Effective Minority Carrier Diffusion Length Research Articles

Analysis of Surface Passivation and Laser Firing on Thin-Film Silicon Solar Cells Via Light-Beam Induced Current

Liquid phase crystallized silicon solar cells on glass have recently demonstrated 15.1% efficiency using a heterojunction interdigitated back contact cell architecture and an absorber thickness of 14 $\mu$ m. One of the key enabling developments was a new method to first passivate electron contact fingers with a-Si:H(i) and then locally laser fire them to maintain a low contact resistance. In this work, high resolution, light-beam induced current measurements (LBIC) were used to analyze this approach. Charge collection was observed to have increased from 0.13 ${\text{mAcm}}^{-2}$ to 0.9 ${\text{mAcm}}^{-2}$ under the electron contact which is a sevenfold increase. Using 520, 642, 932, and 1067 nm wavelengths of incident light, external quantum efficiency was mapped in regions including grain boundaries, dislocation defects, shunts, defect-free regions, and laser fired spots. Reduction of charge collection in the laser fired spots was limited to diameters of 50–20 $\mu$ m, depending on whether electrical recombination or optical losses dominated. Effective minority carrier diffusion length under the majority carrier contacts was obtained by fitting of LBIC measurements. It was observed to have improved from 20.5 $\mu$ m to 22.7–40.4 $\mu$ m and up to 89.0 $\mu$ m in the best case. Based on this, wider contact fingers and improved surface passivation at the electron contact is encouraged in the near future to achieve efficiencies $\geq$ 16%.

Carrier‐Selective Contact Based Silicon Solar Cells Processed at Room Temperature using Industrially Feasible Cz Wafers

For the broad use of solar photovoltaic devices, the device fabricated at commercially viable silicon wafers at room temperature is more preferable to harvest abundant solar energy. Silicon heterojunction solar cells at room temperature, based on carrier‐selective layers without using any specified surface passivation layer on the silicon wafer is fabricated. Industrially feasible Cz n‐type non‐textured silicon wafers having the bulk lifetime of 300 µs are used for cell fabrication. The molybdenum oxide (MoOx) and lithium fluoride (LiFx) are used as hole‐ and electron‐selective layers, respectively. The highest conversion efficiency of >15% from the simple architecture of Ag/TCO/MoOx/n‐Si/LiFx/Al is achieved. The internal quantum efficiency of ≈96% is observed in the shorter wavelength region, whereas to understand relatively less response between 800 and 1100 nm wavelength region; effective minority carrier diffusion lengths are estimated. The authors also confirm the inversion layer formation in the silicon after MoOx contact from temperature dependent capacitance‐voltage measurements, and quantify the crucial built‐in‐potential of ≈0.69 V from the cell structure due to the high work function of MoOx layer.

Potential of interdigitated back-contact silicon heterojunction solar cells for liquid phase crystallized silicon on glass with efficiency above 14%

Liquid phase crystallization of silicon (LPC-Si) on glass is a promising method to produce high quality multi-crystalline Si films with macroscopic grains. In this study, we report on recent improvements of our interdigitated back-contact silicon heterojunction contact system (IBC-SHJ), which enabled open circuit voltages as high as 661mV and efficiencies up to 14.2% using a 13µm thin n-type LPC-Si absorbers on glass. The influence of the BSF width on the cell performance is investigated both experimentally and numerically. We combine 1D optical simulations using GenPro4 and 2D electrical simulations using Sentaurus™ TCAD to determine the optical and electrical loss mechanisms in order to estimate the potential of our current LPC-Si absorbers. The simulations reveal an effective minority carrier diffusion length of 26µm and further demonstrate that a doping concentration of 4 × 1016cm−3 and a back surface field width of 60µm are optimum values to further increase cell efficiencies.

Analysis of Local Minority Carrier Diffusion Lengths in Liquid-Phase Crystallized Silicon Thin-Film Solar Cells

We develop a method to quantify the local minority carrier diffusion lengths in interdigitated back-contact solar cells having a 10- µ m-thick liquid-phase crystallized (LPC) Si absorber by light-beam induced current (LBIC) measurements. The method is verified by 2-D simulations of the LBIC signals using ASPIN3. The effective minority carrier diffusion lengths determined this way range between 33 and 44 µ m inside a grain, which proves that advanced cell concepts like an interdigitated back contact (IBC) system are well suited for the LPC absorbers. Furthermore, the method has the potential to help improve the optimization of contact system geometries, and it may be used to understand the influences of different grain orientations and improve the LPC-Si absorber fabrication process.

Internal quantum efficiency analysis of plasmonic textured silicon solar cells: surface plasmon resonance and off-resonance effects

Silver nanoparticles (Ag NPs) of various sizes and concentration were integrated on textured silicon solar cells for further confinement of incident light, generated photocurrent modifications were investigated using spectrally resolved short-circuit current measurements. Internal quantum efficiency (IQE) spectra were used for quantifying the effective minority carrier diffusion lengths (Leff) of plasmonic cells in the long wavelength region (850 < λ < 1020 nm). The Leff of an optimized plasmonic solar cell enhanced to 431 µm compared to 338 µm of the bare cell, which is due to interacting Ag NPs' scattered fields, leading to enhanced light absorption in the plasmonic cell. Despite the enhanced Leff values, the overall generated photocurrent reduced with Ag NPs which is due to the significant losses near the surface plasmon resonant region. Reduced IQE of plasmonic cells near and below the surface plasmon resonant region is due to size-dependent parasitic absorption and enhanced back scattering of Ag NPs, and a modified surface recombination process due to Ag NPs' strong near-fields.

Hydrogen Evolution from Pt/Ru-Coated p-Type WSe2 Photocathodes

Crystalline p-type WSe(2) has been grown by a chemical vapor transport method. After deposition of noble metal catalysts, p-WSe(2) photocathodes exhibited thermodynamically based photoelectrode energy-conversion efficiencies of >7% for the hydrogen evolution reaction under mildly acidic conditions, and were stable under cathodic conditions for at least 2 h in acidic as well as in alkaline electrolytes. The open circuit potentials of the photoelectrodes in contact with the H(+)/H(2) redox couple were very close to the bulk recombination/diffusion limit predicted from the Shockley diode equation. Only crystals with a prevalence of surface step edges exhibited a shift in flat-band potential as the pH was varied. Spectral response data indicated effective minority-carrier diffusion lengths of ∼1 μm, which limited the attainable photocurrent densities in the samples to ∼15 mA cm(-2) under 100 mW cm(-2) of Air Mass 1.5G illumination.

Highly Efficient Charge Separation and Collection across in Situ Doped Axial VLS-Grown Si Nanowire p–n Junctions

VLS-grown semiconductor nanowires have emerged as a viable prospect for future solar-based energy applications. In this paper, we report highly efficient charge separation and collection across in situ doped Si p-n junction nanowires with a diameter <100 nm grown in a cold wall CVD reactor. Our photoexcitation measurements indicate an internal quantum efficiency of ~50%, whereas scanning photocurrent microscopy measurements reveal effective minority carrier diffusion lengths of ~1.0 μm for electrons and 0.66 μm for holes for as-grown Si nanowires (d(NW) ≈ 65-80 nm), which are an order of magnitude larger than those previously reported for nanowires of similar diameter. Further analysis reveals that the strong suppression of surface recombination is mainly responsible for these relatively long diffusion lengths, with surface recombination velocities (S) calculated to be 2 orders of magnitude lower than found previously for as-grown nanowires, all of which used hot wall reactors. The degree of surface passivation achieved in our as-grown nanowires is comparable to or better than that achieved for nanowires in prior studies at significantly larger diameters. We suggest that the dramatically improved surface recombination velocities may result from the reduced sidewall reactions and deposition in our cold wall CVD reactor.

晶体硅太阳电池数值模拟软件及其应用

A software for the numerical simulation of crystalline silicon solar cells is developed.Photocarrier continuity equation,effective mobility,effective minority carrier diffusion length and carrier recombination on crystal boundary of multicrystalline silicon are taken into account in the physical model.The software outputs the numerical simulation of crystalline silicon solar cells by numerical results and graphics.As the simulation results close to experimental data,the software can be a good guide for the design and production of crystalline silicon solar cells.

Combination of silicon nitride and porous silicon induced optoelectronic features enhancement of multicrystalline silicon solar cells

AbstractThe effects of antireflection (ARC) and surface passivation films on optoelectronic features of multicrystalline silicon (mc‐Si) were investigated in order to perform high efficiency solar cells. A double layer consisting of Plasma Enhanced Chemical Vapor Deposition (PECVD) of silicon nitride (SiNx) on porous silicon (PS) was achieved on mc‐Si surfaces. It was found that this treatment decreases the total surface reflectivity from about 25% to around 6% in the 450‐1100 nm wavelength range. As a result, the effective minority carrier diffusion length, estimated from the Laser‐beam‐induced current (LBIC) method, was found to increase from 312 µm for PS‐treated cells to about 798 µm for SiNx/PS‐treated ones. The deposition of SiNx was found to impressively enhance the minority carrier diffusion length probably due to hydrogen passivation of surface, grain boundaries and bulk defects. Fourier Transform Infrared Spectroscopy (FTIR) shows that the vibration modes of the highly suitable passivating Si‐H bonds exhibit frequency shifts toward higher wavenumber, depending on the x ratio of the introduced N atoms neighbors. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Reduction of absorption loss in multicrystalline silicon via combination of mechanical grooving and porous silicon

AbstractSurface texturing of silicon wafer is a key step to enhance light absorption and to improve the solar cell performances. While alkaline‐texturing of single crystalline silicon wafers was well established, no efficient chemical solution has been successfully developed for multicrystalline silicon wafers. Thus, the use of alternative new methods for effective texturization of multicrystalline silicon is worth to be investigated. One of the promising texturing techniques of multicrystalline silicon wafers is the use of mechanical grooves. However, most often, physical damages occur during mechanical grooves of the wafer surface, which in turn require an additional step of wet processing‐removal damage. Electrochemical surface treatment seems to be an adequate solution for removing mechanical damage throughout porous silicon formation. The topography of untreated and porous silicon‐treated mechanically textured surface was investigated using scanning electron microscopy (SEM). As a result of the electrochemical surface treatment, the total reflectivity drops to about 5% in the 400‐1000 nm wavelength range and the effective minority carrier diffusion length enhances from 190 µm to about 230 µm (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Enhancement of photovoltaic properties of multicrystalline silicon solar cells by combination of buried metallic contacts and thin porous silicon

Photovoltaic properties of buried metallic contacts (BMCs) with and without application of a front porous silicon (PS) layer on multicrystalline silicon (mc-Si) solar cells were investigated. A Chemical Vapor Etching (CVE) method was used to perform front PS layer and BMCs of mc-Si solar cells. Good electrical performance for the mc-Si solar cells was observed after combination of BMCs and thin PS films. As a result the current–voltage ( I– V) characteristics and the internal quantum efficiency (IQE) were improved, and the effective minority carrier diffusion length (Ln) increases from 75 to 110 μm after BMCs achievement. The reflectivity was reduced to 8% in the 450–950 nm wavelength range. This simple and low cost technology induces a 12% conversion efficiency (surface area = 3.2 cm 2). The obtained results indicate that the BMCs improve charge carrier collection while the PS layer passivates the front surface.

Stability of silicon based homojunction and heterojunction solar cells under space conditions

Two different types of n-a-Si:H/p-c-Si heterostructure solar cells with intrinsic buffer layer, one type with and one type without back surface field (BSF), and one purely crystalline silicon solar cell have been irradiated with protons at 1.7 MeV. For both types of heterojunctions and for the c-Si homojunction cells a similar dose been found for which degradation starts and also a similar decrease of the effective minority carrier diffusion length with increasing irradiation dose. For one type of heterojunction cells we observed a small increase of the collection efficiency in the blue part of the spectrum after irradiation what can be explained only by modifications of the interface between the crystalline silicon and the amorphous silicon or of the thin intrinsic a-Si:H layer. As a new tool for the evaluation of the solar cell degradation due to proton irradiation, electroluminescence measurements under forward bias have been used. The emission spectra confirms that for doses up to 5 × 10 12 protons/cm 2, silicon band-to-band recombination is the most important contribution to the light emission and the emission efficiency decreases monotonically with increasing proton dose.

Hydrogen passivation of newly developed EMC-multi-crystalline silicon

Continuous casting with an electromagnetic cold crucible is a promising way of producing multi-crystalline (mc) silicon on an industrial basis for solar cell production. Casting equipment, capable of producing ingots of 130×130 mm 2 cross-section and 600 mm length has been developed and installed. Thorough characterisation of the produced material revealed a relatively high defect density (grain boundaries, dislocations) and small grain sizes of 1–4 mm in diameter. Although the effective minority carrier diffusion length is high on as-cut wafers (>150 μm) it decreases during solar cell processing to values of 50 μm. This can be attributed to standard high temperature processing steps that lead to redistribution and agglomeration of residual impurities (e.g. metals, carbon) at extended crystallographic defects which then act as strong recombination centres. In order to passivate these recombination centres, a PECVD SiN x-layer is deposited which acts as a source of hydrogen and also as an anti-reflective coating. During the firing of the screen-printed metal contacts through the SiN x-layer, atomic hydrogen is released from this layer and diffuses into the bulk of the wafers where it saturates dangling bonds and passivates impurities at crystal defects. After this treatment the minority carrier diffusion length can be restored to values of around 100 μm on finished solar cells.

A critical evaluation of the effective diffusion length determination in crystalline silicon solar cells from an extended spectral analysis

This note draws attention to possible errors when characterizing silicon solar cells by means of the effective minority carrier diffusion length L/sub b/ as defined in the paper on "Numerical modeling of textured silicon solar cells using PC1D" by Basore (1990). The approximations underlying the analytical expression for L/sub eff/ are critically reviewed. Their impact on the determination of the minority carrier bulk diffusion length L/sub b/ from L/sub eff/ is discussed for a 250-/spl mu/m thick Si solar cell. It is found that considerable errors occur in the case of diffusion lengths larger than the cell thickness even when neglecting any measurement uncertainty.

High-efficiency drift-field thin-film silicon solar cells grown on electronically inactive substrates

A drift-field in the base region of a solar cell can enhance the effective minority-carrier diffusion length, thus increasing the long-wavelength spectral response and energy-conversion efficiency. Silicon thin-films of 20–32 μm thickness as a cell base layer were grown by liquid-phase epitaxy (LPE) on electronically inactive heavily doped p ++-type CZ silicon substrates. Growth was performed from In Ga solutions, and in a purified Ar 4% H 2 forming gas ambient, rather than pure H 2. The Ga dopant concentration was tailored throughout the p-type film to create a drift-field in the base layer of the solar cell. An independently confirmed efficiency of 16.4% was achieved on such an LPE drift-field thin-film silicon solar cell with a total cell area of 4.11 cm 2. Substrate thinning, combined with light trapping which is encouraged by the textured front surface and a highly reflective aluminium rear surface, is demonstrated to improve the long-wavelength response and thus, increase cell efficiency by a factor of up to 23.7% when thinned to a total cell thickness of 30 μm.

Advanced, Thin, Polycrystalline Silicon-Film™ Solar Cells on Low-Cost Substrates

AbstractThin, polycrystalline silicon solar cells have the potential for the realization of a 15%, lowcost photovoltaic product. As a photovoltaic material, polycrystalline material is abundant, benign, and electrically stable. The thin-film polycrystalline silicon solar cell design achieves high efficiency by incorporating techniques to enhance optical absorption, ensure electrical confinement, and minimize bulk recombination currents. AstroPower's approach to a thin-film polycrystalline silicon solar cell technology is based on the Silicon-Film™ process, a continuous sheet manufacturing process for the growth of thin films of polycrystalline silicon on low-cost substrates. A new barrier layer and substrate have been developed for advanced solar cell designs. External gettering with phosphorus has been employed to effect significant improvements leading to effective minority carrier diffusion lengths greater than 250 micrometers in the active silicon layer. Light trapping has been observed in 60-micrometer thick films of silicon grown on the new barrier-coated substrate. An efficiency of 12.2% in a 0.659 cm2 solar cell has been achieved with the advanced structure.

Simulation of electron-beam-induced-current profiles for thin-film heteroj unction analysis

Abstract The application of the electron-beam-induced-current (EBIC) method to thin-film heterojunction solar cells is discussed. The aim is to extract the effective minority carrier diffusion length Leff in the absorbing layer of the cell which is closely related to the diffusion length of light-generated charge carriers. Current versus primary electron energy profiles for the planar EBIC configuration are simulated by considering the effect of back-surface recombination and backscatter-ing of primary electrons at the back contact. It is shown that for an absorber layer with thickness in the range of the diffusion length, the accuracy limit for the determination of Leff is reduced to 30% owing to the influence of interface recombination at the back contact.

Silicon solar cell of 16.8 μm thickness and 14.7% efficiency

Liquid phase epitaxy provides a new impetus for thin film photovoltaics based on silicon; we apply this method for about 20-μm-thick solar cells with high efficiencies. The analysis of internal quantum efficiency measurements reveals that the open circuit voltages around 660 mV arise from an excellent electronic quality of our thin silicon films. Their effective minority carrier diffusion lengths range up to 317 μm, a value that exceeds the thickness of the layers by an order of magnitude. The conversion efficiencies exceed 14% with a record value of 14.7% for a 16.8-μm-thick epitaxial layer without special antireflecting coatings. The high open circuit voltages promise a possible boost of the efficiency toward 20% by applying light trapping schemes to optimize the short circuit current.

Gettering in polycrystalline silicon solar cells

Polycrystalline silicon wafers were subjected to extra thermal treatments (0–120 min, 500–800 °C) during the processing of solar cells. It was found that the bulk effective minority carrier diffusion length was enhanced significantly upon annealing, provided the front and back sides of the wafers were doped with phosphorus and aluminium respectively, prior to the anneal. The optimum anneal temperature was 700 °C, yielding an increase of over 10% in diffusion length and a solar cell efficiency improvement of 0.5% absolute, compared with standard cells. Light-beam-induced current measurements have shown that the thermal treatment is most effective in regions with an initially small minority carrier diffusion length. In a separate model experiment with intentionally contaminated wafers, the concentration of metallic impurities such as gold, nickel and copper in the aluminium-doped layer was found to be 1000 times larger than the solid solubility of these metals in silicon at 700 °C. From these observations, it is concluded that gettering of impurities occurs during the annealing and that the aluminium-doped layer (produced by low cost screen-printing process) provides an effective sink for impurities.

Effects of chemical and electrochemical etching on polycrystalline thin films of CuGaSe2

Electrochemical and, especially, chemical oxidative etching drastically improves the photoresponse of liquid electrolyte/CuGaSe2-on-Mo junctions. This is expressed in decreased effective doping levels and increased effective minority carrier diffusion lengths. It is accounted for by removal of highly defective surface layers, which also leads to an increase in the barrier height, as judged from a positive shift of the flat band potential (on the electrochemical scale). The etching effects are seen clearly in Zn/CuGaSe2 devices, by electron beam-induced current. This last method also reveals a supra-grain structure, which is tentatively explained by thermal stress-induced strain at the Mo-CuGaSe2 interface.