Crystalline Silicon Heterojunction Solar Cells Based On Inorganic Carrier Selective Materials

This thesis aims to develop a high-efficiency and low-cost single-crystalline silicon(c-Si)heterojunction solar cell based on inorganic carrier selective materials to replace traditional p-n homojunction and amorphous silicon/c-Si heterojunction.At the interfaces between the carrier selective materials and c-Si,photogenerated carriers can be selectively separated and collected.In such solar cells,some efficiency-limiting physical factors induced by heavy doping,such as Auger recombination and free-carrier absorption,etc.can be avoided.Meanwhile,it is easy to fabricate without dangerous gases or high-temperature processes.Therefore,they are expected to become a new-generation c-Si solar cells.In this work,non-stoichiometric inorganic molybdenum oxide(MoOx)and lithium fluoride(LiFx)are chosen as a hole and an electron selective material to fabricate c-Si heterojunction solar cells.X-ray photoelectron spectroscopy(XPS)is used to characterize the elements,stoichiometry,conduction band,valence band and defect states within MoOx and LiFx films.It is found that MoOx and LiFx in contact with c-Si form interfacial barriers to block electrons and holes,which can selectively separate and extract holes and electrons,respectively.Then,MoOx and LiFx based c-Si heterojunction solar cells are fabricated through thermal evaporation.The MoOx and LiFx thicknesses are optimized,respectively.It is found that the maximum power conversion efficiency of 9.77%occurs when the LiFx film thickness is 1.5 nm and the MoOx film thickness is 20 nm.In order to reduce the c-Si surface reflection and further improve the power conversion efficiency of the solar cells,alkali solution is used to etch c-Si into random micropyramids.This significantly reduces the surface reflection of c-Si.However,due to the metal ion contaminations,a large number of recombination centers form at the heterojunction interfaces,resulting in a large series resistance and a small shunt resistance.An "S"-type I-V curve is even observed,leading to a greatly reduced power conversion efficiency.To avoid any contaminations during the processes,an ITO film is deposited on top of the MoOx/c-Si heterojunction.A reasonable design in thickness makes the ITO and MoOxthin films anti-reflective,particularly at the peak wavelength of the solar spectrum.Due to the increase of the photocurrent,the power conversion efficiency is also increased by 4%to 10%.Meanwhile,a microstructured polydimethylsiloxane(PDMS)film is also prepared by using the micropyramid c-Si as a template.Due to the good refractive-index matching and scattering characteristics,the sunlight is effectively coupled into the c-silicon cell,leading to an improvement of 13%in power conversion efficiency(10.87%),compared with that of the solar cells without the anti-reflective PDMS film.In a word,the conclusions and findings in this thesis provide a foundation for the fabrication and optimization of c-Si heterojunction solar cells based on inorganic carrier selective materials.