Research Update: The electronic structure of hybrid perovskite layers and their energetic alignment in devices

Olthof, Selina

doi:10.1063/1.4960112

Cited by 64 publications

(79 citation statements)

References 45 publications

(44 reference statements)

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“…This value is comparable with the optical band gap of 1.63 eV reported for this type of quadruple cation perovskite27 . We note that an overestimation of the band gap (from UPS and IPES) compared to the optical gap can be due to the linear extrapolation42,43 , and our results are consistent with examples reported in literature[42][43][44][45] . A lower band gap would be expected if the extrapolation was based on logarithmic plots, but due to the uncertainty of cross-section effects and rather large experimental broadening in IPES we refrain from such procedures here.…”

supporting

confidence: 92%

High open circuit voltages in pin-type perovskite solar cells through strontium addition

Caprioglio

Wolff

et al. 2019

Sustainable Energy Fuels

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The incorporation of even small amounts of strontium (Sr) into lead-based quadruple cation hybrid perovskite solar cells results in a systematic increase of the open circuit voltage (Voc) in pin-type perovskite solar cells. We demonstrate via transient and absolute photoluminescence (PL) experiments how the incorporation of Sr significantly reduces the non-radiative recombination losses in the neat perovskite layer and specifically at the perovskite/C60 interface. We show that Sr segregates at the perovskite surface, where it induces important changes of morphology and energetics. Notably, the Sr-enriched surface exhibits a wider band gap and a more n-type character, accompanied with significantly stronger surface band bending. As a result, we observe a significant increase of the quasi-Fermi level splitting in the neat perovskite by reduced surface recombination and more importantly, a strong reduction of losses attributed to non-radiative recombination at the interface to the C60 electron-transporting layer. The resulting solar cells exhibited a Voc of 1.18 V, which could be further improved to nearly 1.23 V through addition of a thin polymer interlayer, bringing the non-radiative voltage loss to only 110 meV. Our work shows that simply adding a small amount of Sr to the precursor solutions induces a beneficial surface modification in the perovskite, without requiring any post treatment, resulting in high efficiency solar cells with power conversion efficiency (PCE) up to 20.3%. Our results demonstrate very high Voc values and efficiencies in Srcontaining quadruple cation perovskite pin solar cells and highlight the imperative importance of addressing and minimizing the recombination losses at the interface between perovskite and charge transporting layer. IntroductionOrganic-inorganic halide perovskites are considered one of the most promising materials for photovoltaic applications due to their rather easy and low-cost fabrication, as well as outstanding optoelectronic properties. Notably, these semiconductors combine a high absorption coefficient with panchromatic absorption of light 1 with a long carrier diffusion length 2,3 , allowing efficient photon absorption and charge extraction for a typical active layer thickness of only 500 nm. Another peculiarity of hybrid perovskites is that defects create mostly shallow energy levels, allowing high open circuit voltage (Voc) and long carrier lifetime 4 . Moreover, perovskite materials can be obtained from in-nature abundant precursors, which potentially reduce further the costs of future large scale production. Despite the fact that the first full solid state perovskite solar cell was reported only in 2012, with a power conversion efficiency (PCE) of 9.7% 5 , this technology has experienced a tremendous improvement 6-8 , currently reaching a record PCE of 22.7% 9 . Regardless of the state of the art of perovskite solar cells and their astonishing performances, the metrics fill factor (FF) andVoc are currently still limiting their PCE. Thus, in order to achiev...

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confidence: 92%

High open circuit voltages in pin-type perovskite solar cells through strontium addition

Caprioglio

Wolff

et al. 2019

Sustainable Energy Fuels

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“…On the Evap‐2 film, an additional interface dipole (evidenced by a sole shift of the VL) is formed. The interface dipole can have various reasons . It can be speculated that it is due to NPB‐induced cation rearrangement, as it only appears at the perovskite film with the highest density of CH 3 + at the surface.…”

Section: Resultsmentioning

confidence: 99%

Surface CH₃NH₃⁺ to CH₃⁺ Ratio Impacts the Work Function of Solution‐Processed and Vacuum‐Sublimed CH₃NH₃PbI₃ Thin Films

Wang

Zhuo

et al. 2019

Adv Materials Inter

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CH3NH3PbI3 thin films are fabricated using several representative synthesis methods such as spin‐coating, evaporation, and a combination of the two. These methods, which frequently occur in reported literatures, use the same precursors PbI2 and CH3NH3I but differ in how the two are mixed. It is found that the latter plays a vital role in determining the surface morphology, composition, and grain size of the films, even when the same stoichiometric ratio of the precursors is used. X‐ray photoelectron spectroscopy reveals that the amount of CH3+‐type defects, which results from CH3NH3I dissociation, is sensitive to both the physical state of CH3NH3I and the order of mixing sequence. The variation of the CH3NH3+:CH3+ ratio also affects the valence band and the work function of the corresponding films, as revealed by ultraviolet photoelectron spectroscopy. Furthermore, the energy‐level alignment between the perovskite film and a model hole transport layer, N,N′‐di(1‐naphthyl)‐N,N′‐diphenylbenzidine (NPB) is examined. It is found that the CH3NH3+:CH3+ ratio correlates with the offsets between the valence band maximum of perovskite film and the highest occupied molecular orbital of NPB as well, and the energy‐level alignment with the dual‐source, coevaporated CH3NH3PbI3 film is most suitable for efficient hole transport.

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“…152,153 The latter have been reported to be a consequence of the natural presence in MoS2 of sulfur vacancies, [153][154][155][156][157][158] impurities 159,160 and defect. [161][162][163][164] Quantum confinement effects open the MoS2 optical bandgap (from 1.4 eV for the flakes to > 3.2 eV for the QDs), raising the minimum energy of the CB of MoS2 (from -4.3 eV for the flakes to -2.2 eV for the QDs) above the energy of LUMO of MAPbI3 (between -4.0 [134][135][136][137] and -3.7 eV 114,138,139 ), thus providing electron-blocking properties. Hole-extraction and electron-blocking properties of MoS2 QDs synergistically suppress the interfacial recombination losses observed in benchmark devices (fluorine doped tin oxide (FTO)/compact TiO2 (cTiO2)/mesoporous TiO2 (mTiO2)/MAPbI3/2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD)/Au), 34,165 and in previous cell architectures exploiting native MoS2 flakes as ABLs.…”

Section: Introductionmentioning

confidence: 99%

MoS₂ Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of a CH₃NH₃PbI₃ Perovskite Solar Cell with an Efficiency of over 20%

et al. 2018

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Interface engineering of organic-inorganic halide perovskite solar cells (PSCs) plays a pivotal role in achieving high power conversion efficiency (PCE). In fact, perovskite photoactive layer needs to work synergistically with the other functional components of the cell, such as charge transporting/active buffer layers and electrodes. In this context, graphene and related twodimensional materials (GRMs) are promising candidates to tune "on demand" the interface properties of PSCs. In this work, we fully exploit the potential of GRMs by controlling the optoelectronic properties of hybrids between molybdenum disulfide (MoS2) and reduced graphene oxide (RGO) as hole transport layer (HTL) and active buffer layer (ABL) in mesoscopic methylammonium lead iodide (CH3NH3PbI3) perovskite (MAPbI3)-based PSC. We show that zero-dimensional MoS2 quantum dots (MoS2 QDs), derived by liquid phase exfoliated MoS2 flakes, provide both holeextraction and electron-blocking properties. In fact, on the one hand, intrinsic n-type doping-induced intra-band gap states effectively extract the holes through an electron injection mechanism. On the other hand, quantum confinement effects increase the optical band gap of MoS2 (from 1.4 eV for the flakes to > 3.2 for QDs), raising the minimum energy of its conduction band (from -4.3 eV for the flakes to -2.2 eV for QDs) above the one of conduction band of MAPbI3 (between -3.7 and -4 eV) and hindering electron collection. The van der Waals hybridization of MoS2 QDs with functionalized reduced graphene oxide (f-RGO), obtained by chemical silanization-induced linkage between RGO and (3-mercaptopropyl)trimethoxysilane, is effective to homogenize the deposition of HTLs or ABLs onto the perovskite film, since the two-dimensional (2D) nature of RGO effectively plug the pinholes of the MoS2 QDs films. Our "graphene interface engineering" (GIE) strategy based on van der Waals MoS2 QD/graphene hybrids enable MAPbI3-based PSCs to achieve PCE up to 20.12% (average PCE of 18.8%). The possibility to combine quantum and chemical effects into GIE, coupled with the recent success of graphene and GRMs as interfacial layer, represents a promising approach for the development of next-generation PSCs. Figure 1. (a) Sketch of mesoscopic MAPbI3-based PSC exploiting MoS2 QDs:f-RGO hybrids as both HTL and ABL. (b) Scheme of the energy band edge positions of the materials used in the different components of the assembled mesoscopic MAPbI3-based PSC. The energy band edge positions of MoS2 flakes and MoS2 QDs were determined from OAS and UPS measurements detailed along the text, while those of the other materials were taken from literature: FTO, 52 TiO2, 52 MAPbI3, 134-139 spiro-OMeTAD 52 and Au 52 . (c) State-of-the-art and predicted PCE evolution for PSCs, highlighting the synergistic potential of GIE and the formulation of advanced perovskite chemistries. The RGO flakes are effective to plug the pinholes MoS2 QDs films, thus to homogenize the HTL. The choice of the functionalization for RGO relies on the bifunctional r...

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Research Update: The electronic structure of hybrid perovskite layers and their energetic alignment in devices

Cited by 64 publications

References 45 publications

High open circuit voltages in pin-type perovskite solar cells through strontium addition

High open circuit voltages in pin-type perovskite solar cells through strontium addition

Surface CH₃NH₃⁺ to CH₃⁺ Ratio Impacts the Work Function of Solution‐Processed and Vacuum‐Sublimed CH₃NH₃PbI₃ Thin Films

MoS₂ Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of a CH₃NH₃PbI₃ Perovskite Solar Cell with an Efficiency of over 20%

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Research Update: The electronic structure of hybrid perovskite layers and their energetic alignment in devices

Cited by 64 publications

References 45 publications

High open circuit voltages in pin-type perovskite solar cells through strontium addition

High open circuit voltages in pin-type perovskite solar cells through strontium addition

Surface CH3NH3+ to CH3+ Ratio Impacts the Work Function of Solution‐Processed and Vacuum‐Sublimed CH3NH3PbI3 Thin Films

MoS2 Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of a CH3NH3PbI3 Perovskite Solar Cell with an Efficiency of over 20%

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Surface CH₃NH₃⁺ to CH₃⁺ Ratio Impacts the Work Function of Solution‐Processed and Vacuum‐Sublimed CH₃NH₃PbI₃ Thin Films

MoS₂ Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of a CH₃NH₃PbI₃ Perovskite Solar Cell with an Efficiency of over 20%