Close-Space Sublimation-Deposited Ultra-Thin CdSeTe/CdTe Solar Cells for Enhanced Short-Circuit Current Density and Photoluminescence

Summary

This work describes the complete fabrication process of thin absorber cadmium selenium telluride/cadmium telluride photovoltaic devices for enhanced efficiency. The process utilizes an automated in-line vacuum system for close-space sublimation deposition that is scalable, from fabrication of small area research devices as well as large-scale modules.

Abstract

Developments in photovoltaic device architectures are necessary to make solar energy a cost-effective and reliable source of renewable energy amidst growing global energy demands and climate change. Thin film CdTe technology has demonstrated cost-competitiveness and increasing efficiencies due partially to rapid fabrication times, minimal material usage, and introduction of a CdSeTe alloy into a ~3 μm absorber layer. This work presents the close-space sublimation fabrication of thin, 1.5 µm CdSeTe/CdTe bilayer devices using an automated in-line vacuum deposition system. The thin bilayer structure and fabrication technique minimize deposition time, increase device efficiency, and facilitate future thin absorber-based device architecture development. Three fabrication parameters appear to be the most impactful for optimizing thin CdSeTe/CdTe absorber devices: substrate preheat temperature, CdSeTe:CdTe thickness ratio, and CdCl₂ passivation. For proper sublimation of the CdSeTe, the substrate temperature prior to deposition must be ~540 °C (higher than that for CdTe) as controlled by dwell time in a preheat source. Variation in the CdSeTe:CdTe thickness ratio reveals a strong dependence of device performance on this ratio. The optimal absorber thicknesses are 0.5 μm CdSeTe/1.0 μm CdTe, and non-optimized thickness ratios reduce efficiency through back-barrier effects. Thin absorbers are sensitive to CdCl₂ passivation variation; a much less aggressive CdCl₂ treatment (compared to thicker absorbers) regarding both temperature and time yields optimal device performance. With optimized fabrication conditions, CdSeTe/CdTe increases device short-circuit current density and photoluminescence intensity compared to single-absorber CdTe. Additionally, an in-line close-space sublimation vacuum deposition system offers material and time reduction, scalability, and attainability of future ultra-thin absorber architectures.

Introduction

Global energy demand is quickly accelerating, and the year 2018 demonstrated the fastest( 2.3%) growth rate in the last decade¹. Paired with increasing awareness of the effects of climate change and the burning of fossil fuels, the need for cost-competitive, clean, and renewable energy has become abundantly clear. Of the many renewable energy sources, solar energy is distinctive for its total potential, as the amount of solar energy that reaches earth far exceeds global energy consumption².

Photovoltaic (PV) devices directly convert solar energy to electrical power and are versatile in scalabi....

Protocol

CAUTION: Gloves must be worn when handling substrates to prevent film contamination and material-to-skin contact. This fabrication process requires the handling of structures containing cadmium compounds; therefore, a lab coat and gloves should be worn in the lab at all times.

1. Substrate cleaning

1. Place TCO-coated glass substrates (9.1 cm x 7.8 cm) in a stainless steel rack with ample spacing such that cleaning solution and compressed air can be applied to each glass face.
2. Blow any dust off the substrates using a nitrogen compressed air hose.
3. Place the rack in an ultrasonic cleaner (UC1) and fill with isopro....

Results

The addition of CdSeTe to a thin CdTe absorber improves device efficiency through superior absorber material quality and higher short-circuit current density (J_SC). Figure 3A and Figure 3B, (adapted from Bothwell et al.⁸) show PL and TRPL, respectively, for the single CdTe absorber and CdSeTe/CdTe bilayer absorber devices. Both PL and TRPL measurements clearly show improved photoluminescence with the CdSeTe/CdTe bilayer absorbe.......

Discussion

Thin bilayer CdSeTe/CdTe photovoltaic devices demonstrate improvements in efficiency compared to their CdTe counterparts because of better material quality and increased current collection. Such enhanced efficiencies have been demonstrated in bilayer absorbers greater than 3 μm^5,7, and now with optimized fabrication conditions, it has been demonstrated that increased efficiencies are also achievable for thinner, 1.5-μm bilayer absorbers.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Alpha Step Surface Profilometer	Tencor Instruments	10-00020	Instrument for measuring film thickness
CdCl2 Material	5N Plus	N/A	Material for absorber passivation treatment
CdSeTe Semiconductor Material	5N Plus	N/A	P-type semiconductor material for absorber layer
CdTe Semiconductor Material	5N Plus	N/A	P-type semiconductor material for absorber layer
CESAR RF Power Generator	Advanced Energy	61300050	Power generator for MgZnO sputter deposition
CuCl Material	Sigma Aldrich	N/A	Material for absorber doping
Delineation Material	Kramer Industries Inc.	Melamine Type 3 60-80 mesh	Plastic beading material for film delineation
Glovebox Enclosure	Vaniman Manufacturing Co.	Problast 3	Glovebox enclosure for film delineation
Gold Crystal	Kurt J. Lesker Company	KJLCRYSTAL6-G10	Crystal for Te evaporation thickness monitor
HVLP and Standard Gravity Feed Spray Gun Kit	Husky	HDK00600SG	Applicator spray gun for Ni paint back contact application
MgZnO Sputter Target	Plasmaterials, Inc.	PLA285287489	N-type emitter layer material
Micro 90 Glass Cleaning Solution	Cole-Parmer	EW-18100-05	Solution for initial glass cleaning
NSG Tec10 Substrates	Pilkington	N/A	Transparent-conducting oxide glass for front electrical contact
Super Shield Ni Conductive Coating	MG Chemicals	841AR-3.78L	Conductive paint for back contact layer
Te Material	Sigma Aldrich	MKBZ5843V	Material for back contact layer
Thickness Monitor	R.D. Mathis Company	TM-100	Instrument for programming and monitoring Te evaporation conditions
Thinner 1	MG Chemicals	4351-1L	Paint thinner to mix with Ni for back contact layer
Ultrasonic Cleaner 1	L & R Electronics	Q28OH	Ultrasonic cleaner 1 for glass cleaning
Ultrasonic Cleaner 2	Ultrasonic Clean	100S	Ultrasonic cleaner 2 for glass cleaning
UV/VIS Lambda 2 Spectrometer	PerkinElmer	166351	Spectrometer used for transmission measurements on CdSeTe films