Diffusion Length and Langevin Recombination of Singlet and Triplet Excitons in Organic Heterojunction Solar Cells

Abstract

We derived new expressions for the diffusion length of singlet and triplet excitons by using the Föster and Dexter transfer mechanisms, respectively, and have found that the diffusion lengths of singlet and triplet excitons are comparable. By using the Langevin recombination theory, we derived the rate of recombination of dissociated free charges into their excitonic states. We found that in some organic polymers the probabilities of recombination of free charge carriers back into the singlet and triplet states are approximately 65.6 and 34.4 %, respectively, indicating that Langevin-type recombination into triplet excitons in organic semiconductors is less likely. This implies that the creation of triplet excitons may be advantageous in organic solar cells, because this may lead to dissociated free charge carriers that can be collected at their respective electrodes, which should result in better conversion efficiency.

Limitations and Perspectives on Triplet‐Material‐Based Organic Photovoltaic Devices

“…It is well known that the lifetime of triplet excitons is usually about three orders of magnitude longer than those of singlet excitons [50] while the binding energy of the triplet excitons is higher than those of singlet excitons due to the attractive exchange interaction of the same spin orientation. The long lifetime of triplets may facilitate charge generation and enhanced Jsc.…”

Charge-carrier-mobility-dependent-open-circuit-voltage-in-organic-and-hybrid-solar-cells

“…Research interest in organic solar cells (OSCs) is currently on the increase mainly because of their cost effectiveness, flexibility, easy fabrication techniques, large scale production and the potential integration of OSCs into a wide variety of devices [1][2][3][4]. The development of new materials for photovoltaic applications coupled with device optimization has led to a dramatic increase in OSCs' performance in recent years [5].…”

“…A major research focus now lies in finding ways for further optimization of the power conversion efficiency (PCE), guided by a deeper understanding of the fundamental processes that influence the photovoltaic properties of OSCs [6]. The following four processes of OSCs and organic hybrid solar cells (OHSCs) make them remarkably different from their inorganic counterparts: i) photon absorption and exciton generation; ii) diffusion of excitons to the donor acceptor (DA) interface; iii) dissociation and charge separation at the interface; and iv) carrier collection by the electrodes [1,2]. These four processes have to be sufficiently efficient to reduce or eliminate energy losses leading to reduction in the short-circuit current density J sc and open-circuit voltage V oc , and hence, reduction in the power conversion efficiency of OSCs and OHSCs.…”

Study of intersystem crossing mechanism in organic materials

“…Direct conversion of sun light into electrical energy in OSC involves four electronic processes: (i) exciton generation after photon absorption, (ii) exciton diffusion to the donor acceptor (D-A) interface, (iii) dissociation of excitons at the D-A interface, and iv) charge carriers transport to the electrodes [5]. Excitons excited in organic semiconductors can be in singlet (S) and triplet (T) spin configurations and as a consequence, both singlet and triplet excitons can be excited in OSCs.…”