Mason Terry

Isotextured Silicon Solar Cell Analysis and Modeling 1: Optics

A comprehensive investigation reveals three useful approximations to the optical behavior of isotextured silicon solar cells. First, we confirm experimentally that front-surface reflectance is accurately modeled with “spherical cap” geometry. Second, we find that light reflected from the surface has a Lambertian distribution. Random upright pyramid texturing results in a less favorable distribution so that, when encapsulated, photogeneration in an isotextured cell approaches 99% of that achieved in an equivalent pyramidally textured device. Third, we perform ray tracing simulations to determine the 1-D photogeneration profile beneath isotexture. On their first pass, rays traverse the substrate at angle θ1 with respect to the macroscopic normal such that they are distributed according to cos(3 θ1/2). This approximation to the ray trajectory establishes, for isotexture, a useful simulation tool that has been available for application to pyramidally textured devices for two decades. This paper is followed by a contribution that investigates recombination at isotextured surfaces, coupling results with optical analyses to model the performance of isotextured solar cells.

Rapid Thermal Annealing and Hydrogen Passivation of Polycrystalline Silicon Thin-Film Solar Cells on Low-Temperature Glass

The changes in open-circuit voltage (Voc), short-circuit current density (Jsc), and internal quantum efficiency (IQE) of aLuminum induced crystallization, ion-assisted deposition (ALICIA) polycrystalline silicon thin-film solar cells on low-temperature glass substrates due to rapid thermal anneal (RTA) treatment and subsequent remote microwave hydrogen plasma passivation (hydrogenation) are examined. Voc improvements from 130 mV to 430 mV, Jsc improvements from 1.2 mA/cm2 to 11.3 mA/cm2, and peak IQE improvements from 16% to > 70% are achieved. A 1-second RTA plateau at 1000°C followed by hydrogenation increases the Jsc by a factor of 5.5. Secondary ion mass spectroscopy measurements are used to determine the concentration profiles of dopants, impurities, and hydrogen. Computer modeling based on simulations of the measured IQE data reveals that the minority carrier lifetime in the absorber region increases by 3 orders of magnitude to about 1 nanosecond (corresponding to a diffusion length of at least 1 μm) due to RTA and subsequent hydrogenation. The evaluation of the changes in the quantum efficiency and Voc due to RTA and hydrogenation with computer modeling significantly improves the understanding of the limiting factors to cell performance.

Silicon ink selective emitter process: Optimization of selectively diffused regions for short wavelength response

The Innovalight Cougar™ Platform is a portfolio of simple to implement technologies that, when combined with Innova-light Silicon Ink, enables the manufacture of a selective emitter solar cell with a non-masking single-step diffusion. Cell efficiencies of up to 19% have been achieved. Innova-light Silicon Ink is a highly engineered silicon nanoparticle colloidal dispersion, implemented for both high volume ink-jet and screen deposition, and further optimized to be produced and delivered in commercial volumes. A particular feature of the selective emitter structure is its enhanced quantum efficiency in the short wavelength region as result of reduced emitter recombination. In this report we demonstrate how the properties of the selectively doped regions on the front surface of the cell affect the short wavelength response of a Cougar cell. In particular, the importance of the emitter dopant profile for minimizing emitter recombination and the broad processing window of the Cougar cell structure with respect to emitter doping are illustrated. Further, the effect of the ink finger width on the short-wavelength response of the cell and the trade-off between the loss of short circuit current (Jsc) and metal-to-ink pattern alignment are demonstrated.

All screen-printed 18% homogeneous emitter solar cells using high volume manufacturing equipment

The Innovalight Cougar™ Platform is a portfolio of simple to implement technologies that, when combined with Innovalight Silicon Ink, enables the manufacture of a selective emitter solar cell with a non-masking single-step diffusion. Cell efficiencies of up to 19% have been achieved. Innovalight Silicon Ink is a highly engineered silicon nanoparticle colloidal dispersion, implemented for both high volume ink-jet and screen deposition, and further optimized to be produced and delivered in commercial volumes [1].

Isotextured Silicon Solar Cell Analysis and Modeling 2: Recombination and Device Modeling

We extend our analysis of isotextured silicon solar cells by 1) examining experimentally the role played by isotexture in determining the surface recombination velocity at silicon surfaces and 2) combining these experimental results with our model for photogeneration in order to simulate in one dimension typical solar cell devices with isotextured surfaces. We examine both undiffused and diffused n-type isotextured silicon surfaces, and we find that the rate of surface recombination usually decreases with increasing isotexture etch depth. However, when undiffused surfaces are passivated with hydrogenated SiO2 or SiNx, surface recombination velocity is, counterintuitively perhaps, found to be independent of surface texture-this is despite a surface area that is up to 1.9-fold larger than a planar equivalent. We demonstrate the utility of our analysis of isotextured surfaces by simulating various device structures in one dimension. In one case, where device parameters are chosen to approximate a typical screen-printed cell with full-area back surface field, simulation results indicate that the optimal isotexture etch depth is 1-3 μm. This optimum etch depth is slightly below the one deduced from published experimental results, indicating that surface recombination on samples observed in this study is uniquely independent of isotexture morphology.

Efficiency gain of Silicon Ink selective emitter solar cells at module level

Innovalight™ previously described an industrially viable approach to fabricate selective emitter (SE) cells based on screen-printed Silicon Ink. In this report we describe the performance of these cells incorporated into photovoltaic modules fabricated at Hanwha SolarOne (formerly Solar-Fun) in Qidong, Jiangsu Province, China. We demonstrate the conservation of the performance gain of Silicon Ink SE cells versus homogeneous emitter (HE) cells after module fabrication. This study is the first systematic and large-scale comparison of full size SE and HE solar panels produced with industry standard materials and facilities. Standard glass and EVA encapsulant used in HE module production at Hanwha SolarOne was used for both the HE and Silicon Ink SE modules. Overall, the cell-to-module (CTM) performance ratio for output power of Silicon Ink SE cells was 94.8 % compared to 95.6 % for HE cells. The fact that the CTM performance ratio for Silicon Ink SE modules was only 0.8 % lower than for HE modules, demonstrates that the performance gains of Silicon Ink SE cells were successfully transferred into the solar module. Ten Modules fabricated on the module production line at Hanwha SolarOne consisting of 54 Silicon Ink 156mm × 156mm SE cells produced at Innovalight have achieved efficiencies of 16.9% (related to the area under the glass, excluding the frame). The Silicon Ink SE modules showed efficiency improvement of over 0.94% (absolute) compared to HE modules fabricated under identical conditions. The Silicon Ink SE modules were submitted to an internationally accredited testing agency and passed certification of PV modules according to the guidelines of IEC-61215.

Silicon Ink selective emitter solar cells for optimum module performance

Innovalights Cougar™ Platform is a selective emitter (SE) process based on a screen-printed patternof Silicon Ink. The Cougar Platform has been licensed to several leading solar companies. The performance gain from the Cougar SE process is well documented at the cell level [1]. In this report we describe the optimization of the Cougar cell for photovoltaic (PV) modules. Ray tracing modeling [2] has been used to optimize the performance of encapsulated Cougar cells, taking into account the optical properties of surface texture, encapsulant materials, and cell anti-reflection coating layers. Anti-reflection coatings are made from silicon nitride (SiNx), the optical characteristics of which were determined from ellipsometry and UV-Vis spectroscopy. Modeling predicts different cell-to-module (CTM) values for short-circuit current (Isc), depending on the SiNx characteristics. Mini-modules were constructed to verify modeling results. In addition to optical modeling, fill factor losses in the mini-modules are investigated and analyzed. Accelerated lifetime testing was also performed. The Cougar Platform shows equivalent reliability performance compared to traditional, homogeneous emitter (HE) solar cells.

Modelling isotextured silicon solar cells and modules

We describe a one dimensional model for isotextured silicon solar cells. Combined optical and recombination analyses provide the tools required to predict the performance of isotextured cells; the utility of these tools is demonstrated by comparison with industrially fabricated screen-printed cells with full area back surface field. In this particular demonstration, inaccurate predetermination of front surface recombination reduces the predictive capability of the model. We measure the angular distribution of light from isotextured surfaces, showing that, when encapsulated with typical pottants beneath glass, current generation in isotextured cells approaches 99% of that achieved in random pyramid textured equivalents. This represents a reduction in the performance difference between the two textures when operating in air (not encapsulated); in this case, current generation in an isotextured device is 96% of that calculated beneath random pyramids. We calculate the short circuit current of photovoltaic modules comprising cast-mono silicon solar cells; when encapsulated beneath glass and EVA, isotexturing, rather than alkaline etching, maximises photogeneration in cells with less than 84% monocrystalline (<;100>;) surface area.

Holistic view of interactions in modules affecting durability - adhesion and snail trails

We are introducing our program to study and understand the interactions of photovoltaic module component materials and their impact on module durability from a holistic point of view. While the impact of backsheets and encapsulants on module durability has been studied to a great extent, the role of metallization pastes is significantly less understood. One durability-related aspect of metallization pastes is their adhesion strength to the tabbing ribbons. We have developed methods to control adhesion over a wide range and are among the first to quantify the impact of adhesion on module durability. We present our finding that adhesion does not have a measurable impact on module durability under the conditions tested, even for ultra-low adhesion conditions below 1 N/mm. Another module durability topic that requires a holistic view of interactions between module components is the phenomenon of snail trails on the surface of PV modules, which has become a widespread problem encountered by a large number of module makers and solar farms around the world. Through detailed studies, we have previously been able to link the root cause of snail trails to chemistry in the ethylene-vinyl acetate (EVA), which reacts with silver in the presence of moisture introduced via micro-cracks. We present our continued efforts to develop an in-depth understanding of the mechanisms behind snail trail formation. We investigate the influence of micro-cracks as well as the material properties of encapsulants and backsheets on the rates of water ingression and subsequent snail trail formation. We also discuss the development and effectiveness of accelerated test methodologies replacing the current time-consuming field aging test method to establish snail trail susceptibility and rate of formation and apply them to specific module designs. Data from module field exposure tests is presented to correlate with lab accelerated aging tests.

Holistic reliability: Accelerated testing of metallization

Understanding metallization reliability is crucial as two-thirds of all defects detected in fielded modules is directly related to cell and metallization problems. This coupled with the reduction in metallization laydown by 70% over the last 5 years poses a risk to long term module reliability. A critical issue is separating intrinsic vs. extrinsic effects on failure modes seen under testing of silver paste durability. We have varied the flux used in soldering of the ribbons to the cell and seen differentiation in durability. From these learnings we have then tested different silver pastes under damp heat and pressure cooker testing. Finally, we present a series of aged adhesion with temperature cycling to develop accelerated testing of adhesion.