1.0eV GaAs基InAs量子点太阳能电池

1.0eV GaAs based InAs quantum dot solar cells

三结InGaP/GaAs/Ge太阳能电池理论设计中加入带隙为1.0 eV的材料代替带隙为1.4 eV的GaAs中间电池有助于解决多结串联电池的电流阻塞效应实现电流匹配,然而带隙为1.0 eV的InGaAs和GaInNAs外延困难.我们利用分子束外延方法外延得到In0.15Ga0.85As量子阱,InAsdots-in-well量子点以调整太阳能电池带隙.X射线衍射谱中观察到了量子阱的多级卫星衍射峰,量子阱界面陡峭.扫描透射图显示量子点呈金字塔状,量子点的高度约为12 nm,底边长约为27 nm.由原子力显微镜图可知,量子点密度约为2×10^(10) cm^(-2).低温光致发光谱显示量子点在As4束流下呈双模分布.光电流响应谱显示InAsdots-in-well量子点太阳能电池吸收波长可以达到1300 nm,相应于带隙约为1.0 eV.器件J-V特性显示短路电流相比于GaA s标准p-i-n电池增加了37.8%.这表明该InAsdots-in-well量子点太阳能电池有望改善多结太阳能电池的设计中的电流阻塞效应,实现电流匹配,在多结太阳能电池的设计中具有广阔的应用前景.

There are great improvements in III-V semiconductor solar cells including GaA s single junction solar cell and tandem solar cells in last decades. The common commercially used triple-junction solar cell mostly utilize a Ge bottom cell with an（In）GaA s and InG a P middle and top cell respectively. The（In）GaA s middle cell absorbs limited solar spectrum and show minimum short-circuit current in multiple junction solar cell, which determine the overall current of the cell. Theoretically, higher energy conversion efficiency will be obtained when introducea subcell with 1.0 eV band gap in series to the In GaP/GaA s/Ge tandem structures. Lattice-matched GaI nN As and lattice-mismatched In_（0.3）Ga_（0.7）As are two common choices for achieving 1.0 eV band gap. However, the addition of N to GaA s has detrimental effects on the material quality, limiting its utility to approaches that required photocurrent. And the lattice mismatch makes thick In_（0.3）Ga_（0.7）As heterogeneous epitaxial difficult. The incorporation of low band gap nanoscale materials, such as quantum well and quantum dots, into current limiting junction of a multijunction solar cell can tuning the effective bandgap and enhance the low-energy photon absorption, thereby increasing the short-circuit current. The samples are grown by solid-source molecular beam epitaxy（V80H） on p-GaA s（001） substrates using an As4 source. Sample A is a standard GaA s solar cell with PIN structure. Sample B contains 10 period 8 nm In_（0.15）Ga_（0.85）As QW and sample C contains 10 period In As dots-in-well structure. The intrinsic region of these samples was at the same length of 630 nm to obtain the same build-in electric field. Besides the instinct region, the three samples are designed the same. The ohmic contact layer＇s doping concentration is ND/NA=3×1018 cm-3 and its thickness is 300 nm. The emitter doping concentration is ND=5×1017 cm-3 and its thickness is 150 nm; The base doping concentration is NA=5×10^（17） cm^（-3） and its thickness is 300 nm. The As 4 pressure and the In As QDs growth rate are 1×10-5 Torr and 0.1 ML/s, respectively. The XRD pattern of sample B（QWSC） shows clearly defined and intense satellite peaks indicating steep interface and periodicity quantum well. However, the XRD pattern of sample C（the In As dots in well solar cell） shows blurring diffraction peaks which means increased interface roughness. Uniform QD structure with density of 2×10^（10） cm^（-2） can be observed in SEM. From the STEM, we can see well-defined unrelated pyramidal quantum dots separated by a nominal 50 nm GaA s spacing. From the contrast, the pyramid In As QDs are estimated to have the lateral dimension of 27 nm and the height of 12 nm. The PC response of sample A rise at 600 nm and fall off at 870 nm, which is corresponding to the Ga As bandgap according to the PL peak position; the PC response of QWSC（sample B） rise at 600 nm and fall off at 990 nm, which is corresponding to the In_（0.15）Ga_（0.85）As bandgap in PL spectrum. The incorporation of In_（0.15）Ga_（0.85）As quantum wells in GaA s standard solar extend the absorption of solar spectrum from 900 nm to 1000 nm. The wavelength region from 700 nm to 900 nm in PC of QWSC is weaker than the GaA s standard solar cell. The insertion of In As QDs into QWSC can extend the infrared photon absorption to the wavelength of 1300 nm. The insertion of In As quantum dots into In0.15Ga0.85As/GaA s quantum well solar cell extend the absorption of infrared light and the Jsc reaches 13.68 mA/cm^2, giving a 37.8% increase in short-circuit current over the baseline solar cell current. Because the voltage of multiple junction solar cell is the sum of the sub cell＇s voltage. The voltage decreasement with the insertion of In As quantum dots do not influence the overall voltage of multiple junction solar cell. In summary, the incorporation of quantum well and quantum dots in GaA s solar cell result in extended solar spectrum absorption and contribute to the sub-band gap current gains for GaA s solar cell. Especially, InA s dots-in-well solar cell can extend the absorption spectrum to the wavelength of 1300 nm, and increase by 37.8% in short-circuit current, which shows great potential in mitigating current mismatch of multiple junction solar cell. Furthermore, designing new multiple junction solar cell becomes possible.