The article "Device Physics of Polymer: Fullerene Bulk Heterojunction Solar Cells" by Blom et al. reviews the fundamental processes and limitations governing the operation of polymer-fullerene bulk heterojunction (BHJ) solar cells. The key aspects discussed include: 1. **Exciton Creation and Diffusion**: The creation of excitons in conjugated polymers and their diffusion lengths are crucial for efficient charge generation. Exciton diffusion lengths are typically 5-20 nm, and their effective width in BHJ devices is limited by the exciton diffusion length. 2. **Dissociation of Charge Carriers**: The dissociation of bound electron-hole pairs at the donor/acceptor interface is essential for charge separation. This process is influenced by the strength of the electric field and the temperature, with strong fields required to dissociate excitons. 3. **Charge Transport in Blends**: The mobility of electrons and holes in polymer-fullerene blends is a critical factor. While electron mobilities are generally higher than hole mobilities, blending with fullerene can enhance hole mobility, leading to more balanced charge transport. 4. **Extraction of Charge Carriers at Electrodes**: The work function of the electrodes affects the open-circuit voltage (VOC) and the efficiency of the solar cells. The VOC is strongly coupled to the reduction potential of the fullerene acceptor, and the choice of electrode material can significantly impact performance. The article provides a comprehensive overview of the device physics, highlighting the importance of optimizing the morphology, composition, and electrode design to enhance the efficiency of polymer-fullerene BHJ solar cells.The article "Device Physics of Polymer: Fullerene Bulk Heterojunction Solar Cells" by Blom et al. reviews the fundamental processes and limitations governing the operation of polymer-fullerene bulk heterojunction (BHJ) solar cells. The key aspects discussed include: 1. **Exciton Creation and Diffusion**: The creation of excitons in conjugated polymers and their diffusion lengths are crucial for efficient charge generation. Exciton diffusion lengths are typically 5-20 nm, and their effective width in BHJ devices is limited by the exciton diffusion length. 2. **Dissociation of Charge Carriers**: The dissociation of bound electron-hole pairs at the donor/acceptor interface is essential for charge separation. This process is influenced by the strength of the electric field and the temperature, with strong fields required to dissociate excitons. 3. **Charge Transport in Blends**: The mobility of electrons and holes in polymer-fullerene blends is a critical factor. While electron mobilities are generally higher than hole mobilities, blending with fullerene can enhance hole mobility, leading to more balanced charge transport. 4. **Extraction of Charge Carriers at Electrodes**: The work function of the electrodes affects the open-circuit voltage (VOC) and the efficiency of the solar cells. The VOC is strongly coupled to the reduction potential of the fullerene acceptor, and the choice of electrode material can significantly impact performance. The article provides a comprehensive overview of the device physics, highlighting the importance of optimizing the morphology, composition, and electrode design to enhance the efficiency of polymer-fullerene BHJ solar cells.