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Exciton Dissociation at Metal-Organic Interface and The induced Surface Loss in BHJ Solar Cells Wenchao Yang, Deli Li and Chang-Qin Wu Department of Physics, Fudan University, Shanghai, 200433, PRC The pursuit of new energy resources that is clean and renewable is always a hot topic of research. Organic solar cells attracts very much attention due to their desirable properties like inexpensive, flexible over their inorganic counterparts. Bulkhetrojunction (BHJ) solar cell emerges as the most efficient and prominent type of organic solar cells in recent years. Experiments of transient photovoltage (TPV) find that there is possibly intensive exciton dissociation occurring at the anode (metal)-organic interface, which leads to an innegligible loss of free electrons, resulting the so-called surface loss. Starting from the device model and including the exciton dynamics as well, we calculate the J-V curves in different interfacial dissociation rates, and evaluate the magnitude of the surface loss and its influence to the short circuit current, open circuit voltage and the efficiency of the device. The role of bimolecular recombination is also investigated. Background: Bulk Heterojunction Organic Solar Cells Models and Equations for Simulation N ex 2 N ex D k0 N ex ( x) N ex G ( x) R t x 2 p 1 J p Gc R t e x n 1 J n Gc R t e x E 1 V 1 1 L J J ( x ) dx t L t 0 r L 0 1 k0 , G ( x) g 0 exp( x La ), g 0 I 0 La interface Two types of organic solar cells: (a) the bilayer device and (b) the recently popular bulk heterojunction one. Both of them have a sandwiched structure, which consists of a transparent anode of ITO, an active layer of organic semiconducting material, and a metal cathode, like Al. illumination The schematic illustration of the mechanism and electronic process of BHJ solar cell. (1) Excitons are generated in the donor under illumination. (2) They diffuse to the donor/acceptor interface and experience a charge transfer to convert into Coulomb bounded e-h pairs. (3) The pairs can dissociate into free carriers. These free electrons and holes are extracted under the built-in field by the electrodes, giving the photocurrent. Current Voltage Characteristics D Gc ( x) N ex , R (np ni2 ) ITO Organic Cathode To consider the interfacial dissociation explicitly, the active layer is divided into the two parts of interface (red) and the bulk. They are of different exciton dissociation rates. Device model combined with the exciton evolution equations. The four coupled equations are solved to obtain the steady state current, under the boundary condition of N ex ( x) 0 x x 0, L J n / p J th J re Interfacial Dissociation Proportion The current-voltage curves are shown when changing the interfacial dissociation rates (left-upper corner) or bulk dissociation rates (right). The short circuit current is decreased with increasing interfacial dissociation rate or decreasing bulk dissociation rate. The upper figure gives a quantitative relation between the short circuit current and the interfacial dissociation time using different interface thickness, suggesting the interfacial effect is restricted to 1nm away from the anode. d 0 L 0 ( x) N ex ( x)dx ( x) N ex ( x)dx . Exciton density distribution under short circuit condition The depletion of excitons in the interface at large dissociation rates result in the saturation of interfacial dissociation proportion . Interfacial Dissociation at Open Circuit Condition Compensation Voltage decreasing Induced by Surface Loss Photocurrent versus the applied voltage. In Ohmic contacts, the compensation voltage of the photocurrent decreases with the increasing of interfacial dissociation rates. This effect is more obvious than the decreasing of short circuit Current. Interfacial dissociation proportion decreases at high bias voltage, for the bimolecular free carrier recombination play an important role, which increases the exciton density in the bulk region . Conclusions & References • The short circuit current decreasing slightly with increasing of interfacial dissociation rate, until saturation at very large rate or small dissociation time. The decreasing of bulk dissociation rate can lead to the decreasing of short circuit current as well, and the effect is much more obvious . • The open circuit voltage dose not change with the increasing of interfacial dissociation rate in Schottky contact device, while it decreases at Ohmic Contact one, this can be seen more clearly from the plot of net photocurrent versus the applied voltage. • The interfacial dissociation proportion increases with the interfacial dissociation rate, varying conversely with the short circuit current. Its saturation can be understood with the help of exciton density profile. The excitons are equivalently quenched at the interface. • The bimolecular recombination can reduce the interfacial dissociation proportion at high bias voltage in Ohmic contact device. [1] C. Deibel and V. Dyakonov, Rep. Prog. Phys. 73, 096401 (2010). [2] C. Deibel, T. Strobel, and V. Dyakonov, Phys. Rev. Lett. 103, 036402 (2009). [3] T. Strobel, C. Deibel and V. Dyakonov, Phys. Rev. Lett. 105, 266602 (2010).