Kevin A. Bush

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.

Perovskite-perovskite tandem photovoltaics with optimized band gaps

Tandem perovskite cells The ready processability of organic-inorganic perovskite materials for solar cells should enable the fabrication of tandem solar cells, in which the top layer is tuned to absorb shorter wavelengths and the lower layer to absorb the remaining longer-wavelength light. The difficulty in making an all-perovskite cell is finding a material that absorbs the red end of the spectrum. Eperon et al. developed an infrared-absorbing mixed tin-lead material that can deliver 14.8% efficiency on its own and 20.3% efficiency in a four-terminal tandem cell. Science, this issue p. 861 A mixed tin-lead perovskite material with a narrow band gap enables efficient tandem solar cells. We demonstrate four- and two-terminal perovskite-perovskite tandem solar cells with ideally matched band gaps. We develop an infrared-absorbing 1.2–electron volt band-gap perovskite, FA0.75Cs0.25Sn0.5Pb0.5I3, that can deliver 14.8% efficiency. By combining this material with a wider–band gap FA0.83Cs0.17Pb(I0.5Br0.5)3 material, we achieve monolithic two-terminal tandem efficiencies of 17.0% with >1.65-volt open-circuit voltage. We also make mechanically stacked four-terminal tandem cells and obtain 20.3% efficiency. Notably, we find that our infrared-absorbing perovskite cells exhibit excellent thermal and atmospheric stability, not previously achieved for Sn-based perovskites. This device architecture and materials set will enable “all-perovskite” thin-film solar cells to reach the highest efficiencies in the long term at the lowest costs.

Towards enabling stable lead halide perovskite solar cells; interplay between structural, environmental, and thermal stability

Metal halide perovskite solar cells are rapidly becoming increasingly competitive with conventional PV technologies. While their efficiencies have been often touted as exceptional, they have received a lot of criticism for an apparent lack of stability. This perspective describes some of the most pressing stability concerns facing perovskite solar cells, and describes some of the recent advances made in this area. We will demonstrate that the solutions to the areas of structural, thermal, and environmental stability are closely linked, and that rational design of the perovskite and careful encapsulation can result in efficient and stable perovskite solar cells. We will conclude with some very promising results, demonstrating perovskite solar cells passing an IEC damp heat stability test.

Design and understanding of encapsulated perovskite solar cells to withstand temperature cycling

The performance of perovskite solar cells has rapidly increased above 22%, and their environmental stability is also progressing. However, the mismatch in thermal expansion coefficients and low fracture energy of layers in perovskite solar cells raise a concern as to whether devices can withstand mechanical stresses from temperature fluctuations. We measured the fracture energy of a perovskite film stack, which was shown to produce 23.6% efficiency when incorporated in a monolithic perovskite-silicon tandem. We found that the fracture energy increased by a factor of two after 250 standardized temperature cycles between −40 °C and 85 °C and a factor of four after laminating an encapsulant on top of the stack. In order to observe how the increased mechanical stability translated from film stacks to device performance and reliability, we carried out a comparative study of perovskite solar cells packaged between glass and two commonly used encapsulants with different elastic moduli. We demonstrated that solar cells encapsulated with a stiffer ionomer, Surlyn, severely decreased in performance with temperature cycling and delaminated. However, the solar cells encapsulated in softer ethylene vinyl acetate withstood temperature cycling and retained over 90% of their initial performance after 200 temperature cycles. This work demonstrates a need for an encapsulant with a low elastic modulus to enable mechanical stability and progress toward 25 year operating lifetime.

Synthesis and use of a hyper-connecting cross-linking agent in the hole-transporting layer of perovskite solar cells

Solution-processed organic semiconducting materials feature prominently in modern optoelectronic devices, especially where low-cost and flexibility are specific goals, such as perovskite solar cells. Their intrinsic solubility, poor cohesion and lack of adhesion to underlying substrates, however, curtail their scope of application and durability. To overcome this, a mechanically stiff, light-activated, tetra-azide cross-linking agent, 1,3,5,7-tetrakis-(p-benzylazide)-adamantane (TPBA), has been developed to transform solution processed organic polymers into solvent-resistant and mechanically tough films. The use of 3-azidopropyltrimethoxysilane (AzPTMS) has been developed as a light-activated adhesion promotor, enabling mechanical testing of toughened, cross-linked polymers. Lithium bis(trifluoromethane)sulfonimide (LiTFSI) doped poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine, poly(triaryl amine) (PTAA), a hole-transporting material used in perovskite solar cells, has been selected as a candidate system for demonstrating the utility of TPBA to transform a fragile and highly-soluble hole-transporting organic semiconductor into a mechanically tough and solvent-resistant semiconducting composite. TPBA enables the solvent resistance and mechanical toughness of PTAA to be tuned without compromising the electronic functionality of the semiconducting material. While increasing the fracture toughness of PTAA by over 300%, TPBA cross-linking also enables fabrication of perovskite solar cells with increased photovoltaic efficiencies in n–i–p and p–i–n geometries, and promotes adhesion of the doped polymer to the perovskite layer, mitigating interfacial device failure.

Encapsulating perovskite solar cells to withstand damp heat and thermal cycling

Perovskite solar cells (PSCs) are highly promising, but they are mechanically fragile, composed of layers with mismatches in thermal expansion coefficients, and known to decompose in the presence of heat and moisture. Here we show the development of a glass–glass encapsulation methodology for PSCs that enables them to pass the industry standard IEC 61646 damp heat and thermal cycling tests. It is important to select a thermally stable perovskite composition to withstand the encapsulation process at 150 °C and design a cell that minimizes metal diffusion. Moreover, the package needs an edge seal to effectively prevent moisture ingress and an inert encapsulant with an appropriate elastic modulus to hold the package together while allowing for compliance during temperature fluctuations. Our work demonstrates that industrially relevant encapsulation techniques have the potential to enable the commercial viability of PSCs.

In Situ Measurement of Electric-Field Screening in Hysteresis-Free PTAA/FA0.83Cs0.17Pb(I0.83Br0.17)3/C60 Perovskite Solar Cells Gives an Ion Mobility of ∼3 × 10–7 cm2/(V s), 2 Orders of Magnitude Faster than Reported for Metal-Oxide-Contacted Perovskite Cells with Hysteresis

We apply a series of transient measurements to operational perovskite solar cells of the architecture ITO/PTAA/FA0.83Cs0.17Pb(I0.83Br0.17)3/C60/BCP/Ag, and similar cells with FA0.83MA0.17. The cells show no detectable JV hysteresis. Using photocurrent transients at applied bias we find a ∼1 ms time scale for the electric field screening by mobile ions in these cells. We confirm our interpretation of the transient measurements using a drift-diffusion model. Using Coulometry during field screening relaxation at short circuit, we determine the mobile ion concentration to be ∼1 × 1018/cm3. Using a model with one mobile ion species, the concentration and the screening time require an ion mobility of ∼3 × 10-7 cm2/(V s). As far as we know, this article gives the first direct measurement of the ion mobility and concentration in a fully functional perovskite solar cell. The measured ion mobility is 2 orders of magnitude higher than the highest estimates previously determined using perovskite solar cells and perovskite thin films, and 3 orders of magnitude higher than is frequently used in modeling hysteresis effects. We provide evidence that the fast field screening is due to mobile ions, as opposed to dark injection and trapping of electronic carriers.

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.

Cross-linkable styrene-functionalized fullerenes as electron-selective contacts for robust and efficient perovskite solar cells

Styrene functionalized fullerene derivatives have been designed for use as electron-selective contacts in perovskite solar cells. Unlike films of PC61BM and C60 fullerene, films of these styrene-functionalized fullerenes (SFFs) can be transformed into a solvent resistant material through thermal curing. Conventional-geometry perovskite solar cells utilizing cured and uncured thin films of SFFs on titania were fabricated and tested for PCE and fracture resistance, and compared to cells employing C60. These cells displayed significant improvements in the fracture resistance (> 200 %) while exhibiting only a 7% drop in PCE (13.8 % vs 14.8 % PCE), with larger VOC and JSC values in comparison to the C60 control cell. Inverted cells fabricated with SFFs displayed an even greater increase in fracture resistance (> 400 %) with only a 6 % reduction in PCE (12.3 % vs 13.1 %) in comparison to those utilizing PC61BM.