Enhanced Open‐Circuit Voltage in High Performance Polymer/Fullerene Bulk‐Heterojunction Solar Cells by Cathode Modification with a C60 Surfactant

Polymer solar cells (PSCs) represent a unique alternative renewable energy source combining the potential benefits of comparatively low-cost fabrication, solution processing, and wide application through the versatility of device design.[1,2] The advantages demonstrated by PSCs over their inorganic counterparts have generated extensive research efforts over the past decade, which have resulted in an impressive body of literature detailing vast improvements in device performance made through molecular design[3–7] and process and device engineering.[3,8–10] Power conversion efficiencies (PCEs) over 7% have recently been reported for bulk-heterojunction (BHJ) type PSCs due to significant strides in polymer design and morphological control of the photoactive blend.[7] Despite these promising advances, there remain a variety of technical issues to be addressed in order for PSCs to mature. The most obvious of these issues is the significantly lower photovoltaic performance as compared to traditional siliconbased and dye-sensitized solar cells. One of the most critical factors determining device performance is the nature of the active layer/electrode interface, which has particularly important implications for the open-circuit voltage (VOC). In contrast to bilayer solar cells, the photoactive layer of BHJ PSCs typically consists of intimately mixed domains of p-type semiconducting polymer donor and n-type fullerene acceptor such that a bicontinuous pathway for charge transport through the layer to the electrodes is achieved. In the ideal case, the polymer and fullerene domains are sufficiently phase-segregated along the vertical charge-carrier transport axis which results in having only electron-(hole-)transporting material at the cathode (anode).[12] Due to the inherently complicated drying dynamics of the photoactive blend layer upon spincoating, this is often not the case,[13,14] and the device performance suffers accordingly via charge recombination at the electrode interfaces. In addition, proper energy level alignment at these interfaces is critical to reduce contact resistance and ensure a maximum VOC and optimized device performance. To this end, several

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