Doç. Dr. Selçuk YERCİ
Prof. Dr. Raşit TURAN, Doç. Dr. Hüsnü Emrah ÜNALAN
The aim of SiZetSolar is to combine novel advanced industrial crystalline silicon (cSi) and CdTe cells on a hybrid CdZnTe (CZT)/cSi tandem platform for higher performance at low cost.
The project will apply advances in fundamental science of photovoltaic (PV) materials to boost both cell and module efficiency and improve service lifetime to reduce manufacturing costs. It will focus on advancing industrially-relevant PV technologies to impact the market within 10 years. The objectives include demonstration of 27% cell and 23% module efficiencies, reaching a TRL of 5 and preparation of detailed levelised cost of electricity (LCOE) and life-cycle assessment reports. The project will contribute to Turkey-UK economies in lowering the LCOE to below 0.2 €/kWh in Southern Europe by increasing PV efficiency thanks to a novel tandem technology.
Industrially relevant highest laboratory efficiency of a solar cell (SC) using a single absorber material (or bandgap) is 26.7% fabricated by cSi, holding >90% of the PV market. Tandem SC concept is considered as the most feasible method of increasing the efficiency of cSi SCs while maintaining the low-cost. The International Technology Roadmap of Photovoltaics (version 2017) predicts that tandem SCs with cSi bottom subcells will be in the market by 2020. CdTe is the second leading material in the PV market after cSi and it strictly meet the accepted environmental requirements . Alloying of CdTe with Zn widens its bandgap, with a CZT/cSi tandem already showing noticeable potential .
In this project, the GÜNAM team will develop the high efficiency cSi bottom subcell based on a newly suggested device scheme: the DASH (dopant free asymmetric heterocontact) cell . The DASH device is a particularly promising heterojunction cell concept for building an efficient tandem cell, which uses distinct electron and hole transport layers unlike other cSi SC architectures . We are targeting demonstration of a 21% efficient cSi subcell, which will then be optimized for operation at longer wavelengths using novel light trapping structures. Adopted to tandem cell operation, the bottom subcell performance will inevitably be sacrificed, with our target for the cSi bottom cell (with top subcell laid over) being to surpass 10%.
CZT top subcells with a bandgap 1.7-1.8 eV will be fabricated by the CSER team by metalorganic chemical vapor deposition (MOCVD). Two approaches will be followed in SC fabrication: (i) polycrystalline (poly)-CZT on glass and (ii) single-crystal-CZT on sacrificial lattice-matching substrates. Both CZT SC structures will be multi-stacked onto the cSi subcell to operate on 4 terminal (4T) basis. The poly-CZT subcell will be processed similar to CSER’s poly-CdTe devices, which stands at 16.1% efficiency . Incorporation of Zn widens the bandgap and increases the cell voltage. Epitaxial CZT devices can outperform poly-CZT due to better material properties, e.g. lack of grain boundary recombination and higher dopant activation. Our target is to achieve 17% cell efficiency for either of the CZT subcells. To enhance tandem performance, highly transparent conductors will be developed based on new metal-oxide films and metal nanowire meshes. Light trapping schemes will be introduced to effectively capture photons in the top and bottom subcells. Tandem devices will then be evaluated for stability and outdoor applications. A thorough cost assessment and life cycle analysis will be performed for the high performance cells by an SME, iTechSolar, in Turkey. This will provide key information allowing Turkey and UK to position themselves at the forefront of the next generation PV technologies. Researchers of GÜNAM and CSER worked successfully in collaboration on a project funded in 2015 under this program, in which they developed extremely-thin absorber CdTe solar cells with high performance, published a high-impact article, performed joint conference presentations, and made several fruitful visits between these centres.