# Presentation: 775474

## 13.9%-efficient transparent CdTe cell with NIR transmission of ~50%

_**X. Wu**_**, J. Zhou, A. Duda, J.C. Keane, **
**T.A. Gessert, Y. Yan, and R. Noufi**** **
***National Renewable Energy Laboratory (NREL)***
** ****May 6, 2005**

## Outline

**Introduction**
**Improvements of transmission**
**	****in near-infrared (NIR) region**
**Device results**
**Conclusions**

## Introduction

- In the polycrystalline thin-film tandem cell project, the most critical work is to make high-efficient (>15%) top cell with higher bandgap (1.5-1.8 eV) and higher transmission (>70%) in the NIR wavelength region
- Several high bandgap (>1.5 eV) cells with efficiencies of ~11%, such as CGS, CIGSS, AIGS, and CZT are being investigated
- CdTe cell can also be as a candidate for top cell, because it’s state-of-the-art single junction devices are available (16.5%), although its bandgap (1.48 eV) is slightly lower as a top cell in a dual-junction device

## Losses of NIR transparency in a top cell 

**Absorption of the back contact, including interfacial layer and ITO transparent electrode**
**Absorption of the front transparent conductive oxide (TCO) contact**
**Absorption of the sub-bandgap of semiconductors**
**Reflection of device**

## Device structure of top CdTe cell

## 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.**

## XRD patterns of etched CdTe films

**(a)**** ****Etch in NP (88:1:35 **
**H****3****PO****4**** : HNO****3**** : H****2****O)**

**Etch in BM (0.2%**
**Br****2****/Methanol)**

## A of Te layers prepared by NP etch and BM etch

## Absorption of a CuxTe layer

## Optical properties of TCO films

**CTO film has the lowest absorption in the region of 400-1300 nm.**

## Near-zero sub-bandgap absorptionof nano-CdS:O and CdTe films

- A

- T

- R

## AR coating layers on the CdTe backside 

- Reflection of a CTO/ZTO/CdS/CdTe/CuxTe cell in the near-IR region can be reduced by optimizing device structure and AR coating layers on the CdTe backside
- With AR coating layers (ITO/MgF2), the NIR transmission can be improved by ~15%, due to the reduction of both A & R

## Summary of losses of NIR transmission

| T loss mechanism  | T loss
(%) | Target of T loss (%) |
| --- | --- | --- |
| A of back contact (1) CuxTe
                         (2) ITO | ~ 15
5-25 | ~ 10
5-10 |
| Front TCO contact
CTO | 
5-10 | 
3-8 |
| Sub-bandgap A of semiconductors | 
~ 0 | 
~ 0 |
| Reflection of device | 10-20 | 10-15 |
| Total | 35-70 | 28-43 |

## 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**

## Transparent CdTe Top Cell

- Voc = 806.1 mV
- Jsc = 24.97 mA/cm2
- FF = 69.22%
- Efficiency = 13.94%
- Area = 0.41 cm2

## NIR transmittance

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

## CdTe/CIS tandem device structure

**CdTe top cell**

-    **CIS bottom cell**

## 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

**Develop a technique to use thinner Cu****x****Te with lower band gap of ~ 1.08 eV as a back-contact for making high-efficiency CdTe cell with high NIR transmission**
**Demonstrate NREL-confirmed total-area 13.9%-efficient CTO/ZTO/nano-CdS:O/CdTe/Cu****x****Te cell with ~50% NIR transmission and 15.3%-efficient CdTe/CIS mechanically stacked tandem cell, exceeding the FY2006 milestone (15%) in DOE/NREL’s HiPerf program **
**It is possible to make transparent CdTe cells with efficiencies of ~15% and NIR transmission of 60%-70%, and ~18%-efficient polycrystalline CdTe/CIS tandem cell**

## Acknowledgments

- 	The authors would like to thank D. Albin, R.G. Dhere, C. DeHart, K. Ramanathan, J. AbuShama, J.S. Ward, K. Emery, T. Moriarty, C. Mack, S. Wei, M. Symko-Davies, T.J. Coutts, and J.P. Benner for their technical contributions and support.
- 	This work is supported by the U.S. Department
- of Energy under Contract No. DE-AC36-99GO10337 to NREL.