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硅异质结太阳电池载流子选择性接触研究
中文摘要

晶体硅太阳电池能量转换效率不断提高,单晶硅PERC太阳电池的生产线效率已达22%,非晶硅薄膜硅异质结(Silicon heterojunction,SHJ)太阳电池的实验室效率高达26.6%,但与晶体硅太阳电池理论效率极限(29.4%)还有相当距离。接触是太阳电池高复合活性金属界面和光子吸收层之间的区域,随着硅片质量的提高,接触处复合损失的影响将愈加明显,被认为是接近理论效率极限的最后障碍。如何逾越这一障碍,是否有一种物理模型解释不同结构硅太阳电池的工作原理,是否能够比较不同材料集成到硅太阳电池结构中的性能表现, c-Si的表面态又会带来什么影响,这些问题可藉由载流子选择性接触来回答。 载流子选择性接触可以实现低少子复合和有效多子输运,是高效晶体硅太阳电池的发展方向,因此研究载流子选择性接触具有重要的理论意义和实际应用价值。载流子选择性接触通常包括载流子选择性输运层(如(p)a-Si:H薄膜)和钝化缓冲层(如(i)a-Si:H薄膜)两部分,选择性输运载流子的同时钝化c-Si表面。本文在综述SHJ太阳电池的物理机制和优化设计的基础上引入载流子选择性接触物理模型,开展模拟和实验工作,主要研究内容和结果如下: (1)利用一维太阳电池模拟软件AFORS-HET模拟载流子选择性输运层材料的固定电荷和功函数对(n)c-Si能带弯曲的影响,发现形成明显的(n)c-Si能带弯曲,固定电荷密度要高于10¹¹ ㎝⁻²,而功函数要低于(n)c-Si的功函数或接近(n)c-Si的价带边。这些研究可为选取载流子选择性输运层材料提供依据。 (2)利用一维太阳电池模拟软件wxAMPS模拟(p⁺)c-Si/(n)c-Si/(n⁺)c-Si、 (p)a-Si:H/(i)a-Si:H/(n)c-Si/(i)a-Si:H/(n)a-Si:H和(n)MoO〓/(n)c-Si/(n)TiO〓三种结构硅太阳电池,分析能带结构和载流子浓度的空间分布,发现接触处都形成(n)c-Si能带弯曲和电导率的不对称,形成载流子选择性接触。本文进而提出用接触处的电导率比值S来评价载流子选择性大小,(p⁺)c-Si、(p)a-Si:H和MoO〓与(n)c-Si接触的S值分别为10⁹(μ/μ〓)、10¹³(μ〓/μ〓)和10¹⁷(μ〓/μ〓)。 (3)利用wxAMPS模拟界面态对SHJ太阳电池载流子选择性接触的影响并解释物理机理。(n)c-Si衬底的SHJ太阳电池,(p)a-Si:H接触端异质结的界面态是类施主缺陷态,会俘获空穴聚集正电荷,一旦界面态密度达到10¹² eV⁻¹㎝⁻²,就有足够的正电荷来屏蔽(p)a-Si:H中固定负电荷对(n)c-Si能带弯曲的影响,降低载流子浓度不对称、电导率不对称和载流子选择性。 (4)在不同氢稀释度条件下沉积(i)a-Si:H钝化缓冲层去钝化(n)c-Si表面,分析钝化物理机理,提出一种分析异质结界面态密度的方法。氢稀释度从SiH₄:H₂=4:5上升到SiH₄:H₂=4:15,更均匀、更紧凑、更少体缺陷的(i)a-Si:H薄膜将覆盖更多的(n)c-Si表面,同时H原子迅速到达(n)c-Si表面、进入(i)a-Si:H薄膜从而饱和(n)c-Si表面和(i)a-Si:H薄膜中的悬挂键,使界面态密度从6.9×10¹¹ eV⁻¹㎝⁻²下降到1.7×10¹¹ eV⁻¹㎝⁻²。这些结果从载流子选择性接触钝化效果角度定量显示了SHJ太阳电池制备中氢比例控制的作用。 (5)分析(i)a-Si:H薄膜中氢含量、硅氢键合模式、质量密度和薄膜中的体结构缺陷,发现薄膜中氢含量增加时质量密度下降,体结构缺陷由二空位占主导过渡到三空位占主导。质量密度下降涉及两个机制:一是更多的H原子代替了Si原子,二是占主导的体结构缺陷的变化。这些结果说明了控制(i)a-Si:H薄膜中氢含量的重要性。 (6)在实验室国产设备条件下制备了(n)c-Si衬底SHJ太阳电池样品,以晶体硅太阳电池的理论最佳性能参数为标准,对其功率损失和填充因子损失进行了定量分析,并指出了包括优化异质结界面化学钝化和载流子选择性接触等SHJ太阳电池性能改进方向。 关键词:硅异质结;太阳电池;选择性接触;界面特性

英文摘要

Crystalline silicon solar cell products are well developed and widely used. Currently the energy conversion efficiency of commercially produced monocrystalline silicon PERC solar cell has reached 22%, what is more, the energy conversion efficiency of amorphous silicon thin film silicon heterojunction (SHJ) solar cells is as high as 26.6% in laboratory. This is, however, still quite lower than the theoretical efficiency limit of crystalline silicon solar cell (29.4%). Contact is the region between the highly recombination active metal interface and the photon absorption layer. With the improvement of silicon wafer quality, recombination loss at the contacts will become more obvious, which is also considered as the last obstacle to approach to the theoretical efficiency limit. How to overstep this obstacle, whether there has a physical model to explain the working principles of different structural silicon solar cells, whether it can compare the performance of different materials integration into silicon solar cell structure and what effect the surface states of c-Si will have, which can be answered by the carrier-selective contacts. The carrier-selective contacts can execute low minority-carrier recombination and effective majority-carrier transport, which are becoming the development direction of high efficiency silicon solar cells. Therefore, the research of carrier-selective contacts has great theoretical significance and practical application value. Carrier-selective contacts generally consist of two parts, that is, the carrier-selective transport layer such as (p)a-Si:H thin film and the passivation buffer layer such as (i)a-Si:H thin film. So, a carrier-selective contact is selective transport for one kind of charge carrier, while simultaneously passivating the c-Si surface. The physical mechanism and optimal design of silicon heterojunction (SHJ) solar cells are reviewed and the carrier-selective contacts physical model is introduced, then simulation and experiment are carried out. The main research contents and results are as follows: (1)The effect of fixed charge and work function on the (n)c-Si band bending are simulated using one-dimensional solar cell simulation software AFORS-HET. In order to acquire an apparent (n)c-Si band bending, the fixed charge density must be higher than 10¹¹ ㎝⁻², and the work function is lower than the work function of (n)c-Si or close to the (n)c-Si valence band edge. These studies can provide certain guidelines for choosing of carrier-selective transport layers. (2)Based on one-dimension solar cell simulation software wxAMPS, three different silicon solar cell structures are numerically simulation, which includes a diffused homojunction silicon solar cell [(p⁺)c-Si/(n)c-Si/(n⁺)c-Si], two SHJ solar cells [(p)a-Si:H/(i)a-Si:H/(n)c-Si/(i)a-Si:H/(n)a-Si:H and (n)MoO〓/(n)c-Si/(n)TiO〓], The energy band structures and the spatial distributions of carrier concentrations are discussed and the physical mechanisms of carrier-selective contacts are analyzed. The (n)c-Si band bending and asymmetric conductivities are acquired, which lead to form carrier selective contacts. We further propose the ratio of conductivity, S, to evaluate the carrier selectivity and the S values of (p⁺)c-Si, (p)a-Si:H and MoO〓 contacted with (n)c-Si are 10⁹(μ〓/μ〓), 10¹³(μ〓/μ〓) and 10¹⁷(μ〓/μ〓), respectively. (3)The effect of interface states on the carrier-selective contacts of SHJ solar cell with amorphous silicon is also simulated based on wxAMPS. For a SHJ solar cell with (n)c-Si substrate, the interface states are donor-like defects and will capture holes, then form a positive charge region. Once the interface state density reaches 10¹² eV⁻¹ ㎝⁻², there will has enough positive charges to shield the effect of fixed negative charge on (n)c-Si band bending, so the asymmetric carrier concentration and asymmetric conductivity decrease and the carrier selectivity degradate. (4)The (i)a-Si:H thin films are deposited on (n)c-Si surfaces with different hydrogen dilution, the passivation mechanism is analyzed and a method to calculate the values of interface state density is proposed. During the hydrogen dilution promotion, the (i)a-Si:H films with more uniform, compact microstructure and fewer bulk defects can cover more (n)c-Si surface, meanwhile H atoms can quickly reach (n)c-Si surface and enter (i)a-Si:H films to saturate dangling bonds, so the values of interface state density decline from 6.9×10 ¹¹ eV⁻¹㎝⁻² to 1.7×10 ¹¹ eV⁻¹㎝⁻². These results quantitatively demonstrate the role of hydrogen proportional control in the preparation of SHJ solar cells from the perspective of carrier selective contact passivation. (5)The hydrogen content, the hydrogen bonding mode, the mass density and the volume deficiency of (i)a-Si:H film are analyzed. The mass density decreases with the increase of hydrogen content, and volume deficiency has a transition from divacancy dominated to trivacancy dominated. The decrease of mass density involves two mechanisms, that is, the substitution of Si-atoms with H-atoms and volume deficiency transition. These results illustrate the importance of controlling the hydrogen content in (i)a-Si:H thin films. (6)The samples of (n)c-Si substrate SHJ solar cell was prepared under the condition of domestic equipment in the laboratory. The power loss and fill factor loss are quantitatively analyzed with the optimum theoretical performance parameters of the crystalline silicon solar cell. The directions in performance improving of SHJ solar cells including optimizing the chemical passivation at the heterojunction interface and the carrier-selective contacts are proposed. Key Words: silicon heterojunction; solar cell; carrier-selective contacts; interface properties

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