TY  - JOUR
A1  - Fritsch, Tobias
A1  - Kurpiers, Jona
A1  - Roland, Steffen
A1  - Tokmoldin, Nurlan
A1  - Shoaee, Safa
A1  - Ferron, Thomas
A1  - Collins, Brian A.
A1  - Janietz, Silvia
A1  - Vandewal, Koen
A1  - Neher, Dieter
T1  - On the interplay between CT and singlet exciton emission in organic solar cells with small driving force and its impact on voltage loss
JF  - Advanced energy materials
N2  - The interplay between free charge carriers, charge transfer (CT) states and singlet excitons (S-1) determines the recombination pathway and the resulting open circuit voltage (V-OC) of organic solar cells. 
By combining a well-aggregated low bandgap polymer with different blend ratios of the fullerenes PCBM and ICBA, the energy of the CT state (E-CT) is varied by 130 meV while leaving the S-1 energy of the polymer (ES1\[{E_{{{\rm{S}}_1}}}\]) unaffected. 
It is found that the polymer exciton dominates the radiative properties of the blend when ECT\[{E_{{\rm{CT}}}}\] approaches ES1\[{E_{{{\rm{S}}_1}}}\], while the V-OC remains limited by the non-radiative decay of the CT state. 
It is concluded that an increasing strength of the exciton in the optical spectra of organic solar cells will generally decrease the non-radiative voltage loss because it lowers the radiative V-OC limit (V-OC,V-rad), but not because it is more emissive. 
The analysis further suggests that electronic coupling between the CT state and the S-1 will not improve the V-OC, but rather reduce the V-OC,V-rad. 
It is anticipated that only at very low CT state absorption combined with a fairly high CT radiative efficiency the solar cell benefit from the radiative properties of the singlet excitons.
KW  - external quantum efficiency
KW  - organic photovoltaics
KW  - ternary blends
KW  - voltage losses
Y1  - 2022
U6  - https://doi.org/10.1002/aenm.202200641
SN  - 1614-6832
SN  - 1614-6840
VL  - 12
IS  - 31
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Poelking, Carl
A1  - Benduhn, Johannes
A1  - Spoltore, Donato
A1  - Schwarze, Martin
A1  - Roland, Steffen
A1  - Piersimoni, Fortunato
A1  - Neher, Dieter
A1  - Leo, Karl
A1  - Vandewal, Koen
A1  - Andrienko, Denis
T1  - Open-circuit voltage of organic solar cells
BT  - interfacial roughness makes the difference
JF  - Communications physics
N2  - Organic photovoltaics (PV) is an energy-harvesting technology that offers many advantages, such as flexibility, low weight and cost, as well as environmentally benign materials and manufacturing techniques. Despite growth of power conversion efficiencies to around 19 % in the last years, organic PVs still lag behind inorganic PV technologies, mainly due to high losses in open-circuit voltage. Understanding and improving open circuit voltage in organic solar cells is challenging, as it is controlled by the properties of a donor-acceptor interface where the optical excitations are separated into charge carriers. Here, we provide an electrostatic model of a rough donor-acceptor interface and test it experimentally on small molecule PV materials systems. The model provides concise relationships between the open-circuit voltage, photovoltaic gap, charge-transfer state energy, and interfacial morphology. In particular, we show that the electrostatic bias generated across the interface reduces the photovoltaic gap. This negative influence on open-circuit voltage can, however, be circumvented by adjusting the morphology of the donor-acceptor interface. 
Organic solar cells, despite their high power conversion efficiencies, suffer from open circuit voltage losses making them less appealing in terms of applications. Here, the authors, supported with experimental data on small molecule photovoltaic cells, relate open circuit voltage to photovoltaic gap, charge-transfer state energy, and donor-acceptor interfacial morphology.
Y1  - 2022
U6  - https://doi.org/10.1038/s42005-022-01084-x
SN  - 2399-3650
VL  - 5
IS  - 1
PB  - Nature portfolio
CY  - Berlin
ER  -