【作者】 沈玲；

【导师】 马忠权；

【作者基本信息】 上海大学 ， 凝聚态物理， 2013， 博士

【摘要】 本论文针对低成本和高效率这两个太阳电池研究方向，开展如下方面的研究：（1）利用脉冲激光沉积技术制备掺铝的氧化锌薄膜（简称ZnO:Al或AZO），表征不同温度和气压下的形貌和光学性质，希望通过揭示温度和气压对薄膜生长的影响，获得实现平整致密的宽带隙高质量薄膜的优化参数区间。随后采用磁控溅射技术制备ZnO:Al/c-Si异质结太阳电池。为更深刻地认识这种异质界面的复杂性质，在ZnO:Al/c-Si的界面用快速热氧化的方法生长一层超薄硅氧化层，通过变温I-V的测试来分析载流子的输运机制,以期为ZnO:Al/c-Si异质结太阳电池光伏性能的优化提供解决思路。（2）利用ELO（Epitaxial Lift-Off）技术将基于光刻技术制备的微型砷化镓太阳电池转移至掺有有机发光基团的波导的表面。这种增加电池对光子吸收的设计有望以较低的额外成本带来较大的电池效率增加。此外，利用机械叠层的概念，把高效单块三结电池作为顶部电池，单结锗作为底部电池，制备了类四结结构。折射率匹配的As2Se3薄膜被用作绝缘隔离层而置于顶部和底部电池之间。因为在电学连接和材料选择方面的灵活性，这种类四结结构可以突破传统单块四结电池的电流匹配和晶格匹配限制，具有实现更高总输出效率的潜力。主要研究成果如下：（1）随着生长温度从500C降到200C，AZO的光学带隙从相应的3.44eV增至3.62eV，发生蓝移现象。700C的热处理使得光学带隙降至3.30eV到3.35eV之间。XRD揭示结晶质量随生长温度升高而改善。借助存在非晶态时半导体态密度与能量间的关系图，这种蓝移现象被证明是源自氧化锌薄膜中的非晶态，蓝移程度与非晶态和结晶态的相对比重有一定关联性。（2） XRD和SEM的结果表明：氧压从25Pa升至35Pa时，AZO薄膜的衍射峰强度出现大幅增加，表面形貌明显更加致密和平整。这些变化行为迥异于氧压小于25Pa和大于35Pa时所对应的变化行为，表明存在一个从25Pa到35Pa的特殊氧压范围。EDS谱证实：随氧压变小，热稳定性变差，Zn/O原子个数比值也随之增加。结合薄膜生长的热力学和动力学机制，我们猜测存在一个沉积速率随氧压增加而明显增加的氧压区间，并随即通过薄膜厚度的数据被直接证实。至此，一个特殊的具有以下特点的氧压范围被证实：动力学因素的影响有限，氧气压强升高不足以影响粒子的表面扩散能力，却使薄膜的热稳定性得到改善，导致沉积速率随氧气压强升高而升高，有助于形成致密平整的薄膜。（3）面积为1×1cm²的ZnO:Al/p-Si异质结太阳电池在AM1.5D条件下的测试结果显示：典型的转换效率在0.7到1.14之间，与效率1.14相对应的开路电压是400mV,短路电流密度为17.27mA/cm²,填充因子则是16.5。光照I-V曲线呈现的roll over及crossover行为分别被归咎于金属与p型硅之间非理想的欧姆接触和缺陷能级。变温I-V分析结果证实SRH复合主导着正向小偏压下的电流输运。伴随着界面热氧化时间从5分钟延长至10分钟,SRH复合机制的激活能从0.625eV降至0.26eV，直接证实了晶格失配引入的界面态是SRH复合中缺陷能级的主要来源。（4）面积为240×200um²,厚度仅为2.68um的GaAs微型薄膜电池通过转移印刷的方法被置于发光波导的表面。基于理论模型给出的优化参数区间，制备了厚度为150um，发光基团与载体的质量比为0.2wt的发光波导。与仅有镜面式背反射器的设计相比，GaAs电池在发光波导的作用下，短路电流增至前者1.51倍。在散射式反射器与发光波导的组合下，短路电流则增至1.71倍。为了减小界面直接接触所造成的一部分反射损失，在散射式反射器与发光波导界面间制造了一超薄空气层，短路电流增至2.21倍。相对外量子效率测试结果分别指出了发光基团在短波段和背反射器在长波段的电流贡献，证实了这种设计能提高光子收集的优点。（5）利用转移印刷技术制备了三结GalnP/GaAs/lnGaNAsSb电池作为顶部电池和单结锗作为底部电池的机械叠层，折射率匹配的As₂Se₃薄膜作为界面缓冲层。鉴于前人多采用粘性聚合物作为叠层电池中间的界面缓冲层，对Si/粘性聚合物/Ge和Si/As₂Se₃/Ge这两种模拟结构进行的红外反射率测试显示：后者具有更低的反射率，意味着在真实的叠层结构中有更多的长波长光子可被底部Ge电池吸收。光照I-V测试显示：单结Ge电池和单块三结电池在1000个太阳光照下的开路电压分别为0.374V和3.47V，对应的短路电流达到6.99A/cm²和14.5A/cm²。通过特殊的电学互连方案可获得一个具有最大输出功率的两端工作器件，为这种机械叠加概念的实际应用提供了有效的解决方案。更多还原

【Abstract】 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.更多还原