Colin D. Bailie

Thermal and Environmental Stability of Semi-Transparent Perovskite Solar Cells for Tandems Enabled by a Solution-Processed Nanoparticle Buffer Layer and Sputtered ITO Electrode

Thermal and environmental stability of metal halide perovskite solar cells remains a major barrier to their commercialization. The industry standard transparent electrode, ITO, has good optoelectronic properties and high stability. We introduce a robust buffer layer by solution-processing AZO nanoparticles, enabling a sputtered amorphous ITO layer without damaging the underlying device. We make both semitransparent cells (12.3%) and mechanically stacked tandems (12.3% + 5.7% = 18.0%) using monocrystalline-silicon solar cells as the bottom cell. We operate the inverted-architecture, semitransparent perovskite solar cell without additional sealing in ambient atmosphere under one-sun equivalent visible illumination and measure a Ts0 lifetime of 124 hours at 100°C.

Hysteresis and transient behavior in current–voltage measurements of hybrid-perovskite absorber solar cells

Hybrid organo-metal halide perovskites are an exciting new class of solar absorber materials and have exhibited a rapid increase in solar cell efficiencies throughout the past two years to over 17% in both meso-structured and thin-film device architectures. We observe slow transient effects causing hysteresis in the current–voltage characterization of these devices that can lead to an over- or underestimation of the solar cell device efficiency. We find that the current–voltage (IV) measurement scan direction, measurement delay time, and light and voltage bias conditions prior to measurement can all have a significant impact upon the shape of the measured IV light curves and the apparent device efficiency. We observe that hysteresis-free light IV curves can be obtained at both extremely fast and slow voltage scan rates but only in the latter case are quasi-steady-state conditions achieved for a valid power conversion efficiency measurement. Hysteretic effects are also observed in devices utilizing alternative selective contacts but differ in magnitude and time scale, suggesting that the contact interfaces have a big effect on transients in perovskite-absorber devices. The transient processes giving rise to hysteresis are consistent with a polarization response of the perovskite absorber that results in changes in the photocurrent extraction efficiency of the device. The strong dependence of the hysteresis on light and voltage biasing conditions in thin film devices for a period of time prior to the measurement suggests that photo-induced ion migration may additionally play an important role in device hysteresis. Based on these observations, we provide recommendations for correct measurement and reporting of IV curves for perovskite solar cell devices.

A 2-terminal perovskite/silicon multijunction solar cell enabled by a silicon tunnel junction

With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm2 2-terminal monolithic perovskite/silicon multijunction solar cell with a VOC as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%.

Melt-infiltration of spiro-OMeTAD and thermal instability of solid-state dye-sensitized solar cells

A method for achieving complete pore-filling in solid-state dye-sensitized solar cells termed melt-infiltration is presented: after the customary solution-processed deposition of spiro-OMeTAD, the device is heated above the glass transition temperature of spiro-OMeTAD to soften the material and allow capillary action to pull additional spiro-OMeTAD from the overlayer reservoir into the pores. The pore-filling fraction increases from 60-65% to 90-100% as a result of melt-infiltration. The organic D-π-A dye used in this study is found to withstand the thermal treatment without performance loss, unlike ruthenium-based dyes. Through our experiments, we find that the 4-tert-butylpyridine (tBP) additive, commonly used in dye-sensitized solar cells, evaporates from the device during heat treatment at temperatures as low as 85 °C. This significantly impacts device performance, potentially excluding its use in commercial applications, and demonstrates the need for a more thermally stable tBP alternative. Melt-infiltration is expected to be a viable method for achieving complete pore-filling in systems where volatile additives are not required for operation.

The importance of dye chemistry and TiCl4 surface treatment in the behavior of Al2O3 recombination barrier layers deposited by atomic layer deposition in solid-state dye-sensitized solar cells

Atomic layer deposition (ALD) was used to fabricate Al(2)O(3) recombination barriers in solid-state dye-sensitized solar cells (ss-DSSCs) employing an organic hole transport material (HTM) for the first time. Al(2)O(3) recombination barriers of varying thickness were incorporated into efficient ss-DSSCs utilizing the Z907 dye adsorbed onto a 2 μm-thick nanoporous TiO(2) active layer and the HTM spiro-OMeTAD. The impact of Al(2)O(3) barriers was also studied in devices employing different dyes, with increased active layer thicknesses, and with substrates that did not undergo the TiCl(4) surface treatment. In all instances, electron lifetimes (as determined by transient photovoltage measurements) increased and dark current was suppressed after Al(2)O(3) deposition. However, only when the TiCl(4) treatment was eliminated did device efficiency increase; in all other instances efficiency decreased due to a drop in short-circuit current. These results are attributed in the former case to the similar effects of Al(2)O(3) ALD and the TiCl(4) surface treatment whereas the insulating properties of Al(2)O(3) hinder charge injection and lead to current loss in TiCl(4)-treated devices. The impact of Al(2)O(3) barrier layers was unaffected by doubling the active layer thickness or using an alternative ruthenium dye, but a metal-free donor-π-acceptor dye exhibited a much smaller decrease in current due to its higher excited state energy. We develop a model employing prior research on Al(2)O(3) growth and dye kinetics that successfully predicts the reduction in device current as a function of ALD cycles and is extendable to different dye-barrier systems.

Optical loss analysis of monolithic perovskite/Si tandem solar cell

Coupling perovskite and silicon solar cells in a tandem configuration is considered an attractive method to increase conversion efficiency beyond the single-junction Shockley-Queisser limit. While a mechanically-stacked perovskite/silicon tandem solar cell has been demonstrated, a method to electrically couple perovskite and silicon solar cell in a monolithic configuration has not been demonstrated. In this contribution, we design and demonstrate a working monolithic perovskite/silicon tandem solar cell, enabled by a silicon tunnel junction, with a VOC of 1.58 V. We further discuss possible efficiency loss mechanisms and mitigation strategies.

Thermal and environmental stability of semi-transparent perovskite solar cells for tandems by a solution-processed nanoparticle buffer layer and sputtered ITO electrode

Thermal and environmental stability of metal halide perovskite solar cells remains a major barrier to their commercialization. The industry standard transparent electrode, ITO, has good optoelectronic properties and high stability. We introduce a robust buffer layer by solution-processing AZO nanoparticles, enabling a sputtered amorphous ITO layer without damaging the underlying device. We make both semitransparent cells (12.3%) and mechanically stacked tandems (12.3% + 5.7% = 18.0%) using monocrystalline-silicon solar cells as the bottom cell. We operate the inverted-architecture, semitransparent perovskite solar cell without additional sealing in ambient atmosphere under one-sun equivalent visible illumination and measure a Ts0 lifetime of 124 hours at 100°C.

Mechanically stacked and monolithically integrated perovskite/silicon tandems and the challenges for high efficiency

With the advent of high-bandgap perovskites, the opportunity now exists to make tandems with perovskites on top of silicon. We have prototyped a mechanically stacked tandem, achieving 17.9% certified efficiency using a perovskite cell with a silver nanowire mesh electrode. We have also prototyped a monolithically integrated tandem on silicon, with the two subcells electronically connected by band-to-band tunneling in the silicon. The primary challenges to propelling perovskite/silicon tandems into a high-efficiency (>25%) regime are spiro-OMeTAD parasitic absorption, perovskite crystal quality, stability of a perovskite with a 1.8 eV bandgap, perovskite environmental stability, and transparent electrode quality and stability.