Conspectus 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 N 2,N 2,N 2′,N ...2′,N 7,N 7,N 7′, octakis-(4-methoxyphenyl)-9,9-spirobi-9,9′-spirobi9H-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.
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 (V
). For instance, spiro-OMeTAD and P3HT have a comparable HOMO level of ~5.2 eV vs vacuum even though a difference in V
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 (J
) at 1 sun.
In this work we report how crucial is the correct design of the porphyrin sensitizers in Dye Sensitized Solar Cells (DSSCs). Only a single atom change switches-onthe efficiency from 2-3% to over 10% ...under standard measurement conditions. We used the 2,1,3-benzothiadazole (BDT) group, as a pi -conjugated linker, for the porphyrin LCVC01, a thiophene moiety for the porphyrin LCVC02 and also the furan group for the LCVC03 porphyrin, as molecular spacers between the BDT fragment and the molecule anchoring group, respectively. These three porphyrins were investigated for their application in DSSC devices. All the devices were characterized and found to achieve a record cell efficiency of 10.5% for LCVC02 but only 3.84% and 2.55% for LCVC01 and LCVC03 respectively. On one hand, the introduction of a thiophene, instead of a furan group, illustrates the importance of introducing a chemical group as a spacer, such as thiophene, between the BDT and the anchoring group. On the other hand, the selection of this group has to be correct because the change of a single atom increases the charge recombination rate and decreases the device performance. These changes can be rationalized by analyzing the dye dipoles and their interactions.
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.
In this work, we analyze the carrier recombination kinetics and the associated charge carrier density in methylammonium lead iodide perovskite (MAPI) solar cells that use mesoporous TiO2 as selective ...contact (m-MAPI) and flat solar cells (without the mesoporous TiO2, f-MAPI), which are the most common device architectures for perovskite solar cells. The use of PIT-PV (photoinduced transient photovoltage) and L-TAS (laser transient absorption spectroscopy) showed that for devices that cannot reach efficiencies close to 19% there is a slow component of the photovoltage decay that corresponds to a charge recombination pathway for carrier losses responsible for the lower device efficiency. Moreover, we have also identified a primary interfacial charge recombination pathway for carrier losses that is common in all devices studied, independent of their efficiency or their device structure, which we have associated with the recombination reaction between electrons in the perovskite and holes in the organic semiconductor material used as the selective contact.
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.
A novel semiconductor organic molecule, denoted as VC89, having A-D-D1-D-A structure, was synthesized and all relevant physical and chemical features for its application in solar cells were ...investigated. The structure comprises 2-ethylhexoxy substituted BDT (donor D1 unit) as a core and a dicyano acceptor unit (DCV) as the terminal acceptor group (A) linked through cyclopentadithiophene (CDT) (donor D) moiety. The BHJ OSC VC89:PC71BM (1:2), processed with chloroform (CF) as solvent, showed an overall power conversion efficiency (PCE) of 4.63% with short circuit current J SC = 9.28 mA/cm2, open circuit voltage V OC = 0.96 V, and fill factor (FF) = 0.52. When the active layer was processed using DIO as a solvent additive (3% v/v in CF), the corresponding solar cell showed a PCE of 6.05% with J SC = 10.96 mA/cm2, V OC = 0.92, and FF = 0.60. The PCE was further improved to 6.66% with J SC = 11.68 mA/cm2, V OC = 0.92, and FF = 0.62, when the DIO/CF (3% v/v)-processed active layer was treated with THF vapors (solvent vapor annealing, SVA). The increase in PCE was due to the enhancement in both the J SC and FF due to the use of the dicyano groups as electron acceptor units. On one hand, J SC is determined by the enhancement of the film light absorbance, which is reflected in a better IPCE and better charge collection. On the other hand, we show herein that the use of solvent annealing after treatment with chemical additives also leads to better nanomorphologies that substantially improve the solar cell efficiency.
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 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.
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
VC62
was designed and synthesized. ...The optical and electrochemical properties of
VC62
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 C
71
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.
A conjugated acceptor-donor-acceptor with a Zn-porphyrin core and di-cyanovinyl substituted thiophene connected at meso positions was designed, synthesized and used in BHJ solar cells. The PCE of the devices reached 5.24%.
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