This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 4789–4795 4789 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 4789–4795 Design of energy band alignment at the Zn 1x Mg x O/Cu(In,Ga)Se 2 interface for Cd-free Cu(In,Ga)Se 2 solar cellsw Chang-Soo Lee, a Liudmila Larina, a Young-Min Shin, a Essam A. Al-Ammar b and Byung Tae Ahn* a Received 6th February 2012, Accepted 7th February 2012 DOI: 10.1039/c2cp40355b The electronic band structure at the Zn 1x Mg x O/Cu(In 0.7 Ga 0.3 )Se 2 interface was investigated for its potential application in Cd-free Cu(In,Ga)Se 2 thin film solar cells. Zn 1x Mg x O thin films with various Mg contents were grown by atomic layer deposition on Cu(In 0.7 Ga 0.3 )Se 2 absorbers, which were deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The electron emissions from the valence band and core levels were measured by a depth profile technique using X-ray and ultraviolet photoelectron spectroscopy. The valence band maximum positions are around 3.17 eV for both Zn 0.9 Mg 0.1 O and Zn 0.8 Mg 0.2 O films, while the valence band maximum value for CIGS is 0.48 eV. As a result, the valence band offset value between the bulk Zn 1x Mg x O(x = 0.1 and x = 0.2) region and the bulk CIGS region was 2.69 eV. The valence band offset value at the Zn 1x Mg x O/CIGS interface was found to be 2.55 eV after considering a small band bending in the interface region. The bandgap energy of Zn 1x Mg x O films increased from 3.25 to 3.76 eV as the Mg content increased from 0% to 25%. The combination of the valence band offset values and the bandgap energy of Zn 1x Mg x O films results in the flat (0 eV) and cliff (0.23 eV) conduction band alignments at the Zn 0.8 Mg 0.2 O/Cu(In 0.7 Ga 0.3 )Se 2 and Zn 0.9 Mg 0.1 O/Cu(In 0.7 Ga 0.3 )Se 2 interfaces, respectively. The experimental results suggest that the bandgap energy of Zn 1x Mg x O films is the main factor that determines the conduction band offset at the Zn 1x Mg x O/Cu(In 0.7 Ga 0.3 )Se 2 interface. Based on these results, we conclude that a Zn 1x Mg x O film with a relatively high bandgap energy is necessary to create a suitable conduction band offset at the Zn 1x Mg x O/CIGS interface to obtain a robust heterojunction. Also, ALD Zn 1x Mg x O films can be considered as a promising alternative buffer material to replace the toxic CdS for environmental safety. Introduction Cu(In,Ga)Se 2 (CIGS) thin film solar cells are expected to play a leading role in the photovoltaic market due to their high efficiency of around 20%. 1 The conventional structure of a CIGS solar cell is ZnO/CdS/CIGS, where CdS deposited by chemical bath deposition (CBD) has been regarded as the best buffer material for high efficiency thus far. However, Cd is a toxic metal element, and the CBD process produces much chemical waste. Thus, for the ecologically sustainable development of the energy system, a Cd-free material which can be deposited by a non-CBD process should be developed for environmental safety. Over the last decade, the Zn 1x Mg x O film has attracted an increasing interest as an alternative Cd-free buffer material for CIGS solar cells. 2–5 The Zn 1x Mg x O film has several advantages, including its variable bandgap value. Indeed, the bandgap can be varied in the range from 3.2 to 3.9 eV by controlling the chemical composition of the ternary compound. 6–9 The electronic band structure at the Zn 1x Mg x O/CIGS interface can be optimized by using the Zn 1x Mg x O buffer with various bandgap energies to enhance the efficiency of CIGS solar cells. To date, several methods have been developed for Zn 1x Mg x O film deposition, including RF magnetron sputtering, 9 pulsed laser deposition, 6 ultrasonic spray pyrolysis, 7 metal organic vapor phase epitaxy, 10,11 and atomic layer deposition (ALD). 2 Among them, the most common deposition method is RF magnetron sputtering. However, highly energetic particles in plasma cause serious damage to the surface of CIGS absorbers which affects the quality of the Zn 1x Mg x O/CIGS interface, and in turn the device performance. Recently, to avoid this problem, Zn 1x Mg x O films have been deposited by ALD. The associated processing parameters have been well established. 2 Because ALD is characterized by the alternate exposure of chemical species with self-limiting surface reactions, ALD has several advantages over other deposition methods. These advantages include accurate composition control, a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Republic of Korea. E-mail: btahn@kaist.ac.kr b Department of Electrical Engineering, King Saud University, 11451, Riyadh, 2454, Kingdom of Saudi Arabia w Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cp40355b PCCP Dynamic Article Links www.rsc.org/pccp PAPER Published on 01 March 2012. Downloaded by Chungnam National University on 07/11/2014 08:14:00. View Article Online / Journal Homepage / Table of Contents for this issue