Structure–Property Relations in Polymer:Fullerene Blends for Organic Solar Cells

Organic solar cells consist of thin films combining an electron donor (often a conjugated polymer) with an electron acceptor (often a fullerene derivative), in a blend commonly referred to as bulk heterojunction material. Charge separation between the donor and the acceptor leads to the generation of carriers, which can be extracted from photovoltaic devices in the form of photocurrent. The generation mechanism of free, extractable charges has caused a lot of controversial discussion in literature. Our research has shown that all the steps involved in charge generation are strongly dependent on the arrangement of the donor and the acceptor (i.e. the structure) of the bulk heterojunction

Organic photovoltaics: pushing the knowledge of interfaces

The use of a spectroscopy technique called pump–push–probe electro-absorption provides insight into the energetic landscape of nanostructured donor–acceptor interfaces in bulk-heterojunction organic solar cell

Spectroscopic Tools to Investigate the Electrochemical Doping Kinetics and Efficiency in Organic Semiconductors

Understanding the electrochemical doping of organic semiconductors plays a crucial role in the current development of organic electronics. In this short review, we present how temperature- and time- dependent visible-near-infrared (Vis-NIR) spectro-electrochemistry and terahertz spectroscopy, combined with multivariate curve resolution analysis, can inform on the fundamental mechanisms governing the doping kinetics and efficiency of two archetypal semiconducting polymers (PEDOT and P3HT). We highlight the experimental procedures and data analysis performed to access (i) the thermodynamic parameters driving the extent and dynamics of electrochemical reactions in doped systems and (ii) how the density and nature of charged species (polarons, bipolarons) impact the charge carrier delocalization, effective THz mobility and hence short-range conductivity

Photophysics and Charge Generation in Low Energy-offset Blends for Organic Solar Cells

The power conversion efficiency of organic solar cells has seen a huge improvement in recent years with state-of-the-art solar cells showcasing efficiencies of ~18.5 %, which is approaching the performance of inorganic and hybrid-perovskite solar cell technologies. This improvement can be mainly attributed to the discovery of highly efficient donor:acceptor blends with a near-zero energetic offset between the molecular orbital levels of the donor and the acceptor component. A distinctive feature of the high efficiency, low energy-offset blends is that they exhibit a concomitant increase in the short-circuit density and the open-circuit voltage of the solar cell. High open-circuit voltage results from the reduced photon energy loss in the exciton dissociation step, while a high short-circuit current density can be attributed to an efficient charge generation process. The reasons for the efficient exciton dissociation and subsequent separation of Coulomb bound electron-hole pair at negligible driving force is not well understood and, in this short review, we highlight recent results which shed light on the mechanism of charge generation in low energy-offset blends

A new single shot THz detection strategy with electro-optic sampling

We present a new technique for single shot Terahertz detection in electro-optics sampling (EOS) with a narrowband probe pulse shaped using a Fabry-Pérot etalon. The technique allows tdetection in the frequency domain using a high-resolution CCD spectrometer. The technique is simple and sensitive. It has a high time resolution and can be simply implemented in a standard EOS scanning experiment

The role of excitons and free charges in the excited-state dynamics of solution-processed few-layer MoS₂ nanoflakes

Solution-processed semiconducting transition metal dichalcogenides are emerging as promising two-dimensional materials for photovoltaic and optoelectronic applications. Here, we have used transient absorption spectroscopy to provide unambiguous evidence and distinct signatures of photogenerated excitons and charges in solution- processed few-layer MoS₂ nanoflakes (10–20 layers). We find that photoexcitation above the direct energy gap results in the ultrafast generation of a mixture of free charges in direct band states and of excitons. While the excitons are rapidly trapped, the free charges are long-lived with nanosecond recombination times. The different signatures observed for these species enable the experimental extraction of the exciton binding energy, which we find to be ∼80 meV in the nanoflakes, in agreement with reported values in the bulk material. Carrier-density-dependent measurements bring new insights about the many-body interactions between free charges resulting in band gap renormalization effects in the few-layer MoS₂ nanoflakes

Bipolarons rule the short-range terahertz conductivity in electrochemically doped P3HT

Doping of organic semiconductor films enhances their conductivity for applications in organic electronics, thermoelectrics and bioe- lectronics. However, much remains to be learnt about the proper- ties of the conductive charges in order to optimize the design of the materials. Electrochemical doping is not only the fundamental mechanism in organic electrochemical transistors (OECTs), used in biomedical sensors, but it also represents an ideal playground for fundamental studies. Benefits of investigating doping mechanisms via electrochemistry include controllable doping levels, reversibility and high achievable carrier densities. We introduced here a new technique, applying in situ terahertz (THz) spectroscopy directly to an electrochemically doped polymer in combination with spectro- electrochemistry and chronoamperometry. We evaluate the intrin- sic short-range transport properties of the polymer (without the effects of long-range disorder, grain boundaries and contacts), while precisely tuning the doping level via the applied oxidation voltage. Analysis of the complex THz conductivity reveals both the mobility and density of the charges. We find that polarons and bipolarons need to co-exist in an optimal ratio to reach high THz conductivity (B300 S c

What drives the kinetics and doping level in the electrochemical reactions of PEDOT:PSS?

The electrochemical dedoping and redoping processes of a thin poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film immersed in an electrolyte are studied at different temperatures with time-resolved spectroelectrochemistry in the visible and near-infrared range. The spectral signatures of neutral, polaronic, and bipolaronic states of PEDOT are resolved using multivariate curve resolution analysis. Kinetic modeling of their dynamics reveals that both the dedoping and redoping are sequential processes and occur within a few hundred milliseconds in the system. Evaluation of the temperature-dependence with the Van't Hoff, Arrhenius, and Eyring formalisms highlights the role of entropy in both the establishment of the redox equilibrium at a given voltage bias and the reaction rates. This study provides a significant understanding of the fundamental mechanisms determining the level and rate of the electrochemical processes in PEDOT:PSS and will help tailor the design of faster and more efficient bioelectronic devices based on mixed ionic–electronic conductors