Optimization of polymer photovoltaic cells with bulk heterojunction layers hundreds of nanometers thick: modifying the morphology and cathode interface

In polymer photovoltaic (PV) cell, it is desirable to use a relatively thick polymer semiconductor film in order to maximize the light absorption, and to achieve better controllability and reproducibility of the film in manufacturing processes. However, the low fill factor due to restricted charge transport and extraction at large film thickness serially limits the performance of the polymer PV cell. In this work, we investigate the factors that can impact the device performances as film thickness is increased. We also introduce ways to help alleviate these problems in thick BHJ PVs. Our measurement results, based on the space-charge limited-current (SCLC) model and the photo-induced carrier extraction by linearly increasing voltage (photo-CELIV) method, show that the thicker BHJ devices have relatively low electron mobility compared with hole mobility, which directly correlates with high contact resistance at the top cathode interface that prevents efficient transport of photo-generated electrons. Specifically, we found that the newly introduced ESSENCIAL fabrication process helps improve the blend donor and acceptor domain morphologies; and adding an ultrathin C60 layer at the cathode interface helps improve the surface morphology and significantly reduce the contact resistance. The effects of the added thin C60 layer on PV cells were further studied by examining several important diode characteristics. Our results proved that this layer not only decreases the contact resistance at the cathode but also improves the hole-blocking, thereby providing significantly suppressed recombination at the cathode interface. Consequently, the fabricated PV devices optimized in morphology and interface show significantly improved internal quantum efficiency (IQE) as compared with the thermally annealed conventional PV cells, leading to 5.11% PCE from a P3HT:PCBM blend system. The modifications to the fabrication of BHJ PV cells described in this work allow for photoactive layers to be hundreds of nanometers thick for efficient light absorption and better controllability.