TY  - JOUR
A1  - Yuan, Jun
A1  - Zhang, Chujun
A1  - Qiu, Beibei
A1  - Liu, Wei
A1  - So, Shu Kong
A1  - Mainville, Mathieu
A1  - Leclerc, Mario
A1  - Shoaee, Safa
A1  - Neher, Dieter
A1  - Zou, Yingping
T1  - Effects of energetic disorder in bulk heterojunction organic solar cells
JF  - Energy & environmental science
N2  - Organic solar cells (OSCs) have progressed rapidly in recent years through the development of novel organic photoactive materials, especially non-fullerene acceptors (NFAs). Consequently, OSCs based on state-of-the-art NFAs have reached significant milestones, such as similar to 19% power conversion efficiencies (PCEs) and small energy losses (less than 0.5 eV). Despite these significant advances, understanding of the interplay between molecular structure and optoelectronic properties lags significantly behind. For example, despite the theoretical framework for describing the energetic disorder being well developed for the case of inorganic semiconductors, the question of the applicability of classical semiconductor theories in analyzing organic semiconductors is still under debate. A general observation in the inorganic field is that inorganic photovoltaic materials possessing a polycrystalline microstructure exhibit suppressed disorder properties and better charge carrier transport compared to their amorphous analogs. Accordingly, this principle extends to the organic semiconductor field as many organic photovoltaic materials are synthesized to pursue polycrystalline-like features. Yet, there appears to be sporadic examples that exhibit an opposite trend. However, full studies decoupling energetic disorder from aggregation effects have largely been left out. Hence, the potential role of the energetic disorder in OSCs has received little attention. Interestingly, recently reported state-of-the-art NFA-based devices could achieve a small energetic disorder and high PCE at the same time; and interest in this investigation related to the disorder properties in OSCs was revived. In this contribution, progress in terms of the correlation between molecular design and energetic disorder is reviewed together with their effects on the optoelectronic mechanism and photovoltaic performance. Finally, the specific challenges and possible solutions in reducing the energetic disorder of OSCs from the viewpoint of materials and devices are proposed.
Y1  - 2022
U6  - https://doi.org/10.1039/d2ee00271j
SN  - 1754-5692
SN  - 1754-5706
VL  - 15
IS  - 7
SP  - 2806
EP  - 2818
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Kroh, Daniel
A1  - Eller, Fabian
A1  - Schötz, Konstantin
A1  - Wedler, Stefan
A1  - Perdigón-Toro, Lorena
A1  - Freychet, Guillaume
A1  - Wei, Qingya
A1  - Dörr, Maximilian
A1  - Jones, David
A1  - Zou, Yingping
A1  - Herzig, Eva M.
A1  - Neher, Dieter
A1  - Köhler, Anna
T1  - Identifying the signatures of intermolecular interactions in blends of PM6 with Y6 and N4 using absorption spectroscopy
JF  - Advanced functional materials
N2  - In organic solar cells, the resulting device efficiency depends strongly on the local morphology and intermolecular interactions of the blend film. Optical spectroscopy was used to identify the spectral signatures of interacting chromophores in blend films of the donor polymer PM6 with two state-of-the-art nonfullerene acceptors, Y6 and N4, which differ merely in the branching point of the side chain. From temperature-dependent absorption and luminescence spectroscopy in solution, it is inferred that both acceptor materials form two types of aggregates that differ in their interaction energy. Y6 forms an aggregate with a predominant J-type character in solution, while for N4 molecules the interaction is predominantly in a H-like manner in solution and freshly spin-cast film, yet the molecules reorient with respect to each other with time or thermal annealing to adopt a more J-type interaction. The different aggregation behavior of the acceptor materials is also reflected in the blend films and accounts for the different solar cell efficiencies reported with the two blends.
KW  - charge-transfer states
KW  - Frank-Condon analysis
KW  - morphology
KW  - organic solar cells
Y1  - 2022
U6  - https://doi.org/10.1002/adfm.202205711
SN  - 1616-301X
SN  - 1616-3028
VL  - 32
IS  - 44
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
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  - Grischek, Max
A1  - Caprioglio, Pietro
A1  - Zhang, Jiahuan
A1  - Pena-Camargo, Francisco
A1  - Sveinbjornsson, Kari
A1  - Zu, Fengshuo
A1  - Menzel, Dorothee
A1  - Warby, Jonathan H.
A1  - Li, Jinzhao
A1  - Koch, Norbert
A1  - Unger, Eva
A1  - Korte, Lars
A1  - Neher, Dieter
A1  - Stolterfoht, Martin
A1  - Albrecht, Steve
T1  - Efficiency Potential and Voltage Loss of Inorganic CsPbI2Br Perovskite Solar Cells
JF  - Solar RRL
N2  - Inorganic perovskite solar cells show excellent thermal stability, but the reported power conversion efficiencies are still lower than for organic-inorganic perovskites. This is mainly caused by lower open-circuit voltages (V(OC)s). Herein, the reasons for the low V-OC in inorganic CsPbI2Br perovskite solar cells are investigated. Intensity-dependent photoluminescence measurements for different layer stacks reveal that n-i-p and p-i-n CsPbI2Br solar cells exhibit a strong mismatch between quasi-Fermi level splitting (QFLS) and V-OC. Specifically, the CsPbI2Br p-i-n perovskite solar cell has a QFLS-e center dot V-OC mismatch of 179 meV, compared with 11 meV for a reference cell with an organic-inorganic perovskite of similar bandgap. On the other hand, this study shows that the CsPbI2Br films with a bandgap of 1.9 eV have a very low defect density, resulting in an efficiency potential of 20.3% with a MeO-2PACz hole-transporting layer and 20.8% on compact TiO2. Using ultraviolet photoelectron spectroscopy measurements, energy level misalignment is identified as a possible reason for the QFLS-e center dot V-OC mismatch and strategies for overcoming this V-OC limitation are discussed. This work highlights the need to control the interfacial energetics in inorganic perovskite solar cells, but also gives promise for high efficiencies once this issue is resolved.
KW  - CsPbI2Br
KW  - efficiency potentials
KW  - inorganic perovskites
KW  - photoluminescence
KW  - solar cells
KW  - voltage losses
Y1  - 2022
U6  - https://doi.org/10.1002/solr.202200690
SN  - 2367-198X
VL  - 6
IS  - 11
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Pena-Camargo, Francisco
A1  - Thiesbrummel, Jarla
A1  - Hempel, Hannes
A1  - Musiienko, Artem
A1  - Le Corre, Vincent M.
A1  - Diekmann, Jonas
A1  - Warby, Jonathan
A1  - Unold, Thomas
A1  - Lang, Felix
A1  - Neher, Dieter
A1  - Stolterfoht, Martin
T1  - Revealing the doping density in perovskite solar cells and its impact on device performance
JF  - Applied physics reviews
N2  - Traditional inorganic semiconductors can be electronically doped with high precision. Conversely, there is still conjecture regarding the assessment of the electronic doping density in metal-halide perovskites, not to mention of a control thereof. This paper presents a multifaceted approach to determine the electronic doping density for a range of different lead-halide perovskite systems. Optical and electrical characterization techniques, comprising intensity-dependent and transient photoluminescence, AC Hall effect, transfer-length-methods, and charge extraction measurements were instrumental in quantifying an upper limit for the doping density. The obtained values are subsequently compared to the electrode charge per cell volume under short-circuit conditions ( CUbi/eV), which amounts to roughly 10(16) cm(-3). This figure of merit represents the critical limit below which doping-induced charges do not influence the device performance. The experimental results consistently demonstrate that the doping density is below this critical threshold 10(12) cm(-3), which means << CUbi / e V) for all common lead-based metal-halide perovskites. Nevertheless, although the density of doping-induced charges is too low to redistribute the built-in voltage in the perovskite active layer, mobile ions are present in sufficient quantities to create space-charge-regions in the active layer, reminiscent of doped pn-junctions. These results are well supported by drift-diffusion simulations, which confirm that the device performance is not affected by such low doping densities.
Y1  - 2022
U6  - https://doi.org/10.1063/5.0085286
SN  - 1931-9401
VL  - 9
IS  - 2
PB  - AIP Publishing
CY  - Melville
ER  - 
TY  - JOUR
A1  - Raoufi, Meysam
A1  - Hörmann, Ulrich
A1  - Ligorio, Giovanni
A1  - Hildebrandt, Jana
A1  - Pätzel, Michael
A1  - Schultz, Thorsten
A1  - Perdigon-Toro, Lorena
A1  - Koch, Norbert
A1  - List-Kratochvil, Emil
A1  - Hecht, Stefan
A1  - Neher, Dieter
T1  - Simultaneous effect of ultraviolet radiation and surface modification on the work function and hole injection properties of ZnO thin films
JF  - Physica Status Solidi.  A , Applications and materials science
N2  - The combined effect of ultraviolet (UV) light soaking and self-assembled monolayer deposition on the work function (WF) of thin ZnO layers and on the efficiency of hole injection into the prototypical conjugated polymer poly(3-hexylthiophen-2,5-diyl) (P3HT) is systematically investigated. It is shown that the WF and injection efficiency depend strongly on the history of UV light exposure. Proper treatment of the ZnO layer enables ohmic hole injection into P3HT, demonstrating ZnO as a potential anode material for organic optoelectronic devices. The results also suggest that valid conclusions on the energy-level alignment at the ZnO/organic interfaces may only be drawn if the illumination history is precisely known and controlled. This is inherently problematic when comparing electronic data from ultraviolet photoelectron spectroscopy (UPS) measurements carried out under different or ill-defined illumination conditions.
KW  - charge injection across hybrid interfaces
KW  - energy-level alignments
KW  - hybrid metal oxides
KW  - organic interfaces
Y1  - 2020
U6  - https://doi.org/10.1002/pssa.201900876
SN  - 1862-6300
SN  - 1862-6319
VL  - 217
IS  - 5
SP  - 1
EP  - 6
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Tait, Claudia E.
A1  - Reckwitz, Anna
A1  - Arvind, Malavika
A1  - Neher, Dieter
A1  - Bittl, Robert
A1  - Behrends, Jan
T1  - Spin-spin interactions and spin delocalisation in a doped organic semiconductor probed by EPR spectroscopy
JF  - Physical chemistry, chemical physics : PCCP ; a journal of European chemical societies
N2  - The enhancement and control of the electrical conductivity of organic semiconductors is fundamental for their use in optoelectronic applications and can be achieved by molecular doping, which introduces additional charge carriers through electron transfer between a dopant molecule and the organic semiconductor. Here, we use Electron Paramagnetic Resonance (EPR) spectroscopy to characterise the unpaired spins associated with the charges generated by molecular doping of the prototypical organic semiconductor poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)TCNQ) and tris(pentafluorophenyl)borane (BCF). The EPR results reveal the P3HT radical cation as the only paramagnetic species in BCF-doped P3HT films and show evidence for increased mobility of the detected spins at high doping concentrations as well as formation of antiferromagnetically coupled spin pairs leading to decreased spin concentrations at low temperatures. The EPR signature for F(4)TCNQ-doped P3HT is found to be determined by spin exchange between P3HT radical cations and F(4)TCNQ radical anions. Results from continuous-wave and pulse EPR measurements suggest the presence of the unpaired spin on P3HT in a multitude of environments, ranging from free P3HT radical cations with similar properties to those observed in BCF-doped P3HT, to pairs of dipolar and exchange-coupled spins on P3HT and the dopant anion. Characterisation of the proton hyperfine interactions by ENDOR allowed quantification of the extent of spin delocalisation and revealed reduced delocalisation in the F(4)TCNQ-doped P3HT films.
Y1  - 2021
U6  - https://doi.org/10.1039/d1cp02133h
SN  - 1463-9076
SN  - 1463-9084
VL  - 23
IS  - 25
SP  - 13827
EP  - 13841
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Perdigón-Toro, Lorena
A1  - Le Quang Phuong, 
A1  - Zeiske, Stefan
A1  - Vandewal, Koen
A1  - Armin, Ardalan
A1  - Shoaee, Safa
A1  - Neher, Dieter
T1  - Excitons dominate the emission from PM6
BT  - Y6 solar cells, but this does not help the open-circuit voltage of the device
JF  - ACS energy letters / American Chemical Society
N2  - Non-fullerene acceptors (NFAs) are far more emissive than their fullerene-based counterparts. Here, we study the spectral properties of photocurrent generation and recombination of the blend of the donor polymer PM6 with the NFA Y6. We find that the radiative recombination of free charges is almost entirely due to the re-occupation and decay of Y6 singlet excitons, but that this pathway contributes less than 1% to the total recombination. As such, the open-circuit voltage of the PM6:Y6 blend is determined by the energetics and kinetics of the charge-transfer (CT) state. Moreover, we find that no information on the energetics of the CT state manifold can be gained from the low-energy tail of the photovoltaic external quantum efficiency spectrum, which is dominated by the excitation spectrum of the Y6 exciton. We, finally, estimate the charge-separated state to lie only 120 meV below the Y6 singlet exciton energy, meaning that this blend indeed represents a high-efficiency system with a low energetic offset.
Y1  - 2021
U6  - https://doi.org/10.1021/acsenergylett.0c02572
SN  - 2380-8195
VL  - 6
IS  - 2
SP  - 557
EP  - 564
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Pranav, Manasi
A1  - Benduhn, Johannes
A1  - Nyman, Mathias
A1  - Hosseini, Seyed Mehrdad
A1  - Kublitski, Jonas
A1  - Shoaee, Safa
A1  - Neher, Dieter
A1  - Leo, Karl
A1  - Spoltore, Donato
T1  - Enhanced charge selectivity via anodic-C60 layer reduces nonradiative losses in organic solar cells
JF  - ACS applied materials & interfaces
N2  - Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. 
This work uses a neat C-60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. 
These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. 
Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C-60 interlayer on MoO3, is attributed to an enhanced built-in potential. 
The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. 
The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. 
Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.
KW  - nonradiative losses
KW  - molybdenum oxide
KW  - organic solar cells
KW  - interfacial layers
KW  - charge selectivity
Y1  - 2021
U6  - https://doi.org/10.1021/acsami.1c00049
SN  - 1944-8244
SN  - 1944-8252
VL  - 13
IS  - 10
SP  - 12603
EP  - 12609
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - GEN
A1  - Phuong, Le Quang
A1  - Hosseini, Seyed Mehrdad
A1  - Sandberg, Oskar J.
A1  - Zou, Yingping
A1  - Woo, Han Young
A1  - Neher, Dieter
A1  - Shoaee, Safa
T1  - Quantifying quasi-fermi level splitting and open-circuit voltage losses in highly efficient nonfullerene organic solar cells
T2  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2  - The power conversion efficiency (PCE) of state-of-the-art organic solar cells is still limited by significant open-circuit voltage (V-OC) losses, partly due to the excitonic nature of organic materials and partly due to ill-designed architectures. Thus, quantifying different contributions of the V-OC losses is of importance to enable further improvements in the performance of organic solar cells. Herein, the spectroscopic and semiconductor device physics approaches are combined to identify and quantify losses from surface recombination and bulk recombination. Several state-of-the-art systems that demonstrate different V-OC losses in their performance are presented. By evaluating the quasi-Fermi level splitting (QFLS) and the V-OC as a function of the excitation fluence in nonfullerene-based PM6:Y6, PM6:Y11, and fullerene-based PPDT2FBT:PCBM devices with different architectures, the voltage losses due to different recombination processes occurring in the active layers, the transport layers, and at the interfaces are assessed. It is found that surface recombination at interfaces in the studied solar cells is negligible, and thus, suppressing the non-radiative recombination in the active layers is the key factor to enhance the PCE of these devices. This study provides a universal tool to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.
T3  - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1384 
KW  - nonfullerene acceptors
KW  - organic solar cells
KW  - quasi-Fermi level
KW  - splitting
KW  - quasi-steady-state photoinduced absorptions
KW  - surface
KW  - recombinations
KW  - voltage losses
Y1  - 2020
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-570018
SN  - 1866-8372
IS  - 1
ER  -