Homogeneously Miscible Fullerene Inducing Vertical Gradient in Perovskite Thin-Film Towards Highly Efficient Solar Cells

One of the strategies for obtaining high PCE in PSCs is to use additives for perovskite films.&lt;font size="1"&gt; &lt;/font&gt;Small amounts of added materials in the perovskite precursor function as a crystal growth template and a crystal growth retarder, which increase the perovskite crystal size and subsequently reduce the interfacial area of grain boundaries. This, in effect, decreases the charge traps that exist at the grain interfaces. Moreover, some additives remain in the perovskite films even after the fabrication, passivating the perovskite defect sites via the Lewis acid-base coordination and sometimes function as charge bridgesor charge extractors. A variety of materials have been reported to function as additives and passivation layers, which range from polymers to nanocarbons, such as fullerene derivatives. Among the fullerene derivatives, phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) has shown promising prospects owing to its high electron affinity and appropriate bandgap. Accordingly, various fullerene derivatives synthesized by modifying PC₆₁BM have been reported. However, there is a significant challenge in using those fullerene derivatives; they are immiscible with polar solvents while most perovskite precursors are dissolved in polar solvents. Conventionally, fullerene additives are either introduced during the antisolvent stage using chlorobenzene (CB) or dispersed directly in the perovskite precursor solution. This restricts the amount of the additives that can be added, and the device architecture to the inverted-type which gives a lower PCE than the normal-type counterparts.<br/>In this work, we attached, triethylene glycol monomethyl ether (TEG) chains to PC₆₁BM to greatly enhance its miscibility with the perovskite precursor. The synthesized TEG-attached PC₆₁BM, [6,6]-phenyl-C₆₁-butyric acid 2-[2-(2-methoxyethoxy)ethoxy]ethyl ester (PC₆₁B-TEG), and a branched substituent with two TEGs-attached PC₆₁BM, [6,6]-phenyl-C₆₁-butyric acid 1-methyl 3-[2-(2,5,8,11-tetraoxadodec-1-yl)-4,7,10,13-tetraoxatetradec-1-yl] ester (PC₆₁B-BiTEG) were highly polar and miscible with the perovskite precursor. Here, we explored three ways to introduce novel fullerene derivatives into PSCs: direct mixing with the precursor prior to film fabrication, overcoating the electron-transporting layer, and overcoating + waiting between drop-casting and spin-coating of the perovskite precursor solution on the overcoated layer of PC₆₁B-TEG to induce gradient dispersion into the perovskite precursor. The initial optimization and tests were conducted using MAPbI₃ (CH₃NH₃PbI₃)-based PSCs. The results showed that the devices in which PC₆₁B-TEG was directly added to the perovskite precursor gave a high open-circuit voltage (<i>V</i>_OC), while the overcoating and waiting approaches yielded a high fill factor (FF). We postulate that such a tradeoff arises from the balance between the perovskite grain defect passivation and the electron-transporting (hole-blocking) effect. In either case, an average PCE of ~19.6% was obtained, which is much higher than ~17% of the reference devices without PC₆₁B-TEG. Contrary to our expectation, the newly synthesized PC₆₁B-BiTEG was not as effective as PC₆₁B-TEG, because of its high-lying highest occupied molecular orbital (HOMO) level, which induced charge recombination. Applying the fullerene additive to a more advanced formamidinium/methylammonium mixed cation lead halide (FA_0.65MA_0.35PbI₃₋_��Cl_��) system using a combined approach of direct mixing with the precursor and overcoating + waiting techniques manifested even greater effectiveness. A PCE of 23.34% was exhibited when PC₆₁B-TEG was used in FA_0.65MA_0.35PbI₃₋_��Cl_��-based PSCs. The same device was taken to a national laboratory for verification and a forward and reverse bias-combined efficiency of 21.24% was officially certified. This, to the best of our knowledge, is one of the highest efficiencies reported among the fullerene additive-used PSCs reported thus far.