Heterojunctions underpin the design and performance of virtually all devices based on conventional semiconductors. While metal halide perovskites have received intense attention for applications in photoconversion and optoelectronics, these devices are often hybrid, containing interfaces between the perovskite and metal oxide or organic semiconductor layers. Heterojunctions between two perovskite layers could enable new paradigms in device engineering, but to date, their formation has remained limited due to difficulty in fabricating multilayers and facile ion diffusion across interfaces. Here, sequential solution and vapor processing is used to successfully fabricate perovskite/perovskite heterojunctions comprising three-dimensional APbX3/CH3NH3SnX3 [A = CH(NH 2)2, CH3NH3, or Cs; X = I or Br] layers. Heterojunction stability is investigated leading to the identification of two pairings that are stable for >1500 h at room temperature. By probing mixing as a function of composition and grain size, we propose general design rules for the realization of stable perovskite/perovskite heterojunctions.

The efficiency of organic light-emitting devices (OLEDs) is often limited by roll-off, where efficiency decreases with increasing bias. In most OLEDs, roll-off primarily occurs due to exciton quenching, which is commonly assumed to be active only above device turn-on. Below turn-on, exciton and charge carrier densities are often presumed to be too small to cause quenching. Using lock-in detection of photoluminescence, we find that this assumption is not generally valid; in fact, luminescence can be quenched by >20% at biases below turn-on. We show that this low-bias quenching is due to hole accumulation induced by intrinsic polarization of the electron transport layer (ETL). Further, we demonstrate that selection of non-polar ETLs or heating during deposition minimizes these losses, leading to efficiency enhancements of >15%. These results reveal design rules to optimize efficiency, clarify how ultrastable glasses improve OLED performance, and demonstrate the importance of quantifying exciton quenching at low bias.

Host-guest structures are used in most state-of-the-art organic light-emitting devices (OLEDs), where host molecules serve to dilute guest molecules, transport charge, and confine excitons on the guest. Hosts are often critical to achieving high efficiency and stability, yet predicting and understanding the effect of host properties on device stability is a persistent design challenge which slows the discovery of new OLED materials. Studying closely related carbazole hosts, we find that hosts which form an intermolecular triplet excimer state show faster degradation. Screening for excimer formation could accelerate discovery of stable host materials.

We report an approach to form periodic patterns in single layers of organic semiconductors by a simple annealing process. When heated, a crystallization front propagates across the film, producing a sinusoidal surface structure with wavelengths comparable to that of near-infrared light. These surface features initially form in the amorphous region within a micrometer of the crystal growth front, probably due to competition between crystal growth and surface mass transport. We demonstrate control over these patterns by varying processing conditions and substrate properties. This phenomenon could be exploited for the self-assembly of microstructured organic optoelectronic devices.

In this work, we studied the kinetics of OLED degradation. We found that device photoluminescence (PL) stability is more sensitive to exciton density, whereas the exciton formation (EF) stability is more strongly impacted by charge carriers. Through this work, we further developed new measurement techniques to quantify the various pathways of OLED efficiency loss.