Conference Programme

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Overview
Session
F-04: Copper Zinc Tin Sulphide (CZTS) based photovoltaic
Time:
Tuesday, 20/Jun/2017:
1:30pm - 3:30pm

Session Chair: David Mitzi, Duke University
Session Chair: Xiaojing Hao, The University of New South Wales, Sydney
Location: Rm 336

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Presentations
1:30pm - 2:00pm
Invited

Over 14% Efficiency of Directly Sputtered Cu(In,Ga)Se2 Absorbers without Post-selenization by Post-treatment of Alkali Metals

Chih-Huang LAI, Chia-Hao HSU, Wei-Hao HO

National Tsing Hua University, Taiwan

Directly sputtering from a single CIGS target has been demonstrated to be a promising process to produce efficient CIGS absorbers without post-selenization. Due to the nature of sputtering, absorbers with excellent uniformity can be prepared. Moreover, CIGS thin films can be prepared without using any toxic substances. The major disadvantage of the process is that Se supply during CIGS formation is fixed by the Se content in the compound target, which has a Se-to-metal ratio of close to one. Therefore, an excess of Se, which is crucial to ensuring absorber quality can hardly be realized by sputtering from a single CIGS target. A deficient supply of Se results in the formation of VSe and InCu donor defects in CIGS thin films, which is detrimental to cell performance. To overcome these issues with Se deficiency, we have previously reported that Se deficiency can be overcome by sputtering from a Se-rich CIGS target or by extra Na doping. However, the cell efficiency of the single-target sputtering process without post-selenization seems to be limited to 10-11%.

In this work, we demonstrate a 14% efficiency CIGS solar cell by sputtering from a single CIGS target without post-selenization, which is to date the highest efficiency obtained with such a process. The improved cell efficiency was achieved through sequential NaF and KF post-deposition treatments. Moreover, unlike studies in which the post-treat-treatment (PDT) was performed by co-evaporating NaF or KF with Se, our PDT was done without an extra Se supply to avoid complexity in both processing and the interpretation of experimental results. The effects of Na and K on the properties of quaternary-sputtered CIGS thin films and solar cells are discussed in detail, providing additional understanding of the beneficial role of alkali metals. These understandings provide tuning knobs for further enhancing efficiency of CIGS solar cells.


2:00pm - 2:30pm
Invited

Above 10% Efficient CZTSSe Thin Film Solar Cell Fabricated by Magnetron Sputtering Method

Yi ZHANG, Jianjun LI, Li WU

Nankai University, China

Thin film solar cells based on Cu2ZnSn(S,Se)4 (CZTSSe) light absorbing materials continue to attract attention from both scientific and industrial community owing to their advantage of using earth abundant and low cost source elements. The remarkable performance enhancement of CZTSSe solar cells to over 12% has indicated the huge potential of this type of new PV technology to deliver cost-effective solar electricity to meet the demand of rapidly increasing electricity usage in today’s world.

Herein, we present the study of fabrication of high performance CZTSSe thin film solar cells with efficiency above 10% using magnetron sputtering deposition. First, CZTSSe film with different stacking sequence are investigated systematically to uncover the growth mechanism of CZTSSe layer and the effect of stacking order in precursor films on the performance of solar cells. It is disclosed that Cu layer should be prepared as the top layer and S will be substituted mostly because of its high saturation vapor pressure. Because Mo/CZTSSe interface is unstable and the interfacial MoSe2/MoS2 will be formed during high temperature annealing process, which increases the series resistance and deteriorate the performance of CZTSSe solar cells dramatically. For this problem, a simple, feasible and effective way to control the formation of interfacial MoSe2 in fabricating Cu2ZnSnSe4 (CZTSe) solar cells without any additional barrier layer is studied. On this basis, the growth of CZTSSe film under controllable Se vapour composition and impact of low Cu content on solar cell efficiency are studied and a 10.4% efficiency CZTSe solar cell is fabricated. We also study the effect of depletion region width of CZTSSe on the performance of solar cell.


2:30pm - 3:00pm
Invited

Cu2ZnSnSxSe4-x (CZTSSe) Based Alternative Earth Abundant Photoactive Materials for Solar Energy Harvesting

Sudip K. BATABYAL

Amrita Centre for Industrial Research & Innovation (ACIRI), Amrita School of Engineering, Amrita Vishwa Vidyapeetham, Amrita University, India

Kesterite-based Cu2ZnSnSxSe4-x (CZTSSe) materials has attracted huge research attentions as a non-toxic, earth abundant photoabsorbing material for thin film solar cells. During the last few years impressive research work were carried out to improve the device performance of CZTSSe based solar cells. The development of CZTSSe was forced by the replacing the In and Ga from the Cu2InGaSxSe4-x (CIGSSe) device. But still now the performance of CZTSSe thin film solar cells in comparisons to CIGSSe is rather low and still stays at the level of 12% . The main limiting factors for the performance of CZTSSe based devices are the intrinsic limitations of CZTSSe materials, such as secondary phases, anti-site defects, life time of carriers and so on. This prompted to develop some new alternative photoactive materials based on CZTSSe for thin film solar cells. Materials with analogous structures similar band gaps and absorption coefficients as CZTSSe such as Cu2MSnS4 are of special interest where M is the suitable earth abundant metal cations, which replaces the Zn either partial or full. Here we report the materials properties and device data of some of the Cu2MSnS4 materials.


3:00pm - 3:15pm
Oral

In Situ Growth of Metal and Metal Chalcogenides Nanoparticle Within Metal Oxide Thin Film For Photovoltaic Solar Cell Application

Bhola PAL, Satyaveer SINGH

Indian Institute of Technology (Banaras Hindu University), India

We have explored the situ growth of metal nanoparticles (like Ag) and metal chalcogenides semiconductors (like Ag2S and Cu2S) within sol-gel derived metal oxide thin film, in view of the development of low cost large area thin film solar cell. This thin film growth consists of three successive steps including sol-gel derived ion conducting thin film fabrication containing loosely bound light ion (Li+) followed by ion-exchanged (with Ag+ or Cu+) and subsequent reduction process (for metal nanoparticle) or sulfurization process (for metal chalcogenides semiconductor). Ion-exchange has been done by immerging ion conducting thin film within silver nitrate (AgNO3 or CuNO3) solution whereas for reduction has been done by dipping the ion-exchanged thin film within aqueous NaBH4. In this process smooth and large area Ag-metal oxide thin film containing Ag particles ranging from 10-20 nm were readily obtained. The size and metal NP distance could be controlled by changing reaction conditions. Similarly, for Ag2S and Cu2S growth Ag+ (or Cu+) ion-exchanged sample was dipped in Na2S solution to form Ag2S (or Cu2S)-metal oxide thin film containing Ag2S (or Cu2S) particles.


3:15pm - 3:30pm
Oral

Cation Substitution for Reduced Defects and Improved Performance in Kesterite Solar Cells

Shin Woei LEOW1, Asim GUCHHAIT1, Stener LIE2, Ying Fan TAY2, Joel TAN2, Wenjie LI2, Lydia Helena WONG2

1Energy Research Institute @ NTU, Nanyang Technological University, Singapore; 2Material Science & Engineering, Nanyang Technological University, Singapore

Chalchogenide thin film photovoltaics (PV) have made great strides in performance, with current efficiencies of Cu(In,Ga)Se2 (CIGSe) and CdTe exceeding 20%, rivaling those of Si PV cells. However, the toxicity of Cd and scarcity of In and Cd remains a concern for the wide spread use of such materials. It is unsurprising that developments in Kesterite Cu2ZnSnS4 (CZTS) have attracted much attention as a competing replacement, due to the abundance and benign nature of its constituent elements. This is not without drawbacks as CZTS efficiencies have lagged far behind, with CZTS and Cu2ZnSnSxSe4-x (CZTSSe) having power conversion efficiency (PCE) of 8% and 12% respectively. This has been ascribed to the high levels of secondary phases and CuZn anti-site defects present in the material, due to the small stable phase formation region of stoichiometric CZTS. Consequently, the introduction of deep acceptor levels and low carrier mobility have been reported. In response, work on cation substitution of Cu and Zn have been shown to control phase formation, crystallinity, and anti-site formation in CZTS films. In our work, small amounts of heavier ions such as Ag, Mn and Cd were added to precursor CZTS solutions to be spin-coated. Precise mixtures of ion additives offered a greater measure of control over material band gap compared to furnace selenization. Cd-Zn substitution in CZTS was demonstrated to raise device efficiency from 5.3% to 9.24% while the addition of Ag reduced CuZn ant-site formation improving device Voc and efficiency



 
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