Improving stability of organic photovoltaic devices: the time-resolved in-situ structural/morphological monitoring approach

Barbara Paci^a, Amanda Generosi^a, Marco Guaragno^a, Valerio Rossi Albertini^a, Francesco Silvestri^a^,^b, Francesca Brunetti^b, Giampaolo Susanna^b, Ivan Davoli^b

^aISM-CNR

^bUniversity of Rome (Tor Vergata), Via del Politecnico, 1, Roma, Italy

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Barbara Paci, 354
Publication date: 5th February 2015

The present work addresses the crucial problem of aging effects in Organic Photovoltaics (OPVs). OPV devices have become a thrilling area of study due to several potential benefits: light weight, flexibility and low cost of production. Devices based on bulk heterojunction (BHJ) photoactive layers exhibit a large potential for PV energy conversion.In such systems, ultrafast photo-induced electrontransfer from a conjugated polymer (as donor) and a soluble fullerene derivative(as acceptor) occurs. Recently, notable efficiency improvements were obtained: BHJ devices power conversion efficiencies of up to 8% were reached¹ and inverted structure cells reached a record of 9.2%.^2,3 Still, novel approaches, OPV materials and architectures are required to enhance the device long-term stability: an important role is played by the structural, morphological and interfacial proprieties, strictly connected to the optical and transport properties of device. Our approach, in the research of the driving mechanisms of OPV device degradation, is to clarify the role of structural/morphological and interface properties of the different layers composing these multi-layered systems and their interfaces, a key point to evaluate^4-6. Here, we discuss our recent results on devices using PTB7:PCB70M blend as the photoactive layer in a direct architecture with PEDOT:PSS as hole transporting layer and Ca/Al as cathode. The samples have been realized using standard spin coating technique and thermal annealing to dry the different layers. The effect of vacuum treatment has been evaluated on the efficiency and aging of the samples. Our unconventional approach is based time-resolved Energy Dispersive X-ray Diffraction (EDXD) applied jointly with in-situ atomic force microscopy (AFM). Such cross-monitoring allowed retrieving information on the structural and morphological degradation pathways. Comparative studies investigating both reference systems and systems submitted to vacuum treatments were conducted. In particular, time-resolved EDXD analysis was applied in-situ, under illumination to monitor the photoactive polymer conformational properties, essential for preserving the BHJ nanoscale morphology of the donor–acceptor network. The treated devices showed improved PV stability, as demonstrated by a comparative time-resolved PCE study under light soaking. In conclusion, in-situ structural/morphological monitoring, by joint EDXD and AFM techniques, is a powerful tool to address aging effects in OPV devices. As a final result, we obtained both an enhancement of the device performance and an increase of the device lifetime.
Figure 1. Time-resolved EDXD and AFM analysis of BHJ OPV device.
[1] S. H. Park, A. Roy, S. Beaupré, S. Cho, N. Coates, J. Sun Moon, D. Moses, M. Leclerc, K. Lee, A. J. Heeger Nature Photon. 2009,3, 297–303. [2] Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y, Cao, Nature Photonics, 2012. 6, 591–595 [3] G.Susanna, L.Salamandra, C.Ciceroni, F.Mura, T.M. Brown, A. Reale, M. Rossi, A. Di Carlo, F. Brunetti, 8.7% Power conversion efficiency polymer solar cell realized with non-chlorinated solvents, Solar Energy Materials and Solar Cells 2015, 134, 194-198, [3] B. Paci, A. Generosi, V. Rossi Albertini, G. Spyropoulos, E. Stratakis, E.; Kymakis, Enhancement of photo/thermal stability of organic bulk heterojunction photovoltaic devices via gold nanoparticles doping of the active layer, Nanoscale 2012, 4, 7452 – 7459. [4] B. Paci, A. Generosi, D. Bailo , V. Rossi Albertini, G.Spyropoulos, E. Stratakis, E.; Kymakis “ Enhanced Structural Stability and Performance Durability of Bulk Heterojunction Photovoltaic Devices Incorporating Metallic Nanoparticles” Advanced Functional Materials 2011, 21, pp. 3578-3582. [5] B. Paci, D. Bailo, V. Rossi Albertini, J.Wright, C. Ferrero, G.D. Spyropoulos, E. Stratakis, E. Kymakis, “Spatially-Resolved In-Situ Structural Study of Organic Electronic Devices with Nanoscale Resolution: The Plasmonic Photovoltaic Case Study”, Advanced Matererials 2013, 25, 4760-4765.

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