**2. Historical development of HIT solar cells**

One of the successful applications of hydrogenated amorphous silicon (a-Si:H) is in crystalline silicon heterojunction (HJ) solar cells. Fuhs et al (1974) first fabricated heterojunction silicon solar cells, where the absorber is P (N) type c-Si, while the emitter N

Computer Modeling of Heterojunction

with Intrinsic Thin Layer "HIT" Solar Cells: Sensitivity Issues and Insights Gained 277

cells is that it has excellent temperature dependence characteristics and its efficiency does not deteriorate as much as that of diffused junction c-Si cells at higher temperatures (Sakata et al, 2000). The efficiency of HIT cells deteriorates by 0.33%/ C with increase of temperature while it is 0.45%/ C for conventional c-Si solar cells. This means HIT cells would generate more output power in summer time than its diffused junction counterpart.

References Wafer Solar cell output parameters Emitter &

SANYO press release N Textured 39.50 729 0.800 23 PECVD Schmidt et al, 2007 N Textured 39.3 639 0.789 19.8 RF-PECVD

Wang et al, 2010,2008 P Textured 36.20 678 0.786 19.3 HWCVD

Das et al, 2008 N Textured 35.68 694 0.741 18.4 PECVD

Olibet et al, 2010, 2007 N Flat 34.0 680 0.82 19.1 VHF-

Sritharathikhun et al 2008 N Textured 35.20 671 0.76 17.9 VHF-

Table 1. Summary of best perfoemances of HIT solar cells on P- and N-type c-Si wafer.

reported 17.5% cell efficiency with a similar cell structure.

Damon-Lacoste, 2008 P Flat 33.0 664 0.778 17.1 PECVD Fujiwara & Kondo, 2009 N Flat 32.79 631 0.764 17.5 PECVD

Inspired by the outstanding performance of Sanyo HIT cells, many research groups throughout the world have been working with these cells and a-Si:H layers have been deposited by PECVD, hot-wire CVD (HWCVD) and very-high-frequency PECVD (VHF-PECVD). A summary of the best HIT solar cells reported till date is given in Table 1. We find that currently, no group has been able to duplicate what Sanyo has achieved in terms of cell efficiency. Very few groups have reached beyond 19% efficiency: Helmholtz Zentrum Berlin on N-type textured wafers (Schimdt et al, 2007) and the National Renewable Energy Laboratory (NREL) on P-type textured wafers (Wang et al, 2008, 2010) have achieved this feat. Good results have also been obtained by the group of EPFL, IMT, Neuchâtel, Switzrland with high open-circuit voltsge (Voc) on flat wafers. The P-type HIT cell of Damon Lacoste et al (2008) from LPICM-Ecole Polytechnique, France also deserves mention. Here the efficiency is limited by the lower short-circuit current density (Jsc) characteristic of flat wafers. The difficulty in attaining the Sanyo HIT cell efficiency illustrates that the a-Si:H/c-Si HJ is indeed a very challenging structure to understand. Therefore, over the last decade scientists are using detailed computer modeling to fully understand the structure. In the next section we will briefly review the computer modeling of HIT solar cells. Recently a few groups have started fabricating HIT cells with intrinsic hydrogenated amorphous silicon oxide (I-a-SiO:H) as the buffer layer between crystalline and doped amorphous silicon. Sritharathikhun et al (2008) have achieved 17.9% cell efficiency with P-c-SiO:H /N-c-Si cell structure and I-a-SiO:H as the buffer layer. A group from AIST (Fujiwara et al, 2009) has

mA cm-2

P 34.3 629 0.79 17.4

N 35.30 664 0.745 17.2

Voc mV

P 32 690 0.74 16.3 PECVD

Fill factor

 %

Type Surface Jsc

BSF deposition technique

PECVD

(P) a-Si:H layer is deposited by the standard plasma-enhanced chemical vapor deposition (PECVD) technique at ~200ºC. However the efficiency achieved was much lower than in c-Si solar cells. In the early 80's Prof. Y. Hamakawa and his co-workers [Osuda et al, 1983] predicted the relevance of a-Si:H /c-Si stacked solar cells in silicon applications. Following the study of Prof. Hamakawa, many research groups world wide became interested in the technological development of a-Si:H/c-Si heterojuction solar cells as an alternative to traditional diffused emitter solar cells. It was almost a decade later that Sanyo began work in 1990 on the growth of low temperature junctions on c-Si and developed a new type of heterojunction solar cells called ACJ-HIT (Artificially Constructed Junction- Heterojunction with Intrinsic Thin layer), now shortened to "HIT", with a conversion efficiency of 18.1% (Tanaka et al, 1992) that has thereafter been continuously improved to yield an outstanding 22% efficiency in 100 cm2 solar cells (Taguchi et al, 2005). Moreover Sanyo also achieved 19.5% efficiency in mass production (Tanaka et al, 2003). The innovation that made this possible was the introduction of thin films of intrinsic a-Si:H on either side of the c-Si wafer, to passivate the defects on its surface, that were responsible for the low efficiency of the earlier heterojunction cells [Fuhs et al, 1974]. A low recombination surface velocity of 15 cm/s has been demonstrated for passivation by intrinsic a-Si:H by Wang et al (2005). This is as good as the best dielectric surface passivation, such as by SiO2 and amorphous silicon nitride (SiNx) (Meier et al, 2007). More importantly, the a-Si:H I-layer can be inserted between the c-Si and a doped layer without significant restriction to carrier transport. The device structure of HIT cells that has been developed by Sanyo is shown in Fig. 1. This cell is fabricated with CZ N-type wafer of thickness ~250 m. The emitter (doped) layer, passivating intrinsic layers and the doped BSF layer of the cell are all thin films (a-Si:H) and deposited by the PECVD technique at ~200ºC. The device terminates with a TCO anti reflection coating followed by metallic electrodes.

Fig. 1. Schematic diagram of HIT cell proposed by SANYO

HIT cells have (1) potential for high efficiency, (2) very good surface passivation: low surface recombination velocity, (3) low processing temperature - all processes occur at ~ 200C resulting in low thermal budget, (4) reduced material cost (low temperature processing permits the use of thinner wafers), leading to overall cost reduction and (5) excellent stability- since the base material of the structure continues to be c-Si. With nearly 19 years of steady progress, in 2009, the best HIT solar cells have recorded a efficiency of 23% over a 100.4 cm2 cell area (press release SANYO, 2009). Another advantage of HIT solar

(P) a-Si:H layer is deposited by the standard plasma-enhanced chemical vapor deposition (PECVD) technique at ~200ºC. However the efficiency achieved was much lower than in c-Si solar cells. In the early 80's Prof. Y. Hamakawa and his co-workers [Osuda et al, 1983] predicted the relevance of a-Si:H /c-Si stacked solar cells in silicon applications. Following the study of Prof. Hamakawa, many research groups world wide became interested in the technological development of a-Si:H/c-Si heterojuction solar cells as an alternative to traditional diffused emitter solar cells. It was almost a decade later that Sanyo began work in 1990 on the growth of low temperature junctions on c-Si and developed a new type of heterojunction solar cells called ACJ-HIT (Artificially Constructed Junction- Heterojunction with Intrinsic Thin layer), now shortened to "HIT", with a conversion efficiency of 18.1% (Tanaka et al, 1992) that has thereafter been continuously improved to yield an outstanding 22% efficiency in 100 cm2 solar cells (Taguchi et al, 2005). Moreover Sanyo also achieved 19.5% efficiency in mass production (Tanaka et al, 2003). The innovation that made this possible was the introduction of thin films of intrinsic a-Si:H on either side of the c-Si wafer, to passivate the defects on its surface, that were responsible for the low efficiency of the earlier heterojunction cells [Fuhs et al, 1974]. A low recombination surface velocity of 15 cm/s has been demonstrated for passivation by intrinsic a-Si:H by Wang et al (2005). This is as good as the best dielectric surface passivation, such as by SiO2 and amorphous silicon nitride (SiNx) (Meier et al, 2007). More importantly, the a-Si:H I-layer can be inserted between the c-Si and a doped layer without significant restriction to carrier transport. The device structure of HIT cells that has been developed by Sanyo is shown in Fig. 1. This cell is fabricated with CZ N-type wafer of thickness ~250 m. The emitter (doped) layer, passivating intrinsic layers and the doped BSF layer of the cell are all thin films (a-Si:H) and deposited by the PECVD technique at ~200ºC. The device terminates with a TCO anti

reflection coating followed by metallic electrodes.

Fig. 1. Schematic diagram of HIT cell proposed by SANYO

HIT cells have (1) potential for high efficiency, (2) very good surface passivation: low surface recombination velocity, (3) low processing temperature - all processes occur at ~ 200C resulting in low thermal budget, (4) reduced material cost (low temperature processing permits the use of thinner wafers), leading to overall cost reduction and (5) excellent stability- since the base material of the structure continues to be c-Si. With nearly 19 years of steady progress, in 2009, the best HIT solar cells have recorded a efficiency of 23% over a 100.4 cm2 cell area (press release SANYO, 2009). Another advantage of HIT solar cells is that it has excellent temperature dependence characteristics and its efficiency does not deteriorate as much as that of diffused junction c-Si cells at higher temperatures (Sakata et al, 2000). The efficiency of HIT cells deteriorates by 0.33%/ C with increase of temperature while it is 0.45%/ C for conventional c-Si solar cells. This means HIT cells would generate more output power in summer time than its diffused junction counterpart.


Table 1. Summary of best perfoemances of HIT solar cells on P- and N-type c-Si wafer.

Inspired by the outstanding performance of Sanyo HIT cells, many research groups throughout the world have been working with these cells and a-Si:H layers have been deposited by PECVD, hot-wire CVD (HWCVD) and very-high-frequency PECVD (VHF-PECVD). A summary of the best HIT solar cells reported till date is given in Table 1. We find that currently, no group has been able to duplicate what Sanyo has achieved in terms of cell efficiency. Very few groups have reached beyond 19% efficiency: Helmholtz Zentrum Berlin on N-type textured wafers (Schimdt et al, 2007) and the National Renewable Energy Laboratory (NREL) on P-type textured wafers (Wang et al, 2008, 2010) have achieved this feat. Good results have also been obtained by the group of EPFL, IMT, Neuchâtel, Switzrland with high open-circuit voltsge (Voc) on flat wafers. The P-type HIT cell of Damon Lacoste et al (2008) from LPICM-Ecole Polytechnique, France also deserves mention. Here the efficiency is limited by the lower short-circuit current density (Jsc) characteristic of flat wafers. The difficulty in attaining the Sanyo HIT cell efficiency illustrates that the a-Si:H/c-Si HJ is indeed a very challenging structure to understand. Therefore, over the last decade scientists are using detailed computer modeling to fully understand the structure. In the next section we will briefly review the computer modeling of HIT solar cells. Recently a few groups have started fabricating HIT cells with intrinsic hydrogenated amorphous silicon oxide (I-a-SiO:H) as the buffer layer between crystalline and doped amorphous silicon. Sritharathikhun et al (2008) have achieved 17.9% cell efficiency with P-c-SiO:H /N-c-Si cell structure and I-a-SiO:H as the buffer layer. A group from AIST (Fujiwara et al, 2009) has reported 17.5% cell efficiency with a similar cell structure.

Computer Modeling of Heterojunction

where *p(*

  **Energy (eV)**

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

**Ec**

**EF**

AM1.5 light and short-circuit conditions.

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

**0.001 0.01 0.1 1 10 100 10**

**Position (microns)**

**Ev** <sup>p</sup>

**EF**

**P-Type HIT cell**

n

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

with Intrinsic Thin Layer "HIT" Solar Cells: Sensitivity Issues and Insights Gained 279

experimental conditions (in this case under 100 mW cm-2 of AM1.5 light), and at thermodynamic equilibrium respectively; while R is the recombination rate in the c-Si wafer. The lifetime, calculated in this manner, is in general, position-dependent; however over a large region inside the c-Si wafer, away from the edges, it is a constant and it is this value that is taken to be the minority carrier lifetime in the wafer. van Cleef et al (1998a,b) and

The generation term in the continuity equations has been calculated using a semi-empirical model (F. Leblanc et al, 1994) that has been integrated into the ASDMP modeling program (Chatterjee et al, 1996, Palit et al, 1998). Both specular interference effects and diffused reflectance and transmittance due to interface roughness can be taken into account. The complex refractive indices for each layer of the structure, required as input to the modeling program, have been measured by spectroscopic ellipsometry. In all cases studied in this article, experimentally or by modeling, light enters through the transparent conducting oxide (TCO)/emitter window, which is taken as x = 0 on the position and referred to as the front contact. Voltage is also applied at x = 0. The BSF/ metal contact at the back of the c-Si wafer is taken as x = L on the position scale, where L is the total thickness of all the semiconductor layers of the device. This back contact is assumed to be at ground potential.

Fig. 2. Energy band diagram for HIT solar cells on P and N type wafers under 100 mW of

The calculated energy band diagrams for typical HIT cells on P- and N-type wafers, with passivated surface defects and under 100 mW of AM1.5 light, 0 volts, are shown in Fig. 2.

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

**E F**

**p**

**E F**

**n**

**0.001 0.01 0.1 1 10 100 1000**

**Position (microns)**

**Ev**

**Ec**

**N-Type HIT cell**

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

Kanevce et al (2009) have also used the DOS model to simulate their HIT cells.

*n)*, *p(n)* and *p0(n0)* are the minority carrier lifetime, its density under the given

#### **2.1 Detailed one-dimensional computer modeling of HIT solar cells:-**

Pioneering work in detailed electrical modeling of a-Si:H solar cells was done by Hack and Schur (1985). Other notable models in this respect are the model AMPS (McElheny et al, 1988, Arch et al, 1991) by S. J. Fonash's group at the Pennsylvania State University, USA, the model of Guha's group (Guha et al, 1989), the ASDMP program by P. Chatterjee (Chatterjee, 1992, 1994, 1996), the ASPIN program of Smole and Furlan (1992) and the ASA program by von der Linden et al (1992). Regarding detailed electrical-optical models, which include textured surfaces and light-trapping kinetics to some extent, the first global electrical-optical model developed in the world was when ASDMP was integrated (Chatterjee et al, 1996) to a semi-empirical optical model by Leblanc et al (1994). This program also takes account of specular interference effects for cells with flat surfaces. Later the developed AMPS program (D-AMPS – Plà et al, 2003) and the ASA package, developed at the Delft University of Technology (Zeman et al, 2000) also introduced light trapping effects.

Modeling of HIT cells was started by van Cleef et al (1998 a,b) using the AMPS computer code (McElheny et al, 1988), which however does not have a proper built-in optical model; and the derivative of the AMPS program (D-AMPS), where a fairly good optical model has been introduced (Plà et al, 2003). The numerical PC program AFORS-HET (Stangl et al, 2001, Froitzheim et al, 2002) has been developed especially for simulating HIT solar cells. The latter has recently also been extended to include light-trapping effects. The ASA program in its later version (Zeman et al, 2000) models both the electrical and optical properties of HIT cells. The PC-1D program (Basore, 1990, Basore et al, 1997), developed at the University of News South Wales, Australia for modeling textured mono-crystalline silicon solar cells, has also been fairly successful in modeling HIT cells. The program ASDMP by Chatterjee et al (1994,1996), has also been extended to model N-a-Si:H/P-c-Si type front (with a heterojunction only on the emitter side) (Nath et al, 2008) HIT cells and subsequently used to model double hetreojunction solar cells both on N- and P-type substrates ( Datta et al, 2008, 2009, Rahmouni et al, 2010).

#### **2.1.1 Simulation model ASDMP**

We will discuss this model in a little more detail, since it has been used in all simulations in this chapter. The "Amorphous Semiconductor Device Modeling Program (ASDMP) " (Chatterjee et al, 1996, Palit et al, 1998 ), originally conceived to model amorphous silicon based devices, has been extended to also model c-Si and "HIT" cells (Nath et al, 2008). This one-dimensional program solves the Poisson's equation and the two carrier continuity equations under steady state conditions for the given device structure, without any simplifying assumptions, and yields the dark and illuminated current density - voltage (J-V), the quantum efficiency (QE) and the photo- and electro-luminescence characteristics of HIT cells. Its electrical part is described in P. Chatterjee (1994, 1996). The gap state model used in these calculations for the amorphous layers, consists of the tail states and the two Gaussian distribution functions to simulate the deep dangling bond states, while in the c-Si part, the tail states absent. The lifetime of the minority carriers inside the N(P) -c-Si wafer may be estimated using the formula:

$$
\tau\_p \approx \frac{p - p\_0}{R} \quad \text{or} \quad \tau\_n \approx \frac{n - n\_0}{R} \,\,\,\,\tag{1}
$$

Pioneering work in detailed electrical modeling of a-Si:H solar cells was done by Hack and Schur (1985). Other notable models in this respect are the model AMPS (McElheny et al, 1988, Arch et al, 1991) by S. J. Fonash's group at the Pennsylvania State University, USA, the model of Guha's group (Guha et al, 1989), the ASDMP program by P. Chatterjee (Chatterjee, 1992, 1994, 1996), the ASPIN program of Smole and Furlan (1992) and the ASA program by von der Linden et al (1992). Regarding detailed electrical-optical models, which include textured surfaces and light-trapping kinetics to some extent, the first global electrical-optical model developed in the world was when ASDMP was integrated (Chatterjee et al, 1996) to a semi-empirical optical model by Leblanc et al (1994). This program also takes account of specular interference effects for cells with flat surfaces. Later the developed AMPS program (D-AMPS – Plà et al, 2003) and the ASA package, developed at the Delft University of

Modeling of HIT cells was started by van Cleef et al (1998 a,b) using the AMPS computer code (McElheny et al, 1988), which however does not have a proper built-in optical model; and the derivative of the AMPS program (D-AMPS), where a fairly good optical model has been introduced (Plà et al, 2003). The numerical PC program AFORS-HET (Stangl et al, 2001, Froitzheim et al, 2002) has been developed especially for simulating HIT solar cells. The latter has recently also been extended to include light-trapping effects. The ASA program in its later version (Zeman et al, 2000) models both the electrical and optical properties of HIT cells. The PC-1D program (Basore, 1990, Basore et al, 1997), developed at the University of News South Wales, Australia for modeling textured mono-crystalline silicon solar cells, has also been fairly successful in modeling HIT cells. The program ASDMP by Chatterjee et al (1994,1996), has also been extended to model N-a-Si:H/P-c-Si type front (with a heterojunction only on the emitter side) (Nath et al, 2008) HIT cells and subsequently used to model double hetreojunction solar cells both on N- and P-type substrates ( Datta et al,

We will discuss this model in a little more detail, since it has been used in all simulations in this chapter. The "Amorphous Semiconductor Device Modeling Program (ASDMP) " (Chatterjee et al, 1996, Palit et al, 1998 ), originally conceived to model amorphous silicon based devices, has been extended to also model c-Si and "HIT" cells (Nath et al, 2008). This one-dimensional program solves the Poisson's equation and the two carrier continuity equations under steady state conditions for the given device structure, without any simplifying assumptions, and yields the dark and illuminated current density - voltage (J-V), the quantum efficiency (QE) and the photo- and electro-luminescence characteristics of HIT cells. Its electrical part is described in P. Chatterjee (1994, 1996). The gap state model used in these calculations for the amorphous layers, consists of the tail states and the two Gaussian distribution functions to simulate the deep dangling bond states, while in the c-Si part, the tail states absent. The lifetime of the minority carriers inside the N(P) -c-Si wafer

**2.1 Detailed one-dimensional computer modeling of HIT solar cells:-** 

Technology (Zeman et al, 2000) also introduced light trapping effects.

2008, 2009, Rahmouni et al, 2010).

**2.1.1 Simulation model ASDMP** 

may be estimated using the formula:

<sup>0</sup>

*p*

*p p R*

or <sup>0</sup> *<sup>n</sup>*

*n n R*

, (1)

where *p(n)*, *p(n)* and *p0(n0)* are the minority carrier lifetime, its density under the given experimental conditions (in this case under 100 mW cm-2 of AM1.5 light), and at thermodynamic equilibrium respectively; while R is the recombination rate in the c-Si wafer. The lifetime, calculated in this manner, is in general, position-dependent; however over a large region inside the c-Si wafer, away from the edges, it is a constant and it is this value that is taken to be the minority carrier lifetime in the wafer. van Cleef et al (1998a,b) and Kanevce et al (2009) have also used the DOS model to simulate their HIT cells.

The generation term in the continuity equations has been calculated using a semi-empirical model (F. Leblanc et al, 1994) that has been integrated into the ASDMP modeling program (Chatterjee et al, 1996, Palit et al, 1998). Both specular interference effects and diffused reflectance and transmittance due to interface roughness can be taken into account. The complex refractive indices for each layer of the structure, required as input to the modeling program, have been measured by spectroscopic ellipsometry. In all cases studied in this article, experimentally or by modeling, light enters through the transparent conducting oxide (TCO)/emitter window, which is taken as x = 0 on the position and referred to as the front contact. Voltage is also applied at x = 0. The BSF/ metal contact at the back of the c-Si wafer is taken as x = L on the position scale, where L is the total thickness of all the semiconductor layers of the device. This back contact is assumed to be at ground potential.

Fig. 2. Energy band diagram for HIT solar cells on P and N type wafers under 100 mW of AM1.5 light and short-circuit conditions.

The calculated energy band diagrams for typical HIT cells on P- and N-type wafers, with passivated surface defects and under 100 mW of AM1.5 light, 0 volts, are shown in Fig. 2.
