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Towards Long-term Stable and Highly Efficient Colloidal Quantum Dot Solar Cells

Final Report Summary - SECQDSC (Towards Long-term Stable and Highly Efficient Colloidal Quantum Dot Solar Cells)

In the course of the project, we conducted research activities to (1) improve the power conversion efficiency, (2) improve the stability, and (3) identify the limiting factors for device performance (such as charge collection and recombination, hysteresis, light soaking effect, degradation pathway) of lead based hybrid perovskite solar cells. Devices were tested in different conditions and various device structures, and different film deposition methods were implemented. All these research activities have met the research objective of our project, namely, improving the efficiency and stability of lead based hybrid solar cell, and the research results are as follows:
(1) We successfully improved the stability of the hybrid solar cells under ambient condition by using several hydrophobic n-type polymers as electron extraction layers in hybrid solar cells, these solar cells after 270 minutes exposure to ambient conditions, display 80% of their initial efficiency. This is outstanding when compared to the performances of the commonly used electron extracting layer based on the fullerene derivative (PCBM), which in the same time frame reduces its efficiency to 0.35% as shown in Fig. 1. Moreover, we identified the electron mobility of the electron extracting layer as one of the limiting factors towards efficient hybrid solar cells. By using the high electron mobility polymers P(NDI2OD-T2) and P(NDI2DT-T2), devices with efficiency higher than 10% have been obtained. Our results were published in a peer-reviewed high impact journal. (Shao et al. Journal of Materials Chemistry A 2016, 4, 2419).
(2) We succeeded in improving the electrical and photo stability as well as efficiency of the hybrid solar cells by using a new fullerene derivative, namely PTEG-1 as electron extraction layer (see Figure 2). The slow photo-response (light soaking effect) in device performance is a phenomenon, which causes electrical instability in the output of hybrid solar cells. We find that the trap-assisted recombination at perovskite/electron extraction layer interface leads to such unstable power output. The electron donating properties of PTEG-1 helps to suppress the trap-assisted recombination and thus improve the electrical stability as well as the efficiency of perovskite solar cells, which in this case is higher than 15.7% (see Fig. 2). In addition, the stability of the solar cells under prolonged light exposure is also improved by using PTEG-1 as electron extraction layer. The results were published in peer-reviewed high impact factor journal (S. Shao et al., Energy & Environmental Science 2016, DOI: 10.1039/c6ee01337f.)
(3) We investigated how the microstructure of the perovskite film affects the device performance. We find that the device with non-compact perovskite morphology shows severe light soaking effect, with the efficiency improved from 3.7% to 11.6% with light soaking. While the device with the compact perovskite morphology shows negligible light soaking effect, with efficiency slightly increased from 11.4% to 11.9% after light soaking. We demonstrate that interface electron traps at the grain boundaries as well as at the crystal surface dominate the light soaking effect. Severe trap-assisted recombination takes place in solar cells using non-compact perovskite films, while it is effectively eliminated in devices with a compact perovskite film. Therefore, the light soaking phenomenon in HPSCs can be effectively eliminated with the improvement of the active layer morphology. Our results were submitted to Advanced Functional Materials (Under revision).
(4) We investigated how the crystallization process of perovskite film affects the device performance. We find that the hypophosphorous acid stabilizer can increase the speed of the crystallization process of the perovskite film and help to form uniform, compact perovskite film with lower defect concentration. As a result, the devices show enhanced device performance due to lower charge recombination process compared to that without stabilizer (manuscript in preparation).

The potential impact of the project:
With the improved efficiency and stability, our research promotes lead based hybrid perovskite solar cells closer to the commercialization, which has great impact on sustainable development of economy and society. The electricity produced from the solar energy by silicon based photovoltaic modules is still limited by the high investment due to the complex production process and high material consumption. Compared to the traditional silicon based solar cells, the lead based hybrid perovskite solar cells promises to have higher efficiency, lower fabrication cost, lower weight, higher flexibility, and thus amenable to a wide range of lighting conditions in practical applications, such as building-integrated photovoltaics (BIPV), automotive industry, greenhouses, sensor networks, etc. The development of low-cost, sustainable and environmental friendly hybrid solar cells will sharply increase the clean energy production and participating to the energy transition towards a sustainable economic growth. Thus, by helping in reducing the consumption of fossil fuels, our research will promote realization of European ‘20-20-20’ targets, i.e. by 2020, 20% increase in energy efficiency and 20% lower greenhouse gas emissions than 1990. This will allow citizens to live a more secure and healthy life with clean soil, water and air. Moreover, knowledge users not only from the same research fields and discipline, but also from different research fields will utilize our research results. For example, the research results on hybrid solar cells can be extended to other optoelectronic devices, such as perovskite based photodetectors and light emitting diodes.

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