TY  - JOUR
A1  - Zuo, Guangzheng
A1  - Shoaee, Safa
A1  - Kemerink, Martijn
A1  - Neher, Dieter
T1  - General rules for the impact of energetic disorder and mobility on nongeminate recombination in phase-separated organic solar cells
JF  - Physical review applied
N2  - State-of-the-art organic solar cells exhibit power conversion efficiencies of 18% and above. These devices benefit from the suppression of free charge recombination with regard to the Langevin limit of charge encounter in a homogeneous medium. It is recognized that the main cause of suppressed free charge recombination is the reformation and resplitting of charge-transfer (CT) states at the interface between donor and acceptor domains. Here, we use kinetic Monte Carlo simulations to understand the interplay between free charge motion and recombination in an energetically disordered phase-separated donor-acceptor blend. We identify conditions for encounter-dominated and resplitting-dominated recombination. In the former regime, recombination is proportional to mobility for all parameters tested and only slightly reduced with respect to the Langevin limit. In contrast, mobility is not the decisive parameter that determines the nongeminate recombination coefficient, k(2), in the latter case, where k2 is a sole function of the morphology, CT and charge-separated (CS) energetics, and CT-state decay properties. Our simulations also show that free charge encounter in the phase-separated disordered blend is determined by the average mobility of all carriers, while CT reformation and resplitting involves mostly states near the transport energy. Therefore, charge encounter is more affected by increased disorder than the resplitting of the CT state. As a consequence, for a given mobility, larger energetic disorder, in combination with a higher hopping rate, is preferred. These findings have implications for the understanding of suppressed recombination in solar cells with nonfullerene acceptors, which are known to exhibit lower energetic disorder than that of fullerenes.
Y1  - 2021
U6  - https://doi.org/10.1103/PhysRevApplied.16.034027
SN  - 2331-7019
VL  - 16
IS  - 3
PB  - American Physical Society
CY  - College Park
ER  - 
TY  - JOUR
A1  - Zu, Fengshuo
A1  - Wolff, Christian Michael
A1  - Ralaiarisoa, Maryline
A1  - Amsalem, Patrick
A1  - Neher, Dieter
A1  - Koch, Norbert
T1  - Unraveling the Electronic Properties of Lead Halide Perovskites with Surface Photovoltage in Photoemission Studies
JF  - ACS applied materials & interfaces
N2  - The tremendous success of metal-halide perovskites, especially in the field of photovoltaics, has triggered a substantial number of studies in understanding their optoelectronic properties. However, consensus regarding the electronic properties of these perovskites is lacking due to a huge scatter in the reported key parameters, such as work function (Φ) and valence band maximum (VBM) values. Here, we demonstrate that the surface photovoltage (SPV) is a key phenomenon occurring at the perovskite surfaces that feature a non-negligible density of surface states, which is more the rule than an exception for most materials under study. With ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe, we evidence that even minute UV photon fluxes (500 times lower than that used in typical UPS experiments) are sufficient to induce SPV and shift the perovskite Φ and VBM by several 100 meV compared to dark. By combining UV and visible light, we establish flat band conditions (i.e., compensate the surface-state-induced surface band bending) at the surface of four important perovskites, and find that all are p-type in the bulk, despite a pronounced n-type surface character in the dark. The present findings highlight that SPV effects must be considered in all surface studies to fully understand perovskites’ photophysical properties.
KW  - lead halide perovskite films
KW  - ultraviolet photoelectron spectroscopy
KW  - Kelvin probe
KW  - surface band bending
KW  - surface photovoltage
KW  - surface states
Y1  - 2019
U6  - https://doi.org/10.1021/acsami.9b05293
SN  - 1944-8244
SN  - 1944-8252
VL  - 11
IS  - 24
SP  - 21578
EP  - 21583
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Pingel, Patrick
A1  - Arvind, Malavika
A1  - Kölln, Lisa
A1  - Steyrleuthner, Robert
A1  - Kraffert, Felix
A1  - Behrends, Jan
A1  - Janietz, Silvia
A1  - Neher, Dieter
T1  - p-Type Doping of Poly(3-hexylthiophene) with the Strong Lewis Acid Tris(pentafluorophenyl)borane
JF  - Advanced electronic materials
N2  - State-of-the-art p-type doping of organic semiconductors is usually achieved by employing strong -electron acceptors, a prominent example being tetrafluorotetracyanoquinodimethane (F(4)TCNQ). Here, doping of the semiconducting model polymer poly(3-hexylthiophene), P3HT, using the strong Lewis acid tris(pentafluorophenyl)borane (BCF) as a dopant, is investigated by admittance, conductivity, and electron paramagnetic resonance measurements. The electrical characteristics of BCF- and F(4)TCNQ-doped P3HT layers are shown to be very similar in terms of the mobile hole density and the doping efficiency. Roughly 18% of the employed dopants create mobile holes in either F-4 TCNQ- or BCF-doped P3HT, while the majority of doping-induced holes remain strongly Coulomb-bound to the dopant anions. Despite similar hole densities, conductivity and hole mobility are higher in BCF-doped P3HT layers than in F(4)TCNQ-doped samples. This and the good solubility in many organic solvents render BCF very useful for p-type doping of organic semiconductors.
KW  - charge carrier transport
KW  - charge transfer
KW  - conductivity
KW  - molecular doping
KW  - organic semiconductors
Y1  - 2016
U6  - https://doi.org/10.1002/aelm.201600204
SN  - 2199-160X
VL  - 2
PB  - Wiley-Blackwell
CY  - Hoboken
ER  - 
TY  - JOUR
A1  - Li, Hongguang
A1  - Babu, Sukumaran Santhosh
A1  - Turner, Sarah T.
A1  - Neher, Dieter
A1  - Hollamby, Martin J.
A1  - Seki, Tomohiro
A1  - Yagai, Shiki
A1  - Deguchi, Yonekazu
A1  - Möhwald, Helmuth
A1  - Nakanishi, Takashi
T1  - Alkylated-C-60 based soft materials regulation of self-assembly and optoelectronic properties by chain branching
JF  - Journal of materials chemistry : C, Materials for optical and electronic devices
N2  - Derivatization of fullerene (C-60) with branched aliphatic chains softens C-60-based materials and enables the formation of thermotropic liquid crystals and room temperature nonvolatile liquids. This work demonstrates that by carefully tuning parameters such as type, number and substituent position of the branched chains, liquid crystalline C-60 materials with mesophase temperatures suited for photovoltaic cell fabrication and room temperature nonvolatile liquid fullerenes with tunable viscosity can be obtained. In particular, compound 1, with branched chains, exhibits a smectic liquid crystalline phase extending from 84 degrees C to room temperature. Analysis of bulk heterojunction (BHJ) organic solar cells with a ca. 100 nm active layer of compound 1 and poly(3-hexylthiophene) (P3HT) as an electron acceptor and an electron donor, respectively, reveals an improved performance (power conversion efficiency, PCE: 1.6 + 0.1%) in comparison with another compound, 10 (PCE: 0.5 + 0.1%). The latter, in contrast to 1, carries linear aliphatic chains and thus forms a highly ordered solid lamellar phase at room temperature. The solar cell performance of 1 blended with P3HT approaches that of PCBM/P3HT for the same active layer thickness. This indicates that C-60 derivatives bearing branched tails are a promising class of electron acceptors in soft (flexible) photovoltaic devices.
Y1  - 2013
U6  - https://doi.org/10.1039/c3tc00066d
SN  - 2050-7526
VL  - 1
IS  - 10
SP  - 1943
EP  - 1951
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Fischer, Florian S. U.
A1  - Trefz, Daniel
A1  - Back, Justus
A1  - Kayunkid, Navaphun
A1  - Tornow, Benjamin
A1  - Albrecht, Steve
A1  - Yager, Kevin G.
A1  - Singh, Gurpreet
A1  - Karim, Alamgir
A1  - Neher, Dieter
A1  - Brinkmann, Martin
A1  - Ludwigs, Sabine
T1  - Highly Crystalline Films of PCPDTBT with Branched Side Chains by Solvent Vapor Crystallization: Influence on Opto-Electronic Properties
JF  - Advanced materials
N2  - PCPDTBT, a marginally crystallizable polymer, is crystallized into a new crystal structure using solvent-vapor annealing. Highly ordered areas with three different polymer-chain orientations are identified using TEM/ED, GIWAXS, and polarized Raman spectroscopy. The optical and structural properties differ significantly from films prepared by standard device preparation protocols. Bilayer solar cells, however, show similar performance.
Y1  - 2015
U6  - https://doi.org/10.1002/adma.201403475
SN  - 0935-9648
SN  - 1521-4095
VL  - 27
IS  - 7
SP  - 1223
EP  - 1228
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Di Pietro, Riccardo
A1  - Nasrallah, Iyad
A1  - Carpenter, Joshua
A1  - Gann, Eliot
A1  - Kölln, Lisa Sophie
A1  - Thomsen, Lars
A1  - Venkateshvaran, Deepak
A1  - Sadhanala, Aditya
A1  - Chabinyc, Michael
A1  - McNeill, Christopher R.
A1  - Facchetti, Antonio
A1  - Ade, Harald W.
A1  - Sirringhaus, Henning
A1  - Neher, Dieter
T1  - Coulomb Enhanced Charge Transport in Semicrystalline Polymer Semiconductors
JF  - Advanced functional materials
Y1  - 2016
U6  - https://doi.org/10.1002/adfm.201602080
SN  - 1616-301X
SN  - 1616-3028
VL  - 26
SP  - 8011
EP  - 8022
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Neher, Dieter
A1  - Kniepert, Juliane
A1  - Elimelech, Arik
A1  - Koster, L. Jan Anton
T1  - A New Figure of Merit for Organic Solar Cells with Transport-limited Photocurrents
JF  - Scientific reports
N2  - Compared to their inorganic counterparts, organic semiconductors suffer from relatively low charge carrier mobilities. Therefore, expressions derived for inorganic solar cells to correlate characteristic performance parameters to material properties are prone to fail when applied to organic devices. This is especially true for the classical Shockley-equation commonly used to describe current-voltage (JV)-curves, as it assumes a high electrical conductivity of the charge transporting material. Here, an analytical expression for the JV-curves of organic solar cells is derived based on a previously published analytical model. This expression, bearing a similar functional dependence as the Shockley-equation, delivers a new figure of merit a to express the balance between free charge recombination and extraction in low mobility photoactive materials. This figure of merit is shown to determine critical device parameters such as the apparent series resistance and the fill factor.
Y1  - 2016
U6  - https://doi.org/10.1038/srep24861
SN  - 2045-2322
VL  - 6
SP  - E2348
EP  - E2349
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Albrecht, Steve
A1  - Vandewal, Koen
A1  - Tumbleston, John R.
A1  - Fischer, Florian S. U.
A1  - Douglas, Jessica D.
A1  - Frechet, Jean M. J.
A1  - Ludwigs, Sabine
A1  - Ade, Harald W.
A1  - Salleo, Alberto
A1  - Neher, Dieter
T1  - On the efficiency of charge transfer state splitting in polymer: Fullerene solar cells
JF  - Advanced materials
KW  - organic solar cells
KW  - charge generation
KW  - geminate recombination
KW  - charge transfer states
KW  - driving force
KW  - excess energy
KW  - morphology
KW  - spectroelectrochemistry
Y1  - 2014
U6  - https://doi.org/10.1002/adma.201305283
SN  - 0935-9648
SN  - 1521-4095
VL  - 26
IS  - 16
SP  - 2533
EP  - 2539
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Zen, Achmad
A1  - Bilge, Askin
A1  - Galbrecht, Frank
A1  - Alle, Ronald
A1  - Meerholz, Klaus
A1  - Grenzer, Jörg
A1  - Neher, Dieter
A1  - Scherf, Ullrich
A1  - Farrell, Tony
T1  - Solution processable organic field-effect transistors utilizing an alpha,alpha '-dihexylpentathiophene- based swivel cruciform
Y1  - 2006
UR  - http://pubs.acs.org/doi/full/10.1021/ja0573357
U6  - https://doi.org/10.1021/Ja0573357
ER  - 
TY  - JOUR
A1  - Zu, Fengshuo
A1  - Schultz, Thorsten
A1  - Wolff, Christian Michael
A1  - Shin, Dongguen
A1  - Frohloff, Lennart
A1  - Neher, Dieter
A1  - Amsalem, Patrick
A1  - Koch, Norbert
T1  - Position-locking of volatile reaction products by atmosphere and capping layers slows down photodecomposition of methylammonium lead triiodide perovskite
JF  - RSC Advances
N2  - The remarkable progress of metal halide perovskites in photovoltaics has led to the power conversion efficiency approaching 26%. However, practical applications of perovskite-based solar cells are challenged by the stability issues, of which the most critical one is photo-induced degradation. Bare CH3NH3PbI3 perovskite films are known to decompose rapidly, with methylammonium and iodine as volatile species and residual solid PbI2 and metallic Pb, under vacuum under white light illumination, on the timescale of minutes. We find, in agreement with previous work, that the degradation is non-uniform and proceeds predominantly from the surface, and that illumination under N-2 and ambient air (relative humidity 20%) does not induce substantial degradation even after several hours. Yet, in all cases the release of iodine from the perovskite surface is directly identified by X-ray photoelectron spectroscopy. This goes in hand with a loss of organic cations and the formation of metallic Pb. When CH3NH3PbI3 films are covered with a few nm thick organic capping layer, either charge selective or non-selective, the rapid photodecomposition process under ultrahigh vacuum is reduced by more than one order of magnitude, and becomes similar in timescale to that under N-2 or air. We conclude that the light-induced decomposition reaction of CH3NH3PbI3, leading to volatile methylammonium and iodine, is largely reversible as long as these products are restrained from leaving the surface. This is readily achieved by ambient atmospheric pressure, as well as a thin organic capping layer even under ultrahigh vacuum. In addition to explaining the impact of gas pressure on the stability of this perovskite, our results indicate that covalently "locking" the position of perovskite components at the surface or an interface should enhance the overall photostability.
Y1  - 2020
U6  - https://doi.org/10.1039/d0ra03572f
SN  - 2046-2069
VL  - 10
IS  - 30
SP  - 17534
EP  - 17542
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  -