Lifetime and encapsulation study of organic solar cells (LEOsc)

《巴黎协定》设定了到2050年实现净零排放的目标，以应对全球变暖；这刺激了对能源转型的大量研究。光伏技术通过将太阳辐射转化为电能而不排放温室气体，满足了日益增长的能源需求，受到了极大的关注。然而，全球光伏部署仍然很低(2021年全球只有5%的电力来自光伏技术)，市场目前由能源密集型常规晶体硅光伏主导。为了促进能源转换，有机太阳能电池(OSCs)等新型光伏技术正被广泛研究，以实现柔性和轻量化的应用。然而，OSC必须满足未来商业化在效率、寿命和成本方面的要求。与传统硅相比，OSC的生产和安装成本预计将非常低。在提高OSC效率方面不断取得进展，实际上到目前为止已经达到了约20%的具有竞争力的高PCE(功率转换效率)。而OSC的寿命仍然大大低于无机电池，还有很大的改进空间。由于OSC的降解主要是由于暴露在潮湿和氧气中造成的，因此对细胞进行封装是提高OSC预期寿命的最直接的方法之一。在这种背景下，本联谊会的研究集中在OSC的终身研究上。研究工作包括两个方面：封装层的开发和OSC的非本征降解机理研究。具体地说，将利用原子层沉积(ALD)技术来制备封装层。ALD是一种基于不同前驱体之间的自限幅反应的技术，它可以沉积出均匀致密的薄膜。薄膜的均匀性使得水和氧气的渗透率极低，从而使胶囊膜能够更长时间地保护OSCs。该研究组将系统地研究ALD技术的所有封装策略，即：(1)用于层压的阻挡膜。这一策略允许预先制造阻挡层膜，然后将OSCs与阻挡层膜层压。这种方案的一个巨大优点是模块的整个加工可以在卷到卷的配置中完成，并且ALD条件不受敏感有机物的限制。(2)直接薄膜封装。这种策略是可取的，以最大限度地减少机械应力、对屏障的磨损和器件污染。TFE的主要问题是封装工艺条件受到有机物的高度限制。在这两种策略中，将开发具有低透湿性和足够机械耐用性的OSC封装层，还将开发中间层作为ALD材料的更好生长表面，以确保薄膜是完美的。将提高沉积速度，以增加ALD的产量。对不同策略中的封装性能进行比较将解决OSC的最佳封装配置。此后，将使用最佳封装来保护最先进的OSC，并将对受保护的OSC进行加速测试和室外寿命测试，以表征其寿命。还将研究包裹细胞的外在降解，以解决细胞分解问题。根据退化机理，对封装层提出优化策略。我的目标是将OSC的寿命提高到与硅相当的水平。在奖学金结束时，将在牛津物理的创新和企业经理(Phillip Tait)和我们的行业合作伙伴的帮助下进行扩大试验。

The Paris Agreement set the goal to reach Net-Zero emission by 2050 to tackle global warming; this has stimulated much research into energy transition. Photovoltaic technology receives great attention as it can meet increasing energy demands by converting solar radiation into electricity without greenhouse gas emissions. However, global PV deployment is still low (only 5% of global electricity came from PV technology in 2021) and the market is currently dominated by energy-intensive conventional crystalline silicon PV. To facilitate the energy transition, novel photovoltaic technologies, such as organic solar cells (OSCs), are being intensively studied for flexible & lightweight applications. However, OSCs must fulfil requirements in efficiency, lifetime, and cost for future commercialisation. The cost of OSC production and installation is expected to be very low compared to conventional Si. Continuing developments have been made in improving OSC efficiency and indeed competitive high PCE (power conversion efficiency) of about 20% has been reached so far. While OSC lifetime is still substantially lower than that of inorganic cells and there is much room for improvement. Since OSC degradation is mainly caused by exposure to moisture and oxygen, encapsulation of the cells is one of the most straightforward ways to improve OSC life expectancy. In this context, this Fellowship research focusses on the OSC lifetime study. The proposed tasks involve two aspects, encapsulation layer development and OSC extrinsic degradation mechanism study.Specifically, the atomic layer deposition (ALD) technique will be utilized for the fabrication of the encapsulation layer. ALD is a technique based on the self-limiting reaction between distinct precursors, which can deposit dense thin films with excellent uniformity. The uniformity of the film allows the water and oxygen permeation rate to be extremely low, thus the encapsulation film can protect OSCs longer. The Fellowship will systematically investigate all the encapsulation strategies by the ALD technique, namely: (1) Barrier films for lamination. This strategy allows the possibility of manufacturing the barrier films in advance, then OSCs will be laminated with barrier films. A huge advantage of this scheme is the whole processing of a module can be done in a roll-to-roll configuration and the ALD conditions are not limited by the sensitive organics. (2) Direct thin-film encapsulation (TFE). This strategy is desirable to minimize mechanical stress, abrasion to the barrier, and device contamination. The main issue of TFE is the encapsulation processing conditions are highly constrained by the organics. In both strategies, OSC encapsulation layers with low moisture permeation and sufficient mechanical durability will be developed, an intermediate layer will also be developed to serve as a better growth surface for ALD materials to ensure the thin films are perfect. The deposition rate will be improved to increase the production throughput of ALD. Comparisons of the encapsulation performance within the different strategies will address the best encapsulation configuration for OSCs. Thereafter, the optimal encapsulation will be utilised for the protection of state-of-the-art OSCs, accelerated tests and outdoor lifetime tests will be performed on the protected OSCs to characterise the lifetime. The extrinsic degradation of encapsulated cells will be also studied to address the cell break-down. Optimisation strategies will be given to the encapsulation layers according to the degradation mechanism. My goal is to push the lifetime of the OSC to a comparable level to that of Si. At the end of the Fellowship, scale-up trials will be undertaken with the help of Oxford Physics' Innovation and Enterprise Manager (Phillip Tait) and our industry partners.