172 results found
Daboczi M, Ratnasingham SR, Mohan L, et al., 2021, Optimal Interfacial Band Bending Achieved by Fine Energy Level Tuning in Mixed-Halide Perovskite Solar Cells, ACS Energy Letters, Vol: 6, Pages: 3970-3981
Most highly efficient perovskite solar cells employ mixed iodide-bromide photoactive layers; however, understanding the beneficial effect of the low (5-15 mol %) bromide content is incomplete. Here, a series of MAPb(I1-xBrx)3 perovskite layers are investigated to understand the origin of the high peak power conversion efficiency (19.2%) observed at small bromide content (0.10 ≤ x ≤ 0.125). For the x = 0.125 perovskite, 200 meV shallower energy levels are revealed, accompanied by a reduced density of trap states and stable tetragonal mixed-halide phase with compressed unit cell. In contrast, the higher bromide content samples (x > 0.125) show deeper energy levels, cubic perovskite crystal structure, and signs of halide segregation. Surface photovoltage measurements unveil an undesirable band bending at the hole transport layer/perovskite interface for MAPbI3 and x > 0.125 mixed-halide layers, which is eliminated for the x = 0.125 perovskite because of its shallower Fermi level, enabling enhanced device performance.
Mohan L, Ratnasingham SR, Panidi J, et al., 2021, Determining out-of-plane hole mobility in CuSCN via the time-of-flight technique to elucidate its function in perovskite solar cells, ACS Applied Materials and Interfaces, Vol: 13, Pages: 38499-38507, ISSN: 1944-8244
Copper(I) thiocyanate (CuSCN) is a stable, low-cost, solution-processable p-type inorganic semiconductor used in numerous optoelectronic applications. Here, for the first time, we employ the time-of-flight (ToF) technique to measure the out-of-plane hole mobility of CuSCN films, enabled by the deposition of 4 μm-thick films using aerosol-assisted chemical vapor deposition (AACVD). A hole mobility of ∼10–3 cm2/V s was measured with a weak electric field dependence of 0.005 cm/V1/2. Additionally, by measuring several 1.5 μm CuSCN films, we show that the mobility is independent of thickness. To further validate the suitability of our AACVD-prepared 1.5 μm-thick CuSCN film in device applications, we demonstrate its incorporation as a hole transport layer (HTL) in methylammonium lead iodide (MAPbI3) perovskite solar cells (PSCs). Our AACVD films result in devices with measured power conversion efficiencies of 10.4%, which compares favorably with devices prepared using spin-coated CuSCN HTLs (12.6%), despite the AACVD HTLs being an order of magnitude thicker than their spin-coated analogues. Improved reproducibility and decreased hysteresis were observed, owing to a combination of excellent film quality, high charge-carrier mobility, and favorable interface energetics. In addition to providing a fundamental insight into charge-carrier mobility in CuSCN, our work highlights the AACVD methodology as a scalable, versatile tool suitable for film deposition for use in optoelectronic devices.
Vasilopoulou M, bin Mohd Yusoff AR, Daboczi M, et al., 2021, High efficiency blue organic light-emitting diodes with below-bandgap electroluminescence, NATURE COMMUNICATIONS, Vol: 12, ISSN: 2041-1723
Clarke AJ, Luke J, Meitzner R, et al., 2021, Non-fullerene acceptor photostability and its impact on organic solar cell lifetime, CELL REPORTS PHYSICAL SCIENCE, Vol: 2, ISSN: 2666-3864
Limbu S, Stewart K, Nightingale J, et al., 2021, Solid-State Ionic Liquid: Key to Efficient Detection and Discrimination in Organic Semiconductor Gas Sensors, ACS APPLIED ELECTRONIC MATERIALS, Vol: 3, Pages: 2152-2163
Marin-Beloqui JM, Toolan DTW, Panjwani NA, et al., 2021, Triplet-Charge Annihilation in a Small Molecule Donor: Acceptor Blend as a Major Loss Mechanism in Organic Photovoltaics, ADVANCED ENERGY MATERIALS, Vol: 11, ISSN: 1614-6832
Kyeong M, Lee J, Daboczi M, et al., 2021, Organic cathode interfacial materials for non-fullerene organic solar cells, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 9, Pages: 13506-13514, ISSN: 2050-7488
Yun D-J, Lee S, Kim SH, et al., 2021, Bevel Structure Based XPS Analysis as a Non-Destructive Chemical Probe for Complex Interfacial Structures of Organic Semiconductors, SMALL METHODS, Vol: 5, ISSN: 2366-9608
Labanti C, Sung MJ, Luke J, et al., 2021, Selenium-substituted non-fullerene acceptors: a route to superior Operational stability for organic bulk heterojunction solar cells., ACS Nano, Vol: 15, Pages: 7700-7712, ISSN: 1936-0851
Non-fullerene acceptors (NFAs) for organic solar cells (OSCs) have significantly developed over the past five years with continuous improvements in efficiency now over 18%. However, a key challenge still remains in order to fully realize their commercialization potential: the need to extend device lifetime and to control degradation mechanisms. Herein, we investigate the effect of two different molecular engineering routes on the widely utilized ITIC NFA, to tune its optoelectronic properties and interactions with the donor polymer in photoactive blends. Heavier selenium (Se) atoms substitute sulfur (S) atoms in the NFA core in either outer or inner positions, and methyl chains are attached to the end groups. By investigating the effects of these structural modifications on the long-term operational stability of bulk-heterojunction OSC devices, we identify outer selenation as a powerful strategy to significantly increase device lifetime compared to ITIC. Combining outer selenation and methylation results in an impressive 95% of the initial OSC efficiency being retained after 450 h under operating conditions, with an exceptionally long projected half-lifetime of 5600 h compared to 400 h for ITIC. We find that the heavier and larger Se atoms at outer-core positions rigidify the molecular structure to form highly crystalline films with low conformational energetic disorder. It further enhances charge delocalization over the molecule, promoting strong intermolecular interactions among acceptor molecules. Upon methylation, this strong intermolecular interaction stabilizes acceptor domains in blends to be resilient to light-induced morphological changes, thereby leading to superior device stability. Our results highlight the crucial role of NFA molecular structure for OSC operational stability and provide important NFA design rules via heteroatom position and end-group control.
Heeney M, Kafourou P, Park B, et al., 2021, One-step six-fold cyanation of benzothiadiazole acceptor Units for air-stable high-performance n-type organic field-effect transistors, Angewandte Chemie International Edition, Vol: 60, Pages: 5970-5977, ISSN: 1433-7851
We report a new high electron affinity acceptor end group for organic semiconductors, 2,1,3-benzothiadiazole-4,5,6-tricarbonitrile (TCNBT). An n-type organic semiconductor with an indacenodithiophene (IDT) core and TCNBT end groups was synthesized by a six-fold nucleophilic substitution with cyanides on a fluorinated precursor, itself prepared by a direct arylation approach. This one-step chemical modification was found to significantly impact the molecular properties: the fluorinated precursor, TFBT IDT, a poor ambipolar semiconductor, was converted into TCNBT IDT, a good n-type semiconductor. The highly electron-deficient end group TCNBT dramatically decreased the energy of the highest occupied and lowest unoccupied molecular orbitals (HOMO/LUMO) compared to the fluorinated analogue and improved the molecular orientation when utilized in n-type organic field-effect transistors (OFETs). Solution-processed OFETs based on TCNBT IDT exhibited a charge carrier mobility of up to µ e ≈ 0.15 cm 2 V -1 s -1 with excellent ambient stability for 100 hours, highlighting the benefits of the cyanated end group and the synthetic approach.
Ratnasingham SR, Mohan L, Daboczi M, et al., 2021, Novel scalable aerosol-assisted CVD route for perovskite solar cells, Materials Advances, Vol: 2, Pages: 1606-1612
Organo-metal halide perovskite research has progressed rapidly, with photovoltaic (PV) devices achieving over 25% power conversion efficiency (PCE). However, scalable production of these devices is an ongoing challenge. We demonstrate the growth of methylammonium lead triiodide (MAPI) films via a novel two-step aerosol-assisted chemical vapour deposition (AACVD) method leading to the first ever perovskite-based PV devices using active layers deposited by AACVD. This is a scalable deposition process, requiring less complex equipment than conventional CVD. Furthermore, our method utilises methanol (MeOH) as the only solvent, as opposed to harmful solvents typically used in perovskite processing. Structural and optical characterization confirms successful formation of MAPI with no secondary phases and an optical bandgap of ∼1.58 eV. The final film had large grains (order of μm), with thickness ranging from 500–1100 nm. These films were used to fabricate working PV devices resulting in a champion PCE of 5.4%. While films demonstrated high structural and compositonal quality, we identified large film roughness as a limiting factor in device PCE, and elucidate the origin of this via detailed study of the film growth, which reveals a unique multi-step film formation process.
Yiwen W, Jinho L, Xueyan H, et al., 2021, Recent progress and Challenges toward highly stable nonfullerene acceptor‐based organic solar cells, Advanced Energy Materials, Vol: 11, ISSN: 1614-6832
Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have made significant breakthrough in their device performance, now achieving a power conversion efficiency of ≈18% for single junction devices, driven by the rapid development in their molecular design and device engineering in recent years. However, achieving long‐term stability remains a major challenge to overcome for their commercialization, due in large part to the current lack of understanding of their degradation mechanisms as well as the design rules for enhancing their stability. In this review, the recent progress in understanding the degradation mechanisms and enhancing the stability of high performance NFA‐based OSCs is a specific focus. First, an overview of the recent advances in the molecular design and device engineering of several classes of high performance NFA‐based OSCs for various targeted applications is provided, before presenting a critical review of the different degradation mechanisms identified through photochemical‐, photo‐, and morphological degradation pathways. Potential strategies to address these degradation mechanisms for further stability enhancement, from molecular design, interfacial engineering, and morphology control perspectives, are also discussed. Finally, an outlook is given highlighting the remaining key challenges toward achieving the long‐term stability of NFA‐OSCs.
Limbu S, Park K-B, Wu J, et al., 2021, Identifying the Molecular Origins of High-Performance in Organic Photodetectors Based on Highly Intermixed Bulk Heterojunction Blends, ACS NANO, Vol: 15, Pages: 1217-1228, ISSN: 1936-0851
Luke J, Correa L, Rodrigues J, et al., 2021, A Commercial Benchmark: Light-Soaking Free, Fully Scalable, Large-Area Organic Solar Cells for Low-Light Applications, ADVANCED ENERGY MATERIALS, Vol: 11, ISSN: 1614-6832
Tseng T-W, Yan H, Nakamura T, et al., 2020, Real-time monitoring of formation and dynamics of intra- and interchain phases in single molecules of polyfluorene, ACS Nano, Vol: 14, Pages: 16096-16104, ISSN: 1936-0851
Poly(9,9-dioctylfluorene) (PFO) is one of the most important conjugated polymer materials, exhibiting outstanding photophysical and electrical properties. PFO is also known for a diversity of morphological phases determined by conformational states of the main chain. Our goal in this work is to address some of the key questions on formation and dynamics of one such conformation, the β-phase, by following in real time the evolution of fluorescence spectra of single PFO chains. The PFO is dispersed in a thin polystyrene film, and the spectra are monitored during the process of solvent vapor annealing with toluene. We confirm unambiguously that the PFO β-phase segments are formed on a true single-chain level at room temperature in the solvent-softened polystyrene. We further find that the formation of the β-phase is a dynamic and reversible process occurring on the order of seconds, leading to repeated spontaneous transitions between the glassy and β-phase segments during the annealing. Comparison of PFO with two largely different molecular weights (Mw) shows that chains with lower Mw form the β-phase segments much faster. For the high Mw PFO chains, a detailed Franck-Condon analysis of the β-phase spectra shows a large distribution of the Huang-Rhys factor, S, and even dynamic changes of this factor occurring on a single chain. Such dynamics are likely a manifestation of changing coherence length of the exciton. Further, for the high Mw PFO chains we observe an additional conformational state, a crystalline γ-phase. The γ-phase formation is also a spontaneous reversible process in the solvent-softened matrix. The phase can form from both the β-phase and the glassy phase, and the formation requires high Mw to enable intersegment interactions in a self-folded chain.
Stewart K, Limbu S, Nightingale J, et al., 2020, Molecular understanding of a pi-conjugated polymer/solid-state ionic liquid complex as a highly sensitive and selective gas sensor, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 8, Pages: 15268-15276, ISSN: 2050-7526
Dong Y, Nikolis VC, Talnack F, et al., 2020, Orientation dependent molecular electrostatics drives efficient charge generation in homojunction organic sol, Nature Communications, Vol: 11, ISSN: 2041-1723
Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges. However, the use of D-A blends intrinsically limits the photovoltage and introduces morphological instability. Here, we demonstrate that polycrystalline films of chemically identical molecules offer a promising alternative and show that photoexcitation of α-sexithiophene (α-6T) films results in efficient charge generation. This leads to α-6T based homojunction organic solar cells with an external quantum efficiency reaching up to 44% and an open-circuit voltage of 1.61 V. Morphological, photoemission, and modelling studies show that boundaries between α-6T crystalline domains with different orientations generate an electrostatic landscape with an interfacial energy offset of 0.4 eV, which promotes the formation of hybridised exciton/charge-transfer states at the interface, dissociating efficiently into free charges. Our findings open new avenues for organic solar cell design where material energetics are tuned through molecular electrostatic engineering and mesoscale structural control.
Kwon S, Pak Y, Kim B, et al., 2020, Molecular-level electrochemical doping for fine discrimination of volatile organic compounds in organic chemiresistors, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 8, Pages: 16884-16891, ISSN: 2050-7488
Jayaram AK, Pitsalidis C, Tan E, et al., 2020, 3D Hybrid Scaffolds Based on PEDOT:PSS/MWCNT Composites (vol 7, 363, 2019), FRONTIERS IN CHEMISTRY, Vol: 8, ISSN: 2296-2646
Wu J, Lee J, Chin Y-C, et al., 2020, Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells, Energy & Environmental Science, Vol: 13, Pages: 2422-2430, ISSN: 1754-5692
In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells. Employing transient optoelectronic and photoemission spectroscopies, we find that this blend exhibits greatly suppressed charge trapping into electronic intra-bandgap tail states compared to other polymer/non-fullerene acceptor solar cells, attributed to lower energetic disorder. The presence of tail states is a key source of energetic loss in most organic solar cells, as charge carriers relax into these states, reducing the quasi-Fermi level splitting and therefore device VOC. DFT and Raman analyses indicate this suppression of tail state energetics disorder could be associated with a higher degree of conformational rigidity and uniformity for the Y6 acceptor. We attribute the origin of such conformational rigidity and uniformity of Y6 to the presence of the two alkyl side chains on the outer core that restricts end-group rotation by acting as a conformation locker. The resultant enhanced carrier dynamics and suppressed charge carrier trapping are proposed to be a key factor behind the high performance of this blend. Low energetic disorder is suggested to be a key factor enabling reasonably efficient charge generation in this low energy offset system. In the absence of either energetic disorder or a significant electronic energy offset, it is argued that charge separation in this system is primarily entropy driven. Nevertheless, photocurrent generation is still limited by slow hole transfer from Y6 to PM6, suggesting pathways for further efficiency improvement.
Daboczi M, Kim J, Lee J, et al., 2020, Towards efficient integrated perovskite/organic bulk heterojunction solar cells: interfacial energetic requirement to reduce charge carrier recombination losses, Advanced Functional Materials, Vol: 30, Pages: 1-8, ISSN: 1616-301X
Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (J sc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in J sc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices.
Nightingale J, Pitsalidis C, Pappa A-M, et al., 2020, Small molecule additive for low-power accumulation mode organic electrochemical transistors, Journal of Materials Chemistry C, Vol: 8, Pages: 8846-8855, ISSN: 2050-7526
A small molecule additive, dodecylbenzenesulfonate (DBSA), is added to the electrolyte in OECTs to improve the device performance.
Luo H, Dimitrov S, Daboczi M, et al., 2020, Nitrogen-Doped Carbon Dots/TiO2 Nanoparticle Composites for Photoelectrochemical Water Oxidation, ACS APPLIED NANO MATERIALS, Vol: 3, Pages: 3371-3381, ISSN: 2574-0970
Iandolo D, Sheard J, Levy GK, et al., 2020, Biomimetic and electroactive 3D scaffolds for human neural crest-derived stem cell expansion and osteogenic differentiation, MRS COMMUNICATIONS, Vol: 10, Pages: 179-187, ISSN: 2159-6859
Lin C-T, Lee J, Kim J, et al., 2020, Origin of open-circuit voltage enhancements in planar Perovskite solar cells induced by addition of bulky organic cations, Advanced Functional Materials, Vol: 30, ISSN: 1616-301X
The origin of performance enhancements in p‐i‐n perovskite solar cells (PSCs) when incorporating low concentrations of the bulky cation 1‐naphthylmethylamine (NMA) are discussed. A 0.25 vol % addition of NMA increases the open circuit voltage (Voc) of methylammonium lead iodide (MAPbI3) PSCs from 1.06 to 1.16 V and their power conversion efficiency (PCE) from 18.7% to 20.1%. X‐ray photoelectron spectroscopy and low energy ion scattering data show NMA is located at grain surfaces, not the bulk. Scanning electron microscopy shows combining NMA addition with solvent assisted annealing creates large grains that span the active layer. Steady state and transient photoluminescence data show NMA suppresses non‐radiative recombination resulting from charge trapping, consistent with passivation of grain surfaces. Increasing the NMA concentration reduces device short‐circuit current density and PCE, also suppressing photoluminescence quenching at charge transport layers. Both Voc and PCE enhancements are observed when bulky cations (phenyl(ethyl/methyl)ammonium) are incorporated, but not smaller cations (Cs/MA)—indicating size is a key parameter. Finally, it demonstrates that NMA also enhances mixed iodide/bromide wide bandgap PSCs (Voc of 1.22 V with a 1.68 eV bandgap). The results demonstrate a facile approach to maximizing Voc and provide insights into morphological control and charge carrier dynamics induced by bulky cations in PSCs.
Hamilton I, Suh M, Kim K, et al., 2020, Organic-inorganic hybrid composites as an electron injection layer in highly efficient inverted green-emitting polymer LEDs, Organic Electronics, Vol: 77, Pages: 1-8, ISSN: 1566-1199
Organic-inorganic hybrid light emitting diodes (HyLEDs) consist of an organic emission layer in combination with at least one metal oxide charge injection layer in an inverted structure. Low temperature, solution processing of metal oxide charge injection layers is one of the key factors in reducing the manufacture cost of HyLEDs. Herein, we report the use of composite materials, comprising conjugated polyelectrolytes (CPE) and zinc oxide nanoparticles (ZnO NPs), as the electron injection layer (EIL) in highly-efficient, green-light-emitting poly (9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) polymer LEDs that are carefully optimised for use in an inverted HyLED architecture for the first time. The composite CPE:ZnO EILs are processed via a room temperature, one-step, solution deposition and enable superior device performance relative to ZnO NPs on their own. We find that specifically, they (i) improve EIL morphology, reducing surface roughness as well as pin-hole size and density, (ii) induce a favourable vacuum level shift for electron injection by coordinate bonding between the CPE and ZnO constituents, and (iii) reduce interfacial quenching by passivation of ZnO chemical defects caused by oxygen vacancies. This work is also the first demonstration that blending ZnO NPs and CPE supports much faster electroluminescence turn-on times (∼7.12 μs) than for traditional ZnO/CPE bilayer devices (∼0.4 s) via ‘locking’ of the CPE mobile ions, as well as higher device performance. This demonstrates good suitability for display applications. After optimisation of the EIL composition and the thickness of the F8BT emissive layer, we achieve promising device efficiencies of 16.5 cd/A and 5.41 lm/W for devices with a 1.1 μm thick F8BT layer, which is particularly relevant for potential roll-to-roll fabrication. These results clearly demonstrate the potential that this organic-inorganic composite EIL material has for the realisation of cheap, scalable
Tan E, Pappa A-M, Pitsalidis C, et al., 2020, A highly sensitive molecular structural probe applied to in-situ biosensing of metabolites using PEDOT:PSS, Biotechnology and Bioengineering, Vol: 117, Pages: 291-299, ISSN: 0006-3592
A large amount of research within organic biosensors is dominated by organic electrochemical transistors (OECTs) that use conducting polymers such as poly(3,4-ethylene dioxythiophene doped with poly(styrenesulfonate) (PEDOT:PSS). Despite the recent advances in OECT-based biosensors, the sensing is solely reliant on the amperometric detection of the bioanalytes. This is typically accompanied by large undesirable parasitic electrical signals from the electroactive components in the electrolyte. Herein, we present the use of in-situ resonance Raman spectroscopy to probe subtle molecular structural changes of PEDOT:PSS associated with its doping level. We demonstrate how such doping level changes of PEDOT:PSS can be used, for the first time, on operational OECTs for sensitive and selective metabolite sensing whilst simultaneously performing amperometric detection of the analyte. We test the sensitivity by molecularly sensing a lowest glucose concentration of 0.02 mM in phosphate buffered saline (PBS) solution. By changing the electrolyte to cell culture media, the selectivity of in-situ resonance Raman spectroscopy is emphasized as it remains unaffected by other electroactive components in the electrolyte. The application of this molecular structural probe highlights the importance of developing biosensing probes that benefit from high sensitivity of the material's structural and electrical properties whilst being complimentary with the electronic methods of detection.
Kasimatis M, Nunez-Bajo E, Grell M, et al., 2019, Monolithic solder-on nanoporous Si-Cu contacts for stretchable silicone composite sensors, ACS Applied Materials and Interfaces, Vol: 11, Pages: 47577-47586, ISSN: 1944-8244
We report a method of creating solderable, mechanically robust, electrical contacts to interface (soft) silicone-based strain sensors with conventional (hard) solid-state electronics using a nanoporous Si-Cu composite. The Si-based solder-on electrical contact consists of a copper-plated nanoporous Si top surface formed through metal-assisted chemical etching and electroplating, and a smooth Si bottom surface which can be covalently bonded onto silicone-based strain sensors through plasma bonding. We investigated the mechanical and electrical properties of the contacts proposed under relevant ranges of mechanical stress for applications in physiological monitoring and rehabilitation. We also produced a series of proof-of-concept devices, including a wearable respiration monitor, leg band for exercise monitoring and Squeeze-ball for monitoring rehabilitation of patients with hand injuries or neurological disorders, to demonstrate the mechanical robustness and versatility of the technology developed, in real-world applications.
Decataldo F, Druet V, Pappa A-M, et al., 2019, BMP-2 functionalized PEDOT:PSS-based OECTs for stem cell osteogenic differentiation monitoring, FLEXIBLE AND PRINTED ELECTRONICS, Vol: 4, ISSN: 2058-8585
Daboczi M, Hamilton I, Xu S, et al., 2019, The origin of open-circuit voltage losses in perovskite solar cells investigated by surface photovoltage measurement, ACS Applied Materials & Interfaces, Vol: 11, Pages: 46808-46817, ISSN: 1944-8244
Increasing the open circuit voltage (Voc) is one of the key strategies for further improvement of the efficiency of perovskite solar cells. It requires fundamental understanding of the complex optoelectronic processes related to charge carrier generation, transport, extraction and their loss mechanisms inside a device upon illumination. Herein we report the important origin of Voc losses in methylammonium lead iodide perovskite (MAPI) based solar cells, which results from undesirable positive charge (hole) accumulation at the interface between the perovskite photoactive layer and the PEDOT:PSS hole transport layer. We show strong correlation between the thickness-dependent surface photovoltage and device performance, unraveling that the interfacial charge accumulation leads to charge carrier recombination and results in a large decrease in Voc for the PEDOT:PSS/MAPI inverted devices (180 mV reduction in 50-nm-thick device compared to 230-nm-thick one). In contrast, accumulated positive charges at the TiO2/MAPI interface modify interfacial energy band bending, which leads to an increase in Voc for the TiO2/MAPI conventional devices (70 mV increase in 50-nm-thick device compared to 230-nm-thick one). Our results provide an important guideline for better control of interfaces in perovskite solar cells to improve device performance further.
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