A Phenomenological Study On The Physical Process Of Metal / Organic Interface In Organic Solar Cells

Organic solar cells(OSC) are some sorts of photovoltaic devices based on organic small molecule material or conjugated polymer. Compared with their inorganic counterparts, they have the advantages of low cost, flexibility and foldable, and easy preparation and manufacturing, thus gain lots of attention and become a hot research area in recent years. OSCs are the central topic of this thesis and I focus on the physical properties of the interfaces between their metal electrodes and organic active layers, i.e. the metal/organic(M/O) interface, and its impacts on the device performance and efficiency. The first chapter is about the application background and various methods of this field. In the second chapter I firstly give an introduction to the three generation of OSCs, and secondly describe elaborately the opto-electronic processes taking place in OSCs in terms of BH J device:exciton generation, diffusion, dissociation, and the generation and transport of free charge carriers. The main method used in this research, the device model, is described in the third chapter.The device model consists of several equations describing the time evolutions of the macroscopic quantities like charge density, electric field and drift and dif-fusion currents, and is applicable to almost all the electronic device. Due to the importance of excitons in organic electronic systems, the exciton dynamic equa-tion is incorporated into the device model, forming an efficient method which can describe both the transient processes under laser pulse and the steady state prop-erties under constant illumination in a whole framework. We phenomenologically investigated the exciton dynamics, M/O interfacial dissociation, surface losses and open circuit voltage in OSCs, and the main results are listed in the following:Transient photovoltage(TPV) experiments in single-layer devices suggest that the exciton dissociation at ITO/organic interface is much stronger that that in the bulk. By diving the device into two regions of interface and bulk, the time-spatial evolution of excitons in the presence of interfacial dissociation. To give a quanti-tative description of the weight interfacial dissociation, the interfacial dissociation proportion(IDP) is defined as the relative proportion of excitons dissociating at the interface to that in the whole device. It is found that IDP increases with the decreasing(increasing) of interfacial dissociation time(rate) and saturate at last. The saturated value depends on the bulk dissociation time. Interfacial dissocia-tion makes the exciton density at the vicinity of the interface reduce a lot, and the exciton is depleted under extremely high dissociation rate, which is the rea-son of IDP’s saturation. The IDPs in both transient and steady state are the same. However, the saturated IDP is a little bit higher under laser pulse which is of much higher intensity than ordinary solar or lamp illumination. For Ohmic contact device, the second order recombination of electron and holes can increase the bulk exciton density near the open circuit condition, and thus reduce IDP. By simulating J—V curves we find The short circuit current decreases with the de-creasing(increasing) of interfacial(bulk) dissociation time, and this is derived from the increasing of surface electron current Jn under efficient interfacial dissociation, and therefore Jn may characterize the interfacial dissociation as well.Surface loss is a serious problem in BHJ solar cells but inaccessible to direct experimental detection. We define a surface loss probability and deduce its ex-pression solely consists of macroscopic quantities, in terms of which it is found that surface losses derive from the flowing of charge carriers into their wrong elec-trons, i.e. electrons entering anode and holes cathode. The numerical simulation suggests that the surface loss probability is a constant small value when far from the compensation voltage. With the increasing of the injection barriers of electron and holes and it becomes larger and its voltage dependence becomes apparent. After taking into account interfacial dissociation, it increases with the interfacial dissociation rate and saturates when the latter approaches infinity. The diffusion of polaron pairs(PP) aslo contributes to the surface losses. If the interfacial dis-sociation rate is large enough, the PP density gradient between the interface and the bulk region is huge and may lead to large diffusion current under high PP diffusivity. Thus the surface loss probability increases with the diffusivity very rapidly in this condition. The damage of surface losses to device performance lies on the decreasing of short circuit current and open circuit voltage. Surface losses can be suppressed by inserting proper interlayers between the metal electrode and the organic active layer.By simulating J-V curves, the logarithmic increasing of open circuit voltage under increasing light intensity is demonstrated. However, the fill factor reduces very much under high illumination. It is found that this can be circumvented by using organic material with imbalanced mobilities in the preparation of OSCs. We aslo investigate the variation of open circuit voltage under different injection barri-ers, and find that it does not decrease with the increasing of barriers monotonically, but has a maximum at0.2eV. This arises from the potential(band) bending at the metal/organic and organic/metal interface due to charge accumulation there in Ohmic contact device. To reach the optimal Voc, the device should be prepared with a certain small injection barrier like0.2eV rather than pure Ohmic contact interface.