CZTSSe Solar Cells from Nanoparticle Inks

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.

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.