Thin Solid Films (2004) precursor. Lenneke H. Slooff *, Martijn M. Wienk, Jan M. Kroon

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1 Thin Solid Films (004) Hybrid TiO :polymer photovoltaic cells made from a titanium oxide precursor a,b, b,c a,b Lenneke H. Slooff *, Martijn M. Wienk, Jan M. Kroon a ECN, Solar Energy, P.O. Box 1, 1755 ZG Petten, The Netherlands b Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands c Dutch Polymer Institute, Eindhoven University of Technology, STW 3.48, P.O. Box 315, 5600 MB Eindhoven, The Netherlands Abstract Hybrid TiO :polymer photovoltaic cells were made from mixtures of titanium(iv) isopropoxide and polyw-methoxy-5-(39,79- dimethyloctyl)-p-phenylene vinylenex (MDMO-PPV) or poly(3-octyl thiophene) (P3OT) via hydrolysis in air. Cells were made with varying titanium(iv) isopropoxide:polymer ratios. Current voltage measurements (at sun equivalent) of a TiO :P3OT (10 vol.% TiO ) photovoltaic cell show a short-circuit current of 0.7 maycm, an open-circuit voltage of 450 mv and a fill factor of 0.41, resulting in a calculated AM1.5 (100 mwycm ) power conversion efficiency of 0.17%. Devices based on MDMO- PPV and TiO (0 vol.% TiO ) show an open-circuit voltage of 600 mv and a short-circuit current of 0.6 maycm (at 0.7 sun equivalent), resulting in a calculated AM1.5 power conversion efficiency of 0.%. 003 Elsevier B.V. All rights reserved. Keywords: Solar cells; Polymers; Titanium dioxide 1. Introduction In photovoltaic cells consisting of conjugatedyconducting polymers as the photoactive materials, light absorption results in the creation of a bound electron hole pair, a so-called exciton. Owing to the high exciton binding energy, the exciton has to be separated at the interface between an electron accepting (acceptor) and an electron donating (donor) material in order to create mobile charge carriers. Only those excitons created within a diffusion length from the charge separation region can participate in charge separation. The exciton diffusion length of the polymers that are typically used in polymer photovoltaics (polyphenylenevinylene and polythiophene), is in the order of several tens of nanometers w1,x. This means that only a thin slab around the interface is contributing to the current. Increasing the layer thickness will increase the light absorption, but not the charge separation and thus efficiency of the cell. In order to improve the efficiency, the average *Corresponding author. Tel.: q ; fax: q address: slooff@ecn.nl (L.H. Slooff). distance to the interface between donor and acceptor has to be on the order of the exciton diffusion length. This is done by making a so-called bulk heterojunction () in which the donor (p-type semiconducting polymer) and acceptor (fullerene, n-type semiconducting polymer, TiO or CdSe) are mixed on a nanometer scale. In these s, it is important that both materials have a percolating path to their respective electrode to ensure efficient charge collection. Previously, we have reported on a new procedure for the preparation of polymer:tio photovoltaic cells, using polyw-methoxy-5-(39,79-dimethyloctyl)-p-phenylene vinylenex (MDMO-PPV) and a TiO precursor w3x. It was shown that charge separation occurs in this polymer:tio blend. In this paper, we will discuss the results of current voltage, as well as EQE measurements of photovoltaic cells consisting of a MDMO-PPV:TiO or a poly(3-octyl thiophene):tio (P3OT:TiO ) with varying polymer:tio ratios.. Experimental Photovoltaic cells were made by spincoating a 75- nm-thick film of electroluminescence-grade poly(3, /04/$ - see front matter 003 Elsevier B.V. All rights reserved. doi: /j.tsf

2 L.H. Slooff et al. / Thin Solid Films (004) Fig. 1. Scanning electron micrographs of the TiO phase of MDMO-PPV:TiO (14 vol.% TiO ) and P3OT:TiO (1 vol.% TiO ) blends. The polymers were removed by a 10 min UV-ozone treatment. ethylene dioxythiophene), doped with poly(styrene sulfonate) (PEDOT:PSS suspension in water, Bayer) on pre-cleaned, patterned indium tin oxide substrates (14 Vysquare, MDT). The PEDOT layer was dried on a hotplate (60 8C) for 5 min. Next, the photoactive layer was deposited by spincoating a blend of polymer:titanium(iv) isopropoxide (Ti(iPrO) 4, Aldrich) in tetrahydrofuran (THF) at 1500 rpm. The spincoat solution was prepared using a THF solution containing 4.5 mgyml MDMO-PPV (Aldrich) and Ti(iPrO) 4 (7 mly ml for a 1:1 (vyv) ratio), or 4.5 mgyml regioregular poly(3-octyl thiophene) (P3OT, Aldrich) and Ti(iPrO) 4 (7 mlyml for a 1:1 (vyv) ratio). For the conversion of Ti(iPrO) 4 into TiO, the samples were kept in air for 4 h and subsequently put into vacuum y6 (10 mbar) for 15 h. Finally, a LiF (1 nm) and aluminium (100 nm) counter electrode were deposited y6 by vacuum evaporation at 10 mbar. Each substrate consists of four cells of 0.1, 0.15, 0.33 and 1.0 cm. Glass substrates were used for UV Vis spectrometry and layer thickness measurements. The reported TiO concentrations are based on the amount of the materials present in the spincoat solution and the density of the pure materials (MDMO-PPV and P3OT 1 mgyml, Ti(iPrO) mgyml and TiO 4. mgyml). Current voltage and spectral response measurements were performed in a glovebox under nitrogen atmosphere using a homebuilt set-up, consisting of a 1 Vy 50 W halogen lamp (set at sun equivalent; 1 sun equivalent is defined as AM1.5 G, 100 mwycm ) and a set of interference filters to create monochromatic light. The spectral response measurements were performed with respect to a calibrated Si solar cell without bias illumination. UV Vis absorption measurements were done using a HP 8453 diode array UV Vis spectrophotometer. The film thickness was determined using a Dektak 8 profiler at a tip force between 1 and.5 mg. Scanning electron microscope images were taken with a JEOL JSM-6330F field emission scanning electron microscope at an accelerating voltage of 5 kv. 3. Results SEM images were taken to investigate if a phase separation on the order of the exciton diffusion length is obtained in blends, prepared using the titanium oxide precursor. Before taking the SEM images the blends were treated for 10 min in a UV-ozon photoreactor to remove the polymer. Fig. 1 shows SEM images of the TiO structure resulting from a P3OT:TiO blend (1 vol.% TiO ) and a MDMO-PPV:TiO blend (14 vol.% TiO ). The TiO phase of the P3OT blend is rather large with a size of nm. The TiO phase of the MDMO-PPV blend shows smaller phases of 0 30 nm. The difference might be due to the tendency of P3OT to aggregate more easily as compared to MDMO-PPV, resulting in larger pore sizes. Changing the volume fraction of TiO with respect to MDMO-PPV or P3OT does not change the phase separation significantly. Fig. (left) shows the absorbance spectrum for a 50- nm-thick P3OT:TiO film and a 55-nm-thick MDMO- PPV:TiO film, both with 14 vol.% TiO. The spectrum of P3OT is slightly red-shifted compared to the spectrum of MDMO-PPV and the peak absorbance is somewhat lower. Fig. 3 shows the peak absorbance as a function of the TiO concentration for both materials. For the P3OT films, the absorbance decreases upon increasing the TiO concentration due to the decrease in polymer concentration. The measured film thickness for these films is almost independent of TiO concentration and is nm. However, the decrease in absorbance for the MDMO-PPV film (1%) is smaller than the expected 44% based on the polymer concentration. This difference can be explained by the measured increase in film thickness (41%).

3 636 L.H. Slooff et al. / Thin Solid Films (004) Fig.. Left: Extinction spectra of polymer:tio blend films on glass. The TiO fraction is roughly 14 vol.%. Film thickness is approxi- mately 55 nm for the MDMO-PPV:TiO blend and 50 nm for the P3OT:TiO blend. Right: EQE vs. wavelength for a P3OT:TiO photovoltaic cell (10 vol.% TiO ) and a MDMO-PPV:TiO pho- tovoltaic cell (0 vol.%). The TiO concentration is chosen for max- imum EQE. Next, photovoltaic cells were characterised. The results are shown in Fig. 4 for both MDMO-PPV:TiO and P3OT:TiO cells as a function of the TiO concentration. The open-circuit voltage (V oc) drops slightly for increasing TiO concentration and is on average 150 mv higher for MDMO-PPV devices (V oc s 550 mv) as compared to P3OT devices (V s400 mv). oc The short-circuit current (I ) of the MDMO-PPV cells sc first increases and then decreases as the TiO concentra- tion increases. The fill factor (FF) also first increases, but then levels off. As a result, the maximum power Fig. 3. Peak extinction at 490 nm as a function of the TiO concentration for MDMO-PPV:TiO and P3OT:TiO blend films. The solid lines are a linear fit through the data. point (MPP) of the MDMO-PPV devices has a maximum of 0.07 mwycm (measured at 0.7 sun equivalent) at a TiO concentration of 18 vol.%. For the P3OT devices, the maximum of the MPP occurs at a TiO concentration of 1 vol.% and is 0.03 mwycm, which is lower as compared to MDMO-PPV devices. It should be mentioned that these results are measured within 1 h after device fabrication. As will be shown below, the I V characteristics of the device changes substantially in time, resulting in maximum power conversion efficiencies that are higher than that based on the initial cell performance. One would expect that due to the larger distance to the polymer:tio interface (Fig. 1), the fraction of excitations that result in charge separation Fig. 4. Results of current voltage measurements on MDMO-PPV:TiO (solid squares) and P3OT:TiO (open triangles) photovoltaic cells as a function of the TiO concentration. Shown are the V, I, FF and MPP. The solid lines are a guide to the eye. oc sc

4 L.H. Slooff et al. / Thin Solid Films (004) Fig. 5. MPP, measured at 0.7 sun equivalent, of a P3OT:TiO photovoltaic cell (10 vol.% TiO ) vs. the time elapsed after preparation of the cell. would be reduced in the P3OT blends compared to the PPV blends. However, the performance of the P3OT and PPV devices is somewhat comparable. This might indicate a larger exciton diffusion length for P3OT compared to PPV, but this has to be studied in more detail. All devices were stored in dark in a glovebox under nitrogen atmosphere. Under these conditions the MDMO-PPV:TiO devices show an initial increase in Isc of 100% followed by a drastic decrease. After days there is hardly any PV response left. The P3OT:TiO devices on the other hand show a completely different behaviour as is shown in Fig. 5. The MPP of a 10 vol.% TiO cell initially increases by a factor of, which is mainly due to an increase in I sc. After roughly one day a maximum in the MPP is observed after which it slowly decreases in time. Although it is known that P3OT is more stable than MDMO-PPV, it is not yet clear why there is such a large difference in the device stability. After the initial increase in performance, the EQE was measured and is shown in Fig. (right) fora10 vol.% P3OT:TiO cell and a 0 vol.% MDMO- PPV:TiO cell. The TiO concentration in these devices was chosen to give the highest power conversion efficiency. The EQE spectra are similar to the absorbance spectra, showing that the polymer is the photoactive material in these cells. The maximum EQE is 0.13 for MDMO-PPV at a wavelength of 474 nm and 0.10 for P3OT at a wavelength of 480 nm. Taking the integral of the overlap of these EQE spectra with the AM1.5 spectrum, normalised at 100 mwycm, the AM1.5 Isc can be calculated, assuming a linear depend- ence of the current on light intensity. This results in a I of 1 maycm for both the P3OT device and the sc MDMO-PPV device and an AM1.5 power conversion efficiency of 0.17 and 0.%, respectively. These efficiencies are still rather low if compared to other polymer photovoltaic cells like e.g. MDMO-PPV:fullerene photovoltaic cells, for which efficiencies up to 3% are reported w4 6x. It should be noted, however, that the conversion of the Ti(iPrO) into TiO occurs at room 4 temperature. It is thus expected that only amorphous TiO is formed. This will hamper the electron transport in the film and as a result, the currents remain rather low. Further experiments have to show that increased crystallinity of the TiO phase should lead to better transport properties and higher performances. 4. Conclusions A new, simple procedure is reported for preparing hybrid TiO :polymer photovoltaic cells, in which a continuous interpenetrating network of TiO is created inside a thin conjugated polymer film. I V measurements on MDMO-PPV:TiO show an increase in Isc with increasing TiO concentration up to a TiO concentration of 0 vol.%, after which the current decreases again. Cells with a of P3OT and TiO show a maximum at a TiO concentration of 10 vol.%. The performance of the devices changes substantially in time when stored under nitrogen atmosphere. Both MDMO-PPV and P3OT devices show an initial increase in efficiency of a factor, which is mainly due to an increase in short-circuit current. However, after 1 days the MDMO-PPV devices do not show a photovoltaic effect anymore, whereas the P3OT devices show only a small decrease in MPP in time (;0% of the maximum MPP over 3 weeks). On the basis of EQE measurements at the maximum performance, the maximum AM1.5 power conversion efficiency is calculated to be 0.% for the PPV:TiO photovoltaic cell (0 vol.% TiO ) and 0.17% for the P3OT:TiO photovoltaic cell (10 vol.% TiO ). Acknowledgments This work was done with financial support of the Dutch Polymer Institute (DPI). Marijke Roos (ECN) is acknowledged for taking the SEM images. References w1x T.J. Savenije, J.M. Warman, A. Goossens, Chem. Phys. Lett. 87 (1998) 148. wx J.J.M. Halls, K. Pichler, R.H. Friend, S.C. Moratti, A.B. Holmes, Appl. Phys. Lett. 68 (1996) 310.

5 638 L.H. Slooff et al. / Thin Solid Films (004) w3x P.A. van Hal, M.M. Wienk, J.M. Kroon, W.J.H. Verhees, L.H. Slooff, W.J.H. van Gennip, P. Jonkheijm, R.A.J. Jansen, Adv. Mat. 15 (003) 118. w4x S.E. Shaheen, C.J. Brabec, F. Padinger, P. Fromherz, J.C. Hummelen, N.S. Sariciftci, Appl. Phys. Lett. 78 (001) 841. w5x C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mat. 11 (001) 15. w6x J.M. Kroon, M.M. Wienk, W.J.H. Verhees, J.C. Hummelen, Thin Solid Films 403 (00) 3.