# Presentation: 422009

## High Efficiency, High Rate Fabrication of a-Si, a-SiGe, nc-Si based solar cells

- X. Cao, W. Du, Y. Ishikawa, C. Das, X. Yang, A. Vijh* and X. Deng*
- University of Toledo
- *Midwest Optoelectronics, LLC
- April 17, 2006
- NREL a-Si Team Meeting

## Fabrication and Optimization of nc-Si:H, a-Si:H and a-SiGe:H n-i-p solar cells using VHF PECVD at High Deposition Rates 

**Xinmin  Cao and Xunming Deng**
**University of Toledo**
**April 17, 2006**
**NREL a-Si Team Meeting**

## Fabrication and optimization of VHF nc-Si:H n-i-p single-junction solar cells 

VHF a-Si:H/nc-Si:H tandem-junction solar cell

a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cell

Fabrication of VHF a-SiGe bot-cells @ 12 Å/s

Fabrication of VHF a-SiGe mid-cells @ 9 Å/s

Summary   

**VHF a-Si:H/nc-Si:H tandem-junction solar cell**
**a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cell**
**Fabrication of VHF a-SiGe bot-cells @ 12 Å/s**
**Fabrication of VHF a-SiGe mid-cells @ 9 Å/s**
**Summary   **

_**Outlines**__** **_

##  1.  Fabrication and optimization of VHF nc-Si:H n-i-p single junction cells

- _Deposition conditions_:
- a). Deposition pressure in the region of 0.1-0.3 Torr;
- b). [Si2H6]/[H2] mix-gases with grading profile during deposition;
- c). No shower-head; Laminating gas flow across substrate-cathode spacing;
- d). 70 MHz VHF plasma power of 6.0 ~ 60 W, Tsub of  200 ~ 300 C;
- d). Deposition rates of 2 ~ 15 Å/s;
- e). Use of 12 nm nc-Si:H n-layer and 16 nm a-Si:H p-layer;
- f). 4” x 4” stainless steel substrates coated with Ag/ZnO back-reflector.

## 1-1).    6.6% VHF nc-Si:H single-junction solar cells at a rate of ~ 10 Å/s

| Table 1-1. AM1.5 IV data for 10 Å/s nc-Si:H n-i-p single-junction solar cells |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- |
| Cell Number | Voc
(V) | Jsc
(mA/cm2) | FF
(%) | Initial Efficiency (%) | Rs
(Ohm*cm2) | Room Voc
(V) |
| GD1569-2.11 | 0.445 | 20.8 | 70.9 | 6.56 | 2.61 | 0.106 |
| GD1569-2.12 | 0.453 | 21.3 | 69.0 | 6.65 | 2.65 | 0.109 |
| GD1569-2.13 | 0.463 | 20.9 | 67.8 | 6.55 | 2.67 | 0.118 |
| GD1569-2.14 | 0.472 | 20.1 | 67.2 | 6.36 | 2.73 | 0.143 |

**1-1).    6.6% VHF nc-Si:H single-junction solar cells at a rate of ~ 10 Å/s**

- _Achievements_:
- i). VHF nc-Si:H i-layer deposition rate of ~ 10 Å/s;
- ii). nc-Si:H cell initial efficiency of  = 6.6% with FF of ~70%, Voc of ~0.45 V and Jsc of ~ 21 mA/cm^2.

- 1-1-1). IV data of VHF nc-Si:H n-i-p single-junction solar cells

## 1-1-2). IV and QE plots of VHF nc-Si:H single-junction solar cell

| Fig.1. AM1.5 and dark I-V plots of 10 Å/s nc-Si:H nip solar cell using VHF-PECVD | Fig. 2. Quantum efficiency plot for cell whose I-V data are shown in Fig. 1. |
| --- | --- |

## 1-1-3). Light-induced degradation of 10 Å/s VHF nc-Si:H cells

| Table 1-2. IV parameters of VHF nc-Si cell as a function of light soaking time |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
| Cell (no.) | Voc
(V) | Jsc
(mA/cm^2) | FF | 
(%) | /
(%) |
| GD1500_2.21, 22, 23
average, Initial  | 0.466  | 19.93  | 0.634  | 5.89  | - |
| 100 hrs LS | 0.464  | 20.42  | 0.635  | 6.02  | +2.2 |
| 1000 hrs LS | 0.450  | 20.14  | 0.644  | 5.83  | -1.0 |
| 3200hrs LS | 0.451  | 20.41  | 0.652  | 6.00  | +1.9 |

- During the first 100 hours of light soaking, a small increase in Jsc and **** was found.
- There is almost no light-induced degradation for long-term light soaking.
- This sample was prepare in a highly crystalline growth regime and exhibited low open circuit voltages below 0.5V.

## 1-2).    7.8% VHF nc-Si:H single-junction solar cells at a rate of ~ 3 Å/s

| Fig. 3. IV curve of a VHF nc-Si:H single-junction solar cell at a deposition rate of 3 Å/s. | Fig. 4. QE curve of an VHF nc-Si:H solar cell whose IV data is shown in Fig 1. |
| --- | --- |

- 1-2-1). IV and QE plots of 3 Å/s VHF nc-Si:H single-junction solar cell

## 1-2-2). Light-soaking stability of 3 Å/s VHF nc-Si:H cells

| Table 1-3. IV parameters of VHF nc-Si cell as a function of light soaking time |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
| Cell (no.) | Voc
(V) | Jsc
(mA/cm^2) | FF | 
(%) | /
(%) |
| GD1759 initial | 0.529 | 22.3  | 0.66  | 7.8  | - |
| 100 hrs LS | 0.531  | 21.2  | 0.68 | 7.7  | -1.2 |
| 1000 hrs LS | 0.514  | 21.1  | 0.68  | 7.4  | -5.1 |

- 7.8% initial and 7.4% stable active-area (0.25 cm^2) cell efficiencies for VHF nc-Si cell have been achieved.
- Good red spectral respond with integrated Jsc = 8.59 mA/cm^2 over 650 – 1000 nm.
- Small light-induced degradation.

## IV and QE plots of 3 Å/s VHF a-Si:H single-junction solar cell on SS

** ****2. VHF a-Si:H/nc-Si:H tandem-junction solar cell **

## IV and QE plots of a-Si/nc-Si tandem-junction cell with 3Å/s VHF a-Si top-cell and 10 Å/s VHF nc-Si bot-cell

- Preliminary effort on VHF a-Si/nc-Si tandem-cell with efficiency of 9.3%. A thick top-cell is expected to get a current-match.

##  3.  a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cell using nc-Si:H bot-cell

| Table 3-1. IV data of a-Si/a-SiGe/nc-Si cells. |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- |
| Triple-cell No. | structure | i-layer t (min) | Voc
(V) | Jsc
(mA/cm2) | FF | 
(%) |
| C1
initial | RF a-Si top-cell | 85 | 2.132 | 7.23 | 0.672 | 10.3 |
|  | RF a-SiGe mid | 60 |  |  |  |  |
|  | VHF nc-Si bot | 25 |  |  |  |  |
| C2
initial | VHF a-Si top-cell | 9 | 2.098 | 7.61 | 0.691 | 11.0 |
|  | VHF a-SiGe mid | 14 |  |  |  |  |
|  | VHF nc-Si bot | 28 |  |  |  |  |
| C3
initial | VHF a-Si top | 14 | 2.183 | 7.55 | 0.752 | 12.4 |
|  | VHF a-SiGe mid | 25 |  |  |  |  |
|  | VHF nc-Si bot | 60 |  |  |  |  |
| C3
100 hrs LS |  |  | 2.138 | 7.29 | 0.702 | 11.0 |
| C3
1000 hrs LS |  |  | 2.116 | 7.38 | 0.704 | 11.0 |

** ****3.  a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cell using nc-Si:H bot-cell**

## IV and QE plots of VHF a-Si/a-SiGe/nc-Si triple-junction cell at a rate of 3 Å/s

- Using nc-Si:H cell as component bottom-cell, 12.4% initial and 11% stable cell efficiencies in a-Si/a-SiGe/nc-Si triple-junction structure have been achieved.
- Very good FF of 0.752 for triple-junction cell using nc-Si:H cell as current-limited cell.

##  4.  Fabrication of VHF a-SiGe:H bot-cell @ ~10 Å/s

| Table 4-1. IV data of VHF a-SiGe bot-cells with i-layer thickness dependence |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Cell (no.) | i-layer thickness
/ time | Voc
(V) | Jsc (mA/cm^2)
 | QE Jsc
(350-1000nm
/650-1000m) | FF | 
(%) |  red
(%)
(650-1000nm) |
| GD1825_4.13 | 150 nm /  2’ | 0.602 | 21.45 | 20.34/7.73 | 0.650 | 8.40 | 3.02 |
| GD1858_4.13 | 180 nm / 2’30” | 0.565 | 22.25 | 21.10/8.58 | 0.613 | 7.70 | 2.97 |
| GD1860_4.13 | 210 nm /  3’ | 0.554 | 23.99 | 21.96/9.31 | 0.579 | 7.69 | 2.99 |

- By using H2 dilution grading in i-layer deposition, VHF a-SiGe bot-cells showed  Jsc values > 21 mA/cm^2.
- With increasing i-layer thickness, the blue QE was the same, the red QE Jsc increased, but Voc and FF dropped.
-    However, red over 650 -1000 nm kept constant of ~3%

**VHF a-SiGe bottom cells with i-layer thickness dependence**

## QE curves of VHF a-SiGe bottom cells with different i-layer thickness

## VHF a-SiGe:H bot-cells with Ge/Si ratio dependence

| Table 4-2. IV data of 10 Å/s a-SiGe bot-cells by VHF PECVD |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Cell (no.) | i-layer gas flow
Si2H6/[20%]GeH4/H2 | i-layer power/time  | Voc
(V) | Jsc (mA/cm^2)
 | QE Jsc
(350-1000nm
/650-1000m) | FF | 
(%) |
| GD1799_4.11
initial | 4.0/13-20/* | 30W / 3’ | 0.562 | 22.04 | 21.17/9.27 | 0.588 | 7.28 |
| GD1799_4.11
100 hrs LS |  |  | 0.546 | 20.55 | 19.71/8.58 | 0.541 | 6.07 |
| GD1802_4.33
initial | 3.5/10-16/* | 30W / 2’30” | 0.612 | 21.64 | 20.20/7.78 | 0.651 | 8.72 |
| GD1802_4.33
100 hrs LS |  |  | 0.593 | 20.60 | 19.18/7.36 | 0.612 | 7.51 |
| GD1803_4.12
initial | 4.0/10-16/* | 30W / 2’15” | 0.671 | 21.19 | 19.54/7.02 | 0.668 | 9.50 |
| GD1803_4.12
100 hrs LS |  |  | 0.647 | 19.99 | 18.71/6.69 | 0.638 | 8.25 |

- With increasing Ge/Si ratio, Voc droped, but red QE increased.
- Good IV performance and light-soaking stability with 10 Å/s VHF a-SiGe bot-cell made under high H2 dilution.

##  5.  Fabrication of VHF a-SiGe:H mid-cell @ 9 Å/s

| Table 5-1. IV data of VHF a-SiGe cells |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
| Cell (no.) | i-layer 
thickness/ time | Voc
(V) | Jsc (mA/cm^2) | FF | 
(%) |
| GD1860_4.13
a-SiGe bot-cell | 210 nm /  3’ | 0.554 | 23.99 | 0.579 | 7.69 |
| GD1863_4.13
 a-SiGe mid-cell | 270 nm /  5’ | 0.820 | 20.72 | 0.666 | 11.32 |

- _Deposition conditions:_
- Gas flows [Si2H6]/[20%GeH4/H2]/[H2] = 4.4 / 5.5-6.6 / 350-420, VHF power P = 40W,  p = 0.45 Torr, T = 300 C, t = 5 min, r = 9 Å/s

** ****5.  Fabrication of VHF a-SiGe:H mid-cell @ 9 Å/s**

## QE curves of VHF a-SiGe:H mid-cells

## QE curves of VHF a-SiGe/a-SiGe tandem-cells

**QE curves of VHF a-SiGe/a-SiGe tandem-cells**

## 6.6% nc-Si:H single-junction solar cells with VHF nc-Si:H i-layer deposition rate of ~ 10 Å/s made using RF nc-Si n-layer and a-Si p-layer. No light-induced degradation for nc-Si cells with high crystalline volume fraction.

7.8% initial and 7.4% stable cell efficiencies for 3 Å/s VHF nc-Si:H bot-cell were achieved.

Using nc-Si:H cell as component bottom-cell, 12.4% initial and 11% stable cell efficiencies in a-Si/a-SiGe/nc-Si triple-junction structure have been achieved.

Preliminary effort on ~10 Å/s VHF a-SiGe bot-cell and a-SiGe mid-cell made under high H2 dilution.

- 7.8% initial and 7.4% stable cell efficiencies for 3 Å/s VHF nc-Si:H bot-cell were achieved.
- Using nc-Si:H cell as component bottom-cell, 12.4% initial and 11% stable cell efficiencies in a-Si/a-SiGe/nc-Si triple-junction structure have been achieved.
- Preliminary effort on ~10 Å/s VHF a-SiGe bot-cell and a-SiGe mid-cell made under high H2 dilution.

**Summary **

## Effect of underlying substrates on VHF nc-Si:H i-layer  deposition

- Xinmin Cao and Xunming Deng
- University of Toledo

## Effect of underlying substrates on VHF nc-Si:H i-layer deposition

| a-Si n-layer, 
no nc-Si seed layer at n/i interface | a-Si n-layer, 
15 nm nc-Si seed layer at n/i interface  | nc-Si n-layer with high crystalline,
no nc-Si seed layer at n/i interface  |
| --- | --- | --- |

- _Deposition conditions:_
- Gas flows [Si2H6]/[H2] = 2.3 sccm / 240 sccm, VHF power P = 8W,  p = 0.2 Torr, T = 400 C, r = 3 Å/s , d = 1600 nm,
- With or without nc-Si seed layer with high crystalline phase at n/i interface.

## Quantum efficiency plots for VHF nc-Si cells using a-Si or nc-Si n-layer, with or without seed layer at n/i interface.

| Raman spectrum of 490 nm nc-Si:H n-layer with a crystalline volume fraction of about 60% and crystallite size of about 5 nm.
12 nm thick nc-Si:H n-layer was used for VHF nc-Si cells. |
| --- |

##  Dark and light IV curves crossed over for cells using a-Si n-layer and without nc-Si seed layer; low FF. That is believed to be due to a high amorphous phase in the beginning of i-layer.

-  Using seed layer at n/i interface, FF and cell efficiency were greatly improved for cells with a-Si n-layer; No change in Jsc; IV curves not crossed over.
-  Using nc-Si n-layer and without seed layer, nc-Si:H cells reached very good performance. Very high Jsc of 24.4 mA/cm^2.

## High-Rate VHF PECVD Fabrication of a-SiGe:H Bottom Cells Using SiH4Wenhui Du and Xunming DengUniversity of ToledoApril 17, 2006Presented at NREL a-Si Team Meeting

## Objective

Fabricate narrow bandgap a-SiGe solar cells at a high deposition rate using low-cost precursor

Approach

Use a shower-head type design for gas inlet in the deposition chamber

- Fabricate narrow bandgap a-SiGe solar cells at a high deposition rate using low-cost precursor
- Approach
- Use a shower-head type design for gas inlet in the deposition chamber

## Deposition Parameters

- VHF-PECVD (60-70 MHz)
- 2 kinds of Si sources: Si2H6 and SiH4 for comparison
- High deposition rate Rd (8-10 A/s)
- Pressure 0.2-1 Torr
- Varied electrode spacing
- Ts ~300C
- Shower-head cathode design

## High Rate a-SiGe Bottom Cells for Using Si2H6 Gas 

|  | Rd
(A/s) | Voc
 (V) | Jsc
 (mA/cm2) | FF
 (%) | Eff 
(%) | Note | R |
| --- | --- | --- | --- | --- | --- | --- | --- |
| GD1622 | 15-9 | 0.588 | 20.4 | 63 | 7.57 | 60W,3' | 125 |
| GD1633 | 10.5 | 0.631 | 17.9 | 68.2 | 7.70 | 30W 2' | 125 |
| GD1623 | 9.5 | 0.617 | 18.5 | 64.1 | 7.33 | 30W 3' | 125 |
| GD1778 | 10 | 0.573 | 18.7 | 63.8 | 6.84 | 30W, 4' | 150 |
| GD1632 | 8.8 | 0.589 | 19.5 | 64.1 | 7.36 | 20W 2.5' | 125 |
| GD1647 | 8.5 | 0.619 | 19.3 | 65.8 | 7.87  | 20W 3.5' | 150 |

## Optimized a-SiGe solar cell IV Performance for Si2H6 case

## Determine R for Phase Transition Region Using SiH4 Gas

- R ( H2/SiH4 ) =12 corresponds to a-Si to nc-Si phase transition region. Best Voc can reach 1.02V in a-Si:H solar cell (~5A/s).

## Optimized a-SiGe Solar Cell IV Results for Using SiH4 Gas

|  | Rd 
(A/s) | d 
(nm) | Voc 
(V) | Jsc
(mA/cm2) | FF | EFF | note |
| --- | --- | --- | --- | --- | --- | --- | --- |
|  |  |  |  |  |  |  |  |
| GD1867 | 9.5 | 220 | 0.648 | 20.3 | 69.7 | 9.20 |  |
|  |  |  | 0.664 | 19.9 | 65.4 | 8.60 |  |
| GD1877 | 11 | 210 | 0.629 | 19.1 | 63.2 | 7.56 |  |
| GD1878 | 8.5 | 210 | 0.643 | 20.0 | 62.2 | 7.99 |  |
| GD1647 | 8.5 |  | 0.619 | 19.3 | 65.8 | 7.87  | Si2H6 |
|  |  |  | 0.593 | 19.8 | 62.8 | 7.36  | w/t SH |

- Optimized a-SiGe Solar Cell IV Results for Using SiH4 Gas

- High rate a-SiGe solar cells have a lower Jsc than  standard RF-made cells, while Voc and FF are comparable between these 2 kinds of cells.
- Solar cells located at the center have lower Voc than those at the edge and corners, though shower-head cathode was used.

## Summary for high rate deposition of a-SiGe cell using shower-head reactor design

- Using SiH4/GeH4, a-SiGe bottom solar cells were obtained at Rd ~10 A/sec with 0.64 V  of Voc, 0.62 of FF and 20 mA/cm2 of Jsc.
- Using Si2H6/GeH4, a-SiGe bottom solar cells have reached: Voc=0.612 V, Jsc=21.6 mA/cm2, FF=0.65 and Eff=8.6%, while Rd=8.5A/s

## DC bias voltage effect on nc-Si:H film deposition and its solar cells

- Yasuaki Ishikawa and Xunming Deng
- University of Toledo
- April 17, 2006
- NREL a-Si Team Meeting

- Deposition rate
- Crystallinity
- Cell performances

## System configuration

- Bias voltage to substrate;         -43V ~ 43V
- Low pass filter
- Deposited area: 3 × 3 inch
- Electrode gap: 0.5 ~ 3.2 cm (std.=1.5 cm)
- Pressure=8 Torr, Power=60 W, H2/Si2H6=67, Ts=400oC

- 1

- 2

- 3

- 4

- 5

- 6

- Gas
- inlet

- RF

- Matching
- Box

- Schematic diagram of deposition chamber for intrinsic layer

- heater

- movable

- Batteries

- Substrate
- Ceramics (support for #3)
- Substrate supporter (rail)
- Single wire for applying voltage
- Shower-head electrode
- Cathode cap

## Deposition rate

- Constant at around 9.5 Å/s at various bias voltage
- No effect on the deposition rate

## Crystallinity

- Maintained the orientation property (random)
- Significant varied the crystal volume fraction
- The error of *X**c* ... within 5%

## Cell performances

- +20V gives the best cell performances.

## Hydrogen dilution dependency

- +21 V; best in *X**c*~50% series
*R* = 66 gives the best Effi.
- Require more data points

## EQE spectra

- Enhanced green part response
- Bulk quality will be improved by appropriate positive bias.

## R dependency

## Summary for Bias Voltage dependence for nc-Si solar cell fabrication

- Used a reactor with shower-head gas flow inlet
- Nanocrystalline Si layer made at high rate
- +20V bias voltage enhances solar cell performance. Eff=7.8% for SJ nc-Si cell.
- No effect on deposition rate
- Strong effect on crystallinity
- Hydrogen dilution R=66 more desirable

## 6.6% nc-Si:H single-junction solar cells with VHF nc-Si:H i-layer deposition rate of ~ 10 Å/s made using RF nc-Si n-layer and a-Si p-layer. No light-induced degradation for nc-Si cells with high crystalline volume fraction.

7.8% initial and 7.4% stable cell efficiencies for 3 Å/s VHF nc-Si:H bot-cell were achieved.

Using nc-Si:H cell as component bottom-cell, 12.4% initial and 11% stable cell efficiencies in a-Si/a-SiGe/nc-Si triple-junction structure have been achieved.

Preliminary effort on ~10 Å/s VHF a-SiGe bot-cell and a-SiGe mid-cell made under high H2 dilution.

- 7.8% initial and 7.4% stable cell efficiencies for 3 Å/s VHF nc-Si:H bot-cell were achieved.
- Using nc-Si:H cell as component bottom-cell, 12.4% initial and 11% stable cell efficiencies in a-Si/a-SiGe/nc-Si triple-junction structure have been achieved.
- Preliminary effort on ~10 Å/s VHF a-SiGe bot-cell and a-SiGe mid-cell made under high H2 dilution.

**Summary for High Efficiency Solar Cell Fabrication**

##  Dark and light IV curves crossed over for cells using a-Si n-layer and without nc-Si seed layer; low FF. That is believed to be due to a high amorphous phase in the beginning of i-layer.

-  Using seed layer at n/i interface, FF and cell efficiency were greatly improved for cells with a-Si n-layer; No change in Jsc; IV curves not crossed over.
-  Using nc-Si n-layer and without seed layer, nc-Si:H cells reached very good performance. Very high Jsc of 24.4 mA/cm^2.

## Summary for high rate deposition of a-SiGe cell using shower-head reactor design

- Using SiH4/GeH4, a-SiGe bottom solar cells were obtained at Rd ~10 A/sec with 0.64 V  of Voc, 0.62 of FF and 20 mA/cm2 of Jsc.
- Using Si2H6/GeH4, a-SiGe bottom solar cells have reached: Voc=0.612 V, Jsc=21.6 mA/cm2, FF=0.65 and Eff=8.6%, while Rd=8.5A/s

## Summary for Bias Voltage dependence for nc-Si solar cell fabrication

- Used a reactor with shower-head gas flow inlet
- Nanocrystalline Si layer made at high rate
- +20V bias voltage enhances solar cell performance. Eff=7.8% for SJ nc-Si cell.
- No effect on deposition rate
- Strong effect on crystallinity
- Hydrogen dilution R=66 more desirable