Sodium is a key dopant in thin film photovoltaic cells with reported benefits including promotion of grain growth, passivation of grain boundaries and increased carrier concentration in chalcopyrite and kesterite based solar cells. Research-grade devices fabricated in substrate configuration often rely on diffusion of Na from a soda lime glass substrate into the photovoltaic absorber layer during high temperature processing. However, for samples on flexible substrates such as foils and plastics, this is not available and requires alternative approaches. In this work, we fabricate Earth-abundant Cu 2 ZnSn(S,Se) 4 thin film solar cells from nanoparticle inks on flexible molybdenum substrates and demonstrate a simple, low-cost route to incorporating Na in solution thereby making it compatible with large area, high volume manufacturing. The technique is verified to improve the device efficiency relative to a reference flexible device built on molybdenum foil.
For successful long-term
deployment and operation of kesterites
Cu
2
ZnSn(S
x
Se
1–
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.
Ge doping is employed to aid the recrystallisation of Cu2ZnSnS4 nanocrystals. Opto-electrical properties are presented to describe Ge incorporation with a focus on the electronic interface between the Ge:Cu2ZnSn(S, Se), absorber and CdS buffer layer.
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