SnO_x Atomic Layer Deposition on Bare Perovskite—An Investigation of Initial Growth Dynamics, Interface Chemistry, and Solar Cell Performance

Abstract

High-end organic–inorganic lead halide perovskite semitransparent p–i–n solar cells for tandem applications use a phenyl-C 61 -butyric acid methyl ester (PCBM)/atomic layer deposition (ALD)-SnO x electron transport layer stack. Omitting the PCBM would be preferred for manufacturing, but has in previous studies on (FA,MA)Pb(Br,I) 3 and (Cs,FA)Pb(Br,I) 3 and in this study on Cs 0.05 FA 0.79 MA 0.16 PbBr 0.51 I 2.49 (perovskite) led to poor solar cell performance because of a bias-dependent light-generated current. A direct ALD-SnO x exposure was therefore suggested to form a nonideal perovskite/SnO x interface that acts as a transport barrier for the light-generated current. To further investigate the interface formation during the initial ALD SnO x growth on the perovskite, the mass dynamics of monitor crystals coated by partial p–i–n solar cell stacks were recorded in situ prior to and during the ALD using a quartz crystal microbalance. Two major finds were made. A mass loss was observed prior to ALD for growth temperatures above 60 °C, suggesting the decomposition of the perovskite. In addition, a mostly irreversible mass gain was observed during the first exposure to the Sn precursor tetrakis(dimethylamino)tin(IV) that is independent of growth temperature and that disrupts the mass gain of the following 20–50 ALD cycles. The chemical environments of the buried interface were analyzed by soft and hard X-ray photoelectron spectroscopy for a sample with 50 ALD cycles of SnO x on the perovskite. Although measurements on the perovskite bulk below and the SnO x film above did not show chemical changes, additional chemical states for Pb, Br, and N as well as a decrease in the amount of I were observed in the interfacial region. From the analysis, these states and not the heating of the perovskite were concluded to be the cause of the barrier. This strongly suggests that the detrimental effects can be avoided by controlling the interfacial design.

Tailoring the Surface of Metal Halide Perovskites to Enable the Atomic Layer Deposition of Metal Oxide Contacts

“…43 Hultqvist et al also observed this phenomenon but instead suggested that the formation of Sn−Br oxides could explain the peaks at higher binding energy. 44 When a perovskite film with the PbS overlayer is exposed to the same 10 cycles of ALD SnO 2 , these higher binding energy bromine species are still present, but their relative concentration is reduced by ca. 30%.…”

“…ALD relies on reactions between gaseous precursor molecules and a substrate surface to initiate film growth. Reports in literature have shown that these precursors can adversely react with the perovskite surface to alter its structure and composition, leading to nonideal growth behavior and a poor perovskite/contact interface. ,− To date, most successful applications of ALD contacts above the perovskite have included an additional contact layer in between to act as a buffer.…”

Inorganic top electron transport layer for high performance inverted perovskite solar cells

“…Many researchers have continuously worked on finding inorganic alternatives to boost higher PCE and stability with lower cost. Compared with organic top ETL, inorganic top ETL has the advantages of (a) facile fabrication process, 50,64,168,[202][203][204][205] (b) high intrinsic stability, 3,4,18,32,50,[206][207][208][209] (c) easily adjustable optical and electronic properties, 3,28,34,35,42,153,159,208 (d) low-temperature or no post-treatment, 26,41,50,155,168,190,203 (e) low cost, 68,163,187,210,211 (f) additional function as passivate layer. 7,36,175 All the advantages of inorganic top ETL accelerate its application in high-performance inverted PSCs.…”

“…The PSC performance has been found to be better than the organic ETL based devices. 1,4,48,147,148,207,219 This means that SnOx is also an efficient top ETL.…”

“…2 Although the PCE value is some lower, inverted PSCs have some special advantages, such as much lower hysteresis and promising application on tandem solar cells, which makes them a famous star among the new-generation photovoltaic devices. [3][4][5] As an important functional layer in high performance inverted PSC, electron transport layer on top of perovskite film (namely top ETL) play an irreplaceable role. Known to be a star ETL, fullerene and it derivative such as [6,6]phenyl-C61-butyric acid methyl ester (PCBM) has been widely used in inverted PSCs, exhibiting negligible hysteresis.…”