Preparation And Properties Of Cu（InGa）Se₂ Thin Films And Solar Cells By E-beam Evaporation

As sustainable and green energy,solar energy is abundant for storage.Solar cells are able to convert solar energy into electric energy,which have a great application prospect.As a second-generation solar cell,Cu（In Ga）Se₂（CIGS）thin film solar cell has lots of advantages.CIGS is a semiconductor which possesses tunable direct bandgap,high optical absorption coefficient(～10⁵cm^-1)and wide range of solar spectral response characteristics.Moreover,CIGS material is cost-effective and it avoids the problem of element toxicity.These superiorities make CIGS be an ideal absorption layer material.In this thesis,CIGS thin films were prepared by electron beam evaporation.The effects of process parameters,Sb and Ca doping on the properties of CIGS thin films and solar cells were studied.Flexible and Cd-free CIGS solar cells were also fabricated.In this paper,two-step process was adopted to prepare CIGS thin films.In first step,metal stack precursors were deposited by electron-beam（e-beam）evaporation and the precursors were selenized in second step.Firstly,CIGS films were prepared on soda-lime glass at room temperature to investigate the Cu and Ga contents on the properties of CIGS films and solar cells.Subsequently,the optimal element content ratios were adopted to prepare CIGS films and substrate temperature and post-annealing process were studied.As the substrate temperature increased,the crystal quality of CIGS film improved.When the substrate temperature was higher than 300℃,Mo Se₂phase was detected in the film,which indicated the quasi-ohmic contact between Mo and CIGS.The CIGS solar cell fabricated at 300 ^oC exhibited the optimal photoelectric property and achieved the highest conversion efficiency of 7.1%.Furthermore,when the annealing process was adopted at 400℃,the efficiency of solar cell showed further improvement of 9.0%.Aiming at the problems such as the requirement for relatively low substrate temperature during deposition of precursors and difficulty on controlling the Cu content and element distribution gradient in the surface region of the films,Sb doping was carried out.Sb elements were doped through depositing upper Sb and lower Sb respectively in the precursor stacks.It is discovered that Sb doping may introduce structural change and stress in films.The crystallization quality and compactness of CIGS films was improved after Sb doping.Cu/Cu⁺redox couple mechanism was proposed to study the energy band of pristine and Sb-doped films.It is learned that the Fermi level of surface of CIGS film moved to higher level after doping of upper Sb layer.The efficiency of CIGS solar cell was improved from 3.3%to 7.2%after doping with 20 nm upper Sb.A further improved efficiency of 9.6%of Sb-doped solar cell was achieved through increasing the substrate temperature to 300℃.The results of SCAPS simulation showed that the doping of upper Sb shallowed the defect energy,which was beneficial to the photovoltaic performance of device.In CIGS film doped with lower Sb layer,most Sb elements accumulated near the CIGS/Mo interface and Cu_2-xSe phase was detected.It was also found that the defect energy of solar cell doped with lower Sb was deeper than that doped with upper Sb through SCAPS simulation,which explained the inferior performance of device.Aiming at the limitation issue of distribution of elements in the surface region of CIGS films,Ca elements were doped in the CIGS absorption layer through post-deposition treatment（PDT）of Ca F₂films.The results showed that the uniformity of composition distribution in CIGS thin films was improved with a certain thickness of Ca F₂.The increase of Ca F₂doping amount would enhance the Cu depletion of surface of CIGS films.Subsequently,PDT and pre-deposition treatment of Ca were compared.It was found that Ca elements hardly diffused to the surface of CIGS films.Moreover,Ca F₂accumulated at the interface of Mo/CIGS would deteriorate the quasi-ohmic contact between the absorption layer and back electrode.The Fermi level of surface of CIGS films doped with Ca by PDT process moved to lower level and thus the films present enhanced p-type conduction,which decreased the barrier height of CIGS/Cd S interface.Furthermore,SCAPS software was used to simulate the CIGS device.It was concluded that the defect level of CIGS solar cell doped with Ca by PDT process was shallower.The conversion efficiency of CIGS solar cell doped with 20 nm Ca F₂by PDT was the optimal,which was 2.9%higher than that of pristine solar cell.A further improved efficiency of 9.3%of Ca-doped solar cell was achieved through increasing the substrate temperature to 300℃.To further expand the application range of CIGS solar cells,flexible CIGS solar cells without diffusion barrier were fabricated on 50 and 100μm thick Ni and Ti foil substrates.Due to the different crystal orientation of substrates,the CIGS films prepared on Ti and Ni foils exhibited（112）and（220）preferred orientation respectively.It is discovered that the metal elements in Ti foils could barely diffuse into the absorption layer.Compared to those on Ni foils,CIGS films prepared on Ti foils present better compactness and larger grains.Consequently,CIGS solar cells fabricated on Ni foils exhibited inferior photoelectric performance,while the conversion efficiency of cells fabricated on 50 and 100μm Ti foils were 4.9%and 6.6%,respectively.The bending test of flexible CIGS solar cells fabricated on Ti foils were subsequently carried out.After100 bending cycles,the efficiency of cells fabricated on 50 and 100μm Ti foils decreased by1.0%and 2.6%,respectively.To fabricate environment-friendly and non-toxic CIGS solar cells,a-Si thin films deposited by e-beam evaporation instead of Cd S films were used as buffer layer to fabricate CIGS/a-Si heterojunction solar cells.The results showed that the conduction band alignment of CIGS and a-Si is a favorable spike,which is more desirable in heterojunction solar cell.The as-prepared CIGS/a-Si interface was compact and no pinholes were observed.As the thickness of a-Si buffer layers increased,the photoelectric properties of CIGS solar cells were improved.The conversion efficiency of CIGS solar cell with 60 nm a-Si buffer layer was the highest,which achieved 5.0%.Subsequently,SCAPS was used to simulate CIGS/a-Si heterojunction solar cells.The simulation results implied that the defect level of the solar cell with 60 nm a-Si buffer layer was the shallowest.Moreover,it manifested that the increase of bandgap and decrease of conduction band energy of absorption layers were beneficial to the performance enhancement of CIGS/a-Si heterojunction solar cells.According to the simulation results,the Ga/（Ga+In）ratios of CIGS films were modulated to adjust the band gap and conduction band energy.Finally,the solar cell based on CIGS absorption layer with GGI ratio of 0.40 present the optimal photoelectric performance,which achieved the efficiency of 6.6%.