To make high-performance semiconductor devices, a good ohmic contact between the electrode and the semiconductor layer is required to inject the maximum current density across the contact. Achieving ohmic contacts requires electrodes with high and low work functions to inject holes and electrons respectively, where the work function is the minimum energy required to remove an electron from the Fermi level of the electrode to the vacuum level. However, it is challenging to produce electrically conducting films with sufficiently high or low work functions, especially for solution-processed semiconductor devices. Hole-doped polymer organic semiconductors are available in a limited work-function range, but hole-doped materials with ultrahigh work functions and, especially, electron-doped materials with low to ultralow work functions are not yet available. The key challenges are stabilizing the thin films against de-doping and suppressing dopant migration. Here we report a general strategy to overcome these limitations and achieve solution-processed doped films over a wide range of work functions (3.0-5.8 electronvolts), by charge-doping of conjugated polyelectrolytes and then internal ion-exchange to give self-compensated heavily doped polymers. Mobile carriers on the polymer backbone in these materials are compensated by covalently bonded counter-ions. Although our self-compensated doped polymers superficially resemble self-doped polymers, they are generated by separate charge-carrier doping and compensation steps, which enables the use of strong dopants to access extreme work functions. We demonstrate solution-processed ohmic contacts for high-performance organic light-emitting diodes, solar cells, photodiodes and transistors, including ohmic injection of both carrier types into polyfluorene-the benchmark wide-bandgap blue-light-emitting polymer organic semiconductor. We also show that metal electrodes can be transformed into highly efficient hole- and electron-injection contacts via the self-assembly of these doped polyelectrolytes. This consequently allows ambipolar field-effect transistors to be transformed into high-performance p- and n-channel transistors. Our strategy provides a method for producing ohmic contacts not only for organic semiconductors, but potentially for other advanced semiconductors as well, including perovskites, quantum dots, nanotubes and two-dimensional materials.

While thermodynamic detailed balance limits the maximum power conversion efficiency of a solar cell, the quality of its contacts can further limit the actual efficiency. The criteria for good contacts to organic semiconductors, however, are not well understood. Here, by tuning the work function of poly(3,4-ethylenedioxythiophene) hole collection layers in fine steps across the Fermi-level pinning threshold of the model photoactive layer, poly(3-hexylthiophene):phenyl-C61-butyrate methyl ester, in organic solar cells, we obtain direct evidence for a non-ohmic to ohmic transition at the hole contact that lies 0.3 eV beyond its Fermi-level pinning transition. This second transition corresponds to reduction of the photocurrent extraction resistance below the bulk resistance of the cell. Current detailed balance analysis reveals that this extraction resistance is the counterpart of injection resistance, and the measured characteristics are manifestations of charge carrier hopping across the interface. Achieving ohmic transition at both contacts is key to maximizing fill factor without compromising open-circuit voltage nor short-circuit current of the solar cell.

The power conversion effi ciency (PCE) of solar cells or photovoltaic cells is given by the product of their short-circuit photocurrent density J sc , fi ll factor (FF) and open-circuit voltage V oc . For organic solar cells, in which the photoactive layer (PAL) is an organic semiconductor (OSC) comprising intimatelymixed donor (D) and acceptor (A) phases, these performance parameters depend on a complex interplay of several processes, some more strongly related to materials and processing while others to cell design. [ 1 ] In brief, the overall effi ciency of the solar-to-electrical energy conversion chain depends on the following effi ciencies. The cell absorption effi ciency for the incident solar photons is determined primarily by the PAL but infl uenced by optical interference effects. The photocarrier generation effi ciency depends on the fraction of excitons that can dissociate to charge-separated pairs through processes that are still under debate, although this is known to be a property of the electronic structure and morphology of the DA interface. Then the collection effi ciency is determined by internal (non-geminate) recombination losses. The maximum electrochemical potential difference available from the carriers depends on electronic structure and other effects. The current-voltage ( JV ) characteristic under solar irradiation refl ects the outcome of all of these processes, which are heavily coupled. [2][3][4] Nevertheless signifi cant advances have occurred in some aspects, such as the correlation of cell properties with PAL and the DA morphology. [ 5 ] The maximum V oc available has been suggested to vary linearly with the energy gap between the highest-occupied molecular orbital (HOMO) band edge of D and the lowest unoccupied molecular orbital (LUMO) band edge of A, subjected to a loss of a few tenths of an eV, often thought to be 0.3 eV. [ 6,7 ] However a number of electrical parameters of the cells are not known accurately, including the relevant electronic energies, and so it is not possible to be certain about the magnitude of this offset. Recent literature for example has tended to emphasize the correlation of V oc with disparate sources of losses, including binding energy of the charge-transfer exciton, [ 8 ] internal recombination, [ 9 ] or sub-gap absorption through a quasi-equilibrium detailed balance. [ 10 ] Nevertheless the V bi is a more fundamental parameter of the cell [11][12][13] and hence a natural starting point to understand the factors that limit V oc in high-performance organic solar cells. However the V bi of even prototypical P3HT:PCBM cells has not been directly established, but estimated between 0.6 and 0.9 V. [ 14 ] Because of strong para meter coupling, this hampers Here, using crosslinked P3HT network:PCBM cells with predefi ned ultrafi ne donor-acceptor morphology and very high internal quantum effi ciencies, the built-in potential V bi is measured to decouple and reliably extract other key parameters of the cells. Using the refi ned device parameters, the ge...

Recently it has been reported that Nafion oligomers, i.e., 2‐(2‐sulfonatotetrafluoroethoxy)‐2‐trifluoromethyltrifluoroethoxyfunctionalized oligotetrafluoroethylenes, also called perfluorinated ionomers (PFIs), can be blended into poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDT:PSSH) films to increase their workfunctions beyond 5.2 eV. These PFI‐modified films are useful for energy‐level alignment studies, and have been proposed as hole‐injection layers (HILs). It is shown here however that these HILs do not provide sufficiently fast hole transfer into adjacent polymer semiconductor layers with ionization potentials deeper than ≈5.2 eV. X‐ray and ultraviolet photoemission spectroscopies reveal that these HILs exhibit a molecularly‐thin PFI overlayer that sets up a surface dipole that provides the ultrahigh workfunction. This dipolar layer persists even when the subsequent organic semiconductor layer is deposited, as evidenced by measurements of the diode built‐in potentials. As a consequence, the PFI‐modified HILs produce a higher contact resistance, and a lower equilibrium density of holes at the semiconductor contact than might have been expected from simple thermodynamic considerations of the reduction in hole‐injection barrier. Thus the use of insulating dipolar surface layers at the charge‐injection contact to tune its workfunction to match the relevant transport level of the semiconductor is of limited utility to achieve ohmic contact in these devices.

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