Recently, the advent of non-fullerene acceptors (NFAs) made it possible for organic solar cells (OSCs) to break the 10% efficiency barrier hardly attained by fullerene acceptors (FAs). In the past five years alone, more than hundreds of NFAs with applications in organic photovoltaics (OPVs) have been synthesized, enabling a notable current record efficiency of above 15%. Hence, there is a shift in interest toward the use of NFAs in OPVs. However, there has been little work on the stability of these new materials in devices. More importantly, there is very little comparative work on the photostability of FA versus NFA solar cells to ascertain the pros and cons of the two systems. Here, we show the photostability of solar cells based on two workhorse acceptors, in both conventional and inverted structures, namely, ITIC (as NFA) and [70]PCBM (as FA), blended with either PBDB-T or PTB7-Th polymer. We found that, irrespective of the polymer, the cell structure, or the initial efficiency, the [70]PCBM devices are more photostable than the ITIC ones. This observation, however, opposes the assumption that NFA solar cells are more photochemically stable. These findings suggest that complementary absorption should not take precedence in the design rules for the synthesis of new molecules and there is still work left to be done to achieve stable and efficient OSCs.

However, time-of-flight (TOF) experiments and Monte Carlo (MC) simulations have shown that if the degree of both energetic and positional disorder is high enough, the fluctuations of the intersite distances create fast diffusion routes. [5] These routes increase the mobility of carriers located in high energetic states leading to their extraction before they have the time to thermalize, thus creating dispersion in current extraction on a short time scale. [5] Although the relative importance of the dispersive effect on OPV devices performance is conflicting in the literature, [6] new reports have suggested that it is necessary to consider the influence of the dispersion effect in current extraction and that steadystate mobilities are not relevant to describe the transport in OPV devices. [6b,c] In their study, they set up a MC simulation and an optoelectrical measurement based on a laser light pulse from which they measured the charge carrier mobility on a time scale of 100 fs after the exciton generation. [6b,c] In the first nanoseconds, they observed a charge carrier mobility orders of magnitude higher than the one measured by steady-state measurements. This high mobility is due to the carriers being excited on the upper part of the DOS that are extracted before losing their energy through thermalization. [6b,c] The authors conclude that the steadystate mobilities are not pertinent to make relevant statements on OPV performance and that the thermalization loss plays a key role in the extraction.On the other hand, van der Kaap and Koster used a MC simulation to show that in an organic diode the thermalization has a limited impact on the performance. [6a] In addition, other reports have also demonstrated that SCLC analysis can be successfully applied to organic solar cells and gives valuable insight into materials transport properties. [4b,7] In fact, many studies have shown that the power conversion efficiency (PCE) [4b] and fill factor (FF) [7] depend on the steady-state mobilities. Furthermore, it has also been shown that drift-diffusion simulations, which assume a near-equilibrium state and use steady-state mobilities as an input, successfully describe the characteristics of organic transistors, light-emitting diodes, and solar cells. [7a,b,8] Transient signals of OPV devices have also been well reproduced by drift-diffusion simulation. For example, Albrecht et al. have been able to fit the time-delayed collection field (TDCF) signal using drift-diffusion simulations. Even though they also see that during the first 50 ns after the charge generation a small effect of mobility relaxation has to be taken into account in order to reproduce the transient signal, it still shows that those simulations are suitable to describe the transient behavior of organic solar cells. [9] These studies raise Charge transport in organic photovoltaic (OPV) devices is often characterized by steady-state mobilities. However, the suitability of steady-state mobilities to describe charge transport has recently been called...

In recent years, the efficiency of organic solar cells (OSCs) has increased to more than 13%, although different barriers are on the way for reaching higher efficiencies. One crucial barrier is the recombination of charge carriers, which can either occur as the bulk recombination of photogenerated charges or the recombination of photogenerated charges and electrodic induced charges (EICs). This work studies the impact of EICs on the recombination lifetime in OSCs. To this end, the net recombination lifetime of photogenerated charge carriers in the presence of EICs is measured by means of conventional and newly developed transient photovoltage techniques. Moreover, a new approach has been introduced to exclusively measure the bulk recombination lifetime, i.e., in the absence of EICs; this approach was conducted by depositing transparent insulating layers on both sides of the OSC active layer. An examination of these approaches on OSCs with different active layer materials, thicknesses, and varying light intensities determined that the EICs can only reduce the recombination lifetime of the photogenerated charges in OSCs with very weak recombination strength. This work supports that for OSCs with highly reduced recombination strength, eliminating the recombination of photogenerated charges and EICs is critical for achieving better performance. Therefore, the use of a proper blocking layer suppresses EIC recombination in systems with very weak recombination.

To gauge whether our experimental data are influenced by RC-time issues, the experiment was repeated with different load resistors for measuring the current, a much smaller device area (1 mm 2 ), and a faster time constant of the LED switching (≈50 ns). [2] In other words, a faster setup was used. The resulting extraction rates at V min are very similar to the rates in the OP: a load resistor of 50 Ω (as used in the OP) or 100 Ω give very similar extraction rates of 1.9 and 1.6 µs −1 respectively giving a mobility 5-5.9 × 10 −8 m 2 V −1 s −1 (expected ≈3 × 10 −8 m 2 V −1 s −1 see OP). We even performed an extra experiment at short-circuit for the PTB7:PC[70]BM device (see Figure 1a) at the shortcircuit condition, where the RC-time should have the biggest impact as the extraction is faster there. We find that there is very little difference in the extraction rate depending on the load resistance when it is smaller than 200 Ω. We also performed additional drift-diffusion simulations which include the load resistor and different device areas characteristic for typical measurement conditions, see Figure 1b. The parameters were chosen such that they are representative for PTB7:PC[70]BM solar cells (see Table 1 for details). These simulations reveal a negligible effect of the RC time on the extraction rate at V min , meaning that measurements at V min provide accurate values of the charge carrier mobility.In their comment, the authors also point out that "the conclusion that the equilibrium mobility describes the extraction of all charges clearly cannot be drawn." It is obvious, however, that any mobility would always correspond to a distribution of extraction times. The point of the steady-state mobility that we discuss in the OP is to describe the mean mobility and not the mobility of all the charges.There are many details that determine whether a Monte-Carlo simulation is accurate or not: the donor-acceptor morphology, dark injection and contacts, Coulomb interactions, recombination, generation of carriers, size of the simulation volume to name but a few. [3][4][5][6] The statement that "For an infinite device, electrons would relax to the highest (holes to the lowest) of the equilibrium energy (σ 2 /kT) and the quasi-Fermi energy" is misleading. As shown by Bässler, charge carriers only relax to σ 2 /kT if carriercarrier interaction can be excluded, i.e., in an empty device. [6] The MC model as shown in ref.[6], Figure 4b uses a perfect sink as the electrodes, which means that the carrier density near the electrodes vanishes, and the device is empty in dark. In a real solar cell, however, the electrodes are not just perfect sinks but also induce charge carriers. The injection of charge carriers by the electrodes is especially important in thin film devices [8][9][10] such as organic solar cells. MC simulations [4] and experiments [8] on diodes have shown that in the presence of an ohmic contact (as used to optimize the open-circuit voltage) the device is not empty and, therefore, the charge carriers do not relax to...

To assess if salt-doping leaves anions and cations in the film, limiting the conductivity, one can apply a bias voltage and monitor the conductivity over time. If the doping is limited by unwanted ions, then the conductivity will increase with time.