Organic solar cells (OSCs) have been extensively studied and significant improvements have been demonstrated in
recent years. Along with the excitement in technology development, the accurate measurement of OSCs has become
critical for the healthy development of this promising technology. The limited absorption and spectral response of
organic based solar cells could lead to significant derivation in solar cell measurement. In this paper, we will discuss
several issues in the measurement of organic solar cells, including spectral mismatch factor, elimination of the
mismatch by proper selection of reference cell, external quantum efficiency testing, device area issue etc. Results on
both polymer based bulk hetero-junction solar cell and small molecule based solar cell will be presented.
To increase the absorption of sunlight in polymer solar cells a large active layer thickness is desired. This, however, is
limited by the short charge carrier diffusion lengths in the active organic materials. Efficient light harvesting can be
achieved in organic solar cells by using a tandem structure. However, fabricating a tandem structure for polymer solar
cells presents its own difficulties. Since the polymer film is solution processed, spin-coating multiple layers in tandem
can result in significant damage to the underlying layers. This problem can be overcome by fabricating separate PV cells
and stacking them in tandem. Here, we report a multiple-device stacked structure where two polymer photovoltaic cells
are stacked together with the help of a multi-layer semi-transparent electrode, made of lithium fluoride (LiF) / aluminum
(Al) / gold (Au) metal layers. The semi-transparent electrode is used as the top contact in the bottom cell to efficiently
transmit the unabsorbed photons to the upper cell. Maximum transparency of up to 80% is achieved for the semitransparent
cathode. In the stacked structure, the open circuit voltage and the short circuit current are twice those of a
single cell. As a result, power conversion efficiency of up to 2.6% is achieved, which is double than that of a single cell.
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