Periodic Reporting for period 1 - cSiOnGlass (Development of high-quality crystalline silicon layers on glass)
Reporting period: 2015-05-01 to 2017-04-30
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 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.