TY  - JOUR
A1  - Armin, Ardalan
A1  - Durrant, James R.
A1  - Shoaee, Safa
T1  - Interplay Between Triplet-, Singlet-Charge Transfer States and Free Charge Carriers Defining Bimolecular Recombination Rate Constant of Organic Solar Cells
JF  - The journal of physical chemistry : C, Nanomaterials and interfaces
N2  - Despite the myriad of organic donor:acceptor materials, only few systems have emerged in the life of organic solar cells to, exhibit considerable reduced bimolecular recombination, with respect to the random encounter rate given by the Langevin equation. Monte Carlo simulations have revealed that the rate constant of the formation of electron-hole bound states depends on the random encounter of opposite charges and is nearly given by the Langevin equation for the domain sizes relevant to efficient bulk heterojunction systems. Recently, three studies :suggested that charge transfer states dissociating much faster than their decay rate to the ground state, can result in reduced bimolecular recombination by lowering the recombination rate to the ground state as a loss pathway. A separate study identified another loss pathway and suggested that forbidden back electron transfer from triplet charge transfer states to triplet excitons is a key to achieving reduced recombination. Herein we further explain the reduced bimolecular recombination by investigating the limitations of these two proposals. By solving kinetic rate equations for a BHJ system with realistic rates, we show that both of these previously presented conditions must only be held at the same time fora system to exhibit non-Langevin behavior. We demonstrate that suppression of both of the parallel loss channels of singlet and triplet states can be achieved through increasing the dissociation rate of the charge transfer states; a crucial requirement to achieve a high charge carrier extraction efficiency.
Y1  - 2017
U6  - https://doi.org/10.1021/acs.jpcc.7b04825
SN  - 1932-7447
VL  - 121
SP  - 13969
EP  - 13976
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Neusser, David
A1  - Sun, Bowen
A1  - Tan, Wen Liang
A1  - Thomsen, Lars
A1  - Schultz, Thorsten
A1  - Perdigon-Toro, Lorena
A1  - Koch, Norbert
A1  - Shoaee, Safa
A1  - McNeill, Christopher R.
A1  - Neher, Dieter
A1  - Ludwigs, Sabine
T1  - Spectroelectrochemically determined energy levels of PM6:Y6 blends and their relevance to solar cell performance
JF  - Journal of materials chemistry : C, Materials for optical and electronic devices
N2  - Recent advances in organic solar cell performance have been mainly driven forward by combining high-performance p-type donor-acceptor copolymers (e.g.PM6) and non-fullerene small molecule acceptors (e.g.Y6) as bulk-heterojunction layers. A general observation in such devices is that the device performance, e.g., the open-circuit voltage, is strongly dependent on the processing solvent. While the morphology is a typically named key parameter, the energetics of donor-acceptor blends are equally important, but less straightforward to access in the active multicomponent layer. Here, we propose to use spectral onsets during electrochemical cycling in a systematic spectroelectrochemical study of blend films to access the redox behavior and the frontier orbital energy levels of the individual compounds. Our study reveals that the highest occupied molecular orbital offset (Delta E-HOMO) in PM6:Y6 blends is similar to 0.3 eV, which is comparable to the binding energy of Y6 excitons and therefore implies a nearly zero driving force for the dissociation of Y6 excitons. Switching the PM6 orientation in the blend films from face-on to edge-on in bulk has only a minor influence on the positions of the energy levels, but shows significant differences in the open circuit voltage of the device. We explain this phenomenon by the different interfacial molecular orientations, which are known to affect the non-radiative decay rate of the charge-transfer state. We compare our results to ultraviolet photoelectron spectroscopy data, which shows distinct differences in the HOMO offsets in the PM6:Y6 blend compared to neat films. This highlights the necessity to measure the energy levels of the individual compounds in device-relevant blend films.
Y1  - 2022
U6  - https://doi.org/10.1039/d2tc01918c
SN  - 2050-7526
SN  - 2050-7534
VL  - 10
IS  - 32
SP  - 11565
EP  - 11578
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  -