Formation of cubic perovskite alloy containing the ammonium cation of 2D perovskite for high performance solar cells with improved stability

The perovskite solar cells have demonstrated to be strong competitors for conventional silicon solar cells due to their remarkable power conversion efficiency. However, their structural instability is the biggest obstacle to commercialization. To address these issues, we prepared (CH₃NH₃)_1−x(HC(NH₂)₂)_xPbI₃ (CH₃NH₃ = MA, HC(NH₂)₂ = FA) perovskite alloys that contain ethylammonium (EA, CH₃CH₂NH₃⁺) and benzylammonium (BA, C₆H₅CH₂NH₃⁺) cations with no new additional two-dimensional (2D) perovskite phases. The crystal structures of alloy perovskites exhibit the cubic phase, which decreased the cation disorder and the intrinsic instability compared to 3D MAPbI₃ perovskite. The band gaps of the alloy perovskites are almost the same as the corresponding 3D perovskites, which exhibit a high refractive index, a large absorption coefficient, and paramagnetic properties for the production of high performance photovoltaic devices. After we constructed the solar cell with the configuration of regular (n–i–p) solar cells using the alloy perovskites, the power conversion efficiencies (PCE) of the MA_0.83EA_0.17PbI₃ perovskite solar cell showed the highest efficiency, which was 10.22%, under 1 sun illumination.

We prepared (MA)_1−x(FA)_xPbI₃ (CH₃NH₃ = MA, HC(NH₂)₂ = FA) perovskite alloys that contain ethylammonium (CH₃CH₂NH₃⁺) and benzylammonium (C₆H₅CH₂NH₃⁺) cations with no new additional two-dimensional perovskite phases.     

Highly crystalline CsPbI2Br films for efficient perovskite solar cells via compositional engineering

All-inorganic CsPbI₂Br shows high thermal stability for promising application in perovskite solar cells (PSCs). The performance of PSCs is significantly affected by their morphology and crystallinity induced by compositional ratio, solvent/anti-solvent engineering and post thermal annealing. In this study, the compositional ratio effect of two precursors, PbI₂ and CsBr, on the power conversion efficiency (PCE) of a device with ITO/SnO₂/CsPbI₂Br/Spiro-MeOTAD/Au structure was investigated. With the assistance of anti-solvent chlorobenzene, perovskite with a PbI₂ : CsBr ratio of 1.05 : 1 showed a high quality thin film with higher crystallinity and larger grain size. In addition, the molar ratio of precursors PbI₂ and CsBr improved the PCE of the PSCs, and the PSCs fabricated using the perovskite with an optimal ratio of PbI₂ and CsBr exhibited a PCE of 13.34%.

All-inorganic CsPbI₂Br shows high thermal stability for promising application in perovskite solar cells (PSCs).     

Potassium iodide reduces the stability of triple-cation perovskite solar cells

The addition of alkali metal halides to hybrid perovskite materials can significantly impact their crystallisation and hence their performance when used in solar cell devices. Previous work on the use of potassium iodide (KI) in active layers to passivate defects in triple-cation mixed-halide perovskites has been shown to enhance their luminescence efficiency and reduce current–voltage hysteresis. However, the operational stability of KI passivated perovskite solar cells under ambient conditions remains largely unexplored. By investigating perovskite solar cell performance with SnO₂ or TiO₂ electron transport layers (ETL), we propose that defect passivation using KI is highly sensitive to the composition of the perovskite–ETL interface. We reconfirm findings from previous reports that KI preferentially interacts with bromide ions in mixed-halide perovskites, and – at concentrations >5 mol% in the precursor solution – modifies the primary absorber composition as well as leading to the phase segregation of an undesirable secondary non-perovskite phase (KBr) at high KI concentration. Importantly, by studying both material and device stability under continuous illumination and bias under ambient/high-humidity conditions, we show that this secondary phase becomes a favourable degradation product, and that devices incorporating KI have reduced stability.

The addition of alkali metal halides to hybrid perovskite materials can significantly impact their crystallisation and hence their performance when used in solar cell devices.     

ZnO nanowire embedded TiO2 film as an electrode for perovskite CsPbI2Br solar cells

A comparative study was conducted to look into the impact of various electron transporting films on the performance of perovskite CsPbI₂Br solar cells. The solar cells with ZnO nanowires embedded TiO₂ as an electrode outperformed those with pure TiO₂ or pure ZnO. The enhanced performance is ascribed to the synergetic effect of both TiO₂/ZnO constituent properties. In particular, an appropriate amount of ZnO nanowires embedded in TiO₂ films could optimize the properties of the electron transporting layer by improving electron transport, light harvesting, and overall photovoltaic performance, leading to the power conversion efficiency as high as 10.53%.

A comparative study was conducted to look into the impact of various electron transporting films on the performance of perovskite CsPbI₂Br solar cells.     

Improved performance and stability of perovskite solar cells with bilayer electron-transporting layers

Zinc oxide nanoparticles (NPs) are very promising in replacing the phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as electron-transporting materials due to the high carrier mobilities, superior stability, low cost and solution processability at low temperatures. The perovskite/ZnO NPs heterojunction has also demonstrated much better stability than perovskite/PC₆₁BM, however it shows lower power conversion efficiency (PCE) compared to the state-of-art devices based on perovskite/PCBM heterojunction. Here, we demonstrated that the insufficient charge transfer from methylammonium lead iodide (MAPbI₃) to ZnO NPs and significant interface trap-states lead to the poor performance and severe hysteresis of PSC with MAPbI₃/ZnO NPs heterojunction. When PC₆₁BM/ZnO NPs bilayer electron transporting layers (ETLs) were used with a device structure of ITO/poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA)/MAPbI₃/PC₆₁BM/ZnO NPs/Al, which can combine the advantages of efficient charge transfer from MAPbI₃ to PC₆₁BM and excellent blocking ability of ZnO NPs against oxygen, water and electrodes, highly efficient PSCs with PCE as high as 17.2% can be achieved with decent stability.

Perovskite solar cells with PC₆₁BM/ZnO nanoparticles bilayer electron-transporting layers were achieved with a power conversion efficiency of 17.2% and decent stability.

Organic–inorganic hybrid perovskite solar cells (PSCs) have recently attracted tremendous attention because of their excellent photovoltaic efficiencies.^1–4 Since the initial results published in 2009 with efficiencies about 4% using a typical dye-sensitized solar cell structure with liquid electrolyte,⁵ significant progress has been made in device performance through developing high quality film processing methods,^6–10 tuning the perovskite composition,^11–15 optimizing the device architectures^16,17 and synthesizing new hole/electron transport materials.^18–21 Recently, a certified record power conversion efficiency (PCE) of 22.7% was achieved.²² Despite of the success in obtaining dramatically improved PCE, there are certain concerns about the stability and cost towards commercialization. For the state-of-the-art PSCs, perovskites are susceptible to degradation in moisture and air, thus the charge transport materials should prevent the perovskite from exposure to such environments.^20,23–25 One the other hand, PSCs also suffer from the high cost of widely used organic charge transport materials such as 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD), phenyl-C_61/71-butyric acid methyl ester (PC_61/71BM).^3,18,26 As alternatives, inorganic materials such as CuSCN,²⁷ CuI,²⁸ CuGaO₂,²⁰ and NiO_x ^29,30 which can be acted as hole transport materials and ZnO,^31,32 SnO₂ ^12,33,34 and TiO₂ ^10,35 which can be acted as electron transport materials are widely studied. Among them, metal oxide nanoparticles (NPs) are very promising in replacing the organic counterparts due to the high carrier mobilities, superior stability, low cost and solution processability at low temperatures.^16,31,33The perovskite/ZnO NPs heterojunction has been demonstrated much better stability than perovskite/PCBM,²³ however it shows lower PCE compared to the state-of-art devices based on perovskite/PCBM heterojunction.^36–38 Thus in this paper, we systematically studied the charge transfer and recombination at CH₃NH₃PbI₃ (MAPbI₃) and ZnO NPs or PC₆₁BM interfaces and tried to fabricate devices with high PCE and super stability simultaneously. We demonstrated that insufficient charge transfer from MAPbI₃ to ZnO NPs and significant interface trap-states lead to the poor performance and severe hysteresis of PSCs based on MAPbI₃/ZnO NPs heterojunction, while the devices based on MAPbI₃/PC₆₁BM show high PCE and negligible hysteresis due to the efficient charge transfer from MAPbI₃ to PC₆₁BM and less recombination at the interface. On the other hand, the MAPbI₃/ZnO NPs devices show excellent stability in air because of the excellent capping ability of ZnO NPs while the stability of MAPbI₃/PC₆₁BM devices is very poor. Thus, we fabricated the PSCs with bilayer electron-transporting layers (ETLs) with the device structure of ITO/poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA)/MAPbI₃/PC₆₁BM/ZnO NPs/Al, trying to combine the advantages of efficient charge extraction ability of PC₆₁BM and excellent blocking ability of ZnO NPs against oxygen, water and electrode, and finally device with PCE as high as 17.2% was achieved with decent stability.     

Segregation-free bromine-doped perovskite solar cells for IoT applications

Perovskite solar cells have attracted much attention as next-generation solar cells because of their high efficiency and low fabrication costs. Moreover, perovskite solar cells are a promising candidate for indoor energy harvesting. We investigated the effect of bandgap tuning on the characteristics of triple cation-based perovskite solar cells under fluorescent lamp illumination. According to the current density–voltage curves, perovskite solar cells with a wider bandgap than the conventional one exhibited improved open-circuit voltage without sacrificing short-circuit current density under fluorescent lamp illumination. Moreover, the wider bandgap perovskite films including a large amount of bromine in the composition did not show phase segregation, which can degrade the photovoltaic performance of perovskite solar cells, after fluorescent lamp illumination. Our results demonstrate the facile strategy to improve the performance of perovskite solar cells under ambient lighting and great potential of perovskite solar cells for indoor applications such as power sources for the internet of things.

Heavily Br-doped PSCs have a segregation-free effect in low-illuminance conditions, leading to the highest performances.     

Influence of the cathode microstructure on the stability of inverted planar perovskite solar cells

One of the main challenges for perovskite solar cells (PSC) is their environmental stability, as oxygen and water induced aging may result in mobile decomposition compounds, which can enhance the recombination rate and react with charge carrier extraction layers or the contact metallization. In this contribution the importance of the microstructure of the contact metallization on the environmental cell stability is investigated. For this purpose, the storage stability of inverted planar methylammonium lead iodide (MAPI)-based perovskite solar cells without encapsulation is tested, using the metals aluminum (Al), silver (Ag), gold (Au) and nickel (Ni) as representative cathode materials. For this study, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis of the different electrodes as well as the perovskite is correlated with PSC device current–voltage (J–V) and impedance measurements. Our findings substantiate that the metal microstructure has a significant influence on the PSC aging properties. While a strong perovskite decomposition and iodide diffusion to the contacts were detected for devices using Al, Ag or Au cathodes with a polycrystalline microstructure, these effects were strongly reduced when Ni metallization was employed, where a nanocrystalline microstructure was exhibited under the chosen process conditions.

The microstructure of the metal contact has a significant influence on the PSC aging properties. In this contribution we show that perovskite decomposition and iodide diffusion can be suppressed by using nanocrystalline Ni metallization.     

Enhanced thermal stability of inverted perovskite solar cells by interface modification and additive strategy

Inverted perovskite solar cells (PSCs) have recently gained increasing attention because of the long operation lifetime achieved. However, bathocuproine (BCP): a commonly used buffer layer in inverted PSCs, is experimentally confirmed by us to show fast aggregation at the temperature of 85 °C, which is the protocol temperature required by the International Electrotechnical Commission (IEC) standard. This thermal instability of the BCP interfacial layer makes long-term thermal stability of inverted PSCs questionable. Simply removing or replacing it can directly lead to an inferior PCE of a device. We solve this problem by removing the BCP layer and simultaneously increasing the thickness of C₆₀, which obtains a high efficiency of 18% comparable with the device with BCP. This is possibly attributed to the extended migration path of carriers from C₆₀ to metal electrode Ag, consequently reducing the carrier accumulation at the interface. In addition to the interfacial modification, the addition of ionic liquid: BMIMBF₄ into perovskite can further improve a device''s thermal stability by its effective suppression of perovskite decomposition. The devices with 0.4 mol% of BMIMBF₄ exhibit promising thermal stability by retaining 80% of their initial PCE after thermal aging of 400 h at 85 °C.

The thermal stability of PSCs has been enhanced synergistically by removing BCP, increasing C₆₀ thickness and iconic liquid addition.     

Improved efficiency and air stability of two-dimensional p-i-n inverted perovskite solar cells by Cs doping

Two-dimensional perovskite solar cells (2-D PSCs) have attracted much research attention in recent years because they are more stable in a regular environment than three-dimensional (3-D) ones are. In this study, we doped Cs into 2D perovskite (BA₂(MA)₂Pb₃I₁₀) films as the absorbing layers of the 2-D p-i-n inverted PSCs to investigate the influence of the Cs doping concentration on the properties of the 2-D perovskite films and the fabricated solar cells. Cs doping clearly improves the power conversion efficiency (PCE) and air stability of the PSCs. Doping perovskite with 10% Cs (the best doping concentration in this study) can increase the PSC efficiency from 7.98% to 10.11%. Scanning electron microscopy indicates the improved surface quality and crystallinity by Cs doping. However, excess Cs doping degrades the PCE of the PSCs. Furthermore, 10% Cs doped PSCs show air stability superior to that of undoped ones in unpackaged humidity environments. After exposure to 55% relative humidity (RH) in 19 °C air for 300 h, the PCE of the PSC decreased by only 39%, in contrast to 84% for the undoped PSC.

Improved efficiency and air stability of two-dimensional p-i-n inverted perovskite solar cells by 10% Cs doping.     

Improved efficiency and stability of flexible perovskite solar cells by a new spacer cation additive

Flexible perovskite solar cells (PSCs) have attracted tremendous attention due to their potential application in portable and wearable electronics. However, the photoelectric conversion efficiency (PCE) of flexible PSCs is still far lower than that of usual rigid PSCs. Moreover, the mechanical stability of flexible PSCs cannot meet the needs of commercial applications because of the cracking of perovskite grains caused by bending stress. Here, we introduced a spacer cation additive (2-(chloromethyl) pyridine hydrochloride, CPHC) within the perovskite organic precursor to improve the device PCE and its mechanical stability. We observed that the CPHC spacer cation additive could simultaneously facilitate the crystallization of perovskite and stitch the grain boundaries to improve the flexibility. Compared to the 17.64% PCE of the control devices, the target flexible PSCs achieved a more highly efficiency over 19% with an improved mechanical stability (87.2% of the initial PCE after the 1000 cycles with the bending radius R = 6 mm). In addition, compared to methylammonium or formamidinium cation, due to the stronger hydrophobic and larger activation energy barrier for the ion migration of the CPHC spacer cation, the device retained over 80% of the initial PCE after 30 days storage in an ambient environment.

A new type organic spacer CPHC acts as an adhesive between perovskite grains to improve the efficiency and mechanical stability of flexible perovskite solar cells.     

A novel 3-methylthiophene additive to boost the performance and stability of perovskite solar cells

Perovskite solar cells (PSCs) have emerged as a practical candidate for new-generation photovoltaic devices to meet global energy demands. Recently, researchers'' attempts have been focused on the crucial issues related to PSCs, i.e., stability and performance. In this research, MAPbI₃-based PSCs were prepared via a two-step deposition process. To boost the power conversion efficiency (PCE) of the prepared PSCs, an additive engineering approach was employed. A novel 3-methylthiophene (MTP) organic molecule was added to the methylammonium iodide (MAI)/isopropanol (IPA) solution precursor. The additive improved the crystallinity of the perovskite layer, which indicates a more desirable film with lower surface defects and larger particle size. Modified PSCs reduced carries recombination rate at the interfacial of perovskite/hole transport layer (HTL), and the charge transport process is facilitated due to a desirable delocalized π-electron system of the MTP additive. The PCE of PSCs in the presence of MTP additive improved from 12.32% to 16.93% for pristine devices. Importantly, MTP-based PSCs showed higher ambient air stability due to the hydrophobic structure of MTP compared to pristine PSCs.

Perovskite solar cells (PSCs) have emerged as a practical candidate for new-generation photovoltaic devices to meet global energy demands.     

Carbon electrode engineering for high efficiency all-inorganic perovskite solar cells

Carbon-based inorganic perovskite solar cells (PSCs) have demonstrated an excellent performance in the field of photovoltaics owing to their simple fabrication techniques, low-cost and superior stability. Despite the lower efficiency of devices with a carbon electrode compared with the conventional structure, the potential applications in large scale have attracted increasing attention. Herein, we employ a mixed carbon electrode inorganic PSC by incorporating one-dimensional structure carbon nanotubes (CNTs) and two-dimensional Ti₃C₂-MXene nanosheets into a commercial carbon paste. This mixed carbon electrode, which is different from the pure carbon electrode in showing a point-to-point contact, provides a network structure and multi-dimensional charge transfer path, which effectively increases the conductivity of the carbon electrode and carriers transport. A respectable power conversion efficiency of 7.09% is obtained through carbon/CNT/MXene mixed electrode in CsPbBr₃-based solar cells.

A carbon/CNT/MXene mixed electrode in CsPbBr₃ solar cells provides a network structure and multi-dimensional charge transfer path, which effectively increases the conductivity of the carbon electrode and carriers transport.     

Spacer layer design for efficient fully printable mesoscopic perovskite solar cells

The spacer layer is a key component of fully printable mesoscopic perovskite solar cells, but its precise characteristics are far from being understood in relation to the device design. In the present work, we perform a detailed systematic study on the effects of spacer parameters, such as size of building blocks, layer thickness, etc., on properties of the perovskite filler, insulating ability and performance of fully printable mesoscopic perovskite solar cells by combining the techniques of time-resolved photoluminescence, high-resolution TEM, insulating resistance measurements, impedance spectroscopy and J–V characteristics. Drawing on the deep understanding from these studies, we formulate key principles, which are anticipated to guide the design of the advanced spacer layer for fully printable mesoscopic perovskite solar cells.

Key principles and reasonable routes are proposed to advance the spacer layer design for fully printable mesoscopic perovskite solar cells.

Lead halide perovskite (PVSK) as a promising semiconducting material has been introduced as a light harvesting semiconductor because of its ease of fabrication and excellent physical properties, such as tunable bandgap, strong absorbance, long carrier diffusion length and shallow intrinsic trap state level.^1–9 Extremely flat and compact perovskite thin film with large crystal size has gained particular attention to boost power conversion efficiency (PCE) by sequential deposition method, vapor deposition, solvent-annealing, solvent engineering, hot-casting method, intramolecular exchange methods, and additive, etc.^10–16 Benefiting from rapid improvements in formation of high quality perovskite thin film, a certified PCE of 25.2% has been achieved.¹⁷ However, illumination stability in real environment still remains a serious challenge due to the inherent moisture and UV sensitivity of perovskite. Moreover, using expensive and rare metals as back contact, such as gold and silver, may limit large-scale production in the future. As the competing architecture of perovskite solar cells, TiO₂/spacer/carbon (abbreviated as TSC) films based fully printable mesoscopic perovskite solar cells (FP-MPSC) have attracted a lot of researchers due to their low cost and printable large-scale production process.^18–21 In this type of solar cell, carbon can efficiently collect hole from perovskite layer even without any other hole transporting materials.^22,23 Most importantly, FP-MPSC could work with excellent illumination stability and heat-stress stability by filling TSC films with (5-AVA)_xMA_1−xPbI₃ (5-AVA = 5-aminovaleric acid, MA = methylammonium), although the efficiency of 12.8% is still far behind from the most efficient solar cell.^16,21,24Spacer, as an important part of FP-MPSC, plays a crucial role in obtaining high performance device. Basically, the spacer layer mainly burdens triple important tasks in the efficient mesoscopic perovskite solar cells. Firstly, the core function of spacer is to separate anode and cathode and to prevent electrons in TiO₂ from transporting directly to carbon electrode. The separating property of spacer depends on spacer particle sizes, morphology, materials, etc. This requires that spacer layer has no cracking and has wide bandgap. Secondly, the perovskite confined in the mesopores of spacer layer can absorb photons transmitted through perovskite/TiO₂ composite layer and have contribution to photocurrents. Thirdly, the holes produced in the perovskite/TiO₂ composite layer have to go through perovskite/spacer composite layer to reach carbon electrode. And the electrons produced in the perovskite/spacer composite layer have to go through perovskite/spacer composite layer to reach TiO₂ electrode. Because spacer layer has these important functions, some research on spacer layer have been carried out. Recently, Al₂O₃ or ZrO₂ spacer layer was compared with respect to their pore size.²⁵ However, conclusion of the effect of pore size in two different materials was incomplete. The effect of spacer layer thickness was simply discussed both in monolithic dye-sensitized solar cells and FP-MPSC.^20,26 The morphology of spacer layer was also improved to increase PCE of FP-MPSC.²⁷ Although these researches made some progress, there are no clear standards that what should an ideal spacer layer satisfy. Therefore, it is urgent to carry out detailed study on how the parameters of spacer affect the above functions and performance of mesoscopic printable perovskite solar cells.In the present study, the effects of size of building blocks of spacer layer, thickness of spacer layer on property of perovskite crystals, insulating property, and performance of mesoscopic perovskite solar cells were investigated in details. Based on these deep understandings, critical principles to design advanced spacer layer are proposed.ZrO₂ is used as spacer material due to its large band gap and high conduction band energy level. There are five different sizes of spacer building blocks in this study. The average particle sizes of spacer are measured to be about 5 nm, 10 nm, 20 nm, 60 nm and 100 nm, respectively, and hereafter referred to as S5, S10, S20, S60, S100 spacer, respectively. SEM images of as-prepared spacer films using these building blocks are shown in Fig. 1. X-ray diffraction patterns of spacer film with different particle sizes are presented in Fig. 1f, indicating that the five spacer films were all tetragonal crystal phase as majority phase. From Scherrer equation, the crystal sizes of spacer building blocks were calculated to be about 5 nm, 10 nm, 20 nm, 30 nm, 30 nm, respectively. These results indicated that the S60 and S100 particles are consisted of 30 nm sized crystal ZrO₂. Fig. 1 presents that there is a large difference in surface morphology with particle size increasing. There are cracks in S5 and S10 spacer films and micrometer scale pores exist in the S100 spacer, while the surface of S20 and S60 are very uniform without defects.Open in a separate windowFig. 1SEM images of spacer films with particle size of 5 nm (a), 10 nm (b), 20 nm (c), 60 nm (d) and 100 nm (e), respectively. (f) XRD patterns of spacer film with different building block sizes.During solvent evaporation of perovskite precursor, perovskite crystal growth is restricted by randomly interconnected mesopores of spacer film, leading to nanoscale crystal size and random crystal orientation, as observed by high resolution transmission electron microscope (Fig. 2a), in which clear crystal lattices of perovskite crystals can be distinguished from spacer particles. Meanwhile, the mesopores of spacer film is fulfilled with perovskite materials, providing continuous channels for charge carriers. The crystal size of perovskite material in spacer film is strongly influenced by mesopore size of spacer layer, as seen in XRD intensity of perovskite at 2θ of about 14.2° (Fig. 2b). There is an apparent trend that the intensity increased with increasing the particle size of spacer film. As a reference, perovskite was also deposited on bare glass, which exhibited the best crystallinity. The calculated sizes of perovskite from XRD spectra are 4.3 nm, 3.9 nm, 6.2 nm, 11.5 nm, and 12.6 nm, respectively, for the S5, S10, S20, S60, and S100 spacer layer. Apparently, the sizes of perovskite crystals confined in the spacer layer are smaller than the average pore sizes of spacer layer measured by N₂ absorption/desorption isotherms (Table S1^†). The high-resolution TEM image also gives consistent results. For example, the sizes of perovskite crystals confined in the S20 spacer layer are between 8.4 nm to 12.7 nm measured in the TEM image (Fig. 2a). The infiltrated perovskite started to nucleate onto the heterogeneous surface of spacer building blocks with high surface area, resulting to multiple nucleation centers and small crystal size. In order to evaluate the effect of spacer particle size on physiochemical properties of perovskite, the band-edge emission spectra of perovskite/spacer composite film were measured in Fig. 2c. The band-edge emission spectrum of perovskite deposited on bare glass peaked at 762 nm (with photon energy of 1.627 eV). As the particle size of spacer film decreased, a blue shift of the band-edge photoluminescence occurred, and linewidth broadened. The peak position of perovskite emission spectra can be tuned in the range of 33 nm through varying the pore size of spacer film. The increase of emission line width at grain boundaries can be attributed to disorder and defects of perovskite,^14,15 which also led to the decrease of lifetime in time-resolved PL (Fig. 2d). Perovskite film grown on glass has the lifetime of 141.9 ns. However, perovskite grown in spacer film decreased to 0.5 ns, 8.4 ns, 24.1 ns, 37.2 ns, 53.7 ns, for S5, S10, S20, S60, S100, respectively.Open in a separate windowFig. 2(a) High resolution transmission electron microscope image of perovskite/S20 spacer film composite. (b) XRD of perovskite/spacer film composite. (c) Steady PL emission spectra and (d) time-resolved PL of perovskite/spacer film composite.The particle size of spacer also has large effect on insulating ability of spacer films with the same thickness. FTO/spacer/carbon configuration was designed to measure the insulating ability of spacer layers. In ideal conditions, the resistance between carbon and FTO, defined as insulating resistance (R_I), should be infinite, indicating that there is not any leakage current from ideal insulating spacer. However, all of the measured resistance has finite values, summarized in 28–31Photovoltaic parameters of mesoscopic perovskite solar cells based on spacer with different building block sizes

Differences in photoinduced optical transients in perovskite absorbers for solar cells

Methylammonium lead iodide films and powdered crystals were studied by time-resolved absorption and emission spectroscopy on the time scales from femtoseconds to nanoseconds. Strikingly different transient absorption signals were observed, changing from strong long-wavelength band-edge bleach to weak signatures of band-shift, which depended on the absorber form (films or polycrystals) and preparation method (stoichiometric or non-stoichiometric). The observed differences were correlated with the variation in absorption and emission spectra, changes in photo-induced carrier lifetimes and solar cell efficiency. These differences also pointed out that similar perovskite absorbers can provide significantly different transient responses and emphasize that special care must be taken when interpolating the obtained findings to the processes occurring in the most efficient devices.

Striking differences in transient absorption signal are observed for perovskite absorbers prepared with different methods.     

Efficiency and stability enhancement of perovskite solar cells using reduced graphene oxide derived from earth-abundant natural graphite

Graphene – two-dimensional (2D) sheets of carbon atoms linked in a honeycomb pattern – has unique properties that exhibit great promise for various applications including solar cells. Herein we prepared two-dimensional (2D) reduced graphene oxide (rGO) nanosheets from naturally abundant graphite flakes (obtained from Tuv aimag in Mongolia) using solution processed chemical oxidation and thermal reduction methods. As a proof of concept, we used our rGO as a hole transporting material (HTM) in perovskite solar cells (PSCs). Promisingly, the use of rGO in the hole transporting layer (HTL) not only enhanced the photovoltaic efficiency of PSCs, but also improved the device stability. In particular, the best performing PSC employing rGO nanosheets exhibited a power conversion efficiency (PCE) of up to 18.13%, while the control device without rGO delivered a maximum efficiency of 17.26%. The present work demonstrates the possibilities for solving PSC issues (stability) using nanomaterials derived from naturally abundant graphite sources.

Solution processed reduced graphene oxide nanosheets have been prepared from naturally abundant graphite flakes and used to enhance the efficiency and stability of perovskite solar cells.     

Atomic layer deposition of metal oxides for efficient perovskite single-junction and perovskite/silicon tandem solar cells

Aluminum-doped and undoped zinc oxide films were investigated as potential front and rear contacts of perovskite single and perovskite/silicon tandem solar cells. The films were prepared by atomic layer deposition (ALD) at low (<200 °C) substrate temperatures. The deposited films were crystalline with a single-phase wurtzite structure and exhibit excellent uniformity and low surface roughness which was confirmed by XRD and SEM measurements. Necessary material characterizations allow for realizing high-quality films with low resistivity and high optical transparency at the standard growth rate. Spectroscopic ellipsometry measurements were carried out to extract the complex refractive index of the deposited films, which were used to study the optics of perovskite single junction and perovskite/silicon tandem solar cells. The optics was investigated by three-dimensional finite-difference time-domain simulations. Guidelines are provided on how to realize perovskite solar cells exhibiting high short-circuit current densities. Furthermore, detailed guidelines are given for realizing perovskite/silicon tandem solar cells with short-circuit current densities exceeding 20 mA cm⁻² and potential energy conversion efficiencies beyond 31%.

The necessity of thin and highly doped metal oxide films is discussed for realizing efficient perovskite single and perovskite/silicon tandem solar cells.     

Development of moisture-proof polydimethylsiloxane/aluminum oxide film and stability improvement of perovskite solar cells using the film

A method for enhancing the moisture barrier property of polydimethylsiloxane (PDMS) polymer films is proposed. This is achieved by filling the PDMS free volume with aluminum oxide (AlO_x). To deposit AlO_x inside PDMS, thermal atomic layer deposition (ALD) is employed. The PDMS/AlO_x film thus produced has a 30 nm AlO_x layer on the surface. Its water vapor transmission rate (WVTR) is 5.1 × 10⁻³ g m⁻² d⁻¹ at 45 °C and 65% relative humidity (RH). The activation energy of permeability with the PDMS/AlO_x film for moisture permeation is determined to be 35.5 kJ mol⁻¹. To investigate the moisture barrier capability of the PDMS/AlO_x layer, (FAPbI₃)_0.85(MAPbBr₃)_0.15/spiro-OMeTAD/Au perovskite solar cells are fabricated, and encapsulated by the PDMS/AlO_x film. To minimize the thermal damage to solar cells during ALD, AlO_x deposition is performed at 95 °C. The solar cells exposed to 45 °C-65% RH for 300 h demonstrate less than a 5% drop in the power-conversion efficiency.

A method for enhancing the moisture barrier property of polydimethylsiloxane (PDMS) polymer films is proposed. This is achieved by filling the PDMS free volume with aluminum oxide (AlO_x).     

Room-temperature synthesized SnO2 electron transport layers for efficient perovskite solar cells

Tin oxide (SnO₂) is widely used as electron transport layer (ETL) material in perovskite solar cells (PSCs). Numerous synthesis methods for SnO₂ have been reported, but they all require a proper thermal treatment for the SnO₂ ETLs. Herein we present a simple method to synthesize SnO₂ nanoparticles (NPs) at room temperature. By using butyl acetate as a precipitator and a proper UV–Ozone treatment to remove Cl residuals, excellent SnO₂ ETLs were obtained without any thermal annealing. The highest power conversion efficiency (PCE) of the prepared PSCs was 19.22% for reverse scan (RS) and 18.79% for forward scan (FS). Furthermore, flexible PSCs were fabricated with high PCEs of 15.27%/14.74% (RS/FS). The low energy consuming SnO₂ ETLs therefore show great promise for the flexible PSCs'' commercialization.

Room-temperature processed SnO₂ as electron transport layer materials for efficient flexible perovskite solar cells.     

Mixed lead source precursors for producing light absorption layers of perovskite solar cells

Beside the conventional perovskite precursors with lead halides as lead sources, non-halide lead sources provide additional tools for tuning the properties of perovskite layers, and lead acetate is a promising candidate for non-halide lead sources. In this work, we develop the perovskite precursor with a mixed non-halide lead source by partially replacing lead acetate with lead thiocyanate. Scanning electron microscopy and X-ray diffraction measurements indicate that lead thiocyanate additive can remarkably increase the size of perovskite grains and the crystallization of perovskite layers. And the cross-sectional investigation illustrates that the penetration of perovskite materials into TiO₂ porous layers also can be improved by the lead thiocyanate additive. As a consequence, the recombination process and charge extraction process of devices are improved. By optimizing the quantity of lead thiocyanate, the power conversion efficiency of devices is increased from 14.0% to 17.2%, and the stability of devices is elevated simultaneously.

Lead thiocyanate is incorporated into lead acetate precursors for improving the efficiency and stability of perovskite solar cells.     

Wetting-induced formation of void-free metal halide perovskite films by green ultrasonic spray coating for large-area mesoscopic perovskite solar cells

A void-free metal halide perovskite (MHP) layer on a mesoscopic TiO₂ (m-TiO₂) film was formed via the wetting-induced infiltration of MHP solution in the m-TiO₂ film via a green ultrasonic spray coating process using a non-hazardous solvent. The systematic investigation of the behavior of ultrasonic-sprayed MHP micro-drops on the m-TiO₂ film disclosed that the void-free MHP layer on the m-TiO₂ film can be formed if the following conditions are satisfied: (1) the sprayed micro-drops are merged and wetted in the mesoscopic scaffold of the m-TiO₂ film, (2) the MHP solution infiltrated into the m-TiO₂ film by wetting is leveled to make a smooth wet MHP film, and (3) the smooth wet MHP film is promptly heat treated to eliminate dewetting and the coffee ring effect by convective flow in order to form a uniform void-free MHP layer. A void-free MHP layer on the m-TiO₂ film was formed under optimal ultrasonic spray coating conditions of substrate temperature of ∼30 °C, spray flow rate of ∼11 mL h⁻¹, nozzle to substrate distance of ∼8 cm, and MHP solution-concentration of ∼0.6 M under a fixed scan speed of 30 mm s⁻¹ and purged N₂ carrier gas pressure of 0.02 MPa. The mesoscopic MHP solar cells with an aperture area of 0.096, 1, 25, and 100 cm² exhibited 17.14%, 16.03%, 12.93%, and 10.67% power conversion efficiency at 1 sun condition, respectively.

A void-free metal halide perovskite (MHP) layer on a mesoscopic TiO₂ (m-TiO₂) film was formed via the wetting-induced infiltration of MHP solution in the m-TiO₂ film via a green ultrasonic spray coating process using a non-hazardous solvent.