石墨烯/Si3N4/Si纳米阵列异质结的界面态、电荷传输及稳定性

Graphene/silicon heterojunction composite materials have been widely studied as a promising solar cell material at home and abroad. But the low barrier height will lead to larger leakage current, and there is a serious charge recombination problem on the unmodified silicon surface. So in this research, the idea is proposed that silicon nitride is used as an intermediate barrier layer, and it is also used as a passivation layer of silicon nanoarray in core shell structure for constructing the structure of heterojunction. The photovoltaic performance of graphene/silicon nitride/silicon nanoarray heterojunction solar cells is investigated fundamentally through the combination of theory and experiment methods. The feature dimension and interface binding states of heterojunction, as well as the effects on the energy band and carrier transport, combining the evolution law of photovoltaic properties under different conditions, are calculated and analyzed to establish the reasonably theoretical model of heterojunction solar cell. The photovoltaic property and conversation efficiency of device are investigated experimentally for the purpose of verifying and correcting the model. The high efficiency heterojunction solar cell theoretical and forecasting models are accurately established on basis of summaring various factors and relationships between each other. The objective of the present study is to clarify the high conversation efficiency reason and the influence mechanism of interface state on carrier transport mechanism, and to provide the theoretical and technical foundation for preparing the graphene based heterojunction composite and developing a new generation of heterojunction solar cell devices.

石墨烯/硅异质结是国内外太阳电池材料研究的热点，但低的势垒高度会导致较大的漏电流，未修饰的硅表面存在严重的电荷复合问题。为此，本项目提出以氮化硅作为中间阻挡层，又作为硅纳米阵列钝化层形成核壳结构的思路，通过理论与实验相结合的方法对石墨烯/Si3N4/Si纳米阵列异质结太阳电池的光伏性能进行基础科学研究。理论计算与分析各组分特征尺寸和界面态及其对异质结能带、载流子传输的影响规律，研究不同条件下光伏性能的演化规律，建立合理的异质结太阳电池理论模型。实验研究电池的光伏性能及转换效率，并对理论研究中提出的模型进行验证和修正，在对各种影响因素及相关性理论总结的基础上建立准确的高效异质结太阳电池理论及定量预测模型。本项目力图阐明异质结电池产生高效转换效率的成因及界面态对载流子传输行为影响机制等关键基础科学问题，为制备石墨烯基异质结复合材料及研发新一代异质结太阳电池基础器件奠定理论和技术基础。

石墨烯基异质结是太阳电池中的研究热点，项目对氮化硅/石墨烯异质结进行了研究。结果表明，对于单项氮化硅而言，在富Si条件下表层更容易形成N空位缺陷，N空位的存在使导带下移，价带顶远离费米能级，中间带出现了新的附加能进，对载流子迁移有很大影响，N空位越接近表层，介电损耗越小；Si空位存在使得体系具有P型半导体的特征，越接近内层，附加能带范围越窄，电子局域程度越大，态密度峰值越尖锐。对于石墨烯/氮化硅异质结研究发现氮化硅表面原子有明显靠近石墨烯层的趋势，石墨烯层没有出现褶皱状，Nt1与C环中心相对的结构更为稳定，层间距为2.914 Å，能带结构类似于两部分能带的叠加，并保留了狄拉克锥的形态，能隙值为67 meV，费米能级接近价带顶，说明形成的是p型肖特基接触，当层间距从4.0 Å降低到2.2 Å时，狄拉克点的带隙值从17 meV增加到283meV，说明层间距的改变可以调节带隙；当层间距从2.0 Å增加到2.885Å，再到4.0 Å时，p型肖特基势垒高度从0.03eV增加到0.72eV，最后增加到1.04eV，但n型肖特基势垒高度从4.08eV降低到3.6eV，最后收敛到3.32eV，虽然层间距发生变化，但p型肖特基接触一直存在。施加外加电场时，可以看到明显的p型肖特基接触，在-0.05V/ Å时，肖特基势垒高度为0.48eV，低于n型肖特基势垒高度的3.77eV，表明界面形成的是p型肖特基接触；当负的电场值从-0.05V/ Å增加到-0.1V/ Å，再到-0.35V/ Å时，p型肖特基势垒高度从0.48eV降低到0.31eV，再逐渐降低到0.72eV，而n型肖特基势垒高度从3.77eV增加到3.99eV，最后增加到3.82eV；当施加正电场时，系统仍然保持p型肖特基接触，在0.05-0.2 V/ Å范围内，但是肖特基势垒的变化规律与之前的相反。值得关注的是，当施加的正电场超过0.2V/ Å时，发生了p型向欧姆接触的转变，这是由于外加电场下发生了电荷转移，从而影响了费米能级的移动。这些结果为石墨烯基异质结在太阳电池上的进一步应用奠定了基础。