Intrinsic efficiency limits in low-bandgap non-fullerene acceptor organic solar cells. Nature Materials.

There is an enormous potential in applying conjugated polymers in novel organic opto-electronic devices such as light emitting diodes and solar cells. Although prototypes and first products exist, a comprehensive understanding of the fundamental processes and energetics involved during photoexcitation is still lacking and limits further device optimisations. Here we report on a unique analysis of the excited states involved in charge generation by photoexcitation. On the model system poly(3-hexylthiophene) (P3HT), we demonstrate the general applicability of our novel approach. From photoemission spectroscopy of occupied and unoccupied states we determine the transport gap to 2.6 eV, which we show to be in agreement with the onset of photoconductivity by spectrally resolved photocurrent measurements. For photogenerated singlet exciton at the absorption edge, 0.7 eV of excess energy are required to overcome the binding energy; the intermediate charge transfer state is situated only 0.3 eV above the singlet exciton. Our results give direct evidence of energy levels involved in the photogeneration and charge transport within conjugated polymers.

The frontier molecular energy levels of organic semiconductors are decisive for their fundamental function and efficiency in optoelectronics. However, the precise determination of these energy levels and their variation when using different techniques makes it hard to compare and establish design rules. In this work, the energy levels of 33 organic semiconductors via cyclic voltammetry (CV), density functional theory, ultraviolet photoelectron spectroscopy, and low‐energy inverse photoelectron spectroscopy are determined. Solar cells are fabricated to obtain key device parameters and relate them to the significant differences in the energy levels and offsets obtained from different methods. In contrast to CV, the photovoltaic gap measured using photoelectron spectroscopy (PES) correlates well with the experimental device VOC. It is demonstrated that high‐performing systems such as PM6:Y6 and WF3F:Y6, which are previously reported to have negligible ionization energy (IE) offsets (ΔIE), possess sizable ΔIE of ≈0.5 eV, determined by PES. Using various D–A blends, it is demonstrated that ΔIE plays a key role in charge generation. In contrast to earlier reports, it is shown that a vanishing ΔIE is detrimental to device performance. Overall, these findings establish a solid base for reliably evaluating material energetics and interpreting property–performance relationships in organic solar cells.

while fullerene-based OSCs are only 10% efficient. [13] To achieve this milestone, various design strategies have been explored, for example, modification/manipulation of the Y6 acceptor side chain design, [7] the use of ternary mixtures with a vertical phase distribution, [9] the chemical modification via chlorination [8] or a variation of a fused-ring acceptor block of the donor polymer. [10] Currently, NFAs match their inorganic counterparts in terms of current generation, but are lacking with regard to their open-circuit voltage. [14] Efficiency losses can be traced back to energy losses during the photon to free charge conversion, and are in generally lower than in the fullerene-based cells. [15][16][17] Free charge generation in organic solar cells is comprised of two steps. During the first step, a photogenerated exciton dissociates at the donor-acceptor interface into an interfacial charge transfer (CT) state. During this process, the ionization energy or electron affinity offset at the heterojunction provides the driving force for the hole or electron transfer. It is known that this offset should exceed a threshold value in order to enable efficient dissociation of the excited state. [18][19][20] For NFAs, only ionization energy offsets are relevant, because of the fast energy transfer from donors to acceptors. [20] During the second step of charge separation, the interfacial CT state dissociates into a pair of free charges, or the charge separated (CS) state. This dissociation is expected to be an endothermic process, and the exact mechanism behind the driving force for this process is still under debate. [21][22][23][24][25][26] It is, however, one of the key processes in OSCs, since the energetics and dynamics of the dissociating CT state determines the open circuit voltage of organic heterojunctions. [25,[27][28][29] Both steps involved in the free charge generation can be optimized by an appropriate design of the donor-acceptor pair. The main difficulty in formulating generic chemical design rules for OSC materials is that any changes to the chemical structure simultaneously modify the open-circuit voltage, V oc , the short-circuit current, J sc , and the fill factor of the solar cell. [30][31][32][33][34][35] Without knowing how these changes correlate with each other, it is impossible to formulate clear design rules and hence speed up the discovery of efficient donor-acceptor combinations.In this work, we identify the microscopic origin of such correlations and propose clear chemical design rules for NFAs. Efficiencies of organic solar cells have practicallydoubled since the development of non-fullerene acceptors (NFAs). However, generic chemical design rules for donor-NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor-acceptor interface. It is shown that an acceptor-donor-acceptor molecular architecture, and molecular alignment parallel to the interface, results in energy level bending that destabilizes the charge transfer state...

Annealing of TQ1:N2200 photovoltaic blends reduces geminate charge recombination, without compromising charge extraction, leading to higher photocurrents and device efficiency.

The slow decay of charge carriers in polymer-fullerene blends measured in transient studies has raised a number of questions about the mechanisms of nongeminate recombination in these systems. In an attempt to understand this behavior, we have applied a combination of steady-state and transient photoinduced absorption measurements to compare nongeminate recombination behavior in films of neat poly(3-hexyl thiophene) (P3HT) and P3HT blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Transient measurements show that carrier recombination in the neat P3HT film exhibits second-order decay with a recombination rate coefficient that is similar to that predicted by Langevin theory. In addition, temperature dependent measurements indicate that neat films exhibit recombination behavior consistent with the Gaussian disorder model. In contrast, the P3HT:PCBM blend films are characterized by a strongly reduced recombination rate and an apparent recombination order greater than two. We then assess a number of previously proposed explanations for this behavior, including phase separation, carrier concentration dependent mobility, non-encounter limited recombination, and interfacial states. In the end, we propose a model in which pure domains with a Gaussian density of states are separated by a mixed phase with an exponential density of states. We find that such a model can explain both the reduced magnitude of the recombination rate and the high order recombination kinetics and, based on the current state of knowledge, is the most consistent with experimental observations.Comment: 9 pages, 4 figures; corrected a few minor typos and grammatical error

currently at the forefront of efficient OSCs and outperform binary systems in terms of power conversion efficiency (PCE), largely due to improved light absorption and mor phology, especially if novel nonfullerene acceptors are employed. [2] Ternary solar cells use a single (ternary blend) photo active layer for photon harvesting and thus are less challenging and costly to realize, compared to tandem solar cells, [3] which have the potential of even higher efficien cies. [4] However, in TSCs, the third compo nent of the blend not only improves light harvesting, but also plays an active role in the photophysical processes including exciton and chargecarrier dynamics and can, in fact, also influence the blend's morphology. [5] This is of critical impor tance in small moleculebased devices, where intermolecular charge transport dominates. [6] Often, the third component in ter nary solar cells enhances the PCE by improving the shortcircuit current (J SC ) [5c] and opencircuit voltage (V OC ), which both can be tuned by the donor/ acceptor composition. [2d,5b] The chal lenge here is, however, to improve all device parameters, while maintaining or increasing the fill factor (FF) of the ternary organic solar cell. [7] Here, we study the photophysics of such a highperformance allsmallmolecule ternary solar cell com posed of DR3TBDTT (DR3) [8] as an electron donor in combi nation with a nonfullerene smallmolecule acceptor, namely ICC6IDTIC (or ICC6) [9] and PC 71 BM as the third component, shown to enhance the device performance. The fully opti mized ternary devices (1:1:0.4 wt%, DR3:ICC6:PC 71 BM) yield an average PCE of 10.8% with a FF of 72%, V OC of 0.87 V, and J SC of 16.3 mA cm −2 . Using picosecond-nanosecond (ps-ns) transient absorption (TA) spectroscopy, we selectively excite each component of the blend and probe the processes following photoexcitation. We find that excitation of PC 71 BM molecules results in fast singlet energy transfer to ICC6, sub sequently followed by hole transfer to DR3. We confirm this observation by TA and timeresolved photoluminescence (TRPL) spectroscopy on binary blends of PC 71 BM:ICC6, showing fast energy transfer from PC 71 BM to ICC6. Our TA studies demon strate that the increased external quantum efficiency (EQE) of the ternary blend is due to higher mobility of charge carriers in Ternary organic solar cells (OSCs) are among the best-performing organic photovoltaic devices to date, largely due to the recent development of nonfullerene acceptors. However, fullerene molecules still play an important role in ternary OSC systems, since, for reasons not well understood, they often improve the device performance, despite their lack of absorption. Here, the photophysics of a prototypical ternary small-molecule OSC blend composed of the donor DR3, the nonfullerene acceptor ICC6, and the fullerene derivative PC 71 BM is studied by ultrafast spectroscopy. Surprisingly, it is found that after excitation of PC 71 BM, ultrafast singlet energy transfer to ICC6 competes efficiently with cha...

Figure A. Dependence of the maximum of the photoconductivitiy normalised to absorbed number of photons versus fluence determined at different temperatures. At an intensity of 25 µJ/cm 2 /pulse the signals are not maximal due to the occurrence of sub ns second order recombination processes.