Study On Passivation Of A-Si Thin Films Based On Soft Plasma Treatment

Faced with the increasingly serious energy and environmental issues,more and more attention has been paid to photovoltaic solar energy technologies.Among them,crystalline silicon solar cell technology has been highly industrialized but its efficiency has reached a bottleneck.In order to further improve cell efficiency and reduce cost in achieving grid parity,a large number of PV manufacturing companies,universities a nd research institutes have been devoting to a variety of low-cost and high-efficiency crystalline Si solar cells.As one kind of high-efficiency crystalline silicon solar cells,amorphous silicon/crystalline silicon heterojunction solar cell is expected to further reduce the cost,and thus has become a research hotspot in the industry.For heterojunction solar cells,the key to achieving high conversion efficiency is that amorphous silicon films can achieve excellent passivation of dangling bonds on crysta lline silicon surfaces.Commonly used methods for preparing amorphous silicon thin films include plasma enhanced chemical vapor deposition,inductively coupled plasma chemical vapor deposition and hot-wire chemical vapor deposition.However,for the former two deposition techniques,the deposition process may induce surface damage onto the substrate due,while the third one can easily cause the low temperature epitaxial growth of silicon,which reduces the quality of the thin film.In this paper,we used an advanced and soft plasma deposition technology to prepare amorphous silicon thin films,and systematically studied their passivation performances on crystalline silicon surface.The specific research contents are as follows:(1)The configuration,principle and advancement of the self-developed soft plasma deposition system(i.e.CCEP: Capacitively Coupled Electrodeless Plasma)was introduced and the experimental procedures for depositing amorphous silicon films were described in detail.(2)Amorphous silicon thin films with different quality were prepared by changing the soft plasma processing parameters(deposition pressure,power,temperature,and hydrogen/silane flow ratio,etc.).Fourier transform infrared spectroscopy,Rama n spectroscopy,Ellipsometry,and Scanning electron microscopy were used to study the microstructure of the films.The influence of the processing parameters on the microstructure of amorphous silicon thin films was analyzed by the hydrogen content and microstructure factor of the deposited thin films.The results show that the films deposited at the working pressure of 65 Pa possess a higher hydrogen content of ~17.6% and a better compactness with the minimum microstructure factor of 0.24.No matter when the working pressure increases or decreases,the hydrogen content decreases and the microstructure factor increases simultaneously,resulting in worse compactness of the deposited films.The films deposited at an applied RF power of 100 W has a higher hydro gen content of 18.2% and the best compactness with the minimum microstructure factor of 0.28.Too low or too high applied power may degrade the quality of the deposited films.The former case is due to the fact that silane cannot be completely decomposed,while the latter may be related to the surface damage caused by the ion bombardment effect because of the higher power.The films deposited at temperature of 50 ? exhibit the highest hydrogen content of 21.3% and a relatively loose microstructure with a microstructure factor of 0.35.As the deposition temperature increases,the deposited films become dense r but the hydrogen content decreases at the same time.When the hydrogen/silane flow ratio(R)was R=0,the hydrogen content of the film was up to 24.27%,and the microstructure factor was relatively higher.The hydrogen content gradually decreases but the films became relatively denser with the increase in hydrogen/silane ratio.The hydrogen content of the film was 20.16%,and the film had the best compactness with the minimum microstructure factor of 0.25 for the case of R=5.When the flow ratio(R)continued to increase,excessively high hydrogen dilution may induce defects into the deposited films,leading to poor film microstructure quality.(3)Amorphous silicon thin films were deposited on the double side of n-type(resistivity: 0.7-1 ?·cm,thickness: 430 ?m)crystalline silicon wafers based on the soft plasma.The surface recombination velocity was calculated based on the measured minority lifetime and open circuit voltage from a minority lifetime tester.The effect of different processing parameters on the passivation performance of amorphous silicon films was systematically studied.The results show that as the deposition pressure increases,the passivation performance of the film increases due to the increasing hydrogen content and the decreasing microstructure factor.When the deposition pressure further increases,the microstructure factor of the film increases resulting in worse compactness and pas sivation performance.As the deposition power increases,the passivation performance of the film increases first and then decreases.Too low or too high applied power may lead to worse compactness,resulting in worse passivation performance.When the temperature is 50 ?,the film has the highest hydrogen content.As the deposition temperature increases,the deposited films become more dense but the hydrogen content decreases at the same time,the corresponding passivation performance decreases first and then increases.W hen the temperature is too high,the passivation performance of the deposited film decreases due to the significantly reducing hydrogen content.When the hydrogen/silane flow ratio is relatively low,the porous microstructure formed in the film results in poor film passivation performance.With the increase in hydrogen/silane ratio,the microstructure factor of the film decreases gradually and the passivation performance of the film is improved.If the silane is over diluted,the microstructure factor of the film increases again and the passivation performance of the film decreases.Finally,at the optimized conditions(deposition pressure: 65 Pa,power: 100 W,temperature: 50 ?,hydrogen silane flow ratio: R=5),the amorphous silicon film passivated crystalline silicon wafer obtained minority lifetime up to 445 ?s and the open circuit voltage reached 696 mV,the surface recombination velocity was as low as 48 cm/s,and the passivation performance of the film was significantly improved.