Front Electrode Research Articles

Environmental Stability Evaluation of Aluminium Doped Zinc Oxide (AZO) Transparent Electrodes Deposited at Low Temperature for Solar cells

The degradation of transparent electrodes’ electrical conductivity under environmental conditions is considered as a major failure mode for solar cells’ long-term efficiency. In this paper, AZO thin films were subjected to the International Electrotechnical Commission (IEC) 61646 test to examine their environmental stability and suitability as front electrodes for solar cells. To explore the interplay between AZO deposition parameters and environmental stability, AZO films were deposited by radio frequency magnetron sputtering at different parameters and without external heating. The conductivity stability evolution upon the testwas investigated via studying the AZO electrical, structural, and morphological characteristics at different deposition conditions. A direct dependence was identified between the samples’ conductivity degradation rates and the samples’ structural and morphological characteristics including grain size, grain boundary density, surface roughness, and compactness. The samples’ resistivity increases linearly over the test period due to both electron density and mobility degradations. Improved stability was observed for thicker AZO samples (360 nm) originating from enhanced grain size, surface profile, and compactness. These samplesmaintained solar cells' applicable sheet resistance of 21.24 Ω/sq (ρ=7.64×10-4 Ω.cm) following the test. The conducted aging studies demonstrated that manipulating the AZO films growth process via optimizing the deposition parameters is an effective pathway for low-temperature deposited electrodes with enhanced environmental stability

Preparation of Photo-Bioelectrochemical Cells With the RC-LH Complex From Roseiflexus castenholzii.

Roseiflexus castenholzii is an ancient green non-sulfur bacteria that absorbs the solar energy through bacteriochlorophylls (BChls) bound in the only light harvesting (LH) complex, and transfers to the reaction center (RC), wherein primary charge separation occurs and transforms the energy into electrochemical potentials. In contrast to purple bacteria, R. castenholzii RC-LH (rcRC-LH) does not contain an H subunit. Instead, a tightly bound tetraheme cytochrome c subunit is exposed on the P-side of the RC, which contains three BChls, three bacteriopheophytins (BPheos), two menaquinones, and one iron for electron transfer. These novel structural features of the rcRC-LH are advantageous for enhancing the electron transfer efficiency and subsequent photo-oxidation of the c-type hemes. However, the photochemical properties of rcRC-LH and its applications in developing the photo-bioelectrochemical cells (PBECs) have not been characterized. Here, we prepared a PBEC using overlapped fluorine-doped tin oxide (FTO) glass and Pt-coated glass as electrodes, and rcRC-LH mixed with varying mediators as the electrolyte. Absence of the H subunit allows rcRC-LH to be selectively adhered onto the hydrophilic surface of the front electrode with its Q-side. Upon illumination, the photogenerated electrons directly enter the front electrode and transfer to the counter electrode, wherein the accepted electrons pass through the exposed c-type hemes to reduce the excited P+, generating a steady-state current of up to 320 nA/cm2 when using 1-Methoxy-5-methylphenazinium methyl sulfate (PMS) as mediator. This study demonstrated the novel photoelectric properties of rcRC-LH and its advantages in preparing effective PBECs, showcasing a potential of this complex in developing new type PBECs.

Review of technology specific degradation in crystalline silicon, cadmium telluride, copper indium gallium selenide, dye sensitised, organic and perovskite solar cells in photovoltaic modules: Understanding how reliability improvements in mature technologies can enhance emerging technologies

AbstractA comprehensive understanding of failure modes of solar photovoltaic (PV) modules is key to extending their operational lifetime in the field. In this review, first, specific failure modes associated with mature PV technologies, such as crystalline silicon (c‐Si), copper indium gallium selenide (CIGS) and cadmium telluride (CdTe), are framed by sources of specific failure modes, their development from the early‐developmental stages onwards and their impact upon long term performance of PV modules. These failure modes are sorted by both PV technology and location of occurrence in PV modules, such as substrate, encapsulant, front and rear electrode, absorber and interlayers. The second part of the review is focused on emerging PV technologies, such as perovskites solar cells, dye sensitised and organic PVs, where due to their low to medium technology readiness levels, specific long‐term degradation mechanisms have not fully emerged, and most mechanisms are only partially understood. However, an in‐depth summary of the known stability challenges associated with each emerging PV technology is presented. Finally, in this paper, lessons learned from mature PV technologies are reviewed, and considerations are given in to how these might be applied to the further development of emerging technologies. Namely, any emerging PV technology must eventually pass industry‐standard qualification tests, while warranties for the lifetime of modern c‐Si‐based modules might be extended beyond the existing warranted life of 25 years.

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Sequential optimization of highly efficient all inorganic CsGeI3 perovskite solar cell by numerical simulation

This study models and optimizes the CsGeI3-based perovskite solar cell with an n-i-p for practical fabrication. This simulation determined that power conversion efficiency (PCE) has practically saturated over 700 nm of thickness, and the increase rate is barely 0.15 percent up to 1100 nm. a thickness of 700 nm as the ideal thickness for further investigation. Proposed structure exhibits lower activation energy of 1.40 eV and higher thermal stability compared to others. Defect tolerance limits are found for the interfaces are 1 × 1015 and 1 × 1016 respectively. At visible light wavelengths of 400–800 nm, the quantum efficiency is quite high. At the hole transport layer/metal junction, the built-in potential (V bi) was determined in the range between 0.81 and 0.96 V. When fluorine-doped tin oxide is utilized as the front electrode and gold as the back electrode, the device’s performance is at its best.

Power conversion efficiency of 25.26% for silicon heterojunction solar cell with transition metal element doped indium oxide transparent conductive film as front electrode

AbstractIn this paper, to improve the power conversion efficiency (Eff) of silicon heterojunction (SHJ) solar cells, we developed the indium oxide doped with transition metal elements (IMO) as front transparent conductive oxide (TCO) layer combined with microcrystalline silicon (μ‐Si:H(n+)) for SHJ solar cell. The optical and electrical properties as well as structures of hydrogenated IMO (IMO:H) films were studied and compared to conventional indium tin oxide (ITO) film. Such IMO:H films have high carrier mobility over 70 cm2/V·s, high transmittance, and low free carrier absorption, which leads to a high short circuit current density exceeding 40 mA/cm2. In addition, the low sheet resistance and contact resistivity of the IMO:H films contribute to the high fill factor of the solar cell. The average Eff of solar cells with IMO:H + μ‐Si:H(n+) is improved by 0.4% compared with that of the solar cells with ITO + a‐Si:H(n+). Finally, a certified efficiency up to 25.26% (total area, 274.5 cm2) was achieved.

Optically Rough and Physically Flat Transparent Conductive Substrates with Strong Far-Field Scattering

Optically rough and physically flat transparent conductive (OR-PF) substrates facilitate the performance improvement of optoelectronic devices and functional glasses via simultaneously enabling high-quality growth of functional layers and effective light management. This paper studies the effect of the interface morphology of the hole array pattern (HAP) and the pillar array pattern (PAP) on the far-field scattering properties of OR-PF substrates fabricated by spin-coating Al-doped ZnO (AZO) on nanoimprint-patterned glasses for improving the performance of superstrate-type thin-film solar cells. Theoretical calculation based on bidirectional scattering distribution function predicts that HAP with a period of 1.5 μm and a diameter of 1.3 μm [HAP(P1.5D1.3)] and the PAP(P1.0D0.5) interface morphology have a haze ratio in transmission (HT) of around 64% and a scatter angle of larger than 34°. The fabricated AZO/HAP(P1.5D1.3) and AZO/PAP(P1.0D0.5) show a flat surface with a σrms of less than 9 nm, a high visible light transmittance of over 86%, a sheet resistance of about 30 Ω/sq, and strong far-field scattering. In particular, AZO/PAP(P1.0D0.5) possesses an average HT of over 11% at the wavelength range of 600-850 nm and an angular intensity distribution of over 1.5% at an azimuthal angle of around 55°, indicating stronger far-field scattering than the conventional pyramid-textured B-doped ZnO (BZO/F). Compared to the flat substrate, AZO/PAP generates an implied Jsc gain of 16.2% in a CH3NH3PbI3 photoactive layer with a thickness of 300 nm under normal incidence at the wavelength range of 550-800 nm. For 60° incidence, AZO/PAP(P1.0D0.5) enables an implied Jsc gain of 2.3% with respect to BZO/F. As applied to the front electrode of CH3NH3PbI3 thin-film solar cells, compared to BZO/F, AZO/PAP(P1.0D0.5) would enable a gain of up to 16.7 and 11.2% in photoelectric conversion efficiency for the 0 and 60° incidence, respectively.

Zn1-xGexOy Passivating Interlayers for BaSi2 Thin-Film Solar Cells.

BaSi2 is a promising absorber material for next-generation thin-film solar cells (TFSCs). For high-efficiency TFSCs, a suitable interlayer should be found for every light absorber. However, such an interlayer has not been studied for BaSi2. In this study, we investigated amorphous Zn1-xGexOy films as interlayers for BaSi2. The Zn/Ge atomic ratio in the Zn1-xGexOy film and the optical band gap depend on the substrate temperature during sputtering deposition. A suitable i-Zn1-xGexOy/BaSi2 heterointerface with spike-type conduction band offset was achieved when Zn1-xGexOy was deposited on BaSi2 at 50 °C. Furthermore, photoresponsivity measurements revealed that Zn1-xGexOy has an excellent surface passivation effect on BaSi2. When the thickness of Zn1-xGexOy was 2 nm, a high photoresponsivity of 0.9 A/W was obtained for a 500 nm thick BaSi2 layer at a wavelength of 780 nm under an applied bias voltage of 0.5 V between the front and rear electrodes, where the photoresponsivity in the short-wavelength region was significantly improved compared with that of BaSi2 capped with an amorphous Si layer. X-ray photoelectron spectroscopy revealed that the Zn1-xGexOy films suppressed the oxidation of the BaSi2 surface, decreasing the carrier recombination rate. This is the first demonstration of passivation interlayers for BaSi2 with suitable band alignment for carrier transport and surface passivation effects.

Single-step process for transparent conductive ZnO:Ga films with uniform ultrahigh haze by oblique angle deposition

Increasing the haze of front electrodes of solar cells while retaining high optical transmittance is beneficial for increasing the power conversion efficiency. However, conventional methods of fabricating hazy films require additional etching steps or materials. Moreover, depositing large-area transparent conductive oxides with uniform ultrahigh haze is challenging. Here, we combine the oblique angle deposition and the atmospheric pressure plasma jet to produce GZO with a uniform ultrahigh haze of > 80% (non-uniformity ~1.35%), high transmittance of 88% (referenced to the substrate), and resistivity of 1.96 × 10–3 Ω cm in a single step. We show that the high haze is caused by the “pre-deposition” of adsorbed particles on the bare substrate downstream and that the upstream dummy area should be avoided for high uniformity. Unlike existing methods, our method produces uniform films with ultrahigh haze and good transmittance in a single step without additional etching/ materials or changing parameters during operation.

Topology optimization of the front electrode patterns of solar cells based on moving wide Bezier curves with constrained end

Topology optimization of the front electrode patterns of solar cells based on moving wide Bezier curves with constrained end

High haze Ga and Zr co-doped zinc oxide transparent electrodes for photovoltaic applications

Creating textured front electrodes to improve optical path length is promising to enhance the power conversion efficiency of solar cells. Deposition of transparent conductive oxides with textured surfaces usually requires additional processing steps, such as etching and coating nanoparticles. However, this makes the process complicated and inefficient. We demonstrate a one-step fabrication process to deposit high haze gallium and zirconium co-doped zinc oxides (GZO:Zr) prepared by atmospheric pressure plasma jets. GZO:Zr (2 at%) films achieve a low resistivity (7.88 × 10–4 Ω cm), a high haze (34.8%), and a great FoM (8.22 × 10−3 Ω−1). The haze factor increases from 7.19% to 34.8% (+384%) when 2% Zr is doped into GZO films. Such an enhancement of haze is attributed to the increased surface roughness and deterioration of crystallinity. AFM results show that roughness increases from 18.4 to 122 nm after 2% Zr is doped. SEM images show that spherical particles appeared on the film surface when Zr was doped into GZO films. Unlike conventional methods, our method produces hazy transparent electrodes in one step without changing any operational parameters and is suitable for industrial-scale mass production. Our findings pave the way for new applications of co-doping in transparent conductive oxides.

Role of front and back contacts in the performance of TiO2/CuO heterojunction solar cells

In this work, the effects of front and back contact work functions on the performance of a TiO2/CuO heterojunction solar cell were studied using numerical simulations. Our findings revealed a high device efficiency of ∼ 18 %, and VOC ∼ 0.83 V can be obtained when the work function values of the front transparent conducting oxide (TCO) electrode are below 4.6 eV, provided the metal at the back contact has a work function above 5.1 eV. The underlying cause of this observation can be explained by the carrier concentration profiles across the device, which indicate the formation of a higher potential barrier for the majority carriers at front contact/emitter interface, for front contact work functions (ϕFC) above 4.6 eV, due to which relatively lesser electrons are collected at the electrode. Compared to this, a lower potential barrier for the majority carriers develops when ϕFC < 4.6 eV, implying higher electron collection at the front electrode. At the absorber/back contact interface, formation of a higher potential barrier for the holes impedes their collection when the metal back contact with work function values below 5.1 eV.

Investigation of In2O3:SnO2 films with different doping ratio and application as transparent conducting electrode in silicon heterojunction solar cell

Transparent conducting oxide films were performed from SnO 2 -doped In 2 O 3 with different doping ratio of 97:3 and 95:5 (ITO) by pulsed DC magnetron sputtering in argon and oxygen atmosphere. The effect of oxygen content on the structural, electrical and optical properties of films was investigated. Proper oxygen content improved the properties of ITO films. All films showed the polycrystalline structures. The optimal Hall mobility and resistivity reached 32.2 cm 2 /V·S and 4.5 × 10 −4 Ω cm, respectively. The average transmittance in the 400–1100 nm wavelength range was above 88 %. The optimized ITO films with Lightly and heavily doped ratio are appropriately chosen as front and back electrode of silicon heterojunction solar cells respectively, and the conversion efficiency of more than 24 % were obtained. • Different Sn-doped In 2 O 3 transparent conductive films (ITO) were investigated by magnetron sputtering. • Lightly and heavily doped ITO films present good transmittance and electrical properties respectively. • Lightly and heavily doped ITO films were used appropriately as the front and back electrodes respectively. • Efficiency of the copper electroplating silicon heterojunction solar cell can reach 24.34 %.

Evaluation method of the light-trapping structure for a transparent thin-film silicon solar cell with low-illuminance condition

Transparent hydrogenated amorphous silicon (a-Si:H) thin-film solar cells were fabricated for building-integrated photovoltaic (BIPV) windows, and light-trapping structures were formed on aluminum-doped zinc oxide (ZnO:Al) front electrodes to increase the output current and power under low-irradiance conditions in buildings. The degree of light-trapping was controlled by varying the etching time for texturing the ZnO:Al electrodes. As the etching time increased from 5 to 20 s, the surface area and average haze increased from 102.02 to 116.27 μm2, and 3.4 to 14.6% in the absorption wavelength range of 400–600 nm, respectively. The introduction of the light-trapping structure can increase the photogenerated current but causes defects in the cell. The current and defect density both depend on the light intensity; the highest power conversion efficiency of the cell was obtained in different light-trapping structures depended on the light intensity. To evaluate the light-trapping effect independent of the light intensity, we theoretically derived the effective absorbance and current density loss parameters using rate equations, which were in good agreement with the measurements. The presented methods can be applied to various thin-film solar cells requiring light-trapping structures to effectively utilize low-illuminance conditions.

Facile preparation of hazy Ga-doped ZnO electrodes by atmospheric pressure plasma jet

Increasing the optical path length without degrading optoelectronic properties of front electrodes of solar cells is promising to enhance the power conversion efficiency. This can be achieved by increasing the haze (H) of the electrode through texturing, while retaining high transmittance (Tt) and low sheet resistance (Rs). However, it is challenging to balance these properties simultaneously since they are mutually influenced. Here, we present a facile, low-cost, and single-run method of preparing gallium-doped zinc oxide (GZO) with high H (>25%), high Tt (>90% in the visible range, referenced to glass), and low Rs (6.62 Ω sq−1) using atmospheric pressure plasma jet. First, we increase the nozzle-substrate distance, or working distance (WD), to deposit a thin, rough GZO as a haze enhance (HE) layer and immediately deposit a standard GZO layer using the same apparatus. By tailoring the roughness of the HE layer using WD and scanning pitch, we successfully produce high-quality hazy GZO that have competitive figures of merit of 6.3 × 10−2 Ω−1 (referenced to the glass) and 2.1 × 10−2 Ω−1 (overall), and H = 25%. Also, unlike conventional methods, our approach does not require vacuum, etching, additional materials, or change of machine/tooling, which can significantly reduce the operational complexity.

Thermal Stability of the Copper and the AZO Layer on Textured Silicon

Transparent conductive oxide (TCO) film is the most widely used front electrode in silicon heterojunction (SHJ) solar cells. A copper metallization scheme can be applied to the SHJ process. The abundance of zinc in the earth’s crust makes aluminum-doped zinc oxide (AZO) an attractive low-cost substitute for indium-based TCOs. No work has focused on the properties of the copper and AZO layers on the textured silicon for solar cells. This work deposited an aluminum-doped zinc oxide layer and copper metal layer on textured (001) silicon by a sputtering to form Cu/AZO/Si stacks. The structures of Cu/AZO/Si are characterized by scanning electron microscope (SEM), scanning transmission electron microscope (STEM), and energy-dispersive X-ray spectrometer (EDS). The results show that the copper thin film detached from AZO in the valley of the textured silicon substrate at a temperature of 400 °C. Additionally, the gap between the copper and AZO layers increases as temperature increases, and the 65 nm thickness AZO layer was found to be preserved up to 800 °C.

Fabrication and characterization of sputtered Mg and F co-doped ZnO thin films with different substrate temperature for silicon thin-film solar cell applications

Transparent conducting oxide thin films have been extensively studied for various optoelectronic devices. In this paper, we investigated the influence of substrate temperature on structure, morphology, electrical and optical properties of Mg and F co-doped ZnO (MFZO) thin films. The MFZO films were deposited on glass substrates by radio frequency magnetron sputtering at a temperature ranging from room temperature to 200 °C. Experimental results indicate that small amounts of Mg (3 at%) and F (6 at%) co-doping does not change the (002) oriented hexagonal wurtzite structure of ZnO, but it can improve the crystallinity and optoelectronic properties by increasing the substrate temperature. For all MFZO films, the average transmittance in the visible region exceeds 91%. As the substrate temperature rises to 200 °C, the electrical resistivity of the film drops to 1.18 × 10−3 Ω cm. The high figure of merit (FOM) of the film deposited at 200 °C, 1.36 × 10−2 Ω−1 or 2.96 × 104 Ω−1 cm−1, makes it a transparent conductor of choice for photovoltaic applications. By etching with a dilute HCl solution, the surface morphology of the as-deposited MFZO film was modified to a crater-like texture, resulting in a significant increase in the average haze in the visible region from 4.5% to 52.3%. By employing the developed high-FOM and high-haze MFZO film as the front electrode of a-Si:H thin-film solar cells, a 30% increase in conversion efficiency was achieved under AM 1.5 G illumination.

Optimized front TCO and metal grid electrode for module‐integrated perovskite–silicon tandem solar cells

AbstractTo unlock the full potential of perovskite–silicon tandem solar cells with &gt;30% efficiency at presumably low cost, the transparent conductive oxides (TCOs) and metal grid at the front side need to be adapted compared to classical silicon heterojunction (SHJ) solar cells. By means of optical and electrical modelling, we consider the main aspects to optimize the front electrode for the tandem case, where in contrast to silicon single junction devices, there are (i) different optical properties including a lower refractive index of the perovskite absorber (ii) about half the current, thus quarter the resistive power losses for the same series resistance contribution (iii) lateral transport at the front needs to be provided solely by the front TCO layer and (iv) lower thermal stability of the perovskite, which affects TCO deposition conditions and results in a less efficient sintering of the silver screen printing pastes. This study concludes that compared to silicon heterojunction cells, the thickness of the front TCO should be reduced from 75 nm to around 20 nm, resulting in less parasitic absorption and a potential cost reduction of 1.46 €ct/cell for ITO. We investigate the impact of different front metallization including plating and silver screen printing and showcase that for multi‐wire interconnection concepts, the number of wires can be reduced from 18 wires to 9 or even less depending on the front metallization. Finally, we give an outlook on the silver consumption and levelized cost of electricity.

High‐Performance Si Photocathode Enabled by Spatial Decoupling Multifunctional Layers for Water Splitting

AbstractSpatial decoupling of light absorption and catalytic reaction is a promising approach to improve the efficiency and stability of Si photoelectrodes. Herein, patterned Ag dots (PADs) are fabricated as the front electrode on a glass layer on commercial SiNx‐coated monocrystalline p‐n+ Si by an industrial manufacture strategy. It is found that the electron tunneling glass layer offers excellent protection to Si photocathode in an acidic electrolyte (0.5 M H2SO4). The SiNx layer for antireflection and surface passivation also resists the corrosive electrolyte. Under 1 sun (AM 1.5G) illumination, PADs‐decorated photocathode with a 0.75 mm dot‐spacing exhibits a saturation photocurrent of 36.1 mA cm−2 and a photovoltage of 0.61 V when Pt is electrodeposited as the hydrogen evolution catalyst on PADs. The applied bias photo‐to‐current conversion efficiency (ABPE) reaches 9.7%. This performance is enabled by the simultaneous optimization of light absorption and collection of photoexcited electrons. The photocurrent can remain stable for about 100 h at 0 V versus the reversible hydrogen electrode (RHE). This study identifies a new combination of multifunctional spatial‐decoupling layers that is efficient in both light absorption, transfer of photoexcited electrons, and stable in an acidic electrolyte.

High brightness silicon nanocrystal white light-emitting diode with luminance of 2060 cd/m2

High brightness Si nanocrystal white light-emitting diodes (WLED) based on differentially passivated silicon nanocrystals (SiNCs) are reported. The active layer was made by mixing freestanding SiNCs with hydrogen silsesquioxane, followed by annealing at moderately high temperatures, which finally led to a continuous spectral light emission covering red, green and blue regimes. The photoluminescence quantum yield (PLQY) of the active layer was 11.4%. The SiNC WLED was composed of a front electrode, electron transfer layer, front charge confinement layer, highly luminescent active layer, rear charge confinement layer, hole transfer layer, textured p-type Si substrate and aluminum rear electrode from top to bottom. The peak luminance of the SiNC WLED achieved was 2060 cd/m2. The turn-on voltage was 3.7 V. The chromaticity of the SiNC WLED indicated white light emission that could be adjusted by changing the annealing temperature of the active layer with color temperatures ranging from 3686 to 5291 K.