Organic Semiconductor Solar Cell Simulation

Organic Semiconductor Solar Cells are a promising alternative for an inexpensive and light weight replacement of dominating the market Silicon based Solar Cells. A number of publications comparing existing currently technologies are available (see recent IEEE Spetrum publication and references therein) reporting cell with up to 6.1 percent energy-conversion efficiency. There is still no simulator that would allow to accurately simulate Organic Semiconductor Solar Cells.

Proposed Organic Semiconductor Solar Cell Simulation Technology

MicroTec is currently used by a few solar cell companies/research centers (see below) for practical crystalline and multi crystalline silicon solar cell design and research. Most (if not all) of the conventional simulators deal with charge carriers directly produced by light and therefore are not directly applicable to Organic Semiconductor Solar Cells (OSSC) and require significant modifications.

Modifications Required for Organic Semiconuctor Solar Cell Simulation

In conventional solar cells photogeneration creates electron/hole pairs in contrast to Organic Semiconductor Solar Cell (e.g. CuPC/PV) structures where excitons are being generated. These excitons then diffuse to a region with a proper energy spectrum that causes their dissociation. Usually dissociation occurs at a heterojunction or at a contact. In order to be able to simulate such devices new species must be accounted for in the simulation such as excitons. Although primarily singlet exciton state is considered due to its higher diffusion coefficient, there is a possibility that in some instances triplet states also could be important due to their higher life-time values. Dissociation rate in heterostructure solar cells will be spatially dependent and needs to be characterized using experimental data and numerical simulation.

As a result of the exciton dissociation, polarons are produced in Organic Semiconductor Solar Cells (self-trapped electrons and holes) that create electrical current collected by electrodes. Electrical transport in Organic Semiconductor Solar Cells is usually related to hopping conductance or tunneling but it can still be characterized by effective mobility depending on the electric field and temperature, although this dependence is quite different from the mobility in conventional semiconductors and is a few orders of magnitude lower in value.

Another significant difference that should be taken into account in Organic Semiconductor Solar Cell Simulation is the effective density of states which are normally considered as a narrow Gaussian spectra located close to the valence and conductance band edges. It is normally treated numerically as a single energy level. Typically Boltzmann statistics is used when simulating Organic Semiconductor Solar Cells.

For Organic Semiconductor Solar Cells optical optimization of the layer structure is very important due to rather small layer thickness (1 to 100 nm range). Calculation of the light intensity taking into account interference of all layers is crucial to provide realistic cell efficiency.

In the current proposal we suggest implementing all the required modifications listed above and implement a one-dimensional simulator for Organic Semiconductor Solar Cells. As a starting point we used our previously developed software tool CharSim [1] for simulation of dynamic behavior of charged defects in a semiconductor under irradiation. Although this system looks very different from what we have in OSSC, the system of equations solved is quite similar and requires only modifications in models of physical parameters.

[1] O.V.Bobrikova, M.S.Obrecht, V.F.Stas', “Charge states of primary radiation defects and the defect formation processes in the space charge region of the silicon diode structures”, Soviet Physics: Semiconductors, v.25, N 5, pp.501-507.

Current Status of Organic Semiconductor Solar Cell Simulation

Currently MicroTec is used by the following companies for Simulation and Design of crystalline and multi crystalline Solar Cells: Advent Solar, New Mexico, USA, Centrotherm, Germany, Ultradots, California, USA, and Energy Centre Netherlands, the Hague, Netherlands.

Challenges to Be Overcome for Organic Semiconductor Solar Cell Simulation

We have already resolved most, if not all, challenges in numerical solution of the required equations. Challenges we still have to overcome are related to the models of physical parameters for Organic Semiconductor Solar Cells such as mobilities, life-times, effective density of states, dissociation rates, cross-sections for trapping and recombination, optical parameters and contact characteristics. Models published in the literature are very simple and may not be suitable for Organic Semiconductor Solar Cell Simulation; therefore additional characterization work is a crucial part of this project.

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