Effects Of Recombination On The Performance Of Organic Solar Cells

The renewable energy resources will soon force their way into the presently usedpower plants as today’s main energy sources will be scarce in the near future. Becauseof low-cost fabrication of large areas and the possibility to use flexible substrates,organic solar cells have been increasing rapidly for recent years. The bulkheterojunction (BHJ) organic solar cells have attracted much attention due to itstechnological application. It clarifies which operating mechanisms govern the solar celloverall performance, and establishes the achievable values for short-circuit current,open-circuit voltage and fill factor. This thesis is based on BHJ organic solar cells, andPoisson and continuity equations can be solved iteratively by a numerical schemeproposed by Gummel and Scharfetter:1. A simulation based on charge transfer state recombination of BHJ organic solarcells, OC1C10-PPV/PCBM, are given. The distribution of potential, current density,carrier density, net generation rate, electric field, and energy level on short circuit andopen circuit are simulated. All electrical parameters restrict each other.2. Recombination in BHJ organic solar cells is the key loss mechanism, and itdirectly affects characteristic parameters such as power conversion efficiency,short-circuit current, open-circuit voltage, and fill factor. However, whichrecombination mechanism dominates the loss in organic materials is under discussion.In this work, we simulate state-of-art BHJ solar cells using five different models forinfinite surface recombination velocity, i.e., constant recombination, Langevinrecombination, charge transfer state recombination, trap-assisted recombination, andrecombination via tail. All processes are strongly dependent on charge carrier mobilityand exhibit a similar recombination distribution in active layer. For higher mobility, allmodels present the same behavior along with the increased mobility, whereas, there areslight differences in open-circuit voltage between trap/tail model and other ones atlower mobility, resulting from the interaction between photo-carriers and dark-carriers.