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Timestamp: 2019-04-21 17:10:48+00:00

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The efficiency of today’s most efficient organic solar cells is primarily limited by the ability of the active layer to absorb all the sunlight. While internal quantum efficiencies exceeding 90% are common, the external quantum efficiency rarely exceeds 70%. Light trapping techniques that increase the ability of a given active layer to absorb light are common in inorganic solar cells but have only been applied to organic solar cells with limited success. Here, we analyze the light trapping mechanism for a cell with a V-shape substrate configuration and demonstrate significantly improved photon absorption in an 5.3%-efficient PCDTBT:PC70BM bulk heterojunction polymer solar cell. The measured short circuit current density improves by 29%, in agreement with model predictions, and the power conversion efficiency increases to 7.2%, a 35% improvement over the performance in the absence of a light trap.
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Fig. 1 Geometry of V-shaped geometric light trap for an organic solar cell for specified folding angles (planar / 60 degrees / 30 degrees) The rays schematically show the benefits of folding the solar cell structure on the number of bounces incident sunlight makes and thus the light absorption in the cell. Jsc is defined as the photocurrent generated inside the V-trap divided by aperature size (0.15 cm2). (b) Photon absorption in the active layer for a specified folding angle (planar / 60 degrees / 30 degrees) We note that the geometric distance of the cell (x axis) is normalized to the projected distance (aperature distance) of a V-trap. (c) Fabrication steps for V-shaped PCDTBT:PC70BM OPV used on the presented experiments.
Fig. 2 Absorption quantum efficiency for a specified folding angle. (a) Planar (b) 60 degrees (c) 30 degrees.
Fig. 3 Short circuit current as a function of a V-folding angle. Blue line indicates theoretical results and red dots show the experimental result.
Fig. 4 (a) Open circuit voltage (Red dot) and Fill factor (blue dot) measured separately at each side, and total short circuit current (green line) as a function of the folding angle for the V-shape configuration. (b) Power conversion efficiency as a function of a folding angle of the V-shape configuration.
Fig. 5 Effect of the incident angle of the illumination (θi) on the short circuit current density (Jsc). Blue line shows the current generation at 30 degrees of the V-shaped cell and the red line shows the same for a planar cell.

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