Modelling MEH-PPV:PCBM (1:4) bulk heterojunction solar cells

Basic solar cell characteristics were examined in an organic bulk heterojunction device. The active layer is a spincoated organic blend of a pmaterial (MEH-PPV) and an n-material (the fullerene derivative PCBM), sandwiched between a transparent ITO-PEDOT/PSS electrode and an Al/LiF back-contact. We carried out light and dark I-V and spectral response measurements and measured the transparency of the active layer. Modelling of organic bulk-heterojunction solar cells is difficult because of the complicated geometry and because of the microscopic scale of the internal mechanisms. We applied an effective medium model in which the whole p-n nanostructure is represented by one single effective semiconductor layer, which then is fed into a standard solar cell device simulator, e.g. SCAPS. First, a parameter set was built up that simulates the measured characteristics of the MEH-PPV / PCBM blend cells. The dark and illuminated I-V, and the spectral response measurements are fitted fairly well. A parameter set for the effective medium model was sought and found that describes (with qualitative agreement) the measured I-V curves. This parameter was then used for a numerical parameter study. Critical issues for cell performance were identified, and their influence quantified (based on the actual parameter set for MEHPPV-PCBM cells). First, the cell performance is sensitive to the electron and hole mobilities in the materials: the efficiency increases monotonously with increasing mobility, over many decades. The sensitivity is about 25 % relative efficiency gain for an order of magnitude of increase of the mobilities. Second, the poor absorption of the solar illumination is a cause of weak performance. We investigated the influence on cell thickness, and of the absorption characteristic.