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

Abstract

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...

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

“…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.…”

“…The reason is that lower contact selectivity causes larger losses due to surface recombination as was shown in former work. [27][28][29][30] And these losses decrease as well for larger driving forces (lower V pre ), resulting in a similar field-dependence of the collected charge. An important difference is the impact of the charge carrier mobility on the losses: while for highly selective contacts larger mobilities lead to reduced losses the losses due to surface recombination in the case of non-selective contacts increase with increasing mobilities as shown previously.…”

Effect of Imbalanced Charge Transport on the Interplay of Surface and Bulk Recombination in Organic Solar Cells

“…Hence, assuming more imbalanced charge transport effectively decreases the difference between σ p and σ n close to the anode. This can be understood in terms of a further loss of selectivity or an virtual increase of the injection barrier height [8,22]. Figure 6 shows V oc as a function of µ p for a fixed electron mobility of µ n = 10 −4 cm 2 /Vs and different barrier heights ϕ an .…”

Watching Space Charge Build Up in an Organic Solar Cell

“…Space charge effects in OPVs may occur due to (unintentional) doping, either within the active layer or in the vicinity of the contacts . Another source of space charge is imbalanced charge transport . If there is a significant mismatch between the mobility of electrons () and holes () in the active layer, charges will accumulate at the electrode extracting the slower carrier.…”