Non-Fullerene Acceptors with an Extended π-Conjugated Core: Third Components in Ternary Blends for High-Efficiency, Post-Treatment-Free Organic Solar Cells.

The synthesis of four non-fullerene acceptors (NFAs) with a "A-π-D-π-A" structure, in which the electron-donating core is extended, was achieved. The molecules differed by the nature of the solubilizing groups on the π-spacer and/or the presence of fluorine atoms on the peripheral electron-accepting units. The optoelectronic properties of the molecules were characterized in solution, in thin film, and in photovoltaic devices. The nature of the solubilizing groups had a minor influence on the optoelectronic properties but affected the organization in the solid state. On the other hand, the fluorine atoms influenced the optoelectronics properties and increased the photo-stability of the molecules in thin films. Compared to reference ITIC, the extended molecules showed a wider absorption across the visible range and higher lowest unoccupied molecular orbital energy levels. The photovoltaic performances of the four NFAs were assessed in binary blends using PM6 (PBDB-T-2F) as the donating polymer and in ternary blends with ITIC-4F. Solar cells (active area 0.27 cm2 ) showed power conversion efficiencies of up to 11.1 % when ternary blends were processed from non-halogenated solvents, without any thermal post-treatment or use of halogenated additives, making this process compatible with industrial requirements.

Benzothiadiazole Aryl-amine Based Materials as Efficient Hole Carriers in Perovskite Solar Cells.

Four hole transport materials (HTMs) based on a benzothiadiazole (BT) central core have been synthesized and successfully employed in triple-cation mixed-halide perovskite solar cells (PSCs), reaching 18.05% solar to energy conversion efficiency. The synthesis of these HTMs follows the push-and-pull approach to modulate the HOMO energy level by combining the BT group as an electron acceptor and diphenyl- and triphenyl-amines as electron donors. Here we show that despite adjusting the HOMO energy level to that of the perovskite is a believed requisite to achieve efficient interfacial hole transfer, additional factors must be taken into account to design novel and efficient HTMs, such as a high hole mobility, solubility in organic solvents, and thermal stability.

Advances in the Synthesis of Small Molecules as Hole Transport Materials for Lead Halide Perovskite Solar Cells.

Over hundreds of new organic semiconductor molecules have been synthesized as hole transport materials (HTMs) for perovskite solar cells. However, to date, the well-known N2, N2, N2', N2', N7, N7, N7', octakis-(4-methoxyphenyl)-9,9-spirobi-[9,9'-spirobi[9 H-fluorene]-2,2',7,7'-tetramine (spiro-OMeTAD) is still the best choice for the best perovskite device performance. Nevertheless, there is a consensus that spiro-OMeTAD by itself is not stable enough for long-term stable devices, and its market price makes its use in large-scale production costly. Novel synthetic routes for new HTMs have to be sought that can be carried out in fewer synthetic steps and can be easily scaled up for commercial purposes. On the one hand, synthetic chemists have taken, as a first approach, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the spiro-OMeTAD molecule as a reference to synthesize molecules with similar energy levels, although these HOMO and LUMO energy levels often have been measured indirectly in solution using cyclic voltammetry. On the other hand, the "spiro" chemical core has also been studied as a structural motif for novel HTMs. However, only a few molecules incorporated as HTMs in complete functional perovskite solar cells have been capable of matching the performance of the best-performing perovskite solar cells made using spiro-OMeTAD. In this Account, we describe the advances in the synthesis of HTMs that have been tested in perovskite solar cells. The comparison of solar cell efficiencies is of course very challenging because the solar cell preparation conditions may differ from laboratory to laboratory. To extract valuable information about the HTM molecular structure-device function relationship, we describe those examples that always have used spiro-OMeTAD as a control device and have always used identical experimental conditions (e.g., the use of the same chemical dopant for the HTM or the lack of it). The pioneering work was focused on well-understood organic semiconductor moieties such as arylamine, carbazole, and thiophene. Those chemical structures have been largely employed and studied as HTMs, for instance, in organic light-emitting devices. Interestingly, most research groups have reported the hole mobility values for their novel HTMs. However, only a few examples have been found that have measured the HOMO and LUMO energy levels using advanced spectroscopic techniques to determine these reference energy values directly. Moreover, it has been shown that those molecules, upon interacting with the perovskite layer, often have different HOMO and LUMO energies than the values estimated indirectly using solution-based electrochemical methods. Last but not least, porphyrins and phthalocyanines have also been synthesized as potential HTMs for perovskite solar cells. Their optical and physical properties, such as high absorption and good energy transfer capabilities, open new possibilities for HTMs in perovskite solar cells.

Charge Injection, Carriers Recombination and HOMO Energy Level Relationship in Perovskite Solar Cells.

We present a comparative study between a series of well-known semiconductor polymers, used in efficient organic solar cells as hole transport materials (HTM), and the state-of-the art material used as hole transport material in perovskite solar cells: the spiro-OMeTAD. The observed differences in solar cell efficiencies are studied in depth using advanced photoinduced spectroscopic techniques under working illumination conditions. We have observed that there is no correlation between the highest occupied molecular orbital (HOMO) energy levels of the organic semiconductors and the measured open-circuit voltage (VOC). For instance, spiro-OMeTAD and P3HT have a comparable HOMO level of ~5.2 eV vs vacuum even though a difference in VOC of around 200 mV is recorded. This difference is in good agreement with the shift observed for the charge vs voltage measurements. Moreover, hole transfer from the perovskite to the HTM, estimated qualitatively from fluorescence quenching and emission lifetime, seems less efficient for the polymeric HTMs. Finally, the recombination currents from all devices were estimated by using the measured charge (calculated using photoinduced differential charging) and the carriers' lifetime and their value resulted in accordance with the registered short-circuit currents (JSC) at 1 sun.

Selective Organic Contacts for Methyl Ammonium Lead Iodide (MAPI) Perovskite Solar Cells: Influence of Layer Thickness on Carriers Extraction and Carriers Lifetime.

We have fabricated MAPI solar cells using as selective contacts PEDOT:PSS polymer for holes and PCBM-C70 fullerene derivative for electrons. The thickness of MAPI, PCBM-C70, and PEDOT:PSS layers has been varied in order to evaluate the contribution of each layer to the final device performance. We have measured the devices capacitance under illumination and the charge carrier's lifetime using photoinduced time-resolved techniques. The results show that in this kind of devices the limiting layer is the PCBM-C70 due to its relative reduced mobility compared to PEDOT:PSS that makes the control of the fullerene thickness crucial for device optimization. Moreover, capacitive measurements show differences for the devices having different PCBM-C70 layer thicknesses in contrast with the measurements on the different PEDOT:PSS thickness. These give indications about holes and electrons storage and their distribution.

Fully Solution-Processed n-i-p-Like Perovskite Solar Cells with Planar Junction: How the Charge Extracting Layer Determines the Open-Circuit Voltage.

Fully solution-processed direct perovskite solar cells with a planar junction are realized by incorporating a cross-linked [6,6]-phenyl-C61-butyric styryl dendron ester layer as an electron extracting layer. Power conversion efficiencies close to 19% and an open-circuit voltage exceeding 1.1 V with negligible hysteresis are delivered. A perovskite film with superb optoelectronic qualities is grown, which reduces carrier recombination losses and hence increases V oc .

Diarylamino-substituted tetraarylethene (TAE) as an efficient and robust hole transport material for 11% methyl ammonium lead iodide perovskite solar cells.

We report the synthesis and characterisation of tetra{4-[N,N-(4,4'-dimethoxydiphenylamino)]phenyl}ethene () as an efficient and robust hole transport material for its application in methyl ammonium lead iodide (MAPI) perovskite solar cells. The solar cells show light-to-energy conversion efficiencies as high as 11.0% under standard measurement conditions without the need of additional dopants.

Efficient bulk heterojunction solar cells based on solution processed small molecules based on the same benzo[1,2-b:4, 5-b']thiophene unit as core donor and different terminal units.

We report the synthesis, characterization, and optical and electrochemical of properties of two novel molecules DRT3-BDT (1) and DTT3-BDT (2), comprising the same BDT central core (donor) and different end capped acceptor units, i.e. rhodanine with ethyl hexyl and thiazolidione with ethylhexyl, respectively, linked via an alkyl-substituted terthiophene (3 T) π-conjugation bridge. The electrochemical properties of these small molecules indicate that their energy levels are compatible with energy levels of PC71BM for efficient exciton dissociation. These molecules have been used as electron donors along with PC71BM as an electron acceptor, for the fabrication of solution processed "small molecule" bulk heterojunction (BHJ) solar cells (smOPV). The device prepared from optimized 1:PC71BM(1:1) processed cast from DIO (3%v)/CF solvent exhibited a power conversion efficiency of 6.76% with Jsc = 11.92 mA cm(-2), Voc = 0.90 and FF = 0.63. The device with 2:PC71BM under the same conditions showed a lower PCE of 5.25% with Jsc = 10.52 mA cm(-2), Voc = 0.86 and FF = 0.56. The AFM, TEM and PL quenching measurements revealed that the high Jsc is a result of the appropriate morphology and exciton dissociation. The performances were compared for the devices based on two small molecules. The higher Jsc for device 1 was attributed to its better nanoscale phase separation, smooth surface and higher carrier mobility in the 1:PC71BM blend film. Moreover, the higher value of FF for the 1:PC71BM based device was ascribed to a good balance between the electron and hole mobilities.

Synthesis, optical and electrochemical properties of the A-π-D-π-A porphyrin and its application as an electron donor in efficient solution processed bulk heterojunction solar cells.

A conjugated acceptor-donor-acceptor (A-π-D-π-A) with the Zn-porphyrin core and the di-cyanovinyl substituted thiophene (A) connected at meso positions denoted as was designed and synthesized. The optical and electrochemical properties of were investigated. This new porphyrin exhibits a broad and intense absorption in the visible and near infrared regions. Bulk-heterojunction (BHJ) solution processed organic solar cells based on this porphyrin, as electron donor material, and PC71BM ([6,6]-phenyl C71 butyric acid methyl ester), as electron acceptor material, were fabricated using THF and a pyridine-THF solvent exhibiting a power conversion efficiency of 3.65% and 5.24%, respectively. The difference in efficiencies is due to the enhancement of the short circuit current J(sc) and FF of the solar cell, which is ascribed to a stronger and broader incident photon to current efficiency (IPCE) response and a better balanced charge transport in the device processed with the pyridine-THF solvent.

Dye molecular structure device open-circuit voltage correlation in Ru(II) sensitizers with heteroleptic tridentate chelates for dye-sensitized solar cells.

Dicarboxyterpyridine chelates with π-conjugated pendant groups attached at the 5- or 6-position of the terminal pyridyl unit were synthesized. Together with 2,6-bis(5-pyrazolyl)pyridine, these were used successfully to prepare a series of novel heteroleptic, bis-tridentate Ru(II) sensitizers, denoted as TF-11-14. These dyes show excellent performance in dye-sensitized solar cells (DSCs) under AM1.5G simulated sunlight at a light intensity of 100 mW cm(-2) in comparison with a reference device containing [Ru(Htctpy)(NCS)(3)][TBA](3) (N749), where H(3)tctpy and TBA are 4,4',4"-tricarboxy-2,2':6',2"-terpyridine and tetra-n-butylammonium cation, respectively. In particular, the sensitizer TF-12 gave a short-circuit photocurrent of 19.0 mA cm(-2), an open-circuit voltage (V(OC)) of 0.71 V, and a fill factor of 0.68, affording an overall conversion efficiency of 9.21%. The increased conjugation conferred to the TF dyes by the addition of the π-conjugated pendant groups increases both their light-harvesting and photovoltaic energy conversion capability in comparison with N749. Detailed recombination processes in these devices were probed by various spectroscopic and dynamics measurements, and a clear correlation between the device V(OC) and the cell electron lifetime was established. In agreement with several other recent studies, the results demonstrate that high efficiencies can also be achieved with Ru(II) sensitizers that do not contain thiocyanate ancillaries. This bis-tridentate, dual-carboxy anchor configuration thus serves as a prototype for future omnibearing design of highly efficient Ru(II) sensitizers suited for use in DSCs.