Internal electric field and fill factor of amorphous silicon (a-Si:H) solar cells

The electric field E within the i-layer of hydrogenated amorphous silicon (a-Si:H) solar cells strongly affects the cell performances, and, specifically, the fill factor FF. It governs the drift length Ldrift = μτE which is the crucial parameter limiting charge collection. Ideally, a constant electric field is assumed across the i-layer, whereas in real devices, it is deformed by charged band tail states and dangling bonds. If the i-layer is too thick or has a high density of charged defects, E is deformed and reduced. To determine theoretically the charge states of band tails and dangling bonds, we must know the carrier density profiles within the i-layer. Here, the SunShine program is used to determine carrier generation profiles within i-layers of pincells on TCO-covered glass substrates. A classical model for transport and electron/hole capture is employed to determine charge conditions of band tail states and dangling bonds. Results are: (a) charged dangling bonds are predominant for the electric field deformation, affecting the output performance of the cell; (b) this effect is very pronounced especially in degraded cells; (c) it is independent of light intensity; (d) it accounts for performance breakdown of thick, degraded a-Si:H cells. Calculated results are confronted with experimental observations (measurements of FF, collection voltage Vcoll and external quantum efficiency EQE) on pin-type solar cells of 100, 200, 300, and 400 nm thickness produced at IMT Neuchâtel, in initial and degraded state. Ldrift is evaluated via Vcoll, determined here with the method of variable intensity measurements (VIM). Trends observed are explained to full satisfaction

Modeling potential-induced degradation (PID) in crystalline silicon solar cells: from acceleratea-aging laboratory testing to outdoor prediction

We present a mathematical model to predict the effect of potential-induced degradation (PID) on the power output of c-Si modules in different climates. For the experimental part, we manufacture mini-modules made of two c-Si p-type cells, and use accelerated ageing laboratory testing performed at different combinations of stress factors (temperature, relative humidity, and voltage). By modeling the effect of each stress factor in a step-wise approach, we obtain a model for the PID at constant stress conditions, which agrees well with models that can be found in the literature for full-size modules. Our model is obtained complementing existing models by introducing a term that describes a linear dependence of module’s power degradation on the magnitude of the applied voltage. Since in field installations PV modules are connected in strings and exposed to different potential – and, therefore, stress – levels, this latter term is needed to approach real field conditions. Finally, we present the first attempts to model PID outdoor degradation in different climate conditions based on the proposed model and on the indoor-determined coefficients for the devices tested. The outdoor prediction model makes use of Typical Meteorological Year (TMY) data for a specific location

New Generation Transparent LPCVD ZnO Electrodes for Enhanced Photocurrent in Micromorph Solar Cells and Modules

ZnO bilayer films were deposited by low-pressure chemical vapor deposition in a single process step by controlling the differential doping of the nucleation and bulk parts of the layers. The resulting 2-μm-thick films are characterized by low free-carrier absorption and electron mobility over 40 cm2 /Vs. They, therefore, combine high transparency in the infrared region and moderate sheet resistance that can be lowered below 20 Ω/sq. These properties make ZnO bilayers ideal candidates as electrodes for the development of micromorph thin-film solar cells with enhanced photogenerated current. The potential of such bilayer front electrodes for a further power improvement and cost reduction of industrial micromorph tandem modules is currently investigated at Oerlikon Solar. The first experiments already show a promising gain in the bottom μc-Si:H cell photogenerated current compared with the current generated with modules deposited on standard uniformly doped ZnO single-layer front contacts

Light-Management Strategies for Thin-Film Silicon Multijunction Solar Cells

Light management is of crucial importance to reach high efficiencies with thin-film silicon multijunction solar cells. In this contribution, we present light-management strategies that we recently developed. This includes high quality absorber materials, low-refractive index intermediate reflectors, and highly transparent multiscale electrodes. Specifically, we show the fabrication of high-efficiency tandem devices with a certified stabilized efficiency of 12.6%, triple-junction solar cells with a stabilized efficiency of 12.8%, recently developed smoothening intermediate reflector layers based on silicon dioxide nanoparticles, and periodic-on-random multiscale textures

In-Situ Monitoring of Moisture Ingress in PV Modules Using Digital Humidity Sensors

A new in-situ moisture monitoring technique for photovoltaic (PV) modules is proposed using miniature digital humidity and temperature sensors. The sensors were embedded in three different ethylene-vinyl-acetate (EVA) stacks and proved to be resistant to lamination conditions. The fact that they are in direct contact to EVA does not affect their performance, since their saturated relative humidity (RH) reading is proportional to the external RH in the air. By exposing the sensors to elevated temperature and RH conditions, water vapor transmission rate of the backsheet and diffusion coefficient of the EVA can be determined. Obtained coefficients agree with reference values within their measurement uncertainties. Besides determining material moisture ingress properties, this monitoring technique is also applicable for long-term outdoor PV module monitoring. It shall provide valuable location and installation specific information of RH and temperature stress conditions, especially as feedback information to manufacturers of materials and PV modules

TOWARDS BETTER UNDERSTANDING OF LONG-TERM STABILITY IN THIN FILM MICROCRYSTALLINE SILICON SOLAR CELLS

High-efficiency p-i-n microcrystalline silicon single-junction solar cells are fabricated and their longterm stability is investigated. We focus in particular on the stability of the absorber layer after atmospheric storage in absence of light and compare non-encapsulated devices with different cell design and crystalline volume fractions. Furthermore, an innovative stack of transparent conductive oxide is used as front electrode in order to investigate the effect of small surface roughness on the electrical properties and the stability of solar cells. Finally, an outstanding single-junction microcrystalline cell showing 10.4% conversion efficiency without anti-reflection coating is presented

Self-Patterned Nanoparticle Layers for Vertical Interconnects: Application in Tandem Solar Cells

We demonstrate self-patterned insulating nanoparticle layers to define local electrical interconnects in thin-film electronic devices. We show this with thin-film silicon tandem solar cells, where we introduce between the two component cells a solution-processed SiO2 nanoparticle layer with local openings to allow for charge transport. Because of its low refractive index, high transparency, and smooth surface, the SiO2 nanoparticle layer acts as an excellent intermediate reflector allowing for efficient light management