How Molecules with Dipole Moments Enhance the Selectivity of Electrodes in Organic Solar Cells – A Combined Experimental and Theoretical Approach

Würfel, Uli; Seßler, M.; Unmüssig, Moritz; Hofmann, Nils; List, Mathias; Mankel, Eric; Mayer, Thomas; Reiter, Günter; Bubendorff, Jean-Luc; Simon, Laurent; Kohlstädt, Markus

doi:10.1002/aenm.201600594

Cited by 40 publications

(47 citation statements)

References 70 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…The devices with a LiF/Al electrode show a reasonable high V OC as compared to literature for the particular donor:acceptor blend system and we therefore conclude that the electron and hole contacts are selective. [31][32][33] In contrast, we observe a distinct decrease of the V OC when using Cu as electron contact, hence its selectivity is strongly reduced. The corresponding experimental and simulated JV-curves are shown in Figure 2.…”

Section: Resultsmentioning

confidence: 48%

“…The effect that increased surface recombination comes along with a reduction of the EL signal was demonstrated in a previous work. [16,33] In these devices, the electron conductivity is lowered in the vicinity of the nonsufficiently selective electron contact such that the injection is impeded. This makes holes to be transported from the hole contact (where they are injected) through the whole photoactive layer to the electron contact where they recombine nonradiatively at its surface.…”

Section: Electroluminescence Measurementsmentioning

confidence: 99%

See 1 more Smart Citation

On the Impact of Contact Selectivity and Charge Transport on the Open‐Circuit Voltage of Organic Solar Cells

Spies

List

Sarkar

et al. 2016

Advanced Energy Materials

Self Cite

View full text Add to dashboard Cite

However, in comparison to established inorganic-based photovoltaic technologies organic solar cells possess still a much lower efficiency. [2,[9][10][11][12] The efficiency η of solar cells is determined bywhere J SC is the short-circuit current density, V OC is the open-circuit voltage, FF is the fill factor, and P in is the incident power density. The relatively poor transport properties (low mobilities) of the disordered organic materials [13,14] applied in OSC lead to a strong accumulation of charge carriers under current flow and thus to an increase in nongeminate recombination. [10] Consequently, the FF is reduced and for cases of extremely low charge carrier mobilities also J SC , which results in a low overall power conversion efficiency. [10,11,15] Further, due to the relatively low permittivity of the organic materials a significant fraction of the absorbed photon energy is required as driving force for the exciton dissociation of the coulombic bound electron-hole pair (offset between the lowest unoccupied molecular orbitals of donor and acceptor) which lowers the achievable power conversion efficiency significantly. [9] In addition to the latter, a key loss mechanism originates from additional nonradiative recombination pathways. Besides the nongeminate recombination via trap states as a first order recombination process, the recombination at the electrodes, i.e., surface recombination, is an additional loss channel. Surface recombination in OSC was investigated intensively in different perspectives which indicates the importance of a thorough understanding of the underlying mechanisms. [16][17][18][19][20] Kirchartz et al. derived the ideality factor differentially by the light intensity dependence of the open-circuit voltage and demonstrated that the presence of surface recombination leads to the occurrence of an ideality factor n id < 1. [18] But also the charge carrier mobility µ dependent efficiency of OSC has been investigated in terms of surface recombination. It seems to be counterintuitive to assume that the device efficiency is maximum at a finite optimum mobility. Yet, this case is in detail discussed by Deibel et al. and Kirchartz et al., who showed that an optimum mobility exists in the presence of not ideally selective contacts and that faster kinetics by mobilities higher than the optimum will result in a reduction of the overall efficiency. [21,22] TressThe selectivity of electrodes of solar cells is a critical factor that can limit the overall efficiency. If the selectivity of an electrode is not sufficient both electrons and holes recombine at its surface. In materials with poor transport properties such as in organic solar cells, these surface recombination currents are accompanied by large gradients of the quasi-Fermi energies as the driving force. Experimental results from current-voltage characteristics, advanced photo-and electroluminescence as well as charge extraction of three different photoactive materials are shown and compared to drift-diffusion simulations. It can be conc...

show abstract

Section: Resultsmentioning

confidence: 48%

Section: Electroluminescence Measurementsmentioning

confidence: 99%

On the Impact of Contact Selectivity and Charge Transport on the Open‐Circuit Voltage of Organic Solar Cells

Spies

List

Sarkar

et al. 2016

Advanced Energy Materials

Self Cite

View full text Add to dashboard Cite

show abstract

“…However, a significant change of the field‐dependence of the extracted charges is not observed in our simulations as can be seen in Figure S2, Supporting Information. The reason is that lower contact selectivity causes larger losses due to surface recombination as was shown in former work . And these losses decrease as well for larger driving forces (lower V pre ), resulting in a similar field‐dependence of the collected charge.…”

Section: Measurement Principle Of Tdcfsupporting

confidence: 64%

“…A variation of the injection barriers and thus of the magnitude of these accumulations will be discussed further below. Although it is not possible to access exact values for these accumulations via experiments a more general approach shows that large differences in conductivities of electrons and holes on their way to the contacts are indeed the fundamental requirement for carrier selectivity and thus good device performance . In all practical cases these required differences in conductivities are, to the best of our knowledge, realized as differences in the concentrations of electrons and holes.…”

Section: Measurement Principle Of Tdcfmentioning

confidence: 99%

Apparent Field‐Dependence of the Charge Carrier Generation in Organic Solar Cells as a Result of (Bimolecular) Recombination

Würfel

Unmüssig

2018

Solar RRL

Self Cite

View full text Add to dashboard Cite

Charge carrier generation in organic solar cells is often reported to depend on the electric field. This is, however, not measured directly but derived from transient charge carrier extraction experiments based on the time delayed collection field (TDCF) method. In this work, numerical simulations of TDCF experiments are presented which – when analyzed in the same way as reported in literature – result in a field‐dependence of charge carrier generation despite the fact that a field‐independent generation is used. This discrepancy is shown to be caused by recombination of photogenerated charge carriers occurring in the time range prior to and during extraction. This apparent field‐dependence becomes more pronounced for larger recombination coefficients and decreasing charge carrier mobilities, very much in accordance with experimental TDCF data from literature. Even an apparent voltage‐ and time‐dependence of the bimolecular recombination coefficient is reproduced in the simulations although a constant, voltage‐independent one is used. These findings strongly question whether TDCF is an appropriate method to detect a potential field‐dependence of the photocurrent generation and the recombination coefficient. Our study shows that all experimental results can consistently be explained without the assumption of a field‐dependence of the charge carrier generation and the bimolecular recombination coefficient.

show abstract

“…33 The greater the dipole moment is, the greater the polarity of the molecule. In addition, the molecular structure can be approximately regarded as an electron cloud and a molecular skeleton composition.…”

Section: Polarizabilitymentioning

confidence: 99%

Tuning the performance of the non-fullerene organic solar cells by the polarizability

Qin

Dai

et al. 2018

RSC Adv.

View full text Add to dashboard Cite

We report here the synthesis and characterizations of a novel series of acceptor copolymers with a broad absorption band. The acceptor polymers were synthesized as a copolymer of perylenediimide (PDI) and naphthalene imide (NDI) along with dithieno[3,2-b:pyrroles (DTP). When the dipole moment and polarizability of the acceptor polymer are compared, it is observed that when the dipole moment decreases, the polarizability becomes larger. The polarizability of polymers containing PDI is significantly greater than those containing NDI, and their polarizability change is in accordance with the change in the transient fluorescence lifetime. It was also found that the power conversion efficiency of the non-fullerene solar cell was strongly correlated to polarizability. The results demonstrate that the polarizability can be utilized to screen novel donor and acceptor polymers for the design and synthesis of high-performance solar cells.

show abstract

How Molecules with Dipole Moments Enhance the Selectivity of Electrodes in Organic Solar Cells – A Combined Experimental and Theoretical Approach

Cited by 40 publications

References 70 publications

On the Impact of Contact Selectivity and Charge Transport on the Open‐Circuit Voltage of Organic Solar Cells

On the Impact of Contact Selectivity and Charge Transport on the Open‐Circuit Voltage of Organic Solar Cells

Apparent Field‐Dependence of the Charge Carrier Generation in Organic Solar Cells as a Result of (Bimolecular) Recombination

Tuning the performance of the non-fullerene organic solar cells by the polarizability

Contact Info

Product

Resources

About