DASH太阳能电池电子传输层的电学输运与界面钝化研究

DASH (Dopant-free Asymmetric Heterocontacts) solar cell, which is a hot spot in the solar research field in 2016, is not based on p-n junction and consists of two different metal-semiconductor heterocontacts. These two complementary carrier-selective heterocontacts provide a negligible resistance to the collected carrier (synonymous with a low contact resistivity) whilst simultaneously ‘blocking’ the other carrier (equivalent to low contact recombination) via either conduction or valence band barrier. DASH solar cells can avoid drawbacks of mainstream doped pn junctions such as doping-induced parasitic absorption, Auger recombination and other heavy doping effect. To overcome the issue of serious carrier recombination losses at the interface of electron-selective heterocontact, a DASH solar cell with the new MoOx/a-Si:H(i)/c-Si(n)/SiOx/LiFx/Al structure is proposed by inserting the ultra-thin tunneling SiOx film with fixed charges as the passivation layer at c-Si(n)/LiFx/Al interface. Combined with the presence of field passivation effect induced by LiFx/Al electron-selective layer due to its low work function, the additional field passivation effect of tunneling SiOx would lead to the large valence band bending than single high-low junction or schottky junction, and further block the flow of holes via the higher valence band barrier compared with HIT, TOPcon and the present DASH solar cells, suppressing the carrier recombination and thus further improving Voc and efficiency. In addtion, the combination of LiFx/Al ohmic contact and ultra-thin SiOx with high electron tunneling efficiency also further reduces the contact resistivity of electron-selective layer.

DASH（Dopant-free Asymmetric Heterocontacts）电池是一种无掺杂无pn结的新型异质结电池，其利用对一种载流子形成传输层而对另一载流子形成高势垒阻挡层的金半接触来分离和收集载流子，可以避免由掺杂而引起的寄生光学吸收、俄歇复合等诸多效率损失。针对DASH电池的电子传输层/晶体硅界面存在严重载流子复合损失的难题，本项目提出了一种具有MoOx/a-Si:H(i)/c-Si(n)/SiOx/LiFx/Al结构的新型DASH电池，将带有固定电荷的超薄氧化层插入电子传输层/晶体硅界面作为界面钝化和电子隧穿层。具有场钝化效应的超薄氧化层与具有更低功函数的LiFx/Al电子传输层结合，使电子传输层/晶体硅界的场钝化叠加，会产生比单一高低结或者金-半接触更大的能带弯曲和空穴势垒，比现有DASH电池和TOPcon电池更加抑制空穴在n型体硅衬底背面的界面复合，有望提高效率。

DASH电池一种新型无p-n结的异质结电池，其利用金属-半导体接触来选择性分离和收集载流子，会从物理机制上避免传统晶体硅电池的掺杂工程引起的效率损失。目前，DASH电池仍面临电子传输层的材料的热稳定性不足、与晶体硅电池制造工艺不兼容以及晶体硅界面存在较大复合等挑战。. 为应对这些挑战，我们研究了新颖的、具有高热稳定的碳化钛薄膜材料的电子传输特性，并用磁控溅射方法生长并有效调制了碳化钛薄膜的微观结构及电学性质。碳化薄膜为含有部分氧的非晶镶嵌纳米晶粒的混合相结构，功函数被调控在电子传输层的阈值4.3电子伏以下，电阻率可在1.0E-2至1.0E-4欧姆·厘米调节，与n型硅可形成接触电阻率约为36毫欧姆·厘米的平方的欧姆接触，表现出良好的电子输运特性。. 通过浓硝酸氧化生长的1.4纳米厚的氧化硅可辅助聚苯乙烯磺酸钝化晶体硅界面，使少子寿命从1毫秒提高2.8毫秒，展现了优异的钝化功能。而原子层沉积方法生长的氧化钛薄膜少子寿命可达2.98毫秒，达到甚至超过常规钝化膜对晶体硅的钝化效果。. 最后，碳化钛应用为太阳能电池的电子传输层，将电池的开路电压、填充因子和转化效率都有提升。最终，我们制备出17.1%的最高效率的DASH电池。碳化钛层提高了电池对近红外线的光谱响应，可归结于其较低的功函数导致的场钝化和电子输运效应。本工作结果对异质结电池、耐热电子传输材料以及界面钝化等方面的研究有一定的参考价值，且制备工艺与现在的主流商用晶体硅电池制造工艺兼容，具有较大的实际应用潜力。