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Register a new userSequentially Deposited versus Conventional Nonfullerene Organic Solar Cells: Interfacial Trap States, Vertical Stratification, and Exciton Dissociation
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Bulk-heterojunction (BHJ) non-fullerene organic solar cells prepared from sequentially deposited donor and acceptor layers (sq-BHJ) have recently been promising to be highly efficient, environmentally friendly, and compatible with large area and roll-to-toll fabrication. However, the related photophysics at donor-acceptor interface and the vertical heterogeneity of donor-acceptor distribution, critical for exciton dissociation and device performance, are largely unexplored. Herein, steady-state and time-resolved optical and electrical techniques are employed to characterize the interfacial trap states. Correlation with the luminescent efficiency of interfacial states and its non-radiative recombination, interfacial trap states are characterized to be about 50% more populated in the sq-BHJ than as-cast BHJ (c-BHJ), which probably limits the device voltage output. Cross-sectional energy-dispersive X-ray spectroscopy and ultraviolet photoemission spectroscopy depth profiling directly vizualize the donor-acceptor vertical stratification with a precision of 1-2 nm. From the proposed needle model, the high exciton dissociation efficiency is rationalized. Our study highlights the promise of sequential deposition to fabricate efficient solar cells, and points towards improving the voltage output and overall device performance via eliminating interfacial trap states.
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Ternary organic solar cells (TOSC) are currently under intensive investigation, recently reaching a record efficiency of 17.1%. The origin of the device open-circuit voltage (VOC), already a multifaceted issue in binary OSC, is even more complex in TOSCs. Herein, we investigate two ternary systems with one donor (D) and two acceptor materials (A1, A2) including fullerene and non-fullerene acceptors. By varying the ratio between the two acceptors, we find the VOC to be gradually tuned between those of the two binary systems, D:A1 and D:A2. To investigate the origin of this change, we employ ultra-violet photoemission spectroscopy (UPS) depth profiling, which is used to estimate the photovoltaic gap in the ternary systems. Our results reveal an excellent agreement between the estimated photovoltaic gap and the VOC for all mixing ratios, suggesting that the energetic alignment between the blend components varies depending on the ratio D:A1:A2. Furthermore, our results indicate that the sum of radiative and non-radiative losses in these ternary systems is independent of the blend composition. Finally, we demonstrate the superiority of UPS over X-ray photoemission spectroscopy (XPS) depth profiling in resolving compositional profiles for material combinations with very similar chemical, but dissimilar electronic structures, as common in TOSCs.
We investigate nongeminate recombination in organic solar cells based on copper phthalocyanine (CuPc) and C$_{60}$. Two device architectures, the planar heterojunction (PHJ) and the bulk heterojunction (BHJ), are directly compared in view of differences in charge carrier decay dynamics. We apply a combination of transient photovoltage (TPV) experiments, yielding the small perturbation charge carrier lifetime, and charge extraction measurements, providing the charge carrier density. In organic solar cells, charge photogeneration and recombination primarily occur at the donor--acceptor heterointerface. Whereas the BHJ can often be approximated by an effective medium due to rather small scale phase separation, the PHJ has a well defined two-dimensional heterointerface. To study recombination dynamics in PHJ devices most relevant is the charge accumulation at this interface. As from extraction techniques only the spatially averaged carrier concentration can be determined, we derive the charge carrier density at the interface $n_{int}$ from the open circuit voltage. Comparing the experimental results with macroscopic device simulation we discuss the differences of recombination and charge carrier densities in CuPc:C$_{60}$ PHJ and BHJ devices with respect to the device performance. The open circuit voltage of BHJ is larger than for PHJ at low light intensities, but at 0.3 sun the situation is reversed: here, the PHJ can finally take advantage of its generally longer charge carrier lifetimes, as the active recombination region is smaller.
Engineering a low singlet-triplet energy gap ({Delta}EST) 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 =3.8x10^5 cm^-1) and a relatively large {Delta}EST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (260 {mu}s), but in aggregated films, BF2 generates intermolecular CT (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 resolves the conflicting requirements of fast radiative emission and low {Delta}EST.
Charge transfer in polymer devices represents a crucial, though highly inaccessible stage of photocurrent generation. In this article we propose studying the properties and behaviour of organic solar cells through the modification of photocurrent generation when an external magnetic field is applied. By allowing the parameters of our theoretical model not to be constrained to any specific material, we are able to show that not only a modest external magnetic field leads to a significant increase in photocurrent intensity, but also how such magnetic field can be used to study in detail the energy levels and transition rates within the polymer compound. Systematic exploration of key properties in organic composites thus can lead to highly optimised devices in which a magnetic field produces an enhancement in the efficiency of polymer solar cells.
Despite many advances towards improving the stability of organic photovoltaic devices, environmental degradation under ambient conditions remains a challenging obstacle for future application. Particularly conventional systems employing fullerene derivatives are prone to oxidise under illumination, limiting their applicability. Herein, we report on the environmental stability of the small molecule donor DRCN5T together with the fullerene acceptor PC70BM. We find that this system exhibits exceptional device stability, mainly due to almost constant short-circuit current. By employing ultrafast femtosecond transient absorption spectroscopy we attribute this remarkable stability to two separate mechanisms: 1) DRCN5T exhibits high intrinsic resistance towards external factors, showing no signs of deterioration. 2) The highly sensitive PC70BM is stabilised against degradation by the presence of DRCN5T through ultrafast long-range energy transfer to the donor, rapidly quenching the fullerene excited states which are otherwise precursors for chemical oxidation. We propose that this photoprotective mechanism be utilised to improve the device stability of other systems, including non-fullerene acceptors and ternary blends.
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