MAPbI3 Layer Research Articles

In CH3 NH3 PbI3 Perovskite Film, the Surface Termination Layer Dominates the Moisture Degradation Pathway.

Theoretical studies have shown that surface terminations, such as MAI or PbI layers, greatly affect the environmental stability of organic-inorganic perovskite. However, until now, there has been little effort to experimentally detect the existence of MAI or PbI terminations on MAPbI3 grains, let alone disclose their effects on the humidity degradation pathway of perovskite solar cell. Here, we successfully modified and detected the surface terminations of MAI and PbI species on polycrystalline MAPbI3 films. MAI-terminated perovskite film followed the moisture degradation process from MAPbI3 to hydrate MAPbI3 ⋅H2 O and then into PbI2 , with penetration of water molecules being the main driving force leading to the degradation of MAPbI3 layer by layer. In contrast, for the PbI-terminated perovskite film in a humid atmosphere, a deprotonation degradation pathway was confirmed, in which the film preferentially degraded directly from MAPbI3 into PbI2 , here the iodine defects played a key role in promoting the dissociation of water molecules into OH- and further catalyzing the decomposition of perovskite.

Crystal Reorientation and Amorphization Induced by Stressing Efficient and Stable P–I–N Vacuum‐Processed MAPbI3 Perovskite Solar Cells

Herein, the long‐term stability of vacuum‐deposited methylammonium lead iodide (MAPbI3) perovskite solar cells (PSCs) with power conversion efficiencies (PCEs) of around 19% is evaluated. A low‐temperature atomic layer deposition (ALD) Al2O3 coating is developed and used to protect the MAPbI3 layers and the solar cells from environmental agents. The ALD encapsulation enables the MAPbI3 to be exposed to temperatures as high as 150 °C for several hours without change in color. It also improves the thermal stability of the solar cells, which maintain 80% of the initial PCEs after aging for ≈40 and 37 days at 65 and 85 °C, respectively. However, room‐temperature operation of the solar cells under 1 sun illumination leads to a loss of 20% of their initial PCE in 230 h. Due to the very thin ALD Al2O3 encapsulation, X‐ray diffraction can be performed on the MAPbI3 films and completed solar cells before and after the different stress conditions. Surprisingly, it is found that the main effect of light soaking and thermal stress is a crystal reorientation with respect to the substrate from (002) to (202) of the perovskite layer, and that this reorientation is accelerated under illumination.

Graded interface engineering of 3D/2D halide perovskite solar cells through ultrathin (PEA)2PbI4 nanosheets

2D halide perovskites have emerged as promising materials because of their stability and passivation effect in perovskite solar cells (PSCs). However, the introduction of bulky organic ammonium cations from 2D halide perovskites would decrease the device performance generally compared to the traditional 3D MAPbI3. Incorporation of ultrathin 2D halide perovskite nanosheets (NSs) with 3D MAPbI3 could address this issue. Herein, we report a rationally designed PSCs with dimensional graded 3D/2D MAPbI3/(PEA)2PbI4 heterojunction, in which 2D (PEA)2PbI4 NSs were synthesized and incorporated between 3D MAPbI3 and hole-transporting layer. Besides the significantly improved stability, a notable increasement in power conversion efficiency (PCE) of 20% was obtained for the 3D/2D perovskite solar cells due to the favourable band alignment among (PEA)2PbI4 NSs and the other components. The graded structure of MAPbI3/(PEA)2PbI4 would upshift the energy level continuously, which enhances the hole extraction efficiency thus reduces the interface charge recombination, leading to the increasements of VOC from 1.04 V to 1.07 V, JSC from 21.81 mA/cm2 to 23.15 mA/cm2 and the fill factor from 67.89% to 74.78%, and therefore an overall PCE of 18.53%.

Enhanced Photovoltaic Properties of Perovskite Solar Cells by Employing Bathocuproine/Hydrophobic Polymer Films as Hole-Blocking/Electron-Transporting Interfacial Layers.

In this study, we improved the photovoltaic (PV) properties and storage stabilities of inverted perovskite solar cells (PVSCs) based on methylammonium lead iodide (MAPbI3) by employing bathocuproine (BCP)/poly(methyl methacrylate) (PMMA) and BCP/polyvinylpyrrolidone (PVP) as hole-blocking and electron-transporting interfacial layers. The architecture of the PVSCs was indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/MAPbI3/[6,6]-phenyl-C61-butyric acid methyl ester/BCP based interfacial layer/Ag. The presence of PMMA and PVP affected the morphological stability of the BCP and MAPbI3 layers. The storage-stability of the BCP/PMMA-based PVSCs was enhanced significantly relative to that of the corresponding unmodified BCP-based PVSC. Moreover, the PV performance of the BCP/PVP-based PVSCs was enhanced when compared with that of the unmodified BCP-based PVSC. Thus, incorporating hydrophobic polymers into BCP-based hole-blocking/electron-transporting interfacial layers can improve the PV performance and storage stability of PVSCs.

Improved photostability of inverted-structure perovskite solar cells with high power conversion efficiency by inserting CuI between PEDOT and MAPbI3 layers

Inverted-structure perovskite devices with PEDOT and PEDOT/CuI as hole transport layers (HTLs) were prepared on Glass/ITO substrates. Surface morphology observation of the CuI revealed that the CuI grew on PEDOT in island mode. A concentration of 20 mg/ml was necessary for continuous CuI formation on the PEDOT. Optimization of the annealing time and the concentration of CuI precursor solution led to a PCE of 16.5% for a device with PEDOT/CuI as HTL, which was slightly higher than that (15%) with PEDOT as HTL. The slight enhancement of PCE was due to the faster hole transport efficiency and improved light-harvesting in the visible light region. Additionally, the photostability of the devices was greatly enhanced by the insertion of the CuI layer. Analysis indicated that the insertion of the CuI decreased the transmittance of light with wavelengths within 320–410 nm, which was the main reason for the enhanced photostability.

Incorporating of Lanthanides Ions into Perovskite Film for Efficient and Stable Perovskite Solar Cells.

Since Yan's work, incorporation of some lanthanide elements, such as Eu and Nd, into MAPbI3 layer has been proven to be a powerful strategy on improving the permanence of the perovskite solar cells (PSCs). However, a comprehensive configuration has not been given for different lanthanide elements doping while the mechanism has not been clarified. Herein, the incorporation of various lanthanides ions (Ln3+ = Ce3+ , Eu3+ , Nd3+ , Sm3+ , or Yb3+ ) into perovskite films to largely enhance the performance of PSCs is presented. Arising from the enlarged grain size and crystallinity of perovskite film upon Ln3+ ions doping, the efficiency and stability of PSCs are significantly improved. Extraordinarily, PSCs with Ce3+ doping achieve the best performance, with a champion power conversion efficiency (PCE) of 21.67% in contrast to 18.50% for pristine PSCs, and outstanding long-term and UV irradiation stability. Such high performance of PSCs after Ce3+ doping originates from special Ce3+ /Ce4+ redox pair and the unique 4f-5d absorption in the UV region. Finally, the flexible PSCs with low-temperature preparation are explored. Considering the richer deposition of cerium element in the earth and lower price, the findings may provide new opportunities for developing low-cost, highly efficient, air/UV stable, and flexible PSCs.

Enhanced performance of CH3NH3PbI3 perovskite solar cells via interface modification using phenyl ammonium iodide derivatives

Interface modification in perovskite solar cells is a key factor for achieving high power conversion efficiency by suppressing electron-hole recombination and accelerating charge carrier extraction. Here, we use a series of phenyl ammonium derivatives, phenyl ammonium iodide (PAI), benzyl ammonium iodide (BAI), and phenyl ethyl ammonium iodide (PEAI), to modify the interface between methylammonium lead triiodide (MAPbI3) perovskite and Spiro-OMeTAD as a hole transport layer in solar cell devices. The structural and optical properties of the perovskite films are studied and the results reveal the formation of two-dimensional perovskite interfacial layers on the surface of the MAPbI3 film modified with PEAI and BAI whereas the MAPbI3 layer modified with PAI gives an interface layer with slightly different properties compared to the two-dimensional perovskite. Impedance spectroscopy shows that the charge transport resistance of the interface engineered solar cells decreases when compared to pristine MAPbI3. In addition, slower open-circuit voltage decay and longer carrier lifetime are also observed for the modified cells which in total lead to the improvement of the photovoltaic performance. The investigation therefore gives insight in the effect of interface modifications, and especially how different sizes of the molecular interface modifier results in different interface formation and characteristics.

Switching between spectral broadening and narrowing of the exciton absorption band of a CH3NH3PbI3 film on altering the polarity of an applied electric field

Electric field-induced broadening and narrowing of an exciton absorption band have been observed to depend on the polarity of the applied electric field for methylammonium lead triiodide perovskite (MAPbI3) as a thin film sandwiched between a conducting film of fluorine-doped tin oxide (FTO) and an insulating film of poly(methyl methacrylate) (PMMA). The width of the absorption band increased or decreased when the direction of the applied field was toward FTO or PMMA, respectively, indicating that the spectral broadening and narrowing become switchable on altering the direction of the applied electric field. When titanium oxide (TiO2) in a compact layer was introduced between the MAPbI3 and FTO layers, the linear electric field effect that depended on the polarity was either not observed or decreased significantly although the quadratic field effects on the exciton absorption band were similarly observed with or without the TiO2 layer.

Highly Efficient Thermally Co-evaporated Perovskite Solar Cells and Mini-modules

Summary The rapid improvement in the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has prompted interest in bringing the technology toward commercialization. Capitalizing on existing industrial processes facilitates the transition from laboratory to production lines. In this work, we prove the scalability of thermally co-evaporated MAPbI3 layers in PSCs and mini-modules. With a combined strategy of active layer engineering, interfacial optimization, surface treatments, and light management, we demonstrate PSCs (0.16 cm2 active area) and mini-modules (21 cm2 active area) achieving record PCEs of 20.28% and 18.13%, respectively. Un-encapsulated PSCs retained ∼90% of their initial PCE under continuous illumination at 1 sun, without sample cooling, for more than 100 h. Looking toward tandem and building integrated photovoltaic applications, we have demonstrated semi-transparent mini-modules and colored PSCs with consistent PCEs of ∼16% for a set of visible colors. Our work demonstrates the compatibility of perovskite technology with industrial processes and its potential for next-generation photovoltaics.

Low-Temperature (

A scan-mode low-temperature (<40 °C) atmospheric-pressure helium (He) dielectric-barrier discharge jet (DBDjet) is applied to treat nickel oxide (NiO) thin films for p–i–n perovskite solar cells (PSCs). Reactive plasma species help reduce the trap density, improve the transmittance and wettability, and deepen the valence band maximum (VBM) level. A NiO surface with the lower trap density surface of NiO allows better interfacial contact with the MAPbI3 layer and increases the carrier extraction capability. MAPbI3 can better crystallize on a more hydrophilic NiO surface, thereby suppressing charge recombination from the grain boundary and the interface. Further, the deeper VBM allows better band alignment and reduces the probability of nonradiative recombination. NiO treatment using He DBDjet with a scan rate of 0.3 cm/s can improve PSC efficiency from 13.63 to 14.88%.

Cesium carbonate modified electron transport layer for improving the photoelectric conversion efficiency of planar perovskite solar cells

TiO2 is a commonly used material in the electron transport layers (ETLs) of perovskite solar cells (PSCs) but its defects restrict the development of PSCs. In this study, cesium carbonate (Cs2CO3) was used to modify the TiO2 ETLs because of its excellent electron injection ability. The new structure of the PSC was FTO/TiO2/Cs2CO3/perovskite (MAPbI3)/sprio-OMETAD/back electrode. As expected, adding Cs2CO3 increased the champion photoelectric conversion efficiency (PCE) from 9.2% to 12.8% in comparison with unmodified solar cells, and the device maintained 78% of the original efficiency after 250 h. Moreover, the reduction of defects in the TiO2 ETLs reduced the coincidence probability of carriers after modification with Cs2CO3. Due to the excellent electron injection ability of Cs2CO3, the modified ETLs yielded lower work functions and smaller energy level barriers, which makes the energy levels between the TiO2 ETL and the MAPbI3 layer well-matched, and reduced the carrier coincidence probability.

Highly stable inverted methylammonium lead tri-iodide perovskite solar cells achieved by surface re-crystallization

A long-term operational stability over 1000 hours in the inverted type perovskite solar cells based on the MAPbI3 layer is demonstrated under ionic defect-free conditions.

CVD growth of perovskite/graphene films for high-performance flexible image sensor

Hybrid perovskite possesses excellent photoelectric properties, including large light-absorption capacity and high carrier mobility, and is an ideal light-absorbing material for photoelectric devices. The grain size and compactness of hybrid perovskite are key factors affecting the performance of photoelectric devices. The photocurrent and photoresponsivity of these devices are relatively low because of the rapidly recombined photoexcited electron-hole pairs in hybrid perovskite. Herein, we develop a facile two-step chemical vapor deposition (CVD) method to synthesize a high-quality van der Waals (vdWs) MAPbI3/graphene heterostructure for high-performance image sensor. We introduced inorganic sources (PbI2) to vdWs epitaxially grown PbI2 film on a seamless graphene monolayer film template through CVD. Methylammonium iodide (MAI) was then reintroduced to prepare the vdWs MAPbI3/graphene heterostructure. The MAPbI3 layer is composed of densely packed, large-size grains and displays a smooth surface. High photoresponsivity of 107 A/W is achieved in the corresponding photodetector. Inspired by the human visual system, we designed a flexible photodetector array containing (24 × 24) pixels, achieving perfect image recognition and color discrimination. Our study may greatly facilitate the construction of high-performance optoelectronic devices in artificial retina, biomedical imaging, remote sensing, and optical communication.

Ferroelectric Poling of Methylammonium Lead Iodide Thin Films

AbstractSeemingly contradictory reports on polar domains and their origin have surrounded the controversial discussion about the ferroelectricity of the methyl ammonium lead iodide (MAPbI3) thin films that are commonly employed in perovskite solar cells. In this work, microscopic modulations of the polar domain patterns upon application of an electric poling field are correlated with macroscopic changes to the currents through the MAPbI3 layer. Piezoresponse force microscopy is used to monitor the widening, narrowing, generation or extinction of polar domains, as well as shifts of the domain walls at room temperature under an in‐plane electric poling field that is applied between two laterally organized electrodes. This poling leads to a net polarization of individual grains and the thin film itself. Macroscopically, this net polarization results in a persistent shift of the diode characteristics that is measured across the channel between the electrodes. Both the modulation of the polar domains upon electric poling and the concurrent persistent shift of the electric currents through the device are the unambiguous hallmarks of ferroelectricity, which demonstrate that MAPbI3 is a ferroelectric semiconductor.

MAPbI3/agarose photoactive composite for highly stable unencapsulated perovskite solar cells in humid environment

Perovskite solar cells, bearing the merits of facile preparation and remarkable efficiency, has great potential for bringing the photovoltaic industry to a new generation. The photovoltaic market demands high-efficiency, high stability and low-cost fabrication of perovksite solar cells, especially stability to the humid environment for operation. Here, MAPbI3/agarose photoactive material for humid stable unencapsulated devices has been proposed. These solar cells have been operated in ambient humid environment without glove box, exhibiting efficiency up to 14.66% and retain 90% of its PCE after 1392 h and 60% of initial PCE after 1972 h in ambient humid environment (RH>70%) without encapsulation. FTIR and XPS measurements reveal two critical factors for the improved stability. The molecular level interactions between agarose and MAPbI3 passivates the grain boundaries of perovskite thus preventing its degradation. Moreover, the formation of Li+-agarose complex at the interface between perovskite layer and hole conductive layer, effectively prevents the water uptake of MAPbI3 layer. Both effects of passivation and minimization of hygroscopicity of LiTFSI by agarose lower the decomposition speed of perovskite, which obviously increases the power efficiency and stability of device.

Influence of TiO2 layer on ultimate efficiencies for planar and nano-textured CH3NH3PbI3 solar cells

The influence of TiO2 layer on the ultimate efficiencies, η, i.e. efficiencies without considering carrier recombination, for the planar and nano-textured CH3NH3PbI3 (MAPbI3) perovskite solar cells (SCs) are investigated. In planar TiO2/MAPbI3 heterojunction SCs, in order to achieve the largest power conversion efficiency (PCE), the TiO2 layer thickness, d1, is important. With the finite difference time domain (FDTD) method, we demonstrated that when the MAPbI3 layer thickness, d2, is 250 nm, which is a common-most MAPbI3 layer thickness for perovskite SCs, η achieves maximum when d1 is 80 nm. Fabricating nano textures on SC surface is an important method to improve the PCE. We studied the effects of d1 and d2 on the optimized η, η0, for two kinds of nano-textured perovskite SCs: the SCs with the nontextured TiO2 layer and the column-shaped nano hollow (CLH) textured MAPbI3 layer, defined as the nontextured-TiO2/CLH-MAPbI3 SCs, and the SCs with CLH textured TiO2 and CLH textured MAPbI3 layers of the same hollow axes and radius, defined as the CLH-(TiO2/MAPbI3) SCs. Generally, when d1 and d2 are fixed, η0 for the CLH-(TiO2/MAPbI3) SC is larger than that for the nontextured-TiO2/CLH-MAPbI3 SC by ca. 5%.

Facile precursor stoichiometry engineering for efficient inverted perovskite solar cells without any dopants

Full coverage of MAPbI3 layer could be easily obtained by two-step method, while the perovskite precursor stoichiometry ratio is very sensitive to the performance of the perovskite solar cells (PSCs). Here, we demonstrated the high quality MAPbI3 film can be formed on the hydrophobic poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) without any surface treatment and dopant. We systematically adjusted the stoichiometry ratio of the first PbI2:MAI precursor from 1.1:0.3 to 1.3:0.3 and the surface morphology as well as optoelectronic properties of perovskite films were also measured. Benefits from the more uniform surface, higher crystallinity and lower trap states, the high photoelectric conversion efficiency (PCE) of 18.38% and lower hysteresis were achieved based on the optimized ratio of PbI2:MAI. That is one of highest performance of the inverted PSCs based on free-dopants both in MAPbI3 active layer and PTAA hole transport layer. In addition, the devices also showed good stability, and the PCE could maintain 73.85% of its initial value when they were exposed in air and 95.63% in N2 for 216 h. This work suggested a facile precursor stoichiometry engineering to obtain efficient and stable MAPbI3 PSCs based on free-dopants.

Elongated Lifetime and Enhanced Flux of Hot Electrons on a Perovskite Plasmonic Nanodiode.

A fundamental understanding of hot electron transport is critical for developing efficient hot-carrier-based solar cells. There have been significant efforts to enhance hot electron flux, and it has been found that a key factor affecting the hot electron flux is the lifetime of the hot electrons. Here, we report a combined study of hot electron flux and the lifetime of hot carriers using a perovskite-modified plasmonic nanodiode. We found that perovskite deposition on a plasmonic nanodiode can considerably improve hot electron generation induced by photon absorption. The perovskite plasmonic nanodiode consists of MAPbI3 layers covering a plasmonic-Au/TiO2 Schottky junction that is composed of randomly connected Au nanoislands deposited on a TiO2 layer. The measured incident photon-to-electron conversion efficiency and the short-circuit photocurrent show a significantly improved solar-to-electrical conversion performance of this nanodiode. Such an improvement is ascribed to the improved hot electron flux in MAPbI3 caused by effective light absorption from near-field enhancement of plasmonic Au and the efficient capture of hot electrons from Au nanoislands via the formation of a three-dimensional Schottky interface. The relation between the lifetime and flux of hot electrons was confirmed by femtosecond transient absorption spectroscopy that showed considerably longer hot electron lifetimes in MAPbI3 combined with the plasmonic Au structure. These findings can provide a fundamental understanding of hot electron generation and transport in perovskite, which can provide helpful guidance to designing efficient hot carrier photovoltaics.

MAPbI3 Incorporated with Carboxyl Group Chelated Titania for Planar Perovskite Solar Cells in Low-Temperature Process.

Low-temperature, solution-processed, highly efficient hybrid organic/inorganic perovskite planar heterojunction solar cells were fabricated by incorporating reactive crystalline titania (h-TAc) into MAPbI3 layers. The h-TAc was prepared by the sol-gel reaction at low temperature followed by solvothermal treatment. The photoelectrical properties of the solar cells with h-TAc were analyzed. The incorporation with 0.85-wt% h-TAc showed the highest power conversion efficiency (PCE, 15.9%), increasing 69% compared to the pristine cell. The enhancement arose from large-grained microstructures, leading to a low rate of charge recombination. The carboxyl groups chelated on the surface of h-TAc revealed a strong attraction to lead ions, which are significantly helpful to MAPbI3 crystal growth.

Pb clustering and PbI2 nanofragmentation during methylammonium lead iodide perovskite degradation

Studying defect formation and evolution in MethylAmmonium lead Iodide (MAPbI3) perovskite layers has a bottleneck in the softness of the matter and in its consequent sensitivity to external solicitations. Here we report that, in a polycrystalline MAPbI3 layer, Pb-related defects aggregate into nanoclusters preferentially at the triple grain boundaries as unveiled by Transmission Electron Microscopy (TEM) analyses at low total electron dose. Pb-clusters are killer against MAPbI3 integrity since they progressively feed up the hosting matrix. This progression is limited by the concomitant but slower transformation of the MAPbI3 core to fragmented and interconnected nano-grains of 6H-PbI2 that are structurally linked to the mother grain as in strain-relaxed heteroepitaxial coupling. The phenomenon occurs more frequently under TEM degradation whilst air degradation is more prone to leave uncorrelated [001]-oriented 2H-PbI2 grains as statistically found by X-Ray Diffraction. This path is kinetically costlier but thermodynamically favoured and is easily activated by catalytic species.