The emerging field of stacked layers (double- and even multi-layers) in organic photovoltaic cells is reviewed. Owing to the limited absorption width of organic molecules and polymers, only a small fraction of the solar flux can be harvested by a single-layer bulk heterojunction photovoltaic cell. Furthermore, the low charge-carrier mobilities of most organic materials limit the thickness of the active layer. Consequently, only part of the intensity of the incident light at the absorption maximum is absorbed. A tandem or multi-junction solar cell, consisting of multiple layers each with their specific absorption maximum and width, can overcome these limitations and can cover a larger part of the solar flux. In addition, tandem or multi-junction solar cells offer the distinct advantage that photon energy is used more efficiently, because the voltage at which charges are collected in each sub-cell is closer to the energy of the photons absorbed in that cell. Recent developments in both small-molecule and polymeric photovoltaic cells are discussed, and examples of photovoltaic architectures, geometries, and materials combinations that result in tandem and multi-junction solar cells are presented.
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