cell prepared using a co-sublimation technique in which the perovskite layer is sandwiched in between organic electron and hole blocking layers. [ 2a ] This confi guration leads to stable and reproducible photovoltaic devices that do not suffer from strong hysteresis effects and when optimized lead to effi ciencies close to 15%. [ 10 ] From previous studies, [ 7,11 ] it is known that charge trapping does occur in (solution-processed) perovskite layers. From time-resolved photoluminescence spectroscopy studies, it was concluded that trap assisted recombination is non-radiative. [ 12 ] This would indicate that traps infl uence the recombination mechanism in an actual device. Here, we investigate the charge recombination in working devices, taking advantage of the fact that they are diodes, by examining the low-voltage part of the J-V characteristics, from which the diode ideality factor can be determined. The diode ideality factor is a measure of the steepness of the J-V characteristics in the low-voltage region. As fi rst described theoretically by Sah et al. for a classical semiconductor p-n junction, trap-assisted recombination changes the diode ideality factor up to a value of 2, [ 13 ] where it normally has a value of 1 in the case that recombination is absent or when only direct free-carrier recombination occurs. Such behavior has been observed also in (undoped) organic-semiconductor diodes, as also expected theoretically for a metal-insulator-metal diode. [ 14 ] A typical perovskite solar cell consists of an intrinsic semiconductor (perovskite) layer sandwiched between (organic) charge-blocking layers. As such, charge recombination should be confi ned to the perovskite layer and trap-assisted recombination can be exposed by investigating the diode ideality factor.Using these devices based on co-sublimated CH 3 NH 3 PbI 3 perovskite layers, we have prepared double-carrier, hole-only and electron-only devices by changing the organic charge extraction/blocking layers and or electrode materials used. We fi nd that trap-assisted recombination via electron traps is present as a non-radiative loss mechanism in co-sublimated perovskite devices.In order to investigate the diode behavior of perovskite solar cells, co-sublimed perovskite layers were prepared and sandwiched in between polyTPD and PCBM electron-and holeblocking layers, respectively. The layout of the device (see experimental section for more details) and the typical performance of these devices under 1 sun illumination are shown in Figure 1 .As can be seen, there is only a small difference in the current density ( J ) versus voltage ( V ) curve as a function of scan sweep direction (Figure 1 B). It has been reported that the scan direction has a large infl uence on the J -V curve of metal-oxide containing perovskite devices, leading to a strong hysteresis

Electron transport in semiconducting polymers is usually inferior to hole transport, which is ascribed to charge trapping on isolated defect sites situated within the energy bandgap. However, a general understanding of the origin of these omnipresent charge traps, as well as their energetic position, distribution and concentration, is lacking. Here we investigate electron transport in a wide range of semiconducting polymers by current-voltage measurements of single-carrier devices. We observe for this materials class that electron transport is limited by traps that exhibit a gaussian energy distribution in the bandgap. Remarkably, the electron-trap distribution is identical for all polymers considered: the number of traps amounts to 3 × 10(23) traps per m(3) centred at an energy of ~3.6 eV below the vacuum level, with a typical distribution width of ~0.1 eV. This indicates that the electron traps have a common origin that, we suggest, is most likely related to hydrated oxygen complexes. A consequence of this finding is that the trap-limited electron current can be predicted for any polymer.

A fullerene bisadduct can enhance the efficiency of polymer:fullerene bulk heterojunction solar cells. The bisadduct has a LUMO that is 100 meV higher compared to that of [6,6]‐phenyl C61 butyric acid methyl ester (PCBM). This increases the open‐circuit voltage of polymer:fullerene bulk heterojunction solar cells based on poly(3‐hexylthio phene) and bisadduct PCBM to 0.73 V, while maintaining high fill factors and currents.

This article reviews the basic physical processes of charge transport and recombination in organic semiconductors. As a workhorse, LEDs based on a single layer of poly(p-phenylene vinylene) (PPV) derivatives are used. The hole transport in these PPV derivatives is governed by trap-free space-charge-limited conduction, with the mobility depending on the electric field and charge-carrier density. These dependencies are generally described in the framework of hopping transport in a Gaussian density of states distribution. The electron transport on the other hand is orders of magnitude lower than the hole transport. The reason is that electron transport is hindered by the presence of a universal electron trap, located at 3.6 eV below vacuum with a typical density of ca. 3 × 10¹⁷ cm⁻³. The trapped electrons recombine with free holes via a non-radiative trap-assisted recombination process, which is a competing loss process with respect to the emissive bimolecular Langevin recombination. The trap-assisted recombination in disordered organic semiconductors is governed by the diffusion of the free carrier (hole) towards the trapped carrier (electron), similar to the Langevin recombination of free carriers where both carriers are mobile. As a result, with the charge-carrier mobilities and amount of trapping centers known from charge-transport measurements, the radiative recombination as well as loss processes in disordered organic semiconductors can be fully predicted. Evidently, future work should focus on the identification and removing of electron traps. This will not only eliminate the non-radiative trap-assisted recombination, but, in addition, will shift the recombination zone towards the center of the device, leading to an efficiency improvement of more than a factor of two in single-layer polymer LEDs.

Origin of the dark-current ideality factor in polymer Wetzelaer, G. A. H.; Kuik, M.; Lenes, M.; Blom, P. W. M.

The trap-assisted recombination of electrons and holes in organic semiconductors is investigated. The extracted capture coefficients of the trap-assisted recombination process are thermally activated with an identical activation energy as measured for the hole mobility μ(p). We demonstrate that the rate limiting step for this mechanism is the diffusion of free holes towards trapped electrons in their mutual Coulomb field, with the capture coefficient given by (q/ε)μ(p). As a result, both the bimolecular and trap-assisted recombination processes in organic semiconductors are governed by the charge carrier mobilities, allowing predictive modeling of organic light-emitting diodes.

Barrier-free (Ohmic) contacts are a key requirement for efficient organic optoelectronic devices, such as organic light-emitting diodes, solar cells, and field-effect transistors. Here, we propose a simple and robust way of forming an Ohmic hole contact on organic semiconductors with a high ionization energy (IE). The injected hole current from high-work-function metal-oxide electrodes is improved by more than an order of magnitude by using an interlayer for which the sole requirement is that it has a higher IE than the organic semiconductor. Insertion of the interlayer results in electrostatic decoupling of the electrode from the semiconductor and realignment of the Fermi level with the IE of the organic semiconductor. The Ohmic-contact formation is illustrated for a number of material combinations and solves the problem of hole injection into organic semiconductors with a high IE of up to 6 eV.

The research on transparent conductive electrodes is in rapid ascent in order to respond to the requests of novel optoelectronic devices. The synergic coupling of silver nanowires (AgNWs) and high-quality solution-processable exfoliated graphene (EG) enables an efficient transparent conductor with low-surface roughness of 4.6 nm, low sheet resistance of 13.7 Ω sq −1 at high transmittance, and superior mechanical and chemical stabilities. The developed AgNWs-EG films are versatile for a wide variety of optoelectronics. As an example, when used as a bottom electrode in organic solar cell and polymer light-emitting diode, the devices exhibit a power conversion efficiency of 6.6% and an external quantum efficiency of 4.4%, respectively, comparable to their commercial indium tin oxide counterparts.