Device-level Nanomorphology Evolution and Evaluation of P3HT:PCBM Bulk Heterojunction Polymer Solar Cells

In all our CGMD simulations of the BHJ nanomorphologies above the simulation cells were periodic in all the dimensions of the simulation box, which is good for modeling nanomorphology evolution in the bulk. However, in real BHJ OPV devices the BHJ layer is sandwiched by the cathode and anode (see schematic device architecture displayed in Fig. ), which may lead to donor/acceptor phase separation in both vertical and lateral dimensions. In this study a set of CG models including the anode (PEDOT:PSS)/BHJ (P3HT:PCBM)/cathode (Aluminum) layers was developed, see the regions enclosed in blue-dashed boxes in Fig. . With such CG model, we were able to simulate both vertical and lateral P3HT/PCBM phase separation under different thermal annealing sequences. In addition. we also investigated the dependence of device architecture (standard vs. inverted architectures) on the BHJ nanomorphologies. The P3HT:PCBM film morphologies of standard and inverted architectures from our CGMD simulations were displayed in the left and right panels of the region enclosed in yellow-dashed box in Fig. , respectively. Note that the BHJ film thickness was around 110 nm, which is compatible with those in most experiments. The two rightmost panels of Fig.  display the vertical P3HT concentration profiles of standard and inverted architectures. To evaluate the BHJ morphologies more accurately, we collaborated with Prof. Baskar Ganapathysubramanian at Iowa State University to analyze the BHJ nanomorphologies using the multi-resolution graph-based morphology analysis framework his group developed. From extensive morphology analysis, we found that the P3HT-depletion region next to the P3HT-rich region close to the PEDOT:PSS anode does not affect the overall P3HT domain connectivity; however, the existence of depletion region did lead to longer hole transport pathway due to reduction of available transport paths, thereby increasing the possibilities of recombinations. Our CGMD simulations also revealed the formation of P3HT-rich region at the free surface during thermal annealing regardless of bottom electrode materials, which is in excellent agreements with experimental observations. This paper has been published in the ACS Applied Materials [IMAGE png] Interfaces in 2014.

Figure: CG models of all components in the BHJ devices (regions enclosed in blue-dashed boxes); morphologies of the anode/BHJ/cathode after CGMD thermal annealing simulations (regions enclosed in yellow-dashed box, left panel: standard architecture, right panel: inverted architecture); rightmost two panels: vertical P3HT concentration profiles of both standard and inverted architecture.