Improved Surface Passivation Research Articles

Optimization of Photon and Electron Collection Efficiencies in Silicon Solar Microcells for Use in Concentration‐Based Photovoltaic Systems

AbstractUltrathin silicon solar microcells (μ‐cells) afford a means to reduce semiconductor material consumption and can be integrated with concentration optics to improve their power density. A μ‐cell design is described that optimizes electron and photon collection with enhanced efficiency for solar concentration applications. An interdigitated back contact (IBC) design improves carrier collection partially due to larger contact coverage while further enabling optimization of the μ‐cell front surface. To do so, a silicon nitride antireflection thin film coating is utilized to enhance photon absorption and improve surface passivation. Performance of IBC μ‐cells is compared to an optimized top contact design and improves μ‐cell conversion efficiencies from 9.9 to 13.7%. Improvements at 1 Sun are amplified under concentration and increase power densities at 20 Suns to 346 from 192 mW cm−2 due to minimized series resistance. Benefits afforded by IBC μ‐cells are exemplified following their integration into a dual concentrator system, affording photon collection capacities for direct and diffuse irradiance. A traditional lens concentrates direct light while a luminescent solar concentrator (LSC) collects diffuse photons otherwise not utilized by passive optics. Addition of the LSC increases maximum power densities on clear and cloudy days, providing concentration for the latter and further increasing the power density.

ESCORTing proteins directly from whole cell-lysate for single-molecule studies

We have developed a method for Enzymatic Sortase-assisted Covalent Orientation-specific Restraint Tethering (ESCORT) recombinant proteins onto surfaces directly from cell-lysate. With an improved surface passivation method, we obviate the cumbersome purification steps even for single molecule studies that demand high purity in the sample. We demonstrated high-specificity of the method, high-passivity of the surface and uncompromised functional integrity of anchored proteins using single molecule fluorescence and force-mapping. We anticipate that this method will substantially reduce the investment by way of time, money and energy in the area of single molecule studies.

Highly efficient air-stable colloidal quantum dot solar cells by improved surface trap passivation

While the power conversion efficiency (PCE) of colloidal quantum dot (CQD) solar cells can reach > 10%, the major obstacle for charge extraction and energy loss in such devices is the presence of surface trap sites within CQDs. In this work, highly trap-passivated PbS CQDs were developed using a novel iodide based ligand, 1-propyl-2,3-dimethylimidazolium iodide (PDMII). We examined the effects of PDMII on the surface quality of PbS-CQDs and compared them with TBAI, which is the best-selling iodide based ligand. By using PDMII, improved surface passivation with reduced sub-bandgap trap-states compared to TBAI was achieved. The reduced trap density resulted in enhanced charge extraction with diminished energy loss (0.447eV) in the devices. Solar cell devices using our PDMII based CQDs displayed high PCE and air stability. The certified PCE of our PDMII based devices reached 10.89% and was maintained at 90% after 210 days of air storage.

Compound Quantum Dot-Perovskite Optical Absorbers on Graphene Enhancing Short-Wave Infrared Photodetection.

Colloidal quantum dots (QDs) combined with a graphene charge transducer promise to provide a photoconducting platform with high quantum efficiency and large intrinsic gain, yet compatible with cost-efficient polymer substrates. The response time in these devices is limited, however, and fast switching is only possible by sacrificing the high sensitivity. Furthermore, tuning the QD size toward infrared absorption using conventional organic capping ligands progressively reduces the device performance characteristics. Here we demonstrate methods to couple large QDs (>6 nm in diameter) with organometal halide perovskites, enabling hybrid graphene phototransistor arrays on plastic foils that simultaneously exhibit a specific detectivity of 5 × 1012 Jones and high video-frame-rate performance. PbI2 and CH3NH3I co-mediated ligand exchange in PbS QDs improves surface passivation and facilitates electronic transport, yielding faster charge recovery, whereas PbS QDs embedded into a CH3NH3PbI3 matrix produce spatially separated photocarriers leading to large gain.

PH-dependent optical properties of N-acetyl-L-cysteine-capped ZnSe(S) nanocrystals with intense/stable emissions

In the present study, a series of aqueous-based ZnSe(S) nanocrystals (NCs) was prepared at different solution pH ranging from 8 to 11.9, and using N-acetyl-L-cysteine (NAC) as capping agent. In addition to zinc blende structure, the X-ray diffraction studies demonstrated the quantum size regime of the ZnSe(S) NCs. To gain further insight toward the influence of the quantum confinement and pH values on optical properties of the as-prepared NCs, their UV-visible absorption and photoluminescence spectra were systematically analyzed. The absorption spectra experienced a red shift from ~340 to ~382 nm as the pH increased from 8.0 to 11.9, indicating the growth of the as-prepared ZnSe(S) NCs. The emission spectra also show the obvious red shift and the relative area of excitonic to trap emission, firstly increases from pH = 8.0 to 10.7, and then decreases by further increasing of the solution pH. The initial behavior might be due to the improved surface passivation of the trap dangling states by better deprotonation of thiol groups in NAC, whereas at pH >10.7, the faster growth rate of the ZnSe(s) NCs may lead to the formation of many defect sites. All of these phenomena were combined in the scheme which displays the effect of quantum confinement and solution pH on variation of the excitonic and trap-related emissions.

Impact of thermal processes on multi-crystalline silicon

Fabrication of modern multi-crystalline silicon solar cells involves multiple processes that are thermally intensive. These include emitter diffusion, thermal oxidation and firing of the metal contacts. This paper illustrates the variation and potential effects upon recombination in the wafers due to these thermal processes. The use of light emitter diffusions more compatible with selective emitter designs had a more detrimental effect on the bulk lifetime of the silicon than that of heavier diffusions compatible with a homogenous emitter design and screen-printed contacts. This was primarily due to a reduced effectiveness of gettering for the light emitter. This reduction in lifetime could be mitigated through the use of a dedicated gettering process applied before emitter diffusion. Thermal oxidations could greatly improve surface passivation in the intragrain regions, with the higher temperatures yielding the highest quality surface passivation. However, the higher temperatures also led to an increase in bulk recombination either due to a reduced effectiveness of gettering, or due to the presence of a thicker oxide layer, which may interrupt hydrogen passivation. The effects of fast firing were separated into thermal effects and hydrogenation effects. While hydrogen can passivate defects hence improving the performance, thermal effects during fast firing can dissolve precipitating impurities such as iron or de-getter impurities hence lower the performance, leading to a poisoning of the intra-grain regions.

Improved Glass Surface Passivation for Single-Molecule Nanoarrays.

Single-molecule fluorescence techniques provide a critical tool for probing biomolecular and cellular interactions with unprecedented resolution and precision. Unfortunately, many of these techniques are hindered by a common problem, namely, the nonspecific adsorption of target biomolecules. This issue is mostly addressed by passivating the glass surfaces with a poly(ethylene glycol) (PEG) brush. This is effective only at low concentrations of the probe molecule because there are defects inherent to polymer brushes formed on glass coverslips due to the presence of surface impurities. Tween-20, a detergent, is a promising alternative that can improve surface passivation, but it is incompatible with living cells, and it also possesses limited selectivity for glass background over metallic nanoparticles, which are frequently used as anchors for the probe molecules. To address these issues, we have developed a more versatile method to improve the PEG passivation. A thin film of hydrogen silsesquioxane (HSQ) is spin-coated and thermally cured on glass coverslips in order to cover the surface impurities. This minimizes the formation of PEG defects and reduces nonspecific adsorption, resulting in an improvement comparable to Tween-20 treatment. This approach was applied to single-molecule nanoarrays of streptavidin bound to AuPd nanodots patterned by e-beam lithography (EBL). The fluorescence signal to background ratio (SBR) on HSQ-coated glass was improved by ∼4-fold as compared to PEG directly on glass. This improvement enables direct imaging of ordered arrays of single molecules anchored to lithographically patterned arrays of metallic nanodots.

Development of High-efficiency Industrial p-type Multi-crystalline PERC Solar Cells with Efficiency Greater Than 21%

In recent years, rather fine-grained multicrystalline Si material seems to have outperformed most quasi-mono material in industrial production. We have developed an industrial process for PERC cells, specifically suitable for 156×156 mm2 fine-grained multicrystalline p-type wafers, and we reached cell efficiencies of up to 21.25% with a Voc of 667.8mV, which seems to be the highest reported efficiency and Voc for industrial p-type mc silicon solar cells. In this work, we describe recent changes to our process sequence, adjust a measurement method to low saturation currents, and give a detailed loss analysis with the help of numerical device simulation. The improved surface passivation and optimized doping profile have reduced the emitter saturation current to 25.6 fA/cm2, which is the main contribution to the cell efficiency improvements. Particular attention is paid to fill factor losses due to recombination rate saturation in the base. But Voc is still dominated by recombination at the rear contacts. Accordingly, improvement strategies need be adjusted.

Effect of Gradient Alloying on Photoluminescence Blinking of Single CdSxSe1–x Nanocrystals

Alloyed CdSxSe1–x semiconductor nanocrystals (NCs) were obtained from a one-pot synthesis at reduced temperature with moderate quantum yield. Comprehensive structural characterizations of the CdSSe NCs reveal that the NCs have gradient alloyed structure, with Cd evenly distributed over the entire NC, Se rich in the center, and S rich in the outer region. This is due to the difference in the nucleation kinetics of S and Se precursors. Optical studies at the single NC level show that the NCs have reduced photoluminescence blinking, increased “on” time fraction, and good photostability, in comparison with CdSe NCs. The incorporation of sulfur composition in the alloy NCs improves surface passivation and in turn protects the NCs from (photo)oxidation. The gradual change in the NC composition from center to outer regions creates a smooth “interface”, compared to core/shell NCs. These factors lead to reduced nonradiative rates in the NCs, improving their emitting properties.

Fundamental understanding, impact, and removal of boron-rich layer on n-type silicon solar cells

Most boron diffusion technologies result in the formation of an undesirable boron-rich layer (BRL) on the emitter surface. This paper reports on a study of the impact of gradual etching of the BRL on n-type silicon solar cell performance. It is found that gradual removal of the BRL improves surface passivation and bulk lifetime in the finished cell, while over-etching of the BRL results in a sharp decrease in fill factor due to the increased n-factor and series resistance. It is shown that the optimum chemical etching of the BRL formed as a byproduct of the screen-printed boron emitter diffusion used in this study raised the cell efficiency by ~0.5%, resulting in 20.0% efficient large area (239cm2) n-type solar cells. The change in BRL thickness and morphology as a function of chemical etching time was investigated by TEM and AES measurements to explain the quantitative impact of BRL removal on cell performance.

Influence of post-hydrogenation upon electrical, optical and structural properties of hydrogen-less sputter-deposited amorphous silicon

Amorphous silicon (a-Si) is common in the production of technical devices and can be deposited by several techniques. In this study intrinsic and doped, hydrogen-less amorphous silicon films are RF magnetron sputter deposited and post-hydrogenated in a remote hydrogen plasma reactor at a temperature of 370°C. Secondary ion mass spectrometry of a boron doped (p) a-Si layer shows that the concentration of dopants in the sputtered layer becomes the same as present in the sputter-target. Improved surface passivation of phosphorous doped 5Ωcm, FZ, (n) c-Si can be achieved by post-hydrogenation yielding a minority carrier lifetime of ~360μs finding an optimum for ~40nm thin films, deposited at 325°C. This relatively low minority carrier lifetime indicates high disorder of the hydrogen-less sputter deposited amorphous network. Post-hydrogenation leads to a decrease of the number of localized states within the band gap. Optical band gaps (Taucs gab as well as E04) can be determined to ~1.88eV after post-hydrogenation. High resolution transmission electron microscopy and optical Raman investigations show that the sputtered layers are amorphous and stay like this during post-hydrogenation. As a consequence of the missing hydrogen during deposition, sputtered a-Si forms a rough surface compared to CVD a-Si. Atomic force microscopy points out that the roughness decreases by up to 25% during post-hydrogenation. Nuclear resonant reaction analysis permits the investigation of hydrogen depth profiles and allows determining the diffusion coefficients of several post-hydrogenated samples from of a model developed within this work. A dependency of diffusion coefficients on the duration of post-hydrogenation indicates trapping diffusion as the main diffusion mechanism. Additional Fourier transform infrared spectroscopy measurements show that hardly any interstitial hydrogen exists in the post-hydrogenated a-Si layers. The results of this study open the way for further hydrogen diffusion experiments which require an initially unhydrogenated drain layer.

Colloidal nanocomposite of reduced graphene oxide and quantum dots for enhanced surface passivation in optoelectronic applications

Colloidal graphene/PbS quantum dots (QDs) hybrid nanostructures are fabricated with chemical grafting in one-pot solution methods. In the hybrid nanocomposites, PbS QDs are decorated on the reduced graphene oxide (rGO) nanosheets (NSs). By employing X-ray photoelectron spectroscopy (XPS) analysis, it is shown that the rGO NSs are bonded to PbS nanocrystals through oxygen functional groups, leading to improved surface passivation and electrical conductivity in the hybrids. The results obtained by the recordings of time-resolved photoluminescence spectra, field effect mobility, and photovoltaic performance revealed that rGO grafted to PbS QD composite structures provided better charge transport by 16 times compared to PbS alone, which is attributed to suppressed charge recombination and improved interfacial charge transport processes. Thus, the developed hybrid photoactive film enhanced open circuit current (Jsc) and power conversion efficiency (PCE) by 12% and 14% correspondingly.

Grain Boundary Passivation in Multicrystalline Silicon Using Hydrogen Sulfide

A new grain boundary passivation method for multicrystalline silicon using hydrogen sulfide has been developed in this work. It has the added benefit of both hydrogen and sulfur for grain boundary passivation. Minority carrier lifetime of the samples is measured to monitor the effect of passivation. It is found that sulfur passivation takes place at higher temperatures, ∼100°C higher, than hydrogen passivation, and sulfur passivation results in much higher lifetime gains than hydrogen passivation. Post-annealing in ambient further improves the lifetime of the samples, which is attributed to improved surface passivation on the p-type silicon samples by aluminum oxide. The highest lifetime gain achieved after post-annealing is 6750% over the control sample for hydrogen sulfide annealed samples vs. ∼2400% for forming gas annealed samples. Post-annealing also improves the stability of the passivation.

Effect of laser damage etching on i-PERC solar cells

Passivated emitter and rear cells (PERC) are considered to be the next-generation silicon solar cell technology because of their significant efficiency gain originating from improved surface passivation and optical reflectivity. However, the complex fabrication process of PERC solar cells is a major hindrance to its mass production. Lasers are widely used for solar cell fabrication because of their potentially high throughput and selectivity. Laser ablation can, however, cause unintended laser-induced damage such as surface melting, heat affected zone, microcracks, point defects etc., which can negatively affect the solar cell's performance. Also, the damage can be amplified depending on the laser's operating condition. This work focuses on the effect of laser-induced damage removal by chemical etching, regardless of laser specification condition, on the silicon substrate after laser ablation. The laser ablated and etched surface were characterized by an optical microscope, electrochemical capacitance voltage (ECV) profiling and quasi-steady-state photoconductance (QSSPC). An industrial-type PERC (i-PERC) cell was fabricated after optimization of the chemical-etching process and the cell performance was evaluated. A maximum efficiency of 19.73% was obtained on a 156 mm × 156 mm crystalline-silicon solar cell. In addition, damage etching was found to have a significant effect on the increase of the shunt resistance (Rsh) and i-Voc.

An improved surface passivation method for single-molecule studies.

We herein report a surface passivation method for in vitro single-molecule studies, which more efficiently prevents non-specific binding of biomolecules as compared to the polyethylene glycol surface. The new surface does not perturb the behavior and activities of tethered biomolecules. It can also be used for single-molecule imaging in the presence of high concentrations of labeled species in solution. Reduction in preparation time and cost is another major advantage.

Guidelines and limitations for the design of high-efficiency InGaN single-junction solar cells

Astract Indium gallium nitride (InGaN) alloys offer great potential for high-efficiency photovoltaics, yet theoretical promise has not been experimentally demonstrated. Several major challenges remain including polarization effects, suitable p-type doping, improved surface passivation, and growth of thick, high-quality InGaN layers. In this paper, we present numerical simulations of InGaN p-i-n single-junction solar cells to provide guidelines for performance improvement through optimization of device structures given achievable material characteristics. The performance of both InGaN/GaN heterojunction devices that are presently achievable and InGaN homojunction solar cells that should be feasible in the future are investigated through the calculation of characteristic parameters: short-circuit current density, open-circuit voltage, and conversion efficiency. These simulations study the effect of indium content, thickness, and background doping of the unintentionally-doped InGaN absorbing layer on the performance of both InGaN solar-cell designs. While the maximum efficiency of a p-i-n InGaN/GaN heterojunction solar cell with low indium composition is 11.3%, the conversion efficiency of heterojunction devices with high indium composition needed for longer wavelength absorption drastically reduces because of polarization effects. Above an indium composition of 45%, the modeled heterojunction devices do not operate as solar cells. Using presently achievable values of minority carrier lifetime and surface recombination velocity, the maximum efficiency of InGaN single-homojunction solar cells with optimized parameters is ~17%, which is significantly smaller than the theoretical maximum energy-conversion efficiency for single-junction cells.

Surface passivation of multicrystalline silicon wafers by porous silicon combined with an ultrathin nanoparticles aluminum coating film

In this paper, we demonstrate that we may efficiently improve surface passivation of multi-crystalline silicon (mc-Si) while combining formation of porous silicon (PS) and deposition of ultrathin aluminum (Al) film. Aluminum Nanoparticles were deposited by thermal evaporation onto PS formed on mc-Si wafers. Optoelectronic properties of Al/PS/mc-Si and Al/mc-Si treated samples were investigated before and after annealing in the 400–700 °C temperature range. The surface passivation effectiveness was pointed out based on minority carrier lifetime and photoluminescence measurements. It was found that, at a minority carrier density Δn = 1015 cm−3, the effective minority carrier lifetime increases from 1.5 μs (for the bare mc-Si wafer) to about 6 and 14 μs before and after thermal annealing, respectively. FTIR analyses show strong correlation between the minority carrier lifetime values and hydrogen and Al passivation. Major beneficial effect of the co-presence of Al and Al–O on the optoelectronic properties is also demonstrated. The reflectivity of Al/PS treated mc-Si decrease significantly at 500 nm as compared to untreated mc-Si (from 31 % for untreated mc-Si wafers to 8 % for Al/PS treated ones), which is due to the roughly ordered structure and to the Al nanoparticles.

Organic-silicon heterojunction solar cells on n-type silicon wafers: The BackPEDOT concept

We measure saturation current densities down to J0=80fA/cm2 for organic-silicon heterojunctions with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as an organic layer. This remarkably low J0 value corresponds to implied open-circuit voltages around 690mV, demonstrating the high-efficiency potential of this novel junction type. However, experimentally realized organic-silicon heterojunction solar cells showed relatively moderate efficiencies so far, typically below 12%. We demonstrate in this study that these solar cells were limited by the fact that the organic-silicon junction was localized on the cell front, resulting in a significant parasitic light absorption within the PEDOT:PSS layer. In addition, the rear surface of these front-junction solar cells was either poorly passivated or not passivated at all. In this paper, we overcome these limitations by proposing a back-junction organic-silicon solar cell, the so-called “BackPEDOT” cell. We show that placing PEDOT:PSS on the rear side instead of the front surface avoids parasitic light absorption within the PEDOT:PSS and allows for an improved surface passivation. We fabricate and characterize BackPEDOT solar cells and achieve very high open-circuit voltages of up to 663mV and short-circuit current densities of up to 39.7mA/cm2. Despite the relatively high series resistances of our first BackPEDOT cells, we achieve an energy conversion efficiency of 17.4%. The measured pseudo efficiency of the best cell of 21.2% suggests that our novel BackPEDOT cell concept is indeed suitable for easy-to-fabricate high-efficiency solar cells after some further optimization to reduce the contact resistance between the PEDOT and the n-type silicon wafer. Based on realistic assumptions we conclude that Back PEDOT cells have an efficiency potential exceeding 22%.

Wet-chemical Treatment for Improved Surface Passivation of Textured Silicon Heterojunction Solar Cells

Silicon heterojunction (SHJ) solar cells constantly gain more attention due to their low cost and relatively high efficiency. An important aspect of these solar cells is the incorporation of intrinsic hydrogenated amorphous silicon (a-Si:H) layers at each side of the c-Si wafer, which has increased the efficiency potential due to the excellent surface passivation. By applying a randomly textured instead of a double-side polished wafer, optical enhancement is achieved resulting in significant reflection reduction and high short-circuit current densities (Jsc). However, texturing-induced defects lead to an a-Si:H/c-Si interface with increased recombination, which limits the open circuit voltage (Voc) of the SHJ device after using the same cleaning treatment as for the flat wafer. Thus, a one-to-one transfer of process parameters from flat to textured c-Si substrate is not necessarily appropriate and a different wet-chemical treatment is needed. In this work, a chemical treatment is demonstrated, which leads to an improved surface passivation.

Process Control of Reactive Sputter Deposition of AlO $_{x}$ and Improved Surface Passivation of Crystalline Silicon

In this paper, we investigate the relationship between the deposition-process parameters of reactively sputtered aluminium oxide films and the passivation of silicon surfaces. A method of tuning the deposition process has been established that results in a reduced level of surface recombination, where surface recombination velocities as low as 8.5 cm/s have been achieved on 1 Ω·cm n-type monocrystalline silicon. We find that in order to achieve good surface passivation, the deposition need to be conducted at low power density and at high deposition pressure. We have found that effective passivation is achieved when a sputtering target is close to being fully oxidized-indicated by deposition rate - likely resulting in films that are less aluminium rich. Additionally, Fourier-transform-infrared spectroscopy measurements were used for film characterization; the correlation between effective lifetime and the integrated absorption of all Al and O related bonds shows that films with lower absorption in the wavenumber range 500-1060 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> result in better passivation.