Studies Of ZnO:Al/c-Si Heterojunction Photovoltaic Properties And Highly-efficient Microscale Ⅲ-Ⅴ Compound-based Solar Cells

The advanced PV materials and novel solar cells with low cost and highefficiency, respectively, have been the main two concerns for the researchers in thephotovoltaic field. With these purposes, the studies have been conducted here in thedissertation:（1） Aluminium doped Zinc oxide （AZO） has been grown by the methodof pulsed lased deposition, the morphology and optical properties have beencharacterized and analysed with different growth temperature and ambient pressure.An optimal parameter range of flat and dense film with high crystalline quality issupposed to obtain by revealing the impact of growth temperature and ambientpressure on film properties. ZnO:Al/c-Si heterojunction solar cells have beensubsequently prepared with the commercially cost-effective magnetron sputteringtechnique. To get deeper understanding of the complicated interfacial properties inZnO:Al/c-Si heterojunction, an ultra-thin silicon oxide has been grown by rapidthermal oxidization. The dependence of I-V curves on temperature has been studiedto explore the transport mechanisms, which are expected to help the optimization ofthe heterojunction photovoltaic performance.（2） The microscale GaAs solar cells have been fabricated by lithography technologywith MOCVD-grown GaAs epitaxial stack and subsequently transferred onto the topsurface of luminescent waveguide with the ELO （Epitaxial Lift-Off） technique.Thisdesign with more photon absorption is expected to achieve a large boost inconversion efficiency with relatively low extra cost. On the other side, with theconception of mechanical stacking, monolithic three junction solar cells based on Ⅲ-Ⅴ compounds have been taken as top cells and then mounted on the surface of thebottom single-junction Ge solar cells. The As₂Se₃films with matched refractiveindex have been put in between the top and bottom cells as insulating layers.Considering the flexibility of electrical interconnection and material options, theconfigurations have been expected to achieve higher power output than theconventional monolithic four-junction solar cells, which have to meet the needs of lattice match and current match.The major results are summarized as follows:（1） With the growth temperature decreasing from500C to200C, the optical bandgap has a blue shift, increasing from3.44eV to3.62eV correspondingly. After thesubsequent thermal annealing at700C, the values of the optical band gapimmediately decline to the range of3.30eV to3.35eV. XRD results indicate crystalquality get improved with growth temperature increases. Based on the energydiagram of state density versus energy in semiconductor with amorphous phase, thephenomenon of blue shift is confirmed to be intrinsic to the amorphous phase in zincoxide films and the degree of blue shift is dependent on the ratio of amorphous/crystalline phase.（2） The results of XRD and SEM show: with the oxygen pressure increasing from25Pa to35Pa, the diffraction intensity has an abrupt increase and surfacemorphology apparently become more dense and flat. These behaviors are differentfrom what happen in the range of oxygen pressure smaller than25Pa or larger than35Pa. That means an existence of a special oxygen pressure range. EDS resultsfurther confirm that the thermal stability gets improved and the atom ratio of Zn/Oincreases with oxygen pressure decreases. Based on the thermodynamicmechanism of film growth, we speculated that the deposition rate will increase withoxygen pressure in a given oxygen pressure range, which has been subsequentlytestified by the characterization of film thickness. Thus a special pressure range hasbeen confirmed with the following features: the dynamic factor has negligible effecton film quality, therefore increasing oxygen pressure will not affect the atomdiffusion ability too much but improve the film thermal stability. Consequently, thedeposition rate is increased with oxygen pressure, leading to a dense and smoothsurface morphology.（3） The ZnO:Al/c-Si heterojunction solar cell with the area of1×1cm²underAM1.5D have been studied: Typical conversion efficiency is in the range of0.7%to 1.14%and corresponding to the1.14%conversion efficiency the open circuit voltageis400mV, the short-circuit current density is17.27mA/cm²and fill factor is16.5%.The rollover and cross-over behavior of light I-V curves have been respectivelyattributed to the non-ideal omic contact and defect levels. The temperaturedependence of dark I-V curves helps to confirm that the SRH recombinationdominates the carrier transport in the small forward bias range. Prolonged thermaloxidation time leads to a decrease in activation energy of SRH recombination, whichfurther confirms: the interfacial states induced by lattice mismatch are the mainsource of defect levels in SRH recombination. These defect levels cause currentleakage under small reverse bias and result in smaller Rsh, which is reflected by theobvious change around the short-circuit current in light I-V. Meanwhile, these defectlevels cause more recombinational current in light than in dark condition for thesmall forward bias, leading to the “cross over” behavior. All the above resultsindicate that the interfacial states have a vitally negative effect on the performance ofn-AZO/p-Si hete-rojunction.（4） The microscale GaAs solar cells with the area of240×200um²and thethicknesses of2.68um have been transferred and printed onto the top surface ofluminescent waveguides. The luminescent waveguides have been made with thethicknesses150um and the weight ratio of dopant luminophores and matrix polymer0.2wt%. These parameters fall within the optimal setup range based on thetheoretical modeling results. Compared to the reference of GaAs on specular backreflector, the short-circuit current has achieved a1.51×increase with theincorporation of luminescent waveguide. Moreover, a1.71×increase of short-circuitcurrent has been obtained with the configuration of diffusive back reflector andluminescent waveguide. To reduce the interfacial reflectance losses from thecontradiction between total internal reflection （TIR） and diffusive back reflection, anuntrathin air gap is formed in between diffusive back reflector and luminescentwaveguide. With this modified configuration, the short-circuit current has a2.21× increase. Besides, relative EQE measurements help to elaborate how theluminophores and back reflector in different configurations contribute to currentincrease, which reflects the advantage of photo absorption in this design.（5）The monolithic three-junction GalnP/GaAs/lnGaNAsSb solar cells have beentaken as top cells and then mounted on the surfaces of single-junction Ge solar cells.To reduce the interfacial reflectance losses from the mismatch of refractive indexes,As₂Se₃thin film is chosen to be the interfacial buffer layer in between the top andbottom solar cells. Given that the adhesive polymers are usually taken as interfacialbuffer layers by former researchs, the measurements of infrared reflectance havebeen conducted with the mimic stacking structures of Si/polymer/Ge and Si/As₂Se₃/Ge to compare the interfacial optical properties. It turns out that the latterstructure has lower reflectance, which means more long-wavelength photons can beabsorbed by the bottom Ge solar cells in real staking structures. The photovoltaicproperties in330suns condition have been obtained: corresponding to single-junctionGe and three-junction GalnP/GaAs/lnGaNAsSb solar cells, the open circuit voltagesare0.35V and3.5V, and the short-circuit current are6.5mA/cm²and10mA/cm². aspecial parallel electrical interconnection of one top cell and ten bottom cells inseries can be used to realize a two-terminal device with a maximum output power.Which provides this mechanical stacking with an effective solution to practicalapplication.