CZTSSe Solar Cells from Nanoparticle Inks

Lead Research Organisation: Northumbria University
Department Name: Fac of Engineering and Environment

Abstract

Future energy demand can be addressed by using renewable and inexhaustible solar energy, providing clean, unlimited, economical and green energy. The world global photovoltaic (PV) capacity currently stands at >140 GW and is expected to reach levels of 1 TW within the next decade. Electricity generation from the sun employing PV technology is currently dominated by Si-based PV and requires expensive equipment and process and schemes for cost reduction on a large scale are limited. Thin film technologies such as CdTe and Cu(In,Ga)Se2 (CIGS), provide a lower cost alternative primarily due to the use of in-line and low-temperature processes. While considerable efforts have been made to increase efficiency and reduce costs, thin film PV currently relies on scarce and therefore expensive resources and/or toxic elements. Alternative thin film materials would therefore provide routes to reduce PV cost-per-watt while still exhibiting lower input energy requirements. Solar cells based on Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers offer such an alternative.

Despite its young history CZTSSe record efficiency stands at 12.6% and the major limitations are (i) a lower than expected open circuit voltage accompanied by a low efficiency at converting and collecting carriers from low energy photons; (ii) the difficulty in controlling the kesterite crystal structure throughout the fabrication process; and (iii) the use of hydrazine, a highly toxic chemical, in the fabrication process to achieve the record efficiencies. This project will use nanocrystal dispersions (inks) of CZTS fabricating from hot injection as the starting material. This technique can reliably control crystal structure, composition and doping and does not present any environmental risks. Inks are easily spin coated or sprayed on substrates and a heat treatment under selenium rich atmosphere promotes grain growth without loss of the crystal structure. In order to fabricate record efficiencies using this technique the microstructure of the absorber and back contact layers need to be engineered to provide large grains extending the full thickness of the absorber combined with a small interfacial layer to ensure a good ohmic contact. This will be achieved by the removal of long hydrocarbon chained ligand in the nanocrystal fabrication alongside modifications of the selenization procedures. In addition the role of substrates and process impurities affecting devices performances will be quantified. I will produce nanoparticle inks, solar absorber and PV devices and demonstrate world leading results.

Planned Impact

Climate change and global warming are amongst the top five recognised grand challenges faced by humanity in the next 20 years. The need for sustainable and renewable energy is undeniable and amongst the mix of green energy sources photovoltaics (PV) offers the most potential. PV is currently under exploited partly due to the relatively low efficiencies but primarily due to perceived high cost. In order to reduce cost at an accelerated rate, new materials, new processes and alternative designs must be developed. This proposal investigates a material not yet on the market but with potential greater than what is currently available in the thin film sector.

Developing renewable energies and in particular photovoltaics is a key priority for the UK to maintain and reinforce security in its energy supply as national fossil fuel resources dwindle. Nanoparticle inks are well positioned to offer a low cost alternative for generating electricity with specific interest to supply the wearable electronic market, portable consumer product and building integrated photovoltaics. My industrial collaborator Big Solar Ltd a local SME in the Northeast of England is focussed on developing flexible plastic PV. The potential cost reductions compared to flat glass encapsulated modules are enormous (glass represents up to 30% of the costs of CdTe thin film modules) and Big Solar's new approach of cell design give them significant scope for becoming a new technology leader in the field. However Big Solar is still looking for the best possible light absorbing material and CZTS nanoparticle inks are ideal candidates. Big Solar is a potential partner for future spin-off projects funded directly (under non-disclosure agreements), via InnovateUK (KTPs or otherwise) or with other partners in, for example in EU Horizon 2020 projects. I have also identified Nanoco Technologies (developing CIGS inks) and BIPVco (piloting flexible printed CIGS based PV solution) as additional partners to make the transition from CIGS to CZTS.

The training of the postdoctoral researcher and a university funded PhD student with skills relevant to Energy Systems (InnovateUK Catapult Centres) will benefit both the individuals and UK industry. The experience gained will include semiconductor handling, materials, measurement and instrumentation, device fabrication and characterisation. They will receive training in public understanding and engagement and will be involved in exhibition and outreach work through Think Physics (see Pathways to Impact document). This will benefit the individuals and also attract wider interest from the public by promoting the spirit of science and highlighting everyday impacts which arise from this research.
 
Description Key discovery#1: a single layer absorber without the fine grain layer was achieved using a alternative ligand (FA instead of OLA) (published in Solar Energy) and also by altering the soft baking recipe using OLA; however it was not possible to fabricate devices due to the high porosity of the layers, yielding shunts.

Key discovery #2: Devices were fabricated without the fine grain layer at the back interface by using a dual absorber layer OLA+FA. The removal of the fine grain layer yields a lower back contact barrier height and slightly improved open circuit voltage. Grain boundaries/voids seem to be passivated via OLA-CZTS small grains (published in Solar Energy).

Key discovery #3: a comparative study of high/low purity chemicals for the fabrication of the nanoparticle inks showed that the purity does not affect the performance of our devices and that the performance are limited by the front layer (n-type matting layer). We showed that however the higher purity leads to a reduce potential fluctuations which should increase the output voltage. (published in J Applied Physics D). Ongoing work is looking into improving this part of the structure.

Key discovery #4: Na doping of flexible CZTSSe solar cells on Mo foil. We have developed a new method of doping the CZTS nanoparticle ink during the ink synthesis and as a result have improve grain formation and uniformity in devices prepared on Mo foil (under revision J Mat Science).
Exploitation Route Na doping method can be use widely.
We aim to show that it is possible to replace the toxic CdS n-layer by a more benign and more transparent layer (publication being written)
Sectors Energy

 
Description North East Centre for Energy Materials
Amount £1,833,000 (GBP)
Funding ID EP/R021503/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 10/2017 
End 03/2021
 
Description Big Solar Ltd 
Organisation Big Solar Ltd
Country United Kingdom 
Sector Private 
PI Contribution Processing of inks from Big Soalr Ltd: (i) device fabrication and (ii) selenisation precursors (summer 2017). JV measurements of Big Solar devices (Feb 2018)
Collaborator Contribution Big Solar is providing laser curing of ink, one of the work packages of this project
Impact Still ongoing
Start Year 2016
 
Description Christophe Labbe (Caen-CNRS) - TRPL 
Organisation Caen University
Country France 
Sector Academic/University 
PI Contribution None at this stage
Collaborator Contribution CL to perform TRPL on CZTSSe absorebr (high/low putiry adn new soft baking procedure)
Impact None yet available
Start Year 2018
 
Description GD-OES @ Horiba Scientific (France) 
Organisation Horiba
Department Horiba Jobin Yvon IBH Ltd
Country United Kingdom 
Sector Private 
PI Contribution Provided samples and references for calibrations to Horiba (Paris)
Collaborator Contribution Data received Nov 2016, informing current research. YQ visited Horiba in June 2017 for trainig and data analysis in June 2017. New set of data recevied in Autumn 2017 on XX samples (Mo foil and Mofilm/foil)
Impact Not yet published (partial results to be presented at PVSAT-13)
Start Year 2016
 
Description Jon Major - SIMS Analysis & DLTS JVT 
Organisation University of Liverpool
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution NU (GZ) performed SIMS analysis on CdTeSe samples (May-July 2017). Resutls submitted for publication to Solar Energy Materials (revise form Feb 2018): "Incorporation of CdSe layers into CdTe thin film solar cells Solar Energy Materials and Solar Cells" by Tom Baines, Guillaume Zoppi, Leon Bowen, Thomas P. Shalvey, Silvia Mariotti, Ken Durose, Jonathan D. Major
Collaborator Contribution JM (LU) performed DLTS, C-V and JV(T) on high/low purity absorber and devices (oct 17 Feb 18). Those resutls will be included in EMRS persentation/manuscriot (unconfimred at time of report) "Spectroscopy of nanoparticle inks based CZTSSe solar cells" by Stephen Campbell, Yongtao Qu, Jonathan D Major, Vincent Barrioz, Neil S Beattie and Guillaume Zoppi
Impact SIMS CdTe: not possible to achieve grading Se profile at thsi stage. DLTS Results needs further analsyis befor eany conclusion can take place
Start Year 2016
 
Description Phil Dale Luxembourg university 
Organisation University of Luxembourg
Country Luxembourg 
Sector Academic/University 
PI Contribution Invited seminar talk (GZ July 2016)
Collaborator Contribution Invited seminar talk (PD July 2016)
Impact None. Investigated proposal ideas but close overlap between the two research groups make it difficult.
Start Year 2010
 
Description Big Bang Fair 2017 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Big bang Fair 2017, July 2017, Northumbria University. Approximately 100 pupils from a range of local schools attended my stand where I presented the research nanoparticle for solar cells.
Year(s) Of Engagement Activity 2017
 
Description Impact Publication : "Photovoltaic Paint" 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Publication distribution:
To 35'000 stakeholder readers including funding agencies, policymakers, NGO's, universities, academic agencies, research institutes and private sector
· Each publication is distributed through IngentaConnect.com the world's largest platform for scholarly information, used by 1.5 million visitors a month and with 30'000 libraries registered. Each article is hosted on Ingentaconnect.com open access, receives a CrossRef DOI, is indexed in all major search platforms including Google Scholar and is deposited in Portico.
· Selected research and industry events

DOI to follow, to be printed in Spet/Oct edition of magazine
Year(s) Of Engagement Activity 2018
URL https://www.ingentaconnect.com/content/sil/impact/2018/00002018/00000009/art00015
 
Description Scientist of the week 2018 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Scientist of the week event to primary schools organised by NUSTEM (Feb 2018)
Year(s) Of Engagement Activity 2018