In this application the layer is used to collect current from the PV cells (and thus needs high conductivity) but must also have high transparency to avoid light energy loses.
TCO Process Requirements
a. Film growth rates : Glass line based PV TCO films, whilst related to Low E TCOs, (for example both are based on F.Sn02) have some significant differences. Most obviously, the films are thicker. Low E TCOs are typically between 250 and 350nms. Typical PV TCOs requirements lead to film thicknesses of 500-800nms. Additionally, PV glass substrates tend to be thinner than Low E requirements (which are most commonly 4 or 6mm). This leads to potential for faster glass line speeds (and thus cost savings) but a further challenge for growth rates.
b. PV Process compatibility : The glass and CVD layer stack forms the platform for the subsequent PV processing. The CVD layers need to withstand downstream process conditions which can (depending on PV absorber technology and process employed) involve reactive plasmas, vacuum, thermal load (up to 400C or more)
c. Long term stability: All PV cells need to meet tests which simulate 20 year lifetimes. These normally include UV, humidity and temperature cycling for thousands of hours. CVD films (particularly bath based) perform well on such test.
d. Process Flexibility: Dedicated PV CVD lines, may need to capability to adapt to new products (e.g. to tune the stack to the developing PV technology. A further requirement may be in additional (PV relevant) layers such as Anti-reflection or self/easy clean. All bath based technology is the most flexible CVD process technology.
e. Cost targets: Significant progress has been made in recent years, in reducing PV cell costs. A key medium term objective for PV technology is to meet grid parity. To achieve this every aspect of the process must be targeted. Dedicated PV TCO Glass lines will make a significant contribution to this objective
APCVD is successfully used for Industrial scale production of coated glasses as substrates for subsequent PV layer growth. It has also been used for coating on metal foils for the same subsequent processing. CVD Technologies technology has licensed it's technology for this application to a number of companies.
Such coating technology can be exploited either on-line (e.g. on the glass production line, or on metal roll to roll processing) or off line on glass plates or silicon wafers.
Reel to reel manufacturing of PV cells incorporating CVD Technologies technology is currently in production operation. Dedicated Float Glass lines with integrated CVD capability, are now being developed to meet the rapid growth forecast for thin film PV manufacturing in the coming years, Such lines give rise to many new technology and operational challenges. Advanced CVD technology is essential to meet the performances, throughput and cost targets sought. All in-Bath based coating technology has many advantages. CVD Technologies and Stewart Engineers have combined their capabilities to offer AcuraCoat on-line CVD systems to meet this requirement.