# Presentation: 004971

## Advances in CdTe R&D at NREL

**X. Wu **
**Material and device:**
** ****J. Zhou, J.C. Keane, R.G. Dhere, D.S. Albin, T.A. Gessert, C. DeHart, X. Li**
**A. Duda, S. Ward, T. Coutts, and R. Noufi**
**Charecrization:**
**Y. Yan, G. Teeter, D.H. Levi, S. Asher, C. Perkins, H.R. Moutinho, **
**B. To, M.J. Romero, K. Emery, T. Moriarty, C. Mack, and P. Sheldon**
**Basic division:**
**S. Wei, Y. Zhang**
***National Renewable Energy Laboratory (NREL)***
** ****November 8, 2005**

## Outline

**Introduction**
**16.5%-efficient polycrystalline CdTe thin-film solar cell**
**“****One heat-up step” CdTe manufacturing processes**
**13.9-efficient transparent CdTe solar cell**
**Summary**

## Introduction

- CdTe has been recognized as a very promising material for thin-film solar cells, because of its near optimum bandgap of ~1.5 eV and its high absorption coefficient (>5x105/cm)
- Small-area(1 cm2) CdTe cell with efficiency of 16% and large-area (2’x4’) CdTe module with total-area efficiency of 8-10% have been demonstrated
- CdTe modules demonstrated high performance, low cost, and large-scale manufacturing ability to attract capital investments.  CdTe solar cell become a fast growing business. Total CdTe module products will reach ~30 MW in 2005 & ~95 MW in 2007
- DOE/NREL PV program strongly support CdTe R&D through Thin-Film Partnership Program, PVMaT, and in-house R&D
- This presentation review partial CdTe R&D approaches at NREL

## 16.5%-efficient Polycrystalline CdTe thin-film solar cell

## Issues Associated with Conventional SnO2/CdS/CdTe Devices

**Conductivity and transmission of conventional SnO****2****-TCO do not provide adequate design latitude to improve cell efficiency**
**CdS window layer has low bandgap (~2.4 eV)**
**Reducing CdS thickness to improve J****sc**** can adversely impact device V****oc**** and FF**
**Device adhesion problems can limit CdCl****2**** treatment process optimization, as well as device performance and yield **

## NREL Developed Novel Materials and Device Structure - 1

**	****  ****Developed a novel process to prepare high-quality Cd****2****SnO****4**** (CTO) TCO films **(*US patent# 6,221,495;  *
*	**J of electronic Materials, ****25****, No. 6, pp. 935;  *
*	**Thin Solid Film, ****286****, pp. 274*)** **

## NREL Developed Novel Materials and Device Structure - 2

**  ****Replaced SnO****2**** TCO with Cd****2****SnO****4 ****in CdTe cells, yielding improved J****sc**** and FF ***(US patent# 5,922,142;  Proc. 14th NREL/SNL PV meeting, pp. 693;  *
*26th IEEE PVSC, pp. 347)*

## NREL Developed Novel Materials and Device Structure - 3

**  ****Developed and Integrated high-resistivity Zn****2****SnO****4**** (ZTO) buffer layer, yielding improved device performance and reproducibility ***(US patent# 6,169,246;  Proc. 2nd NCPV meeting, pp. 37;  *
*JAP, ****89****, No. 8, pp. 4546)*

## NREL Developed Novel Materials and Device Structure - 4

**  ****Developed sputtered CdS:O films with Nanostructure and higher optical bandgap, yielding improved J****sc**** ***(US patent under process, *
*29th IEEE PVSC, pp.531; *
*Phys. Stat. Sol., pp.1062)*

## NREL Developed Novel Materials and Device Structure - 5

**        ****Developed a modified CTO/ZTO/CdS/CdTe device structure, and fabricated high-efficiency CdTe cells ***(28th IEEE PVSC, pp. 470; 17th European PVSEC; Solar Energy, ****77****, pp.803)*

## High-EfficiencyCTO/ZTO/CdS/CdTe Cells

| 
Cell # | Voc
(mV) | Jsc
(mA/cm2) | FF
(%) | 
(%) | Area
(cm2) |
| --- | --- | --- | --- | --- | --- |
| W547-A | 847.5 | 25.86 | 74.45 | 16.4 | 1.131 |
| W553-A | 849.9 | 25.50 | 74.07 | 16.1 | 1.029 |
| W566-A | 842.7 | 25.24 | 76.04 | 16.2 | 1.116 |
| W567-A | 845.0 | 25.88 | 75.51 | 16.5 | 1.032 |
| W597-B | 835.6 | 25.25 | 76.52 | 16.1 | 0.961 |
| W608-B | 846.3 | 25.43 | 74.24 | 16.0 | 1.130 |
| W614-B | 842.2 | 25.65 | 74.67 | 16.1 | 0.948 |

## I-V Curve of world-record CdTe Cell

- Voc = 845.0 mV
- Jsc = 25.88 mA/cm2
- FF = 75.51%
- = 16.5%
- Area = 1.032 cm2

## Voc improvement

- To achieve CdTe cell with efficiency of more than 18%, our work focus on Voc improvement
- Voc improvement :
- Optimize device process to improve junction quality (reduce A & J0) and reduce back barrier height;
- Study defects that limit doping in CdTe film
- Achieved an NREL-confirmed Voc of 858 mV in a CdTe cell with an efficiency of 15.6%

## Novel manufacturing processes

## Conventional Manufacturing Processes

**Use the conventional SnO****2****/CdS/CdTe device structure                      (requiring a thicker CdS layer)**
**Mix “wet” and “dry” processes                                                              (yielding hazardous liquid waste and low throughput)**
**Utilize undesirable heat-up and cool-down process segments          (consuming time and increasing thermal budget)**

## Novel manufacturing processes

- Developed “one heat-up step” process on borosilicate glass, *(US patent# 6,137,048; X. Wu and P. Sheldon, Proc. of 16**th** European PVSEC, p341-344, 2000)*
- Developed “one heat-up step” process on commercial soda-lime glass/SnO2 substrates, *(X. Wu and J. Zhou, Proc. of 19**th** European PVSEC, p1271, 2004)*

## Manufacturing Process 1

**CTO, ZTO and CdS layers are deposited on borosilicate glass at room temperature by RF sputtering**
**Only a single heat-up segment in the entire process**
**Complete recrystallization of all three layers (CTO, ZTO, and CdS), and interdiffusion at each interface (CTO/ZTO, ZTO/CdS, and CdS/CdTe) during the CdTe deposition step**

## Device performance

- Voc = 833.8 mV
- Jsc = 24.06 mA/cm2
- FF = 73.29%
- Efficiency  = 14.7%
- Area = 1.159 cm2

## Manufacturing process 2

- Soda-lime
- Glass/SnO2

- ZTO          CdS:O
-       Sputtering
- Room-temperature

-       CdTe
-   CSS or VTD
-     ~570°C

- V-CdCl2
- treatment
- 410-C

- Back-contact
- ZnTe:Cu
- or CuxTe
- Sputtering

- Metal
- Sputtering
- Room
- -temperature

## Device results

| Cell # | Voc
(mV) | Jsc
(mA/cm2) | FF
(%) | Efficiency*
(%) | Area
(cm2) |
| --- | --- | --- | --- | --- | --- |
| 
W898-B | 
828.7 | 
23.37 | 
72.50 | 
14.0 | 
1.001 |
| 
W900-B | 
829.4 | 
23.48 | 
74.07 | 
14.4 | 
0.923 |
| 
W901-B | 
831.2 | 
23.32 | 
72.18 | 
14.0 | 
1.027 |
| 
W902-A | 
823.6 | 
23.16 | 
74.28 | 
14.2 | 
0.917 |
| 
W902-B | 
830.1 | 
23.43 | 
73.71 | 
14.3 | 
1.062 |

- * NREL confirmed total-area efficiency.

## Potential benefits

- Reduce thermal budget and energy payback time
- Reduce process time - increase throughput
- Low cost soda-lime/SnO2 substrates
- High yield: clean interface, no adhesion problem (due to ZTO buffer layer)
- Potential for completely dry processes
- High-efficiency, excellent uniformity, and easy to scale-up
- Provide a novel process to prepare CdTe modules with efficiencies of 10%-12%

## 13.9%-efficient transparent CdTe solar cell 

## High-bandgap polycrystalline thin-film champion cells with the opaque back contact

| Affiliate | Absorber | Eg 
(eV) | Voc
(mV) | Jsc
mA/cm2 | FF
(%) | Effi
(%) | Reference |
| --- | --- | --- | --- | --- | --- | --- | --- |
| IEC | Cu(InGa)(SeS)2
(CIGSS) | 1.50 | 826 | 20.4 | 64.5 | 10.9* | Thin Solid Film, In press, 2004 |
| NREL | CuGaSe (CGS) | 1.64 | 823 | 18.6 | 66.8 | 10.2* | 31st IEEE PVSC, 2005 |
| AGU | Ag(In0.2Ga0.8) Se2 (AIGS) | 1.70 | 949 | 17.0 | 57.7 | 9.3 | 2005 Spring MRS |
| IEC | Cu(InAl)Se2 (CIAS) | 1.66 | 763 | 16.7 | 61.5 | 7.8 | 29th IEEE PVEC, p519, 2002 |
| HMI | Cu(InGa)S2 (CIGS) | 1.65 | 831 | 17.1 | 71.2 | 10.1 | Thin Solid Film, p266, 2002 |
| HMI | Cu(InGa)S2 (CIGS) | 1.53 | 774 | 21.6 | 73.7 | 12.3 | Thin Solid Film, p266, 2002 |
| HMI | CuInS2 (CIS) | 1.50 | 729 | 21.8 | 72 | 11.4 | Sol. Energ. Mat. Sol. C 67,
P159, 2001 |
| IEC | Cd0.95Zn0.05Te (CZT) | 1.53 | 780 | 23.0 | 64 | 11.5 | 31st IEEE PVSC, 2005 |
| GIT & Ametek | CdMnTe (CMT) | 1.53 | 680 | 20.6 | 44.2 | 6.0 | 20th IEEE PVSC, p1477, 1988 |
| USF | CdSe | 1.70 | 250 | 14.0 | 53 | 1.9 | 19th E PVSEC,  p1651, 2004 |
| NREL | CdTe | 1.48 | 845 | 25.9 | 75.5 | 16.5* | 17th E PVSEC, p995, 2001 |

***: NREL-confirmed efficiency**

## New approach on transparent back contact

**Almost all of R&D activities focused on developing higher E****g**** back-contact material, namely: E****g**** of back-contact > E****g**** of top cell**
**In this work, we present a different  approach: using thinner low-bandgap back-contact material (namely: E****g**** of back-contact < E****g**** of top cell) to make high transmission in NIR region **

## Poly-CdTe cells with a transparent back contact

| Affiliate | Back-contact | Eg 
(eV) | Device efficiency
(%) | Device transmission
(%) | Reference |
| --- | --- | --- | --- | --- | --- |
| Ametek 
& IEC | ZnTe:Cu | 2.26 | 9.4 | ~40 | 20th IEEE PVSC, 
p1448, 1988 |
| IEC | ZnTe:Cu | 2.26 | 10.1 | Film transmission: 60%-85% | 2005 Spring
MRS |
| NREL | ZnTe:Cu | 2.26 | 8.9 | 20~30 | NCPV PV Review Meeting, 2003 |
| UT | ZnTe:N | 2.2 | 6 | Film transmission: 75% | 29th IEEE PVSC, 
p539, 2002 |
| ETH & CREST | ITO | ~4.2 | 7.9 | No reported | Progress in PV: R&A, 12, p33, 2004 |
| NREL | CuxTe | <1.08 | 13.9* | 60-40 | This work |

## Preparation of CuxTe back contact

-   **Using a three-step process to prepare Cu****x****Te back contact :**
**  ****1. Prepare a Te-rich layer on CdTe back side by a chemical etch;**
**  ****2. Deposit a thin Cu layer by electron-beam **
**evaporation;**
**  ****3. Post-anneal in N****2**** or He at temperature of 200°C - 300°C for 30 minutes.**

## Device structure of top CdTe cell

## Transparent CTO/ZTO/nano-CdS:O/CdTe/CuxTe top cells

| Cell # | Voc
(mV) | Jsc
(mA/cm2) | FF
(%) | Efficiency*
(%) | Area
(cm2) |
| --- | --- | --- | --- | --- | --- |
| 1 | 809.8 | 24.87 | 66.71 | 13.4 | 0.406 |
| 2 | 783.9 | 25.65 | 66.58 | 13.4 | 0.413 |
| 3 | 815.4 | 24.75 | 66.62 | 13.4 | 0.409 |
| 4 | 798.9 | 24.98 | 68.01 | 13.6 | 0.417 |
| 5 | 806.1 | 24.97 | 69.22 | 13.9 | 0.406 |

***:** **NREL-confirmed total-area efficiency**

## NIR transmittance

- IR transmission: ~60%-40% in the wavelength range between 850 nm and 1300 nm.

## CdTe/CIS tandem device structure

**CdTe top cell**
**    ****(1.48 eV)**

-   **CIS bottom cell**
**       ****(0.96 eV)**

## NREL–confirmed I-V parameters of a CdTe/CIS tandem solar cell

| Device | Voc
(mV) | Jsc
(mA/cm2) | FF
(%) | Efficiency
(%) |
| --- | --- | --- | --- | --- |
| CTO/ZTO/CdS/CdTe/CuxTe | 786.2 | 25.55 | 68.89 | 13.84 |
| CdS/CIS
(Through CdTe cell) | 
357.9 | 
6.06 | 
68.01 | 
1.47 |
| CdTe/CIS |  |  |  | 15.31 |

## Summary

**Developed several novel materials, including CTO TCO, ZTO buffer layer, and nanocrystalline CdS:O window layer, and a modified CdTe device structure. Demonstrated a CTO/ZTO/CdS/CdTe cell with an NREL-confirmed total-area efficiency of 16.5%.**
**Developed “one heat-up step” manufacturing processes on either borosilicate glass or commercial SL/SnO****2**** substrates, which provided an attractive alternative for producing CdTe modules with high efficiency, high throughput, and low cost.**
**Demonstrated NREL-confirmed total-area 13.9%-efficient CTO/ZTO/nano-CdS:O/CdTe/Cu****x****Te cell with more than 50% NIR transmission, and 15.3%-efficient CdTe/CIS mechanically stacked tandem cell.**

## Acknowledgments

- 	This work is supported by the U.S. Department of Energy under Contract No. DE-AC36-99GO10337 to NREL.