shuhui

By using chloroform‐assisted HCl method, smooth and full coverage MAPbI₃ films are obtained at room temperature under ambient air condition (relative humidity, 30%). A maximum efficiency of 17.72% is achieved for the MAPbI₃ solar cells fabricated via this method. Good reproducibility is demonstrated for the MAPbI₃ solar cells when relative humidity during device fabrication is between 0% and 30%.

Abstract

The power conversion efficiency of perovskite solar cells has been boosted rapidly, it has so far exceeded that of commercial polycrystalline silicon solar cells. This has prompted great interest in large‐scale production and deployment of perovskite solar cells. However, state‐of‐the‐art perovskite solar cells are fabricated inside gloveboxes and further annealing at high temperatures (typically at >100 °C for 30 min) is needed. These two required conditions are not compatible with, either in the respect to high‐throughput or thermal budget, a feasible industrial production process. By eliminating the two requirements, the deposition of perovskite films both at room temperature and under ambient air condition will make the scalable roll‐to‐roll fabrication scheme feasible. Here, the anti‐solvent (chloroform) washing is introduced to the previously developed hydrochloride‐assisted method and demonstrate that the room‐temperature method can be carried out under ambient air condition for MAPbI₃ film deposition. Through this new procedure, a power conversion efficiency as high as 17.72% is achieved for MAPbI₃ planar devices fabricated under a relative humidity of 30% at room temperature. Further, it is revealed that the room‐temperature process MAPbI₃ films show a near monoexponential decay pathway with a long photoluminescence lifetime of >400 ns.

Halide perovskites in photovoltaic applications have recently achieved an impressive 23% efficiency. However, the devices' long‐term stability falls short of industrial requirements. This review focuses on the effect of the nano‐/microstructure of perovskite as well as its interfacial structure with contact layers on its stability, and hence, provides a critical view of the strategies toward stable perovskite solar cells.

Abstract

Halide perovskites have emerged recently as a promising candidate for the next generation of photovoltaics. Power conversion efficiencies for laboratory‐scale devices surpass those of established technologies, such as multicrystalline silicon. However, perovskite solar cells lose their initial efficiency rapidly due to the convolution of several degradation factors, which hinder the process of industrialization. In this review, the important role of the nano‐ and microstructure of the perovskite layer in the performance and stability of the device are discussed. The defects located predominantly at the grain boundaries within the perovskite film and at the interface of the perovskite with the other materials can compromise the devices' stability. Thus, lowering the surface and interface concentration of defects is a key approach toward long‐term stable perovskite solar cells.

In order to reduce the cost of hole transporting materials (HTMs) for perovskite solar cells, the amide‐bond is introduced in the backbone resulting in a straightforward synthesis. Despite the lack of conjugation, the here presented HTM (EDOT‐Amide‐TPA) outperforms state‐of‐the‐art materials in performance, showing over 20% power conversion efficiency, and stability, which is assigned to the unique properties of the amide‐bond.

Abstract

State‐of‐the‐art perovskite‐based solar cells employ expensive, organic hole transporting materials (HTMs) such as Spiro‐OMeTAD that, in turn, limits the commercialization of this promising technology. Herein an HTM (EDOT‐Amide‐TPA) is reported in which a functional amide‐based backbone is introduced, which allows this material to be synthesized in a simple condensation reaction with an estimated cost of <$5 g⁻¹. When employed in perovskite solar cells, EDOT‐Amide‐TPA demonstrates stabilized power conversion efficiencies up to 20.0% and reproducibly outperforms Spiro‐OMeTAD in direct comparisons. Time resolved microwave conductivity measurements indicate that the observed improvement originates from a faster hole injection rate from the perovskite to EDOT‐Amide‐TPA. Additionally, the devices exhibit an improved lifetime, which is assigned to the coordination of the amide bond to the Li‐additive, offering a novel strategy to hamper the migration of additives. It is shown that, despite the lack of a conjugated backbone, the amide‐based HTM can outperform state‐of‐the‐art HTMs at a fraction of the cost, thereby providing a novel set of design strategies to develop new, low‐cost HTMs.

Polymer–fullerene bulk‐heterojunction layers made by spontaneous spreading on a water surface and subsequently transferred to glass substrates are used to fabricate unique bilayer–ternary solar cells that possess two complementary absorber layers stacked on top of each other. Surprisingly, these novel devices can exhibit external quantum efficiencies exceeding 100% under bias illumination.

Abstract

A new method is presented to fabricate bilayer organic solar cells via sequential deposition of bulk‐heterojunction layers obtained using spontaneous spreading of polymer–fullerene blends on a water surface. Using two layers of a small bandgap diketopyrrolopyrrole polymer–fullerene blend, a small improvement in power conversion efficiency (PCE) from 4.9% to 5.1% is obtained compared to spin‐coated devices of similar thickness. Next, bilayer–ternary cells are fabricated by first spin coating a wide bandgap thiophene polymer–fullerene blend, followed by depositing a small bandgap diketopyrrolopyrrole polymer–fullerene layer by transfer from a water surface. These novel bilayer–ternary devices feature a PCE of 5.9%, higher than that of the individual layers. Remarkable, external quantum efficiencies (EQEs) over 100% are measured for the wide bandgap layer under near‐infrared bias light illumination. Drift‐diffusion calculations confirm that near‐infrared bias illumination can result in a significant increase in EQE as a result of a change in the internal electric field in the device, but cannot yet account for the magnitude of the effect. The experimental results indicate that the high EQEs over 100% under bias illumination are related to a barrier for electron transport over the interface between the two blends.

Double cation substitution in Cu₂ZnSnS₄ (CZTS) by partially substituting Cu with Ag and Zn with Cd is shown to alter the characteristics of acceptor defects and deep defects responsible for non‐radiative recombination. This synergistic effect of Cd and Ag reflects in power conversion efficiency of 10.1% (10.8% active area) obtained in the (Cu,Ag)₂(Zn,Cd)SnS₄ system.

Abstract

The performance of many emerging compound semiconductors for thin‐film solar cells is considerably lower than the Shockley–Queisser limit, and one of the main reasons for this is the presence of various deleterious defects. A partial or complete substitution of the cations presents a viable strategy to alter the characteristics of the detrimental defects and defect clusters. Particularly, it is hypothesized that double cation substitution could be a feasible strategy to mitigate the negative effects of different types of defects. In this study, the effects of double cation substitution on pure‐sulfide Cu₂ZnSnS₄ (CZTS) by partially substituting Cu with Ag, and Zn with Cd are explored. A 10.1% total‐area power conversion efficiency (10.8% active‐area efficiency) is achieved. The role of Cd, Ag, and Cd + Ag substitution is probed using temperature‐dependent photoluminescence, time‐resolved photoluminescence, current–voltage (IV), and external quantum efficiency (EQE) measurements. It is found that Cd improves the photovoltaic performance by altering the defect characteristics of acceptor states near the valence band, and Ag reduces nonradiative bulk recombination. It is believed that the double cation substitution approach can also be extended to other emerging photovoltaic materials, where defects are the main culprits for low performance.

Dopant‐free hole transporting materials for perovskite solar cells are reviewed. The efficiency growth of perovskite solar cells (PSCs) based on dopant‐free hole transporting materials (HTMs) is highlighted. Hydrophilic chemical dopants can accelerate the degradation of the perovskite layer. Dopant‐free HTMs play a significant role in both efficiency and stability of PSCs.

This article reviews various dopant‐free hole transporting materials (HTMs) used in perovskite solar cells (PSCs) in three main categories including inorganic, polymeric, and small molecule HTMs. PSCs have undergone rapid progress, achieving power conversion efficiencies (PCEs) above 22%. With their low production cost and high efficiencies, PSCs are considered promising next‐generation solar cell technology. In all developed architectures for PSCs, including planar and mesoscopic with conventional and inverted structures, HTMs play a significant role in determining the photovoltaic performance of PSCs. Using p‐type dopants, however, is considered a common strategy to increase the hole conductivity of HTM, which is usually compensated by a more complicated fabrication procedure, higher production costs, and lower stability of PSC. Although several reviews on HTMs have been published, progress on dopant free HTMs needs to be reviewed and analyzed. Here, a review covering most of the published reports on dopant‐free HTMs is presented, and the device structure and fabrication method, HTM layer deposition techniques, and the efficiency and the stability of PSCs are addressed during discussions in each main category. Finally, an outlook on stability and PCE growth in PSCs based on dopant‐free HTMs is presented.

Design principles of the thermal and redox properties of spiro‐based hole transport materials (HTMs) were developed and the relevance of these findings to high‐performance perovskite solar cells (PSCs) is shown. The studies resulted in an HTM characterized by both a low reduction potential (0.7 V) and a high T _g value (>125 °C) to yield a device power conversion efficiency (PCE) of 20.8 % in a PSC.

Abstract

We report design principles of the thermal and redox properties of synthetically accessible spiro‐based hole transport materials (HTMs) and show the relevance of these findings to high‐performance perovskite solar cells (PSCs). The chemical modification of an asymmetric spiro[fluorene‐9,9′‐xanthene] core is amenable to selective placement of redox active triphenylamine (TPA) units. We therefore leveraged computational techniques to investigate five HTMs bearing TPA groups judiciously positioned about this asymmetric spiro core. It was determined that TPA groups positioned about the conjugated fluorene moiety increase the free energy change for hole‐extraction from the perovskite layer, while TPAs about the xanthene unit govern the T _g values. The synergistic effects of these characteristics resulted in an HTM characterized by both a low reduction potential (≈0.7 V vs. NHE) and a high T _g value (>125 °C) to yield a device power conversion efficiency (PCE) of 20.8 % in a PSC.

A template‐assisted solution‐processed technique is developed to fabricate 3D nanowire‐coated macroporous titania thin films with outstanding optical and electrical properties. Perovskite solar cells based on newly prepared TiO₂ electron‐transporting layer deliver an impressive power conversion efficiency of up to 20.1% owing to enhanced light harvesting and facilitated charge collection.

Abstract

Microscopic design and morphological engineering of the semiconducting metal oxide as electron‐transporting layers (ETLs) is of vital importance for optical enhancement, photonic structuring, and charge collection optimization within optoelectronic devices. Herein, nanowire‐coated, branched macroporous titania (BMT) thin films are reported as a new type of ETL prepared by using silica spheres as a sacrificial template, followed by a sol–gel and subsequent alkaline‐assisted etching process. The BMT films feature 3D hierarchical structures and interconnected networks with tunable pore sizes, branch densities, and film thicknesses. The titania films are employed as ETLs in perovskite solar cells (PSCs), resulting in remarkable power conversion efficiencies (PCEs) of 20.1%; a noticeable 16% increase compared with titania nanowire (TNW) ETL‐based counterparts (PCE = 17.3%). The superior device performance of the BMT‐based PSCs can be attributed to the maximized light harvesting and charge collection capabilities. These beneficial properties are derived from the effective infiltration of the perovskite precursor into the titania macropores, efficient light confinement within the macropore structure, and the textured perovskite capping layer, as well as enhanced charge transport and reduced charge recombination through the BMT architecture. This work demonstrates a simple and effective approach for constructing branched macroporous metal‐oxide photoelectrodes toward high‐performance photovoltaic devices.

A very low‐cost and low‐temperature photovoltaic cell based on dopant‐free contact (transition metal oxide/n‐SiNWs/alkali metal salt) and Si nanowires arrays show a power conversion efficiency of 16.9%. Insights into the interaction between the self‐assembling interface passivation, interface polarities, and the performance of the device is demonstrated.

Abstract

Low‐cost and efficient interfacial layer construction with the required charge selectivity and compatibility is necessary for nanostructured solar cells, and the proper integration of the interfacial layer with the light‐trapping system is required to improve the power conversion efficiency of the cell. Herein, low‐cost Si nanowires‐based solar cells with tunneling heterojunctions are developed by the deposition of MoO _x and spin‐coating of Cs₂CO₃ as the carrier‐selective layers. The power conversion efficiency of 16.9% for a device of 4 cm² in area is achieved by Si nanowires solar cells by the self‐assembly of ultra‐thin SiO _x as the surface tunneling passivation layer. Self‐assembly is realized with an ultraviolet O₃ treatment process at room temperature. Quasi‐steady‐state photoconductance, microwave‐detected photoconductance decay, and constant current–voltage measurements are used to characterize the passivation quality and tunneling transportation properties of the ultra‐thin SiO _x layers. Interfacial charge recombination is suppressed and effective carrier tunneling properties are developed by the growth of ≈1.5 nm thick SiO _x layers on the surfaces of the Si nanowires. This proposed Si nanowires solar cell architecture featuring tunneling heterojunctions achieves high performance and may be suitable for fabricating industrialized Si nanowires‐based photovoltaic devices through a cost‐effective, simple, and low‐temperature process.

Efficient nonfullerene organic solar cells (OSCs) are realized by combining a donor polymer, PffBT2T‐TT, and a small‐molecular acceptor, O‐IDTBR, which have identical bandgaps and close energy levels. Despite the small energy offsets for both hole and electron transfer, this system can still achieve efficient charge separation and a high efficiency of 10.4%.

Abstract

State‐of‐the‐art organic solar cells (OSCs) typically suffer from large voltage loss (V _loss) compared to their inorganic and perovskite counterparts. There are some successful attempts to reduce the V _loss by decreasing the energy offsets between the donor and acceptor materials, and the OSC community has demonstrated efficient systems with either small highest occupied molecular orbital (HOMO) offset or negligible lowest unoccupied molecular orbital (LUMO) offset between donors and acceptors. However, efficient OSCs based on a donor/acceptor system with both small HOMO and LUMO offsets have not been demonstrated simultaneously. In this work, an efficient nonfullerene OSC is reported based on a donor polymer named PffBT2T‐TT and a small‐molecular acceptor (O‐IDTBR), which have identical bandgaps and close energy levels. The Fourier‐transform photocurrent spectroscopy external quantum efficiency (FTPS‐EQE) spectrum of the blend overlaps with those of neat PffBT2T‐TT and O‐IDTBR, indicating the small driving forces for both hole and electron transfer. Meanwhile, the OSCs exhibit a high electroluminescence quantum efficiency (EQE_EL) of ≈1 × 10⁻⁴, which leads to a significantly minimized nonradiative V _loss of 0.24 V. Despite the small driving forces and a low V _loss, a maximum EQE of 67% and a high power conversion efficiency of 10.4% can still be achieved.

The influence of the surface effect of 2D layered perovskites before and after mechanical exfoliation is studied. The smooth 2D perovskite is less sensitive to ambient moisture and exhibits a considerably low dark current. This work reveals the strong dependence of the surface condition of 2D hybrid perovskite crystals on their moisture stability and optoelectronic properties.

Abstract

Despite the remarkable progress of optoelectronic devices based on hybrid perovskites, there are significant drawbacks, which have largely hindered their development as an alternative of silicon. For instance, hybrid perovskites are well‐known to suffer from moisture instability which leads to surface degradation. Nonetheless, the dependence of the surface effect on the moisture stability and optoelectronic properties of hybrid perovskites has not been fully investigated. In this work, the influence of the surface effect of 2D layered perovskites before and after mechanical exfoliation, representing rough and smooth surfaces of perovskite crystals, are studied. It is found that the smooth 2D perovskite is less sensitive to ambient moisture and exhibits a considerably low dark current, which outperforms the rough perovskites by 23.6 times in terms of photodetectivity. The superior moisture stability of the smooth perovskites over the rough perovskites is demonstrated. Additionally, ethanolamine is employed as an organic linker of the 2D layered perovskite, which further improves the moisture stability. This work reveals the strong dependence of the surface conditions of 2D hybrid perovskite crystals on their moisture stability and optoelectronic properties, which are of utmost importance to the design of practical optoelectronic devices based on hybrid perovskite crystals.

The primary factor limiting the efficiency of photocurrent generation is identified in polymer:nonfullerene acceptor (NFA) blends and its implications for materials design in the optimisation of organic solar cell performance is discussed. The magnitude of this geminate recombination loss pathway is found to be the key determinant of the efficiency of photocurrent generation in polymer:NFA blend solar cells.

Abstract

Herein, a meta‐analysis of the device performance and transient spectroscopic results are undertaken for various donor:acceptor blends, employing three different donor polymers and seven different acceptors including nonfullerene acceptors (NFAs). From this analysis, it is found that the primary determinant of device external quantum efficiency (EQE) is the energy offset driving interfacial charge separation, ΔE _CS. For devices employing the donor polymer PffBT4T blended with NFA and fullerene acceptors, an energy offset ΔE _CS = 0.30 eV is required to achieve near unity charge separation, which increases for blends with PBDTTT‐EFT and P3HT to 0.36 and ≈1.2 eV, respectively. For blends with PffBT4T and PBDTTT‐EFT, a 100 meV decrease in the LUMO of the acceptor is observed to result in an approximately twofold increase in EQE. Steady state and transient optical data determine that this energy offset requirement is not associated with the need to overcome the polymer exciton binding energy and thereby drive exciton separation, with all blends studied showing efficient exciton separation. Rather, the increase in EQE with larger energy offset is shown to result from suppression of geminate recombination losses. These results are discussed in terms of their implications for the design of donor/NFA interfaces in organic solar cells, and strategies to achieve further advances in device performance.

All‐small‐molecule organic solar cells employing three nonfullerene acceptors (F‐0Cl, F‐1Cl, and F‐2Cl) are investigated. End group chlorination leads to redshifted absorption, enhanced crystallinity, and high electron mobility. F‐2Cl with highest crystallinity gives the best device performances with power conversion efficiency of 9.89 and 10.76%, respectively, when small molecule DRCN5T and DRTB‐T are used as donors.

Abstract

While a wide variety of nonfullerene acceptors are developed and perform well in combination with polymer donors, only a few nonfullerene acceptors can work well with small molecule donors. Here, all‐small‐molecule solar cells with high performance enabled by a new type of small molecule acceptors (F‐0Cl, F‐1Cl, and F‐2Cl), which contain linear alkyl side chains and end groups substituted with various number of chlorine atoms, are reported. End group chlorination leads to redshifted absorption, enhanced crystallinity, and high electron mobility. These properties make them competitive as electron acceptors for all‐small‐molecule solar devices. When combined with two popular small molecule donors DRTB‐T and DRCN5T, these nonfullerene acceptors offer power conversion efficiencies up to 10.76 and 9.89%, which are among the top efficiencies reported in all‐small‐molecule solar cells and indicate the great potential of all‐small‐molecule solar devices.