Intensity Dependence of Current–Voltage Characteristics and Recombination in High-Efficiency Solution-Processed Small-Molecule Solar Cells (original) (raw)
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The Journal of Physical Chemistry Letters
Charge extraction in organic solar cells (OSCs) is commonly believed to be limited by bimolecular recombination of photogenerated charges. However, the fill factor of OSCs is usually almost entirely governed by recombination processes that scale with the first order of the light intensity. This linear loss was often interpreted to be a consequence of geminate or trap-assisted recombination. Numerical simulations show that this linear dependence is a direct consequence of the large amount of excess dark charge near the contact. The first-order losses increase with decreasing mobility of minority carriers, and we discuss the impact of several material and device parameters on this loss mechanism. This work highlights that OSCs are especially vulnerable to injected charges as a result of their poor charge transport properties. This implies that dark charges need to be better accounted for when interpreting electro-optical measurements and charge collection based on simple figures of merit.
Dominating recombination mechanisms in organic solar cells based on ZnPc and C60
Applied Physics Letters, 2013
We investigate the dominating recombination mechanisms in bulk heterojunction solar cells, using a blend of ZnPc and C 60 as model system. Analyzing the open-circuit voltage (V oc ) as a function of illumination intensity, we find that trap-assisted recombination dominates for low light intensities, whereas at 1 sun, direct/bimolecular recombination becomes important. The recombination parameters are not significantly influenced by the blend mixing ratio and are also valid for injected charges. By changing the hole transport layer, recombination at the contact is separately identified as further mechanism reducing V oc at higher light intensities. V C 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4802276\]
Advanced Energy Materials, 2015
to effi ciently photogenerate charges. [ 2,3 ] However, they have low open-circuit voltages and typically cannot be made optically thick while maintaining high fi ll factors. [ 4,5 ] For comparison, the best silicon solar cell has a bandgap of 1.1 eV and an open-circuit voltage of 0.71 V, corresponding to a difference between the bandgap and qV oc of only 0.40 eV. [ 1 ] In contrast, one of the best performing organic solar cells, PTB7:PC 71 BM, has an optical gap of 1.65 eV and an open-circuit voltage of 0.76 V, a difference of 0.89 eV. [ 6 ] The lower qV oc of organic solar cells relative to their optical gaps directly translates into lower power conversion effi ciencies. [ 7 ] Some of this voltage loss is known to occur during the charge generation process when the initial photoexcitation produced by absorbing light is split at the heterointerface between donor and acceptor materials to form a charge transfer (CT) state, which is an interfacial electronic state composed of an electron in the acceptor material and a nearby hole in the donor material that can directly recombine back to the ground state. [ 8 ] In order to provide a driving force for this exciton splitting process to occur, donor and acceptor materials are typically chosen to have electron affi nities that differ by 0.1-0.3 eV, which also reduces qV oc by the same amount. [ 9,10 ] Since the voltage loss between optical absorption and CT state formation is thought to be a necessary tradeoff in order to effi ciently split excitons, V oc is often referenced to the CT state energy rather than the optical gap. [ 4,9-11 ] Even by this metric, however, the voltage is still quite low, with almost all organic solar cells having qV oc between 0.5 and 0.7 eV below the CT state energy. [ 4,12 ] In this work, we explain why the opencircuit voltage of organic solar cells has remained persistently low and develop a theory that provides guidance on how to improve it. Our key results and the relevant energy levels for understanding V oc are summarized schematically in Figure 1. 2. Background Information In order to understand V oc , we will need to build a model that describes how electrons and holes recombine in organic solar cells and how this process depends on voltage. Since our goal is to develop an understanding of V oc that will allow for the Organic solar cells lag behind their inorganic counterparts in effi ciency due largely to low open-circuit voltages (V oc). In this work, a comprehensive framework for understanding and improving the open-circuit voltage of organic solar cells is developed based on equilibrium between charge transfer (CT) states and free carriers. It is fi rst shown that the ubiquitous reduced Langevin recombination observed in organic solar cells implies equilibrium and then statistical mechanics is used to calculate the CT state population density at each voltage. This general result permits the quantitative assignment of V oc losses to a combination of interfacial energetic disorder, non-negligible CT state binding energies, large degrees of mixing, and sub-ns recombination at the donor/acceptor interface. To quantify the impact of energetic disorder, a new temperature-dependent CT state absorption measurement is developed. By analyzing how the apparent CT energy varies with temperature, the interfacial disorder can be directly extracted. 63-104 meV of disorder is found in fi ve systems, contributing 75-210 mV of V oc loss. This work provides an intuitive explanation for why qV oc is almost always 500-700 meV below the energy of the CT state and shows how the voltage can be improved.
The Journal of Physical Chemistry Letters, 2011
O rganic photovoltaics (OPVs) present an opportunity for the low-cost manufacture of solar cells, with devices made from a polymer blended with a functionalized fullerene in a bulk heterojunction (BHJ) structure achieving power conversion efficiencies (PCEs) exceeding 7% in the literature. There is therefore considerable focus on developing methods by which the loss processes limiting the efficient generation and collection of charge carriers in this class of cells may be understood and minimized. In particular, "corrected photocurrent" analyses, based on consideration of the device photocurrent as a function of voltage after subtraction of the corresponding dark current, are widely used tools to analyze the function of such devices. 2À6 Such corrected photocurrents are typically observed to scale linearly with light intensity; this observation has been widely interpreted as indicating that the dominant loss processes limiting device performance scale linearly with charge carrier density in the device. In this Letter, we address the corrected photocurrent analysis and the validity of its underlying assumptions. Additionally, we present a comparison of corrected photocurrent and transient optoelectronic analyses of the same device to demonstrate that the observation of such linear corrected photocurrents cannot be unambiguously employed to determine the order of the underlying loss pathways.