El verbo como causa ejemplar del universo

In the first publication reporting the use of perovskite as a light harvesting material in a PV

device in 2009 by Kojimaet al., CH₃NH₃PbI₃was deposited from 8 wt%γ-butyrolactone (GBL)

solution onto a mesoporous TiO2 film, and allowed to dry, which produced isolated perovskite

nanocrystals decorating the surface of the TiO₂ in a dye-sensitised architecture with a liquid

hole-transporting electrolyte similar to that depicted in Figure 1.11 (a), an η of 3.8% was

reported.[22] This structure was improved upon in 2012 by Kim et al., who replaced the liquid

electrolyte with a solid hole transporter; spiro-OMeTAD, and increased the concentration of

the precursor to 20 wt%, leading to a η >9%.[55]

The next major advance occurred in November 2012 when Leeet al. reported on the use of

Al₂O₃as a scaffold layer for fabricating high coverage perovskite films, in the structure depicted

in Figure 1.11 (b).[39] In that work a 20 wt% precursor solution containing 3:1 ratio of CH₃NH₃I

and PbCl2 in DMF was used, and the film was annealed at 100°C. The use of PbCl2 instead of PbI₂ has since been shown to improve the morphology and interfacial charge transport, while

chloride is not incorporated in the bulk of the perovskite, and most coming off as CH₃NH₃Cl

during the annealing step.[56] The very high charge carrier mobility of the perovskite ensured

that the photogenerated charges could be transported directly to the compact TiO₂layer, since

Al₂O₃is an insulator, and doesn’t accept any charges from the perovskite. Whereas, in the case

of using a mesoporous TiO2, which has lower charge carrier mobilities than the perovskite, the

TiO₂ would have to carry the electrons a longer distance to the electrode, thereby increasing

series resistance and the chance of recombination. The champion device had η >10%.

The use of mixed methylammonium (MA) - formamidinium (FA) cation perovskite was

paper the authors found that the ideal ratio of CH3NH3:HNCHNH3 was 3:2. The inclusion

of HNCH3NH3 reduced the band-gap of the perovskite and led to a higher device Jsc with a

similar Voc and FF compared with the reference MA lead perovskite. Whilst pure FAPbI3 is

more stable in a hexagonal phase known asδ-FAPbI3or the “yellow phase” (which is a different structure from the orthorhombic yellow phases of CsSnI3 and CsPbI3)[58]. In 2016 Jacobsson

et al. systematically tested lead perovskites with varying ratios of MA and FA cations, and I

and Br anions.[59] They found that the perovskite which made devices with the highest η was

(CH3NH3)1/3(HNCHNH3)2/3PbI2.5Br0.5, achieving an η of 20.7%. Saliba et al. reported on the

use of Pb perovskite containing three cations; Cs, MA and FA, resulting in a champion device

with an η of 21.1%, as well as excellent stability.[23] In that work a pillared structure with

Li-doped mesoporous TiO2 ETL, and spiro-OMeTAD HTL with a Au top electrode.

A big advance in planar heterojunction (PHJ) PPVs occurred in 2013 when Liu et al.

demonstrated the use of dual source evaporation as a means to produce thick, highly uniform

perovskite films for use in PHJ PPVs (Figure 1.12), such as that depicted in Figure 1.11 (d).[41]

The optimised ratio of precursor deposition rates for the co-evaporation was determined to be

approximately 4:1 CH3NH3I: PbCl2, and found an optimal thickness of 330 nm. The champion

device had η >15%, with Jsc>20 mA/cm2 and Voc >1 V.

FTO

TiO₂ CH3NH3PbI3

Spiro-OMeTAD Ag

Figure 1.12: Cross-sectional SEM of a device in which the perovskite was prepared using dual-source evaporation. Taken from ref. [41].

Another advance in the fabrication of high quality perovskite films was the introduction

of a two step deposition method: In 2013, Burschka et al. reported PPV devices in which

the perovskite had much higher coverage on mesoporous TiO₂ than in earlier dye-sensitised

type architectures, leading to the pillared structure (Figure 1.11 (c)).[38] This was achieved by

first spin-coating PbI2 onto the mesoporous TiO2 from high concentration, 33 wt% solution,

which resulted in a high loading onto the mesoporous layer. The PbI₂ coated TiO₂ film was

then dipped into a solution of CH3NH3I. The film was then rinsed and dried at 70 °C, forming CH3NH3PbI3. A champion deviceηof 15% was achieved. In 2014, Jeonet al. reported another

novel technique of anti-solvent precipitation.[60] The group used a perovskite precusor solution

with mixed solvent γ-butyrolactone (GBL) and dimethylsulfoxide (DMSO). This solution was

spin-cast onto a mesoporous TiO2 layer, forming an intermediate phase where the materials

were fully mixed and coordinated with DMSO on the slide. Toluene was then dropped onto the

film while the substrate was spinning, causing the precipitation of a dense uniform perovskite

film > 300 nm thick - thicker than the mesoporous TiO2 layer. The champion device had an

η >16%.[60]

The inverted planar architecture (Figure 1.11 (e)) was first reported in 2013, when Jeng

et al. used C60, PC61BM and ICBA as ETLs for CH3NH3PbI3 based PPV, achieving an η of

3.9%.[42] In that work, a very thin film of perovskite, <30 nm, was used, and so the efficiency

was limited by the low thickness, poor film uniformity and crystallite size of the photoactive

layer.[42] In 2014, Kim et al. improved upon this by using mixed DMF and GBL to improve

the morphology, and reported a device η >6%.[61] In 2016 Shaoet al. reported a device with

η > 19%, using a CH3NH3PbI3 film over 500 nm thick,[62] prepared using a method taken

from the literature. First PbI2 was spin-cast onto a PEDOT:PSS HTL and dried at 70 °C. The CH₃NH₃I was then spin-cast on top, and the film was annealed at 100 °C.[63] During the annealing step, the slides were placed in a glass Petri dish, alongside a small amount of

DMF,[64] which resulted in crystals 100s of nm across (Figure 1.13). Shao et al. improved

upon this reported procedure by adding a solvent annealing step after deposition of PC61BM

using dichlorobenzene while also thermally annealing at 100°C.[62] This ordered the fullerene molecules, reducing the trap state density and increasing the potential difference between the

perovskite VB edge and the PC61BM LUMO position, leading to a significant increase in Voc,

reporting a maximum of 1.13 V.

FTO

TiO₂

CH₃NH₃PbI₃

Spiro-OMeTAD

Ag

Figure 1.13: SEM of a perovskite film prepared using solvent annealing technique, scale bar is 2 µm. Taken from ref. [64].

1.8.3

Potential Problems with Perovskite Photovoltaics