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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 616mV 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. [ABSTRACT FROM AUTHOR]

We present the latest results of our back contact back silicon heterojunction (BCB-SHJ) solar cells with punctiform absorber contacts. We discuss the technological challenges and present a basic cell structure reaching an e_ciency of (17.1 _ 0.5)% on our BCB-SHJ solar cells. The main loss mechanisms a_ecting the fill factor and the VOC are identified and are found to be avoided by minor process modifications. [ABSTRACT FROM AUTHOR]

Employing quantitative electron-paramagnetic resonance analysis and numerical simulations, we investigate the performance of thin-film polycrystalline silicon solar cells as a function of defect density. We find that the open-circuit voltage is correlated to the density of defects, which we assign to coordination defects at grain boundaries and in dislocation cores. Numerical device simulations confirm the observed correlation and indicate that the device performance is limited by deep defects in the absorber bulk. Analyzing the defect density as a function of grain size indicates a high concentration of intra-grain defects. For large grains (>2 μm), we find that intra-grain defects dominate over grain boundary defects and limit the solar cell performance. [ABSTRACT FROM AUTHOR]

Abstract: Thin-film silicon solar cells with up to 11.2% stable cell efficiency have been developed on ZnO coated glass substrates on laboratory scale using solely plasma enhanced chemical vapour deposition (PECVD) and magnetron sputtering as deposition techniques for all thin films. To transfer this concept towards large areas, we developed a novel PECVD electrode which proved its capability of providing homogenous amorphous and microcrystalline silicon films on ∼1 m2 sized substrates. Microcrystalline silicon (μc-Si:H) solar cells were realised with efficiencies up to 7.2%. The high open-circuit voltage V oc and fill factor FF of 520 mV and 71%, respectively, demonstrate that high quality μc-Si:H material can be prepared with this large area electrode. Note that finally the entire thin-film structure–including all silicon, TCO and metal films–can be produced using one single equipment platform which complies with the requirements for a cost-effective and large scale mass production. [Copyright &y& Elsevier]

Abstract: This short review article gives an overview on the current status of polycrystalline silicon thin-film solar cells on glass and discusses the future perspectives of this technology. [Copyright &y& Elsevier]

The chemical structure of the interface between silicon thin films and the transparent conductive oxide ZnO:Al has been investigated by hard x-ray photoelectron spectroscopy. By varying the excitation energy between 2010 and 8040 eV, we were able to probe the Si/ZnO interface buried below 12 nm Si. This allowed for the identification of changes induced by solid phase crystallization (SPC). Based on in-situ SPC annealing experiments, we find clear indications that the formation of Si-O bonds takes place at the expense of Zn-O bonds. Hence, the ZnO:Al acts as the oxygen source for the interfacial Si oxidation. [ABSTRACT FROM AUTHOR]

Microcrystalline silicon solar cells with an open-circuit voltage surpassing the 600 mV barrier were prepared by combining the use of a hot wire deposited p-i interface with that of an i layer deposited by radio-frequency parallel plate plasma deposition using controlled SiH4 flow profiling. The control of the bulk and interface properties facilitated effective charge carrier collection from i layers with a crystalline volume fraction as low as ∼30%. Judging from the absorption in the infrared and the excellent charge carrier transport, the optoelectronic properties of this material were still dominated by the crystalline rather than the amorphous phase. [ABSTRACT FROM AUTHOR]

A time-resolved optical emission spectroscopic study identified transient behavior of the excited SiH emission in a parallel plate SiH4/H2 plasma. The transient behavior could be prevented by filling the background gas with H2 prior to plasma ignition. Applying this condition, state-of-the-art microcrystalline silicon (μc-Si:H) could be deposited irrespective of the applied H2 flow, ultimately demonstrated by a 9.5% efficient solar cell deposited from pure SiH4. The results are discussed in terms of SiH4 back diffusion: an initial diffusion flux of SiH4 from the reactor’s dead volume back into the plasma. [ABSTRACT FROM AUTHOR]

The effect of process parameters on the plasma deposition of µc-Si:H solar cells is reviewed in this article. Several in situ diagnostics are presented, which can be used to study the process stability as an additional parameter in the deposition process. The diagnostics were used to investigate the stability of the substrate temperature during deposition at elevated power and the gas composition during deposition at decreased hydrogen dilution. Based on these investigations, an updated view on the role of the process parameters of plasma power, heater temperature, total gas flow rate, and hydrogen dilution is presented. [ABSTRACT FROM AUTHOR]

Abstract: For thin film silicon solar cells with high conversion efficiency and low production costs transparent conducting oxide (TCO) substrates with a high performance are required. TCO layers must have excellent conductivity and high transparency over a wide range in the VIS/NIR wavelength spectrum. Moreover, an advanced light trapping scheme has to be implemented, generally obtained by TCO surface texture. For low production costs it is favorable to keep the TCO and silicon layers as thin as possible. We present methods that accomplish all of these requirements. Tandem solar based on amorphous and microcrystalline silicon deposited on DC magnetron-sputtered zinc oxide (ZnO:Al) coated substrates, which have been capped with silicon and subsequently annealed at 500°C, result in an efficiency improved by 0.4% reaching 12.4%. This is due to the increased carrier mobility, reaching up to 70cm2/Vs, and improved transparency of the ZnO:Al. The high TCO conductivity allows for reducing the layer thickness significantly. In order to demonstrate the potential of combining this improved TCO with alternative light scattering concepts, we show results of a tandem cell on substrates comprising ZnO:Al sputtered on sol–gel coated light-scattering layers with dielectric particles having excellent light trapping properties. Combining both methods will allow for thin film silicon solar cells with high efficiency and potentially low production costs. [Copyright &y& Elsevier]

Abstract: This paper addresses the achievements and challenges in a-Si:H/µc-Si:H tandem solar cell technology including production aspects. As an example we show figures of the module production at Masdar PV, with module conversion efficiencies reaching 10%. The process integration was supported by PVcomB, namely, by developing improved PECVD processes and transferring them to Masdar PV. The important role of the TCO substrate for producing high-efficiency modules at lowest possible costs is discussed. We show the results of tandem cells on thermally annealed ZnO:Al substrates, resulting in an improvement of 0.4–0.5% (abs) as a result of the TCO annealing, reaching a stable efficiency of 12.1% for a 1cm² solar cell and 11.6% for a mini module. The challenges in developing thin film silicon solar modules with higher efficiency than presently reached are discussed. [Copyright &y& Elsevier]

Abstract: The present article gives a summary of recent technological and scientific developments in the field of polycrystalline silicon (poly-Si) thin-film solar cells on foreign substrates. Cost-effective fabrication methods and cheap substrate materials make poly-Si thin-film solar cells promising candidates for photovoltaics. However, it is still the challenge for research and development to achieve the necessary high electrical material quality known from crystalline Si wafers on glass as a prerequisite to harvest the advantages of thin-film technologies. A wide variety of poly-Si thin-film solar cell approaches has been investigated in the past years, such as thermal solid phase crystallization – the only technology that had already been matured to industrial production so far – the seed layer concept where a large-grained seed layer is epitaxially thickened, direct growth of fine grained material, and liquid phase crystallization methods by laser or electron beam. In the first part of this paper, the status of these four different poly-Si thin-film solar cell concepts is summarized, by comparing the technological fabrication methods, as well as the structural and electrical properties and solar cell performances of the respective materials. In the second part, three promising technologies are described in more detail due to their highly auspicious properties regarding material quality and throughput aspects during fabrication: (1) High-rate electron–beam evaporation of silicon for the low-cost deposition of high-quality material, (2) large-area periodic nano- and micro-structuring of poly-Si by the use of imprinted substrates providing a large absorption enhancement by a factor of six at a wavelength of 900nm, (3) liquid-phase crystallization of silicon thin-film solar cells by electron–beam, yielding an excellent poly-Si material quality reflected by an open-circuit voltage of 582mV which has been achieved only very recently. A successful combination of these three complementary technologies is envisaged to be the basis for a prospective low-cost and highly efficient poly-Si solar cell device. [Copyright &y& Elsevier]

Electrically detected electron-spin-echo envelope modulations (ED-ESEEM) were employed to detect hyperfine interactions between nuclear spins and paramagnetic sites, determining spin-dependent transport processes in multilayer thin-film microcrystalline silicon solar cells. Electrical detection in combination with a modified Hahn-echo sequence was used to measure echo modulations induced by 29Si, 31p, and ¹H nuclei weakly coupled to electron spins of paramagnetic sites in the amorphous and microcrystalline solar cell layers. In the case of CE centers in the μc-Si:H i-layer, the absence of ¹H ESEEM modulations indicates that the adjacencies of CE centers are depleted from hydrogen atoms. On the basis of this result, we discuss several models for the microscopic origin of the CE center and conclusively assign those centers to coherent twin boundaries inside of crystalline grains in μc-Si:H. [ABSTRACT FROM AUTHOR]

Abstract: GIXRF–NEXAFS is a combination of X-ray spectroscopy methods which allows for a non-destructive, depth-dependant chemical speciation of layer systems in the range of a few to several hundred nanometers. We applied this technique to a model system for thin-film silicon solar cells, a Si/ZnO layer system, which was investigated in its as-deposited and its annealed state. By means of total reflection at the buried ZnO/Si interface we could gain access to chemical information on the interface. In addition, a diffusion of contaminants from the ZnO into the Si was observed after annealing. [Copyright &y& Elsevier]

Abstract: A planar rear emitter back contact silicon heterojunction (PreBC-SHJ) solar cell design is presented, which combines the advantages of different high efficiency concepts using point contacts, back contacts, and silicon heterojunctions. Electrically insulated point or stripe contacts to the solar cell absorber are embedded within a planar hydrogenated amorphous silicon emitter layer deposited at low temperature on the rear side. The new solar cell design requires less structuring and allows large structure sizes, enabling the use of low-cost patterning technologies such as inkjet printing or screen printing. By means of numerical computer simulation the efficiency potential of back contacted heterojunction solar cells is shown to exceed 24%. First PreBC-SHJ solar cells have been realized and exhibit higher short circuit currents than our state-of-the-art front contacted silicon heterojunction reference solar cells. [Copyright &y& Elsevier]

Abstract: In this report, we discuss the influence of rapid thermal annealing (RTA) on the performance of polycrystalline Si (poly-Si) thin-film solar cells on glass where the poly-Si layers are differently prepared. The first part presents a comprehensive study of RTA treatments on poly-Si thin-films made by solid phase crystallization (SPC) (standard material of CSG Solar AG, Thalheim). By varying both plateau temperature (up to 1050°C) and duration (up to 1000s) of the annealing profile, we determined the parameters for a maximum open-circuit voltage (V OC). In addition, we applied our standard plasma hydrogenation treatment in order to passivate the remaining intra-grain defects and grain boundaries by atomic hydrogen resulting in a further increase of V OC. We found, that the preceding RTA treatment increases the effect of hydrogenation already at comparable low RTA temperatures. The effect on hydrogenation increases significantly with RTA temperature. In a second step we investigated the effect of the RTA and hydrogenation on large-grained poly-Si films based on the epitaxial thickening of poly-Si seed layers. [Copyright &y& Elsevier]

Abstract: An analysis of spin-dependent processes in microcrystalline silicon (μc-Si:H) pin solar cells is presented using pulsed electrically detected magnetic resonance (pEDMR). In this first study it is shown that by modulating the morphology of the n-type contact layer from amorphous to microcrystalline, pronounced changes in the pEDMR spectra may be observed. Due to the fact that pEDMR allows a deconvolution of the spin-dependent signals in time as well as in magnetic field domain, we were able to significantly reduce the complexity of the spectra compared to conventional EDMR. In the samples containing amorphous n-type contact layers we found signals from shallow localized conduction band tail states and phosphorous donor states. Upon replacement of this layer by its microcrystalline counterpart both signals disappeared. Possible spin-dependent transport mechanisms involving paramagnetic states in the various layers are discussed in view of sign and time evolution of the associated pEDMR signals. [Copyright &y& Elsevier]

Abstract: A 13.56 MHz parallel plate hydrogen-diluted silane plasma, operated at high pressure and high power, was used to deposit microcrystalline silicon solar cells with efficiencies of 6–9% at high deposition rates of 0.4–1.2 nm/s. In this regime new challenges arise regarding temperature control, since the high plasma power causes the substrate to heat up significantly during film deposition. We investigated this effect of plasma-induced substrate heating experimentally by means of pyrometric substrate temperature measurements and spectroscopic gas temperature measurements. The substrate temperature was observed to increase by up to 100 K during film deposition, depending on power density and deposition time. Performance of deposited solar cells decreased whenever the plasma induced heating caused a drift outside the ideal temperature window, of around 475 K (∼200 °C). Further analysis related this decrease in performance to the substrate temperature''s influence on film crystallinity and open circuit voltage. [Copyright &y& Elsevier]