Subscriber access provided by La Trobe University Library
Article
On the Impact of Na Dynamics at the Cu2ZnSn(S,Se)4/CdS Interface During Post Low Temperature Treatment of Absorbers Haibing Xie, Simon Lopez-Marino, Tetiana Olar, Yudania Sánchez González, Markus Neuschitzer, Florian Oliva, Sergio Giraldo, Victor IzquierdoRoca, Iver Lauermann, Alejandro Pérez-Rodríguez, and Edgardo Saucedo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12243 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 4, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 34
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
On the Impact of Na Dynamics at the Cu2ZnSn(S,Se)4/CdS Interface During Post Low Temperature Treatment of Absorbers Haibing Xie†*, Simon López-Marino†, Tetiana Olar‡, Yudania Sánchez†, Markus Neuschitzer†, Florian Oliva†, Sergio Giraldo†, Victor Izquierdo-Roca†, Iver Lauermann‡, Alejandro PérezRodríguez†,║, Edgardo Saucedo† †
Catalonia Institute for Energy Research (IREC), Jardins de les Dones de Negre 1, 08930 Sant Adrià del Besòs,Barcelona, Spain.
‡ Institute HeterogenerousMaterial Systems, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany. ║
IN2UB, Departament d’Electrònica, Universitat de Barcelona, Martí i Franquès, 1-11, 08028 Barcelona, Spain.
KEYWORDS: kesterite, Cu2ZnSn(S,Se)4 , thin film, solar cell, sodium, post low temperature treatment (PLTT)
ACS Paragon Plus Environment
1
ACS Applied Materials & Interfaces
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 34
ABSTRACT: Cu2SnZn(S,Se)4 (CZTSSe) solar cells based on earth abundant and non-toxic elements currently achieve efficiencies exceeding 12%. It has been reported that to obtain high efficiency devices, a post thermal treatment of absorbers or devices at temperatures ranging between 150-400 ºC (post low temperature treatment, namely PLTT) is advisable. Recent findings point towards a beneficial passivation of grain boundaries with SnOx or Cu-depleted surface and grain boundaries during the PLTT process, but no investigation about alkali doping are available, although alkali dynamics, especially Na, are systematically reported as crucial within the field. In this work, CZTSSe absorbers were subjected to PLTT process under different temperatures and solar cells were completed. We found a surprisingly behavior where efficiency decrease up to nearly 0% at 200 ºC during the PLTT process, being recovered or even improved at temperatures above 300 ºC. This unusual behavior correlates well with the Na dynamics in the devices, especially with the in-depth distribution of Na in the active CZTSSe/CdS interface region, indicating the key importance of Na spatial distribution on device properties. We present an innovative model for Na dynamics supported by theoretical calculations and additional specially designed experiments to explain this behavior. After optimization of PLTT process, a Se-rich CZTSSe solar cell with 8.3% efficiency was achieved.
INTRODUCTION Cu2SnZn(S,Se)4 (CZTSSe) solar cells have a promising potential to substitute the more mature Cu(In,Ga)Se2 (CIGS) solar cells in the mid or long term, due to their earth abundant and nontoxic constituents. In the last five years, CZTSSe solar cells developed fast, starting from 6.8% in 20091 and recently achieving efficiencies exceeding 12%2. However, the efficiency is still far below their counterpart CIGS solar cells (21.7%)3, mainly due to a large Voc deficit (around 600
ACS Paragon Plus Environment
2
Page 3 of 34
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
mV), which is linked to non-optimized grain boundaries and interfaces4. Recently, IBM reported on the introduction of a post low temperature treatment (PLTT) of CZTSSe absorbers at 375 ºC in air before buffer layer deposition as a key step for high efficiency solar cells exceeding 10%5. The efficiency enhancement was ascribed to Cu-depleted and SnOx-rich grain boundaries. Moreover, an air PLTT of pure selenide Cu2SnZnSe4 (CZTSe) solar cells under 200ºC for large improvement of performance was reported as well6, which correlates the efficiency increase mainly with Cu-poorer Zn-richer surface and grain boundaries. Additionally, PLTT of pure sulfide Cu2ZnSnS4 (CZTS) solar cells or CZTS/CdS was proved to improve efficiency through changing ITO resistivity or forming a hetero-phase epitaxial junction at the CZTS/CdS interface, respectively7,8. Furthermore, another work showed that PLTT of CZTSSe absorbers under inert atmosphere at 200 ºC can improve the efficiency considerably due to the increase of Na concentration in the bulk9. All this suggests that, independently of the S/Se content, the processes employed and the characteristics of the solar cell architecture, the absorbers seem to respond distinctly to the PLTT process. On the other hand, as it is well known, Na is paramount for high efficiency CIGS and CZTSSe solar cells due to passivation of defects in the grain boundaries and interfaces10-12. The impact of Na concentration on device properties is frequently reported and the optimal concentration is considered to be around 0.1at%13,14. However, the role of Na spatial distribution, i.e., Na indepth distribution in absorbers, interfaces and buffer/window layer, is rarely investigated. Recently, V. Fjallstrom et al. reported the important role of Na distribution in active CIGS/CdS interface region during potential induced degradation (PID) process15, which gives insight into the impact of Na spatial distribution on CIGS solar cells performance.
ACS Paragon Plus Environment
3
ACS Applied Materials & Interfaces
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 34
Na out-diffusion from soda-lime glass (SLG) to absorbers is common during thermal treatment. However, in those PLTT processes mentioned above, although SLG substrates were employed, the influence of Na is either not mentioned or is lacking of deeper investigation. Thus, to illustrate the impact of Na during PLTT process and get insights into Na dynamics, CZTSSe absorbers (Cu/(Zn+Sn)=0.77-0.83, Zn/Sn= 1.13-1.18, S/(S+Se)=0.25-0.30) were prepared by a single-step sulfo-selenization process16 and were then re-annealed at temperatures from 150 ºC to 400 ºC under 1bar N2 atmosphere for 1h. Solar cells were completed by etching the absorbers in (NH4)2S solution17, following deposition of CdS and i-ZnO/ITO window layer. The impact of different temperatures during PLTT on the optoelectronic properties, external quantum efficiency (EQE), CZTSSe surface composition and Na concentration is analyzed. Based on the results, an innovative model of Na dynamics is developed to illustrate the performance variation during the PLTT process, showing the key importance of Na spatial distribution control other than Na concentration in this technology. EXPERIMENTAL SECTION Preparation of metallic precursors. Cu/Sn/Cu/Zn metallic stack precursors were deposited by DC-magnetron sputtering (Ac450 Alliance Concepts) onto Mo coated soda-lime glass substrates. Precursor films were approximately 600 nm thick, with compositional ratios of Cu/(Zn+Sn) around 0.75-0.8 and Zn/Sn around 1.18-1.22. Some precursors were prepared using a nanometric Ge layer on top (10 nm thick), that has shown to improve the efficiency of the devices18 in order to confirm the PLTT effect in this type of absorbers. Preparation of Cu2ZnSn(S,Se)4 absorbers. CZTSSe thin films were prepared through a single-step sulfo-selenization of metallic precursors. The singe-step sulfo-selenization was
ACS Paragon Plus Environment
4
Page 5 of 34
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
conducted in a graphite box at 550 ºC for 30 min under S+Se+Sn atmosphere, with an Ar flow maintaining at 1 mbar. To supply S+Se+Sn atmosphere, two small containers were included inside the graphite box, one with 48mg Se+2mg S mixed powder and the other with 5 mg Sn. Post low temperature treatment (PLTT) of absorbers. A 4.2×3.3 cm2 as-fabricated CZTSSe absorber was immediately cut into 7 pieces equally, 1 as reference without PLTT process and the other 6 for PLTT in a quartz boat at temperatures ranging from 150 ºC to 400 ºC under 1 bar N2 atmosphere for 1 h, respectively. Fabrication of Solar cells. Solar cells were completed by etching the absorbers in 22% w/w (NH4)2S solution for 1min, following chemical bath deposition of 50 nm CdS and DC sputtering (Alliance CT100) of 350 nm i-ZnO/ITO window layer. Characterizations. 3×3 mm2 solar cells were scribed and illuminated and dark J-V curves (ABET3000 Solar Simulator) and external quantum efficiency (EQE) spectra (Bentham PVE300 characterization
system)
were
measured.
X-ray
diffraction
(XRD)
(Siemens
D500
diffractometer), Raman spectroscopy (532 nm, LabRam HR800-UV Horiba-Jobin Yvon spectrometer) and scanning electron microscope (SEM) (ZEISS Series Auriga microscope) were employed to investigate the structure and morphology of CZTSSe absorbers. The integral composition of CZTSSe absorbers was checked by X-ray fluorescence (XRF) (Fischerschope XVD). For in-depth composition, especially Na distribution, time-off-flight secondary ion mass spectrometry (TOF-SIMS) was performed onto full devices. TOF-SIMS measurements were performed in an ION-TOF IV equipment, equipped with 25 kV Bi cluster primary ion gun for analysis, and O2 and Cs ion guns for sputtering in-depth profiling modes. The analyzed area was 50×50 µm2 with a cycle time of 100 µs and a time to digital converter (TDC) resolution of 200
ACS Paragon Plus Environment
5
ACS Applied Materials & Interfaces
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 34
ps. For absorbers surface composition, high energy X-ray photoelectron spectroscopy (XPS) measurements were performed at the KMC-1 beamline at the electron storage ring BESSY II Berlin (Germany). The high kinetic energy end station (HIKE) allows to tune excitation the energy (Eex) of the X-ray beam from 2.01 keV up to 12 keV and therefore to increase the information depth from roughly 5 nm up to 25 nm ( where information depth is define as three times mean free path of the excited photoelectrons). All measurements were performed in ultra high vacuum (
