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Development of high-quality crystalline silicon layers on glass

Periodic Reporting for period 1 - cSiOnGlass (Development of high-quality crystalline silicon layers on glass)

Reporting period: 2015-05-01 to 2017-04-30

Photovoltaic electricity generation with solar modules, converting sunlight directly into electricity, plays an important role in the worldwide efforts to increase the share of renewable energy production. Silicon (Si) wafers and its processing to solar cells are the most expensive parts of solar module fabrication. The fabrication of thin (10-20 micrometer (um)) high quality micro-crystalline silicon (mc-Si) layers applying liquid-phase crystallization of Si on glass (LPCSG) by line focus laser is a promising method to reduce Si consumption in solar modules by a factor of 10 and to reduce processing costs (see image of LPCSG absorber).
One objective of the cSiOnGlass project executed at the Leibniz-Institute of Photonic Technologies in Jena, Germany, was the fabrication and investigation of barrier layers (BL) like SiO2 and SiNx deposited on the glass before deposition and crystallization of the Si layer to improve mc-Si layer quality.
Next, an objective of the project was to investigate different glasses like Borofloat 33 from Schott and Corning Eagle for implementation in a large area (several sqm) module or as very thin (100-200 um) glass substrate for a so-called wafer equivalent.
Finally, an objective was the development of a contact-less characterization method to determine the electronic material quality of mc-Si layers on glass.
Concluding, in the cSiOnGlass project considerable progress was made in the determination of the electronic quality of the LPCSG absorbers by using the quasi steady-state photoconductance (QSSPC) method. This method determines the effective charge carrier lifetime (indicator for material quality) in function of the illumination intensity exposed to the sample. Lifetime data from the QSSPC method correlates with lifetime data from photoluminescence (PL) decay measured by time-correlated single photon detection. Effective lifetime of more than 400 ns has been measured in LPCSG absorbers indicating an effective charge carrier diffusion length more than the double of the absorber thickness (10 um). In small (mm) solar cells open circuit voltage of 629 mV on Corning Eagle glass and 603 mV on Schott Borofloat 33 are measured. These values confirm the high electronic material quality of the mc-Si layers after hydrogen passivation which was determined from lifetime data. Sputtered and post annealed SiO2 layer in the thickness range from 100 to 300 nm are effective barrier layer to prevent impurity diffusion from the glass. SiNx layers present issues related to the formation of non-homogenous Si/glass interface after laser crystallization. Glass bending after the laser crystallization is an issue for large scale industrial application and should be focus of further research.
For the deposition and investigation of BL in the cSiOnGlass project, rf-magnetron sputtering equipment was installed for the deposition of hydrogen-free SiO2, SiNx and SiOyNx layers. BL layers were deposited on glass at 200 oC and post-deposition annealing at 500 oC is performed to obtain well-working diffusion barrier layers in the laser crystallization process. Annealed SiO2 layers in the thickness range from 100-300 nm present a good diffusion barrier to suppress impurity diffusion from the glass. Laser crystallization experiments in this work were performed with Si layer on Corning Eagle (CE) glass with 1.1 mm and 0.7 mm thickness and on 3.3 mm Schott Borofloat 33 (B33) glass. The implemented glass size is 2.5 cm x 2.5 cm and 5 cm x 5 cm. Glass bending after the laser crystallization leads to a concave shape at the Si side but does not present an issue for the fabrication of small solar cells or for characterization of the layers on laboratory scale.
For the determination of the electronic material quality of LPCSG layers for the first time the quasi steady-state photoconductance (QSSPC) method was applied. It is shown in this project that by modification of standard equipment (WCT-120, Sinton Instruments) effective charge carrier lifetime can be measured by the QSSPC method in only 5-20 um thick LPCSG absorbers. These lifetime measurements present a dependency of the maximum lifetime on the doping density in n-type LPSCG absorbers. Maximum lifetime of about 400 ns, indicating a diffusion length of more than 20 um in a 10 um thick absorber, is measured in low-doped (0.7 ohmcm) n-type LPCSG absorbers on 3.3 mm B33 glass. On CE glass maximum lifetime of about 100 ns was measured. Details on the development of the QSSPC method for application on LPCSG absorbers is published in three articles and was presented in two oral presentations at international conferences. Charge carrier lifetime was calculated also from the decay of the photoluminescence (PL) generated by picosecond laser and was measured with time-correlated single photon detection equipment. In addition, PL spectra due to the band-to-band recombination in LPCSG layers were measured at room temperature for the first time by using a spectrometer in combination with an optical fiber. Latter is also reported in more detail in an article.
Small (1-2 mm diameter) solar cells were fabricated on LPCSG absorbers on B33 glass and 0.7 mm CE glass by implementation of an amorphous intrinsic/p-type Si hetero emitter and transparent conductive oxide. In two studies concerning surface treatments (etching and texturing) of LPCSG absorbers is shown that data from QSSPC method presents well the average LPCSG absorber material quality (on about 4 cm2) and correlates with data from small solar cells. Open circuit voltage (Voc) of 629 mV was measured in a solar cell on an 8 um thick LPCSG absorber (0.02 ohmcm) on CE glass. For a solar cell on B33•glass the highest Voc of 603 mV was measured on a textured LPCSG absorber (0.2 ohmcm). For the fabrication of interdigitated back contact (IBC) solar cells on LPCSG absorbers the so-called “Cold IBC process” developed at the Polytechnical University of Catalunya (Barcelona) was applied (see G. López et al., Energy Procedia 92 (2016) 652). High quality IBC schemes with diffusion length larger than the absorber thickness were fabricated on LPCSG absorbers.
In the cSiOnGlass project the contact-less QSSPC method is further developed for measurement of LPCSG absorbers with very thin Si layers which presents an important progress beyond the state of the art for determination of absorber quality. Then, the deposition technology for sputtered SiO2 and SiNx used as BL in LPCSG absorbers was installed and investigated. These layers are now available for the research group at IPHT, Jena. Next, the easy measurement of PL spectra of Si at room temperature using an optical fiber coupled to a spectrometer was developed in collaboration with the “Fibre Optics Division” of the host institution. This opens new collaboration opportunities in the field of Si technology. Finally, in collaboration with the Institute of Physics (University of Halle) charge carrier lifetime in mc-Si of LPCSG absorbers was measured for the first time by TCSPD equipment, widely used for microcrystalline Cu(In,Ga)Se2 (CIGS) solar cell material. This presents an interesting overlap of Si and CIGS technology. Furthermore, for the fabrication of IBC solar cells the implementation of dielectric passivation schemes in combination with laser doping was investigated as an alternative process to conventional diffusion of doping impurities.
3.3 mm Borofloat 33 glass with 10 um microcrystalline silicon layer