EPSRC-FNR Collaborative Proposal: Radiative Efficiency in Advanced Sulfide Chalcopyrites for Solar Cells (REACh)

未来的可再生和无碳能源系统将严重依赖太阳能电池产生的电力。在过去的几年里，太阳能组件的价格下降了很多，现在与系统其他部分相关的成本已经成为主要的费用。这些成本与太阳能电池的面积有关。因此，提高太阳能电池的效率仍然是至关重要的，因为这样我们就需要更少的面积来产生相同数量的电力。传统太阳能电池经过几十年的改进，其效率现已接近理论极限。串联太阳能电池提供了一种新的前进方式，我们将两个不同的太阳能电池堆叠在一起，这样每个电池都可以更好地利用太阳光谱，效率变得更高。我们正在研究一种新材料，硫化物黄铜矿，它可以用于薄膜（只使用少量的原材料），是稳定的，并已显示出有前途的效率。我们想用这种材料作为顶部电池，将其与硅等众所周知的太阳能电池材料结合起来，生产出一种串联电池。这些薄膜不是单晶，而是由微小的晶体组成-在微米尺度上-被称为“晶粒”，它们在“晶界”处相互对接。这些晶界可能是有问题的，因为它们不同于完美的晶体，因此是我们可以快速丢失入射阳光产生的电子的地方。此外，晶界可以阻挡电流移动通过太阳能电池。通常，在薄膜中存在许多不同的晶界，一些对太阳能电池有害，而一些是良性的。在这个项目中，我们想了解硫化物黄铜矿太阳能电池中晶界的作用。我们将研究发光，这是太阳能电池材料在受到激光或电子束激发时发出的光，从而产生电子。我们用这种光来检查薄膜的质量。一个好的太阳能电池材料也会发出大量的发光，因为导致发光的电子也是那些在太阳能电池中携带电流的电子-如果它们能够发光，它们就不会在晶界等缺陷处丢失。对于最好的电影，我们将使用电子显微镜，这使我们能够以非常高的空间分辨率研究发光。因此，在电子显微镜中，我们可以检查每个单独的晶界，看看它对发光的影响有多大。然后，我们可以用电子显微镜检查那些不会导致电子损失的晶界的特殊之处。这些信息将帮助我们调整薄膜的生长过程，使其成为更好的太阳能电池，避免生长更具破坏性的晶界类型。

The renewable and carbon free energy system of the future will rely heavily on electricity generated by solar cells. Solar modules have decreased in price so much in the last few years, that now the cost related to the rest of the system has becomes the main expense. These cost are related to the area of the solar cell. Therefore it remains essential to increase the efficiency of solar cells, because then we need less area to produce the same amount of electricity. Conventional solar cells have been refined over several decades and their efficiencies are now approaching theoretical limits. A new way forward is provided by tandem solar cells, where we stack two different solar cells on top of each other, so that each can make better use of the solar spectrum and the efficiency becomes higher. We are working on a new material, sulfide chalcopyrite, which can be used in a thin film (using only small amounts of raw materials), is stable and has already shown promising efficiencies. We want to use this material as the top cell, combining it with well-known solar cell materials like silicon to produce a tandem cell. These thin films are not single crystals but consist of tiny crystals - on the micrometre scale - known as "grains", which are butt up against one another at "grain boundaries". These grain boundaries can be problematic because they differ from the perfect crystal, and are hence places where we can lose the electrons generated by the incoming sunlight at a rapid rate. Also, grain boundaries can block the current moving through the solar cell. Usually, there are many different grain boundaries in a thin film, some are detrimental for the solar cell and some are benign. In this project we want to understand the role of grain boundaries in sulfide chalcopyrite solar cells. We will study luminescence, which is the light that is emitted by a solar cell material when it is excited by a laser or an electron beam, thereby generating electrons. We use this light to check the quality of the thin films. A good solar cell material will also emit a lot of luminescence, because the electrons that lead to light emission are also those that would carry the current in the solar cell - and if they are able to emit light they are not lost at defects like grain boundaries. For the best films we will use an electron microscope, which allows us to study the luminescence with a very high spatial resolution. Thus, in the electron microscope, we can examine each individual grain boundary and see how much it affects the luminescence. We can then check with the electron microscope what is special about those grain boundaries that do not lead to loss of electrons. This information will help us adapt the growth process of our thin films to make them into better solar cells, by avoiding growth of the more damaging types of grain boundary.