Liquid Phase Crystallized Silicon Research Articles

Toward High Solar Cell Efficiency with Low Material Usage: 15% Efficiency with 14 μm Polycrystalline Silicon on Glass

Liquid‐phase‐crystallized silicon (LPC‐Si) is a bottom‐up approach to creating solar cells with the potential to avoid material loss and energy usage in wafer slicing techniques. A desired thickness of silicon (5–40 μm) is crystallized with a line‐shaped energy source, which is a laser, herein. The first part reports the efforts to optimize amorphous silicon contact layers for better surface passivation. The second part covers laser firing on the electron contact. It enables a controllable trade‐off between charge collection and fill factor (FF) by creating a low resistance contact, while preserving a‐Si:H (i) passivation in other areas. Short‐circuit current density (JSC) is observed to be up to 33:1 mA cm−2, surpassing all previously reported values for this technology. Open‐circuit voltage (VOC) of up to 658 mV also exceeded every previous value published at a low bulk doping concentration (1 × 1016 cm−3). Laser firing reduced JSC by 0:6 mA cm−2 on average but improved the FF by 22.5% absolute on average, without any significant effect on VOC. Collectively, these efforts have helped in achieving a new in‐house record efficiency for LPC‐Si of 15.1% and show a potential to reach 16% efficiency in the near future with optimization of series resistance.

Multicrystalline Silicon Thin‐Film Solar Cells Based on Vanadium Oxide Heterojunction and Laser‐Doped Contacts

Liquid phase crystallized (LPC) silicon thin films on glass substrates are a feasible alternative to conventional crystalline silicon (c‐Si) wafers for solar cells. Due to substrate limitation, a low‐temperature technology is needed for solar cell fabrication. While silicon heterojunction is typically used, herein, the combination of vanadium oxide/c‐Si heterojunction as emitter and base contacts defined by IR laser processing of phosphorus‐doped amorphous silicon carbide stacks is explored. LPC solar cells are fabricated using such technologies to identify their issues and advantages with a promising performance of an active‐area efficiency of 5.6%. Apart from the absence of light‐trapping techniques, the relatively low efficiency obtained is attributed to a low lifetime in the LPC silicon bulk. These poor material properties imply a short diffusion length that makes it that only photogenerated carriers in the emitter regions can be collected. Consequently, future devices should show narrower base contact regions, suggesting a shorter‐wavelength laser, combined with longer LPC substrate lifetimes.

All-Thin-Film Tandem Cells Based on Liquid Phase Crystallized Silicon and Perovskites

Combining the emerging perovskite solar cell technology with existing silicon approaches in a tandem cell design offers the possibility for new low-cost high-performance devices. In this study, the potential of liquid phase crystallized silicon (LPC-Si) solar cells as a bottom cell in an all-thin-film tandem device is investigated. By optimizing the current output of a four terminal tandem using optical simulations and state-of-the-art electrical properties of the top and bottom cells, we show that an efficiency of 23.3 $\%$ can be reached, where 7.2 $\%$ are attributed to the LPC-Si bottom cell. Including the potential of future developments of both sub cells, efficiencies of over 28 $\%$ are estimated. Electrical and optical measurements of the bottom cell are performed by attaching a perovskite and a cutoff filter to the front side of the interdigitated back contacted LPC-Si cells. The measurements using a cutoff filter show a high impact of the filtered incident light spectrum on the open circuit voltage of the LPC-Si cell. A comparison of the simulated and measured absorptance shows that especially the optical properties of the transparent conductive oxides and recombination losses in the LPC-Si cause high current losses. Combining the measured data of the filtered LPC-Si cells and the semitransparent perovskite cells, yields a realistic estimation for the efficiency of a state-of-the-art four-terminal tandem device of 19.3 $\%$ .

Assessment of Bulk and Interface Quality for Liquid Phase Crystallized Silicon on Glass

This paper reports on the electrical quality of liquid phase crystallized silicon (LPC-Si) on glass for thin-film solar cell applications. Spatially resolved methods such as light beam induced current (LBIC), microwave photoconductance decay (MWPCD) mapping, and electron backscatter diffraction were used to access the overall material quality, intra-grain quality, surface passivation, and grain boundary (GB) properties. LBIC line scans across GBs were fitted with a model to characterize the recombination behavior of GBs. According to MWPCD measurement, intra-grain bulk carrier lifetimes were estimated to be larger than 4.5 μs for n-type LPC-Si with a doping concentration in the order of 1016 cm−3. Low-angle GBs were found to be strongly recombination active and identified as highly defect-rich regions which spatially extend over a range of 40–60 μm and show a diffusion length of 0.4 μm. Based on absorber quality characterization, the influence of intra-grain quality, heterojunction interface, and GBs/dislocations on the cell performance were separately clarified based on two-dimensional (2-D)-device simulation and a diode model. High back surface recombination velocities of several 105 cm/s are needed to get the best match between simulated and measured open circuit voltage ( V oc), indicating back surface passivation problem. The results showed that V oc losses are not only because of poor back surface passivation but also because of crystal defects such as GBs and dislocation.

Passivation of Liquid‐Phase Crystallized Silicon With PECVD‐SiNx and PECVD‐SiNx/SiOx

Silicon nitride (SiNx) and silicon oxide (SiOx) grown with plasma‐enhanced chemical vapor deposition are used to passivate the front‐side of liquid‐phase crystallized silicon (LPC‐Si). The dielectric layer/LPC‐Si interface is smooth and layers are well‐defined as demonstrated with transmission electron microscopy. Using electron energy loss spectroscopy a thin silicon oxynitride is detected which is related to oxidation of the SiNx prior to the silicon deposition. The interface defect state density (Dit) and the effective fixed charge density (QIL,eff) are obtained from high‐frequency capacitance‐voltage measurements on developed metal‐insulator‐semiconductor structures based on SiOx/SiNx/LPC‐Si and SiOx/SiNx/SiOx/LPC‐Si sequences. Charge transfer across the SiNx/LPC‐Si interface is observed which does not occur with the thin SiOx between SiNx and LPC‐Si. The SiOx/SiNx/LPC‐Si interface is characterized by QIL,eff > 1012 cm−2 and Dit,MG>1012 eV−1 cm−2. With SiOx/SiNx/SiOx stack, both parameters are around one order of magnitude lower. Based on obtained QIL,eff and Dit(E) and capture cross sections for electrons and holes of σn = 10−14 cm s−1 and σp = 10−16 cm s−1, respectively, a front‐side surface recombination velocity in the range of 10 cm s−1 at both interfaces is determined using the extended Shockley‐Read‐Hall recombination model. Results indicate that field‐effect passivation is strong, especially with SiOx/SiNx stack.

Impact of carrier doping on electrical properties of laser-induced liquid-phase-crystallized silicon thin films for solar cell application

Large-grain-size (>1 mm) liquid-phase-crystallized silicon (LPC-Si) films with a wide range of carrier doping levels (1016–1018 cm−3 either of the n- or p-type) were prepared by irradiating amorphous silicon with a line-shaped 804 nm laser, and characterized for solar cell applications. The LPC-Si films show high electron and hole mobilities with maximum values of ∼800 and ∼200 cm2 V−1 s−1, respectively, at a doping level of ∼(2–4) × 1016 cm−3, while their carrier lifetime monotonically increases with decreasing carrier doping level. A grain-boundary charge-trapping model provides good fits to the measured mobility–carrier density relations, indicating that the potential barrier at the grain boundaries limits the carrier transport in the lowly doped films. The open-circuit voltage and short-circuit current density of test LPC-Si solar cells depend strongly on the doping level, peaking at (2–5) × 1016 cm−3. These results indicate that the solar cell performance is governed by the minority carrier diffusion length for the highly doped films, while it is limited by majority carrier transport as well as by device design for the lowly doped films.

Liquid phase crystallized silicon – A holistic absorber quality assessment

In this paper, we report on the current status of absorber attributes in Liquid Phase Crystallized Silicon (LPC-Si) cells. To this end, an absorber doping series (ND = 2·1016/cm³ to 7·1017/cm³) and an absorber thickness variation (14 and 33µm) are evaluated. Best cells from the batches for these series showed open circuit voltages up to 640mV and fill factors above 70%. It is observed that for state-of-the-art cells, thicknesses over 15µm are not beneficial due to limited diffusion lengths. Lower absorber doping concentrations tend to yield longer intra-grain diffusion lengths (Ldiff) and better passivated grain boundaries, which may be due to lower impurity precipitation. The longer Ldiff leads to higher short circuit current densities which over-compensate a decrease in open circuit voltage and fill factor with regards to efficiency. Both front and rear surfaces are sufficiently passivated and at the current status the bulk lifetime has most potential for improvement.

Periodic and Random Substrate Textures for Liquid-Phase Crystallized Silicon Thin-Film Solar Cells

A major limitation in current liquid-phase crystallized (LPC) silicon thin-film record solar cells is optical losses caused by their planar glass–silicon interface. In this study, silicon is grown on nanoimprinted periodically, as well as randomly textured glass substrates, and successfully implemented into state-of-the-art LPC silicon thin-film solar cells. Compared with an optimized planar reference device, both textures enhance absorption of light. Interlayer and process optimization allowed achieving a material quality comparable with the planar reference device. On the random texture, an open-circuit voltage above 630 mV was obtained, as well as an external quantum efficiency exceeding the planar reference device by +3 mA/cm2.

Passivation at the interface between liquid‐phase crystallized silicon and silicon oxynitride in thin film solar cells

AbstractThe passivation quality at the interface between liquid‐phase crystallized silicon (LPC‐Si) and a dielectric interlayer (IL) was investigated in terms of the defect state density at the IL/LPC‐Si interface (Dit) as well as the effective fixed charge density in the IL (QIL,eff). Both parameters were obtained via high‐frequency capacitance–voltage measurements on developed metal–insulator–semiconductor structures based on a molybdenum layer sandwiched between the IL and the glass substrate. Dit and QIL,eff were correlated to the open circuit voltage (Voc) and the integrated external quantum efficiency (Jsc,EQE) obtained on corresponding solar cell structures as well as to Voc and Jsc,EQE results based on two‐dimensional simulations. We found that Dit was reduced by one order of magnitude using a hydrogen plasma treatment (HPT) at 400 °C. Irrespectively of the HPT, QIL,eff was > 1012 cm−2. We suggest that field‐effect passivation dominates chemical passivation at the IL/n‐type LPC‐Si interface. We attribute the significant enhancement of Voc and Jsc,EQE observed after HPT on n‐type LPC‐Si solar cells mainly to improvements of the passivation quality in the n‐type LPC‐Si bulk rather than at the IL/n‐type LPC‐Si interface. For p‐type absorbers, the HPT did not improve Voc and Jsc,EQE significantly. We propose that this is because of an insufficient passivation of bulk defects by positively charged hydrogen, which dominates in p‐type silicon, in combination with an insufficient interface passivation. Copyright © 2016 John Wiley & Sons, Ltd.

Interface passivation of liquid‐phase crystallized silicon on glass studied with high‐frequency capacitance–voltage measurements

The passivation quality at the interface between dielectric interlayer (IL) stacks and liquid‐phase crystallized silicon (LPC‐Si) was investigated by means of high‐frequency capacitance–voltage (C–V) measurements. The developed device structure was based on a molybdenum layer sandwiched between the glass substrate and the IL/LPC‐Si stack. C–V curves were discussed in terms of hysteresis formation and capacitance relaxation. We varied the nitrogen and hydrogen content in the a‐SiOxNy:H layer adjacent to the LPC‐Si and studied the effects on the defect state density at the IL/LPC‐Si interface (Dit) as well as on the effective charge density in the IL (QIL,eff). Both parameters are crucial for the analysis of chemical and field‐effect passivation. Furthermore, the effect of an additional hydrogen plasma treatment (HPT) on Dit and QIL,eff was investigated. A Gaussian‐like defect distribution at around 0.1 eV above the mid gap energy is significantly reduced by the additional HPT. With additional HPT, the lowest Dit and highest QIL,eff at mid gap, i.e., Dit = (3.5 ± 0.7) × 1011 eV−1 cm−2 and QIL,eff = (1.6 ± 0.3) × 1012 cm−2, correspond to the passivation by an a‐SiOxNy:H layer with a low nitrogen and high hydrogen content.

Influence of Barrier and Doping Type on the Open-Circuit Voltage of Liquid Phase-Crystallized Silicon Thin-Film Solar Cells on Glass

We investigate the influence of the barrier type and the absorber doping on the open-circuit voltage of liquid phase-crystallized silicon solar cells on glass. It was found that the use of n-type instead of p-type substrates is the major reason for the recently reported boost of the open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ) up to values of 656 mV, which is by far exceeding the previously reported V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> values of crystalline silicon solar cells on glass. Despite the high doping, locally, an internal quantum efficiency of 90% can be achieved. Therewith, efficiencies of 16% and up should be possible.

Double-side textured liquid phase crystallized silicon thin-film solar cells on imprinted glass

Emerging liquid phase crystallization (LPC) techniques recently rendered a possible substantial progress in the fabrication of high quality crystalline silicon thin-film solar cells on glass. The implementation of an efficient light trapping texture into such LPC silicon devices is still challenging as an excellent bulk material quality and well-passivated interfaces have to be guaranteed. In this paper we present recent advances in light management for LPC silicon thin-film solar cells on imprinted glasses. A double-sided 2µm periodic texture is realized by sandwiching the silicon film during the electron-beam induced crystallization process between an imprinted glass substrate coated with silicon oxide and a silicon oxide capping layer. Amorphous-crystalline silicon (a-Si:H/c-Si) heterojunction solar cells with single sided contacting scheme are fabricated. Textured prototype devices and simultaneously processed planar solar cells exhibit a comparable electronic material quality featuring open circuit voltages above 550mV and efficiencies up to 8.1%. Optical absorption properties of 10µm thick double-side textured silicon films even predict maximum achievable short circuit current densities in solar cells up to 38mA/cm2 assuming zero parasitic absorption.