Nanoscale PAPER - Emmanuel Kymakis

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50 nm) can be used as effective subwavelength scattering elements that couple and trap freely propagating plane waves of the sunlight into the photoactive layer. Owing to enhanced absorption and scattering effects caused by the incorporation of NPs, improved exciton generation and, in turn, higher device efficiency are anticipated. At the same time, the dispersion of NPs into the photoactive layer leads to an improvement in the structural stability as well, giving rise to a slower device degradation rate upon prolonged illumination. This work sheds light on the mechanism behind such stability enhancement by in situ time-resolved EDXR, photoluminescence and Raman spectroscopy as well as device degradation electrical measurements. In particular, the effect of NPs incorporation on the aging performance of BHJ OPV devices is investigated by correlating the morphological characteristics of pristine and NP doped active layers with the photovoltaic properties of the respective OPV devices. It is evidenced that, in addition to further stabilization of the polymer–fullerene blend, the observed improvement can be ascribed to a NPmediated mitigation of the photooxidation effect at the cathode– active layer interface.

2. Experimental 2.1 Au NPs generation The generation of NPs was performed by ultrafast laser ablation of metallic targets (Au/99.98%). This technique provides the possibility of generating a large variety of NPs that are free of both surface-active substances and counter-ions.31,32 The targets were placed into a Pyrex cell and covered by a layer of absolute ethanol. A femtosecond (100 fs @ 1 kHz) laser beam was focused onto the target through the ethanol layer. The cell was mounted on a computer-driven X–Y stage and translated during laser exposure. More experimental details can be found elsewhere.33 Laser irradiation gives rise to a high temperature gradient in the metal bulk and melts a thin layer of the target. A fraction of the molten layer of the target is dispersed into the liquid as NPs. The corresponding absorption spectra were measured using a Perkin-Elmer UV-VIS spectrophotometer. The absorption spectrum of the colloidal Au NPs solution in ethanol exhibits a distinct peak at about 530 nm, close to the theoretically This journal is ª The Royal Society of Chemistry 2012

predicted enhanced absorption due to plasmon resonance (see Fig. S1 in the ESI†). The analysis of a series of TEM images (inset of ESI†) indicates that the NPs exhibit sizes in the 1.5 to 20 nm range, with an average size of 10 nm. 2.2 Device fabrication OPV devices based on the ITO/PEDOT:PSS/P3HT:PCBM/Al structure with Au NPs embedded in the active layer at different concentrations were fabricated and characterized (device areas were 15 mm2). The active layers were fabricated by mixing respective solutions of P3HT and PCBM at a 1 : 1 ratio and Au NPs, in dichlorobenzene and spin-coated on PEDOT:PSS/ITO. The optimum volume ratio of the P3HT:PCBM and the Au NP solutions was 5%.30 Post-fabrication annealing was avoided for the device characterization by time-resolved EDXR, Raman spectroscopy and aging, in order to account for the case of flexible OPVs as well. Furthermore, for the photovoltaic performance evaluation, OPV devices were post-annealed at 140 C for 5 min in a glove box under nitrogen atmosphere. 2.3 Device parameters Current–voltage (I–V) measurements were performed at room temperature using an Agilent B1500A semiconductor device analyzer in air. For photovoltaic characterization the devices were illuminated with 100 mW cm2 power intensity of white light by an Oriel solar simulator with an AM1.5 filter through the glass/ITO side. Experiments were performed on un-encapsulated devices in order to monitor the photovoltaic performance under ambient conditions. 2.4 EDXR/AFM apparatus and procedures The morphological interface properties of P3HT:PCBM films incorporating Au NPs were studied by EDXR performed, in situ, during light exposure and correlated with the degradation of the device efficiency over time. In order to monitor the stability under real conditions, experiments were performed on devices exposed to ambient air. The X-ray reflectivity technique is a powerful tool for studying the surface and interface properties of layered samples.34 In the small angle approximation and far from the materials absorption edges, the reflected intensity can be expressed as a function of the scattering parameter q only. Since q depends on both the reflection angle and the energy of the X-rays, reflectivity measurement can be carried out, in the energy dispersive mode, using a polychromatic beam and performing an energy scan by means of an energy sensitive detector. Upon fitting the reflectivity pattern data, the film morphological parameters (thickness and roughness) can be obtained. Here, the use of EDXR, in situ, minimizes systematic errors and allows the time evolution of the film morphology to be followed with extreme accuracy.35 The EDXR setup is a non-commercial apparatus31 equipped with a tungsten anode X-ray tube and an energy sensitive detector (a EG&G high purity germanium solid-state detector, with 1.5–2% energy resolution in the 15–50 keV energy range used). The Bremsstrahlung spectrum of the X-ray tube is used as the X-ray probe and measurements are performed under static Nanoscale, 2012, 4, 7452–7459 | 7453

conditions, neither a monochromator nor a goniometer being needed. AFM measurements were performed in non-contact mode using a non-commercial air-operating atomic force microscope, placed in the reflectometer optical centre, actually playing the role of a sample holder for the X-ray measurements.16 2.5 Photoluminescence experiments For the PL experiments the devices were placed into a vacuum chamber with optical access. For sample excitation a He–Cd CW laser operating at a wavelength of 325 nm with 35 mW power is used. The PL spectra were measured at room temperature and resolved by using a UV grating and a sensitive, calibrated liquid nitrogen cooled CCD camera. For the PL decay experiments the sample was initially (t < 0 s) illuminated (AM1.5 solar irradiation, 100 mW cm2) under vacuum. For t > 0 s the sample was concurrently exposed to light and air. This led to a continuing decay of the PL that is monitored at certain time intervals. 2.6 Raman apparatus and procedures The Raman spectra of the blends of the pristine and NP-doped OPV devices were measured using a micro-Raman spectrometer (NICOLET ALMEGA XR). A 473 nm laser was used as the excitation source. For each sample, the Raman spectra in the initial as well as degraded state (7 h 30 min under a simulated AM1.5 solar irradiation of 100 mW cm2) were monitored. The spectra from the same area of each sample were monitored in all cases.

3. Results and discussion 3.1 PV properties Fig. 1A displays the illuminated current–voltage (J–V) characteristics of the pristine and OPV devices with 5% NPs concentration. The respective averaged photovoltaic characteristics are summarized in Table 1. It is shown that the incorporation of Au NPs in the active layer induces a significant improvement of both the device short-circuit current (Jsc), by 19%, and the fill factor (FF), by 19%, whereas the open-circuit voltage (Voc) remains constant. As a result, a 42% increase in the device efficiency (h) is obtained for the device with the optimum Au NPs concentration of 5%. The use of surfactant-free NPs appears to be an efficient way to suppress exciton quenching via elimination of recombination pathways taking place on the capping layer of chemically synthesized NPs. Besides this, the laser production method employed gives rise to a rather broad NPs distribution, so that small-sized NPs will contribute to LSPR, while large-sized ones to multiple scattering effects. More details can be found elsewhere.31 As shown in Fig. 1B, the incident photon-to-electron conversion efficiency (IPCE) increases remarkably upon the incorporation of Au NPs, which complies with the enhanced Jsc observed. The IPCE enhancement due to LSPR effects can be firstly attributed to the local enhancement of the incident electromagnetic irradiation field in the vicinity of the small-sized NPs and secondly to multiple scattering by the larger sized NPs. Besides this, it was 7454 | Nanoscale, 2012, 4, 7452–7459

Fig. 1 (A) Current–voltage (J–V) characteristics: comparison of pristine and doped (with 5% NPs concentration) OPV devices. The respective averaged photovoltaic characteristics are summarized in Table 1. (B) Photon-to-electron conversion efficiency (IPCE): comparison of pristine and doped OPV devices (with 5% NPs concentration). A sketch of the cell is shown in the inset.

Table 1 Averaged photovoltaic characteristics

Pristine 5% Au

Jsc (mA cm2)

Voc (V)

FF (%)

h (%)

8.27 9.86

0.6 0.6

53.30 63.50

2.64 3.76

recently demonstrated that the incorporation of NPs in the photoactive layer does not only lead to an increase in the performance but also gives rise to enhanced structural stability of the blend and in turn to slower degradation effects.30 Therefore, the performance enhancement can also be attributed to improvement in the photoactive layer morphology due to the presence of NPs. In the following, the effect of NPs incorporation on the device aging is investigated. It is reported that an important degradation pathway is related to the morphological changes at the interfaces among the various layers composing the device.36 Such changes are crucial and here are probed in situ, under prolonged illumination, by means of timeresolved EDXR and directly correlated with electrical degradation measurements. This journal is ª The Royal Society of Chemistry 2012

3.2 In situ time resolved EDXR studies An OPV device is a multi-layered system (inset of Fig. 1) and in order to identify the separate contribution of each layer to the overall X-ray reflection signal, a specific experimental procedure was followed.37 In particular, devices were characterized by ex situ EDXR after each stage of their fabrication. The corresponding patterns are presented in Fig. S2, ESI.† These preparatory measurements allowed the determination, with high accuracy, of the total reflection edge of each film as well as the scattering length densities of the different layers. Regarding the experiments performed on the complete device, the signal coming from the Al cathode is maximized with respect to that originated from the other layers, in particular from ITO. For this purpose, the sample was slightly tilted with respect to the X-ray beam using a rotating cradle. As a result, the Kiessig fringes at higher scattering vectors of the red curve shown in Fig. S2, ESI,† were attributed to the Al cathode, while the period of oscillations is related to its thickness.38 The fit of these data, according to the Parratt model,39 allows the thickness and roughness of the Al layer to be determined. In the following, a comparative in situ study, based on collection of sequences of X-ray reflection patterns for both the pristine and the Au doped devices, is described. The pristine device (structure: glass/ITO/PEDOT:PSS/P3HT:PCBM/Al) was initially studied in order to detect any modifications in the morphology of the Al cathode layer induced by continuous illumination of the device with a white light lamp (10 mW cm2) for about 20 hours. The results are shown in Fig. 2. As a consequence of light exposure, an increase of the Al layer thickness, d, is observed, while its time dependence is well fitted by a two-step Boltzmann growth (blue and red lines). The overall increase in d is about 4.5 nm, starting from the initial value of 159.5 0.3 nm. Furthermore the cathode layer roughness (joint interface and surface contribution), s, remains unchanged (s ¼ 1.3 0.1 nm). These results are in accordance with interface aging processes observed in similar OPV systems and associated with the formation of an aluminium oxide layer at the cathode– organic layer buried interface.27,39,40 The formation of such oxide layer is attributed to photo-oxidation reactions within the organic film. Indeed, during the cathode preparation by thermal

Fig. 2 Results of data fit (Al-electrode film roughness s vs. time and film thickness d vs. time) obtained as a result of the EDXR experiment performed upon illumination on the pristine PV cell.

This journal is ª The Royal Society of Chemistry 2012

evaporation, Al atoms with high kinetic energy flux can diffuse into the photoactive film. Furthermore, during device operation, oxygen and water molecules present in air may well diffuse into the BHJ and react with Al clusters and polymeric molecules, generating an insulating layer of Al2O3 and degraded polymer molecules.41 Indeed, high-resolution TEM imaging42 revealed a 3–5 nm thick amorphous Al2O3 interfacial layer between the P3HT:PCBM active layer and the Al cathode. Device encapsulation is only partially useful to address the problem of photo-oxidation, since some oxygen is always present in the device (i.e. in ITO or residual from PEDOT:PSS layer deposition) and is not compatible to low-cost deposition processes including ‘‘roll-to-roll’’. Therefore, different approaches must be adopted. For example, buffer layers, acting as electron transport layers, can be interposed between the active film and the metal electrode.43,44 In particular, a LiF barrier film, permeable to electrons, may be used to avoid the direct contact of the active layer with the reactive electrode metal, similarly to what reported for organic light-emitting devices.43 Indeed, it was reported that the use of buffer layers leads to the suppression of photo-oxidation due to the absence of diffusion of Al atoms into the polymer layer, confirmed by XPS45 and X-ray reflectivity analysis.20,27,28 Nevertheless, considering that LiF films exhibit insulating properties, they must be kept extremely thin to enable electrons transport to the electrode, which may compromise their barrier role. Therefore, the identification of alternate approaches to stabilize the cathode interface properties would have a positive impact on the development of OPV devices. For this purpose, we focused on the effect of Au NPs incorporation on the interfacial properties and a second in situ EDXR experiment was performed on the Au doped device (structure: glass/ITO/PEDOT:PSS/ P3HT:PCBM:Au NPs/Al). The corresponding series of EDXR patterns collected, under illumination with the same white light lamp (10 mW cm2), is presented in Fig. 3. The corresponding

Fig. 3 Series of EDXR patterns collected performed upon illumination on the Au doped PV cell. The inset reports the cathode thickness and roughness values determined from the data analysis of each pattern.

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evolution of cathode thickness and roughness values determined from the data analysis are plotted in the inset of Fig. 3. It is observed that, in contrast to the case of the NP-free device, both d and s remain practically unchanged during light exposure. Our results indicate that the morphology of the cathode layer does not undergo any modification during device operation, in contrast to the pristine, NP-free device. This may be attributed to a NPs-mediated suppression of photo-oxidation similar to the result obtained by the deposition of a buffer layer. Photo-oxidation of the electrode buried interface is attributed to the diffusion of molecular oxygen into the active layer. In the presence of Au NPs this process may be inhibited due to their catalytic activity.46 Indeed, as reported in theoretical47,48 and experimental studies,49,50 oxygen is preferentially adsorbed by gold NPs and clusters of them. Alternatively, the presence of NPs may hinder the process behind photoinduced oxidation of the conjugated polymer in the blend.51,52 As presented in Scheme 1, during photo-oxidation of semiconducting polymers, singlet oxygen, formed via energy transfer from the polymer triplet exciton, reacts with the polymer to generate exciton traps. Such traps are topological defects comprised of carbonyl groups formed on the ends of polymer chains and provide an additional nonradiative channel for the polymer singlet excitons. As a result, quenching of polymer luminescence is induced. In our case NPs may play the role of a stabilizer that blocks the action of oxygen. In particular, the triplet excitons may be quenched as a result of the overlap of their energy levels with the plasmon resonance of the embedded NPs, as well as increasing the polymer–NPs contact area.48 Indeed, AFM analysis, reported in Fig. 4, revealed that the incorporation of Au NPs produced a 100% increase in the BHJ surface roughness (4.0 0.2 nm vs. 2.0 0.2 nm). The increased roughness of the NP-doped blend surface, onto which the Al electrode is deposited, results in a large interface between the NPs and the active layer, in particular in proximity of the electrode. To further clarify the role of NPs doping, PL decay measurements, which are a proper mean to quantify singlet quenching in P3HT under oxygen exposure, were performed.

Scheme 1 Schematic of the photo-oxidation process in the polymer: Au NPs active layer. Energy from the polymer triplet excitons excites singlet oxygen, which reacts with the polymer chains to form exciton trap states. The Au NPs embedded into the blend act as quenchers of the triplet excitons and in this way the photooxidation process can be impeded. (1) Absorption; (2) luminescence; (3) system intercrossing; (4) triplet state; (5a and 5b) triplet quenching; (6a and 6b) exciton recombination via trap states. This process is limited in the presence of a triplet quencher.

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Fig. 4 Results of the comparative AFM study of the pristine reference BHJ (A) and Au doped BHJ (B): representative (5 5 mm) and (3 3 mm) images with line profiles, corresponding to lines 1 and 2 in the figures.

Fig. 5 presents the results for the pristine compared to the NPdoped blend. It is observed that the addition of NPs in the blend retards the PL intensity decay rate of the device. Considering that the NPs resonance has an excitation lifetime of a few picoseconds, the donor–acceptor interaction between the comparatively long-lived triplet excitons of P3HT and the NPs will result in a strong quenching of the triplet state and, thus, the photo-oxidation rate. The above results further support the EDXR study and suggest a NP-mediated mechanism for the impediment of the photo-induced degradation effects at the cathode–BHJ interface. Such mitigation effect is similar to the one obtained upon using a buffer barrier layer, while the drawbacks related to its use discussed above are avoided.

Fig. 5 PL decay measurements: comparison of the pristine (black) and NP-doped blend (red).


A second important cause of OPVs failure upon prolonged illumination is due to thermal effects that give rise to the rearrangement of the active layer molecules. As a result changes in the degree of crystallization of P3HT and aggregation in larger clusters of PCBM molecules are observed.53 Indeed, during continuous illumination with more intense light, equal to that of solar radiation, the temperature of the active layer is increased to about 75 C, which in turn facilitates the softening of the polymeric components and favours PCBM diffusion and clustering.14,54,55 Confronting such uncontrolled morphological modifications is a crucial aspect towards the improvement of OPVs stability. In order to investigate the influence of Au NPs on thermally induced degradation effects, in situ Raman spectroscopy of the active layer of the pristine as well as the NP doped device, under prolonged one sun illumination, is performed. Fig. 6 shows such spectra for both types of devices in the asprepared and after 7.5 h of continuous operation under light, while Table 2 summarizes the respective Raman shifts. As shown in Fig. 6A, the characteristic vibration frequencies of P3HT are present in both devices in the as-prepared state.56,57 Following 7 h 30 min of continuous illumination, lowering of Raman peaks’ intensity is observed in both cases (Fig. 6B and C), possibly due to photo-thermal degradation of the polymeric components. It can be speculated that polymer softening under working conditions leads to conformational modifications that affect the vibrational properties of P3HT molecules, leading to

Table 2 Raman shifts corresponding to bands in Fig. 6 Wavenumber (cm1)

Mode

719 999 1085 1200 1371 1445

C–S–C deformation thiophene ring C–C stretching alkyl chain C–H bending alkyl chain C–C stretching/C–H bending alkyl chain C–C stretching thiophene ring C]C stretching thiophene ring

an overall decrease of the Raman intensity. On the other hand, a gradual shift to higher wavenumbers of the peaks related to P3HT (both rings and chains) is observed for the pristine device. This shift indicates the onset of thermally induced rearrangement processes. In particular, the shift of the C–C mode peak position from 1371 cm1 to 1375 cm1 is possibly related to a more planar conformation of the P3HT crystallites, which in turn indicates a modification of the lamellar packing of the polymer.58 In contrast, no such shift is observed in the NP doped device (Fig. 6C), suggesting that the presence of Au NPs preserves the initial polymer morphology. Indeed, incorporation of NPs in the blend is expected to limit the segmental motions of the polymer chains leading to an increase in its glass-transition temperature and in turn to the enhancement of its thermal stability.29 In conclusion, Au NPs act as stabilizers and mitigate the thermally induced degradation of the OPV active layer. It should be emphasized that this is a general effect, different from that described previously regarding the stabilization of the cathode– active layer interface which is strongly related to the nature and size of NPs. 3.3 PV aging

Fig. 6 Raman spectroscopy of the active layers: (A) comparison of the pristine (black line) and the NP doped (red line) device before illumination; the effect of prolonged sun illumination (red line) on the pristine (B) and on the NP doped device (C).


As discussed above, Au NPs doping leads to enhancement of the initial device efficiency, stabilization of morphological properties of the photoactive blend and suppression of photooxidation at the cathode–BHJ interface. The question is whether the observed superior structural and morphological properties of the NP doped blends comply with higher device stability and performance durability against degradation. To investigate this issue, the PV performance of the pristine and NP-doped devices following continuous operation under solar simulating illumination was compared. In order to monitor the stability in conditions similar to outdoor, the experiments were performed with the devices being unencapsulated and exposed to ambient air during operation. Fig. 7 shows the evolution of the respective PV parameters over exposure time for both devices tested. For each data point of the aging curves, a complete I–V characteristic was recorded and the normalized Jsc, FF, Voc and PCE values were subsequently calculated. Among successive measurements, devices were continuously irradiated, in the open circuit mode. For the pristine device (Fig. 7A), an abrupt reduction of the PCE, down to 5% of its initial value, was observed within the first 10 hours. In contrast, in the same time interval, the Au NP doped device (Fig. 7B) preserved 25% of its initial PCE. Degradation of the pristine device is caused essentially by a single-step exponential decay of the Jsc, while Voc is relatively stable over the same time. Such Nanoscale, 2012, 4, 7452–7459 | 7457

the device performance by 42% due to LSPR and scattering effects, and the device lifetime by 3 times. Additionally, timeresolved EDXR monitoring showed that, while pristine devices undergo a progressive photo-oxidation at the cathode buried interface (testified by an increase in the cathode thickness), the doped cells do not show any modification. Strong indication is, therefore, gained that doping the BHJ of OPV devices with Au NPs protects the metallic electrode buried interface against degradation. Additionally, Raman analysis of the devices shows that the NPs are able to stabilize the photoactive polymer conformational properties, preserving the BHJ nanoscale morphology of the donor–acceptor network. As a final result, the Au doped devices are characterized by enhanced stability over time of the PV performance, as demonstrated by a comparative time resolved PCE study. It is important to stress that the superior structural, interfacial and PV stability was obtained on devices that were not subjected to any prior thermal annealing treatment. In conclusion, our unconventional approach indicates that the incorporation of Au NPs protects the photoactive film against degradation and preserves the metallic electrode buried interface, leading to devices of improved efficiency and stability.

Acknowledgements The authors are grateful for the support received from the COST Action MP0902 COINAPO and from the European Science Foundation ORGANISOLAR Activity. The authors acknowledge M. Androulidaki for her support with the PL and IPCE experiments.

Notes and references Fig. 7 PV characteristics, as a function of exposure time, normalized to their initial values for the reference (A) and the Au NP-based solar cells (B).

photocurrent loss is attributed to both the morphological instability of the active layer and the degradation of the active layer–cathode interface confirmed by EDXR experiments. It is important to emphasize that the time scale of the morphological process detected by EDXR well matches the PV aging of the pristine device, indicating that the photo-induced oxidation at the cathode buried interface is an important source of PV degradation. In contrast, the morphological as well as photooxidation stability of the NP doped device revealed by EDXR, Raman spectroscopy and PL decay experiments correlates well with the slower degradation rate of the respective device. Overall, the comparative time-resolved structural and PV performance data show that the incorporation of NPs stabilizes the photoactive layer, allowing the preservation of the OPV cell performance leading to an enhanced stability over time.

4. Conclusions OPV devices using the P3HT:PCBM active layer doped with Au NPs were studied in situ upon illumination. A comparative study allowed us to confirm that the Au NP doped devices are characterized by superior PV performances and stability. Indeed, it was demonstrated that the incorporation of surfactant-free Au NPs in the active layer of OPV devices can significantly enhance 7458 | Nanoscale, 2012, 4, 7452–7459

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