Organic solar cells based on nonfullerene acceptors have recently witnessed a significant rise in their power conversion efficiency values. However, they still suffer from severe instability issues, especially in an inverted device architecture based on the zinc oxide bottom electron transport layers. In this work, we insert a pyrene-bodipy donor–acceptor dye as a thin interlayer at the photoactive layer/zinc oxide interface to suppress the degradation reaction of the nonfullerene acceptor caused by the photocatalytic activity of zinc oxide. In particular, the pyrene-bodipy-based interlayer inhibits the direct contact between the nonfullerene acceptor and zinc oxide hence preventing the decomposition of the former by zinc oxide under illumination with UV light. As a result, the device photostability was significantly improved. The π–π interaction between the nonfullerene acceptor and the bodipy part of the interlayer facilitates charge transfer from the nonfullerene acceptor toward pyrene, which is followed by intramolecular charge transfer to bodipy part and then to zinc oxide. The bodipy-pyrene modified zinc oxide also increased the degree of crystallization of the photoactive blend and the face-on stacking of the polymer donor molecules within the blend hence contributing to both enhanced charge transport and increased absorption of the incident light. Furthermore, it decreased the surface work function as well as surface energy of the zinc oxide film all impacting in improved power conversion efficiency values of the fabricated cells with champion devices reaching values up to 9.86 and 11.80% for the fullerene and nonfullerene-based devices, respectively.
As organic solar cells (OSCs) and perovskite solar cells (PVSCs) move closer to commercialization, further efforts toward optimizing both cell efficiency and stability are needed. As interfaces strongly affect device performance and degradation processes, interfacial engineering by employing various materials as hole transport layers (HTLs) and electron transport layers (ETLs) has been a very active field of research in OSCs and PVSCs. Among them, inorganic materials exhibit significant advantages in promoting device performance due to their excellent charge transporting properties and intrinsic thermal and chemical robustness. In this review, an extensive overview is provided of inorganic semiconductors such as copper‐based ones with emphasis on copper iodide and copper thiocyanate, transition metal chalcogenides, nitrides and carbides as well as hybrid materials based on these inorganic compounds that have been recently employed as HTLs and ETLs in OSCs and PVSCs. Following a short discussion of the main optoelectronic and physical properties that interfacial materials used as HTLs and ETLs should possess, the functionalities of the aforementioned materials as interfacial, charge transport, layers in OSCs and PVSCs are discussed in depth. It is concluded by providing guidelines for further developments that could significantly extend the implementation of these materials in solar cells.
Motivated by the excellent electron-transfer capability of porphyrin molecules in natural photosynthesis, we introduce here the first application of a porphyrin compound to improve the performance of planar perovskite solar cells. The insertion of a thin layer consisting of a triazine-substituted Zn porphyrin between the TiO2 electron transport layer and the CH3NH3PbI3 perovskite film significantly augmented electron transfer toward TiO2 while also sufficiently improved the morphology of the perovskite film. The devices employing porphyrin-modified TiO2 exhibited a significant increase in the short-circuit current densities and a small increase in the fill factor. As a result, they delivered a maximum power conversion efficiency (PCE) of 16.87% (average 14.33%), which represents a 12% enhancement compared to 15.01% (average 12.53%) of the reference cell. Moreover, the porphyrin-modified cells exhibited improved hysteretic behavior and a higher stabilized power output of 14.40% compared to 10.70% of the reference devices. Importantly, nonencapsulated perovskite solar cells embedding a thin porphyrin interlayer showed an elongated lifetime retaining 86% of the initial PCE after 200 h, while the reference devices exhibited higher efficiency loss due to faster decomposition of CH3NH3PbI3 to PbI2.
Effective interface engineering has been shown to play a vital role in facilitating efficient charge-carrier transport, thus boosting the performance of organic photovoltaic devices. Herein, we employ water-soluble lacunary polyoxometalates (POMs) as multifunctional interlayers between the titanium dioxide (TiO) electron extraction/transport layer and the organic photoactive film to simultaneously enhance the efficiency, lifetime, and photostability of polymer solar cells (PSCs). A significant reduction in the work function (W) of TiO upon POM utilization was observed, with the magnitude being controlled by the negative charge of the anion and the selection of the addenda atom (W or Mo). By inserting a POM interlayer with ∼10 nm thickness into the device structure, a significant improvement in the power conversion efficiency was obtained; the optimized POM-modified poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2- 33 ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl-C butyric acid methyl ester (PTB7:PCBM)-based PSCs exhibited an efficiency of 8.07%, which represents a 21% efficiency enhancement compared to the reference TiO cell. Similar results were obtained in POM-modified devices based on poly(3-hexylthiophene) (P3HT) with electron acceptors of different energy levels, such as PCBM or indene-C bisadduct (ICBA), which enhanced their efficiency up to 4.34 and 6.21%, respectively, when using POM interlayers; this represents a 25-33% improvement as compared to the reference cells. Moreover, increased lifetime under ambient air and improved photostability under constant illumination were observed in POM-modified devices. Detailed analysis shows that the improvements in efficiency and stability synergistically stem from the reduced work function of TiO upon POM coverage, the improved nanomorphology of the photoactive blend, the reduced interfacial recombination losses, the superior electron transfer, and the more effective exciton dissociation at the photoactive layer/POM/TiO interfaces.
An interfacial engineering approach was adopted in order to optimize the photovoltaic parameters and the stability of n-i-p planar perovskite solar cells (PSCs). A thin manganese (Mn) porphyrin [(TMePyP)I 4 Mn(AcO)] layer was introduced between the titania (TiO 2 ) electron transport layer (ETL) and the perovskite absorber. The introduction of porphyrin onto the TiO 2 substrate provoked a significant decrease in the work function (W F ), which arose from the large local dipole moment. The modification also provided a more hydrophobic environment that favored the growth of homogeneous and large perovskite crystals. Moreover, the electron charge transport to the ETL was facilitated via the highly paramagnetic character of the Mn porphyrin, whereas the negative impact of humidity and oxygen on the PSC performance was hindered. Density functional theory analysis justified the observed large decrease of the W F and the strong electronic coupling of porphyrin with the TiO 2 compact layer (following the porphyrin deposition), which are beneficial for electron extraction. By combining the Mn porphyrin and the CH 3 NH 3 PbI 3 perovskite, significant enhancement of the stabilized power conversion efficiency by 22% was recorded. The shelf-shield stability was also improved after more than 600 h of storage in the dark under ambient conditions.
In the present work, we effectively modify the TiO electron transport layer of organic solar cells with an inverted architecture using appropriately engineered porphyrin molecules. The results show that the optimized porphyrin modifier bearing two carboxylic acids as the anchoring groups and a triazine electron-withdrawing spacer significantly reduces the work function of TiO, thereby reducing the electron extraction barrier. Moreover, the lower surface energy of the porphyrin-modified substrate results in better physical compatibility between the latter and the photoactive blend. Upon employing porphyrin-modified TiO electron transport layers in PTB7:PCBM-based organic solar cells we obtained an improved average power conversion efficiency up to 8.73%. Importantly, porphyrin modification significantly increased the lifetime of the devices, which retained 80% of their initial efficiency after 500 h of storage in the dark. Because of its simplicity and efficacy, this approach should give tantalizing glimpses and generate an impact into the potential of porphyrins to facilitate electron transfer in organic solar cells and related devices.
A silanol-functionalized polyoxometalate improves the electron collecting interface.
Photovoltaic devices based on organic semiconductors and organo-metal halide perovskites have not yet reached the theoretically predicted power conversion efficiencies while they still exhibit poor environmental stability. Interfacial engineering using suitable materials has been recognized as an attractive approach to tackle the above issues. We introduce here a zinc porphyrin–triazine–bodipy donor−π bridge–acceptor dye as a universal electron transfer mediator in both organic and perovskite solar cells. Thanks to its “push–pull” character, this dye enhances electron transfer from the absorber layer toward the electron-selective contact, thus improving the device’s photocurrent and efficiency. The direct result is more than 10% average power conversion efficiency enhancement in both fullerene-based (from 8.65 to 9.80%) and non-fullerene-based (from 7.71 to 8.73%) organic solar cells as well as in perovskite ones (from 14.56 to 15.67%), proving the universality of our approach. Concurrently, by forming a hydrophobic network on the surface of metal oxide substrates, it improves the nanomorphology of the photoactive overlayer and contributes to efficiency stabilization. The fabricated devices of both kinds preserved more than 85% of their efficiency upon exposure to ambient conditions for more than 600 h without any encapsulation.
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