Towards next-generation multi-junction solar absorbers - new THz probes of charge dynamics in quantum dots

太阳能是最有希望替代石油、天然气和煤炭来产生我们所需能源的能源之一。从太阳照射到地球的阳光足以提供我们所需的一万倍的能量。然而，今天的太阳能电池还不经济；燃烧化石燃料发电仍然更便宜，这阻碍了它们的广泛使用。我们如何使太阳能电池在经济上与化石燃料竞争？有两种方法：使它们更便宜或使它们更有效（或者最好两者兼而有之！）我们今天使用的大多数太阳能电池都是由硅制成的，效率高达20%左右，但制造成本很高。一些更新的、不同类型的电池开始出现，它们制造成本更低，但效率最多只有10%。我们需要开发既便宜又高效的太阳能电池，以与化石燃料竞争。最有希望的方法之一是使用“量子点”（QDs）——由几百个半导体原子组成的微小簇，可以吸收阳光并将其转化为电能。它们既便宜又容易制作。我们可以通过改变量子点的大小来改变被吸收的阳光的颜色。这意味着我们可以很容易地制造出效率更高的“多结”电池，通过使用不同大小的点来吸收更多的阳光。这并不是量子点提高效率的唯一途径。在今天的太阳能电池中，当太阳光被电池吸收时，大约一半来自太阳的能量以热量的形式浪费掉了。然而，在量子点中，可能会发生其他事情——在正常电池中成为废热的能量可以用来产生额外的电力。这被称为“多激子生成”或“MEG”。基于量子点中MEG的太阳能电池的效率可能比目前的技术高出50%。这是一个令人兴奋的前景，但我们仍然需要更好地理解这个过程。我们需要知道阳光被吸收后量子点发生了什么。MEG发生速度极快，且难以研究，因此很难证明MEG是否发生在量子点中。为了解决这个问题，我们开发了超快激光实验，在电流产生时给我们一个快照。我们用一个非常短的激光脉冲来复制阳光，产生电流。然后我们用太赫兹辐射脉冲（非常低能量的红外线）来测量样品中发生了什么。这被当前的运营商非常强烈地吸收了。如果我们改变“泵”脉冲和“探针”脉冲之间的时间，我们可以非常快速地测量电流发生了什么（大约在1/ 10,000,000,000,000秒内）。这让我们对MEG产生的额外电力有了一个测量。我们可以用含有大量原子的半导体样品来做这件事，但是我们使用的传统太赫兹辐射源不够强大，无法研究量子点样品，因为它们非常稀。STFC Daresbury实验室的ASTeC正在开发更高功率的紧凑型太赫兹源。本应用程序的目的是在我们的测量中使用这种STFC技术，使我们能够在非常快的时间尺度上测量量子点（和MEG）中阳光产生的电流。我们将在实验中安装和测试一些STFC太赫兹源。这样的测量对量子点的制造商来说非常重要。在曼彻斯特大学，我们已经与英国领先的量子点制造商Nanoco技术有限公司合作了几年。他们感兴趣的是如何将他们的点应用到未来的太阳能电池中。在他们的内部研究中，他们正在开发使用量子点的太阳能电池原型。在这个项目中，我们将向Nanoco和太阳能行业展示stfc开发的便携式高功率太赫兹源用于量子点测量的价值。在这项可行性研究结束时，我们希望与Nanoco和STFC合作开发这项技术。

Solar power is one of the most promising alternatives to using oil, gas and coal to generate the energy we need. The sunlight that reaches the earth from the Sun is enough to supply all our energy needs 10,000 times over. However, today's solar cells are not yet economic; it is still cheaper to produce power by burning fossil fuels and this is preventing their widespread use.How can we make solar cells economically competitive with fossil fuels? There are two ways: make them more cheaply or make them more efficient (or preferably both!) Most of the solar cells we use today are made from silicon and are up to around 20% efficient but expensive to make. Some newer, different types of cell are beginning to become available which are cheaper to make but are only 10% efficient at most. We need to develop solar cells that are both cheap and efficient enough to compete with fossil fuels.One of the most promising ways to do this is by using 'quantum dots' (QDs) - tiny clusters of a few hundred semiconductor atoms that absorb the sunlight and turn it into electricity. They are cheap and easy to make. We can change the colour of sunlight that is absorbed simply by changing the size of the QD. This means we can easily make a higher-efficiency 'multijunction' cell that absorbs more of the sunlight by using dots of several different sizes.This is not the only way in which QDs can lead to higher efficiency. In today's solar cells, about half of the energy from the Sun is wasted as heat when the sunlight is absorbed by the cell. In QDs, however, something else can happen - the energy that would become waste heat in a normal cell can be used instead to produce extra electricity. This is known as 'multiple exciton generation' or 'MEG'. Solar cells based on MEG in QDs could be up to 50% more efficient than today's technology.This is an exciting prospect but we still need to understand this process better. We need to find out what happens in the QD straight after sunlight is absorbed. MEG occurs extremely fast, and is hard to study, so it is difficult to prove whether MEG is happening in a QD or not. To tackle this, we have developed ultrafast laser experiments that give us a snapshot of the current as it is created. We use a very short laser pulse to replicate the sunlight, creating the current. Then we measure what has happened in the sample using a pulse of terahertz radiation (very low energy infrared). This is absorbed very strongly by the current carriers. If we vary the time between the 'pump' pulse and the 'probe' pulse, we can measure what happens to the current very quickly (in around 1/10,000,000,000th of a second). This gives us a measure of the extra electricity created by MEG.We can do this with semiconductor samples with a very large number of atoms, but the conventional terahertz radiation source we use is not powerful enough to study QD samples, which are very dilute. Much higher power compact terahertz sources are being developed in ASTeC at STFC Daresbury Laboratory. The purpose of this application is to use this STFC technology in our measurements to allow us to measure the current created by sunlight in QDs (and MEG), on very fast timescales. We will install and test a number of STFC terahertz sources in our experiments.Measurements like this are very important to the manufacturers of QDs. At the University of Manchester, we have been collaborating for some years with Nanoco Technologies Ltd, the UK's leading manufacturer of QDs. They are interested in the ways in which their dots might be used in future solar cells. In their in-house research they are developing solar cell prototypes that use QDs. In this project we will demonstrate the value of STFC-developed portable high power terahertz sources for QD measurements to Nanoco and the solar industry. At the end of this feasibility study, we hope to develop the technology in partnership with Nanoco and STFC.