Thermodynamic principles limit the conversion efficiency of a single bandgap organic photovoltaic (OPV) cell to 33%1 . In order to increase efficiency, multiple OPV devices can be combined to cover a larger spectral range of the incident solar spectrum. The most common way of doing this is to mount multiple bandgap cells in tandem or series. However, stacked multijunction systems have limitations, such as current-matching constraints and optical quality of the OPV layer. A separated arrangement with spectrum splitting is a promising alternative to the stacked tandem approach. In this paper, two organic photovoltaic cells with complementary EQE curves are integrated into a holographic spectrum splitting module. The highest possible conversion efficiency of this two-cell combination is quantified assuming an ideal spectral filter as a reference. A spectrum splitting module is built, consisting of a reflective hologram oriented at an angle to split the incident beam into two spectral bands. The holographic beamsplitting system is assembled and studied under a solar simulator. Power output and conversion efficiency of the holographic spectrum splitting system is evaluated in terms of Improvement over Best Bandgap (IoBB) of the two-cell combination. The combined system has a measured improvement over its best single cell of 12.30% under a solar simulator lamp and a predicted improvement of 16.39% under sunlight.
A design is presented for a planar spectrum-splitting photovoltaic (PV) module using Holographic Optical Elements (HOEs). A repeating array of HOEs diffracts portions of the solar spectrum onto different PV materials arranged in alternating strips. Several combinations of candidate PV materials are explored, and theoretical power conversion efficiency is quantified and compared for each case. The holograms are recorded in dichromated gelatin (DCG) film, an inexpensive material which is easily encapsulated directly into the panel. If desired, the holograms can focus the light to achieve concentration. The side-by-side split spectrum layout has advantages compared to a stacked tandem cell approach: since the cells are electrically isolated, current matching constraints are eliminated. Combinations of dissimilar types of cells are also possible: including crystalline, thin film, and organic PV cells. Configurations which yield significant efficiency gain using relatively inexpensive PV materials are of particular interest. A method used to optimize HOE design to work with a different candidate cells and different package aspect ratios is developed and presented. (Aspect ratio is width of the cell strips vs. the thickness of the panel) The relationship between aspect ratio and HOE performance properties is demonstrated. These properties include diffraction efficiency, spectral selectivity, tracking alignment sensitivity, and uniformity of cell illumination.