Paul Sonntag

Silicon Thin-Film Solar Cells on Glass With Open-Circuit Voltages Above 620 mV Formed by Liquid-Phase Crystallization

Liquid-phase crystallization (LPC) using line-shaped energy sources such as laser or electron beam has proven to be a suitable method to grow large grained high-quality silicon films onto commercially well-available glass substrates. In this study, we compare cw-diode laser-crystallized absorbers with electron beam-crystallized material using back contacted back junction solar cells. Furthermore, the influence of the absorber doping concentration thickness on the solar cell performance is studied. Using experimental data obtained on test structures, as well as solar cells and 1-D device simulations, an ideal dopant concentration is determinedtobe 2 - 6 × 1016cm-3, in combination with an absorber thickness of 10-20 μm. Finally, we present a slightly modified cell process to reduce the optical losses, which resulted in conversion efficiencies of up to 11.8%.

Silicon Solar Cells on Glass with Power Conversion Efficiency above 13% at Thickness below 15 Micrometer

Liquid phase crystallized silicon on glass with a thickness of (10–40) μm has the potential to reduce material costs and the environmental impact of crystalline silicon solar cells. Recently, wafer quality open circuit voltages of over 650 mV and remarkable photocurrent densities of over 30 mA/cm2 have been demonstrated on this material, however, a low fill factor was limiting the performance. In this work we present our latest cell progress on 13 μm thin poly-crystalline silicon fabricated by the liquid phase crystallization directly on glass. The contact system uses passivated back-side silicon hetero-junctions, back-side KOH texture for light-trapping and interdigitated ITO/Ag contacts. The fill factors are up to 74% and efficiencies are 13.2% under AM1.5 g for two different doping densities of 1 · 1017/cm3 and 2 · 1016/cm3. The former is limited by bulk and interface recombination, leading to a reduced saturation current density, the latter by series resistance causing a lower fill factor. Both are additionally limited by electrical shading and losses at grain boundaries and dislocations. A small 1 × 0.1 cm2 test structure circumvents limitations of the contact design reaching an efficiency of 15.9% clearly showing the potential of the technology.

Liquid-Phase Crystallized Silicon Solar Cells on Glass: Increasing the Open-Circuit Voltage by Optimized Interlayers for n- and p-Type Absorbers

Liquid-phase crystallization (LPC) has proven to be a suitable method to grow large-grained silicon films on commercially well-available glass substrates. Zone-melting crystallization with high-energy-density line sources such as lasers or electron beams enabled polycrystalline grain growth with wafer equivalent morphology. However, the electronic quality is strongly affected by the material used as the interlayer between the glass and the silicon absorber. Open-circuit voltages above 630 mV, and efficiencies up to 11.8% were demonstrated using n-type absorbers on a sputtered interlayer comprising a triple stack of SiO2/SiNx/SiO2. In this study, we present our results to further improve the device performance by investigating the influence of the interlayer on the open-circuit voltage of the devices and characterize the properties of the absorber and interface using bias light-dependent quantum efficiency data and transmission electron microscopy (TEM) images. Finally, we investigate the applicability of aluminum oxide (Al2O3) for passivation of p-type LPC absorbers.

Smooth anti-reflective three-dimensional textures for liquid phase crystallized silicon thin-film solar cells on glass

Recently, liquid phase crystallization of thin silicon films has emerged as a candidate for thin-film photovoltaics. On 10 μm thin absorbers, wafer-equivalent morphologies and open-circuit voltages were reached, leading to 13.2% record efficiency. However, short-circuit current densities are still limited, mainly due to optical losses at the glass-silicon interface. While nano-structures at this interface have been shown to efficiently reduce reflection, up to now these textures caused a deterioration of electronic silicon material quality. Therefore, optical gains were mitigated due to recombination losses. Here, the SMooth Anti-Reflective Three-dimensional (SMART) texture is introduced to overcome this trade-off. By smoothing nanoimprinted SiOx nano-pillar arrays with spin-coated TiOx layers, light in-coupling into laser-crystallized silicon solar cells is significantly improved as successfully demonstrated in three-dimensional simulations and in experiment. At the same time, electronic silicon material quality is equivalent to that of planar references, allowing to reach Voc values above 630 mV. Furthermore, the short-circuit current density could be increased from 21.0 mA cm−2 for planar reference cells to 24.5 mA cm−2 on SMART textures, a relative increase of 18%. External quantum efficiency measurements yield an increase for wavelengths up to 700 nm compared to a state-of-the-art solar cell with 11.9% efficiency, corresponding to a jsc, EQE gain of 2.8 mA cm−2.

Emitter Patterning for Back-Contacted Si Heterojunction Solar Cells Using Laser Written Mask Layers for Etching and Self-Aligned Passivation (LEAP)

A novel emitter patterning method for back-contacted Si heterojunction solar cells is presented, which combines laser processing and wet etching of a mask layer stack with self-aligned repassivation, thus reducing the process complexity, as compared with the commonly used emitter patterning methods. Lifetime samples demonstrate that with a suitable mask stack, laser scribing can be performed without inducing laser damage to the passivation. Despite nonoptimal wet etch and repassivation processes which currently limit the obtained lifetime, proof-of-concept cells on p-type wafers fabricated using this novel emitter patterning process and lithographically patterned metallization exhibit an open-circuit voltage of 694 mV and pseudo-fill-factors of 83%. With the laser written mask layers for etching and self-aligned passivation process, we have thus developed the proof-of-concept for a simple, lithography free, and contactless emitter patterning method for industrial applications.

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.

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

We developed a method to quantify the local minority carrier diffusion lengths in interdigitated back-contact solar cells having a 10 μm 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 33–44 μm inside a grain, which proves that advanced cell concepts like an IBC system are well suited for the LPC absorbers. Furthermore, the method has the potential to help improving 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.

Liquid phase crystallized silicon solar cells on glass: Material quality and device design

Liquid phase crystallization (LPC) has proven to be a suitable method to grow large-grained silicon films on commercially well available glass substrates. Zone-melting crystallization with high energy density line sources such as lasers or electron beams enabled polycrystalline grain growth with wafer equivalent morphology. A lot of effort was put into interlayer optimization by different groups. Open circuit voltages above 630 mV and efficiencies up to 11.8% were demonstrated using n-type absorbers on a sputtered interlayer between glass substrate and silicon absorber comprising a triple stack of SiO2/SiNx/SiO2. In this work we report on our results to further improve device performance by investigating the influence of the interlayer on the open circuit voltage of the devices and demonstrate first results achieved on an interdigitated back contacted silicon heterojunction solar cell to simplify device fabrication.

Backside contacted solar cells with heterojunction emitters and laser fired absorber contacts for crystalline silicon on glass

Backside contacted cells were fabricated on p- and n-type, liquid phase crystallized silicon on glass. The heterojunction emitter and transparent contacting layer were opened locally using an inkjet printer and the absorber was contacted with laser fired point contacts. An analytical resistance model was derived to determine the resistance losses for the point contact geometry. Using test structures and the model it was found that for p-type cells the main contribution to the cell resistance came from the absorber contact and the absorber. Both were strongly reduced by increasing the absorber doping. For n-type cells the main contributions originated from the emitter and emitter-TCO contact, which was improved by changing the emitter type, resulting in an 11.6 % solar cell.

Facing the challenge of liquid phase crystallizing silicon on textured glass substrates

A major limitation in current liquid phase crystallized (LPC) silicon thin-film record solar cells are optical losses caused by their planar glass-silicon interface. In this study, silicon is grown on nanoimprinted periodically as well as on randomly textured glass substrates and successfully implemented into state-of-the-art LPC silicon thin-film solar cell stacks. By systematically varying every layer the whole sample stack is optimized regarding its anti-reflection ability. Compared to an optimized planar reference device, a reduction of reflection losses by -3.5% (absolute) on the random and by -9.4% (absolute) on the periodic texture has been achieved in the wavelength range of interest.