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Flexible perovskite solar cells (FPSCs) with excellent recoverability show a wide range of potential applications in portable power sources. The recoverability of FPSCs requires outstanding bendability of each functional layer, including the flexible substrates, electrodes, perovskite light absorbers, and charge transport materials. This review highlights the recent progress and practical applications of high‐recoverability FPSCs, and illustrates the routes toward improvement of the recoverability and environmental stability through the choice of flexible substrates and the preparation of high‐quality perovskite films, as well as the optimization of charge‐selective contacts. In addition, we explore the intrinsic properties of each functional layer from the physical perspective and analyze how to select suitable functional layers. Additionally, some effective strategies are summarized, including material modification engineering of selective contacts, additives and interface engineering of interlayers, which can release mechanical stress and increase the power‐conversion efficiency (PCE) and recoverability of the FPSCs. The challenges of making high‐performance FPSCs with long‐term stability and high recoverability are discussed. Finally, future applications and perspectives for FPSCs are discussed, aiming to promote more extensive commercialization processes for lightweight and durable FPSCs. [ABSTRACT FROM AUTHOR]

In general, perovskite solar cells (PSC) with a sensitized or thin-film architecture absorb light from a single-side illumination, and carrier separation and transport only take place inside the active layer of the perovskite film. Herein, we demonstrated a dual-irradiation PSC system in which light passes through both the fluorinated tin oxide (FTO) side and the Au electrode side, resulting in much faster interfacial charge carrier extraction and transportation than that in a single-irradiation system, in which light passes through from either the FTO or semitransparent Au electrode side. This dual-irradiation PSC system with a configuration of FTO/Cl-TiO2/Mp-TiO2/mixed perovskite/spiro-OMeTAD/Au/ITO can form two quasi-interfacial p-n junctions, which occur separately at the interfaces of TiO2/perovskite and perovskite/spiro-OMeTAD. When the PSC device was illuminated simultaneously from both the FTO and Au/ITO sides, the PSC achieved a total power conversion efficiency (PCE) as high as 20.1% under high light intensity (1.4 sun), which is higher than PCE (18.4%) of a single-irradiation system. The time of flight (TOF) photoconductivity, small perturbation transient photovoltaic (TPV), finite-difference time-domain (FDTD) optical simulations, and dual illumination-side-dependent impedance spectroscopy (ISD-IS) were used to authenticate the presence of two quasi-interfacial p-n junctions in the PSC, creating more charge carriers than only one quasi p-n junction, and thus leading to a fast recombination process. [ABSTRACT FROM AUTHOR]

With the power conversion efficiencies of perovskite solar cells (PSCs) exceeding 25%, the PSCs are a step closer to initial industrialization. Prior to transferring from laboratory fabrication to industrial manufacturing, issues such as scalability, material cost, and production line compatibility that significantly impact the manufacturing remain to be addressed. Here, breakthroughs on all these fronts are reported. Carbon‐based PSCs with architecture fluorine doped tin oxide (FTO)/electron transport layer/perovskite/carbon, that eliminate the need for the hole transport layer and noble metal electrode, provide ultralow‐cost configuration. This PSC architecture is manufactured using a scalable and industrially compatible electrospray (ES) technique, which enables continuous printing of all the cell layers. The ES deposited electron transport layer and perovskite layer exhibit properties comparable to that of the laboratory‐scale spin coating method. The ES deposited carbon electrode layer exhibits superior conductivity and interfacial microstructure in comparison to films synthesized using the conventional doctor blading technique. As a result, the fully ES printed carbon‐based PSCs show a record 14.41% power conversion efficiency, rivaling the state‐of‐the‐art hole transporter‐free PSCs. These results will immediately have an impact on the scalable production of PSCs. [ABSTRACT FROM AUTHOR]

The recent studies widely believe that the stability of (PEA)2PbI4/MAPbI3 perovskite solar cells (PSCs) can be progressively increased with the addition of phenethylammonium ion (PEA+). Herein, it is reported on the stability variations after the addition of PEA+ into the MAPbI3 film and it is demonstrated that excess PEA+ has negative effect on the stability of (PEA)2PbI4/MAPbI3 perovskite film. X‐ray photoelectron spectroscopy, nuclear magnetic resonance, and X‐ray absorption fine structure spectroscopy are utilized to testify that the excess PEA+ damages the MAPbI3 crystal structure and generates defects due to the strong coordination between NH3+ from PEA+ and I− from [PbI6]4−. This work reveals that the stability of (PEA)2PbI4/MAPbI3 perovskite films is not proportional to the addition of PEA+, which is of interest for the developing of stable 2D/3D PSCs. [ABSTRACT FROM AUTHOR]

Cesium‐based fully inorganic black‐phase (BP) lead halide perovskites (such as α‐, β‐, and γ‐CsPbI3) with excellent thermal stability and a decently high photovoltaic performance have attracted increasing attention. However, a below 200 °C fabrication process of the desirable BP CsPbI3 has rarely been reported. Herein, the detailed crystal structure evolution of ambient‐air‐stable BP CsPbI3 prepared under low temperature conditions is investigated by exploiting the strong coordination bonding between CO in polyvinylpyrrolidone (PVP) and Pb in CsPbI3 and inflection effect of PVP under annealing. It is found that ambient‐air‐stable BP CsPbI3 films are formed and the energy barrier for the long‐term stable BP CsPbI3 formation is significantly reduced (the required annealing temperature is only 80 °C). After optimization, the highest power conversion efficiencies (PCEs) of ≈4.0% and 10.0% are recorded for the 3% PVP‐added devices with light absorbers annealed at 80 and 160 °C, respectively. More importantly, the 3% PVP device annealed at 160 °C maintains ≈80% of its original PCE after 5 months storage under ambient‐air conditions. [ABSTRACT FROM AUTHOR]

Abstract Hollow rice grain-shaped titanium dioxide (TiO 2) nanostructures (NSs) with the radius of 200–300 nm are fabricated by electrospinning titanium isopropoxide solution and further calcination. The resulting hollow rice grain-shaped TiO 2 NSs are highly porous, which are beneficial to the infiltration of perovskites and provide a large contact area, as the building blocks to construct the mesoporous TiO 2 layer for the large-area (the active area is 1 cm2) CH 3 NH 3 PbI 3 based perovskite solar cells (PSCs). By varying the spin coating speed (2000 rpm, 4000 rpm, and 8000 rpm, respectively), the performance of PSCs changes with different TiO 2 NSs distribution densities. The optimized PSC employing the 4000 rpm spin coating speed exhibits a photovoltaic conversion efficiency (PCE) of 14.2% with the short circuit current density (J SC) of 21.6 mA cm−2, open circuit voltage (V OC) of 1.07 V and fill factor (FF) of 0.61, which is superior to the plain structure based control group with the PCE of 9.6%. Furthermore, the PSC possesses a reproducible PCE value with weak hysteresis in its current density-voltage (J-V) curves. Moreover, photoluminescence (PL) measurements and finite-difference time-domain (FDTD) optical simulations reveal the enhanced fast charge carrier extraction/transport and light absorption in the proposed system, which makes electrospun hollow rice grain-shaped TiO 2 NSs a promising electron transportation material for high-efficiency and large-area photovoltaic devices. Graphical abstract Electrospun hollow rice grain-shaped titanium dioxide (TiO 2) nanostructures (NSs) are used as the building blocks to construct the mesoporous TiO 2 layer for the large-area CH 3 NH 3 PbI 3 based perovskite solar cells (PSCs). Significant enhancements are observed in nearly all photovoltaic parameters of the PSC with rice grain-shaped TiO 2 NSs. The enhanced device performance can be ascribed to the enhanced fast charge carrier extraction/transport and light absorption in the mesoporous TiO 2 /perovskite interface. fx1 Highlights • Electrospun hollow rice grain-shaped TiO 2 nanostructures were used as mesoporous layer of large-area PSCs. • A ~47.9% enhancement of PCE from 9.6% to 14.2% has been obtained for the hollow rice grain-shaped TiO 2 NSs based PSC. • The PCE enhancement is derived from the enhanced fast charge carrier extraction/transport and light absorption. • The stability investigation shows that the device can maintain ~93% of their initial PCE after a period of 60 days. • FDTD method is adopted to simulate the optical power distribution and optical absorption of the proposed system. [ABSTRACT FROM AUTHOR]

A series of thiazolothiazole‐thiophene‐based polymer hole‐transporting materials (HTMs) are synthesized and applied into perovskite solar cells (PSCs). These polymer HTMs have good performance and stability without any dopants. The improved performance and stability of PSCs is attributed to the polymer HTMs' crystallinity which can be controlled by adjusting the thiophene units on the polymer backbone. The highest PCE of the polymer HTMs based PSCs reached 14.02%. The stability of the PSCs was shown by retaining 80% PCE of the original value after stored them in a N2‐filled glovebox for 113 days. A series of thiazolothiazole‐thiophene‐based polymer hole‐transporting materials (HTMs) are synthesized and applied into perovskite solar cells (PSCs). These polymer HTMs have good performance and stability without any dopants. [ABSTRACT FROM AUTHOR]

Abstract: 3D organic–inorganic lead halide perovskites have shown great potential in efficient photovoltaic devices. However, the low stability of the 3D perovskite layer and random arrangement of the perovskite crystals hinder its commercialization road. Herein, a highly oriented 2D@3D ((AVA)2PbI4@MAPbI3) perovskite structure combining the advantages of both 2D and 3D perovskite is fabricated through an in situ route. The highest power conversion efficiency (PCE) of 18.0% is observed from a 2D@3D perovskite solar cell (PSC), and it also shows significantly enhanced device stability under both inert (90% of initial PCE for 32 d) and ambient conditions (72% of initial PCE for 20 d) without encapsulation. The high efficiency of 18.0% and nearly twofold improvement of device stability in ambient compared with pure 3D PSCs confirm that such 2D@3D perovskite structure is an effective strategy for high performance and increasing stability and thus will enable the timely commercialization of PSCs. [ABSTRACT FROM AUTHOR]

Abstract: Mesoporous titania (TiO2) nanoparticles (NPs) were synthesized by treating prehydrolyzed titanium precursors with polyethylene glycol (PEG). By varying the amount of PEG, their different physical properties were tuned appropriately in terms of surface area, pore size/volume, and electrical conductivity. By increasing the amount of PEG in TiO2, the surface area and pore volume of mesoporous TiO2 increased, and no direct correlation with the photovoltaic performance of perovskite solar cells was observed. In comparison, the pore size of mesoporous TiO2 first increased greatly before quickly decreasing, and a similar trend was also observed for electrical conductivity. These implied that the interparticle interaction became much stronger with an optimum amount of PEG added to form mesoporous TiO2 NPs. With an optimum molar ratio of PEG/Ti precursor at 0.75, greatly improved conductivity of mesoporous TiO2 NPs with interconnection as compared to that prepared in the absence of PEG, which was used as an electron transport layer, improved the power conversion efficiency of perovskite solar cells by ≈36%. Their greatly reduced photoluminescence after incorporating the perovskite also revealed a difference in the electron‐injection properties from the perovskite into mesoporous TiO2. This strategy highlights a versatile tool to tune mesoporous structures to achieve a cooperative effect for studying photovoltaic properties in perovskite solar cells. [ABSTRACT FROM AUTHOR]

Photoinduced charge selective carrier extraction by linearly increasing voltage technique allows straightforward assessment of charge transport properties within planar and mesostructured perovskite solar cells with respect to light intensity and signal delay time. Charge sensitive device architecture is realized through implementation of insulating layer between the anode or cathode to prevent extraction of unwanted type of carriers. Resulting behavior of comparatively efficient mesoporous and planar solar cells exhibits well balanced charge transport with slight dependence of charge mobility on applied laser pulse fluence, for given pulse delay times. Very similar charge carrier mobilities are present within mesoporous devices, whereas holes trail approximately half an order of magnitude behind electrons in planar structured specimens. Moreover, dispersive transport is identified in the electron selective devices with titanium oxide electron transporter, suggesting considerable presence of trapping states at the perovskite interface, whereas no such behavior characterizes planar samples. Variation in delay time between laser pulse and extraction ramp only affects initial charge concentration present within the device, while transient outlay remains unchanged, indicating absence of film charging effect. [ABSTRACT FROM AUTHOR]

Tri-cation and dual-anion mixed perovskites have been widely utilized in perovskite solar cell (PSC) applications due to their novel properties such as high absorption, high stability, and low cost. To commercialize the PSCs, further improving the device performance without detrimentally changing the device configuration is important at present. Herein, Au@SiO2 nanoparticles (NPs) are introduced to modify the interface between mesoporous TiO2 (mp-TiO2) and mixed perovskite with increased main photovoltaic parameters of the device, resulting in a ≈29% enhancement of power conversion efficiency (PCE) from 15.8% to 20.3%. The origins of the enhancement have been studied by exploring the optical absorption, optical power distribution, and charge carrier behaviors within the system. The small perturbation transient photovoltage measurement exhibits prolonged charge carrier lifetimes after the Au@SiO2 NPs incorporation, and time of flight photoconductivity measurement shows that charge carrier mobilities of this system are also enhanced. These characteristics make metallic nanostructures a promising functional material in facile tuning of the charge carriers transport and further boosting the PCE of the PSCs. [ABSTRACT FROM AUTHOR]

In recent years, organic-inorganic lead halides attracted widespread interest, mainly due to their impressive photoconversion properties and low-cost solution processing. In this study, we employed small amplitude transient photovoltage and photocurrent spectroscopy to investigate charge transport and recombination properties of perovskite CH3NH3PbI3-xClx solar cell under realistic light harvesting conditions (<1 sun). Cell structure resembles outlay commonly found in organic photovoltaics, with perovskite absorber being sandwiched between two thin layers of organic polymers. Tested device displayed high power conversion efficiency (10.3%), good fill factor and negligible hysteresis effect. Fundamental device parameters were characterized at various open-circuit voltages (Voc) by examination of small voltage and current perturbations created by the low intensity pulsed laser excitations. The obtained results exhibit long charge carrier lifetimes and fast charge transport over the full range of applied optical bias, as well as remarkable diffusion lengths exceeding 1 µm. [ABSTRACT FROM AUTHOR]

Incremental increase in dimethyl sulfoxide (or dimethyl formamide) in ortho-dichlorobenzene solution of poly[N-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) gradually reduces the polymer-solvent interaction, the attraction forces between polymer chains become more dominant, and the polymer chains adopt a tight and contracted conformation with more interchain interactions, resulting in a progressive aggregation in both solutions and films. This was used to fine tune the morphology of PCDTBT/PC71BM based solar cells, leading to improved domain structure and hole mobility in the active layer, and significantly improved photovoltaic performance. The power conversion efficiency increased from 6.0% to 7.1% on devices with an active area of 1.0 cm2. [ABSTRACT FROM AUTHOR]

We combined experimental and computational approaches to tune the thickness of the films in poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT)-based organic solar cells to maximize the solar absorption by the active layer. High power-conversion efficiencies of 5.2% and 5.7% were obtained on PCDTBT-based solar cells when using [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) as the electron acceptor, respectively. The cells are designed to have an active area of 1.0 cm2, which is among the largest organic solar cells in the literature, while maintaining a low series resistance of 5 Ω cm2. [ABSTRACT FROM AUTHOR]

A low band-gap alternating copolymer of indolocarbazole and benzothiadiazole-cored oligothiophene demonstrated balanced crystallinity and solubility; a solar cell combining this polymer with PC61BM in a preliminary test demonstrated power conversion efficiencies of 3.6%. [ABSTRACT FROM AUTHOR]

Organic-inorganic lead halide perovskites have shown great potential in efficient photovoltaic devices. However, there are issues related to device stability and reliability and the high power conversion efficiencies (PCE) are typically demonstrated on cell areas much less than 0.2 cm2. The main challenges which limit high efficiencies in larger area devices lie on the low temperature solution processing methods which typically produce lower quality perovskites with defects (pinholes and traps) and the undesired increase in series resistance with cell area. Herein, the control of the dimethyl sulfoxide (DMSO) adduct intermediate phase for the formation of the defect-free perovskite layer and their suitability for larger area solar cells are investigated. We have also selected different conducting substrates, namely indium tin oxide (ITO) with sheet resistance of 10 Ω/□ and fluorine doped tin oxide (FTO) substrates with sheet resistances of 7 and 15 Ω/□ to characterize the effect of substrate sheet resistance and transparency on the photovoltaic performance in large area devices. We demonstrate high PCEs of 18.2% for small area devices (0.16 cm2) and 15.1% for large area device (2 cm2) using the DMSO-enriched recipe. In addition, enhanced device stability was observed, where the devices sustained 94% of their initial efficiency after 105 days without encapsulation. These results confirm that the fine control of adduct intermediate phase for reduced-defect perovskite film provides a simple and universal solution for larger area, efficient and stable perovskite solar cells. Image 1 • Control of DMSO adduct intermediate phase for formation of defect-free perovskite layer for large area solar cells. • High power conversion efficiencies of 15.1% for 2 cm2 area device. • The device maintained ~94% of their initial efficiency after 105 days without encapsulation. [ABSTRACT FROM AUTHOR]

Lead halide complexes are efficient precursors to obtain high quality perovskite films and high performance solar cells. However, how the lead halide complex affects the crystallization kinetics and thus the film quality of perovskite has not been systematically studied. Herein, two PbI 2 (X) complexes with different coordination strength of ligand X in complexation with PbI 2 (PbI 2 (DMF) and PbI 2 (DMSO)) are synthesized. The correlation between the morphology, evolution of the perovskite formation, defects state, carrier lifetime and the subsequent photovoltaic performance are investigated in details via a two-step coating method with various PbI 2 (X) complexes. We find that PbI 2 (DMSO) derived perovskite film shows larger grains and vertically oriented grain boundaries as well as enhanced photoluminescence intensity and longer carrier lifetime. As a result, highly efficient inverted planar perovskite solar cells with a power conversion efficiency up to 17.0% are achieved from PbI 2 (DMSO) complex under 1 sun illumination. This research could open up a new pathway to further improve the performance of perovskite solar cells through the control of perovskite crystallization kinetics by choosing proper PbI 2 (X) complex precursors. [ABSTRACT FROM AUTHOR]

Morphology control is critical to achieve high efficiency CH 3 NH 3 PbI 3 perovskite solar cells (PSC). In this paper, fluorinated perylene diimide (FPDI) was used as novel organic electron transport material in planar heterojunction perovskite solar cells. The perovskite film was fabricated by sequential vacuum vapor deposition, and the film morphology could be controlled by optimizing the FPDI film morphology with short time solvent spin-coating or solvent vapor annealing (SVA). Dense and uniform perovskite film with high substrate coverage could be obtained when the FPDI film was treated by chloroform SVA for half an hour, and the fill factor (FF) of the perovskite solar cell increased from 30.44% to 55.20%, enhancing the power conversion efficiency (PCE) from 3.23% to 7.44%. The PCE of the best device reached 7.93%, which was comparable to that (8.25%) of the conventional ZnO electron transport layer based perovskite device prepared by the same method. [ABSTRACT FROM AUTHOR]

We report the synthesis of new push–pullconjugated polymers for bulk heterojunction solar cells based on benzo[1,2-b:4,5-b′]dithiophene (push) and thieno[3,4-d]thiazole (TTz) (pull) derivatives. Stille polycondensation between distannyl-BDT derivatives with 4,6-dibromo-2-octylthieno[3,4-d]thiazole led to two alternating copolymers, namely PBDTTTz-1and PBDTTTz-2, which have a bandgap of 1.8 and 1.7 eV, respectively. Both copolymers are stable in air with HOMO energy level of −5.3 eV for PBDTTTz-1and −5.4 eV for PBDTTTz-2. Both copolymers have been tested in bulk heterojunction solar cells, and preliminary results (without any optimization process) show promising power conversion efficiency of 1.4% for PBDTTTz-1and 1.7% for PBDTTTz-2. [ABSTRACT FROM AUTHOR]

A new alternating copolymer of dithienosilole and thienopyrrole-4,6-dione (PDTSTPD) possesses both a low optical bandgap (1.73 eV) and a deep highest occupied molecular orbital energy level (5.57 eV). The introduction of branched alkyl chains to the dithienosilole unit was found to be critical for the improvement of the polymer solubility. When blended with PC71BM, PDTSTPD exhibited a power conversion efficiency of 7.3% on the photovoltaic devices with an active area of 1 cm2. [ABSTRACT FROM AUTHOR]

Low-bandgap regioregular polythiophene derivatives, PTh6BTD, PTh8BTD, and PTh4TTBTD, were synthesized through Stille coupling reaction. These are alternating copolymers of an electron-deficient benzothiadiazole unit and oligothiophene units including hexathiophene, octathiophene, and thieno[3,2-b]thiophene-bridged quarterthiophene, respectively. The polymers are soluble in halogenated solvents such as o-dichlorobenzene, affording good processability in solar cell fabrication. Meanwhile, the synthesized alternating copolymers show much broader absorption than P3HT, covering the spectral region from 350 to 800 nm. DSC analysis showed that all three polymers readily crystallized, indicating highly ordered intermolecular packing, which is beneficial for achieving higher charge carrier mobility. Bulk-heterojunction solar cells using 1:1 w/w PThnBTD:PC61BM ([6,6]-phenyl C61-butyric acid methyl ester) blends as the photovoltaic active layers were fabricated and characterized. Best power conversion efficiency of 1.7% was obtained from PTh8BTD-based devices under a simulated AM 1.5 G solar irradiation of 100 mW/cm2. [ABSTRACT FROM AUTHOR]

On the basis of theoretical models and calculations, several alternating polymeric structures have been investigated to develop optimized poly(2,7-carbazole) derivatives for solar cell applications. Selected low band gap alternating copolymers have been obtained via a Suzuki coupling reaction. A good correlation between DFT theoretical calculations performed on model compounds and the experimental HOMO, LUMO, and band gap energies of the corresponding polymers has been obtained. This study reveals that the alternating copolymer HOMO energy level is mainly fixed by the carbazole moiety, whereas the LUMO energy level is mainly related to the nature of the electron-withdrawing comonomer. However, solar cell performances are not solely driven by the energy levels of the materials. Clearly, the molecular weight and the overall organization of the polymers are other important key parameters to consider when developing new polymers for solar cells. Preliminary measurements have revealed hole mobilities of about 1 x 10-3 cm2·V-1·s-1 and a power conversion efficiency (PCE) up to 3.6%. Further improvements are anticipated through a rational design of new symmetric low band gap poly(2,7-carbazole) derivatives. [ABSTRACT FROM AUTHOR]

A new s-tetrazine-based low-bandgap semiconducting polymer, PCPDTTTz, was designed and synthesized. This is the first solution-processable conjugated polymer with tetrazine in the main chain. This polymer shows good thermal stability and broad absorption covering 450-700 nm. The HOMO and LUMO energy levels were estimated to be -5.34 and -3.48 eV, with an electrochemical bandgap of 1.86 eV. Simple polymer solar cells based on PCPDTTTz and PC71BM exhibit a calibrated power conversion efficiency of 5.4%. [ABSTRACT FROM AUTHOR]

The article discusses the polymer bulk heterojunction (BHJ) solar cells that provide renewable energy sources due to their unique features. The use of the donor-acceptor structures has become an effective strategy for getting low-band-gap polymers and regulating their electronic properties. The thieno[3,4-c]pyrrole-4,6-dione (TPD) unit and related derivatives can be effectively applied to adjust the optical and electronic properties of conjugated polymers.