  Historically, the design of hybrid solar photovoltaic thermal (PVT) systems
has focused on cooling crystalline silicon (c-Si)-based photovoltaic (PV)
devices to avoid temperature-related losses. This approach neglects the
associated performance losses in the thermal system and leads to a decrease in
the overall exergy of the system. Consequently, this paper explores the use of
hydrogenated amorphous silicon (a-Si:H) as an absorber material for PVT in an
effort to maintain higher and more favourable operating temperatures for the
thermal system. Amorphous silicon not only has a smaller temperature
coefficient than c-Si, but also can display improved PV performance over
extended periods of higher temperatures by annealing out defect states from the
Staebler-Wronski effect. In order to determine the potential improvements in
a-Si:H PV performance associated with increased thicknesses of the i-layers
made possible by higher operating temperatures, a-Si:H PV cells were tested
under 1 sun illumination (AM1.5) at temperatures of 25oC (STC), 50oC
(representative PV operating conditions), and 90 oC (representative PVT
operating conditions). PV cells with an i-layer thicknesses of 420, 630 and 840
nm were evaluated at each temperature. Results show that operating a-Si:H-based
PV at 90 oC, with thicker i-layers than the cells currently used in commercial
production, provided a greater power output compared to the thinner cells
operating at either PV or PVT operating temperatures. These results indicate
that incorporating a-Si:H as the absorber material in a PVT system can improve
the thermal performance, while simultaneously improving the electrical
performance of a-Si:H-based PV.
