Photovoltaic characterization of organic solar cells

In recent years organic solar cells (SCs) have reached power conversion efficiencies above 10% [1]. Organic photovoltaics is indeed an intensively pursued research field because it promises high efficiency and low cost SCs. Organic materials have unique and useful optoelectronic properties, their chemical synthesis can be cheap and easy, and can be deposited in the form of thin films even on flexible plastic substrates by simple deposition techniques such as spinning, ink-jet printing and high vacuum thermal evaporation. Here we report results of the photovoltaic characterization of organic SCs having the donor (D)/acceptor (A) heterojunction structure [2]. The SCs, fabricated by vacuum thermal deposition on Indium Tin oxide (ITO)-coated glass substrates, have the structure and layer thicknesses ITO/CuPc (20 nm)/C60 (40 nm)/BCP (12 nm)/Al (80 nm) where CuPc is Copper phtalocyanine used as D organic material, C60 is fullerene used as A organic material and BCP is bathocuproine used as exciton blocking layer [3]. The measurement system shown in Fig.1 is used to test the organic SCs, straight after being taken out from the high vacuum deposition chamber, in ambient atmosphere and without encapsulation. A calibrated halogen lamp (cold light) is used as light source because its spectrum, shown in Fig.2, is very close to the solar spectrum except for a lower power in the NIR with wavelengths above 700 nm. The calibration of the halogen lamp, i.e. the extrapolation of its optical power density, is carried out by using a calibrated Newport 818-UV Si photodiode with 1 cm2 area and known spectral responsivity i.e. photocurrent over incident optical power at each wavelength. The “equivalent responsivity” of the photodiode corresponding to the halogen lamp is calculated by simply weighting the photodiode responsivity with the normalized optical spectrum of the halogen lamp. The desired incident optical power density can be set by simply varying the distance D in Fig.1 between the lamp and the SC surface [4]. The illumination condition used is AM1.0, i.e. vertical incidence and a standard 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-meter Keithley 6487 for increasing incident optical power density. The J-V characteristics of the organic SC is shown in Fig. 2 where one notices that the current density increases with increasing incident optical power density, meaning there are little saturation effects at the used values of incident optical power density. The organic SC exhibits a good photovoltaic effect and this can be ascribed to the fullerene layer used as acceptor and, more important, to the insertion of the exciton blocking layer. The SC exhibits VOC = 0,43 V and JSC = 2,35 mA/cm2 with a fill factor FF ≈ 50%, an external quantum efficiency (electrons/s over incident photons/s) EQE ≈ 5% and a power conversion efficiency ≈ 0,5%. The not-encapsulated SCs are very much sensitive to oxygen and humidity induced degradation and we observed the degradation process in a time interval of 48 hr. Of course the solution to the oxygen and humidity induced degradation problem is the encapsulation of the organic SCs in an inert atmosphere (glove box) with an air tight packaging of the devices. However our results are certainly 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.183-185 [3] B. P. Rand J. Li J. Xue R. J. Holmes M. E. Thompson S. R. Forrest, “Organic Double‐ Heterostructure Photovoltaic Cells Employing Thick Tris(acetylacetonato)ruthenium(III) Exciton‐ Blocking Layers,” Adv. Mater., Vol.17, 2005, pp. 2714-2718. [4] P. Cona, “Realizzazione e caratterizzazione fotovoltaica di celle solari organiche”, Master Degree Thesis in Electronic Engineering, University of Palermo, A.A. 2005-200