Organic photovoltaics (OPVs) operate by separating two electrodes with a layer of organic polymeric material that generates a current between the two ends of the diode. Initial designs of OPVs used a single layer of organic material that, when struck by sunlight, generates excitons due to electrons vacating the Highest Occupied Molecular Orbital (HOMO) and filling the Lowest Unoccupied Molecular Orbital (LUMO). Due to the existing difference of charge between the two ends of the circuit, the excitons flow to the positive electrode, establishing a current. This design, however, yielded extremely low power conversion rates due to the weakness of the electric field established by the electrodes. Bilayer OPVs, which had electron donor and acceptor layers of organic material, were subsequently developed to address this issue. The two layers of the bilayer OPV generated themselves electrostatic forces that helped generate a stronger electric field. Bilayer OPVs still yielded low power conversion rates, however, due to the discrepancy between the diffusion length of excitons and the length of the polymer layer required to absorb enough sunlight. Bulk heterojunction (BHJ) cells combine the donor and acceptor layer of the bilayer OPV into a single bulk layer. This design solves the issue of matching exciton diffusion length with the length of the polymer layer and has so far yielded the highest power conversion rates of any OPV. An central question in growing BHJ cells is the question of what acceptor/donor ratio is optimal for achieving higher conversion rates. Zhang et al. (2011) have demonstrated through their experimentation that low donor concentration (under 10%) and the presence of molybdenum oxide (MoOx) can yield relatively high conversion ratios across a diversity of donor materials.—Alan Hu
Zhang, M., Wang H., Tian H., Geng Y., Tang C. 2011. Bulk Heterojunction Photovoltaic Cells with Low Donor Concentration. Advanced Materials 23, 4960–4964.
Zhang et al. at the University of Rochester and the Changchun Institute of Applied Chemistry grew cells composed of a sequence of indium tin oxide (ITO), molybdenum oxide (MoOx), TAPC:C60/C70, bathophenanthroline (Bphen), and aluminum (Al). As the researchers were primarily interested in measuring the effect of the BHJ layer TAPC:C, the dimensions and composition of the other layers were kept constant: ITO (90nm), MoOx (2nm), Bphen (8nm), Al (100nm). Researchers then modeled and tested the cells for indicators including the short circuit density, voltage, fill factor, power conversion efficiency, calculated hole mobility, and calculated electron mobility. Hole and electron mobility, the speed at which an electron can move through a semiconductor, were calculated with Bässler’s model.
The experiments yielded results that indicated a low concentration of donors was optimal. Voc increased as donor concentration decreased until 5%, below which Voc began to drop quickly. The same was found to be true for Jsc, which increased until donor concentration dropped below 5%. Similarly, the fill factor peaked at 0.53, which occured when donor concentration was between 5% and 10%. High donor concentrations, on the other hand, yielded poorer results: when TAPC concentration was 50%, fill factor was 0.29. External quantum efficiency (EQE) also peaked with a low TAPC concentration. At TAPC 5%, EQE was about 60% while at TAPC 50%, EQE was only 10%.
Researchers also found that the commonly used HOMO/LUMO gap rule did not directly affect Voc as long as donor concentrations were sufficiently low. Instead, the Voc was largely determined by the Schottky barrier, a difference in charge formed between a metal and semiconductor, created between the MoOx and BHJ layers.
Zhang et al. (2011) achieved an impressive conversion ratio of 5.23% in their tests. They concluded that a low donor concentration maximized efficiency in BHJ OPVs and recognized the importance of the MoOx for generating the electric field