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 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
Solar energy is often hailed as the successor to fossil fuels as the planet’s main source of energy. However, solar cells face various issues affecting widespread adoption including prohibitive costs and low energy conversion rates. Currently, multijunction solar cells are the most efficient. These cells are able to boost conversion rates by employing different junctions of semiconductors that utilize different wavelengths of light. The most commonly used semiconductors belong to the III-V group due to perceived advantages over II-VI group semiconductors. Garland et al (2011) argue that II-VI semiconductors are both more efficient and less expensive than III-V semiconductors. Results from models and initial experimentation indicate that II-VI solar cells are 3–4% more efficient than III-V solar cells.—Alan Hu
Garland and colleagues of EPIR Technologies project III-V and II-VI solar cell output with the commonly accepted “standard” model put forth by Xu et al. (2010) in an earlier paper. The model uses a beta coefficient that takes such factors into account as the dimensions and doping (the molecular makeup) of the semiconductor. The beta is calculated through a best fit line describing modeled data and figures from real world performance of the latest III-V solar cell. Projected efficiency is then computed by dividing the sum of the junction outputs by the power input. The researchers supplement their projected figures with real world figures generated through actual experimentation. Garland and colleagues also grew II-VI semiconductors and collected empirical data on the solar cells.
The results from both projections and empirical observation supported the argument in favor of II-VI solar cells. Calculated efficiency for a III-V solar cell under one sun, a measure of sun intensity, was 43.7% whereas the figure for a II-VI solar cell was 49.7%. Empirical observations agree with these results: III-V solar cells were observed to have achieved 38.6% under one sun whereas II-VI solar cells achieved 44.5% under the same conditions.
The study also argues that II-VI solar cells could bring about significant reductions in manufacturing cost. The authors claim that due to the sturdy nature of silicon wafers used in the production of II-VI solar cells and lower costs of growing II-VI crystals, nearly all associated costs of creating II-VI semiconductors are lower than those of creating III-V semiconductors. Specifically, molecular beam epitaxy (MBE), which is a process for growing crystals, can be replaced by a cheaper production line method of production due to the nature of II-VI materials.
The lower cost of II-VI solar cells means that medium concentration photovoltaics can be used instead of high concentration photovoltaics. III-V solar cells were relatively costly; as such, it was cheaper to have fewer solar cells and instead have a system of mirrors that concentrated sunlight onto a small area of solar cells. This meant the costs of a solar energy field were increased by the installation of such tracking systems. The cheaper II-VI solar cells allow for a relatively larger area of solar cells and less complicated tracking systems.
The search for a commercially viable alternative to fossil fuels continues. As long as solar cells are more expensive and less effective than existing energy sources, the widespread adoption of solar energy is unlikely. However, incremental advances in photovoltaic technology are gradually cheapening the cost of solar energy. The improvement of 3–4% in energy conversion rates brought about by the use of II-VI semiconductors is one small step toward a greener future.
D. Xu, T. Biegala, M. Carmody, J. W. Garland, C. Grein, and S. Sivananthan. Proposed monolithic triple-junction solar cell structures with the potential for ultrahigh efficiencies using II–VI alloys and silicon substrate. Applied Physics Letters. 96, 073508(2010).