No abstract

Thin films of a molecular spin crossover iron(ii) complex featuring a photochromic diarylethene-based ligand have been grown by sublimation in ultra-high vacuum on Au(111) and investigated by photoelectron spectroscopies.

Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges. However, the use of D-A blends intrinsically limits the photovoltage and introduces morphological instability. Here, we demonstrate that polycrystalline films of chemically identical molecules offer a promising alternative and show that photoexcitation of α-sexithiophene (α-6T) films results in efficient charge generation. This leads to α-6T based homojunction organic solar cells with an external quantum efficiency reaching up to 44% and an open-circuit voltage of 1.61 V. Morphological, photoemission, and modelling studies show that boundaries between α-6T crystalline domains with different orientations generate an electrostatic landscape with an interfacial energy offset of 0.4 eV, which promotes the formation of hybridised exciton/charge-transfer states at the interface, dissociating efficiently into free charges. Our findings open new avenues for organic solar cell design where material energetics are tuned through molecular electrostatic engineering and mesoscale structural control.

efficiency (PCE). [4,5] The main reason is their low open-circuit voltage (V OC ) as compared to the optical gap (E opt ) of the main absorbing materials. [6] All photovoltaic (PV) technologies suffer from voltage losses, arising from fundamental radiative recombination and parasitic nonradiative recombination. Radiative recombination is inevitable, and is the only recombination process taking place in an ideal solar cell. [7][8][9][10] This process determines the upper limit of the V OC , denoted as the radiative open-circuit voltage V r . In reality, the measured V OC is lower than V r due to the presence of nonradiative decay channels, lowering V r by ΔV nrRau has shown that ΔV nr is proportional to the natural logarithm of the quantum efficiency of emission (EQE EL ). [7] The validity of Equation (1) for OSCs has been shown previously, [11,12] where ΔV nr typically accounts for 0.25-0.40 V of the total voltage losses (ΔV OC = E CT − V OC ). [8,[12][13][14] This is a much higher value than in inorganic and Perovskite solar cells, where ΔV nr ≤0.15 V. [15][16][17] In addition to voltage losses due to radiative and nonradiative recombination, OSCs suffer voltage losses because the photogenerated excitons on the donor (D) or acceptor (A) undergo a charge transfer to form an interfacial charge-transfer (CT) state with energy E CT . However, it has been recently shown that the energy difference between the optical gap of the donor or acceptor and the CT state (E opt − E CT ) can be minimized to less than 0.05 eV [13,18] and even down to 0.01 eV, [6] without sacrificing efficient free charge carrier generation. Therefore, in the OSCs with the currently lowest voltage losses, nonradiative recombination is the main reason for the low V OC as compared to other PV technologies employing absorber with similar optical gaps.In a previous study, we have shown for a whole range of solution and vacuum processed OSCs that ΔV nr correlates with E CT . This led us to the conclusion that nonradiative decay is mediated by CT state decay via electron-phonon coupling. [12] However, in the related OLED technology, the major nonradiative decay channel is mediated by the triplet excited states. [19] In OSCs, triplet states are present on both the D and A materials, and for high voltage OSCs the energy of the lowest energy The best organic solar cells (OSCs) achieve comparable peak external quantum efficiencies and fill factors as conventional photovoltaic devices. However, their voltage losses are much higher, in particular those due to nonradiative recombination. To investigate the possible role of triplet states on the donor or acceptor materials in this process, model systems comprising Zn-and Cu-phthalocyanine (Pc), as well as fluorinated versions of these donors, combined with C 60 as acceptor are studied. Fluorination allows tuning the energy level alignment between the lowest energy triplet state (T 1 ) and the charge-transfer (CT) state, while the replacement of Zn by Cu as the central metal in the Pcs leads to a largely enhanc...

We report on computational studies of the potential of three borane Lewis acids (LAs) (B(C6F5)3 (BCF), BF3, and BBr3) to form stable adducts and/or to generate positive polarons with three...

Multiple charge transfer states in Donor-acceptor heterojunctions with large frontier orbital energy offsets.

Engineering a low singlet-triplet energy gap (ΔEST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 105 cm−1) and a relatively large ΔEST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 μs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔEST in organic DF emitters.

The role of local environmental interactions on the generation of free charge carriers in doped organic layers is investigated. Via a classical micro-electrostatic model, a dual effect of molecular quadrupole moments of host and dopant molecules on doping is demonstrated. Namely, electrostatic interactions ease ionization of the dopant by altering the energy level alignment between the host and the dopant and reduce the barrier for charge dissociation by flattening the energy landscape around the ionized dopants. These results indicate that tailoring molecular quadrupole moments of the host and/or dopant are an attractive strategy toward improved doping efficiency in organic semiconductors.