Chen Weijie

Nanoporous Au film is successfully introduced into perovskite solar cells to replace the typical thermal deposition of metal electrode with a high efficiency of 19.0% on rigid substrate and sustains an excellent bending durability of 98.5% even after 1000 cycles testing on a flexible device, while its facile and recycled utilization significantly reduces the device fabrication cost, noble metal consuming, and environmental pollution.

Abstract

Perovskite solar cells (PSCs) using metal electrodes have been regarded as promising candidates for next‐generation photovoltaic devices because of their high efficiency, low fabrication temperature, and low cost potential. However, the complicated and rigorous thermal deposition process of metal contact electrodes remains a challenging issue for reducing the energy pay‐back period in commercial PSCs, as the ubiquitous one‐time use of a contact electrode wastes limited resources and pollutes the environment. Here, a nanoporous Au film electrode fabricated by a simple dry transfer process is introduced to replace the thermally evaporated Au electrode in PSCs. A high power conversion efficiency (PCE) of 19.0% is demonstrated in PSCs with the nanoporous Au film electrode. Moreover, the electrode is recycled more than 12 times to realize a further reduced fabrication cost of PSCs and noble metal materials consumption and to prevent environmental pollution. When the nanoporous Au electrode is applied to flexible PSCs, a PCE of 17.3% and superior bending durability of ≈98.5% after 1000 cycles of harsh bending tests are achieved. The nanoscale pores and the capability of the porous structure to impede crack generation and propagation enable the nanoporous Au electrode to be recycled and result in excellent bending durability.

Polymer solar cells (PSCs) with conjugated block copolymer are fabricated using a PBDT2T‐b‐N2200 solution in a nonhalogenated solvent such as toluene. PSCs created with an annealed film show the highest power conversion efficiency of 6.43% and an excellent shelf‐life time of over 1020 h owing to their morphological stability.

Abstract

A highly crystalline conjugated donor (D)–acceptor (A) block copolymer (PBDT2T‐b‐N2200) that has good solubility in nonhalogenated solvents is successfully synthesized. PBDT2T‐b‐N2200 shows a broad complementary absorption behavior owing to a wide‐band gap donor (PBDT2T) present as a D‐block and a narrow‐band gap acceptor (N2200) present as an A‐block. Polymer solar cells (PSCs) with conjugated block copolymer (CBCP) are fabricated using a toluene solution and PSC created with an annealed film showing the highest power conversion efficiency of 6.43%, which is 2.4 times higher than that made with an annealed blend film of PBDT2T and N2200. Compared to the blend film, the PBDT2T‐b‐N2200 film exhibits a highly improved surface and internal morphology, as well as a faster photoluminescence decay lifetime, indicating a more efficient photoinduced electron transfer. In addition, the PBDT2T‐b‐N2200 film shows high crystallinity through an effective self‐assembly of each block during thermal annealing and a predominant face‐on chain orientation favorable to a vertical‐type PSC. Moreover, the CBCP‐based PSCs exhibit an excellent shelf‐life time of over 1020 h owing to their morphological stability. From these results, a D–A block copolymer system is one of the efficient strategies to improve miscibility and morphological stability in all polymer blend systems.

By designing N‐functionalized asymmetrical acceptors N7IT and N8IT, the effects of nitrogen (N) atom on reducing nonradiative recombination loss (ΔE ₃) and dipole moment on morphology are revealed.

Abstract

Energy loss (E _loss) consisting of radiative recombination loss (ΔE ₁ and ΔE ₂) and nonradiative recombination loss (ΔE ₃) is considered as an important factor for organic solar cells (OSCs). Herein, two N‐functionalized asymmetrical small molecule acceptors (SMAs), namely N7IT and N8IT are designed and synthesized, to explore the effect of N on reducing E _loss with sulfur (S) as a comparison. N7IT‐based OSCs achieve not only a higher PCE (13.8%), but also a much lower E _loss (0.57 eV) than those of the analogue (a‐IT)‐based OSCs (PCE of 11.5% and E _loss of 0.72 eV), which are mainly attributed to N7IT's significantly enhanced charge carrier density (promoting J _SC) and largely suppressed nonradiative E _loss by over 0.1 eV (enhancing V _OC). In comparison, N8IT, with an extended π‐conjugated length, shows relatively lower photovoltaic performance than N7IT (but higher than a‐IT) due to the less favorable morphology caused by the excessively large dipole moment of the asymmetrical molecule. Finally, this work sheds light on the structure–property relationship of the N‐functionalization, particularly on its effects on reducing the E _loss, which could inspire the community to design and synthesize more N‐functionalized SMAs.

A stable intermediate‐state film is obtained by using teramethylene sulfoxide (TMSO), originating from the formation of stronger coordination bond between TMSO and all perovskite precursors, which extends the annealing window and promotes the formation of a high‐quality film with larger grains and textured surface. 21.14% efficiency is achieved attributable to the improvement of the long‐wavelength response and fill factor.

Abstract

As the power conversion efficiency (PCE) of perovskite solar cells (PSCs) is increased to as high over 25%, it becomes pre‐eminent to study a scalable process with wide processing window to fabricate large‐area uniform perovskite films with good light‐trapping performance. A stable and uniform intermediate‐state complex film is obtained by using tetramethylene sulfoxide (TMSO), which extends the annealing window to as long as 20 min, promotes the formation of a high‐quality perovskite film with larger grains (over 400 nm) and spontaneously forms the surface texture to result in an improved fill factor and open‐circuit voltage (V _oc). Moreover, the superior surface texture significantly increases the long‐wavelength response, leading to an improved short‐circuit current density (J _sc). As a result, the maximum PCE of 21.14% is achieved based on a simple planar cell structure without any interface passivation. Moreover, a large area module with active area of 6.75 cm² is assembled using the optimized TMSO process, showing efficiency as high as 16.57%. The study paves the way to the rational design of highly efficient PSCs for potential scaled‐up production.

Acetic acid (Ac) is used as an antisolvent for preparing perovskite films with excellent optoelectronic properties. Ac is found to not only reduce perovskite film roughness and residual PbI₂ but also generate a passivation effect from the electron‐rich carbonyl group. The best 0.159 cm² devices produce efficiencies of 22.0% for Cs_0.05FA_0.80MA_0.15Pb(I_0.85Br_0.15)₃ and 23.0% for Cs_0.05FA_0.90MA_0.05Pb(I_0.95Br_0.05)₃.

Abstract

Improving the quality of perovskite poly‐crystalline film is essential for the performance of associated solar cells approaching their theoretical limit efficiency. Pinholes, unwanted defects, and nonperovskite phase can be easily generated during film formation, hampering device performance and stability. Here, a simple method is introduced to prepare perovskite film with excellent optoelectronic property by using acetic acid (Ac) as an antisolvent to control perovskite crystallization. Results from a variety of characterizations suggest that the small amount of Ac not only reduces the perovskite film roughness and residual PbI₂ but also generates a passivation effect from the electron‐rich carbonyl group (CO) in Ac. The best devices produce a PCE of 22.0% for Cs_0.05FA_0.80MA_0.15Pb(I_0.85Br_0.15)₃ and 23.0% for Cs_0.05FA_0.90MA_0.05Pb(I_0.95Br_0.05)₃ on 0.159 cm² with negligible hysteresis. This further improves device stability producing a cell that maintained 96% of its initial efficiency after 2400 h storage in ambient environment (with controlled relative humidity (RH) <30%) without any encapsulation.

A polar nonconjugated small molecule ultrathin layer with an intrinsic dipole moment is introduced to modify the work function of indium tin oxide and to optimize the front interface energy level alignment, which contributes to suppressed energy loss and results in a 20.55% efficient electron transport layer–free perovskite solar cell with enhanced open‐circuit voltage short circuit current density and fill factor, simultaneously.

Abstract

Efficient electron transport layer–free perovskite solar cells (ETL‐free PSCs) with cost‐effective and simplified design can greatly promote the large area flexible application of PSCs. However, the absence of ETL usually leads to the mismatched indium tin oxide (ITO)/perovskite interface energy levels, which limits charge transfer and collection, and results in severe energy loss and poor device performance. To address this, a polar nonconjugated small‐molecule modifier is introduced to lower the work function of ITO and optimize interface energy level alignment by virtue of an inherent dipole, as verified by photoemission spectroscopy and Kelvin probe force microscopy measurements. The resultant barrier‐free ITO/perovskite contact favors efficient charge transfer and suppresses nonradiative recombination, endowing the device with enhanced open circuit voltage, short circuit current density, and fill factor, simultaneously. Accordingly, power conversion efficiency increases greatly from 12.81% to a record breaking 20.55%, comparable to state‐of‐the‐art PSCs with a sophisticated ETL. Also, the stability is enhanced with decreased hysteresis effect due to interface defect passivation and inhibited interface charge accumulation. This work facilitates the further development of highly efficient, flexible, and recyclable ETL‐free PSCs with simplified design and low cost by interface electronic structure engineering through facile electrode modification.

A novel emulsion‐based bottom‐up self‐assembly strategy is used to prepare sizable SnO₂ microspheres from oleic acid capped SnO₂ nanorods. Combined with an in‐situ ligand‐stripping strategy, the low‐temperature solution‐processed mesoscopic perovskite solar cells (PSCs) can achieve an efficiency as high as 21.35% with slight hysteresis and good reproducibility. This novel route will greatly expand the material selection range for preparing efficient mesoscopic PSCs.

Mesoporous scaffolds in perovskite solar cells (PSCs) can accelerate the formation of heterogeneous nucleation sites, leading to enhanced quality of perovskite films and uniform perovskite coverage over large areas. Nevertheless, the mesoporous electron transport layers (ETLs) can effectively compensate for the drawback of shorter electron diffusion lengths than their hole counterparts. Therefore, most mesoscopic PSCs usually show superior photovoltaic performance to their planar counterparts. However, mesoporous ETLs, particularly those prepared with metal oxide nanocrystals, often require a high‐temperature sintering process for the removal of residual organics and the improved crystallization of metal oxides. Here, a novel emulsion‐based bottom‐up self‐assembly strategy is used to prepare sizable SnO₂ microspheres from oleic acid capped SnO₂ nanorods. Combined with an in‐situ ligand‐stripping strategy, the low‐temperature solution‐processed mesoscopic PSCs can achieve efficiency as high as 21.35% with slight hysteresis and good reproducibility. In particular, the emulsion‐based bottom‐up self‐assembly strategy is a general way for preparing microspheres from several kinds of semiconductor nanocrystals, so it will greatly expand the material selection range for preparing efficient mesoscopic PSCs and even inverted mesoscopic devices.

Solvation of PbI₂ promotes the intercalation of solvent molecules with formamidinium iodide to form the perovskite phase of FAPbI₃ directly at room temperature. Subsequent annealing completes the conversion and yields high‐quality perovskite films with reduced trap state density and a high power conversion efficiency.

Abstract

The two‐step conversion process consisting of metal halide deposition followed by conversion to hybrid perovskite has been successfully applied toward producing high‐quality solar cells of the archetypal MAPbI₃ hybrid perovskite, but the conversion of other halide perovskites, such as the lower bandgap FAPbI₃, is more challenging and tends to be hampered by the formation of hexagonal nonperovskite polymorph of FAPbI₃, requiring Cs addition and/or extensive thermal annealing. Here, an efficient room‐temperature conversion route of PbI₂ into the α‐FAPbI₃ perovskite phase without the use of cesium is demonstrated. Using in situ grazing incidence wide‐angle X‐ray scattering (GIWAXS) and quartz crystal microbalance with dissipation (QCM‐D), the conversion behaviors of the PbI₂ precursor from its different states are compared. α‐FAPbI₃ forms spontaneously and efficiently at room temperature from P₂ (ordered solvated polymorphs with DMF) without hexagonal phase formation and leads to complete conversion after thermal annealing. The average power conversion efficiency (PCE) of the fabricated solar cells is greatly improved from 16.0(±0.32)% (conversion from annealed PbI₂) to 17.23(±0.28)% (from solvated PbI₂) with a champion device PCE > 18% due to reduction of carrier recombination rate. This work provides new design rules toward the room‐temperature phase transformation and processing of hybrid perovskite films based on FA⁺ cation without the need for Cs⁺ or mixed halide formulation.

Publication date: 18 March 2020

Source: Joule, Volume 4, Issue 3

Author(s): Kai Wang, Congcong Wu, Yuchen Hou, Dong Yang, Wenjie Li, Guodong Deng, Yuanyuan Jiang, Shashank Priya

A soft template‐controlled growth (STCG) method is proposed for the fabrication of a pinhole‐free CsPbI₃ film and the device exhibits an efficiency of 16.04%. By suppressing the inductive effect of defects on the phase transition and utilizing the unique reversibility of the phase transition, the STCG‐based all‐inorganic solar cell retains 90% of its initial efficiency after 3000 h of light soaking and heating.

Abstract

The unfavorable morphology and inefficient utilization of phase transition reversibility have limited the high‐temperature‐processed inorganic perovskite films in both efficiency and stability. Here, a simple soft template‐controlled growth (STCG) method is reported by introducing (adamantan‐1‐yl)methanammonium to control the nucleation and growth rate of CsPbI₃ crystals, which gives rise to pinhole‐free CsPbI₃ film with a grain size on a micrometer scale. The STCG‐based CsPbI₃ perovskite solar cell exhibits a power conversion efficiency of 16.04% with significantly reduced defect densities and charge recombination. More importantly, an all‐inorganic solar cell with the architecture fluorine‐doped tin oxide (FTO)/NiO _x /STCG‐CsPbI₃/ZnO/indium‐doped tin oxide (ITO) is successfully fabricated to demonstrate its real advantage in thermal stability. By suppressing the inductive effect of defects during the phase transition and utilizing the unique reversibility of the phase transition for the high‐temperature‐processed CsPbI₃ film, the all‐inorganic solar cell retains 90% of its initial efficiency after 3000 h of continuous light soaking and heating.

Molecules with controlled electron affinity processed at low temperature are used to tailor conductivity and the energy levels of hole transporting materials (HTMs), enabling fast holes extraction at the HTM/perovskite interface. This method with novel 3,6‐difluoro‐2,5,7,7,8,8‐hexacyanoquinodimethane enables the highest reported power conversion efficiency (PCE) of 22.13% and 20.01% for NiO _x ‐based rigid and flexible perovskite solar cells, respectively.

Abstract

Inverted perovskite solar cells (PSCs) with low‐temperature processed hole transporting materials (HTMs) suffer from poor performance due to the inferior hole‐extraction capability at the HTM/perovskite interfaces. Here, molecules with controlled electron affinity enable a HTM with conductivity improved by more than ten times and a decreased energy gap between the Fermi level and the valence band from 0.60 to 0.24 eV, leading to the enhancement of hole‐extraction capacity by five times. As a result, the 3,6‐difluoro‐2,5,7,7,8,8‐hexacyanoquinodimethane molecules are used for the first time enhancing open‐circuit voltage (V _oc) and fill factor (FF) of the PSCs, which enable rigid‐and flexible‐based inverted perovskite devices achieving highest power conversion efficiencies of 22.13% and 20.01%, respectively. This new method significantly enhances the V _oc and FF of the PSCs, which can be widely combined with HTMs based on not only NiO _x but also PTAA, PEDOTT:PSS, and CuSCN, providing a new way of realizing efficient inverted PSCs.

An n–p homojunction design comprising a perovskite absorber with graded Pb/Sn architecture is realized by a three‐step dynamic spin‐coating procedure. It significantly weakens the dependence on carrier transport layer (CTL) and enables CTL all‐free perovskite solar cells. The tandem‐cell‐like energy band structure expands photon harvesting range and boosts the J _sc to 26 mA cm⁻² without sacrificing V _oc.

Abstract

Organic–inorganic halide perovskites are promising materials for next‐generation photovoltaic device due to their attractive photoelectrical properties such as strong light absorption, high carrier mobility, and tunable bandgap. Generally, perovskite solar cells require carrier transport layers (CTL) to provide a built‐in electric field and reduce the recombination rate. However, the construction of suitable electron‐ and hole‐transport layers is not cost effective, impairing the commercial application of the devices. An n–p perovskite homojunction absorber with a graded bandgap is developed by introducing a three‐step dynamic spin‐coating strategy and variable valence Sn elements. The bandgap of the perovskite absorber is gradually manipulated from 1.53 eV (the bottom) to 1.27 eV (the top). The electronic behavior is also transformed from n‐type (excess PbI₂, the bottom) to p‐type (Sn vacancy, the top) in a very short distance (50 nm). This designed perovskite homojunction electronic structure not only expands the light harvesting range from 800 to 970 nm which provides potential to break the PCE limits, but also promotes oriented carrier transportation and weakens the dependence on CTL. The demonstrated asymmetrical active layer shows a brand‐new approach to simplify the device structure and boost the performance of CTL‐free perovskite solar cells.

A dopant‐free alkoxy‐PTEG device processed with 3‐methylcyclohexanone exhibits 19.9% efficiency and a device with 2‐methyl anisole, which is a reported aromatic food additive, exhibits 21.2% efficiency. In addition, tetraethylene glycol groups can chelate lead ions with moderate strength (K _binding = 2.76), and this strength is considered to be nondestructive to the perovskite lattice to prevent lead leakage.

Abstract

With the recent developments in the efficiency of perovskite solar cells (PSCs), diverse functionalities are necessary for next‐generation charge‐transport layers. Specifically, the hole‐transport layer (HTL) in the various synthesized materials modified with functional groups is explored. A novel donor–acceptor type polymer, alkoxy‐PTEG, composed of benzo[1,2‐b:4,5:b′]dithiophene and tetraethylene glycol (TEG)‐substituted 2,1,3‐benzothiadiazole is reported. The alkoxy‐PTEG exhibits high solubility even in nonaromatic solvents, such as 3‐methylcyclohexanone (3‐MC), and can prevent possible lead leakage via chelation. The optical and electronic properties of alkoxy‐PTEG are thoroughly analyzed. Finally, a dopant‐free alkoxy‐PTEG device processed with 3‐MC exhibits 19.9% efficiency and a device with 2‐methyl anisole, which is a reported aromatic food additive, exhibits 21.2% efficiency in a tin oxide planar structure. The PSC device shows 88% stability after 30 d at ambient conditions (40–50% relative humidity and room temperature). In addition, nuclear magnetic resonance reveals that TEG groups can chelate lead ions with moderate strength (K _binding = 2.76), and this strength is considered to be nondestructive to the perovskite lattice to prevent lead leakage. This is the first report to consider lead leakage and provide solutions to reduce this problem.

This review covers the latest development in dopant‐free organic hole transporting materials which have been employed in different perovskite solar cell structures. The criteria (appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital, high thermal stability, good solubility, high purity, and amorphous nature) for the rational design of these novel pristine dopant‐free organic hole transporting materials as well as their deposition methods and commercialization aspects are discussed.

Abstract

There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long‐term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′‐tetrakis(N,N′‐di‐pmethoxyphenylamino)‐9,9′‐spirbiuorene(Spiro‐OMeTAD) and poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA). HTMs are essential for high‐performance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping‐induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant‐free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low‐cost, effective, and scalable materials for devices with long‐term stability. It summarizes recent literature reports on non‐doped, alternative, and more stable HTMs used in PSCs as essential components for high‐efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant‐free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant‐free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.

The dependence of performance on composition in organic solar cells based on PffBT4T‐2DT polymer with O‐IDTBR or O‐IDFBR as a nonfullerene acceptor is investigated. The effect on morphology is discussed in terms of the interplay between immiscibility, inferred from phase behavior, and polymer aggregation. Morphology is optimized when polymer crystallite interconnectivity and size are balanced.

Abstract

The temperature‐dependent aggregation behavior of PffBT4T polymers used in organic solar cells plays a critical role in the formation of a favorable morphology in fullerene‐based devices. However, there is little investigation into the impact of donor/acceptor ratio on morphology tuning, especially for nonfullerene acceptors (NFAs). Herein, the influence of composition on morphology is reported for blends of PffBT4T‐2DT with two NFAs, O‐IDTBR and O‐IDFBR. The monotectic phase behavior inferred from differential scanning calorimetry provides qualitative insight into the interplay between solid–liquid and liquid–liquid demixing. Transient absorption spectroscopy suggests that geminate recombination dominates charge decay and that the decay rate is insensitive to composition, corroborated by negligible changes in open‐circuit voltage. Exciton lifetimes are also insensitive to composition, which is attributed to the signal being dominated by acceptor excitons which are formed and decay in domains of similar size and purity irrespective of composition. A hierarchical morphology is observed, where the composition dependence of size scales and scattering intensity from resonant soft X‐ray scattering (R‐SoXS) is dominated by variations in volume fractions of polymer/polymer‐rich domains. Results suggest an optimal morphology where polymer crystallite size and connectivity are balanced, ensuring a high probability of hole extraction via such domains.

Malleable and pliable silk‐derived electrodes are fabricated for use in deformable perovskite solar cells with a power conversion efficiency of 10.40%. The devices maintain 92% of initial efficiency after 1000 bends and more than 60% of the initial power after stretching at 50% strain for 50 cycles.

Abstract

For the fabrication of deformable electronic devices, electrodes that are robust against repeated bending, twisting, stretching, folding, reversible plasticizing, and that maintain electrical conductivity, and so on, are required. Malleable and pliable silk‐derived electrodes are fabricated to enable the shape deformation of perovskite solar cells. Moisture‐driven silk‐derived electrodes show reversible plasticization with malleability and pliability, realizing diverse deformation from simple operations (including bending, folding, stretching, etc.) to complicated structures (including flower, bowknot, and paper crane). It is worth noting that the silk‐derived electrodes maintain electrical conductivity (15.8 Ω sq⁻¹) compared to their initial value (15 Ω sq⁻¹) even after suffering from reversible mechanical plasticization of complicated structures. Deformable perovskite solar cells are fabricated with the silk‐derived electrodes and achieve a power conversion efficiency of 10.40%. The devices maintain 92% of the initial efficiency after 1000 bends at a curvature radius of 2.5 mm. The power does not decline at 50% strain and keeps more than 60% of the initial value after stretching for 50 cycles. Malleability and pliability of silk‐derived electrodes benefit the realization of stretchable perovskite solar cells and deformable electronic devices.

This review covers the latest development in dopant‐free organic hole transporting materials which have been employed in different perovskite solar cell structures. The criteria (appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital, high thermal stability, good solubility, high purity, and amorphous nature) for the rational design of these novel pristine dopant‐free organic hole transporting materials as well as their deposition methods and commercialization aspects are discussed.

Abstract

There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long‐term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′‐tetrakis(N,N′‐di‐pmethoxyphenylamino)‐9,9′‐spirbiuorene(Spiro‐OMeTAD) and poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA). HTMs are essential for high‐performance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping‐induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant‐free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low‐cost, effective, and scalable materials for devices with long‐term stability. It summarizes recent literature reports on non‐doped, alternative, and more stable HTMs used in PSCs as essential components for high‐efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant‐free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant‐free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.

Polymer aggregation and miscibility have been demonstrated to influence photovoltaic performance in nonfullerene‐based organic solar cells. Polymers having a strong tendency to aggregate are herein found to undergo aggregation prior to liquid–liquid phase separation and have a higher miscibility with nonfullerene acceptors, resulting in mixed donor–acceptor domains, stronger PL quenching, and a higher exciton dissociation efficiency.

Abstract

Understanding the correlation between polymer aggregation, miscibility, and device performance is important to establish a set of chemistry design rules for donor polymers with nonfullerene acceptors (NFAs). Employing a donor polymer with strong temperature‐dependent aggregation, namely PffBT4T‐2OD [poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3″′‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2″′‐quaterthiophen‐5,5‐diyl)], also known as PCE‐11 as a base polymer, five copolymer derivatives having a different thiophene linker composition are blended with the common NFA O‐IDTBR to investigate their photovoltaic performance. While the donor polymers have similar optoelectronic properties, it is found that the device power conversion efficiency changes drastically from 1.8% to 8.7% as a function of thiophene content in the donor polymer. Results of structural characterization show that polymer aggregation and miscibility with O‐IDTBR are a strong function of the chemical composition, leading to different donor–acceptor blend morphology. Polymers having a strong tendency to aggregate are found to undergo fast aggregation prior to liquid–liquid phase separation and have a higher miscibility with NFA. These properties result in smaller mixed donor–acceptor domains, stronger PL quenching, and more efficient exciton dissociation in the resulting cells. This work indicates the importance of both polymer aggregation and donor–acceptor interaction on the formation of bulk heterojunctions in polymer:NFA blends.

A novel solvent‐free and low‐temperature melting encapsulation strategy enables the full encapsulating operations under an ambient environment. It is found that the strategy not only removes residual oxygen and moisture to prevent the perovskite from phase segregation, but also suppresses the species volatilization to impede absorber decomposition, enabling a perovskite solar cell device with good thermal, moisture, and maximum power point stability.

Abstract

Improving device lifetime is one of the critical challenges for the practical use of metal halide perovskite solar cells (PSCs), wherein a reliable encapsulation is indispensable. Herein, based on an in‐depth understanding of the degradation mechanism for the PSCs, a solvent‐free and low‐temperature melting encapsulation technique, by employing low‐cost paraffin as the encapsulant that is compatible with perovskite absorbers, is demonstrated. The encapsulation strategy enables the full encapsulating operations to be undertaken under an ambient environment. It is found that the strategy not only removes residual oxygen and moisture to prevent the perovskite from phase segregation, but also suppresses the species volatilization to impede absorber decomposition, enabling a PSC devices with good thermal and moisture stability. As a result, the as‐encapsulated PSCs achieve a 1000 h operational lifetime for the encapsulated device at continuous maximum power point output under an ambient environment. This work paves the way for scalable and robust encapsulation strategy feasible to hybrid perovskite optoelectronics in an economic manner.

This study pictures the complex morphological evolution of perovskite thin films when organic cations of different size and concentration are blended together and proposes an effective solution, enables stable performance for tens of hours at the maximum power point, without encapsulation, at 50% relative humidity.

Abstract

The intrinsic instability of lead halide perovskite semiconductors in an ambient atmosphere is one of the most critical issues that impedes perovskite solar cell commercialization. To overcome it, the use of bulky organic spacers has emerged as a promising solution. The resulting perovskite thin films present complex morphologies, difficult to predict, which can directly affect the device efficiency. Here, by combining in‐depth morphological and spectroscopic characterization, it is shown that both the ionic size and the relative concentration of the organic cation, drive the integration of bulky organic cations into the crystal unit cell and the thin film, inducing different perovskite phases and different vertical distribution, then causing a significant change in the final thin film morphology. Based on these studies, a fine‐engineered perovskite is constructed by employing two different large cations, namely, ethyl ammonium and butyl ammonium. The first one takes part in the 3D perovskite phase formation, the second one works as a surface modifier by forming a passivating layer on top of the thin film. Together they lead to improved photovoltaic performance and device stability when tested in air under continuous illumination. These findings propose a general approach to achieve reliability in perovskite‐based optoelectronic devices.

A highly phase‐stable perovskite film without the methylammonium cation is fabricated by introducing cesium chloride in the double cation Cs, formamidinium perovskite precursor, leading to high power conversion efficiency of 20.5% and remarkable long‐term stability. The unencapsulated perovskite solar cell retains about 80% of its initial efficiency after a 1000 h aging study, demonstrating a feasible approach to enhance solar cell efficiency and stability simultaneously.

Abstract

Organic–inorganic metal halide perovskite solar cells (PSCs) have achieved certified power conversion efficiency (PCE) of 25.2% with complex compositional and bandgap engineering. However, the thermal instability of methylammonium (MA) cation can cause the degradation of the perovskite film, remaining a risk for the long‐term stability of the devices. Herein, a unique method is demonstrated to fabricate highly phase‐stable perovskite film without MA by introducing cesium chloride (CsCl) in the double cation (Cs, formamidinium) perovskite precursor. Moreover, due to the suboptimal bandgap of bromide (Br⁻), the amount of Br⁻ is regulated, leading to high power conversion efficiency. As a result, MA‐free perovskite solar cells achieve remarkable long‐term stability and a PCE of 20.50%, which is one of the best results for MA‐free PSCs. Moreover, the unencapsulated device retains about 80% of the original efficiencies after a 1000 h aging study. These results provide a feasible approach to enhance solar cell stability and performance simultaneously, paving the way for commercializing PSCs.

The results of this study mark the first high‐pressure studies on 2D van der Waals perovskites. Pressure studies show that (BA)₂MAPb₂Br₇ (n = 2) layers undergo two unique phase transitions and anomalous bandgap variation related to BA/MA molecules tilting and PbBr₆ octahedra distortion. However, (BA)₂PbBr₄ (n = 1) without MA molecules possesses only one pressure‐induced phase transition, highlighting the unique pressure effects in layered perovskites.

Abstract

The application of high pressure allows control over the unit cell and interatomic spacing of materials without any need for new growth methods or processing while accessing their materials properties in situ. Under these extreme pressures, materials may assume new structural phases and reveal novel properties. Here, unusual phase transition and band renormalization effects in 2D van der Waals Ruddlesden−Popper hybrid lead halide perovskites, which have shown extraordinary optical properties and immense potential in light emission and conversion technologies, are reported. The results show that (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)Pb₂Br₇ (n = 2) layers undergo two distinct phase transitions related to PbBr₆ octahedra, butylammonium (BA), and methylammonium (MA) molecule tilting motion that leads to rather unique/anomalous bandgap variation with pressure. In contrast, (CH₃(CH₂)₃NH₃)PbBr₄ (n = 1) lacks MA molecules and possesses only one pressure‐induced phase transition related to PbBr₆ octahedra and BA tilting. In this range, the bandgap reduces monotonically, much similar to other inorganic semiconductors and display surprisingly large redshift from 3 to 2.4 eV. Together with theoretical calculations, this study offers unique insights into these pressure‐induced changes and extends the understanding of these highly anisotropic layered soft organic perovskite materials under extreme conditions.

Through a strategy of embedding cyclohexane‐1,4‐dione into the thieno[3,4‐b]thiophene unit, a highly electron‐deficient core (TTDO) is synthesized, and the corresponding donor polymer (PBTT‐F) is also developed. The nonfullerene photovoltaic device based on this new donor polymer exhibits an outstanding PCE of 16.1% with a very high fill factor of 77.1%, which demonstrates it a very promising donor for high‐performance solar cells.

Abstract

It is of great significance to develop efficient donor polymers during the rapid development of acceptor materials for nonfullerene bulk‐heterojunction (BHJ) polymer solar cells. Herein, a new donor polymer, named PBTT‐F, based on a strongly electron‐deficient core (5,7‐dibromo‐2,3‐bis(2‐ethylhexyl)benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione, TTDO), is developed through the design of cyclohexane‐1,4‐dione embedded into a thieno[3,4‐b]thiophene (TT) unit. When blended with the acceptor Y6, the PBTT‐F‐based photovoltaic device exhibits an outstanding power conversion efficiency (PCE) of 16.1% with a very high fill factor (FF) of 77.1%. This polymer also shows high efficiency for a thick‐film device, with a PCE of ≈14.2% being realized for an active layer thickness of 190 nm. In addition, the PBTT‐F‐based polymer solar cells also show good stability after storage for ≈700 h in a glove box, with a high PCE of ≈14.8%, which obviously shows that this kind of polymer is very promising for future commercial applications. This work provides a unique strategy for the molecular synthesis of donor polymers, and these results demonstrate that PBTT‐F is a very promising donor polymer for use in polymer solar cells, providing an alternative choice for a variety of fullerene‐free acceptor materials for the research community.

Publication date: 19 February 2020

Source: Joule, Volume 4, Issue 2

Author(s): Eshwar Ravishankar, Ronald E. Booth, Carole Saravitz, Heike Sederoff, Harald W. Ade, Brendan T. O’Connor

Publication date: 19 February 2020

Source: Joule, Volume 4, Issue 2

Author(s): Yiming Li, Jiangjian Shi, Bingcheng Yu, Biwen Duan, Jionghua Wu, Hongshi Li, Dongmei Li, Yanhong Luo, Huijue Wu, Qingbo Meng