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Abstract: Improvements in emitter passivation as well as back contact passivation lead to several high efficiency solar cell concepts. PERC (Passivated Emitter Rear Contact) solar cells on monocrystalline wafers above 21% have been achieved, but the diffusion processes with POCL3 and BBr3 have emerged as one of the limiting factors. The formation of high efficiency emitters by APCVD (Atmospheric Pressure Chemical Vapour Deposition) has several advantages compared to the standard diffusion. Epitaxial emitters can be optimised to match the passivation and metallisation of high efficiency concepts as well as industrial approaches like nickel plating. Simulations by PC1D show the potential of epitaxial emitters by featuring low contact resistance in combination with high shunt resistance, a good blue response and low emitter saturation current (J0e) . The emitter profiles can be designed in depth and in doping in the range from 1x1017cm-3 up to 1x1020cm-3 for p-type and n-type emitters. Simple reference solar cells with J0e= 46 fA/cm2 and efficiencies of η=17.5% on float zone (FZ) and η=16.1% on multicrystalline (mc) wafers have been processed. Furthermore an epitaxial selective emitter has been developed. The deposition process itself has been enhanced to improve the emitter profiles as well as material quality. [Copyright &y& Elsevier]

Abstract: Crystalline silicon thin-film solar cells (CSITF; high temperature approach) on low-cost substrates will only get their chance on the market when they can be introduced into an industrial solar cell process. Therefore, a conductive diffusion barrier intermediate layer (IL) is necessary. The most promising candidate is, in our opinion, SiC. Not just because doping with boron or phosphorous can change the electrical behavior effectively. It also matches well in thermal expansion coefficients with RBSiC (reaction-bonded) ceramics which is a very promising low-cost substrate. The passivation performance and the possibility to use the thin layer as a dopant source for an in situ back surface field are additional benefits. In this paper, we will show that the electrical conductivity of SiC can become sufficiently high with temperature treatments. We will furthermore present FTIR and SIMS measurements which would give information about the changing of bonding conditions and the distribution of the dopant boron with different deposition temperatures and annealing. [Copyright &y& Elsevier]

Abstract: Crystalline silicon thin-film solar cells combine the advantages of the stability and high-efficiency potential of crystalline silicon solar cell technology with the low material utilization of the thin-film solar cell technology. At Fraunhofer ISE the wafer equivalent concept is currently pursued. Within this concept, the active silicon layers are deposited on high-temperature stable substrates. The resulting substrate/layer sandwich can be processed into a solar cell using the same techniques that are used in conventional crystalline silicon wafer solar cell processing, hence the name wafer equivalent. In the present paper we report on how we realized wafer equivalents and explain in detail our development work on processors for both large-area silicon deposition and for zone melting recrystallization. An overview is given on the solar cell results achieved in this area. [Copyright &y& Elsevier]

Abstract. The crystalline silicon thin-film solar cell combines, in principle, the advantages of crystalline silicon wafer-based solar cells and of thin-film solar cell technologies. Its efficiency potential is the highest of all thin-film cells. In the "high-temperature approach" thin silicon layers are deposited on substrates that withstand processing temperatures higher than 1000 Celsius. The basic features of the high-temperature crystalline silicon thin-film cell technology are described and some important results are discussed. [ABSTRACT FROM AUTHOR]

The passivation properties of phosphorus-doped amorphous silicon carbide (a-SixC1-x) layers on monocrystalline Si wafers (floating zone, 1 Ω cm) have been investigated. The cleaning and the deposition process were performed in our two source (microwave, radio frequency) plasma-enhanced chemical vapor deposition reactor. In situ plasma etching and deposition at 350 °C without any following annealing step led to extraordinary low surface recombination velocities of less than 5 cm/s for injection levels between 1×1014 and 1×1015 cm-3. Characterization of the a-SixC1-x layer was done with quasi-steady-state photoconductance, microwave-detected photoconductance, and carrier density imaging techniques. For injection levels of more than 1×1015 cm-3 the effective lifetime was limited only by intrinsic Auger recombination. [ABSTRACT FROM AUTHOR]

The quality and cost of a wafer are important prerequisites for a high-efficiency, low-cost solar cell. Standard wafers are fabricated using a dicing process, which leads to the loss of a substantial amount of material (up to 40%), the kerf loss. The kerf loss is costly, but in standard technologies, it is unavoidable. As the wafer thickness decreases, these losses become more significant. Using an alternative approach, by applying a sacrificial detachment layer on top of a thin seed layer made of porous silicon for a follow-on thick epitaxial growth, so-called EpiWafers can be produced, and kerf losses can be avoided. An advantage of this EpiWafer production process is that the thickness of the wafers can be deliberately chosen according to requirements, e.g. between 50 µm and 250 µm. The wafer production costs are lower than standard wafering techniques if high-throughput porosification tools and chemical vapour deposition reactors are used. In this work, solar cells with n-type wafers epitaxially grown in an atmospheric pressure chemical vapour deposition reactor are presented. The processed wafers feature an n-type doping density of 3×10 16 cm −3 and a mean thickness of 150 µm. The best EpiWafer cell reached an open circuit voltage of 657 mV, a short circuit current of 39.6 mA/cm 2 and a fill factor of 77%. A detailed analysis of the losses caused by stacking faults and cell process issues is presented. The resulting energy conversion efficiency of 20% proves the high efficiency potential of this material. [ABSTRACT FROM AUTHOR]

Abstract: Standard wafer solar cells are made of near-semiconductor quality silicon. This high quality material makes up a significant part of the total costs of a solar module. Therefore, new concepts with less expensive so called solar grade silicon directly based on physiochemically upgraded metallurgical grade silicon are investigated. Metallurgical grade silicon contains large amounts of impurities, mainly transition metals like Fe, Cr, Mn, and Co, which degrade the minority carrier lifetime and thus the solar cell efficiency. A major reduction of the transition metal content occurs during the unidirectional crystallization due to the low segregation coefficient between the solid and liquid phase. A further reduction of the impurity level has to be done by gettering procedures applied to the silicon wafers. The efficiency of such cleaning procedures of metallurgical grade silicon is studied by instrumental neutron activation analysis (INAA). Small sized silicon wafers of approximately 200mg with and without gettering step were analyzed. To accelerate the detection of transition metals in a crystallized silicon ingot, experiments of scanning whole vertical silicon columns with a diameter of approximately 1cm by gamma spectroscopy were carried out. It was demonstrated that impurity profiles can be obtained in a comparably short time. Relatively constant transition metal ratios were found throughout an entire silicon ingot. This led to the conclusion that the determination of several metal profiles might be possible by the detection of only one “leading element”. As the determination of Mn in silicon can be done quite fast compared to elements like Fe, Cr, and Co, it could be used as a rough marker for the overall metal concentration level. Thus, a fast way to determine impurities in photovoltaic silicon material is demonstrated. [Copyright &y& Elsevier]

The development of a low-cost substrate is one of the major technological challenges for crystalline Si thin-film solar cells. Zirconium silicate (ZrSiO4) ceramics is a material which can meet the demanding physical requirements as well as the cost goals. Thin microcrystalline Si films were deposited by atmospheric pressure CVD on ZrSiO4-based ceramic substrates coated with barrier layers. The Si film was transferred into a multicrystalline grain structure by zone-melting recrystallization (ZMR). Film growth was analyzed in situ and correlated with substrate and barrier layer properties. Thin-film solar cells were fabricated from selected coarse-grained films. The best solar cell achieved an efficiency of 8.3% with a short circuit current density of 26.7 mA/cm2. The effective diffusion length obtained from internal quantum efficiency measurements was about 25 μm. [Copyright &y& Elsevier]

Polycrystalline silicon layers have been grown on various alumino-silicate substrates in a rapid thermal chemical vapor deposition (RTCVD) system at high temperatures (> 1000°C). Structural analysis shows a columnar growth with grain sizes up to 15 μm and growth rates up to 5 μm/min. Solar cell devices on this fine-grained Si material result in a short-circuit current of about 13 mA/cm² but a poor open-circuit voltage (< 0.4 V). Larger grains obtained by the zone melting recrystallization (ZMR) technique boosted the current up to 26.1mA/cm², thanks to the light-trapping by the mullite substrate. Best efficiency is 8.2% on a 1 cm² cell made on a 20 μm thick poly-Si layer. c 2002 Elsevier Science B.V. [ABSTRACT FROM AUTHOR]