Cu2ZnSn(S,Se)4 (CZTSSe) is a promising material for thin-film photovoltaics, however, the open-circuit voltage (VOC) deficit of CZTSSe prevents the device performance from exceeding 13% conversion efficiency. CZTSSe is a heavily compensated material that is rich in point defects and prone to the formation of secondary phases. The landscape of these defects is complex and some mitigation is possible by employing non-stoichiometric conditions. Another route used to reduce the effects of undesirable defects is the doping and alloying of the material to suppress certain defects and improve crystallization, such as with germanium. The majority of works deposit Ge adjacent to a stacked metallic precursor deposited by physical vapour deposition before annealing in a selenium rich atmosphere. Here, we use an established hot-injection process to synthesise Cu2ZnSnS4 nanocrystals of a pre-determined composition, which are subsequently doped with Ge during selenisation to aid recrystallisation and reduce the effects of Sn species. Through Ge incorporation, we demonstrate structural changes with a negligible change in the energy bandgap but substantial increases in the crystallinity and grain morphology, which are associated with a Ge-Se growth mechanism, and gains in both the VOC and conversion efficiency. We use surface energy-filtered photoelectron emission microscopy (EF-PEEM) to map the surface work function terrains and show an improved electronic landscape, which we attribute to a reduction in the segregation of low local effective work function (LEWF) Sn(II) chalcogenide phases.
Rafael Jaramillo opened discussion of the introductory lecture by David Mitzi: What is your most trusted approach to accurately measure the absorption coefficient on a new thin film material? David Mitzi replied: The point about significant challenges in making and evaluating rel
Traditional photovoltaics (PV) has made excellent progress over the last decade with decreasing installation costs and a levelised cost of electricity that is competitve with fossil fuel based sources of electricity. This has been achieved using economies of scale manufacturing at a relatively small number of mega-factories that are mostly based in a single geographical region. One consequence of this approach to PV manufacturing is that PV modules are highly standardised and the ubiquitous and low-cost deployment of PV on any surface is still not a commercial reality. There is therefore clear scope to perform the manufacturing research that enables this goal. In this work, slot-die deposition of an inorganic nanoparticle ink is used to create proof-of-concept photovoltaic devices in any geometry and pattern using a low-cost masking technique. The results are discussed in the context of a new design-led approach to photovoltaics manufacturing that has the potential to provide complementary new global electricity capacity for distributed applications in the built-environment.
For successful long-term deployment and operation of kesterites Cu2ZnSn(SxSe1–x)4 (CZTSSe) as light-absorber materials for photovoltaics, device stability and recovery in kesterite solar cells are investigated. A low-temperature heat treatment is applied to overcome the poor charge extraction that developed in the natural aging process. It is suggested that defect states at aged CZTSSe/CdS heterojunctions were reduced, while apparent doping density in the CZTSSe absorber increased due to Cd/Zn interdiffusion at the heterojunction during the annealing process. In situ annealing experiments in a transmission electron microscope were used to investigate the elemental diffusion at the CZTSSe/CdS heterojunction. This study reveals the critical role of heat treatment to enhance the absorber/Mo back contact, improve the quality of the absorber/buffer heterojunction, and recover the device performance in aged kesterite thin-film solar cells.
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