Alternative buffer layer development in Cu(In,Ga)Se2 thin film solar cells
Date
2017
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Cu(In,Ga)Se2-based thin film solar cells are considered to be one of the most
promising photovoltaic technologies. Cu(In,Ga)Se2 (CIGS) solar devices have the
potential advantage of low-cost, fast fabrication by using semiconductor layers of only
a few micrometers thick and high efficiency photovoltaics have been reported at both
the cell and the module levels. CdS via chemical bath deposition (CBD) has been the
most widely used buffer option to form the critical junction in CIGS-based thin film
photovoltaic devices. However, the disadvantages of CdS can’t be ignored -
regulations on cadmium usage are getting stricter primarily due to its toxicity and
environmental impacts, and the proper handling of the large amount of toxic chemical
bath waste is a massive and expensive task. ☐ This dissertation is devoted to the development of Cd-free alternative buffer
layers in CIGS-based thin film solar cells. Based on the considerations of buffer layer
selection criteria and extensive literature review, Zn-compound buffer materials are
chosen as the primary investigation candidates. Radio frequency magnetron sputtering
is the preferred buffer deposition approach since it’s a clean and more controllable
technique compared to CBD, and is readily scaled to large area manufacturing. ☐ First, a comprehensive study of the ZnSe1-xOx compound prepared by reactive
sputtering was completed. As the oxygen content in the reactive sputtering gas
increased, ZnSe1-xOx crystallinity and bandgap decreased. It’s observed that oxygen
miscibility in ZnSe was low and a secondary phase formed when the O2 / (O2 + Ar)
ratio in the sputtering gas exceeded 2%. Two approaches were proposed to optimize
the band alignment between the CIGS and buffer layer. One method focused on the
bandgap engineering of the absorber, the other focused on the band structure
modification of the buffer. As a result, improved current of the solar cell was achieved
although a carrier transport barrier at the junction interface still limited the device
performance. ☐ Second, an investigation of Zn(S,O) buffer layers was completed. Zn(S,O)
films were sputtered in Ar using a ZnO0.7S0.3 compound target. Zn(S,O) films had the
composition close to the target with S / (S+O) ratio around 0.3. Zn(S,O) films showed
the wurtzite structure with the bandgap about 3.2eV. The champion Cu(In,Ga)Se2 /
Zn(S,O) cell had 12.5% efficiency and an (Ag,Cu)(In,Ga)Se2 / Zn(S,O) cell achieved
13.2% efficiency. Detailed device analysis was used to study the Cu(In,Ga)Se2 and
(Ag,Cu)(In,Ga)Se2 absorbers, the influence of absorber surface treatments, the effects
of device treatments, the sputtering damage and the Na concentration in the absorber. ☐ Finally alternative buffer layer development was applied to an innovative
superstrate CIGS configuration. The superstrate structure has potential benefits of
improved window layer properties, cost reduction, and the possibility to implement
back reflector engineering techniques. The application of three buffer layer options –
CdS, ZnO and ZnSe was studied and limitations of each were characterized. The best
device achieved 8.6% efficiency with a ZnO buffer. GaxOy formation at the junction
interface was the main limiting factor of this device performance. For CdS / CIGS and
ZnSe / CIGS superstrate devices extensive inter-diffusion between the absorber and
buffer layer under CIGS growth conditions was the critical problem. Inter-diffusion
severely deteriorated the junction quality and led to poorly behaved devices, despite
different efforts to optimize the fabrication process.