Reducing hole transporter use and increasing perovskite solar cell stability with dual-role polystyrene microgel particles

Perovskite solar cells (PSCs) are a disruptive technology that continues to attract considerable attention due to their remarkable and sustained power conversion efficiency increase. Improving PSC stability and reducing expensive hole transport material (HTM) usage are two aspects that are gaining increased attention. In a new approach, we investigate the ability of insulating polystyrene microgel particles (MGs) to increase PSC stability and replace the majority of the HTM phase. MGs are sub-micrometre crosslinked polymer particles that swell in a good solvent. The MGs were prepared using a scalable emulsion polymerisation method. Mixed HTM/MG dispersions were subsequently spin-coated onto PSCs and formed composite HTM-MG layers. The HTMs employed were poly(triaryl amine) (PTAA), poly(3-hexylthiophene) (P3HT) and Spiro-MeOTAD (Spiro). The MGs formed mechanically robust composite HTMs with PTAA and P3HT. In contrast, Spiro-MG composites contained micro-cracks due the inability of the relatively small Spiro molecules to interdigitate. The efficiencies for the PSCs containing PTAA-MG and P3HT-MG decreased by only ∼20% compared to control PSCs despite PTAA and P3HT being the minority phases. They occupied only ∼35 vol% of the composite HTMs. An unexpected finding from the study was that the MGs dispersed well within the PTAA matrix. This morphology aided strong quenching of the CH3NH3PbI3-xClx fluorescence. In addition, the open circuit voltages for the PSCs prepared using P3HT-MG increased by ∼170 mV compared to control PSCs. To demonstrate their versatility the MGs were also used to encapsulate P3HT-based PSCs. Solar cell stability data for the latter as well as those for PSCs containing composite HTM-MG were both far superior compared to data measured for a control PSC. Since MGs can reduce conjugated polymer use and increase stability they have good potential as dual-role PSC additives.

3 are larger than the respective d SEM values because the particles contracted upon drying.From the d z values the particle volume swelling ratio (Q p = (d z /d z(water) ) 3 ) was calculated as ~ 6.7 and 16.3, respectively, for the MGs dispersed in toluene and CBZ.Clearly, the particles swelled strongly in these two solvents.This trend for Q p is expected because the polymer-solvent interaction parameter for polystyrene in CBZ is lower than that for toluene 2 .Data re-plotted from (a) to enable determination of the shunt resistances.The reciprocal of the gradients for the PSCs with P3HT and P3HT-MG for these devices gave calculated shunt resistances of 5,000 and 9,000  cm 2 , respectively.

Achieving uniform microgel films using spin-coating
As a preliminary study to determine the MG concentration required to form uniform coatings, MG particles dispersed in toluene were spin-coated onto glass.The coverage was estimated from optical micrographs (Fig. S8a -e).Gaps between deposited islands of particles vanished and full coverage for the films occurred at a MG concentration (C MG ) of 3.0 %.The AFM image (Fig. S8f) shows the films consisted of overlapping MGs.The variation of %Coverage with C MG determined from optical the microscopy data is shown in Fig. S8g.

Fig. S2
Fig. S2 Lower magnification AFM images (left hand column) and larger area optical micrographs (right hand column) for PTAA-MG (a and d), P3HT-MG (b and e) and Spiro-MG (c and f) composite films.The yellow arrows in (b) highlight P3HT-rich regions.The red arrows in (f) highlight micro-cracks.

Fig. S7 .
Fig. S7.Measured and corrected PL spectra for (a) films containing P3HT-MG and (b) films containing P3HT.The spectra for P3HT-MG and P3HT were multiplied by 0.48 which gave good fits to the P3HT parts of the MAPI(C)/P3HT or MAPI(C)/P3HT-MG spectra (black dashed lines).The reduced spectra were subtracted from the spectra for MAPI(C)/P3HT or MAPI(C)/P3HT-MG spectra, respectively, to provide the corrected spectra (thick dotted lines).

Fig. S8 .
Fig. S8.MG films deposited from toluene dispersions onto glass using concentrations of 0.5% (a), 1.5% (b), 2.0% (c), 3.0% (d) and 4.6% (e and f).Optical micrographs are show in (a) to (e).A tapping mode AFM image and line profile are shown in (f).The scale bars for the optical micrographs correspond to 20 µm.(g) Variation of surface coverage with MG concentration.

Fig. S9 .
Fig. S9.(a) UV-visible spectrum of a MG film deposited on glass.(b) Contact angle measurement for a MG film deposited on ITO/bl-TiO 2 surface.The C MG value was 4.6%.

Fig. S10 .
Fig. S10.(a) J-V curve and schematic for a PSC encapsulated by a MG film.(b) Cross-sectional SEM for the device from (a).The arrows in (b) highlight flattened MG particles.