Front Electrode Research Articles

(Invited) Electrochemistry for Sustainable Solar Photovoltaics

Commercial solar photovoltaic technologies such as Si and CdTe are traditionally considered to be in a separate domain from electrochemistry. Their device operation is governed by semiconductor physics and their production involves non-electrochemical processes such as diffusion, screen printing, fractional distillation, etc. However, electrochemistry will likely play an indispensable role in sustaining the commercial solar technologies. This talk will discuss three roadblocks to sustainable solar photovoltaics and how electrochemistry can remove these roadblocks: 1) storage of intermittent solar electricity, 2) scarce Ag used in today’s Si solar cells, and 3) high energy input in producing solar Si wafers. An off-grid route is proposed for solar electricity storage based on a closed loop of Zn↔ZnO [1], in which Zn rods are produced in a solar electrolyzer from ZnO. The Zn rods are shipped to homes, offices, factories, and electric vehicles to be inserted into mechanically-recharged Zn/air batteries, for electricity on demand. The spent Zn anodes are collected for regeneration of Zn in the solar electrolyzer. This Zn↔ZnO loop is advantageous over the H2↔H2O loop in terms of theoretical performance and technical readiness. Electroplated Al on Si is proposed to replace the screen-printed Ag electrode in Si solar cells [2]. 18% efficiency has been demonstrated in a Si solar cell with an electroplated Al front electrode and a screen-printed Al back electrode, i.e., an Ag-free all-Al Si solar cell. To overcome the high resistivity of the solar Si wafer, direct Al plating on Si without any seed layer is developed through light-induced Al plating. Direct Al plating on Si drastically simplifies the metallization process for Al, resulting in a significantly-lower cost than competing technologies. Finally, electrochemical refining of metallurgical-grade Si for solar-grade Si has been unsuccessful. While metals up to 99.99% purity are readily produced by electrolysis, solar-grade Si requires a purity of at least 99.99999%. This is coupled with non-electrochemical difficulties in Si purification such as oxidation of the Si anode and crystallinity of the Si cathode. An analysis will be presented on ultrapure materials by electrolysis [3]. The reason for unsuccessful Si electrochemical refining will be discussed through an analogy between electrochemical refining and fractional distillation. A revised method for electrochemical refining of Si is proposed. [1] M. Tao, A Zn↔ZnO loop for terawatt-scale storage of solar electricity, 43rd IEEE Photovoltaic Specialist Conference (Portland, OR, 2016), p. 2011[2] 1. L. Wang, W.-H. Huang, W. J. Shin, M. Tao, B. Deng, and D. Wang, Light-induced plating of aluminum on silicon in a Lewis acidic chronoaluminate ionic liquid, Journal of the Electrochemical Society, 165, D381 (2018)[3] M. Tao, Impurity segregation in electrochemical processes and its application to electrorefining of ultrapure silicon, Electrochimica Acta, 89, 688 (2013)

Analysis of the Bowing Phenomenon for Thin c-Si Solar Cells using Partially Processed c-Si Solar Cells

The silicon wafers for solar cells on which the paste is deposited experience a bowing phenomenon. The thickness of commonly used c-Si wafers is 180 μm or more. When fabricating c-Si solar cells with this wafer thickness, the bowing value is 3 mm or less and the problem does not occur. However, for the thin c-Si solar cells which are being studied recently, the output reduction due to failure during manufacture and cracking are attributed to bowing. In generally, it is known that the bowing phenomenon arises mainly from the paste applied to the back side electrode of c-Si solar cells and the effects of SiNx (silicon nitride) and the paste on the front side are not considered significant. The bowing phenomenon is caused by a difference in the coefficient of expansion between heterogeneous materials, there is the effect of bowing on the front electrode and ARC. In this paper, a partially processed c-Si solar cell was fabricated and a bowing phenomenon variation according to the wafer thicknesses was confirmed. As a result of the experiment, the measured bow value after the firing process suggests that the paste on the front-side indicates a direction different from that of the back-side paste. The bow value increases when Al paste is deposited on SiNx. The fabricated c-Si solar cell was analyzed on basis of the correlation between the bowing phenomenon of the materials and the c-Si wafer using Stoney’s equation, which is capable of analyzing the relationship between bowing and stress. As a result, the bowing phenomenon of the c-Si solar cell estimated through the experiment that the back side electrode is the important element, but also the front electrode and ARC influence the bowing phenomenon when fabricating c-Si solar cells using thin c-Si wafers.

Efficient enhancement of light trapping in the double-textured Al doped ZnO films with nanorod and crater structures

Double-textured Al doped ZnO (AZO) films with nanorod and crater structures were prepared by the simple hydrothermal growth of AZO nanorods on the single-textured AZO films with crater structures. The light trapping capabilities, morphological, structural and optoelectronic properties of double-textured AZO films prepared with different precursor concentrations were demonstrated systematically. Remarkable surface morphology evolution of the double-textured films depending on precursor concentration was observed. Especially, the relationship between surface morphologies and light trapping capabilities of the AZO films with nanorod and crater structures was discussed. The double-textured AZO films having nanorod and crater structures prepared with 0.01 M precursor concentration exhibited low sheet resistance, high visible total transmittance and haze value, which could be used as transparent front electrodes for enhancing the light trapping in solar cells.

Realization of highly conductive and transparent GaZnO/InCdO/GaZnO films for the enhancement of solar cell efficiency

Abstracts In0.024Cd0.976O thin films were inserted between two layers of Ga0.04Zn0.96O films, in an effort to improve the electrical properties of the oxide film. The multilayer structure with an optimum InCdO layer thickness of ∼30 nm showed significantly improved electrical properties without seriously deteriorating the optical properties, with an average optical transmittance of ∼90% over the visible and the NIR range and a resistivity of 7.5 × 10−5 Ω cm. It was also observed that efficiency of the CuInGaSe solar cell devices using the Ga0.04Zn0.96O (50 nm)/In0.024Cd0.976O (30 nm)/Ga0.04Zn0.96O (50 nm) structure as the front electrode shows much higher efficiency (8.5%) than that (4.7%) of the devices with 100 nm Ga0.04Zn0.96O front electrode. This result indicates that the proposed multilayer structure would be a strong candidate for the transparent conducting electrode for the high performance optoelectronic devices.

Application of ALD-Al2O3 in CdS/CdTe Thin-Film Solar Cells

The application of thinner cadmium sulfide (CdS) window layer is a feasible approach to improve the performance of cadmium telluride (CdTe) thin film solar cells. However, the reduction of compactness and continuity of thinner CdS always deteriorates the device performance. In this work, transparent Al2O3 films with different thicknesses, deposited by using atomic layer deposition (ALD), were utilized as buffer layers between the front electrode transparent conductive oxide (TCO) and CdS layers to solve this problem, and then, thin-film solar cells with a structure of TCO/Al2O3/CdS/CdTe/BC/Ni were fabricated. The characteristics of the ALD-Al2O3 films were studied by UV–visible transmittance spectrum, Raman spectroscopy, and atomic force microscopy (AFM). The light and dark J–V performances of solar cells were also measured by specific instrumentations. The transmittance measurement conducted on the TCO/Al2O3 films verified that the transmittance of TCO/Al2O3 were comparable to that of single TCO layer, meaning that no extra absorption loss occurred when Al2O3 buffer layers were introduced into cells. Furthermore, due to the advantages of the ALD method, the ALD-Al2O3 buffer layers formed an extremely continuous and uniform coverage on the substrates to effectively fill and block the tiny leakage channels in CdS/CdTe polycrystalline films and improve the characteristics of the interface between TCO and CdS. However, as the thickness of alumina increased, the negative effects of cells were gradually exposed, especially the increase of the series resistance (Rs) and the more serious “roll-over” phenomenon. Finally, the cell conversion efficiency (η) of more than 13.0% accompanied by optimized uniformity performances was successfully achieved corresponding to the 10 nm thick ALD-Al2O3 thin film.

SnO2-Catalyzed Oxidation in High-Efficiency CdTe Solar Cells.

Interfaces at the front of superstrate CdTe-based solar cells are critical to carrier transport, recombination, and device performance, yet determination of the chemical structure of these nanoscale regions has remained elusive. This is partly due to changes that occur at the front interfaces during high temperature growth and substantive changes occurring during postdeposition processing. In addition, these buried interfaces are extremely difficult to access in a way that preserves chemical information. In this work, we use a recently developed thermomechanical cleaving technique paired with X-ray photoelectron spectroscopy to probe oxidation states at the SnO2 interface of CdTe solar cells. We show that the tin oxide front electrode promotes the formation of nanometer-scale oxides of tellurium and sulfur. Most oxidation occurs during CdCl2/O2 activation. Surprisingly, we show that relatively low-temperature anneals (180-260 °C) used to diffuse and activate copper acceptors in a doping/back contact process also cause significant changes in oxidation at the front of the cell, providing a heretofore missing aspect of how back contact processes can modify device transport, recombination, and performance. Device performance is shown to correlate with the extent of tellurium and sulfur oxidation within this nanometer-scale region. Mechanisms responsible for these beneficial effects are proposed.

Aluminum-Doped Zinc Oxide as Front Electrode for Rear Emitter Silicon Heterojunction Solar Cells with High Efficiency

Transparent conductive oxide (TCO) layers of aluminum-doped zinc oxide (ZnO:Al) were investigated as a potential replacement of indium tin oxide (ITO) for the front contact in silicon heterojunction (SHJ) solar cells in the rear emitter configuration. It was found that ZnO:Al can be tuned to yield cell performance almost at the same level as ITO with a power conversion efficiency of 22.6% and 22.8%, respectively. The main reason for the slight underperformance of ZnO:Al compared to ITO was found to be a higher contact resistivity between this material and the silver grid on the front side. An entirely indium-free SHJ solar cell, replacing the ITO on the rear side by ZnO:Al as well, reached a power conversion efficiency of 22.5%.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

18.78% hierarchical black silicon solar cells achieved with the balance of light-trapping and interfacial contact

18.78% efficiency of crystalline silicon (Si) solar cells was achieved through introducing the combined nanopore/pyramid textures. These hierarchical structures possessed the ultra-low reflectivity with values <6% under the various illumination angles from 0 to 60°, evidencing their remarkable omnidirectional light-trapping capability. Although the greatly improved light-trapping effect of the hierarchical structures was demonstrated, we found that the increased formation durations of Si nanopores led the substantial increase of photoluminescent characteristics that could limit the efficient separation of photogenerated carriers through charge recombination. Moreover, the positive correlation in surface roughness of nanopore arrays with respect to the elongated etching durations was evidenced, and these issues could increase the series resistance of solar cells owing to poor interfacial contact between nanopores and front electrodes. Thus, there existed the optimal combination between two-scale textures critically for both photonic and electrical management of cell design. By optimizing the etching durations for the controlled nanopore formation, the improved light-trapping characteristics and fairly unchanged contact resistance with front electrode was achieved, possessing the improved conversion efficiency with >0.58% of increased value beyond the typical microtexture-based solar cells.

Correlation of Different Electrical Parameters of Solar Cells with Silver Front Electrodes.

This work presents comparison results of the selected electrical parameters of silicon solar cells manufactured with silver front electrodes which were co-fired in an infrared belt furnace in the temperature range of 840–960 °C. The commercial paste (PV19B) was used for the metallization process. Electrical properties of a batch of solar cells fabricated in one cycle were investigated. Three methods were used, including measurement of the current-voltage characteristics (I-V), measurement of contacts’ resistivity using the transmission Line model method (TLM), and measurement of contacts’ resistivity using the potential difference method (PD). This work is focused on both the different metallization temperatures of co-firing of solar cells and measurements using the above-mentioned methods. It is shown that the solar cell parameters measured with three methods have different, but strongly correlated values. Moreover, the comparative analysis was performed of the investigations of the same photovoltaic solar cells using both the TLM method and independent research stands (including one non-commercial and two commercial ones) at three different scientific units. In the PD and TLM methods, the same calculation formulae are used. It can be stated, comparing methods I-V, PD, and TLM, that for each, different parameters are determined to assess the electrical properties of the solar cell.

Sol-gel derived Zn1-xMgxO:Al transparent conductive thin film and its application to thin film solar cells

Al doped zinc magnesium oxide (Zn1-xMgxO:Al) thin films with tunable optical bandgap (3.48 eV–3.66 eV) and resistivity level of 10−3 Ω·cm were prepared by sol-gel process. The effect of the Mg incorporation on the structural, electrical and optical properties was studied by analyzing the results of X-ray diffraction diffractograms, transmittance spectra, Hall measurement and X-ray photoelectron spectroscopy. Mg incorporation influences the crystal structure through reducing the c-axis lattice constant. Blueshift of absorption edge with Mg incorporation manifests the bandgap widening effect. Mg incorporation also increases the film resistivity through decreasing Hall mobility and carrier concentration. Employed as the front electrode, these Zn1-xMgxO:Al thin films improved the blue response of hydrogenated amorphous silicon solar cells through reducing the optical loss in ZnO:Al. Zn1-xMgxO:Al with x ≤ 0.13 enables an open-circuit voltage (Voc) of about 0.9 V and a fill factor (FF) of about 0.7. The Zn0.87Mg0.13O:Al empowers a gain of about 9% in short-circuit current density relative to ZnO:Al via the bandgap widening and the improved refractive-index-matching, while the Zn0.80Mg0.20O:Al deteriorates Voc and FF through increasing the downward band bending at the Zn1-xMgxO:Al/p-a-SiC:H interface. This work paves the way for applying the sol-gel derived Zn1-xMgxO:Al thin films to optoelectronic devices such as ultraviolet light detector, ultraviolet light emitting diodes and solar cells.

Effect of Carboxyl-Functionalized Single Walled Carbon Nanotubes on the Interfacial Barrier Height of Malachite Green Dye Based Organic Device

In this study, we have estimated the interfacial barrier height (Φb) of Indium Tin Oxide (ITO) coated glass/Malachite Green (MG) dye/Aluminium (Al) based organic device and subsequently we have also observed the effect of carboxylfunctionalized SWCNT (COOH-SWCNT) on the Φb. Presence of COOH-SWCNT reduces the interfacial barrier height as SWCNT acts as filler and provides easy path for charge percolation. We have used ITO coated glass and aluminium as front electrode and back electrode respectively to form the organic device. This organic device has been prepared with and without COOH-SWCNT by using spin coating technique. We have measured the steady state current-Voltage (I-V) characteristics of the device to estimate the interfacial barrier height (Φb) of the device. Φb is reduced from 0.67 eV to 0.59 eV in the presence of COOH-SWCNT. We have also estimated the Φb by using Norde’s Method. This method also shows a reduction of interfacial barrier height from 0.72 eV to 0.64 eV due to incorporation of COOH-SWCNT. Both the methods show good consistency with each other. Reduction of the interfacial barrier height in presence of COOH-SWCNT indicates the enhancement of charge injection through the metal-organic dye interface. By suitable doping or addition of COOH-SWCNT within the MG dye it is possible to reduce the barrier height and enhance the current injection through metal-organic dye interface.

Digital printing of electroluminescent devices on textile substrates

Alternating current–driven electroluminescent devices on polyester fabrics were realized using a combination of coating and printing. The PEDOT:PSS front electrode was coated onto the fabric using knife coating. All other layers were digitally printed using a specially modified three-dimensional printer and three-dimensional printing software. Slicing parameters (line distance, printing speed, printing pattern) as well as other hardware parameters and ink viscosity were evaluated for each ink to obtain a good print. Final results show a complex interaction of all investigated parameters. Fully digitally printed electroluminescent devices show a luminescence of 44 lx, but combinations of digital printing and knife coating show a much higher luminescence of up to 128 lx for samples with an even smaller luminous layer thickness.

Effectiveness of Isochronic Tones on Gamma-domain Brainwave Stimulation

The surge of electric pulses and chemical reactions in human brain produce brain waves at five distinct frequency domains between 0.5 Hz and 100 Hz. These domains are associated with different brain activities and characterize different mental states. The cerebral brain cells, which produce the frequency patterns in accordance to the brain activity, can be synchronized to or resonated at a desired frequency by externally stimulating the cells using acoustical means including binaural beats (BB) and isochronic tones (IT). IT are single frequency tones that can be easily generated and perceived by the brain whereas BB are auditory brainstem responses originating from the decomposition of two tones. The states of alertness and focus can be traced to areas within the frontal lobes of the brain at gamma range of brainwave frequencies, i. e. circa 40 Hz and above. This paper presents the preliminary investigation on the effectiveness of IT over BB on stimulating the mental states of alertness and attention on human brain at the gamma domain. A maiden group of ten healthy male human subjects are chosen for this purpose. Electroencephalograph signals of the evoked potential due to resonance of the prefrontal cortex (PFC) with the induced aural stimuli are measured. It is observed that the root-mean-square values of the potentials recorded at the anterior front electrode locations namely, AF3 and AF4, after exposition to IT increased by an average of 15% over that measured post exposition to BB. Similar observation is made at the frontal right (F4) location, which has a greater influence of the mental states mentioned above. The results indicate that IT has an improved effect than BB on the potentials measured at the PFC locations and render a positive influence on the focus and attention mental states of the brain.

A wrinkled structure with broadband and omnidirectional light-trapping abilities for improving the performance of organic solar cells with low defect density.

Fabricating thin film solar cells on the light-trapping structures is an effective way to improve the absorption of the active layer. Here, we report a non-fullerene organic solar cell based on a PBDB-T:ITIC active layer, a wrinkled metal rear electrode, and a MoO3/Ag/ZnS front transparent electrode. Optical characterization shows that the wrinkled metal structure can remarkably increase the absorption of the active layer in a broadband range. The resulting device shows a power conversion efficiency of 8.2%, which increases by 4.6% compared to that of the flat counterpart. Apart from higher absorption, the improved performance can also be ascribed to the efficient charge transport and collection in the device due to the lower defect density, larger interfacial area, and smaller active layer thickness. A device with a power conversion efficiency of 10.19% based on the flat ITO/glass substrate is also achieved. Accordingly, a power conversion efficiency of about 10.66% is predicted under the condition where the wrinkled rear electrode and the ITO front electrode are employed. In addition, the power conversion efficiency of the wrinkled device could increase by about 50.48% compared to that of the flat device under an incident angle of 60 °C, illustrating that a better omnidirectional ability is achieved.

Highly efficient sub-100-nm thermophotovoltaic cells enhanced by spectrally selective two-dimensional metasurface

Great efforts have been devoted to the investigation of selective emitters and filters for improving the conversion efficiency of thermophotovoltaic (TPV) systems. This work proposes a spectrally selective ultrathin cell with two-dimensional (2-D) metasurface for high-efficiency TPV energy conversion. The cell is composed of Ag substrate, ultrathin gallium antimonide (GaSb) photovoltaic layer, and 2-D metasurface comprising symmetric 2-D metallic grating. The spectral-normal absorptance of the proposed structure is calculated by the finite-element method and the results show significant enhancement in the photon absorption comparing to the free-standing GaSb layer with the same thickness, which is attributed to the excitation of magnetic polariton and Fabry–Perot resonance. The effects of using indium tin oxide and graphene as front electrodes on the absorptance of the proposed ultrathin cell are investigated. The TPV system efficiency is also evaluated using the proposed ultrathin cell. The results show that the TPV efficiency with the proposed ultrathin cell can approach 29.1% with a black emitter at 1500 K and can be further increased to be 35.1% using an ideal selective emitter. This work will facilitate the development of next-generation high-efficiency TPV systems.

The Role of Graphene‐Based Derivative as Interfacial Layer in Graphene/n‐Si Schottky Barrier Solar Cells

Schottky‐barrier solar cells (SBSCs) represent low‐cost candidates for photovoltaics applications. The engineering of the interface between absorber and front electrode is crucial for reducing the dark current, blocking the majority carriers injected into the electrode, and reducing surface recombination. The presence of tailored interfacial layers between the metal electrode and the semiconductor absorber can improve the cell performance. In this work, the interface of a graphene/n‐type Si SBSC by introducing a graphene‐based derivative (GBD) layer meant to reduce the Schottky‐barrier height (SBH) and ease the charge collection are engineered. The chemical vapor deposition (CVD) parameters are tuned to obtain the two graphene films with different structure and electrical properties: few‐layer graphene (FLG) working as transparent conductive electrode and GBD layer with electron‐blocking and hole‐transporting properties. Test SBSCs are fabricated to evaluate the effect of the introduction of GBD as interlayer into the FLG/n‐Si junction. The GBD layer reduces the recombination at the interface between graphene and n‐Si, and improves the external quantum efficiency (EQE) with optical bias from 50 to 60%. The FLG/GBD/n‐Si cell attains a power conversion efficiency (PCE) of ≈5%, which increase to 6.7% after a doping treatment by nitric acid vapor.

Optically rough and physically flat TCO substrate formed by coating ZnO thin film on pyramid-patterned glass substrate

An optically rough and physically flat (OR-PF) TCO substrate was developed by spin-coating Al-doped ZnO thin film on glass substrate patterned with random pyramidal morphology to enhance the light absorption in superstrate-type thin-film silicon solar cells without deteriorating the growth of photoactive layers. This substrate possesses flat surface with a root mean square roughness of 10.9 nm, high visible light transmittance of over 86%, a sheet resistance of 29.5 Ω/sq, and moderate light-scattering ability. A surface morphology of random pyramidal morphology was formed on the glass substrate by replicating the pyramidal surface texture of a ZnO thin film with the room-temperature nanoimprint technique. Employed as the front electrode of hydrogenated amorphous Si single-junction solar cells, this OR-PF substrate enabled a Voc of 0.91 eV, an FF of 0.71, a Jsc gain of approximately 8%, and an efficiency gain of approximately 20% with respect to the AZO-coated flat glass substrate. The Jsc gain is attributed to the enhancement in the light absorption of a-Si:H solar cells triggered by the diffraction behaviour of the interfacial pyramidal morphology, while the achievement of large Voc and FF is ascribed to the improvement in the growth quality of the photoactive layers. Based on the calculation results of the bidirectional scattering distribution function, this substrate empowers comparable intensity of the diffraction peaks for 600–700 nm wavelength light but scatters less light away from the specular direction with respect to the conventional pyramid-textured ZnO substrate. This OR-PF substrate would enable efficient photon management and the growth of high-quality photoactive materials simultaneously for thin-film silicon solar cells.

Copper-Based Volumetric Filler Dedicated for Ag Paste for Depositing the Front Electrodes by Printing on Solar Si Cells.

In this work we present research results on a new paste NPCuXX (where: NP—new paste, CuXX—component, XX—a modifier consisting of Ni and other important elements) based on a copper composite (CuXX) for fabrication of front electrodes in silicon solar cells. The CuXX composite is obtained by chemical processing of copper powder particles and can be used in two ways: as an additive to commercially available paste or as a base material for a new paste, NPCuXX. The CuXX offers the possibility to exchange up to 30 and 50 wt.% Ag into Cu, which significantly decreases the solar cells material costs, and therefore, the overall solar cell price. Emphasis was placed on a proper mass suitable fabrication process of the CuXX component. The NPCuXX paste has been applied both to conventional cell structures such as aluminum-back surface field (Al-BSF) and passivated emitter and rear contact (PERC), and finally solar cells with front electrodes deposited by screen-printing method were fabricated and characterized by current-voltage techniques. This paper reports the first implementation of the copper volumetric material into a screen print paste used in a high-temperature metallization process to fabricate the front contacts of Si solar cells with a highest fill factor of 77.92 and 77.69% for the abovementioned structures, respectively.

Crystalline Silicon Solar Cells With Coannealed Electron- and Hole-Selective SiC x Passivating Contacts

We present electron- and hole-selective passivating contacts based on wet-chemically grown interfacial SiO x and overlying in-situ doped silicon carbide (SiC x ) deposited by plasma-enhanced chemical vapor deposition. After annealing at 850 °C, excellent surface passivation on the p -type planar crystalline silicon wafer is obtained for both electron- and hole-selective contacts. Their potential is demonstrated at the device level by employing a simple process flow, in which the junction formation of the two polarities is achieved with a single coannealing step. Both-side-contacted patterning-free planar p -type cells with an area of 4 cm2 and screen-printed metallization reach a fill factor of 83.4% and a open-circuit voltage of 726 mV. Zirconium-doped indium oxide with excellent optoelectrical properties is used as a front electrode. The decrease in the parasitic absorption in the front electrode results in higher photogenerated current. By realizing front-side-textured and rear-side-planar p -type cells, an efficiency of up to 22.6% is achieved.