Air Dependence of Electrical Performance Parameters in Polymeric Organic Solar Cell
Authors: Balderrama Victor Samuel , Sanchez Jose G. , Estrada Magali , Pallares Josep , Ferré-Borrull Josep , Marsal Lluis F 
Published: February 2020

The manufacture under air environment of organic solar cells (OSC) has been the main attention of many research groups in the last years due to analyze and study the stability and degradation process of the photovoltaic devices. Recent advances in polymer-based organic solar cells have been possible due to different approaches such as design of new devices and synthesis of new materials such as small molecule and polymers with low band-gaps, control of the nanoscale morphology with additives, variation of the ratio of the donor/acceptor in the bulk heterojunction, application of thermal or solvent annealing process, among others [1-3]. For large area fabrication, most reports agree that the roll-to-roll process seems to be the most promising in the last years. However, the fabrication process is difficult to realize completely under N2 environment and it is well known that OSC under air environment degrade due to low-stability of some organic semiconductors when exposed under H2O and O2 environment. Tandem structures in combination with organic materials have been manufactured employing roll-to-roll process [4-7] which some of them are manufactured under air environment.

The effects of exposing the samples to air environment during the fabrication process are not well studied, being necessary a more detailed analysis to indicate which the main effects that degraded the device. Results regarding the performance parameters of polymer OSCs under these conditions are not well known yet.

In this scenario, we present the manufacture of OSCs partially under air environment and under N2 environment. The polymer used of low band-gap was poly[[4,8-bis[(2-ethylhexyl)-oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) in combination with [6,6]-phenyl-C71-butyric acid methyl (PC70BM) to get the blend layer. The device inverted structure was manufactured with the stack: indium tin oxide (ITO) / poly[(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN) / PTB7:PC70BM / vanadium oxide (V2O5) / silver (Ag) as is shown in Figure 1a. The PFN and the PTB7:PC70BM layers were prepared and deposited under N2 environment. Afterwards, two groups of solar cells were formed. The first group of samples was taken out of the glove box and exposed under air environment (60% relative humidity at 24 oC) for 10 min before depositing by thermal evaporation the V2O5 and Ag layers. While that the second group of samples were finished the fabrication completely under N2 environment. The last group was made to compare with the first group of samples.

Electrical characterizations like current density–voltage were measured for both groups of inverted OSC samples under light conditions, which are shown in figure 1b. The basic parameters were extracted from J-V curves such as open circuit voltage, short-circuit current density, fill factor and power conversion efficiency.

Results from the first group of cells was observed that the JSC was increased and Rs reduced due to the increment on conductivity of the active blend layer when the samples were exposed for 10 min under air environment before the metal contact deposition. Other electrical test were made such as incident photon-to-current efficiency and absorbance coefficient were obtained from the photovoltaic devices to understand the differences between the two groups of cells, as well as factors that limit the power conversion efficiency. The knowledge of this behavior is important for the fabrication, design and optimization of the OSC and to reduce the effects of manufacturing under air conditions during industrial process development in the transition from the laboratory to the large scale fabrication conditions.

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Acknowledgements

Spanish Ministry of Economy and Competitiveness (MINECO) TEC2012-34397 and TEC2015-71915-REDT, the Catalan Government 2014-SGR-1344, the ICREA under the ICREA Academia Award. Consejo Nacional de Ciencia y Tecnología (CONACYT) Project 237213 in Mexico and National Postdoctoral Fellowship – CONACYT, 2016-2 and adjudge to the CVU No. 227699 in Mexico.

Figure 1