Organic–inorganic halide perovskites incorporating two-dimensional (2D) structures have shown promise for enhancing the stability of perovskite solar cells (PSCs). However, the bulky spacer cations ...often limit charge transport. Here, we report on a simple approach based on molecular design of the organic spacer to improve the transport properties of 2D perovskites, and we use phenethylammonium (PEA) as an example. We demonstrate that by fluorine substitution on the para position in PEA to form 4-fluorophenethylammonium (F-PEA), the average phenyl ring centroid–centroid distances in the organic layer become shorter with better aligned stacking of perovskite sheets. The impact is enhanced orbital interactions and charge transport across adjacent inorganic layers as well as increased carrier lifetime and reduced trap density. Using a simple perovskite deposition at room temperature without using any additives, we obtained a power conversion efficiency of >13% for (F-PEA)2MA4Pb5I16-based PSCs. In addition, the thermal stability of 2D PSCs based on F-PEA is significantly enhanced compared to those based on PEA.
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IJS, KILJ, NUK, PNG, UL, UM
Complex refractive index and dielectric function spectra of organic–inorganic lead halide perovskite alloy thin films are presented, together with the critical-point parameter analysis (energy and ...broadening) of the respective composition. Thin films of methylammonium lead halide alloys (MAPbI 3 , MAPbBr 3 , MAPbBr 2 I, and MAPbBrI 2 ), formamidinium lead halide alloys (FAPbI 3 , FAPbBr 3 , and FAPbBr 2 I), and formamidinium cesium lead halide alloys FA 0.85 Cs 0.15 PbI 3 , FA 0.85 Cs 0.15 PbBrI 2 , and FA 0.85 Cs 0.15 Pb(Br 0.4 I 0.6 ) 3 were studied. The complex refractive index and dielectric functions were determined by spectroscopic ellipsometry (SE) in the photon energy range of 0.7–6.5 eV. Critical point energies and optical transitions were obtained by lineshape fitting to the second-derivative of the complex dielectric function data of these thin films as a function of alloy composition. Absorption onset in the vicinity of the bandgap, as well as critical point energies and optical band transition shift toward higher energies as the concentration of Br in the films increases. Cation alloying (Cs + ) has less effect on the optical properties of the thin films compared to halide mixed alloys. The reported optical properties can help to understand the fundamental properties of the perovskite materials and also be used for optimizing or designing new devices.
In this work, the impact of cation disorder on the electrical properties of biaxially textured Co2ZnO4 and Co2NiO4 thin films grown by pulsed laser deposition are investigated using a combination of ...experiment and theory. Resonant elastic X‐ray diffraction along with conductivity measurements both before and after post‐deposition annealing show that Co2ZnO4 and Co2NiO4 exhibit opposite changes of the conductivity with cation disorder, which can be traced back to their different ground‐state atomic structures, being normal and inverse spinel, respectively. Electronic structure calculations identify a self‐doping mechanism as the origin of conductivity. A novel thermodynamic model describes the non‐equilibrium cation disorder in terms of an effective temperature. This work offers a way of controlling the conductivity in spinels in a quantitative manner by controlling the cation disorder and a new design principle whereby non‐equilibrium growth can be used to create beneficial disorder.
A combination of experiment and theory quantifies the dependence of the conductivity in Co2ZnO4 and Co2NiO4 on the cation disorder. A self‐doping mechanism is identified as the origin of conductivity and a thermodynamic model is used to describe the non‐equilibrium cation disorder in terms of an effective temperature. The conductivity in spinels can be controlled by manipulating the cation disorder.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Titanium oxide (TiOx) nanolayers grown by atomic layer deposition are investigated with respect to their application as carrier selective contacts for crystalline silicon (c-Si) solar cells. Although ...TiOx is known to act as an electron contact, in this work the selectivity of TiOx layers is found to be widely tunable from electron to hole selective depending on deposition conditions, post-deposition treatments, and work function of the metal electrode used. Using TiOx and an intrinsic hydrogenated amorphous silicon buffer layer, solar cell test structure exhibiting open-circuit voltages (Voc) as high as 720 and 650 mV are shown for electron and hole selective contacts, respectively. Surface photovoltage and capacitance-voltage measurements reveal that carrier selectivity is correlated with the amount of c-Si band bending induced by TiOx, which are governed not only by the effective work function difference at the Si/TiOx interface, but also by the negative fixed charge present in the TiOx layer. This new finding is in contrast to the previous model for carrier transport where selectivity is determined only by the asymmetric band offsets at the Si/contact interface. It highlights the influence of induced band bending to produce carrier depletion/inversion conditions, and the importance of its selectivity effect in a c-Si absorber.
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•TiOx has been considered to act as an electron selective contact in Si solar cells.•TiOx is found to act as a hole selective contact as well.•Carrier selectivity is influenced by the capping material and post treatments.•The asymmetric interface band offsets do not account for the carrier selectivity.•The negative fixed charge present in the TiOx plays a role in hole selectivity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We show enhanced efficiency and stability of a high performance organic solar cell (OPV) when the work‐function of the hole collecting indium‐tin oxide (ITO) contact, modified with a ...solution‐processed nickel oxide (NiOx) hole‐transport layer (HTL), is matched to the ionization potential of the donor material in a bulk‐heterojunction solar cell. Addition of the NiOx HTL to the hole collecting contact results in a power conversion efficiency (PCE) of 6.7%, which is a 17.3% net increase in performance over the 5.7% PCE achieved with a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL on ITO. The impact of these NiOx films is evaluated through optical and electronic measurements as well as device modeling. The valence and conduction band energies for the NiOx HTL are characterized in detail through photoelectron spectroscopy studies while spectroscopic ellipsometry is used to characterize the optical properties. Oxygen plasma treatment of the NiOx HTL is shown to provide superior contact properties by increasing the ITO/NiOx contact work‐function by 500 meV. Enhancement of device performance is attributed to reduction of the band edge energy offset at the ITO/NiOx interface with the poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothidiazole) (PCDTBT):6,6‐phenyl‐C61 butyric acid methyl ester PCBM and 6,6‐phenyl‐C71 butyric acid methyl ester (PC70BM) active layer. A high work‐function hole collecting contact is therefore the appropriate choice for high ionization potential donor materials in order to maximize OPV performance.
Enhanced efficiency and stability of a high performance organic solar cell is demonstrated when the hole collecting indium‐tin oxide contact is modified with solution‐processed nickel oxide and the high work‐function contact is matched with the donor ionization potential. We use photoemission spectroscopy to characterize the band edge positions in the contact layer, a multilayer matrix optical model and a diode model to explain the improvements in contact quality in comparison to the hole‐transport layer, polyethylene dioxythiophene:polystyrene sulfonate.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We report the charge carrier recombination rate and spin coherence lifetimes in single crystals of two-dimensional (2D) Ruddlesden–Popper perovskites PEA2PbI4·(MAPbI3) n−1 (PEA, phenethylammonium; ...MA, methylammonium; n = 1, 2, 3, 4). Layer thickness-dependent charge carrier recombination rates are observed, with the fastest rates for n = 1 because of the large exciton binding energy, and the slowest rates are observed for n = 2. Room-temperature spin coherence times also show a nonmonotonic layer thickness dependence with an increasing spin coherence lifetime with increasing layer thickness from n = 1 to n = 4, followed by a decrease in lifetime from n = 4 to ∞. The longest coherence lifetime of ∼7 ps is observed in the n = 4 sample. Our results are consistent with two contributions: Rashba splitting increases the spin coherence lifetime going from the n = ∞ to the layered systems, while phonon scattering, which increases for smaller layers, decreases the spin coherence lifetime. The interplay between these two factors contributes to the layer thickness dependence.
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We report third-order nonlinear coefficient values and decay time kinetics vs. halide composition (CH 3 NH 3 PbBr 3 and CH 3 NH 3 PbBr 2 I), temperature, and excitation wavelength. The maximum values ...of the third-order nonlinear susceptibility χ (3) (∼1.6 × 10 −6 esu) are similar to or larger than many common third-order materials. The source of the nonlinearity is shown to be primarily excitonic in the tribromide film by virtue of its strong enhancement near the exciton resonance. Nonresonant excitation reduces the nonlinearity significantly, as does increasing the temperature. Substitution of one I for one Br also reduces the nonlinearity by at least one order of magnitude, presumably due to the lack of strong exciton resonance in the substituted form. The thin films are stable, highly homogenous (lacking significant light scattering), and simple and inexpensive to fabricate, making them potentially useful in a variety of optoelectronic applications in which wavelength selectivity is important.
We report on investigations of the stability of inverted organic solar cells with ZnO electron collecting interlayer that are solution-processed from zinc acetate (ZnAc) or diethylzinc (deZn) ...precursors. Characterization of the respective solar cells suggests that the two materials initially function similarly in devices, however, we find that devices with ZnO from the deZn precursor are more stable under long-term illumination and load than devices with ZnO from the ZnAc precursor. A dipolar phosphonic acid that reduces the ZnO work function also improved device performance and stability when compared with unmodified ZnAc-based ZnO, but was problematic for deZn-based ZnO. The long-term device degradation analyses shows that the improved devices had increased and significantly more stable open-circuit voltage and fill factor characteristics. Chemical analyses suggests that defects in the ZnO films, most likely interstitial zinc, may be responsible for the observed disparities in stability within organic solar cells.
Natural soiling and the subsequent requisite cleaning of photovoltaic (PV) modules result in abrasion damage to the cover glass. The durability of the front glass has important economic consequences, ...including determining the use of anti-reflective and/or anti-soiling coatings as well as the method and frequency of operational maintenance (cleaning). Artificial linear brush abrasion using Nylon 6/12 bristles was therefore examined to explore the durability of representative PV first-surfaces, i.e., the surface of a module incident to direct solar radiation. Specimens examined include silane surface functionalized-, roughened (etched)-, porous silica-coated-, fluoropolymer-coated-, and ceramic (TiO2 or ZrO2/SiO2/ZrO2/SiO2)-coated-glass, which are compared to monolithic-poly(methyl methacrylate) and -glass coupons. Characterization methods used in this study include: optical microscopy, ultraviolet–visible–near-infrared (UV-VIS-NIR) spectroscopy, sessile drop goniometry, white-light interferometry, atomic force microscopy (AFM), and depth-profiling X-ray photoelectron spectroscopy (XPS). The corresponding characteristics examined include: surface morphology, transmittance (i.e., optical performance), surface energy (water contact angle), surface roughness, scratch width and depth, and chemical composition, respectively. The study here was performed to determine coating failure modes; identify characterization methods that can detect nascent failures; compare the durability of popular contemporary coating materials; identify their corresponding damage characteristics; and compare slurry and dry-dust abrasion. This study will also aid in developing an abrasion standard for the PV industry.
Figure A: Representative images of the experimental configurations for the linear brush abrasion tester, including: (a) isometric view of the slurry tester, (b) isometric view of the dry-dust tester.Artificial linear brush abrasion was examined to explore the durability of representative PV first-surfaces. Specimens examined include silane surface functionalized-, roughened (etched)-, porous silica coated-, fluoropolymer coated-, and ceramic (TiO2 or ZrO2/SiO2/ZrO2/SiO2) coated-glass, which are compared to monolithic-poly(methyl methacrylate) and -glass coupons. Characterization methods included: optical microscopy, UV-VIS-NIR spectroscopy, sessile drop goniometry, white light interferometry, atomic force microscopy (AFM), and depth profiling X-ray photoelectron spectroscopy (XPS). The corresponding characteristics examined include: surface morphology, transmittance (i.e., optical performance), surface energy (water contact angle), surface roughness, scratch-width and -depth, and chemical composition, respectively. The study here was performed to determine coating failure modes; identify characterization methods that can detect nascent failures; compare the durability of popular contemporary coating materials; and identify their corresponding damage characteristics; and compare between slurry and dry dust abrasion. Display omitted
•Correlation between transmittance, yellowness, surface energy, and roughness consistent with history of film degradation.•Damage primarily results from the facets and edges of dust particles and/or brush bristles, i.e., localized abrasion sources.•A variety of longevities from 100 cycles to 1000 cycles to 10000 cycles was observed for the representative specimens here.•Test parameter details as well as post examination suggest improvement of the linear artificial brush abrasion method.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising candidates as the active layer in photovoltaics (PV), particularly for niche applications where high infrared absorbance and/or ...semi-transparent solar cells are desirable. Most current fabrication strategies for SWCNT PV devices suffer from relatively high surface roughness and lack nanometer-scale deposition precision, both of which may hamper the reproducible production of ultrathin devices. Additionally, detailed optical models of SWCNT PV devices are lacking, due in part to a lack of well-defined optical constants for high-purity s-SWCNT thin films. Here, we present an optical model that accurately reconstructs the shape and magnitude of spectrally resolved external quantum efficiencies for ultrathin (7,5) s-SWCNT/C60 solar cells that are deposited by ultrasonic spraying. The ultrasonic spraying technique enables thickness tuning of the s-SWCNT layer with nanometer-scale precision, and consistently produces devices with low s-SWCNT film average surface roughness (Rq of <5 nm). Our optical model, based entirely on measured optical constants of each layer within the device stack, enables quantitative predictions of thickness-dependent relative photocurrent contributions of SWCNTs and C60 and enables estimates of the exciton diffusion lengths within each layer. These results establish routes towards rational performance improvements and scalable fabrication processes for ultra-thin SWCNT-based solar cells.
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