Nicholas Rolston

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.

Scaffold-reinforced perovskite compound solar cells

The relative insensitivity of the optoelectronic properties of organometal trihalide perovskites to crystallographic defects and impurities has enabled fabrication of highly-efficient perovskite solar cells by scalable solution-state deposition techniques well suited to low-cost manufacturing. Fracture analyses of state-of-the-art devices, however, have revealed that both the perovskite active layer and adjacent carrier selective contacts are mechanically fragile—a major obstacle to technological maturity that stands to significantly compromise their thermomechanical reliability and operational lifetimes. We report a new concept in solar cell design, the compound solar cell (CSC), which addresses the intrinsic fragility of these materials with mechanically reinforcing internal scaffolds. The internal scaffold effectively partitions a conventional monolithic planar solar cell into an array of dimensionally scalable and mechanically shielded individual perovskite cells that are laterally encapsulated by the surrounding scaffold and connected in parallel via the front and back electrodes. The CSCs exhibited a significantly increased fracture energy of ∼13 J m−2—a 30-fold increase over previously reported planar perovskite (∼0.4 J m−2)—while maintaining efficiencies comparable to planar devices. Notably, the efficiency of the microcells formed within the scaffold is comparable to planar devices on an area-adjusted basis. This development is a significant step in demonstrating robust perovskite solar cells to achieve increased reliability and service lifetimes comparable to c-Si, CIGS, and CdTe solar cells.

Improved stability and efficiency of perovskite solar cells with submicron flexible barrier films deposited in air

We report on submicron organosilicate barrier films produced rapidly in air by a scalable spray plasma process that improves both the stability and efficiency of perovskite solar cells. The plasma is at sufficiently low temperature to prevent damage to the underlying layers. Oxidizing species and heat from the plasma improve device performance by enhancing both interfacial contact and the conductivity of the hole transporting layer. The thickness of the barrier films is tunable and transparent over the entire visible spectrum. The morphology and density of the barrier are shown to improve with the addition of a fluorine-based precursor. Devices with submicron coatings exhibited significant improvements in stability, maintaining 92% of their initial power conversion efficiencies after more than 3000 h in dry heat (85 °C, 25% RH) while also being resistant to degradation under simulated operational conditions of continuous exposure to light, heat, and moisture. X-ray diffraction measurements performed while heating showed the barrier film dramatically slows the formation of PbI2. The barrier films also are compatible with flexible devices, exhibiting no signs of cracking or delamination after 10 000 bending cycles on a 127 μm substrate with a bending radius of 1 cm.

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.

Improved mechanical adhesion and electronic stability of organic solar cells with thermal ageing: the role of diffusion at the hole extraction interface

Organic photovoltaic (OPV) solar cells are a promising option for cheap, renewable energy, but must improve in their stability. This study examines changes in the IV parameters of inverted OPV devices with thermal ageing and correlates them to changes in the mechanical stability of the devices observed by fracture analysis. In particular, the role that the use of different materials in the hole transport layer (HTL) and metal electrode has in determining the stability (both mechanical and electronic) of the device is studied. Data from a range of characterization techniques (including Kelvin probe analysis and X-ray photoelectron spectroscopy elemental depth profiling) are used to correlate changes in device structure and performance, demonstrating the presence of inter-layer diffusion when silver is used as an electrode material. This inter-diffusion has the beneficial effect of improving the adhesion of the electrode to the device, but is correlated to declines in the performance of the device when used in conjunction with MoO3 as an HTL. An improvement in adhesion is also seen with aluminium electrodes, but without any signs of diffusion, showing that an improvement in the mechanical stability of a device when thermally aged need not come at the expense of performance stability.

Rapid route to efficient, scalable, and robust perovskite photovoltaics in air

We demonstrate a scalable atmospheric plasma route to rapidly form efficient and mechanically robust photoactive metal halide perovskite films in open air at linear deposition rates exceeding 4 cm s−1. Our plasma process uses clean dry air to produce a combination of reactive energetic species (ions, radicals, metastables, and photons) and convective thermal energy to rapidly convert the perovskite precursor solution after spray-coating. Such high energy species dissociate the precursor and superheat the solvent, quickly and efficiently curing the perovskite film. Synchrotron X-ray radiation enabled in situ wide angle X-ray scattering (WAXS) measurements with high time resolution. The ultrafast crystallization kinetics are governed by rapid nucleation and growth during the plasma exposure, followed by continued grain growth during cooling. We deposit pinhole-free, robust CH3NH3PbI3 films with a ten-fold increase in fracture toughness, a key metric for reliability. Planar devices exhibited remarkably consistent performance with 15.7% power conversion efficiency (PCE) without hysteresis and an improved open-circuit voltage (VOC). This excellent performance is attributed to lower defect densities, as measured by external quantum efficiency, steady-state and time-resolved photoluminescence. Large-area devices were made with a strip of 10 samples, and a 13.4% average PCE was measured on a total of 2.4 cm2 electrode area.

Mechanical integrity of solution-processed perovskite solar cells

The fracture of a variety of solution-processed organometal trihalide perovskite solar cells and isolated perovskite layers is reported. The study covers cells in which the perovskites, including planar and mesoporous layers, are deposited by an array of solution-state deposition techniques and feature a wide variety of ancillary organic and inorganic charge transport layers. Understanding the influence of materials selection and fabrication techniques on mechanical stability is an important step towards realizing mechanically robust perovskite cells.

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.