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In Situ Study of Degradation in P3HT-titania Based Solid-state Dye-sensitized Solar Cells Lin Song, Weijia Wang, Stephan Pröller, Daniel Moseguí González, Johannes Schlipf, Christoph J. Schaffer, Kristina Peters, Eva M. Herzig, Sigrid Bernstorff, Thomas Bein, Dina Fattakhova-Rohlfing, and Peter Muller-Buschbaum ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00117 • Publication Date (Web): 06 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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ACS Energy Letters

In Situ Study of Degradation in P3HT-Titania Based Solid-State Dye-Sensitized Solar Cells Lin Song,a Weijia Wang,a Stephan Pröller,b Daniel Moseguí González,a Johannes Schlipf,a Christoph J. Schaffer,a Kristina Peters,c Eva M. Herzig,b Sigrid Bernstorff,d Thomas Bein,c Dina Fattakhova-Rohlfingc and Peter Müller-Buschbaum*,a a

Lehrstuhl für funktionelle Materialien, Physik Department, Technische Universität München.

James-Franck-Str. 1, 85747 Garching, Germany. E-mail: [email protected] b

Herzig Group, Munich School of Engineering, Technische Universität München.

Lichtenbergstr. 4, 85748 Garching, Germany. c

Department of Chemistry and Center for NanoScience (CeNS), Ludwig-Maximilians-

Universität München (LMU). Butenandtstr. 5-13 (E), 81377 Munich, Germany. d

Elettra-Sincrotrone Trieste S.C.p.A.,Strada Statale 14 - km 163.5 in AREA Science Park,

Basovizza, 34149 Trieste, Italy. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

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ABSTRACT The degradation of P3HT-titania based solid-state dye-sensitized solar cells (ssDSSCs) is studied to better understand device aging mechanisms. The correlation of temporal evolution between P3HT crystallite structures and device performance is discussed for the first time using in situ measurements. For comparison, two types of mesoporous titania photoanodes with different pore sizes are prepared. Grazing incidence wide-angle x-ray scattering (GIWAXS) is used in situ under continuous solar illumination to obtain information about the impact of pore size on P3HT crystalline order and on temporal evolution of the P3HT crystallites. The development of the photovoltaic characteristics is explored in parallel. The lattice constants, crystal sizes and volume fraction of crystalline P3HT in the large-pore active layer remain stable over 30 minutes while the volume fraction of crystalline P3HT decreases in the small-pore active layer. Thus, the pore size of titania photoanodes is important for the stability of P3HT-titania based ssDSSCs.

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Mesoscopic dye-sensitized solar cells (DSSCs) have received great attention as potential alternative to conventional silicon photovoltaic devices due to low cost, simple manufacturing processes and high efficiencies of over 12%.1-8 Although DSSCs have achieved commercial breakthrough, concerns over liquid-electrolyte leakage and electrode corrosion hinder their market expansion.9-11 Thus, solid-state inorganic hole-transport materials (HTMs), molecular HTMs, and/or polymeric HTMs have been exploited to replace the liquid electrolyte.12-16 Poly(3hexylthiophene) (P3HT), amongst others, has been widely studied as an HTM in solid-state DSSCs (ssDSSCs) due to its high hole mobility and good solubility in various organic solvents.17-20 Moreover, it has a prominent light-absorbing ability in the visible-light region. To date, considerable advances have been made in ssDSSCs based on mesoporous titania (TiO2) films and P3HT. For example, cells with ruthenium complex, metal-free organic dye and quantum-dot sensitizer have been reported to reach power-conversion efficiencies (PCE) of 2.7%, 3.2% and 5.2%, respectively.21-23 Nevertheless, the stability of these devices is still unverified and aging mechanisms are not completely understood. The present work focuses on the stability and aging of P3HT-titania based ssDSSCs with an organic dye 5-[[4-[4-(2,2-Diphenylethenyl)phenyl]-1,2,3-3a,4,8b-hexahydrocyclopent[b] indol7-yl]methylene]-2-(3-ethyl-4-oxo-2-thioxo-5-thiazolidinylidene)-4-oxo-3-thiazolidineacetic acid (D149). Although this dye provides lower efficiencies as compared to the best sensitizers, its important advantages are low costs, non-toxicity and being well established. Thus, TiO2-D149P3HT based solid-state DSSCs can be understood as a well-established model system. We compare two types of titania photoanodes with different mesopore sizes as a response to many research groups which have reported that the pore size greatly affected the device performance.24,25 Since its impact on aging is still unclear, we focus on aging in the present


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study. The dye-modified titania films are subsequently backfilled with P3HT. Since the P3HT crystallinity has a great impact on the photovoltaic device performance,26-29 we track its crystalline evolution in situ under continuous solar illumination. The development of the photovoltaic characteristics is probed in parallel to the crystalline structure evolution. A kinetic study on the P3HT crystal evolution in hybrid solar cells being illuminated is still missing, especially inside titania mesopores of ssDSSCs. From our study such correlation between P3HT crystalline structure and ssDSSCs device performance is obtained for the first time. We prepare two ordered mesoporous titania films with different pore size, which relies on templating by the diblock copolymer poly(styrene-block-ethylene oxide) (PS-b-PEO) in combination with sol-gel chemistry.18,30-32 Figure 1a and 1b illustrate plan-view scanning electron microscopy (SEM) images of the mesoporous titania films after 500 °C calcination. Both films present an interconnected titania network out of ordered mesopore arrays, however the pore sizes differ. Before calcination, the two samples have the same polymer-titania weight ratio (1:1) but the molecular weight of the polymer templates differs because the molecular weight can be used for tuning pore sizes in polymer assisted sol-gel synthesis.33 The used low molecular weight template polymer leads to small pores with a size of about 12 nm (Figure 1a), whereas a high molecular weight template gives rise to large pores with 40 nm average size (Figure 1b). The samples with small and large mesopore arrays are denoted as small-pore and large-pore, respectively. The large-pore film possesses a remarkably thicker titania framework of around 13 nm compared to the titania wall of about 8 nm in the small-pore sample. This difference is due to the longer PEO chains holding more titania during the sol-gel synthesis as the titania species are bound with PEO domains through covalent bounds.30,34 The insets in Figure 1a and 1b are the Fast Fourier Transform (FFT) patterns of the corresponding SEM


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images. The bright rings demonstrate that the pores distribute isotropically over the surface with a narrow distribution of center-to-center distance in both films. To obtain quantitative information, the power spectral density (PSD) functions are computed by azimuthal integration of the FFT pattern as shown in Figure S1, Supporting Information (SI). The corresponding center-to-center distances are calculated to be around 23.0 nm and 50.0 nm via 2π/q for the samples shown in Figure 1a and b, respectively.

Figure 1. Surface and inner morphologies of mesoporous titania films. SEM plan-views of the a) small-pore and b) large-pore samples after calcination at 500 °C. The insets display the corresponding FFT patterns. c) Horizontal line cuts obtained from 2D GISAXS data. The black and red curves represent the calcined small-pore and large-pore samples, respectively. The grey lines represent the fits to the cuts. The curves are shifted along the intensity axis for clarity of the presentation. Extracted characteristic length scales: d) structure radii and e) the corresponding center-to-center distance of both samples. Green hollow triangles indicate small-sized structure and blue hollow circles indicate large-sized structure. The black crosses in e) are the center-tocenter distances calculated from SEM images.


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Since SEM only provides information about the film surface, grazing-incidence small-angle xray scattering (GISAXS) measurements are performed to gain better insight on the inner film morphology. GISAXS gives structure information with a high statistical relevance as it averages over a macroscopic sample area, allowing tracking of domain sizes and corresponding arrangement in the nano- and meso-scale.35-37 The inner morphology is significant for the solar cell performance as the electron-hole pairs are separated at the titania/dye/HTM interface.38-40 The 2D GISAXS data recorded for both samples and a detailed feature description can be found in Figure S2, SI. For quantitative analysis, horizontal line cuts along qy direction are performed at the titania Yoneda peak position. This peak is located at the critical angle of titania, which is characteristic of the material.41 The horizontal line cuts allow for obtaining lateral structural information of the probed titania samples, like structure sizes and the corresponding center-tocenter distances. The horizontal line cuts of the small-pore and large-pore films are displayed in Figure 1c. The sharp peaks in both curves imply the ordered inner arrangements for both samples. However, the position of these two peaks varies, which is caused by the different predominant structural length scale in the volume of the films. Moreover, weak higher-order peaks are observed (indicated by arrows), suggesting some long-range lateral ordering of the mesopores inside both films. To extract quantitative structural information, the data are fitted in the framework of the distorted wave Born approximation (DWBA) using the local monodisperse approximation (LMA).42,43 Data modeling is described in detail in the Supporting Information. Within this model, form factors with cylindrical shape distributed on a 1D paracrystal are used.4446

From data modeling, two structure sizes (form factors) and two center-to-center distances

(structure factors) are found in both films and displayed in Figure 1d and 1e. Characteristic


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structure radii of (3.9 ± 0.1) nm and (4.7 ± 0.1) nm and their corresponding center-to-center distances of (23.5 ± 0.2) nm and (22.1 ± 0.3) nm are observed in the small-pore film, and the pore sizes are calculated to be (15.7 ± 0.2) nm and (12.7 ± 0.3) nm following the model introduced by Sarkar et. al.46 For the large-pore sample, lateral structure radii of (4.9 ± 0.1) nm and (7.2 ± 0.1) nm with center-to-center distances of (50.0 ± 0.5) nm and (54.0 ± 3.1) nm are obtained, which give rise to the mesopore sizes of (40.2 ± 0.5) nm and (39.6 ± 3.1) nm. Thus, the center-to-center distances of both films obtained from the GISAXS evaluation are in good agreement with the values extracted from the PSD analysis of the SEM data, suggesting that the surface morphology is reproduced within the bulk of the sample. High-temperature calcination gives rise to the mesoporous titania films, and transforms the amorphous titania to the crystalline anatase phase, which is verified by x-ray diffraction (XRD) measurements. The XRD pattern and a detailed feature description are depicted in in Figure S3, SI. Both mesoporous titania films are then backfilled with P3HT after dye (D149) loading. Cross section SEM measurements qualitatively reveal a rather good infiltration for both small-pore and large-pore samples (Figure S4, SI). In situ grazing-incidence wide-angle x-ray scattering (GIWAXS) measurements are performed on these active layers under ambient working conditions to track the evolution of P3HT crystalline order under continuous light illumination. Besides the kinetic study, it is of interest, whether different titania pore sizes affect the key P3HT crystalline properties like lattice distances or crystal sizes. In ssDSSCs based on P3HT the crystalline evolution is expected to be closely related to the performance of the solar cells since P3HT, the medium for the positive charge carrier transport, is limiting the device performance. The 2D GIWAXS data of both active layers at the initial state are illustrated in in Figure S6, SI. A number of appropriate corrections, including solid-angle correction, q-reshaping and


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conversion, efficiency correction and polarization correction, are applied to the raw 2D GIWAXS data in order to retrieve the corrected reciprocal space patterns. These corrections are discussed in detail elsewhere.47 The GIWAXS data display a predominantly edge-on orientation of the P3HT crystals throughout the entire 30 min of solar illumination as the out-of-plane (100) peaks are the dominant scattering signals for both samples. The (100) peak acts as a good indicator of the polymer crystallization and is chosen to parametrize the evolution of the P3HT crystallites. Radially-integrated cuts along qz at lowest accessible qr are performed on the corrected in situ GIWAXS data. The cuts are shown for both samples at their initial state in in Figure S7, SI. For analysis all cuts are fitted with Gaussian functions. The amplitude, mean qposition and crystal size (obtained via the full-width-at-half-maximum (FWHM) and the Scherrer equation under the assumption of constant paracrystallinity) of the (100) peak as a function of illumination time are extracted to understand the P3HT crystallite evolution inside the pores caused by the illumination (see Figure 2).


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Figure 2. Temporal evolution of parameters obtained from fits of the (100) P3HT peaks as measured with GIWAXS under continuous solar illumination. Black hollow circles represent the small-pore active layer, red solid circles and orange solid rectangles represent the first and second (100) peaks of the large-pore active layer, respectively. a) Peak center position; b) crystal size; c) normalized peak intensity. Small-pore intensities are normalized to the initial peak intensity for the small-pore active layer, while all large-pore intensities are normalized to the initial intensity of the first (100) peak for the large-pore active layer.

It has to be noted that the P3HT capping layer is negligible in this work as it is very thin with a thickness of a few nanometers, as shown in in Figure S5, SI. For the small-pore active layer, the P3HT (100) peak shows the strongest intensity at q ≈ 3.84 nm-1, and its (010) peak overlaps with the titania (101) peak at q ≈ 17.78 nm-1 (Figure S7, SI). The (100) peak position remains constant and the peak width is unchanged as well, whereas the (100) peak intensity decreases gradually over time. A constant (100) peak position suggests that the distance between adjacent P3HT backbones is unchanged, and remains at around 1.64 nm as it can be calculated via the peak position, by d=2π/q. This value is in good agreement with the observations in P3HT:[6,6]phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend systems, meaning that the small pores do not have an influence on the crystallization. 26,48 The average crystal size is about 9.4 nm and does not change during illumination as the peak width stays constant within the error bars (Figure 2b). However, the (100) peak intensity decreases with increasing illumination time, indicating a decrease of the volume fraction of crystalline regions in the sample. For the largepore active layer, two (100) peaks are observed at q ≈ 3.84 nm-1 (denoted as the first (100) peak) and q ≈ 4.05 nm-1 (denoted as the second (100) peak), respectively (Figure S7, SI), suggesting that two types of P3HT crystals exist inside the large mesopores. The occurrence of P3HT polymorphism, in the condition of different solvent evaporation rates, was observed by Yuan et al. as well.49 All extracted parameters remain almost unchanged for both (100) peaks during


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illumination. For the first (100) peak, the peak position is constant over time at q ≈ 3.84 nm-1 and the crystal size is about 10.0 nm, which are similar to the counterpart in the small-pore active layer. The peak intensity is also not changing, indicating the volume fraction of crystalline regions does not reduce over time and also ruling out potential radiation damage in our study. For the second (100) peak the peak position is at a larger value of q ≈ 4.05 nm-1, indicating a denser polymer backbone spacing of 1.55 nm (1.63 nm for the first (100) peak). The average crystal size is larger with an average size of 38.3 nm, which is almost the same size as the pore size of the large-pore titania film, indicating that a single P3HT crystallite nearly fills one pore completely. Moreover, the size of these P3HT crystallites is much bigger than that reported commonly in literature for P3HT (about 10 nm).26,29,50 Putting all information together, we infer that the backfilled P3HT crystalline order and stability are strongly influenced by the scaffold pore size. The P3HT inside the large titania pores is more stable than inside the small pores as no intensity loss of the P3HT (100) peaks is observed in the large-pore active layer due to illumination. Michell et al. concluded that the crystallization temperature (Tc) of the infiltrated polymer decreases with pore size of the scaffold.51 Thus, it is believed that the Tc of P3HT is higher in the large-pore active layer than in the small-pore active layer. Moreover, Keller reported that the polymer crystal stability lowers with decreasing size.52 The lower and smaller size of P3HT crystals result in a worse stability of the crystalline regions in the small-pore active layer than in the large-pore active layer, which manifested in decreased versus stable (100) peak intensities in the small-pore versus large-pore active layers, respectively. Additionally, P3HT crystals with a denser packing of the P3HT chains only exist in the large-pore active layer, but their volume fraction is much lower than for the first (100) peak as deduced from the comparison of the respective intensities shown in Figure 2c.


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The two active layers discussed above are individually sandwiched between fluorine-doped tin oxide (FTO) and gold electrodes to assemble ssDSSCs, and then the cells are tested under simulated AM1.5 G illumination. The small-pore cells are examined to have a PCE of (2.09 ± 0.16) %, whereas the large-pore cells perform better with a PCE of (2.64 ±.0.21) %. For both types of solar cells, the champion devices are used for the characterization of the performance degradation. During 30 min illumination, current-voltage (J-V) sweeps are recorded continuously over time. J-V curves of the small-pore and large-pore cells with the best performance are exemplarily shown in Figure 3a. An open-circuit voltage (Voc) of 0.74 V, a short-circuit current density (Jsc) of 6.72 mA cm-2 and a fill factor (FF) of 47% result in a PCE of 2.32% for the small-pore cell, while the performance of the large-pore cell is slightly better with a Voc of 0.71 V, a Jsc of 7.31 mA cm-2, a FF of 58% and a PCE of 2.95%. As compared with DSSCs using liquid electrolytes these PCE values are low, however, for TiO2-D149-P3HT based solid-state DSSCs they rank among the top values. The enhancement of the Jsc and FF in the large-pore cell may be due to the presence of large P3HT crystals (Figure 2b) as larger crystals yield a higher hole-mobility which reduces geminate-pair recombination.28 The temporal evolution of Voc and Jsc for both devices under ambient working conditions is shown in Figures 3b and 3c, respectively. All the parameters are normalized to their individual initial values. Since dye molecules are the dominant absorber material in the system, the light absorption properties are examined over time in accordance with many studies which argue that dyes degrade under solar illumination.53-55 The UV/Vis absorption spectra of the dye-adsorbed small-pore and large-pore titania samples under different illuminating times are shown in Figure S8, SI. The integrated absorbances are normalized to their initial values and depicted in Figure 3b and 3c, respectively. For both cells, the Voc reduces gradually over time. The decrease in dye absorbance is in good


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agreement with the Voc decrease. Thus, the Voc decay is mainly ascribed to the dye degradation in the first 30 minutes. The Jsc of both samples increases during the initial dozens of seconds and then decreases under continuous solar illumination. This initial growth of Jsc is caused by the light soaking effect.56 However, the Jsc later drops faster in the small-pore cells than in the largepore cells. The dye degradation also induces the Jsc decay as the similar tendency of dye and Jsc over time are observed in the large-pore cells (Figure 3c). For the small-pore cells, besides the contribution of dye degradation, the decrease of the P3HT crystallinity and the absence of large P3HT crystals result in a stronger Jsc decay. The mobility of positive charge carriers in the P3HT crystalline regions is several orders of magnitude higher than in the amorphous regions.29,57 Thus, the decrease of the volume fraction of crystalline regions in the small-pore active layer results in a decrease of the mobility of positive charge carriers and causes thereby a decrease of Jsc. The PCE reduction is attributed to the decrease of Voc, Jsc and FF for both photovoltaic devices as shown in Figure S9, SI.

Figure 3. Photovoltaic performance of P3HT-titania based ssDSSCs. a) J-V curves of the smallpore (black) and large-pore (red) cells with the best performance. Temporal evolution of Voc (brown), Jsc (blue) for b) the small-pore and c) large-pore cells, respectively. The hollow pink triangles in b) and c) represent the temporal evolution of integrated absorbance for the dyeadsorbed small-pore and large-pore titania films, respectively. All the parameters are normalized to their initial values.


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In conclusion, we present an in situ degradation study for ssDSSCs with different pore-sized titania photoanodes. The P3HT crystalline behavior is closely related to titania mesopore size. Two types of P3HT crystals with different lattice distances in the (100) direction are observed in the large-pore active layer, while such feature is not found in the small-pore-sized sample. Moreover, the lattice constants, crystal sizes and volume fraction of crystalline P3HT in the large-pore active layer remain stable over 30 minutes under continuous solar illumination while the volume fraction of crystalline P3HT decreases in the small-pore active layer. The Voc loss of both cells is caused by degradation of the organic dye molecules. The Jsc decays faster in the small-pore cells than in the large-pore cells, due to the combined effect of the dye degradation, the decreased P3HT crystallinity and the absence of large P3HT crystals. Thus, the pore size of the titania films strongly impacts the performance and stability of the P3HT-titania based solidstate DSSCs. A stable dye and a titania film with large pores appear to be helpful to stabilize the Voc and Jsc of P3HT-titania based ssDSSCs. ASSOCIATED CONTENT Supporting Information. Experimental details, detailed description of 2D GISAXS and GIWAXS data, GISAXS data modeling, XRD data, SEM images of P3HT infiltration and P3HT capping layer, temporal evolution of absorption spectra obtained from dye-modified titania films and temporal evolution of FF and PCE obtained from P3HT-titania based ssDSSCs can be found in the Supporting Information file. Notes The authors declare no competing financial interest. Acknowledgments


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This work was supported by funding from TUM.solar in the context of the Bavarian Collaborative Research Project “Solar Technologies Go Hybrid" (SolTech), the Excellence Cluster “Nanosystems Initiative Munich” (NIM), the Center for NanoScience (CeNS) and the International Research Training Groups 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS). L.S. and W.W. acknowledge the China Scholarship Council (CSC) and E.M.H. the Energy Valley Bavaria (EVB) of the Munich School of Engineering (MSE). The authors thank Prof. Alexander Holleitner and Peter Weiser for providing access to the SEM.

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