Further inside understanding in degradation processes affecting stability in small molecule solar cells

Sara Pescetelli^a, Aldo Di Carlo^a, Lucio Cinà^a, Antonio Agresti^a^,^b, Tom Aernouts^b, Eszter Voroshazi^b

^aCHOSE - Centre for Hybrid and Organic Solar Energy, University of Rome ‘‘Tor Vergata’’, Via del Politecnico, 1, Roma, Italy

^bIMEC, Thin-Film Photovoltaics, PV department, Kapeldreef, 75, Leuven, Belgium

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, Antonio Agresti, 266
Publication date: 5th February 2015

Despite the considerable understanding of water and oxygen aging effects on the stability of small molecule photovoltaic devices, few studies attempted to investigate the effect of temperature stress by excluding moisture contribution to the degradation processes. In this work, we aim to provide further insights through a systematic device degradation study coupled with opto-electrical characterizations. The tested devices are flat heterojunction solar cell with the following structure: ITO-Glass/MoO₃/ZnPc/C₆₀/BCP/Ag. The optimized encapsulation procedure is based on glue cured glass lid over the device substrate, sealed in inert (nitrogen filled glove box system) and vacuum condition. The calcium test procedure allows us to estimate the lifetime of the encapsulation and the glue edge WVTR (Water Vapor Transmission Rate) under the harshest test conditions used in the applied aging protocol. Three aging tests are performed by storing the samples (i) in the dark conditions in glove-box (GB), (ii) over an hot plate at 85°C in glove-box (GB-85°C) and (iii) in climate chamber @85% of humidity rate and 85°C (CC-85%HR,85°C). All the stresses are performed in dark conditions The first evident effect of temperature on devices performance is an overall decrease in conversion efficiency mainly imputed to a marked reduction in the short circuit current (-20%) while V_OC and FF slightly increase during aging time (Fig.1). For thermal stressed devices, open circuit voltage decay measurements revel an increased charge carriers lifetime τ, testifying an overall decrease in the nongeminate recombination rate. At the same time the decreasing trend of the diode ideality factor “n”, extracted from I-V characteristics, points out a gradually decrease of electron trap states density involving mainly the acceptor material. Finally, the reduction of the reverse saturation current J₀, combined with an increased lifetime, gives a clear indication of a decreased donor/acceptor (D/A) interfacial area [1]. This suggests a temperature induced morphological change in the C₆₀ layer (formation of larger C₆₀ domains). As further confirmation, Raman spectroscopy confirms that thermal stress affects the acceptor material and thus the interaction between acceptor and donor material at the interface. On the other hand devices subjected to moisture attack rapidly show an abrupt decrease in the electrical performance. Oxygen molecules indeed are note to be gradually embedded in C₆₀ molecular structure as p-type impurity, drastically reducing the electron mobility and lifetime[2]. The work demonstrate how the control and optimization of the encapsulation technique combined with a well-defined aging protocol can support the deep comprehension of the degradation mechanisms in order to make the organic photovoltaic reliable and durable.
Fig.1: Time evolution of the devices electrical parameters extracted from I-V characteristics under 1 Sun illumination conditions. Each points is obtained as average considering five devices on the same sample. Black curve are related to the reference sample, stored in glove-box (GB) in inert atmosphere (H2O < 1 ppm, O2 < 1 ppm), blue curve to the sample stored in GB and over a hot-plate at 85°C, red curve to the sample stored in climate chamber (CC) by maintain a constant temperature of 85°C and 85% as humidity rate. All the averaged values are normalized to the first value (t=0)
[1] Vandewal, K.; Widmer, J.; Heumueller, T.; Brabec, C. J.; McGehee, M. D.; Leo, K.; Riede, M.; Salleo, A. Increased Open-Circuit Voltage of Organic Solar Cells by Reduced Donor-Acceptor Interface Area. Advanced Materials 2014, DOI: 10.1002/adma.201400114. [2] Hermenau, M.; Riede, M.; Leo, K.; Gevorgyan, S. A.; Krebs, F. C.; Norrman, K. Water and oxygen induced degradation of small molecule organic solar cells. Solar Energy Materials & Solar Cells 2011, 95, 1268–1277.

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