Supplementary information

Title: “Long-Term Stabilization of Organic Solar Cells using Hydoperoxide Decomposers as Additives”

Journal: Applied Physics A, Springer, 2015

Authors: Vida Turkovica,11*, Sebastian Engmanna,2, Nikos Tsierkezosb, Harald Hoppea, Morten Madsen1, Horst-Günter Rubahn1, Uwe Ritterb, Gerhard Gobscha

Affiliations: a Institute of Physics and Institute of Micro- and Nanotechnologies, Ilmenau University of Technology, 98683 Ilmenau, Germany b Institute of Chemistry and Biotechnology, Ilmenau University of Technology, 98693 Ilmenau, Germany 1 Mads Clausen Institute ,University of Southern Denmark, 6400 Sønderborg, Denmark 2 present address: National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

1 corresponding author: [email protected], +45 65 50 93 58 Bandgaps of tested additives

Additive Ebandgap # (eV) 1 5.6 2 4.3 3 - 4 2.8 5 - SI 1 Optical band gaps of additives, as determined via UV-Vis spectroscopy. Note that the bandgaps of Additives #1 and #5 could not be determined as it was impossible to form films for optical measurements.

Additive concentration optimization by devices efficiency

Efficiency (%) Conc. (%) 0.03 0.3 3 10 20 Additive # 1 1.38 1.54 2.32 1.42 0.29 2 1.35 1.34 1.36 1.09 1.1 3 1.37 1.32 1.36 1.08 0.71 4 1.33 1.34 0.78 0.22 0.05 5 1.31 1.28 1.31 1.15 1.06 Ref 1.48

SI 2 Optimization of additive concentration with regard to efficiency. Optimum concentration is defined as the maximum additive concentration yielding a PCE not smaller than 2/3 of the reference PCE. Parameters of optimized solar cells containing each of the tested additives

Additive # Conc. (%) Jsc (mA/cm²) Voc (mV) FF (%) Efficiency (%) 1 3 6.989 535 62.15 2.32 2 20 6.046 350 51.85 1.1 3 10 6.928 309 50.37 1.08 4 3 6.075 387 33.14 0.78 5 20 5.067 381 55.09 1.06 Reference pre-cat 7.014 389 54.22 1.48 Reference post-cat 7.889 642 60.71 3.08

SI 3 Solar cell parameters of the optimized devices

Morphology

In SI 4 experimentally measured photoluminescence signal of non-degraded P3HT:PCBM films containing hydroperoxide decomposers and a reference without additives are shown. Additives #1 and #5 exhibit an increased photoluminescence as compared to the reference, which is attributed to significantly rougher film morphology including coarsening of the phases, as confirmed by microscopic images. The reduction in the photoluminescence intensity observed for samples containing Additives #3 and #4, can be attributed to the underestimation of the normalized signal due to the absorption of the additive in the wavelength range of the excitation laser and to the capability of the additive to act as an excitation quencher. Additionally, in case of the Additive #4, a significantly lower FF of 33%, as compared to the other samples’ FF reaching over 50% (see SI 2), points to a possibly finer intermixed morphology, resulting in a drastic increase of the recombination of charge carriers and the built up of space charges.[30-33] As AFM offers only the information on the surface morphology, it cannot be used to draw conclusions on the bulk of the active layer. Further electron tomography measurements should be conducted in order to conclude on the suggested morphological improvements with more certainty.

SI 4 (top) Photoluminescence signal of P3HT:PCBM non-degraded films containing each of the UV Absorbers (concentration as noted in SI 3), as well as the reference without any additive. The PL excitation wavelength was 445nm (~2.78eV), which is below the optical gap of each of the additives. (bottom) Optical microscopy images (100x zoom) of the P3HT:PCBM reference film (top left), and films containing Additive #1 (top middle), #2 (top right), #3 (bottom left), #4 (bottom middle), #5 (bottom right)

Bleaching

Blend films containing hydroperoxide decomposers and a reference film without additives on CaF substrates were investigated via UV-Vis and FTIR spectroscopy, both freshly prepared and upon illumination in ambient, as shown in SI 5.

In spite of the coarsening of domains, the polymer order within all films stayed unaltered upon addition of additives, which is manifested in a nearly identical ratio of the A0-0 and A0-1 absorption peaks, and a good overlap of the absorption spectra in the wavelength range 300-800nm after normalization to the maximum polymer absorption. Since all additives have shown large optical band gaps (see Error: Reference source not found), the blend absorption for each of the investigated films only differs in the high energy region.

The films show significant photo-bleaching after 120h of aging under 1000W/m² continuous illumination in air and increased scattering due to the morphological changes and formation of PCBM clusters, see SI 5 (right). Photo-bleaching indicates the corruption of the backbone conjugation, and its magnitude is directly proportional to the number of oxidized thiophene rings. [34]

However, photo-bleaching cannot be used to identify the extent of degradation, as the side chain reactions, which significantly affect the electronic functioning of the device, do not contribute to the visual impression of degradation observable in simple UV-Vis measurements.

SI 5 (left) Normalized UV-Vis absorption spectra of the non-degraded films with additives and one reference with no additive on CaF substrates. Normalization was done with respect to the absorption peak at around 500nm. (right) Relative change in absorption of the films after 120h of continuous 1000W/m² illumination in ambient. Note that all spectra were normalized to the absorption peak at 500nm of the corresponding non-degraded film.

Experimental Details

Materials: In this study, the photochemical stability of organic solar cells active layers consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was investigated. P3HT was purchased from Rieke Metals Inc, PCBM of purity 99 % was delivered by Solenne BV. P3HT:PCBM blends of mass ratio 3:2 were prepared from stock solutions. Five hydroperoxide decomposers, see Error: Reference source not found, were obtained from Sigma-Aldrich (#1 and #4), TCI Europe (#2 and #3), and Rohm and Haas (#5), and dissolved in chlorobenzene. The purity of the compounds lies in the range 95-99 %. Solar Cells: Six UV absorbers, which are common stabilizers for insulating polymers were blended in different mass concentrations with the P3HT:PCBM mixture. Solar cells with active layers containing one of the five compounds, in concentrations of 0.03 %, 0.3 %, 3 %, 10 % and 20 % of total dry weight, and reference cells with no additives added, were prepared inside the glove box (O 2 <

1 ppm, H2O < 1 ppm) via spin-coating on PEDOT:PSS (Clevios PH) coated ITO/glass substrates. Prior to the evaporation of the aluminum cathode, the films were annealed at 160°C for 10 min (pre-cathode annealing). An approximately 150 nm thick cathode was deposited under high vacuum (base pressure smaller than 5 x 10-6 mbar) via thermal evaporation. Completed solar cell devices were encapsulated behind glass inside glovebox. The parameters of the as-cast devices, short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE), were extracted from J(V)-characteristics, which were recorded under AM 1.5 conditions using a Keithley SMU 2400. Do note that the devices were pre-cathode annealed, to ensure comparable morphologies of the films and the devices, which can strongly influence photooxidative stability. Due to such differences in morphology (a thin layer of face-on P3HT is formed on top, representing a barrier to electron transport), the Voc and thus the efficiencies are lower than in the case of post-cathode devices.[35-39] Accelerated long-term stability testing: The tests were conducted under continuous light illumination (1 kWm-²), according to ISOS-3 standards [40] using a home build setup [41, 42]. The solar cell parameters were traced in-situ for over more than 100 h via periodic J(V)-measurements. Two time regimes were observed in the investigated samples, thus a bi-exponential decay was used to quantitatively describe the time dependence of the photovoltaic parameters during illumination[43]: (SI1)

For long-term stability evaluation of recorded data, two values, tburn-in (burn-in period) and tlife (lifetime of the solar cell) [44], are extracted from the fitted curves. The burn-in of the PCE is characterized by a fast decay, described by the time constant t1, while t2 corresponds to a slow degradation process of the solar cell following the burn-in. There is no exact regulation on how to derive these parameters, and the burn-in period is commonly approximated as a point in time where the dynamics of the decay changes. In this study, the burn-in time was defined as the intersection between the first derivations of the two exponential contributions of the fitted curve: (SI2)

Additionally, we defined an additional parameter, the Accumulated Power Generation or APG, to reflect the commercially relevant information concerning the power which can be generated by the device. It is calculated as the integral of extrapolation of PCE measured under continuous illumination (1 kW/m²) for a given period of time (in this study, two years), as depicted in SI 6. This parameter merges the initial and final power conversion efficiency, thus taking into account that a device with initially slightly lower PCE can compensate for it with a slower decay of the PCE, resulting with a higher power accumulation throughout their lifetime.

SI 6 Graphical representation of Accumulated power generation APG Spectroscopy: Polymer:fullerene films were prepared in a similar manner as reported for the solar cell devices, but on CaF2 substrates. Such prepared films were characterized via Fourier transform infrared spectroscopy (FTIR), UV-Vis and photoluminescence (PL) spectroscopy, before and after their degradation inside the stability setup used for solar cell characterization. FTIR spectra were recorded using a JASCO FT/IR-4000 Series spectrometer. Transmission (T) and reflection (R) spectra in the wavelength range 300-800 nm were recorded using Agilent Cary 5000 UV-Vis-NIR spectrometer. The film absorption (A) was calculated via A = 1-T-R, neglecting any light diffraction. PL-spectra were recorded using Avantes SensLine spectrometer AvaSpec-ULS2048XL. The excitation wavelength was 445 nm. Cyclic Voltammetry: In order to determine the redox potentials of the used compounds, namely electro-chemical energy levels indicating possible trap states with respect to the HOMO/LUMO of P3HT:PCBM, cyclic voltammetry measurements were performed. For this purpose, the compounds were dissolved in dichloromethane containing n-tetrabutyl-ammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. The concentration of the supporting electrolyte TBAPF6 and the investigated compounds was 0.1 molL−1 and 10-3 molL−1, respectively. The electrochemistry measurements were performed on electrochemical working station Zahner IM6/6EX. The obtained results were analyzed by means of Thales software (version 4.15). A three electrode system consisted of glassy carbon working electrode, platinum counter electrode, and Ag/AgCl (0.1 M NBu4Cl, acetonitrile) reference electrode, was used for the electrochemistry measurements. All potential values are reported versus Ag/AgCl (0.1 M NBu4Cl, acetonitrile) reference electrode. The cyclic voltammograms were recorded at the scan rate of 0.05 Vs−1 at room temperature. More details concerning the electrochemistry experiments were already reported in previous published articles.[45-47] The trap levels of each of the additives were determined via the empirical relation , where Eonset is the onset of the reduction or oxidation peak, 0.45 V is the value for ferrocene vs Ag/AgCl and 4.8 is the energy level of ferrocene below the vacuum. In case of the materials that could not be reversibly oxidized, the potential given is the irreversible anodic peak potential, the first peak of the CV trace. It must be noted here, that an irreversible peak potential may correspond to within 100 mV of the reversible oxidation potential if the species generated by a reversible electron transfer process is consumed by a rapid, chemical follow-up reaction. [48, 49]

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Appendix

Material Safety Data Sheet ADVAPAK™ NEO-1120 Stabilizer One-pack, as issued by Rohm and Haas Chemicals LLC