Multiple state representation scheme for organic bulk heterojunction solar cells: A novel analysis perspective

Abstract

Abstract. -The physics of organic bulk heterojunction solar cells is studied within a six state model, which is used to analyze the factors that affect current-voltage characteristics, powervoltage properties and efficiency, and their dependence on nonradiative losses, reorganization of the nuclear environment, and environmental polarization. Both environmental reorganization and polarity is explicitly taken into account by incorporating Marcus heterogeneous and homogeneous electron transfer rates. The environmental polarity is found to have a non-negligible influence both on the stationary current and on the overall solar cell performance. For our organic bulk heterojunction solar cell operating under steady-state open circuit condition, we also find that the open circuit voltage logarithmically decreases with increasing nonradiative electron-hole recombination processes.Considerable progress has been achieved in improving the device efficiency of bulk heterojunction (BHJ) organic solar cells, with a recently set record of 10.7%. [1,2] Much of this success came about by searching for promising electron-donor polymers characterized by low optical gap, using fullerene based electron-acceptor derivatives and optimizing the interpenetrated frozen-in microstructures. Such phase-separated blend morphologies are distinguished by a large interface area between the donor and the acceptor phases, which is a prerequisite to tailor most efficient organic photovoltaic solar cells (OPVs). While material design is one successful strategy to improve the OPV setup, another is to focus on the device physics by developing approaches that take into account physical and chemical features of BHJ organic solar cells in order to improve their dynamical operation. In the last two years there appeared several reviews [3][4][5][6][7] and perspective articles [8, 9] about both material designs and device physics giving an excellent account of the state-of-the-art for organic photovoltaics.In the context of solar energy conversion, device physics aims to identify routes for improved cell performance by studying models that account for both the material prop-(a) E-mail: meinax@uos.de erties and the underlying microscopic principles of the energy conversion processes, i. e. structural and energetics system parameters, in order to identify critical factors that affect the overall OPV performance. Thus, the generated free carriers in a photovoltaic device, which can be harvested at the electrodes, are limited by the complex interplay between charge generation, diffusion, and recombination processes. In BHJ organic solar cells the generation of free charge carriers requires that photoinduced excitons (bounded electron-hole pairs) on the donor material must diffuse to and dissociate at the donor-acceptor (D-A) interface before their recombination takes place. This exciton dissociation at the D-A interface starts with the formation of a charge transfer (CT) state (a geminate pair), where the hole and the electron remain at close proximity on thei...

Theoretical limit of power conversion efficiency for organic and hybrid halide perovskite photovoltaics

“…[2][3][4][5][6][7][8][9] Some other approaches have also been developed to study the efficiency, in particular, on the basis of a thermodynamic detailed balance. [10][11][12][13][14] As a practical approach, limits for solar cells were assessed using criteria based on the short circuit currents, open circuit voltage and other quantities.…”

Photocell Optimization Using Dark State Protection

“…One such measure is the energy transfer efficiency, i.e., the probability of an excitation reaching the target electron acceptor after starting from a spatially localized state [17][18][19], but this does not capture all aspects of the process. An alternative approach is placing a system between two electrodes, and measuring the current through them [20][21][22][23]. Dorfman et al proposed a different canonical measure [1]: They consider the entire cycle, from absorbing a photon to extracting work, as a quantum heat engine (QHE).…”

The Molecular Photo-Cell: Quantum Transport and Energy Conversion at Strong Non-Equilibrium

“…The next question we wish to address is the extent to which the quantum nature of the system affects the PV conversion efficiency, a question which is beyond the reach of the formalism presented in Refs. 4 , 5 . The formalism we present here allows us to access, in addition to fully quantum-coherent processes described above, also incoherent processes.…”

“…Here we propose a formalism to study non-equilibrium transport in molecular junctions, and use it to investigate a model for the molecular photo-cell, a single molecular donor-acceptor complex attached to electrodes and subject to external illumination. This model was recently suggested 4 5 6 (and a simpler version in Ref. 7 ) to be the minimal model to describe PV energy conversion in ideal, single-molecule heterojunction organic PV cells.…”

“…In Refs. 4 , 5 PV conversion efficiency was analyzed using the (essentially classical) rate equations for the electronic degrees of freedom. The dynamics and non-equilibrium properties of the phonons and photons were ignored, being considered only within a (non-self-consistent) mean-field approximation and assumed to have equilibrium distributions.…”