Search query: Intrinsic Fermi level of perovskite MAPbI3

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abstract: Controlled doping for adjustable material polarity and charge carrier concentration is the basis of semiconductor materials and devices, and it is much more difficult to achieve in ionic semiconductors (e.g. ZnO and GaN) than in covalent semiconductors (e.g. Si and Ge) due to the high intrinsic defect density in ionic semiconductors. The organic inorganic perovskite material, which is frenetically being researched for applications in solar cells and beyond, is also an ionic semiconductor. Here we present the Ag incorporated organic inorganic perovskite films and planar heterojunction solar cells. Partial substitution of Pb2+ by Ag+ leads to improved film morphology, crystallinity, and carrier dynamics as well as shifted Fermi level and reduced electron concentration. Consequently, in planar heterojunction photovoltaic devices with inverted stacking structure, Ag incorporation results in an enhancement of the power conversion efficiency from 16.0% to 18.4% in MAPbI3 based devices and from 11.2% to 15.4% in MAPbI3 xClx based devices. Our work implies that Ag incorporation is a feasible route to adjust carrier concentrations in solution processed perovskite materials in spite of the high concentration of intrinsic defects.
author_names: Qi Chen

Long-qing Chen

Fengye Ye

Ting Zhao

Feng Tang

Adharsh Rajagopal

Zheng Jiang

Shenlong Jiang

Alex K -Y Jen

Yi Xie

Jinhua Cai

Liwei Chen
title: Ag Incorporated Organic Inorganic Perovskite Films and Planar Heterojunction Solar Cells.
venue: Nano letters
year: 2017

abstract: It is important to change the semiconducting behaviors of CH3NH3PbI3 (MAPbI3) perovskites to meet the needs for different applications, which requires efficient strategies for Fermi level tuning in.
author_names: Ya Xu

Jianyong Feng

Tao Yu

Jiawei Li

Xiaopeng Han

Huiting Huang

Zhaosheng Li

Zhi-gang Zou
title: Elegant Molecular Iodine/Antisolvent Solution Engineering To Tune the Fermi Level of Perovskite CH3NH3PbI3
venue: ACS Applied Energy Materials
year: 2019

abstract: Multi cation metal halide perovskites exhibit desirable performance and stability compared to their mono cation counterparts. However, the study of the photophysical properties and the nature of defect states in these materials is still a challenging and ongoing task. Here, we study bulk and interfacial energy loss mechanism in solution processed MAPbI3 (MAPI) and (CsPbI3)0.05[(FAPbI3)0.83(MAPbBr3)0.17]0.95 (triple cation) perovskites solar cells using absolute photoluminescence (PL) measurements. In neat MAPI films, we find a significantly smaller quasi Fermi level splitting than in triple cation perovskite absorbers which defines the device open circuit voltage. PL measurements at low temperatures ~50 K) on MAPI films demonstrate that emissive subgap states can be effectively reduced using different passivating agents, which lowers the non radiative recombination loss at room temperature. We conclude that whilst triple cation perovskite cells are limited by interfacial recombination, the passivation of surface trap states in MAPI films is the primary consideration for device optimization.
author_names: Shanshan Zhang

Paul E Shaw

Guanran Zhang

Hui Jin

Meiqian Tai

Hong-Cheu Lin

Paul Meredith

Paul L Burn

Dieter Neher

Martin Stolterfoht
title: Defect/interface recombination limited quasi Fermi level splitting and open circuit voltage in mono and triple cation perovskite solar cells.
venue: ACS applied materials interfaces
year: 2020

abstract: Solution processable organic inorganic perovskite solar cells have attracted much attention in the past few years. Energy level alignment is of great importance for performance improvement of perovskite solar cells since it greatly influences charge separation and recombination processes. In this report, we introduce three amide additives, namely, formamide, acetamide, and urea into MAPbI3 perovskite by mixing them directly in perovskite precursor solutions. The Fermi level of MAPbI3 shifts from 4.36 eV to 4.63 eV, 4.65 eV and 4.61 eV respectively with these additives. The charge transfer between perovskite and mp TiO2 is found to be promoted via TRPL spectra and the recombination process in perovskite is suppressed. As a result, the built in electric field (Vbi) of the printable, hole conductor free mesoscopic perovskite solar cells based on these perovskites with amide additives is enhanced, and a highest power conversion efficiency of 15.57% is obtained.
author_names: Shuang Liu

Shubin Li

Jiawen Wu

Qifei Wang

Yue Ming

Deyi Zhang

Yusong Sheng

Yue Hu

Yaoguang Rong

Anyi Mei

Hongwei Han
title: Amide Additives Induced Fermi Level Shift for Improved Performance of Hole Conductor Free, Printable Mesoscopic Perovskite Solar Cells.
venue: The journal of physical chemistry letters
year: 2019

abstract: While the power conversion efficiency of metal halide perovskite (MHP) solar cells has increased enormously, the open circuit voltage, Voc, is still below the conceivable limit. Here, we derive the Fermi level splitting, mF, for various types of noncontacted MHPs, which sets a limit for their achievable Voc, using rate constants and mobilities obtained from time resolved photoconductivity measurements. Interestingly, we find that for vacuum evaporated MAPbI3 and K+ doped (MA,FA,Cs)Pb(I/Br)3, the mF/e values are close to the reported Voc values. This implies that for an improvement of the Voc, charge carrier recombination within the bare perovskite has to be reduced. On the other hand, for MHPs with Cs+ and/or Rb+ addition, the experimental Voc is still below mF/e, suggesting that higher voltages are feasible by optimizing the transport layers. The presented approach will help to select which techniques and transport layers are beneficial to improve the efficiency of MHP solar cells.
author_names: Dengyang Guo

Valentina M Caselli

Eline M Hutter

Tom J Savenije
title: Comparing the Calculated Fermi Level Splitting with the Open Circuit Voltage in Various Perovskite Cells
venue: ACS Energy Letters
year: 2019

abstract: Author(s) Perry, Erin Advisor(s) Chabinyc, Michael L. Abstract: Perovskite solar cells (PSCs) have captured the focus of the photovoltaic field due to their ease of fabrication and high power conversion efficiencies of ~22% In PSCs, the perovskite active material is sandwiched between electron and hole transport layers that serve to extract photogenerated charge carriers and to avoid direct contact between the perovskite layer and metal electrodes in order to prevent degradation. Organic semiconductors are attractive candidates for transport layers because of their low temperature processing and scalability. The major obstacle to use of organic semiconductors in PSCs is their low carrier mobility and carrier concentration, which cause them to act as a parasitic series resistance in devices. In order to overcome transport issues, doping of organic transport layers is performed to passivate trap sites and increase the concentration of free carriers.To date, there have been few reports on understanding how the presence of small molecule dopants can influence the perovskite active layer. Small molecule dopants that have been developed to dope organic semiconductors are also candidates for charge transfer to MAPbI3. The impact of direct charge transfer from small molecules to MAPbI3 was investigated as a model system to provide a means to understand the more complex heterointerface of doped organic blends at perovskite surfaces. We studied the impact of surface doping on passivation of electron traps in MAPbI3. Using spectroscopy and photoconductivity measurements, we determined that at moderate degrees of doping the Fermi level shift will passivate this recombination center. At high degrees of doping, there is an overabundance of charge beyond what is necessary to passivate surface traps, creating an additional pathway for bimolecular recombination. In addition, excessive charge leads to chemical damage of the surface of MAPbI3, resulting in the release of mobile I ions. Given these design principles, the impacts of doping the organic electron transport layer on solar cell performance was investigated. The power conversion efficiency of PSCs showed a rise and fall in performance with increasing doping concentration of the electron transport layer, consistent with observations from surface doping. Optimal solar cell performance was observed at moderate doping concentrations of 1 wt% At this doping concentration, evidence of trap passivation was observed by higher open circuit voltages (VOC) and longer carrier lifetimes. Numerous studies in literature on the impact of doping the electron transport layer have identified a range of factors influencing performance, including changes to carrier mobility, resistance mismatch, energy level alignment, and morphology. This work indicates that the impacts of interfacial charge transfer on surface recombination should be added to this list of considerations.
author_names: Erin E Perry
title: Fermi Level Engineering of the Perovskite and Electron Transport Layer Interface through Charge Transfer Doping
year: 2018

abstract: Lead free inorganic perovskites are promising for optoelectronic applications. Understanding their phase diagram and defect properties is beneficial to predict the stable phase and applications. Here, taking CsSnCl3 as an example, the stability and defect properties are investigated systemically. The results show that the stoichiometric CsSnCl3 can only be grown in a narrow area determined by Sn and Cs chemical potentials. No matter what the Sn condition is, both acceptor defects (Cs and Sn vacancies with low charge states) are formed spontaneously with negative formation energies, and the donor defects are difficult to be formed with high formation energies. Interestingly, these charge state transition levels induced by such stable defects are in the valence band. Meanwhile, no deep acceptor and donor states are formed in the bandgap. In addition, although the Fermi levels can vary in a wide range, the Fermi levels are pinned in the valence band, irrespective of the atmospheric conditions, leading to p type CsSnCl3 with high hole density and low electron density. Such electronic characters are elucidated in detail by the atomic orbitals and structural deformations. Our studies provide an insight view of the defect properties of CsSnCl3 and provide a valuable guideline for CsSnCl3 fabrication and further modulation.Lead free inorganic perovskites are promising for optoelectronic applications. Understanding their phase diagram and defect properties is beneficial to predict the stable phase and applications. Here, taking CsSnCl3 as an example, the stability and defect properties are investigated systemically. The results show that the stoichiometric CsSnCl3 can only be grown in a narrow area determined by Sn and Cs chemical potentials. No matter what the Sn condition is, both acceptor defects (Cs and Sn vacancies with low charge states) are formed spontaneously with negative formation energies, and the donor defects are difficult to be formed with high formation energies. Interestingly, these charge state transition levels induced by such stable defects are in the valence band. Meanwhile, no deep acceptor and donor states are formed in the bandgap. In addition, although the Fermi levels can vary in a wide range, the Fermi levels are pinned in the valence band, irrespective of the atmospheric conditions, leading to p
author_names: Junyu Zhang

Jie Su

Zhenhua Lin

Mengyu Liu

Jingjing Chang

Yue Hao
title: Disappeared deep charge states transition levels in the p type intrinsic CsSnCl3 perovskite
venue: Applied Physics Letters
year: 2019

abstract: Ionic movement is considered awful in perovskite solar cells (PSCs) for relating with the hysteresis and long term instability. However, the positive role of ions to enhance the energy band bending for high performance PSC is always overlooked, let alone reducing the hysteresis. In this work, LiI is doped in CH3NH3PbI3. It is observed that the aggregation of Li+/I tunes the energy level of the perovskite and induces n/p doping in CH3NH3PbI3, which makes charge extraction quite efficient from perovskite to both NiO and [6,6] phenyl C61 butyric acid methyl ester (PCBM) layer. Therefore, in NiO/LiI doped perovskite/PCBM solar cells, Li+ and I modulate the interface energy band alignment to facilitate the electron/hole transport and reduce the interface energy loss. On the other hand, n/p doping enlarges Fermi energy level splitting of the PSCs to improve the photovoltage. The performance of LiI doped PSCs is much higher with reduced hysteresis compared to the undoped solar cells. This work highlights the positive effect of selective ionic doping, which is promisingly important to design the stable and efficient PSCs.
author_names: Jing Zhang

Renjie Chen

Yongzhen Wu

Minghui Shang

Zhaobing Zeng

Yue Zhang

Yuejin Zhu

Liyuan Han
title: Extrinsic Movable Ions in MAPbI3 Modulate Energy Band Alignment in Perovskite Solar Cells
year: 2018

abstract: The organic inorganic perovskite material with better energy alignment in the solar cell device will have a profound impact on the solar cell performance. It is valuable to tune the energy states by element substitution and doping in perovskites. Here, we present that Ca2+ is incorporated into CH3NH3PbI3, which up shifts the valence band maximum and the conduction band minimum, leading to a difference between the bandgap and the Fermi level in the device. Consequently, Ca2+ incorporation results in an enhancement of the photovoltage and photocurrent, achieving a summit efficiency of 18.3% under standard 1 sun (AM 1.5) This work reveals the doped perovskite to improve the solar cell performance by tuning the energy state.
author_names: Chaojie Lu

Jing Zhang

Dagang Hou

Xinlei Gan

Hongrui Sun

Zhaobing Zeng

Renjie Chen

Hui Tian

Qi Xiong

Yue Zhang

Yuanyuan Li

Yuejin Zhu
title: Calcium doped MAPbI3 with better energy state alignment in perovskite solar cells
year: 2018

abstract: As an alternative to methylammonium lead triiodide (MAPbI3) formamidinium lead triiodide (FAPbI3) perovskites have recently attracted significant attention because of their higher stability and smaller band gaps. Here, based on first principles calculations, we investigate systematically the intrinsic defects in FAPbI3. While methylammonium (MA) related defects MAI and IMA in MAPbI3 have high formation energies, we found that formamidinium (FA) related defects VFA, FAI and IFA in FAPbI3 have much lower formation energies. Antisites FAI and IFA create deep levels in the band gap, and they can act as recombination centers and result in reduced carrier lifetimes and low open circuit voltages in FAPbI3 based photovoltaic devices. We further demonstrate that through cation mixing of MA and FA in perovskites the formation of these defects can be substantially suppressed.
author_names: Na Liu

Chiyung Yam
title: First principles study of intrinsic defects in formamidinium lead triiodide perovskite solar cell absorbers.
venue: Physical chemistry chemical physics PCCP
year: 2018