Wavelength selective Perovskite solar cells

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Doctor of Philosophy
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Hayali, Ahmed Mahfoodh Taha

Perovskite solar cells (PvSCs) have attracted numerous attention due to their low manufacturing cost and high efficiency. The efficiency and stability of perovskite solar cells (PvSCs) depend not only on the perovskite film quality, but they are also dependent on the charge carriers of both the electron transport layer (ETL) and hole transport layer (HTL). The ETL needs to be highly transparent and highly conductive. To achieve these properties, titanium dioxide TiO2 has been widely used as an ETL. In particular, compact titanium dioxide (C-TiO2) has been employed as an ETL in most PvSCs. Both surface roughness and uniformity of the C-TiO2 can influence the electrical and optical properties of the film and consequently the efficiency of the PvSCs. The effect of changing the DC sputtering power and the ratio of argon (Ar) to oxygen (O2) plasma on the C-TiO2 films and subsequently on the overall efficiency were studied. The optimum preparation conditions for the C-TiO2 films were obtained when the DC power was set at 200 W and a flow rate of 6 sccm Ar and 12 sccm O2. A maximum power conversion efficiency (PCE) of 15.3% in forward sweep and 16.7% in reverse sweep were achieved under sunlight simulator of 100 mW/cm2. These results indicate that significant improvement in the efficiency can be achieved, by optimizing the C-TiO2 layer.

Doping of the ETL with FK209 cobalt found to enhance the efficiency and performance of the PvSCs. It has been demonstrated in this study that an optimum concentration of 2.5 mg FK209 cobalt in the M-TiO2 has resulted in an efficiency of 15.6% on 0.36 cm2 active device area. The enhanced efficiency is due to the improved conductivity of the ETL while maintaining high transparency and low surface roughness with FK209 doping. Devices fabricated using M-TiO2 doped with FK209 have shown improved performance and are highly reproducible.

Solvents play an important role in the preparation of the ETL and HTL. Lithium salt TFSI employed in HTL and FK209 cobalt TFSI used in ETL are usually dissolved by acetonitrile (ACN) solvent. However, this solvent is toxic and can accelerate the deterioration of the perovskite film, which can also limits any improvement of PvSC performance. This study demonstrated that using ethanol to dissolve FK209 and isopropanol (IPA) to dissolve lithium can be used as an effective alternative to ACN. Ethanol and IPA can slow down the degradation of the perovskite film compared with ACN due to decreasing pinhole defects density and recombination process. This can lead to better stability of perovskite based solar cells. Devices prepared using FK209/ethanol and lithium/IPA have shown good performance with champion device efficiency of 16.4% and the drop in efficiency was 17% over the 40 weeks testing period. The champion device prepared using ACN solvent gave an average efficiency of 15.8% and the efficiency dropped by 22 % over the 40 weeks period. Using the proposed ethanol and IPA solvents offer considerable potentials toward environmentally friendly fabrication process and notable improvement in device performance.

Perovskite solar cells without the ETL have been investigated to simplify fabrication processes by eliminating the use of high temperature annealing; facilitating the deposition of perovskite films directly on the FTO glass substrate. The PCE of the perovskite devices without the ETL was measured at 13%. This means that 17% of efficiency was lost by avoiding the use of the ETL. However, PvSCs without the ETL is an important alternative for simple structure solar cells, and allow the possibility of low temperature fabrication and use of plastic substrate for PvSCs.

This work developed perovskite based solar cells with wavelength selective properties ranging from 500 nm (UV-Vis) to 800 nm (IR). The bandgap tuning was achieved through composition changes mainly Lead (II) iodide PbI2 and Lead (II) bromide PbBr2. High content of iodine displays a photoluminescence (PL) peak at 790 nm, whereas high content of bromine shows a PL peak at 548 nm. The combined composition mixture of (PbI2 and PbBr2) can be fine-tuned to prepare material that absorb light in the visible range (640-660 nm) or any other wavelength between the range from 500 to 800 nm. The corresponding bandgaps were changed and cover a range from narrow bandgap of 1.54eV to wide bandgap of 2.4eV. The average efficiency of the fabricated solar cells ranges from 1% to 15.5% depending on perovskite composition. Wavelength selective perovskite solar cells have potential applications in building integrated PV and in solar operated greenhouses.

In the last part of this research, the organic-inorganic hybrid multilayers Perovskite/Perovskite tandem solar cells (PvTSCs) have been investigated to explore the advantages of combing two wavelength selective materials in the construction of perovskite based solar cells. The PvTSCs have an essential potential to overcome the limitations of Shockley–Quiesser, which has limited the efficiency of single-junction solar cells. PvTSCs have been used to boost the efficiency to much higher levels then single junctions. Materials of wide bandgap are deposited on top cell (~2.3 eV), while materials of narrow bandgap are deposited on bottom cell (~1.6 eV). This study shows that the Perovskite/Perovskite tandem device (Wide bandgap material + Narrow bandgap material) yielded the PCE of 16.5% in the forward sweep and 18.8% in the backward sweep achieved on 0.25 cm2 active area. Whereas, it yielded a PCE of 14.6 % in the forward sweep and 16.2 % in the backward sweep on 0.36 cm2 active area. Using all perovskite tandem solar cells with single layer of ETL and HTL and without recombination junction layer yielded average efficiencies around 15.4%, which is around 14% lower than PvSCs with two ETLs, HTLs and recombination junction layer. For applications with their main drive is low cost, simplifying fabrication processes by eliminating the use of double ETLs will be more attractive.

In summary, this study investigates the fabrication and characterisation of wavelength selective perovskite solar cells with potential, for greenhouses applications and building integrated photovoltaics. To achieve this, the perovskite composition mixture materials (I, Br and Cl) were changed to obtain wide range of bandgaps. Experiments were performed to verify changes in the bandgap as a result of using perovskite compositions with different concentrations. Structural, optical, electrical and device characterizations were examined. The optimum preparation conditions were found for the deposition of the TiO2 together with the optimum concentration of the FK209 doping. We examined an alternative solvent to ACN, explored tandem structures and monitored devices stability over 60 weeks time period. The reliability of the devices was tested by preparing at least 10 samples for each experiments on a relatively large device area of up to 1cm2. Finally, this study demonstrates new Perovskite/Perovskite tandem cells structure that eliminates the need for two carrier transport layers and a recommendation junction layer. The tandem perovskite cells achieved an average efficiency of 17% with a low-cost and simple manufacturing process.

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