Photovoltaic characterization of organic solar cells

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In recent years organic solar cells (SCs) have reached power conversion efficiencies above10% [1]. Organic photovoltaics is indeed an intensively pursued research field because itpromises high efficiency and low cost SCs. Organic materials have unique and usefuloptoelectronic properties, their chemical synthesis can be cheap and easy, and can bedeposited in the form of thin films even on flexible plastic substrates by simple depositiontechniques such as spinning, ink-jet printing and high vacuum thermal evaporation. Here wereport results of the photovoltaic characterization of organic SCs having the donor(D)/acceptor (A) heterojunction structure [2]. The SCs, fabricated by vacuum thermaldeposition on Indium Tin oxide (ITO)-coated glass substrates, have the structure and layerthicknesses ITO/CuPc (20 nm)/C60 (40 nm)/BCP (12 nm)/Al (80 nm) where CuPc is Copperphtalocyanine used as D organic material, C60 is fullerene used as A organic material andBCP is bathocuproine used as exciton blocking layer [3].The measurement system shown in Fig.1 is used to test the organic SCs, straight after beingtaken out from the high vacuum deposition chamber, in ambient atmosphere and withoutencapsulation. A calibrated halogen lamp (cold light) is used as light source because itsspectrum, shown in Fig.2, is very close to the solar spectrum except for a lower power in theNIR with wavelengths above 700 nm. The calibration of the halogen lamp, i.e. theextrapolation of its optical power density, is carried out by using a calibrated Newport818-UV Si photodiode with 1 cm2 area and known spectral responsivity i.e. photocurrent overincident optical power at each wavelength. The “equivalent responsivity” of the photodiodecorresponding to the halogen lamp is calculated by simply weighting the photodioderesponsivity with the normalized optical spectrum of the halogen lamp. The desired incidentoptical power density can be set by simply varying the distance D in Fig.1 between the lampand the SC surface [4]. The illumination condition used is AM1.0, i.e. vertical incidence and astandard value of 100 mW/cm2 for the optical power density.The current density J vs. forward voltage V under illumination is measured by a source-meterKeithley 6487 for increasing incident optical power density. The J-V characteristics of theorganic SC is shown in Fig. 2 where one notices that the current density increases withincreasing incident optical power density, meaning there are little saturation effects at the usedvalues of incident optical power density. The organic SC exhibits a good photovoltaic effectand this can be ascribed to the fullerene layer used as acceptor and, more important, to theinsertion of the exciton blocking layer. The SC exhibits VOC = 0,43 V and JSC = 2,35 mA/cm2with a fill factor FF ≈ 50%, an external quantum efficiency (electrons/s over incidentphotons/s) EQE ≈ 5% and a power conversion efficiency  ≈ 0,5%. The not-encapsulated SCsare very much sensitive to oxygen and humidity induced degradation and we observed thedegradation process in a time interval of 48 hr. Of course the solution to the oxygen andhumidity induced degradation problem is the encapsulation of the organic SCs in an inertatmosphere (glove box) with an air tight packaging of the devices. However our results arecertainly a good starting point for further improvements. References[1] Martin A. Green et al., “Solar cell efficiency tables (version 49)”, Prog. Photovolt: Res. Appl.,Vol.25, 2017, pp.3-13[2] C. W. Tang et al., “Two-Layer Organic Photovoltaic Cell”, Appl. Phys. Lett. Vol.48(2), 1986,pp.18
Original languageEnglish
Title of host publicationSIE2018 Book of Abstracts
Number of pages2
Publication statusPublished - 2018


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