Jan Haschke

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%.

Numerical Simulation of Solar Cells and Solar Cell Characterization Methods: the Open-Source on Demand Program AFORS-HET

Within this chapter, the principles of numerical solar cell simulation are described, using AFORS-HET (automat for simulation of heterostructures). AFORS-HET is a one dimensional numerical computer program for modelling multi layer homoor heterojunction solar cells as well as some common solar cell characterization methods. Solar cell simulation subdivides into two parts: optical and electrical simulation. By optical simulation the local generation rate ) , ( t G x within the solar cell is calculated, that is the number of excess carriers (electrons and holes) that are created per second and per unit volume at the time t at the position x within the solar cell due to light absorption. Depending on the optical model chosen for the simulation, effects like external or internal reflections, coherent superposition of the propagating light or light scattering at internal surfaces can be considered. By electrical simulation the local electron and hole particle densities ) , ( ), , ( t p t n x x and the local electric potential ) , ( t x φ within the solar cell are calculated, while the solar cell is operated under a specified condition (for example operated under open-circuit conditions or at a specified external cell voltage). From that, all other internal cell quantities, such like band diagrams, local recombination rates, local cell currents and local phase shifts can be calculated. In order to perform an electrical simulation, (1) the local generation rate ) , ( t G x has to be specified, that is, an optical simulation has to be done, (2) the local recombination rate ) , ( t R x has to be explicitly stated in terms of the unknown variables φ , , p n , ( ) φ , , ) , ( p n f t R = x . This is a recombination model has to be chosen. Depending on the recombination model chosen for the simulation, effects like direct band to band recombination (radiative recombination), indirect band to band recombination (Auger recombination) or recombination via defects (Shockley-Read-Hall recombination, dangling-bond recombination) can be considered. In order to simulate a real measurement, the optical and electrical simulations are repeatedly calculated while changing a boundary condition of the problem, which is specific to the measurement. For example, the simulation of a i-V characteristic of a solar cell is done by calculating the internal electron and hole current (the sum of which is the total current) as a function of the externally applied voltage. Source: Solar Energy, Book edited by: Radu D. Rugescu, ISBN 978-953-307-052-0, pp. 432, February 2010, INTECH, Croatia, downloaded from SCIYO.COM

PECVD Intermediate and Absorber Layers Applied in Liquid-Phase Crystallized Silicon Solar Cells on Glass Substrates

Liquid-phase crystallized silicon absorber layers have been applied in heterojunction solar cells on glass substrates with 10.8% conversion efficiency and an open-circuit voltage of 600 mV. Intermediate layers of SiOx, SiNx, and SiOxNy, as well as the a-Si:H precursor layer, were deposited on 30 cm × 30 cm glass substrates using industrial-type plasma-enhanced chemical vapor deposition equipment. After crystallization on 3cm × 5cm area using a continuous-wave infrared laser line, the resulting polysilicon material showed high material quality with large grain sizes.

Liquid phase crystallized silicon on glass: Technology, material quality and back contacted heterojunction solar cells

Liquid phase crystallization has emerged as a novel approach to grow large grained polycrystalline silicon films on glass with high electronic quality. In recent years a lot of effort was conducted by different groups to determine and optimize suitable interlayer materials, enhance the crystallographic quality or to improve post crystallization treatments. In this paper, we give an overview on liquid phase crystallization and describe the necessary process steps and discuss their influence on the absorber properties. Available line sources are compared and different interlayer configurations are presented. Furthermore, we present one-dimensional numerical simulations of a rear junction device, considering silicon absorber thicknesses between 1 and 500 mu m. We vary the front surface recombination velocity as well as doping density and minority carrier lifetime in the absorber. The simulations suggest that a higher absorber doping density is beneficial for layer thicknesses below 20 mu m or when the minority carrier lifetime is short. Finally, we discuss possible routes for device optimization and propose a hybride cell structure to circumvent current limitations in device design

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 (VOC) up to values of 656 mV, which is by far exceeding the previously reported VOC 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.

Balance of optical, structural, and electrical properties of textured liquid phase crystallized Si solar cells

Liquid phase crystallized Si thin-film solar cells on nanoimprint textured glass substrates exhibiting two characteristic, but distinct different surface structures are presented. The impact of the substrate texture on light absorption, the structural Si material properties, and the resulting solar cell performance is analyzed. A pronounced periodic substrate texture with a vertical feature size of about 1 μm enables excellent light scattering and light trapping. However, it also gives rise to an enhanced Si crystal defect formation deteriorating the solar cell performance. In contrast, a random pattern with a low surface roughness of 45 nm allows for the growth of Si thin films being comparable to Si layers on planar reference substrates. Amorphous Si/crystalline Si heterojunction solar cells fabricated on the low-roughness texture exhibit a maximum open circuit voltage of 616 mV and internal quantum efficiency peak values exceeding 90%, resulting in an efficiency potential of 13.2%. This demonstrates that high quality crystalline Si thin films can be realized on nanoimprint patterned glass substrates by liquid phase crystallization inspiring the implementation of tailor-made nanophotonic light harvesting concepts into future liquid phase crystallized Si thin film solar cells on glass.

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.

Planar rear emitter back contact amorphous/crystalline silicon heterojunction solar cells (RECASH / PRECASH)

Point / stripe contacted, planar rear emitter back contact amorphous/crystalline Silicon, a-Si:H/c-Si, heterojunction solar cells are presented (RECASH / PRECASH solar cells), combining the high efficiency concepts of silicon heterojunctions (high VOC potential) and back contacts (high ISC potential). Electrically insulated point or stripe contacts to the solar cell absorber are embedded within a low temperature deposited rear side planar amorphous silicon emitter layer. The new contacting schemes for back contacted a-Si:H/c-Si heterojunction solar cells require less structuring and enable the use of low cost patterning technologies which result in a large structure size (i.e. inkjet printing, screen printing). The efficiency potential of back contacted a-Si:H/c-Si heterojunction solar cells (> 24 %) is discussed by means of numerical computer simulation. First RECASH and PRECASH solar cells have been realized and are compared to a conventional front contacted a-Si:H/c-Si heterojunction solar cell (SHJ). The predicted higher short circuit current potential of back contacted a-Si:H/c-Si heterojunction solar cells could be proofed.

The impact of silicon solar cell architecture and cell interconnection on energy yield in hot & sunny climates

Extensive knowledge of the dependence of solar cell and module performance on temperature and irradiance is essential for their optimal application in the field. Here we study such dependencies in the most common high-efficiency silicon solar cell architectures, including so-called Aluminum back-surface-field (BSF), passivated emitter and rear cell (PERC), passivated emitter rear totally diffused (PERT), and silicon heterojunction (SHJ) solar cells. We compare measured temperature coefficients (TC) of the different electrical parameters with values collected from commercial module data sheets. While similar TC values of the open-circuit voltage and the short circuit current density are obtained for cells and modules of a given technology, we systematically find that the TC under maximum power-point (MPP) conditions is lower in the modules. We attribute this discrepancy to additional series resistance in the modules from solar cell interconnections. This detrimental effect can be reduced by using a cell design that exhibits a high characteristic load resistance (defined by its voltage-over-current ratio at MPP), such as the SHJ architecture. We calculate the energy yield for moderate and hot climate conditions for each cell architecture, taking into account ohmic cell-to-module losses caused by cell interconnections. Our calculations allow us to conclude that maximizing energy production in hot and sunny environments requires not only a high open-circuit voltage, but also a minimal series-to-load-resistance ratio.