wangzhaowei

Carbon-based hole transport material (HTM)-free perovskite solar cells (PSCs) have shown much promise for practical applications because of their high stability and low cost. However, the efficiencies of this kind of PSCs are still relatively low, especially for the simplest paintable carbon-based PSCs, in comparison with the organic HTM-based PSCs. This can be imputed to the perovskite deposition methods that are not very suitable for this kind of devices. A solvent engineering strategy based on two-step sequential method is exploited to prepare a high-quality perovskite layer for the paintable carbon-based PSCs in which the solvent for CH₃NH₃I (MAI) solution at the second step is changed from isopropanol (IPA) to a mixed solvent of IPA/Cyclohexane (CYHEX). This mixed solvent not only accelerates the conversion of PbI₂ to CH₃NH₃PbI₃ but also suppresses the Ostwald ripening process resulting in a high-quality perovskite layer, e.g., pure phase, even surface, and compact capping layer. The paintable carbon-based PSCs fabricated from IPA/CYHEX solvent exhibits a considerable enhancement in photovoltaic performance and performance reproducibility in comparison with that from pure IPA, especially on fill factor (FF), owing mainly to the better contact of perovskite/carbon interface, lower trap density in perovskite, higher light absorption ability, and faster charge transport of perovskite layer. As a result, the highest power conversion efficiency (PCE) of 14.38% is obtained, which is a record value for carbon-based HTM-free PSCs. Furthermore, a PCE of as high as 10% is achieved for the large area device (1 cm²), also the highest of its kind.

A solvent engineering strategy based on the two-step sequential method is exploited to prepare a high-quality perovskite layer for high-performance paintable carbon-based, hole transport material-free perovskite solar cells (14.38%). By lowering the solvent polarity for CH₃NH₃I solution, the conversion of PbI₂ is accelerated and the Ostwald ripening process is suppressed, leading to an even perovskite layer with enhanced interface (perovskite/carbon) contact and performance.

Highest reported efficiency cesium lead halide perovskite solar cells are realized by tuning the bandgap and stabilizing the black perovskite phase at lower temperatures. CsPbI₂Br is employed in a planar architecture device resulting in 9.8% power conversion efficiency and over 5% stabilized power output. Offering substantially enhanced thermal stability over their organic based counterparts, these results show that all-inorganic perovskites can represent a promising next step for photovoltaic materials.

The efficiency of perovskite photovoltaic cells is greatly impacted by the anomalous hysteresis effect. It is found that by applying a forward bias prior to testing, device efficiency (η_p) can be significantly improved. This bias poling effect can be stabilized at temperatures ≤250 K. As a result, hysteresis free operation of a planar perovskite photovoltaic cell is achieved at 170 K with η_p of 19%.

Low-temperature solution-processed high-efficiency colloidal quantum dot (CQD) photovoltaic devices are developed by improving the interfacial properties of p–n heterojunctions. A unique conjugated polyelectrolyte, WPF-6-oxy-F, is used as an interface modification layer for ZnO/PbS-CQD heterojunctions. With the insertion of this interlayer, the device performance is dramatically improved. The origins of this improvement are determined and it is found that the multifunctionality of the WPF-6-oxy-F interlayer offers the following essential benefits for the improved CQD/ZnO junctions: (i) the dipole induced by the ionic substituents enhances the quasi-Fermi level separation at the heterojunction through favorable energy band-bending, (ii) the ethylene oxide groups containing side chains can effectively passivate the interfacial defect sites of the heterojunction, and (iii) these effects occur without deterioration in the intrinsic depletion region or the series resistance of the device. All of the figures-of-merit of the devices are improved as a result of the enhanced built-in potential (electric field) and the reduced interfacial charge recombination at the heterojunction. The benefits due to the WPF-6-oxy-F interlayer are generally applicable to various types of PbS/ZnO heterojunctions. Finally, CQD photovoltaic devices with a power conversion efficiency of 9% are achievable, even by a solution process at room temperature in an air atmosphere. The work suggests a useful strategy to improve the interfacial properties of p–n heterojunctions by using polymeric interlayers.

High-efficiency colloidal quantum dot photovoltaic devices (CQDPVs) are developed by improving the interfacial properties of p–n heterojunctions using a conjugated polyelectrolyte as an interface modifier. The conjugated polyelectrolyte effectively improves charge extraction efficiency by enhancing internal electric field and passivating interfacial defects. Using this strategy, CQDPVs with 9% efficiency are able to be fabricated through room temperature solution process.

An attractive method to broaden the absorption bandwidth of polymer/fullerene-based bulk heterojunction (BHJ) solar cells is to blend near infrared (near-IR) sensitizers into the host system. Axial substitution of silicon phthalocyanines (Pcs) opens a possibility to modify the chemical, thermodynamic, electronic, and optical properties. Different axial substitutions are already designed to modify the thermodynamic properties of Pcs, but the impact of extending the π-conjugation of the axial ligand on the opto-electronic properties, as a function of the length of the alkyl spacer, has not been investigated yet. For this purpose, a novel series of pyrene-substituted silicon phthalocyanines (SiPc-Pys) with varying lengths of alkyl chain tethers are synthesized. The UV–vis and external quantum efficiency (EQE) results exhibit an efficient near IR sensitization up to 800 nm, clearly establishing the impact of the pyrene substitution. This yields an increase of over 20% in the short circuit current density (J _SC) and over 50% in the power conversion efficiency (PCE) for the dye-sensitized ternary device. Charge generation, transport properties, and microstructure are studied using different advanced technologies. Remarkably, these results provide guidance for the diverse and judicious selection of dye sensitizers to overcome the absorption limitation and achieve high efficiency ternary solar cells.

A series of silicon phthalocyanines, axially substituted by a pyrene acid group and peripherally incorporated by tert-butyl groups, are introduced as functional near infrared (IR) sensitizers into the P3HT:PCBM system. An efficient near-IR sensitization of up to 750 nm/800 nm and power conversion efficiency improvement up to 50% is achieved. The influence of the alkyl chain length on the performance, transport, and microstructure is extensively studied.

New phenanthridinone-based polymers are designed and synthesized by direct (hetero)arylation polymerization for photo­voltaic applications. Bulk-hetero­junction solar cells prepared in air and a random terpolymer (P3) blended with PC₇₁BM in o-dichlorobenzene lead to a power conversion efficiency (PCE) up to 6.7%. When the same polymer is processed with PC₆₁BM in o-xylene with blade-coating in a chlorine-free system, a PCE of 4.7% is observed.

The morphology features of polymer/small molecule/fullerene ternary organic solar cells are investigated. For the first time, simultaneously enhanced face-on molecular packing for both polymer and small-molecule donor materials is observed. This optimized morphology yields an enhanced performance at 40% small-molecule composition.

It is shown that the performance of inverted organic solar cells can be significantly improved by facilitating the formation of a quasi-ohmic contact via solution-processed alkali hydroxide (AOH) interlayers on top of n-type metal oxide (aluminum zinc oxide, AZO, and zinc oxide, ZnO) layers. AOHs significantly reduce the work function of metal oxides, and are further proven to effectively passivate defect states in these metal oxides. The interfacial energetics of these electron collecting contacts with a prototypical electron acceptor (C₆₀) are investigated to reveal the presence of a large interface dipole and a new interface state between the Fermi energy and the C₆₀ highest occupied molecular orbital for AOH-modified AZO contacts. These novel interfacial gap states are a result of ground-state electron transfer from the metal hydroxide-functionalized AZO contact to the adsorbed molecules, which are hypothesized to be electronically hybridized with the contact. These interface states tail all the way to the Fermi energy, providing for a highly n-doped (metal-like) interfacial molecular layer. Furthermore, the strong “light-soaking” effect is no longer observed in devices with a AOH interface.

Solution-processed alkali hydroxides significantly reduce the work function of metal oxides, such as zinc oxide or aluminum zinc oxide (AZO), and are further proven to effectively passivate defect states in these metal oxides. The interface states with alkali hydroxide-modified AZO contacts tail all the way to the Fermi energy, providing for a highly n-doped (metal-like) interfacial molecular layer.

In article number 1501873, Mansoo Choi and co-workers demonstrate highly efficient transparent conductive oxide (TCO)-free inverted perovskite (CH₃NH₃PbI₃) solar cells by using a graphene transparent electrode. Careful engineering of the interface between the graphene electrode and the hole transport layer enables the highest energy conversion efficiency of 17.1% among TCO-free solar cells to be obtained.

Perovskite-based solar cells manifest often an electrical instability in terms of current-voltage hysteresis. In article number 1501453, Mario Caironi, Annamaria Petrozza and co-workers demonstrate how fullerene based electron extracting layers stabilize the short-circuit photocurrent from the very first J–V scan, thanks to efficient electron extraction, while preconditioning cycles are needed to stabilize the open-circuit voltage owing to interaction between migrating iodide ions and the charge extraction layer, which result in the doping of the fullerene.

A new light trapping architecture to enhance the power conversion efficiency of organic photovoltaics is proposed and implemented. In article number 1501640, Emmanuel Kymakis and co-workers demonstrate that the incorporation of gold nanorods inside the rear buffer layer, leads to the redistribution of photons inside the active medium mainly through efficient light back-scattering, simultaneously increasing the exciton generation and charge collection.