Cu₂ZnSnS₄ (CZTS) is a very promising absorber for solar driven photovoltaics and water splitting applications due to its high theoretical efficiency, low-cost and non-toxicity. Alloying Ge into CZTS is a potential method to improve the efficiency of CZTS-based devices. However, decomposition of Ge-CZTS during a high temperature sulfurization process usually leads to serious Ge element loss and secondary phases, which lower the performance of Ge-CZTS based devices. Moreover, inconsistent optimum Ge content over a wide range was reported in previous studies. To date, there is no reasonable explanation on this unusual phenomenon. In this study, for the first time, we found that an optimum Ge content sensitively depended on sulfur pressure. By increasing sulfur pressure and alloying Ge simultaneously, a high crystalline Ge-CZTS without Ge element loss and secondary phases was obtained, which remarkably increased a half-cell solar to hydrogen efficiency (HC-STH) of a CZTS photocathode 27 times. After further modification, the Ge-CZTS photocathode indicated a stable photocurrent density of 11.1 mA cm⁻² at 0 V_RHE. To the best of our knowledge, it is the highest value among CZTS based photocathodes for solar water splitting. Moreover, the stability of a CZTS photocathode was also improved by increasing sulfur pressure and alloying Ge simultaneously due to higher crystalline of Ge-CZTS film.

中文翻译：

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Ge合金化同时提高硫压提高Cu ₂ ZnSnS ₄光电阴极的效率和稳定性

Cu ₂ ZnSnS ₄CZTS（CZTS）具有很高的理论效率，低成本和无毒特性，是太阳能驱动的光伏和水分解应用中非常有前途的吸收剂。将Ge合金化成CZTS是提高基于CZTS的器件效率的一种潜在方法。然而，在高温硫化过程中，Ge-CZTS的分解通常会导致严重的Ge元素损失和第二相，从而降低基于Ge-CZTS的器件的性能。此外，先前的研究报道了在广泛范围内不一致的最佳锗含量。迄今为止，对于这种异常现象还没有合理的解释。在这项研究中，我们首次发现最佳的Ge含量敏感地取决于硫压力。通过增加硫压并同时使Ge合金化，获得了没有Ge元素损失和第二相的高结晶Ge-CZTS，这使CZTS光电阴极的半电池太阳能到氢效率（HC-STH）显着提高了27倍。进一步修改后，Ge-CZTS光电阴极显示出11.1 mA cm的稳定光电流密度⁻²在0 V _RHE时。据我们所知，它是基于CZTS的用于太阳能水分解的光电阴极中最高的价值。此外，由于较高的Ge-CZTS膜的结晶性，通过增加硫压力和同时使Ge合金化，还改善了CZTS光电阴极的稳定性。