Dependence of InGaN solar cell performance on polarization-induced electric field and carrier lifetime

The effects of Mg-induced net acceptor doping concentration and carrier lifetime on the performance of a p-i-n InGaN solar cell are investigated. It is found that the electric field induced by spontaneous and piezoelectric polariza- tion in the i-region could be totally shielded when the Mg-induced net acceptor doping concentration is sufficiently high. The polarization-induced potential barriers are reduced and the short circuit current density is remarkably increased from 0.21 mA/cm2 to 0.95 mA/cm2 by elevating the Mg doping concentration. The carrier lifetime determined by defect density of i-InGaN also plays an important role in determining the photovoltaic properties of solar cell. The short circuit current density severely degrades, and the performance of InGaN solar cell becomes more sensitive to the polarization when carrier lifetime is lower than the transit time. This study demonstrates that the crystal quality of InGaN absorption layer is one of the most important challenges in realizing high efficiency InGaN solar cells.     

Advantages of InGaN/GaN multiple quantum well solar cells with stepped-thickness quantum wells

InGaN/GaN multiple quantum well (MQW) solar cells with stepped-thickness quantum wells (SQW) are designed and grown by metal-organic chemical vapor deposition. The stepped-thickness quantum wells structure, in which the well thickness becomes smaller and smaller along the growth direction, reveals better crystalline quality and better spectral overlap with the solar spectrum. Consequently, the short-circuit current density (Jsc) and conversion efficiency of the solar cell are enhanced by 27.12% and 56.41% compared with the conventional structure under illumination of AM1.5G (100 mW/cm2). In addition, approaches to further promote the performance of InGaN/GaN multiple quantum well solar cells are discussed and presented.     

MOVPE生长的InGaN/GaN单量子阱的光致发光和光吸收特性

研究了用金属有机物气相外延(MOVPE)法在蓝宝石衬底上生长的In组分浓度保持不变的InGaN/GaN单量子阱结构在室温下的发光特性和光吸收特性.实验结果表明,在InGaN厚度<3nm时,随着样品InGaN势阱层宽度的增加(1nm),光致发光(PL)谱的发光峰值波长出现明显的红移33nm现象,而且发光强度下降8%,谱线半峰全宽(FWHM)展宽,通过对样品的透射、反射光谱研究发现,量子阱层窄(1.5nm)的样品在波长接近红外区时出现无吸收的现象,即R+T达到了100%,而在阱层较宽的样品中没有发现这一现象,对引起这些现象的原因进行了讨论.这些结果有助于开发和优化三族氮化物半导体光电器件的进一步研究工作.     

Numerical investigation on the enhanced carrier collection efficiency of Ga-face GaN/InGaN p-i-n solar cells with polarization compensation interlayers

The impact of the polarization compensation InGaN interlayer between the heterolayers of Ga-face GaN/InGaN?p-i-n solar cells is investigated numerically. Because of the enhancement of carrier collection efficiency, the conversion efficiency is improved markedly, which can be ascribed to both the reduction of the polarization-induced electric field in the InGaN absorption layer and the mitigation of potential barriers at heterojunctions. This beneficial effect is more remarkable in situations with higher polarization, such as devices with a lower degree of relaxation or devices with a higher indium composition in the InGaN absorption layer.     

Enhanced performance of InGaN/GaN multiple quantum well solar cells with double indium content

The performance of a multiple quantum well （MQW） InGaN solar cell with double indium content is investigated. It is found that the adoption of a double indium structure can effectively broaden the spectral response of the external quantum efficiencies and optimize the overall performance of the solar cell. Under AM1.5G illumination, the short-circuit current density （Jsc） and conversion efficiency of the solar cell are enhanced by 65% and 13% compared with those of a normal single-indium-content MQW solar cell. These improvements are mainly attributed to the expansion of the absorption spectrum and better extraction efficiency of the photon-generated carriers induced by higher polarization.     

结构参数对p-i-n结构InGaN太阳能电池性能的影响及机理

研究了器件结构参数对p-i-n结构InGaN单结太阳能电池性能的影响及物理机制. 模拟结果发现: 随着InGaN禁带宽度的增加, InGaN电池的短路电流减小, 但同时开路电压增加, 当InGaN层的禁带宽度为1.5 eV左右时, 同质p-i-n结InGaN电池的效率最高, 并计算了不同厚度的i层对InGaN电池效率的影响. 进一步的计算表明, 适当采用带宽更大的p-InGaN层形成异质p-i-n结InGaN电池可以获得更高效率, 但是p-InGaN层带宽过大也会导致电池的效率急剧下降. 研究还发现, 采用禁带宽度更大的n-InGaN层可以形成背电场, 从而增加p-i-n结InGaN太阳电池的效率. 研究结果表明, 适当选择p-InGaN和n-InGaN禁带宽度形成异质p-i-n结可以提高InGaN太阳能电池效率.     

Effect of Interface Roughness and Dislocation Density on Electroluminescence Intensity of InGaN Multiple Quantum Wells

Effects of interface roughness and dislocation density on the electroluminescence (EL) intensity of InGaN multiple quantum wells (MQWs) are investigated. It is found that the EL intensity increases with the number of satellite peaks in the x-ray diffraction experiments of InGaN MQW samples. It is indicated that the rough interface will lead the reduction of EL intensity of InGaN MQW samples. It is also found that the EL intensity increases with the decrease of dislocation density which is characterized by the x-ray diffraction measurements. It is suggested that the EL intensity of InGaN MQWs can be improved by decreasing the interface roughness and dislocation density.     

MOCVD生长InGaN/GaN MQW紫光LED

利用LP－MOCVD系统生长了InGaN/GaN MQW紫光LED外延片，双晶X射线衍射测试获得了2级卫星峰，室温光致发光谱的峰值波长为399.5nm，FWHM为15.5nm，波长均匀性良好。制成的LED管芯，正向电流20mA时，工作电压在4V以下。     

Enhanced performance of InGaN light-emitting diodes with InGaN and composition-graded InGaN interlayers

The effects of InGaN light-emitting diodes (LEDs) with InGaN and composition-graded InGaN interlayers in the space of multiple quantum wells and electron blocking layer are studied numerically. The electrostatic field, energy band diagrams, carrier concentrations, light–current–voltage performances, and internal quantum efficiency (IQE) are investigated. Simulation results show that the light output power and IQE are both largely improved over the conventional LED structure due to the improvement in hole injection efficiency and electron blocking capability, especially for the LED with composition-graded InGaN interlayer.     

Performance enhancement of an InGaN light-emitting diode with an AlGaN/InGaN superlattice electron-blocking layer

The efficiency enhancement of an InGaN light-emitting diode(LED) with an AlGaN/InGaN superlattice(SL)electron-blocking layer(EBL) is studied numerically,which involves the light-current performance curve,internal quantum efficiency electrostatic field band wavefunction,energy band diagram carrier concentration,electron current density,and radiative recombination rate.The simulation results indicate that the LED with an AlGaN/InGaN SL EBL has better optical performance than the LED with a conventional rectangular AlGaN EBL or a normal AlGaN/GaN SL EBL because of the appropriately modified energy band diagram,which is favorable for the injection of holes and confinement of electrons.Additionally,the efficiency droop of the LED with an AlGaN/InGaN SL EBL is markedly improved by reducing the polarization field in the active region.     

InGaN/GaN multiple quantum well solar cells with an enhanced open-circuit voltage

In this paper,InGaN/GaN multiple quantum well solar cells (MQWSCs) with an In content of 0.15 are fabricated and studied.The short-circuit density,fill factor and open-circuit voltage (V oc) of the device are 0.7 mA/cm 2,0.40 and 2.22 V,respectively.The results exhibit a significant enhancement of V oc compared with those of InGaN-based hetero and homojunction cells.This enhancement indicates that the InGaN/GaN MQWSC offers an effective way for increasing V oc of an In-rich In x Ga 1 x N solar cell.The device exhibits an external quantum efficiency (EQE) of 36% (7%) at 388 nm (430 nm).The photovoltaic performance of the device can be improved by optimizing the structure of the InGaN/GaN multiple quantum well.     

n-ZnO:Al/i-ZnO/n-CdS/p-Cu2ZnSnS4太阳能电池光伏特性的分析

具有高光吸收系数的半导体Cu₂ZnSnS₄ (CZTS)薄膜是一种新型太阳能电池材料. 本文对n-ZnO:Al/i-ZnO/n-CdS/p-CZTS结构的CZTS薄膜太阳能电池进行分析, 讨论CZTS薄膜的掺杂浓度、厚度、缺陷态和CdS薄膜的掺杂浓度、 厚度对太阳能电池转换效率的影响以及太阳能电池的温度特性. 分析表明, CZTS薄膜作为太阳能电池的主要光吸收层, CZTS薄膜的掺杂浓度和厚度的取值对太阳能电池的转换效率有显著影响, CZTS薄膜结构缺陷态的存在会导致太阳能电池性能的下降. CdS缓冲层的掺杂浓度、厚度对太阳能电池光伏特性的影响较小. 经结构参数优化得到的n-ZnO:Al/i-ZnO/n-CdS/p-CZTS薄膜太阳能电池的最佳光 伏特性为开路电压1.127 V、短路电流密度27.39 mA/cm²、填充因子87.5%、 转换效率27.02%,转换效率温度系数为-0.14%/K.     

InGaN/GaN超晶格厚度对Si衬底GaN基蓝光发光二极管光电性能的影响

采用有机金属化学气相沉积技术在Si(111)衬底上生长蓝光多量子阱发光二极管(LED) 结构, 通过在量子阱下方分别插入两组不同厚度的InGaN/GaN超晶格, 比较了超晶格厚度对LED光电性能的影响. 结果显示: 随超晶格厚度增加, 样品的反向漏电流加剧; 300 K下电致发光仪测得随着电流增加, LED发光光谱峰值的蓝移量随超晶格厚度增加而减少, 但不同超晶格厚度的两个样品在300 K下的电致发光强度几乎无差异. 结合高分辨X射线衍射仪、扫描电子显微镜、透射电子显微镜对样品的位错密度和V形坑特征分析, 明确了两样品反向漏电流产生巨大差异的原因是由于超晶格厚度大的样品具有更大的V形坑和V形坑密度, 而V形坑可作为载流子的优先通道, 使超晶格更厚的样品反向漏电流加剧. 通过对样品非对称(105)面附近的X射线衍射倒易空间图分析, 算得超晶格厚度大的样品其InGaN量子阱在GaN上的弛豫度也大, 即超晶格厚度增加有利于减小InGaN量子阱所受的应力. 综合以上影响LED发光效率的消长因素, 导致两样品最终的发光强度相近.     

Two dimensional electron gas in a hybrid GaN/InGaN/ZnO heterostructure with ultrathin InGaN channel layer

We investigated the influence of an ultrathin InGaN channel layer on two-dimensional electron gas (2DEG) properties in a newly proposed hybrid GaN/In_xGa_1−xN/ZnO heterostructure using numerical methods. We found that 2DEG carriers were confined at InGaN/ZnO and GaN/InGaN interfaces. Our calculations show that the probability densities of 2DEG carriers at these interfaces are highly influenced by the In mole fraction of the InGaN channel layer. Therefore, 2DEG carrier confinement can be adjustable by using the In mole fraction of the InGaN channel layer. The influence of an ultrathin InGaN channel layer on 2DEG carrier mobility is also discussed. Usage of an ultrathin InGaN channel layer with a low indium mole fraction in these heterostructures can help to reduce the short-channel effects by improvements such as providing 2DEG with higher sheet carrier density which is close to the surface and has better carrier confinement.     

Comparision between Ga- and N-polarity InGaN solar cells with gradient-In-composition intrinsic layers

Performances of Ga-and N-polarity solar cells(SCs) adopting gradient-In-composition intrinsic layer(IL) are compared.It is found the gradient ILs can greatly weaken the negative influence from the polarization effects for the Gapolarity case,and the highest conversion efficiency(η) of 2.18%can be obtained in the structure with a linear increase of In composition in the IL from bottom to top.This is mainly attributed to the adsorptions of more photons caused by the higher In composition in the IL closer to the p-GaN window layer.In contrast,for the N-polarity case,the SC structure with an InGaN IL adopting fixed In composition prevails over the ones adopting the gradient-In-composition IL,where the highest η of 9.28%can be obtained at x of 0.62.N-polarity SC structures are proven to have greater potential preparations in high-efficient InGaN SCs.     

Enhancing performance of GaN-based LDs by using GaN/InGaN asymmetric lower waveguide layers

The effects of GaN/InGaN asymmetric lower waveguide (LWG) layers on photoelectrical properties of InGaN multiple quantum well laser diodes (LDs) with an emission wavelength of around 416 nm are theoretically investigated by tuning the thickness and the indium content of InGaN insertion layer (InGaN-IL) between the GaN lower waveguide layer and the quantum wells, which is achieved with the Crosslight Device Simulation Software (PIC3D, Crosslight Software Inc.). The optimal thickness and the indium content of the InGaN-IL in lower waveguide layers are found to be 300 nm and 4%, respectively. The thickness of InGaN-IL predominantly affects the output power and the optical field distribution in comparison with the indium content, and the highest output power is achieved to be 1.25 times that of the reference structure (symmetric GaN waveguide), which is attributed to the reduced optical absorption loss as well as the concentrated optical field nearby quantum wells. Furthermore, when the thickness and indium content of InGaN-IL both reach a higher level, the performance of asymmetric quantum wells LDs will be weakened rapidly due to the obvious decrease of optical confinement factor (OCF) related to the concentrated optical field in the lower waveguide.     

引入n型InGaN/GaN超晶格层提高量子阱特性研究

利用金属有机物化学气相淀积技术在蓝宝石衬底上生长了InGaN/GaN量子阱结构. 研究了引入n型InGaN薄层或InGaN/GaN超晶格层的量子阱特性,结果表明通过引入n型InGaN薄层或InGaN/GaN超晶格层缓解了量子阱有源区中的应力,改善了多量子阱表面形貌,减少了V型缺陷密度,而且提高了多量子阱的光致发光强度,从而也改进了LED的发光效率. 关键词： InGaN/GaN多量子阱 原子力显微镜 X射线双晶衍射 光致发光     

Optical excitation study on the efficiency droop behaviors of InGaN/GaN multiple-quantum-well structures

Efficiency droop is generally observed in electroluminescence under high current injection. Optical characterization on efficiency droop in InGaN/GaN multiple-quantum-well structures has been conducted at 12 K. Clear droop behaviors were observed for the sample excited by above-bandgap excitation of GaN with pulse laser. The results show that dislocation is not the crucial factor to droop under high carrier density injection, and Auger recombination just slightly affects the efficiency. The radiative recombination may be mainly affected by a multi-carrier-related process (diffusion and drift with a factor of n ^3.5 and n ^5.5) at the interface between GaN barrier and InGaN well.     

Numerical study of InGaN tandem solar cells with intermediate bands

Theoretical investigations of InGaN tandem solar cells with intermediate bands (IBs) have been conducted through calculating the diode equation taking into account the radiative and nonradiative recombination currents. The calculated maximum ef?ciencies of the double‐junction cell with one IB in each subcell are 57.85% and 68.37% under AM1.5G one‐sun and 46000‐sun illuminations, respectively. It has also been observed that the combined device with the top‐cell bandgaps of 2.9–3.4 eV (2.6–3.4 eV for full concentration) may have an opportunity to realize the application of over 50% efficiency. We suggest that the optimized width of the IB layer be designed in the range of 1–6 μm if its absorption coefficient is 10⁴–10⁵ cm^–1 in the IB region.     

Carrier transport via V-shaped pits in InGaN/GaN MQW solar cells

Carrier transport via the V-shaped pits(V-pits) in InGaN/GaN multiple-quantum-well(MQW) solar cells is numerically investigated. By simulations, it is found that the V-pits can act as effective escape paths for the photo-generated carriers. Due to the thin barrier thickness and low indium composition of the MQW on V-pit sidewall, the carriers entered the sidewall QWs can easily escape and contribute to the photocurrent. This forms a parallel escape route for the carries generated in the flat quantum wells. As the barrier thickness of the flat MQW increases, more carriers would transport via the V-pits. Furthermore, it is found that the V-pits may reduce the recombination losses of carriers due to their screening effect to the dislocations. These discoveries are not only helpful for understanding the carrier transport mechanism in the InGaN/GaN MQW, but also important in design of the structure of solar cells.