Toward the high efficiency Si solar cells, the surface passivation technology is one of the most important issues along with fabrication process, so the a-Si:H/c-Si heterojunction structure is a good candidates for that reason recently. In a-Si:H/c-Si heterojunction solar cell fabrications, the substrate preparation is very important for surface passivation quality. We optimized wet cleaning process and developed analyzing method using spectroscopic ellipsometric technique. As higher <ε2> at 4.25eV as, the interface defect density could be decreased. Through surface smoothing effect, we acquired the higher <ε2> at 4.25eV value 43.02 than common RCA process which matching about 3x1011(cm-2,ev-1) Dit. And then we conducted additional chemical treatment with NH4F solution having very neutral properties can etch the Si surface really slowly with SiOx layer removal simultaneously. As a result, the surface passivation quality was enhanced and the implied Voc was reached over 740mV increased about 3~5mV than normal DHF treatment.
The intrinsic a-Si:H layer is necessary for excellent surface passivation in which the epitaxial or crystalline phase formation need to be prevented. We varied the H2/SiH4 gas ratio in PECVD plasma conditions and found out very critical and abrupt point for H2/SiH4 gas ratios. The minority carrier lifetime (τeff) was maximized at that conditions, which might related to silicon-hydrogen bonding configuration in the a-Si:H thin films. As low Si-H2 or Si-H3 multi bonds as, the Si-H single bonds terminated substrate surface and thin film dangling bonds when just before the phase transformation to epitaxial growth. For the quantized evaluation, we conducted interface defect density (Dit) between intrinsic a-Si:H/c-Si through conductance method in Capacitance-Voltage measurement. The Dit was derived successively by equivalent circuit matching and acquired about 7.6x1011(cm-2ev-1) as a result which corresponds to 0.71V built-in potential. This method can be implicated for intrinsic amorphous Si case like to MOS (metal oxide semiconductor) structure application
The dopant/SiH4 ratios, PH3/SiH4 and B2H6/SiH4 are optimized for surface passivation quality and electrical properties. As PH3 flow rate, the effective minority carrier lifetime (τeff) was changed, and then reached to maximum value at optimal condition for Si-H bonds fraction and peak wavenumber value. The B2H6 flow rate is really more sensitive to surface passivation performance than N type cause by dopant concentration as defect sources over 1.0x1021 (atoms/cm3). In the FT-IR absorption spectrum, the Si-H single bonds are preferred to silicon-hydrogen termination, so the passivation quality is strongly concerned.
In terms of SiH4 base plasma deposition process, SiH* reactive radicals’ relative concentration is depended on their electron temperature (Te). As low electron temperature as, the SiH3* radical can be more produced and favorable to Si- dangling bonds termination. As a result, reduced electron temperature in plasma, leaded the SRV enhancement for 28% and 60% for N and P type a-Si:H case respectively. There is some material alteration regarding to Si-H bonds configuration where the Si-H bond fraction increased and Si-H2 bond decreased contrastively. The RAMAN peak was shifted to crystalline phase direction where the intermediate increased to 10~12% volume fraction about 2.45~4.2 times higher than ND (normal dilution) case. For the excellent surface passivation performance using a-Si:H thin films, it is necessary that the maximizing Si-H single bonds and sustaining conditions just before crystallization through low electron temperature.
The contact resistance (Rc) was analyzed through simple structure using voltage-current sweeping with TLM(or Burgurs) pattern. The ITO electrode was very effective for contact resistance decrease represented liner voltage-current test. The contact resistance was reached to 3.0~9.4x10-3Ωcm2 and 1.1~3.8x10-3Ωcm2 in N and P type a-Si:H, respectively. Finally we fabricated the heterojunction backcontact solar cell through optimized recipes. We accomplished best efficiency about 23.4% in 4cm2 small size cell with very high Voc and Jsc , 723mV and 41.8mA/cm2 respectively. For the 6 inches solar grade wafer scale, we adopted all screen printing technology and acquired 20.7% and 20.5% conversion efficiency under AM 1.5 condition by Ag paste and Cu/Sn electroplated metal electrode each.