indicating high sensitivity to the Voc and FF. The defect density at the rear surface of the c-Si wafer is 1011 cm-2.

Fig. 4. Effect of changing the defect density (shown in units of cm-2) on the front surface of the c-Si wafer under 100 mW cm-2 of AM1.5 light and 0 volts, on (a) the electric field (the inset shows the electric field on an expanded scale over the depletion region) and (b) the band diagram over the front part of the device.

and a collapse of the field over the adjacent depletion region of the c-Si wafer (Fig. 4a inset) for the case with Nss = 3x1013 cm-2. This results in the flattening of the energy bands in the totality of the P-type crystalline silicon wafer (Fig. 4b, dashed lines), and a consequent fall in Voc and FF (Table 4). For the case of low Nss, the space charge region on the P-c-Si wafer is not localized and more field exists up to the neutral zone of the c-Si wafer (Fig. 4a inset and band diagram in Fig. 5b, solid lines); resulting in higher Voc and FF (Table 4).

### **4.3 Influence of the defect density on the rear surface of the c-Si wafer**

Table 5 gives the calculated solar cell output parameters Jsc, Voc, FF and efficiency for different values of the defect density (Nss) on the rear surface of the c-Si wafer (away from the side where light enters). We have again varied Nss between 1011 cm-2 and 3x1013 cm-2, but this time the largest effect is on the fill factor and the short-circuit current density, as seen from Table 5 and Fig. 5 (a). In order to understand why, we have traced the band diagrams for different Nss on the RS, with the Nss at the FS held at 1011 cm-2 (Fig. 5b). We find that the band bending over the depletion region has completely disappeared for the highest value of Nss (3x1013 cm-2) at RS. From our modeling calculations we also note that up to a defect density of ~1012 cm-2 at RS, the solar cell output parameters do not deteriorate appreciably. For higher values of Nss the decrease in Jsc and FF in particular, is extremely rapid, the sensitivity to Voc being relatively small. Experimentally also it has been found that whether or not an intrinsic passivating layer is deposited on the rear face of the P-type c-Si wafer, the

Computer Modeling of Heterojunction

**Trapped hole density (cm-3)**

circuit conditions.

**(a)**

**5. Simulation of N-type HIT solar cells** 

**300 300.02 300.04**

**Position (microns)**

**10<sup>11</sup> 2x10<sup>12</sup> 3x10<sup>13</sup>**

the solar cell performance to various controlling factors.

**5.1 Simulation of experimental results** 

with Intrinsic Thin Layer "HIT" Solar Cells: Sensitivity Issues and Insights Gained 285

Fig. 6. Effect of changing the defect density (shown in units of cm-2) on the rear surface of the c-Si wafer on (a) the trapped hole density, (b) the electron current density, Jn, and (c) the electric field on the holes. Results are shown for 100 mW cm-2 of AM1.5 light under short-

**(b)**

**J**

 **(mA cm-2)**

**n**

**0.001 0.01 0.1 1 10 100 1000**

**Position (microns)**

Simulation of a range of experimental results on HIT cells developed by the Sanyo group and available in the literature (Maruyama et al, 2006, Takahama et al, 1992, Sawada et al, 1994, Taguchi et al, 2008) has been undertaken to extract typical parameters that characterize state-of-the-art HIT cells on N-type c-Si substrates, as well as to gain an insight into carrier transport and the general functioning of these cells. Both "front" HIT cells having an amorphous/ crystalline heterojunction on the emitter side only - where the light enters (Takahama et al, 1992), and "double" HIT cells having heterojunctions on both ends of the c-Si wafer (Maruyama et al, 2006, Sawada et al, 1994, Taguchi et al, 2008) have been simulated. The cells have the structure: ITO/ P-a-Si:H/ I-a-Si:H/ textured N-c-Si/ N-c-Si BSF/ metal (front HIT) (Takahama et al, 1992) and ITO/ P-a-Si:H/ I-a-Si:H/ textured N-c-Si/ I-a-Si:H/ N++-a-Si:H/ metal (double HIT) (Maruyama et al, 2006, Sawada et al, 1994, Taguchi et al, 2008). In Taguchi et al (2008), after depositing the undoped and doped a-Si:H layers on both ends of the c-Si wafer, ITO films were sputtered on both sides, followed by screen-printed silver grid electrodes. Simulation of these cells (Maruyama et al, 2006, Takahama et al, 1992, Sawada et al, 1994) gives us an insight into the parameters that play a crucial role in improving HIT cell performance. On the other hand, the article by Taguchi et al (2008) gives the temperature dependence of the dark current density - voltage characteristics and the solar cell output parameters as a function of the thickness of the intrinsic amorphous layer sandwiched between the emitter P-a-Si:H and the main absorber N-c-Si. A study of the temperature dependence of the dark J-V characteristics is particularly important to understand the carrier transport mechanism in these devices. The parameters extracted by such modeling (Table 3) will be used in the following sections to calculate the sensitivity of

In Table 6 we compare our simulation and experimental results of various HIT cells on Ntype c-Si substrates (Takahama et al, 1992, Sawada et al, 1994, Maruyama et al, 2006). Modeling indicates that improvements in Voc could be brought about (a) by going from a


solar cell output is little affected. From this we may conclude that the defect density on the back wafer surface in the experimental as-deposited condition is probably ≤ 1012 cm-2, as also obtained by modeling the experimental characteristics (Table 2).

Table 5. Calculated values of the solar cell output parameters for different values of the defect density (Nss) on that (rear) surface of the crystalline silicon wafer that is away from the incoming light, indicating that the maximum sensitivity is to the short circuit current density and fill factor. The defect density at the front surface of the c-Si wafer is 1011 cm-2.

In order to understand the sensitivity of the solar cell output to Nss on the RS, we turn to Fig. 6. We note that when Nss on the rear c-Si wafer surface is highest (3x1013 cm-2), there is a huge concentration of trapped holes at the crystalline- amorphous interface on the c-Si wafer side where the high surface defect density exists (dashed line, Fig. 6a). The hole pile-up at the crystalline-amorphous interface slows down the arrival of holes to the back contact (the collector of holes), and encourages the back diffusion of photo-generated electrons in the absorber c-Si wafer. The result is that the electron current is negative over most of the device (Fig. 6b – electron current towards the back contact is negative according to our sign convention). Thus little electron current is collected at the front contact (the collector of electrons, Fig. 6b). In addition, the back-diffusing electrons recombine with the photogenerated holes over most of the absorber c-Si, resulting in poor hole current collection at the back contact. Thus Jsc and FF fall sharply for very high values of Nss at RS. More details can be found in Datta et al (2008).

Fig. 5. Effect of changing the defect density (shown in units of cm-2) on the rear surface of the c-Si wafer on (a) the illuminated current density versus voltage characteristics and (b) the band diagram at 0 volts as a function of position in the device under 100 mW cm-2 AM1.5 light. Results are shown for double heterojunction solar cells having a 4 nm P+-a-SiC:H/ 19 nm P+-a-Si:H BSF structure. The defect density on the front surface is 1011 cm-2 for all cases.

Fig. 6. Effect of changing the defect density (shown in units of cm-2) on the rear surface of the c-Si wafer on (a) the trapped hole density, (b) the electron current density, Jn, and (c) the electric field on the holes. Results are shown for 100 mW cm-2 of AM1.5 light under shortcircuit conditions.
