Solution-processed Films Research Articles

Organic phosphor doped thermoplastics with ultralong and memorable room temperature phosphorescence different from crystals

Organic room temperature phosphorescence (RTP) crystals have been extensively reported but encounter poor mechanical properties and processing and application flexibility, which inspires much interest in developing amorphous RTP polymers. However, the current RTP polymers are mainly solution-processed films, and their thermoplasticity and shape and size flexibility and retainability as well as RTP properties need to be improved. Herein, we dissolve two classes of carbazole derivatives starting from commercial (CCZ) and self-made (LCZ) carbazole into solution of PMMA and find the solution-processed films show very poor RTP properties, however, when they are thermoplastic processed into plastic sheets, ultralong blue-yellow dual RTP and blue RTP are observed after photo-activation, which are distinctly different from their crystal RTP. These results reveal that thermoplasticizing can not only alter dispersion states but also local environment of carbazole derivatives, enabling LCZ derivative without crystal RTP to also emit ultralong molecular RTP in polymer matrix. Since RTP generation of polymer films requires photoactivation and the photoactivated RTP patterns show ultralong memory effect, promising information storage and anti-counterfeiting is enabled. This work demonstrates that organic hetero-cyclic molecules can exhibit very interesting and unpredictable RTP properties in polymers different from crystals, opening up a vast research space and a simple processing strategy for tuning and/or enhancing RTP properties of organic doped polymers.

Cross-linkable molecule in spatial dimension boosting water-stable and high-efficiency perovskite solar cells

Solution-processed perovskite films have high trap densities especially on the perovskite surface that become pathways of moisture invasion, unavoidably leading to perovskite degradation. Although the water resistance or crosslinking organic molecules attached to the perovskite film can partially mitigate the degradation, there is usually a significant trade-off between device power conversion efficiency (PCE) and moisture stability due to the insulating nature of the organic molecules. Here, we designed and synthesized a cross-linkable molecule PETA-G to solve the problem by rationally combining the guanidine and tri-acrylate groups. The guanidine group with three lone pair electrons enables sufficient bonding between lone pair electrons and undercoordinated Pb2+ of the perovskite grain surface; the tri-acrylate group with three crosslink sites has a moderate crosslinking condition that can spatially crosslink, thus forming a compact network on the perovskite surface. The crosslinked PETA-G (CL-PETA-G) can significantly suppress non-radiative recombination and improve the moisture resistance of the perovskite film that even survives when dipped into water, while the charge transport was not influenced. Consequently, the long-term moisture and operational stabilities of the perovskite solar cells based on FA0.92MA0.08PbI3 with CL-PETA-G were dramatically increased, and the devices achieved a remarkable PCE as high as 22.6%.

Understanding the solvent effects on polarity switching and thermoelectric properties changing of solution-processable n-type single-walled carbon nanotube films

Developing high performance solution-processable n-type thermoelectric materials is challenging. In this paper, we report that single-walled carbon nanotubes (SWCNTs) prepared in dimethyl sulfoxide (DMSO) (SWCNTs/DMSO) exhibit a high n-type electrical conductivity of 2298 S/cm at room temperature, which is superior to that of previously reported n-type solution-processable SWCNTs. The maximum n-type electrical conductivity was 3490 S/cm at 373 K. The great electrical conductivity results in a high n-type power factor of 195 μW/m-K2 for SWCNTs/DMSO films at 373 K which is larger than most of the n-type solution-processable SWCNTs. The theoretical calculation indicates that the wrapping morphology of the surfactant on SWCNTs is strongly affected by different solvent polarities which lead to different packing densities of the SWCNT films as demonstrated in the scanning electron microscope images, subsequently affecting the n-type doping efficiency and the n-type electrical conductivity. The electrical conductivity affected by the mixed p- and n-type carriers in the film were discussed. A full SWCNT thermoelectric generator has been fabricated to show the heat-to-electricity conversion ability of the materials. This work reveals a potential method to prepare highly conductive n-type SWCNTs by choosing the proper solvent for optimized thermoelectric performance.

High-Yield Ti3 C2 Tx MXene-MoS2 Integrated Circuits.

It is very challenging to employ solution-processed conducting films in large-area ultrathin nanoelectronics. Here, spray-coated Ti3 C2 Tx MXene films as metal contacts are successfully integrated into sub-10nm gate oxide 2D MoS2 transistor circuits. Ti3 C2 Tx films are spray coated on glass substrates followed by vacuum annealing. Compared to the as-prepared sample, vacuum annealed films exhibit a higher conductivity (≈11 000 S cm-1 ) and a lower work function (≈4.5eV). Besides, the annealed Ti3 C2 Tx film can be patterned through a standard cleanroom process without peeling off. The annealed Ti3 C2 Tx film shows a better band alignment for n-type transport in MoS2 channel with small work function mismatch of 0.06eV. The MoS2 film can be uniformly transferred on the patterned Ti3 C2 Tx surface and then readily processed through the cleanroom process. A large-area array of Ti3 C2 Tx MXene-MoS2 transistors is fabricated using different dielectric thicknesses and semiconducting channel sizes. High yield and stable performance for these transistor arrays even with an 8nm-thick dielectric layer are demonstrated. Besides, several circuits are demonstrated, including rectifiers, negative-channel metal-oxide-semiconductor (NMOS) inverters, and voltage-shift NMOS inverters. Overall, this work indicates the tremendous potential for solution-processed Ti3 C2 Tx MXene films in large-area 2D nanoelectronics.

Emissive Material Optimization for Solution-Processed Exciplex OLEDs

Exciplex or charge transfer blends can be emissive in a similar manner to thermally activated delayed fluorescent materials but without the requirement for covalent linkages. Exciplex blends have been mainly used in organic light-emitting diodes fabricated using thermal evaporation, which is in part due to the poor film forming properties of evaporable materials when processed from solution. In this paper, we report the synthesis of three oxadiazole derivatives designed to have good solubility and compare the properties with commercial 1,3-bis[2-(4-t-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7). The acceptor materials have 4-(2-ethylhexyloxy)phenyl or 9,9-di-n-alkylfluoren-2-yl (where the alkyl groups are either n-propyl or n-hexyl) end groups as opposed to the 4-t-butylphenyl moieties of OXD-7. Neat films of the synthesized oxadiazole derivatives were found to have electron mobilities an order of magnitude higher than that of OXD-7. The oxadiazole derivatives were found to be miscible with the hole transporting 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA) with charge transfer emission from solution-processed films. The 4-(2-ethylhexyloxy)phenyl-oxadiazole derivative exhibited the best overall performance when blended with m-MTDATA, with the solution-processed films having a photoluminescence quantum yield of around 21 ± 3% and simple bilayer OLEDs having a maximum external quantum efficiency of 5.3 ± 0.3%.

Near-unity blue luminance from lead-free copper halides for light-emitting diodes

Copper(I)-based halides are emerging as attractive nontoxic candidates to rival the lead-halide perovskites for optoelectronic applications. Herein, the zero-dimensional organic-inorganic copper halide [N(C2H5)4]2Cu2Br4 was proposed as a novel luminescence material. Its single crystals (SCs) were synthesized with a facile approach, realizing photoluminescence quantum yield (PLQY) up to 97.08% and superior stability. Importantly, TEA2Cu2Br4 SC exhibited a broadband blue emission around 463 nm with a large Stokes shift of 183 nm and a long photoluminescence (PL) lifetime up to 50 μs. In particular, the solution-processed polycrystalline films maintain a high PLQY of 94.73%. Based on temperature-dependent and time-resolved PL measurements and first-principles calculations, we propose a multi-channel phosphorescence mechanism for the broadband blue emission of this novel material. In addition, tunable PL of the polycrystalline films from sky-blue to deep-blue was realized. The solution processed electroluminescence devices employing the multiple blue emitters were readily achieved in the coordinate range of CIE from (0.15, 0.17) to (0.15, 0.04). Notably, the sky-blue EL device achieves a brightness of 85 cd/m2 and an external quantum efficiency of 0.11%. Our work thus inspires the design and synthesis of novel copper-based emitters and paves a way for future applications in white light-emitting diodes and full-color display.

In-Situ polymerization of PEDOT in perovskite Thin films for efficient and stable photovoltaics

Solution-processed polycrystalline perovskite films have detrimental defects in the bulk and surfaces/grain boundaries, especially the uncoordinated lead ions (Pb2+) with low formation energy, limiting both the performance and stability of the photovoltaic devices. In this work, a conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is incorporated into a bulk perovskite film by a facile in-situ polymerization method, where 5-bromo-2,3-dihydro-thieno[3,4-b][1,4]dioxine (BEDOT) as monomer is directly added to a perovskite precursor solution and then in-situ polymerized within the perovskite film during the film formation. The PEDOT is chemically anchored at the surface and grain boundaries of the perovskite film via interaction of sulfur and oxygen atoms in PEDOT with uncoordinated lead ions in the perovskite, passivating the surface and bulk defects and also improving the charge extraction efficiency in the solar cell. Moreover, robust and hydrophobic properties of PEDOT can enhance the thermal and moisture resistance of the perovskite film. As a result, a champion power conversion efficiency (PCE) of 22.58% is achieved, along with significantly enhanced environmental, thermal, and light-soaking stability.

Nanometer-Thick SiN Films as Gas Barrier Coatings Densified by Vacuum UV Irradiation

Gas barrier films are widely used in electronic and packaging applications. They are also critical components of flexible organic light-emitting diodes (FOLEDs) that require high gas barrier performance. Among the various film manufacturing techniques, solution-processed thin-film encapsulation (TFE) represents a low-cost FOLED fabrication method. The nanometer-thick SiN films produced following the vacuum ultraviolet (VUV)-induced densification of solution-processed perhydropolysilazane (PHPS) films in a N2 atmosphere can potentially serve as TFE barrier films. However, the nanometer-thick PHPS densification process has not been examined in sufficient detail. We investigated and discussed the effects of the Si–N bond number, PHPS film composition, and free volume (present in the produced Si–N network) on the VUV-induced PHPS densification process. It was found that VUV irradiation caused rapid hydrogen release and film densification through the formation of Si–N bonds. The results obtained using the X-ray photoelectron spectroscopy and dynamic secondary ion mass spectrometry techniques, and the calculated residual hydrogen ratios, revealed that the film composition was strongly related to the number of residual hydrogen atoms and Si–N bonds. Notably, nanometer-thick PHPS film densification was a relatively slow process, in which the free volume in the Si–N network was considerably reduced by the atomic rearrangement induced by the simultaneous cleavage of several Si–N bonds during VUV irradiation. We believe that the results presented herein can potentially serve as a guideline for developing solution-processed nanometer-thick SiN films with relatively high density and excellent gas barrier performance (that is comparable to that exhibited by vacuum-processed barrier films).

Thickness dependent photodetection properties of solution-processed CuI films: Towards cost-effective flexible visible photodetectors

Herein, solution-processed earth-abundant CuI films are employed for low-cost visible photodetectors on either rigid Si/SiO2 (100 nm) wafers or flexible polyimide (PI) substrates. The CuI film thickness dependent structure, morphology and optical properties are investigated via film analysis methods including X-ray spectroscopy (XPS), elucidating that consistent shift for binding energy position of constituent elements and variation in the elemental composition are highly relevant with photo detective properties (PPs). Among various film thickness, the highest PPs for photodetectors on the Si/SiO2 substrate is obtained for 500 nm condition in the respective of (i) photosensitivity (403.26 ± 20.86%), (ii) photoresponsivity ((65.16 ± 3.35) mA/W), and (iii) specific detectivity ((1.66 ± 0.45) × 1010 Jones). Moreover, flexible photodetectors with the optimized condition show 90% level PPs, as compared with the rigid substrate, anticipating that CuI can be a promising material for flexible photodetectors.

Multimode Operation of Organic-Inorganic Hybrid Thin-Film Transistors Based on Solution-Processed Indium Oxide Films.

Solution-processed metal oxide (MO) thin films have been extensively studied for use in thin-film transistors (TFTs) due to their high optical transparency, simplicity of fabrication methods, and high electron mobility. Here, we report, for the first time, the improvement of the electronic properties of solution-processed indium oxide (InOx) films by the subsequent addition of an organic p-type semiconductor material, here 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), yielding organic-inorganic hybrid TFTs. The addition of TIPS-pentacene not only improves the electron mobility by enhancing the charge carrier percolation pathways but also improves the electronic and temporal stability of the IDS(VG) characteristics as well as reduces the number of required spin-coating steps of the InOx precursor solution. Very interestingly, the introduction of 10 nm TIPS-pentacene films on top of 15 nm InOx layers allows the fabrication of either enhancement- or depletion-mode devices with only minimal changes to the fabrication process. Specifically, we find that when the TIPS-pentacene layer is added on top of the source/drain electrodes, resulting in devices with embedded source/drain electrodes [embedded electrode TFTs (EETFTs)], the devices exhibit an enhancement-mode behavior with an average mobility (μ) of 6.4 cm2 V-1 s-1, a source-drain current ratio (Ion/Ioff) of around 105, and a near-zero threshold voltage (VTH). When on the other hand the TIPS-pentacene layer is added before the source-drain electrodes, i.e., in top-contact electrode TFTs (TCETFTs), a very clear depletion mode behavior is observed with an average μ of 6.3 cm2 V-1 s-1, an Ion/Ioff ratio of over 105, and a VTH of -80.3 V. Furthermore, a logic inverter is fabricated combining the enhancement (EETFTs)- and depletion (TCETFTs)-mode transistors, which shows a potential for the construction of organic-inorganic hybrid electronics and circuits.

Interfacial "Anchoring Effect" Enables Efficient Large-Area Sky-Blue Perovskite Light-Emitting Diodes.

While tremendous progress has recently been made in perovskite light‐emitting diodes (PeLEDs), large‐area blue devices feature inferior performance due to uneven morphologies and vast defects in the solution‐processed perovskite films. To alleviate these issues, a facile and reliable interface engineering scheme is reported for manipulating the crystallization of perovskite films enabled by a multifunctional molecule 2‐amino‐1,3‐propanediol (APDO)‐triggered “anchoring effect” at the grain‐growth interface. Sky‐blue perovskite films with large‐area uniformity and low trap states are obtained, showing the distinctly improved radiative recombination and hole‐transport capability. Based on the APDO‐induced interface engineering, synergistical boost in device performance is achieved for large‐area sky‐blue PeLED (measuring at 100 mm2) with a peak external quantum efficiency (EQE) of 9.2% and a highly prolonged operational lifetime. A decent EQE up to 6.1% is demonstrated for the largest sky‐blue device emitting at 400 mm2.

A Study on Solution-Processed Y2O3 Films Modified by Atomic Layer Deposition Al2O3 as Dielectrics in ZnO Thin Film Transistor

In this work, Y2O3–Al2O3 dielectrics were prepared and used in ZnO thin film transistor as gate insulators. The Y2O3 film prepared by the sol–gel method has many surface defects, resulting in a high density of interface states with the active layer in TFT, which then leads to poor stability of the devices. We modified it by atomic layer deposition (ALD) technology that deposited a thin Al2O3 film on the surface of a Y2O3 dielectric layer, and finally fabricated a TFT device with ZnO as the active layer by ALD. The electrical performance and bias stability of the ZnO TFT with a Y2O3–Al2O3 laminated dielectric layer were greatly improved, the subthreshold swing was reduced from 147 to 88 mV/decade, the on/off-state current ratio was increased from 4.24 × 106 to 4.16 × 108, and the threshold voltage shift was reduced from 1.4 to 0.7 V after a 5-V gate was is applied for 800 s.

Solution-processed CuI films towards flexible visible-photodetectors: Role of annealing temperature on Cu/I ratio and photodetective properties

This study reports the physical and electrical properties of solution spin-coated CuI films on the thermally oxidized Si substrates according to different annealing temperatures. The planar and cross-sectional scanning electron microscopy (FE-SEM) analysis confirm the formation of dense CuI films on SiO2 substrates. The decrease in the chemical compositional ratio (Cu/I) of prepared CuI films with the rise of annealing temperature is confirmed by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analysis. The annealing temperature-dependent study shows that the CuI films annealed at 175 °C for 30 min have the highest photodetection properties (PDPs) under the illumination of a halogen lamp. The higher percentage of recovery (86%) of photocurrent and the lower drift current is obtained for CuI films annealed at 200 °C for 30 min. The wavelength-dependent (RGB illumination) photodetection properties exhibit that CuI films are highly photosensitive under blue LEDs than green and red LEDs. The highest photosensitivity (264.36 ± 10.12%), photoresponsivity (Rph = (45.64 ± 1.68) mA/W), specific detectivity (Dph = (6.56 ± 1.61)× 109 Jones)), normalized photocurrent to dark current ratio (NPDR=(595971.03 ± 31223.18) W−1)), signal to noise ratio (SNR=3.64 ± 0.50), noise equivalent power (NEP=(3.45 ± 1.41) pWHz−1/2)), and linear dynamic range (LDR = 11.23 ± 0.26) dB) for Vb = 1 V, respectively for CuI films under blue-LEDs. For flexible PDPs, 85% of photodetection properties are achieved for flexible CuI films than PDPs on the rigid substrates. The current study reveals that annealing temperature significantly impacts the chemical compositional ratio and photodetection properties of CuI films.

Novel Method for Fabricating Visible-Light Phototransistors Based on a Homojunction-Porous IGZO Thin Film Using Mechano-Chemical Treatment.

A homojunction-structured oxide phototransistor based on a mechano-chemically treated indium-gallium-zinc oxide (IGZO) absorption layer is reported. Through this novel and facile mechano-chemical treatment, mechanical removal of the cellophane adhesive tape induces reactive radicals and organic compounds on the sputtered IGZO film surface. Surface modification, following the mechano-chemical treatment, caused porous sites in the solution-processed IGZO film, which can give rise to a homojunction-porous IGZO (HPI) layer and generate sub-gap states from oxygen-related defects. These intentionally generated sub-gap states played a key role in photoelectron generation under illumination with relatively long-wavelength visible light despite the wide band gap of IGZO (>3.0 eV). Compared with conventional IGZO phototransistors, our HPI phototransistor displayed outstanding optoelectronic characteristics and sensitivity; we measured a threshold voltage (Vth) shift from 3.64 to -6.27 V and an on/off current ratio shift from 4.21 × 1010 to 4.92 × 102 under illumination with a 532 nm green light of 10 mW/mm2 intensity and calculated a photosensitivity of 1.16 × 108. The remarkable optoelectronic characteristics and high optical transparency suggest optical sensor applications.

Digital printing of a novel electrode for stable flexible organic solar cells with a power conversion efficiency of 8.5%

Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) mixed with single-wall nanotubes (SWNTs) (10:1) and doped with (0.1 M) perchloric acid (HClO4) in a solution-processed film, working as an excellent thin transparent conducting film (TCF) in organic solar cells, was investigated. This new electrode structure can be an outstanding substitute for conventional indium tin oxide (ITO) for applications in flexible solar cells due to the potential of attaining high transparency with enhanced conductivity, good flexibility, and good durability via a low-cost process over a large area. In addition, solution-processed vanadium oxide (VOx) doped with a small amount of PEDOT-PSS(PH1000) can be applied as a hole transport layer (HTL) for achieving high efficiency and stability. From these viewpoints, we investigate the benefit of using printed SWNTs-PEDOT-PSS doped with HClO4 as a transparent conducting electrode in a flexible organic solar cell. Additionally, we applied a VOx-PEDOT-PSS thin film as a hole transporting layer and a blend of PTB7 (polythieno[3,4-b] thiophene/benzodithiophene): PC71BM (phenyl-C71-butyric acid methyl ester) as an active layer in devices. Zinc oxide (ZnO) nanoparticles were applied as an electron transport layer and Ag was used as the top electrode. The proposed solar cell structure showed an enhancement in short-circuit current, power conversion efficiency, and stability relative to a conventional cell based on ITO. This result suggests a great carrier injection throughout the interfacial layer, high conductivity and transparency, as well as firm adherence for the new electrode.

Solution-Processed Multistacked Tin-Doped Indium Oxide Nanoparticle Conductors for Cost-Effective Thin Film Heaters

Tin-doped indium oxide (ITO) nanoparticle (NP) films were fabricated as a cost-effective and highly transparent electrode for high-performance thin film heaters (TFHs), using a solution coating process on glass substrate. The electrical and optical properties of the ITO NP films depended on the number of spin coatings and post-annealing with various conditions (temperature, time, and ambient gas). The optimized three layer ITO NP films annealed at 600 °C under ambient N2 showed a low sheet resistance of 14 Ω/sq and a high optical transmittance of 88% in the visible wavelength region of (400–800) nm. Cross-sectional TEM images showed the ITO nanoparticles to be evenly dispersed in the thin film with good crystallinity and well-defined elliptical shape in the range of 10 to 25 nm. X-ray diffraction analysis confirmed that heat treatment increased the size of the ITO NPs. The KPFM result exhibited that as the heat treatment duration increased, the surface roughness of the ITO NP films reduced. X-ray photoelectron spectroscopy surface analysis showed a relatively higher area ratio of oxygen vacancies and a more stable Sn state in the chemical states of the O 1s and Sn 3d5/2 spectra than those of typical ITO films deposited by a magnetron sputtering after heat treatment. Due to the thermal stability and uniformity of the ITO NP film, ITO NP-based TFHs showed a stable time–temperature profile and uniform IR imagery. Therefore, solution-processed ITO NPs films can be applied to a promising low-cost transparent electrode in the next-generation smart window for buildings and automobiles.

Photo-patterned oxide films produced using polymeric metal acrylate for low-voltage thin-film transistors

Solution-processed zinc oxide (ZnO) thin-film transistors (TFTs) have received considerable attention as key elements for future electronics due to the ease of fabricating these TFTs and due to the natural abundance and hence low cost of ZnO. However, issues regarding the patterning of solution-processed ZnO semiconductor films unfortunately continue to pose a barrier to their commercialization. In this study, we optimized nanometer-thick photo-curable zinc acrylate (ZnA) films as ZnO precursors and used these precursors to prepare photo-patterned ZnO semiconductors for the fabrication of n-type TFTs. Solution-processed ZnA films were effectively micro-patterned under UV light, because UV-irradiated ZnA film was not solved in etching solvent as a result of photocrosslinking of diacrylate moiety. Through a sol-gel reaction, wurtzite ZnO crystals were stably formed from the photo-patterned ZnA films. After optimizing the film thickness and by using a photo-patterned ZrOx dielectric layer, the prepared ZnO TFT exhibited stable performances with a low-voltage operation within ±2 V. This approach for developing a ZnO semiconductor film can be used to enable the realization of simple, inexpensive, and environmentally friendly solution-processing techniques in electronic device fields.

Conductivity of solution-processed films of a metal-free chlorophyll derivative: drop casting vs. spin coating

Conductivity of solution-processed films of a metal-free chlorophyll derivative: drop casting vs. spin coating

An ultraviolet-ozone post-treatment to remove the inherent impurities in all-ambient solution-processed CsPbBr3 perovskite films

It is very important to exclude impurities during the material preparation process for the production of high-quality optoelectronic devices. For the all-ambient solution-processed CsPbBr3 films, unexpected inherent impurities could be inadvertently introduced, which would induce electronic trap states and impede carrier transport in the as-derived CsPbBr3-based optoelectronic devices, such as all-inorganic perovskite solar cells (PSCs). Herein, it is found that ultraviolet-ozone (UVO) post-treatment, which is usually applied for cleaning substrates, has the ability to remove these harmful impurities in all-ambient solution-processed CsPbBr3 films. Upon proper UVO post-treatment, the assembled PSC with the architecture of fluorine-doped tin oxide/compact TiO2 (c-TiO2)/CsPbBr3/carbon exhibits an improved power conversion efficiency of 7.82% from 7.37%. X-ray photoelectron spectroscopy measurements demonstrate that the undesirable inherent impurities in all-ambient solution-processed CsPbBr3 films may be related to organic solvents and adsorbates, which can be removed effectively by UVO post-treatment. After UVO post-treatment, photoluminescence and space-charge-limited current measurements reveal that CsPbBr3 films possess reduced trap states. Meanwhile, due to the enhanced contact between the CsPbBr3 film and the subsequently deposited carbon electrode, the charge collection at the interface is effectively promoted. Our work not only finds the influence of the inherent impurities residual in all-ambient solution process, but also proposes a simple and effective strategy to remove these impurities.

Solution-processed molybdenum oxide films by low-temperature annealing for improved silicon surface passivation

Crystalline silicon solar cells are developing towards higher conversion efficiency, however, the surface recombination and Auger recombination from heavily doped silicon are the main limit. Some metal oxides are of great interest because of their selective contact performance with crystalline silicon, which can minimize the carrier recombination in the contact area. Especially, due to the high work function of molybdenum oxide (MoOx), the MoOx film can provide excellent hole transport and electron blocking properties for the silicon surface. Here, we have deposited MoOx films on the surface of silicon wafer by spin-coating a copper hydromolybdate solution. The MoOx film can provide superior passivation quality compared to thermal evaporated one, with the τeff of 110.6 μs (Δn = 1015 cm−3) obtained on n-type Czochralski silicon. Moreover, the passivation of MoOx film can be further improved by annealing at low temperature and under reduced atmosphere environment. The dense surface morphology of amorphous films and formation of Si–O–Si bonds by annealing are the main reasons for improved silicon surface passivation. This work demonstrates the feasibility of using solution-processed transition metal oxides as effective and low-cost carrier-selective passivating contacts for silicon photovoltaic devices.