School of Chemistry
Wu, Qijie, Investigations of the effects of hole transporting materials and additives on mesoscopic perovskite solar cells, thesis, School of Chemistry, University of Wollongong, 2017. https://ro.uow.edu.au/theses1/300
Perovskite solar cells (PSCs) have drawn a large amount of attention during last few years in both academic and industrial areas of solar cell development, due to their simple fabrication, use of low-cost materials and high power conversion efficiencies (PCEs). Several key improvements have been made since the first report of this technology in PCE and stability through interface and material engineering. However, these have been demonstrated almost exclusively for small-scale devices. A key challenge is to successfully translate the fundamental knowledge obtained on small-scale PSCs to practical devices in a larger scale. The aim of this thesis is to investigate the effect of the film qualities (specifically morphological and electronic properties) for each layer on device performance, and then to apply this knowledge to upscaled devices. As a practical outcome, a larger scale PSC with efficiency of 6% with an active area of 2 cm2 is demonstrated. The device was fabricated using solution processing and widely available, low-cost materials.
The aim of the first experimental chapter (chapter III) is to correlate device performance with perovskite (CH3NH3PbI3) films deposition conditions using different solvents (γ-butyrolactone (GBL) or dimethylformamide (DMF)) as well as different deposition methods (single-step or sequential deposition), which are frequently used in the literature, but have not been directly compared. Here, specific concern is paid to practicality and scalability. The sequential deposition is shown be preferable for upscaling. This method results in a lower standard variation of device performance due to the better uniformity. Furthermore, the enhanced light harvesting contributes to a higher Jsc, by using perovskite film deposited by sequential methods.
In chapter IV, different hole transport materials (HTMs), P3HT, PCPDTBT and spiro OMeTAD are studied along with HTM-free devices, in terms of photovoltaic performance and electron lifetimes. The electron blocking effect (to block the electron injection from photoexcited perovskite to cathode) of the HTMs is observed to be a critical parameter, which depends on the HTM’s LUMO level. In addition, the pore infiltration capability of the HTMs is shown to be significantly less important than for solid-state dye sensitized solar cell (ss-DSSC) using MK2 organic dye. Considering the high cost of spiro-OMeTAD and similar PCEs in optimized PSCs, a cheaper HTM, P3HT, is considered to be a more practical HTM for the upscaling purposes.
A further advantage of P3HT is seen with a reduced reliance on additives in chapter V. LiTFSI and tBP commonly used in PSCs to enhance performance. However, their presence may lead to corrosion of the perovskite layer therefore reducing the device stability. This suggests the removal of the additives or diminishing use should be beneficial. In this chapter, the role of additives is evaluated by varying the concentrations. The combined effects of additive and HTMs are examined following layer by layer approach. In addition to improving the electronic conductivity of HTMs, the combination of LiTFSI and tBP are to enhance both Voc and Jsc of perovskite solar cells. The Jsc is increased owing to modification of the perovskite film morphology and electronic properties. The Voc is enhanced as the electron density is increased, as well as the work function of metal conatct being downward shifted due to the charge accumulation at the interface. This is different to the accepted understanding of the effect of additives in solidstate dye sensitized solar cells (ss-DSSCs), where the main effect is to interact with TiO2. Through investigating the effect of additives, a new strategy has been found to eliminate the use of additives, by well controlling the morphology of perovskite and the alignment of energy levels. This also raises the possibility of a wider range of materials to be applied as cathodic contacts in PSCs.
In chapter VI, the scalability of the fabrication of mesoscopic perovskite solar cells is tested by scaling the device area by more than a factor with an active area of 2 cm2, utilizing the results from the previous investigations for each layer. Scalable techniques are introduced in the fabrication process including an ultrasonic spray-coating of compact TiO2 layer and mechanical doctor-blading of active layers (perovskite and HTM) instead of spin coating. Based on these preliminary studies, devices with larger sizes could obtain a comparable efficiency of small-scale devices. It shows a great potential of the perovskite solar cells for further scaling up and large-scale fabrication.
The lessons learned here will help guide the further development of perovskite photovoltaics, particularly with regard to the translation from lab bench to commercial scale.