Improving the Stability of Perovskite Solar Cells
The development focus for next generation photovoltaic technology has been to lower costs while retaining comparable, if not improved, power conversion efficiencies and performance lifetimes in comparison to conventional CdTe, CIGS, and c-Si technologies. Perovskite solar cells are a promising, low-cost, solution-processed solar technology that already exhibit high efficiencies. 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 further technological advance which significantly compromises their thermomechanical reliability and operational lifetimes. For example, in their present planar design, perovskite solar cells are the most fragile class of cells ever tested by our group, offering negligible resistance to fracture and limiting their commercial viability and path to market.
We thus aim to employ strategies for improving the mechanical (as well as chemical) resilience of these potentially disruptive solar technologies. One such strategy is to partition a conventional monolithic perovskite solar cells into an array of microcells by a mechanically-shielding scaffold. These compound solar cells (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. Other areas of focus include encapsulating moisture and oxygen barriers and improvements in flexible perovskite solar cell design.
Figure 1. (a) A perovskite solar cell is partitioned by a mechanically-reinforcing scaffold to form a compound solar cell (CSC). (b) When compared to planar organic (OPV) or perovskite solar cells, CSCs exhibit co-optimization of efficiency and mechanical resilience, a clear deviation from the trend of a performance/resilience tradeoff.
Mechanical Resilience of Organic Semiconductors
Electronically active organic materials are of great interest because they are solution processable, allowing for low-cost roll-to-roll printing of devices, and because of their molecular nature, can potentially exhibit favorable mechanical properties including high mechanical compliance and deformability, enabling exciting new applications such as wearable and implantable electronics and sensors. These materials have been given the moniker “plastic electronics,” leading many researchers to assume that organic semiconductors in general are inherently mechanically deformable. However, this assumption is not justified as many of the semiconducting polymers and small molecules exhibiting high electronic performance are mechanically stiff—with tensile moduli ≥1 GPa—and fracture at strains ≤2%, which is not significantly greater than those of their inorganic counterparts. Understanding and improving the mechanical properties of these materials is thus critical not only for use in roll-to-roll processed solar cells, displays, and sensors that can withstand outdoor and other realistic environments, but also for future applications in inexpensive and stretchable electronics such as solar tarps for disaster relief and low-cost energy for the developing world, and conformable devices for implantable sensors, prostheses, and soft robotics. My research is motivated by the ways in which molecular structure of an organic semiconductor influence the self-assembled nanoscale morphology of a thin film, and how this microstructure influences both the mechanical and electronic properties of these materials, in particular, how softness and charge transport can coexist in the same material.
Figure 2. Thin films of an organic semiconductor being evaluated for microstructural evolution with repeated strain. The absorption spectra is taken periodically and the experimental data is fit with the weakly interacting H-aggregate model to give information about quantity and quality of aggregation in the film. Decreasing aggregation is correlated with poorer electronic performance, but generally more mechanically compliant films. However, in the case of a film being strained cyclically, the decrease in aggregation is actually a sign of film fatiguing.
- B. L. Watson*, N. Rolston*, A. D. Printz*, and R. H. Dauskardt, “Scaffold-Reinforced Perovskite Compound Solar Cells,” Energy and Environmental Science, 2017, in press. DOI: 10.1039/c7ee02185b (*Equal contribution)
- N. Rolston, A. D. Printz, S. R. DuPont, E. Voroshazi, and R. H. Dauskardt, “Effect of Environmental Stressors on the Decohesion Kinetics of Organic Solar Cells,” Solar Energy Materials and Solar Cells, 2017, 170, 239.
- S. E. Root, S. Savagatrup, A. D. Printz, D. Rodriquez, and D. J. Lipomi, “Mechanical Properties of Organic Semiconductors for Stretchable, Highly Flexible, and Mechanically Robust Electronics,” Chemical Reviews, 2017, 117, 6467.
- S. E. Root, M. A. Alkhadra, D. Rodriquez, A. D. Printz, and D. J. Lipomi, “Measuring the Glass Transition Temperature of Conjugated Polymer Thin Films with UV-Vis Absorption Spectroscopy,” Chemistry of Materials, 2017, 29, 2646.
- S. Savagatrup, A. D. Printz, T. F. O’Connor, I. Kim, and D. J. Lipomi, “Efficient Characterization of Bulk Heterojunction Films by Mapping Gradients by Reversible Contact with Liquid Metal Top Electrodes,” Chemistry of Materials, 2017, 29, 389.
- A. D. Printz and D. J. Lipomi, “Competition Between Deformability and Charge Transport in Semiconducting Polymers for Flexible and Stretchable Electronics,” invited review for Applied Physics Reviews, 2016, in review.
- A. D. Printz, A. S-C. Chiang, and D. J. Lipomi, “Analyzing Fatigue Due to Cyclic Loading in Poly(3-alkylthiophene) Thin Films with the Weakly Interacting H-aggregate Model,” Synthetic Metals, 2016, in press.
- E. J. Sawyer, A. V. Zaretski, A. D. Printz, N. de los Santos, A. Bautista-Gutierrez, and D. J. Lipomi, “Large Increase in Stretchability of Organic Electronic Devices by Encapsulation: All-Rubber Solar Cells,” Extreme Mechanics Letters, 2016, in press.
- A. V. Zaretski, S. E. Root, A. Savchenko, E. Molokanova, A. D. Printz, L. Jibril, G. Arya, M. Mercola, and D. J. Lipomi, “Metallic Nanoislands on Graphene as Highly Sensitive Transducers of Mechanical, Biological, and Optical Signals,” Nano Letters, 2016, in press.
- T. F. O’Connor, A. V. Zaretski, S. Savagatrup, A. D. Printz, C. D. Wilkes, M. I. Diaz, D. J. Lipomi, “Wearable Organic Solar Cells with High Cyclic Bending Stability: Materials Selection Criteria,” Solar Energy Materials & Solar Cells, 2015, 144, 438.
- A. D. Printz, A. V. Zaretski, S. Savagatrup, A. S-C. Chiang, and D. J. Lipomi, “Yield Point of Semiconducting Polymer Films on Stretchable Substrates Determined by Onset of Buckling,” ACS Applied Materials & Interfaces, 2015, 7, 23257.
- P. B. Landon, A. H. Mo, A. D. Printz, C. Emerson, C. Zhang, W. Janetanakit, D. A. Colburn, S. Akkiraju, S. Dossou, B. Chong, G. Glinsky, and R. Lal, “Asymmetric Colloidal Janus Particle Formation is Core Size Dependent,” Langmuir, 2015, 31, 9148.
- S. Savagatrup, D. Rodriquez, A. D. Printz, A. B. Sieval, J. C. Hummelen, and D. J. Lipomi, “PCBM and Incompletely Separated Grades of Methanofullerenes Produce Bulk Heterojunctions with Increased Robustness for Ultra-Flexible and Stretchable Electronics,” Chemistry of Materials, 2015, 27, 3902.
- T. F. O’Connor, K. M. Rajan, A. D. Printz, and D. J. Lipomi, “Toward Organic Electronics with Properties Inspired by Biological Tissue,” Journal of Materials Chemistry B, 2015, 3, 4947.
- S. Savagatrup*, A. D. Printz*, H. Wu, K. M. Rajan, E. J. Sawyer, A. V. Zaretski, C. J. Bettinger, and D. J. Lipomi, “Viability of Stretchable Poly(3-heptylthiophene) (P3HpT) for Organic Solar Cells and Field-Effect Transistors,” Synthetic Metals, 2015, 203, 208. (* Equal contribution)
- A. V. Zaretski, H. Moetazedi, C. Kong, E. J. Sawyer, S. Savagatrup, E. Valle, T. F. O’Connor, A. D. Printz, and D. J. Lipomi, “Metal-Assisted Exfoliation (MAE): Green, Roll-to-Roll Compatible Method for Transferring Graphene to Flexible Substrates,” Nanotechnology, 2015, 26, 045301.
- A. D. Printz, S. Savagatrup, D. Rodriquez, and D. J. Lipomi, “Role of Molecular Mixing on the Stiffness of Polymer:Fullerene Bulk Heterojunction Films,” Solar Energy Materials & Solar Cells, 2015, 134, 62.
- S. Savagatrup, E. Chan, S. M. Renteria-Garcia, A. D. Printz, A. V. Zaretski, T. F. O’Connor, D. Rodriquez, E. Valle, and D. J. Lipomi, “Plasticization of PEDOT:PSS by Common Additives for Mechanically Robust Devices and Wearable Sensors,” Advanced Functional Materials, 2015, 4, 13635.
- S. Savagatrup, A. D. Printz, T. F. O’Connor, A. V. Zaretski, D. Rodriquez, E. J. Sawyer, K. Rajan, R. I. Acosta, S. E. Root, and D. J. Lipomi, “Mechanical Degradation and Stability of Organic Solar Cells: Molecular and Microstructural Determinants,” Energy and Environmental Science, 2015, 8, 55.
- E. J. Sawyer, S. Savagatrup, T. F. O'Connor, A. S. Makaram, D. J. Burke, A. V. Zaretski, A. D. Printz, D. J. Lipomi, "Toward Instrinsically Stretchable Organic Semiconductors: Mechanical Properties of High-Performance Conjugated Polymers," Proc. SPIE 9185, Organic Field-Effect Transistors XIII; and Organic Semiconductors in Sensors and Bioelectronics VII, 91850U (October 7, 2014); doi: 10.1117/12.2059098
- P. B. Landon, A. H. Mo, C. Zhang, C. Emerson, A. D. Printz, A. F. Gomez, C. J. DeLaTorre, D. A. M. Colburn, P. Anzenberg, M. Eliceiri, C. O’Connell, and R. Lal, “Designing Hollow Nano Gold Golf Balls,” ACS Applied Materials & Interfaces, 2014, 6, 9937.
- S. Savagatrup, A. D. Printz, T. F. O’Connor, A. V. Zaretski, and D. J. Lipomi, “Molecularly Stretchable Electronics,” Chemistry of Materials, 2014, 26, 3028.
- S. Savagatrup*, A. D. Printz*, D. Rodriquez, and D. J. Lipomi, “Best of Both Worlds: Conjugated Polymers Exhibiting Good Photovoltaic Properties and High Tensile Elasticity,” Macromolecules, 2014, 47, 1981. (* Equal contribution)
- A. D. Printz*, S. Savagatrup*, D. J. Burke, T. N. Purdy, and D. J. Lipomi, “Increased Elasticity of a Low-Bandgap Conjugated Polymer by Random Segmentation for Mechanically Robust Solar Cells,” RSC Advances, 2014, 4, 13635. (* Equal contribution)
- T. F. O’Connor, A. V. Zaretski, B. A. Shiravi, S. Savagatrup, A. D. Printz, M. I. Diaz, and D. J. Lipomi, “Stretching and Conformal Bonding of Organic Solar Cells to Non-Planar Substrates,” Energy & Environmental Science, 2014, 7, 370.
- A. D. Printz, E. Chan, C. Liong, R. S. Martinez, and D. J. Lipomi, “Photoresist-Free Patterning by Mechanical Abrasion of Water-Soluble Lift-Off Resists and Bare Substrates: Toward Green Fabrication of Transparent Electrodes,” PLoS One, 2013, 8, e83939.
Conference Proceedings and Presentations
- A. D. Printz, “Understanding and Improving the Mechanical Stability of Solution-Processable Semiconductors for Flexible and Stretchable Electronics,” Distinguished Young Scholar Seminar Series, University of Washington, Department of Chemical Engineering, Seattle, WA, June 22nd, 2017.
- A. D. Printz, N. Rolston, B. L. Watson, and R. H. Dauskardt, “New Concepts in Reinforced, Segmented Perovskite Solar Cell Design with Polymer Scaffolding,” Materials Research Society Spring Meeting, Phoenix, AZ, April 18th, 2017, abstract ES1.5.05, Poster Presentation.
- A. D. Printz, N. Rolston, B. L. Watson, and R. H. Dauskardt, “Reinforced perovskite solar cells designed with integrated polymer scaffolding for robust, efficient photovoltaics,” American Chemical Society Spring Meeting, San Francisco, CA, April 2nd, 2017, abstract ENFL 57, Oral Presentation. (*Awarded Sci-Mix Poster Presentation for top 10% abstract on April 3rd, 2017)
- A. D. Printz, S. Savagatrup, D. Rodriquez, E. Chan, D. J. Lipomi “Competition between mechanical compliance and charge transport in organic semiconductors for flexible and stretchable electronics” Materials Research Society Fall Meeting, Boston, MA, December 2nd, 2015, abstract B5.02
- A. D. Printz, “Conjugated Polymers for Robust, Stretchable, and Wearable Electronic Devices,” UCSD Jacobs School Research Expo, April 16th, 2015, abstract 21.
- A. D. Printz, S. Savagatrup, D. Rodriquez, E. J. Sawyer, D. J. Lipomi “Influence of Molecular Mixing and Microstructure on the Mechanical Properties of Organic Electronics” Materials Research Society Spring Meeting, San Francisco, CA, April 9th, 2015, abstract D15.15.
- A. D. Printz, A. S.-C. Chiang, S. Savagatrup, D. Rodriquez, D. J. Lipomi “Metrology of Organic Electronics using Elastomeric Substrates: Beyond the Tensile Modulus” Materials Research Society Spring Meeting, San Francisco, CA, April 7th, 2015, abstract LL3.03.
- A. D. Printz, “Highly Compliant Organic Semiconductors: Toward Low-cost Stretchable Electronics,” TSensors Summit La Jolla, November 12th-13th, 2014.
- A. D. Printz, S. Savagatrup, D. Rodriquez, D. J. Lipomi “Best of Both Worlds: Co-Optimization of Mechanical Compliance and Photovoltaic Performance in Conjugated Polymers” Materials Research Society Spring Meeting, San Francisco, CA, April 24th, 2014, abstract R9.09.
- A. D. Printz, “Conjugated Polymers Exhibiting Good Photovoltaic Behavior and High Tensile Elasticity,” UCSD Jacobs School Research Expo, April 17th, 2014, abstract 163.
Honors & Awards
- Best Poster Nomination, Materials Research Society, Spring 2017 Meeting (April 2017)
- Chancellor’s Dissertation Medal, University of California, San Diego (June 2016)
- NanoEngineering Honorable Mention Poster, University of California, San Diego (April 2015)
- NanoEngineering Best Poster, University of California, San Diego (April 2014)