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New unsymmetrical squaraines for organic solar cells (OSCs) are synthesized without using highly toxic reagents. OSCs based on these squaraines exhibit an impressive open-circuit voltage of 0.92 V and power conversion efficiencies approaching 5%.
Non-toxic and air-stable silver bismuth iodide semiconductors are promising light absorber candidates for photovoltaic applications owing to a suitable band gap for multi- or single-junction solar cells. Recently, solution-based film fabrication approaches for several silver bismuth iodide stoichiometries have been investigated. The current work reports on a facile and reproducible two-step coevaporation/annealing approach to deposit compact and pinhole-free films of AgBi 2 I 7 , AgBiI 4 and Ag 2 BiI 5 . X-ray diffraction (XRD) in combination with scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX) analysis reveals formation of pure cubic (Fd$\bar{3}$m) with combining macron]m) phase AgBi 2 I 7 , cubic (Fd$\bar{3}$m) or rhombohedra (R$\bar{3}$m with combining macron]m) phase AgBiI 4 , each with >3 μm average grain size, or the rhombohedral phase (R$\bar{3}$m) Ag 2 BiI 5 with >200 nm average grain size. A phase transition from rhombohedral to cubic structure is investigated via temperature-dependent X-ray diffraction (TD-XRD). Planar-junction photovoltaic (PV) devices are prepared based on the coevaporated rhombohedral AgBiI 4 films, with titanium dioxide (TiO 2 ) and poly(3-hexylthiophene) (P3HT) as electron- and hole-transport layers, respectively. Finally, the best-performing device exhibited a power conversion efficiency (PCE) of as high as 0.9% with open-circuit voltage (VOC) > 0.8 V in the reverse scan direction (with significant hysteresis).
Highly fluorescent N-doped graphene quantum dots (NGQDs) and graphitic carbon nitride quantum dots (CNQDs, g-C 3 N 4 ) were synthesized using a solid-phase microwave-assisted (SPMA) technique. The SPMA method, based on the pyrolysis of citric acid and urea with different recipes, is capable of producing quantum dots with coexisting NGQDs and CNQDs at 280°C within only five minutes. The photoluminescence (PL) emissions from NGQD and CNQDs are strongly dependent on excitation wavelength and the solvent type, i.e., water, ethanol, and N-methyl pyrrolidinone. The unique design of the quantum dots possesses a multiple chromophoric band-gap structure, originated from the presence of g-C 3 N 4 , defect-related emissive traps, and grain boundaries. Thus, an appropriate excitation wavelength induces a conjugated π electron system to fulfill the most probable absorption band, resulting in wavelength-dependent emissions including ultraviolet, visible and infrared light. Here, the quantum yield of the NGQD and CNQD samples can attain as high as 68.1%. Accordingly, one light-emitting device using the combination of the NGQD and CNQD powders embedded polymeric film can emit white-like light with high power conversion efficiency.
A novel class of β-functionalized push–pull zinc opp-dibenzoporphyrins was designed and synthesized as sensitizers for dye-sensitized solar cells. Dibenzoporphyrin YH7 displayed strong push–pull characteristics, which included clearly segregated HOMO/LUMO and fast charge injection from the singlet excited state of zinc porphyrin to the conduction band of TiO 2 . The incorporation of both the phenylethynyl bridge and amino group was critical to achieve the enhanced push–pull effect. The highest PCE value in this series obtained with the YH6 cell was 6.7%, which is comparable to that of the best ruthenium dye, N719 (η = 7.4%), under similar conditions. The high PCE of the YH6 cell is ascribed to its high light harvesting ability and favorable cell kinetics, which is supported by electrochemical impedance spectroscopy studies and femtosecond transient absorption studies. An unusual illumination-induced photodegradation was observed for YH7, which was attributed as the major reason for the unexpected lower PCE of the YH7 cell (η = 5.7%). Furthermore, this work demonstrates the applicability of π-extended porphyrins as a new generation of sensitizers for high performance dye-sensitized solar cells.
Organic photovoltaics have reached high power conversion efficiencies (PCE) using non-fullerene acceptors (NFAs). However, the best NFAs tend to have complex syntheses, limiting scalability. Among polymer donors, regioregular poly(3-hexylthiophene) (P3HT) has the greatest potential for commercialization due to its simple synthesis and good stability, but PCEs have been limited. It is thus imperative to find scalable NFAs that give high PCE with P3HT. Herein we report a zinc( II ) complex of di(naphthylethynyl)azadipyrromethene (Zn(L2) 2 ) as a non-planar NFA that can be synthesized on the gram scale using inexpensive starting materials without chromatography column purification. The NFA has strong absorption in the 600–800 nm region. Time-dependent density-functional theory calculations suggest that the low-energy absorptions can be understood within a four-orbital model involving transitions between π-orbitals on the azadipyrromethene core. OPVs fabricated from P3HT:Zn(L2) 2 blends reached a PCE of 5.5%, and the PCE was not very sensitive to the P3HT:Zn(L2) 2 weight ratio. Due to its shape, Zn(L2) 2 shows isotropic charge transport and its potential as an electron donor is also demonstrated. The combination of simple synthesis, good PCE and photostability, and tolerance to the active material weight ratio demonstrates the potential of Zn(L2) 2 as an active layer material in OPVs.
Poly(N-(5-(5-norbornene-2-carbonyl)oxy)pentyl)-N'-n-hexyl-naphthalene-1,8:4,5-bis(dicarboximide) has been synthesized by esterification of (N-(5-hydroxypentyl)-N'-n-hexyl-naphthalene-1,8:4,5-bis(dicarboximide)) with exo-5-norbornene-2-carboxylic acid, and has been polymerized using the first-generation Grubbs initiator. This side-chain polymer exhibits good transparency throughout the visible (absorption onset at ca. 400 nm), good solubility in common low- and medium-polarity organic solvents, good resistance to dimethylformamide, and appropriate electron affinity for use as an electron-extraction layer in lead-halide perovskite solar cells. The performance of this polymer in n-i-p perovskite solar cells was compared to that of several small-molecule naphthalene-1,8:4,5-bis(dicarboximide) derivatives and of SnO 2 . Solar cells using the polymer exhibited open-circuit voltages of up to 1.02 V, short-circuit currents of over 21 mA cm –2 , and power conversion efficiencies (PCE) reaching 14% which stabilize at 13.8% upon 90 s of illumination. Meanwhile control SnO 2 devices exhibited a PCE of ca. 16%, and small-molecule devices gave PCE values of less than 10%. Here, the devices employing the polymer exhibited improved long-term stability relative to the SnO 2 control devices under continuous illumination.
The solar-to-power power conversion efficiencies (PCEs) of metal halide perovskites (MHP) have improved over the last decade using a wide variety of methods, including composition manipulation, dopant introduction, and interfacial buffers. These methods, however, have taken little regard for the electronic and interfacial effects such alterations may cause within devices under voltage bias stress – a condition required for most device operation. We investigate how halide and cation substitution in MHP structures [specifically, CH 3 NH 3 PbI 2.87 Cl 0.13 and Cs 0.1 (MA 0.17 FA 0.83 ) 0.9 Pb(I 0.83 Br 0.17 ) 3 ] effects the current behavior of devices while under a range of voltage bias stress in both light and dark conditions. Additionally, conducting in depth investigations into the electronic and morphological differences between these two MHP devices, we confirmed their unique voltage bias stress effects are due to intrinsic behavior within the perovskite structure. Importantly, we also determined how crystallite orientation can influence ion migration and therefore alter charge transport and current stability in MHP photovoltaic devices.
Introduction of chlorine in the conjugated side chains significantly improves open-circuit voltage and power conversion efficiency, benefiting from a lower HOMO energy level, well-balanced charge transport and superior nanoscale morphology.
We develop an effective strategy to obtain polymer solar cells with high power conversion efficiency (17%) and remarkable mechanical robustness, by design of polymer donors with aliphatic flexible spacer incorporated into a rigid backbone.
Interfacial layers (IFLs) are ubiquitous in solar cells, but their precise impact on carrier transfer rates and the relation of these rates to performance metrics and the concepts of selectivity and recombination is lacking. We report the use of a well-defined interdigitated back-contact (IBC) silicon solar cell to determine the precise role of such IFLs. We characterize the action of the common IFL spiro-OMeTAD by making it a third contact to the IBC cell. This architecture creates three solar cells within a single structure that, with numerical simulation, provide the exchange current densities (i.e., charge transfer rates) for electrons ( J 0n ) and holes ( J 0p ) and the quasi Fermi-level splitting in the absorber, which measures the balance of generation and recombination. Further, we describe the relation of V oc to contact recombination, the asymmetry in electron/hole collection rates at a single contact (contact selectivity), and the asymmetry in collection rates of the same carrier at separate contacts (carrier selectivity). Relative to bare gold, neat spiro-OMeTAD reduces J 0n and J 0p (their geometric mean ( J 0n J 0p ) 0.5 decreases by 10 4 ), decreasing contact recombination. Addition of the common dopant Li-TFSI and air increase J 0p / J 0n by 10 6 with little effect on ( J 0n J 0p ) 0.5 , increasing contact hole selectivity. The significant increase in V oc observed by introducing spiro-OMeTAD/Li-TFSI IFLs into the cells studied, however, is due to an increase in the carrier selectivity rather than the contact selectivity or recombination of the spiro-OMeTAD-modified contact. Operando measurements further show voltage-dependent changes in the J 0 s, demonstrating that spiro-OMeTAD contributes to hysteresis. Broader context : Solar cells operate by photogenerating excess charge carriers in an absorber material and, in competition with recombination, asymmetrically extracting them at so-called carrier-selective contacts, one that ideally collects only electrons and the other, only holes. Particularly in emerging photovoltaics such as perovskites, thin layers of organic semiconductors or related materials are introduced between the absorber and contact to improve power conversion efficiency. In terms of interfacial charge transfer, a prevailing view is that such interfacial layers improve performance by helping block the collection of the undesired carrier, considered a form of recombination. We use a novel platform to study the simultaneous impact of spiro-OMeTAD, a common interfacial layer, on the collection of electrons, the collection of holes, and on the recombination of electrons and holes. We quantitatively demonstrate that spiro-OMeTAD layers indeed passivate the contact toward recombination, but that larger improvements in the open-circuit voltage, a key cell metric, can come not only from this but also from increasing the asymmetry of the collection of electrons in the system. Further, operando measurements show transient changes in the properties of spiro-OMeTAD which suggest that it contributes to hysteresis phenomena commonly observed in perovskite and other solar cells.
An accurate assessment of bifacial solar cells under concurrent bifacial illumination is critical to evaluate their real photovoltaic performance. Here, we demonstrate bifacial perovskite solar cells with bifacial equivalent power conversion efficiencies of 21% and 26% under concurrent bifacial illumination with an albedo of 0.2 and 0.5, respectively. A detailed balance efficiency limit analysis further reveals the full potential of bifacial perovskite solar cells.
Semitransparent organic solar cells based on sequentially processed heterojunction (SHJ) structure show a power conversion efficiency (PCE) of 12.5% and an average visible transmittance (AVT) of 25.4%, outperforming the BHJ counterpart (PCE = 11.8%, AVT = 22.9%).
Organic solar cells (OSCs) based on donor–acceptor blends have shown a rapid improvement in power conversion efficiency (PCE) now approaching, for small cells, those of the state-of-the art commercial solar modules. However, performance degradation remains one of the most critical impediments for OSC technology commercialization. Ternary solar cells where a third component, for instance an acceptor, is added to a non-fullerene acceptor–polymer donor blend are an effective approach for improving both OSC efficiency and long-term stability. Here, in this work, we study the role of two fullerene acceptors, ET18 and PCBM, as the third component in P D :Y6 blends. These fullerene derivatives significantly enhance the cell stability, which retained >90% of their initial PCEs (13–14%) even after storage in air for 6 months, compared to only ~20% retention for the binary devices. GIWAXS, AFM, in situ impedance spectroscopy and femtosecond transient absorption spectroscopy measurements reveal that the enhanced stability of the ternary devices results from a more robust blend morphology reducing charge recombination in the ternary devices during aging.
Although the recent development of Y-series small-molecule acceptors (SMAs) has led to a dramatic increase in the power conversion efficiency (PCE) of organic solar cells (OSCs), the operational stability (device lifetime) of OSCs is inadequate for commercialization. In this study, we develop a new trimer acceptor (TYT), consisting of three Y-based molecules linked by electron-donating spacers, to realize an OSC with high-performance (PCE > 18%) and -stability (t 80% lifetime > 8000 h under 1 sun illumination, in which t 80% lifetime is the time required for the PCE of the OSC to reach 80% of its initial value). We demonstrate that the trimerization approach affords an acceptor, TYT, with an upshifted lowest unoccupied molecular orbital energy level, which, in turn, affords an efficient OSC with a high open-circuit voltage (0.964 V). In addition, the glass-transition temperature (T g ) of TYT (217 °C) is significantly higher than those of monomer (MYT, T g = 80 °C) and dimer (DYT, T g = 127 °C) acceptors, which effectively suppresses the molecular diffusion of TYT in a blend film with a polymer donor. Accordingly, a TYT-based OSC demonstrates a high PCE (18.2%) and long t 80% lifetime under 1 sun illumination (8454 h), outperforming MYT- and DYT-based OSCs that exhibit PCEs and t 80% lifetimes of 16.4% and 35 h, and 17.3% and 2551 h, respectively. Therefore, this study provides important guidelines for the design of electron acceptors in achieving OSCs with high performance and stability close to a commercial level.
Synthetic design has enabled increasing power conversion efficiency advances in organic photovoltaics (OPV). One continuing knowledge gap is detailed understanding of single-material chemical (photo)stability. Many considerations are based on prior OPV-relevant donor homopolymer systems rather than the next-generation push–pull architectures. Generally, energetic offsets between the lowest occupied molecular orbital of the donor and molecular oxygen are assumed to dictate kinetics of photo-induced charge transfer to a super oxide radical. Herein we determine the ambient-induced photo-degradation pathways, presented as proposed site-specific arrow-pushing mechanisms, for five donor polymers all containing the same push unit – alkylthienyl-substituted-benzodithiophene (BDTT) – but with chemically distinct pull units. The donor-only polymer films were subject to controlled photobleaching in air as an accelerated degradation approach, coupled with simultaneous monitoring of absorptance. Sample subsets were periodically removed and analyzed with X-ray photoelectron spectroscopy, to evaluate near-surface chemical composition and oxygen additions to hetero reporter atoms on either the BDTT push unit or distinct pull unit (i.e., sulphur and nitrogen). Here, this methodology allows us to distinguish between different mechanisms of bond cleavage and formation. Overall, we find that neither polymer redox properties nor individual push or pull unit stability are sufficient to predict photo-oxidative degradation of these polymers. Rather, there is a greater dependence on the susceptibility of unique structural groups within a polymeric system. Alkyl chain oxygen addition is generally the first attack site and direct sulfur oxidation on the conjugated backbone occurs after saturation of the alkyl chain initiation sites. This work provides a standard that could be used to evaluate relative photo-oxidative (in)stability for new OPV materials quickly – prior to time-consuming device optimization – and demonstrates an effective methodology for correlating optical degradation with chemical structure alterations via spectroscopic signatures to guide synthetic design.
Scanning 3DXRD was used to visualize strain localization at grain boundaries with a high spatial resolution of 100 nm.
The development of elastomer-containing photoactive block copolymers with regulated rigid-block lengths results in organic solar cells with high power conversion efficiency (17.3%) and mechanical robustness (toughness = 1.8 MJ m −3 ).
Halide perovskites (HPs) have remarkable optoelectronic properties, and in the last decade their photovoltaic power conversion efficiency and light-emitting diode efficiency have skyrocketed. Despite the surge in research on these burgeoning materials, two key challenges in the field remain: material irreproducibility and instability. Their behavior is especially dynamic in response to environmental stressors, due to complex interactions with the perovskite crystal lattice. Here, in this review, we survey the latest achievements in HP materials research accomplished with the assistance of artificial intelligence (AI), through the implementation of automated experimentation and machine learning (ML) data analysis. Automated synthesis and characterization tackle problems with material irreproducibility by systematically controlling parameters with very high precision, creating massive datasets, and allowing methodical comparisons from which unbiased conclusions can be drawn. AI can reveal otherwise unnoticed trends, inform future experiments with the highest potential information gain, and forecast future performance. The review concludes with a forward viewpoint of how human-assisted closed-loop laboratories and shared databases allow halide perovskite materials’ processing, properties, and performance to be potentially optimized with AI, accelerating the development of highly reproducible and stable optoelectronic devices.