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PUBLISHED ONLINE: 13 SEPTEMBER 2009 | DOI: 10.1038/NMAT
The effect of three-dimensional morphology on
the efficiency of hybrid polymer solar cells
Stefan D. Oosterhout
1
, Martijn M. Wienk
1
, Svetlana S. van Bavel
2,
, Ralf Thiedmann
4
L. Jan Anton Koster
1
, Jan Gilot
1
, Joachim Loos
2
, Volker Schmidt
4
and René A. J. Janssen
1
The efficiency of polymer solar cells critically depends on the intimacy of mixing of the donor and acceptor semiconductors used in these devices to create charges and on the presence of unhindered percolation pathways in the individual components to transport holes and electrons. The visualization of these bulk heterojunction morphologies in three dimensions has been challenging and has hampered progress in this area. Here, we spatially resolve the morphology of 2%-efficient hybrid solar cells consisting of poly(3-hexylthiophene) as the donor and ZnO as the acceptor in the nanometre range by electron tomography. The morphology is statistically analysed for spherical contact distance and percolation pathways. Together with solving the three-dimensional exciton-diffusion equation, a consistent and quantitative correlation between solar-cell performance, photophysical data and the three-dimensional morphology has been obtained for devices with different layer thicknesses that enables differentiating between generation and transport as limiting factors to performance.
E
fficient organic solar cells use a bulk heterojunction of two semiconductors with offset energy levels to create charges. By intimately mixing electron donor and acceptor materials, the intrinsic limitations related to the low mobility and lifetime of excitons in organic semiconductors can be overcome, resulting in effective carrier generation at the extended donor–acceptor interface. The most efficient organic solar cells combine p-type conjugated polymers with n-type fullerenes^1 , which have recently reached efficiencies of 6% (refs 2, 3). Control over the morphology of the blend by the proper choice of processing conditions is generally essential to reach this level of performance and one of the traditional challenges in the field of organic solar cells lies in optimization of the morphology of the mixed layer. As an alternative to fully organic solar cells, hybrid solar cells use a combination of organic and inorganic materials. This concept has been demonstrated by combining semiconducting polymers as the donor with different inorganic materials, including CdSe (refs 4, 5), TiO 2 (refs 6, 7) and ZnO (refs 8–10), as the acceptor. Potential advantages of the inorganic semiconductors are a high dielectric constant, which facilitates carrier generation processes, a high carrier mobility and thermal morphological stability of the blended materials. Ultimately, hybrid cells offer the prospect of actual control over the morphology of the blend by first constructing the inorganic scaffold with the proper layout and dimensions11–13, but until now the best hybrid solar cells were made by simultaneous deposition of the two components. This often involves precarious processing, owing to the rather different nature of the materials involved. These drawbacks can largely be circumvented by the in situ generation of the inorganic semiconductor inside the organic material14,15. In this process, a well-soluble organometallic precursor is deposited from solution together with the semiconducting polymer. During and after this deposition the precursor is converted, by reacting with
(^1) Molecular Materials and Nanosystems, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands, 2 Laboratory of Materials
and Interface Chemistry and Soft Matter Cryo-TEM Research Unit, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands, 3 Dutch Polymer Institute, PO Box 902, 5600 AX Eindhoven, The Netherlands, 4 Institute for Stochastics, Ulm University, 89069 Ulm, Germany. *e-mail: [email protected].
moisture from the surrounding atmosphere, to an inorganic network inside the polymer film. This has provided 1.4%-efficient photovoltaic cells, using diethylzinc as the ZnO precursor and poly(3-hexylthiophene) (P3HT) as the semiconducting polymer^16. The reasons for the reduced efficiency of hybrid solar cells compared with the more efficient polymer/fullerene cells are only partly understood and need to be analysed to further increase the performance. Here, we describe and analyse in situ P3HT/ZnO solar cells that reach a power conversion efficiency of 2%. Photoinduced absorption spectroscopy and three-dimensional (3D) electron microscopy of the active layers, combined with statistical analysis of percolation paths and solving the exciton diffusion equation are used to provide an unprecedented detailed insight into the role of the nanoscale morphology in creating and transporting charges in these bulk heterojunctions. Such analysis enables a unique and quantitative insight into the loss mechanisms associated with morphology that control, and ultimately limit, the power conversion of these rather efficient P3HT/ZnO hybrid solar cells. The active layer of the photovoltaic cells was applied by spin-coating a blend of diethylzinc and P3HT from a mixture of chlorobenzene, toluene and tetrahydrofuran onto a trans- parent electrode, consisting of poly(3,4-ethylenedioxythiophene)/ poly(styrenesulfonate) (PEDOT/PSS) on an indium tin oxide (ITO)-covered glass substrate. During spin-coating, the diethylzinc was exposed to humidity, causing hydrolysis and the formation of Zn(OH) 2. The subsequent condensation reaction is completed by annealing the film at 100 ◦C to form an interpenetrating net- work of ZnO inside the P3HT. Electron diffraction experiments demonstrate the presence of crystalline P3HT and ZnO domains in these active layers (see Supplementary Information). These mixed films do not visibly scatter light, indicating that no coarse phase separation between the organic and inorganic component takes
818 NATURE MATERIALS | VOL 8 | OCTOBER 2009 | www.nature.com/naturematerials
NATURE MATERIALS DOI: 10.1038/NMAT2533 (^) ARTICLES
Current density (mA cm
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Figure 1 | Performance of P3HT/ZnO solar cells. a , Current density/voltage ( J – V ) characteristics of the best P3HT/ZnO device (film thickness is 225 nm) in the dark (black line) and under illumination with simulated solar light (red line). b , EQE of the same device, corrected for the nonlinear response of the current with light intensity. Convolution of this spectral response with the AM1.5G solar spectrum provides an estimated current density of 5.6 mA cm−^2. The fraction of absorbed light, as calculated by optical modelling, using ellipsometry data is included. c , Evolution of the current density of P3HT/ZnO solar cells with the thickness of the active layer. For comparison, the amount of photons absorbed in the active layer is included. d , IQE of solar cells versus thickness of the active layer. The scatter in the data is attributed to differences in the morphology for films of similar thickness, but manufactured independently.
place during the deposition process. An aluminium top electrode completes the device. Several mass ratios of P3HT versus ZnO were tested. A 1: ratio (w/w) was found to give the best performance. This translates into a ZnO volume fraction of about only 20%, noticeably smaller than what is commonly used in polymer/fullerene solar cell devices, where the optimal fullerene content generally exceeds 50%. In addition, the thickness of the P3HT/ZnO active layer was systematically varied between 50 and 250 nm, by adjusting the spin rate during the coating process, while keeping the same P3HT and diethylzinc concentration. Despite some spread in the results, a clear trend is observed: the performance of the devices improves with increasing photoactive layer thickness, mainly as a result of increasing current densities. The overall best performance was obtained for a device with an active layer thickness of 225 nm. Under illumination with 100 mW cm−^2 simulated solar light, this device delivers an open-circuit voltage ( V OC) of 0.75 V, a short-circuit current density ( J SC) of 5.2 mA cm−^2 and a fill factor of 0.52, resulting in an estimated device efficiency of 2.0%. Spectral response measurements (Fig. 1b) reveal a maximal external quantum efficiency (EQE) of 44% at 520 nm. The broad, rather flat shape of the spectral response is related to the thickness of the device. Convolution of the EQE with the AM1.5G spectrum affords a current density of 5.6 mA cm−^2 , which is in reasonable agreement with the value obtained from J – V measurements. It is quite uncommon for polymer solar cells to have an optimal layer thickness significantly larger than 100 nm, because many
Δ T
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Figure 2 | PIA spectra of P3HT/ZnO layers. The normalized PIA spectra of P3HT/ZnO layers used in solar cells recorded at T = 80 K. In the devices, the active layers had thicknesses of 44 nm (black), 54 nm (red) and 151 nm (blue). Spectra were normalized at the 2.2 eV bleaching band.
material combinations suffer from charge collection limitations at larger thicknesses. P3HT/PCBM ([6,6]-phenyl-C 61 -butyric acid methyl ester) is one notable exception^17 , demonstrating that in P3HT carrier mobilities are sufficiently high to allow thick photoactive layers. The good performance of the relatively thick P3HT/ZnO layer is a strong indication that electron collection
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NATURE MATERIALS DOI: 10.1038/NMAT2533 (^) ARTICLES
Table 1 | Calculated volume fraction, exciton quenching and percolation connectivity of P3HT:ZnO bulk heterojunction layers inferred from electron tomography.
Without electrodes With electrodes Film thickness (nm)
ZnO volume fraction (%)
Quenched by ZnO (%)
Quenched by ZnO (%)
Quenched by electrodes (%)
ZnO connected to top (%)
ZnO monotonously connected to top (%)
57 13 ± 4 40 ± 3 32 ± 1 32 ± 1 96 ± 2 93 ± 2 100 21 ± 8 78 ± 5 73 ± 5 7 ± 1 94 ± 5 85 ± 10 167 21 ± 8 83 ± 5 79 ± 6 4 ± 1 92 ± 5 80 ± 12
Probability
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Cumulative probability
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0 5 10 15 20 25 30 Distance (nm)
0 0 2 4 6 8 10 12 14
a b
Figure 4 | Statistical analysis of the 3D morphology. a , Distribution of the probability to find a P3HT voxel at a certain distance from a ZnO domain for mixed P3HT/ZnO films of different thicknesses, calculated from the 3D-TEM data shown in Fig. 3b. b , Cumulative probability to have P3HT within a shortest distance to ZnO. The error margins indicated are obtained from the two most extreme thresholds possible for the binarization of the 3D-TEM data.
through the ZnO phase occurs quite effectively as well, despite the relatively low ZnO content. Thicker P3HT/ZnO cells outperform the thinner devices owing to increasing current density, while the fill factor and open circuit voltage remain constant. The increase of current density with increasing layer thickness is shown in Fig. 1c. For comparison, the number of photons absorbed by the P3HT/ZnO layer is included. The absorption was calculated by optical modelling, integrating over all wavelengths using the complex refractive index of the mixed layer ( ˜n = n + i · k , where n is the refractive index and k is the extinction coefficient) as determined by ellipsometry. The modelling was carried out on the entire stack of layers (glass/ITO/PEDOT:PSS/P3HT:ZnO/Al) in the device and thus accounts for interference effects (see Supplementary Information). Figure 1c shows that the increasing current density with layer thickness is noticeably larger than the absorption increase. This can be quantified with the internal quantum efficiency (IQE), which is the ratio between the measured J SC and the number of absorbed AM1.5 photons per unit area and time. The IQE (Fig. 1d) increases significantly with layer thickness from 20% for 50-nm-thick cells up to 50% for cells with active layer thicknesses >150 nm. Hence, in thicker layers, not only more photons are absorbed, but these photons are also more efficiently transferred into collected current. This is counterintuitive, because one would expect that charge collection becomes more difficult for thicker layers, owing to the larger distances over which the carriers need to be transported and the smaller electric field over the layer. To further investigate the observed IQE differences, photoin- duced absorption (PIA) measurements were carried out on three P3HT/ZnO photovoltaic cells with different layer thicknesses (44, 54 and 151 nm). For all three samples, excitation at 488 nm (2.54 eV) results in photoinduced bleaching bands of neutral P3HT at 1.9, 2.2 and 2.35 eV (Fig. 2, normalized at the bleaching signal
at 2.2 eV to correct for differences in layer thickness). Moreover, the characteristic absorption bands of the polymer radical cation at 0.5 and 1.3 eV are observed, unambiguously demonstrating photoinduced charge generation in this blend^18. The low-energy absorption at 0.5 eV is relatively large because at this energy also electrons injected into the ZnO contribute to the PIA signal8,19. The spectra also exhibit a peak at 1.06 eV, which is related to a P3HT triplet state^18. This indicates that quenching of the initially formed singlet exciton is not quantitative, leaving enough time for intersystem crossing to the triplet state for a significant fraction of the excitations. Without knowing the exact absorption coefficient and lifetime of the different photoinduced states, one cannot quantitatively assess their yields, but comparing the ratio between polaron and triplet bands does provide relevant information about the relative yields. For thicker layers, the radical cation bands increase significantly compared with the triplet absorption, sug- gesting a higher carrier generation efficiency, in accordance with the observed higher IQE for thicker layers. Photoluminescence spectroscopy on P3HT/ZnO films confirmed the incomplete ex- citon quenching. Quantitative analysis was severely hampered by the intrinsic low photoluminescence efficiency in P3HT (ref. 20), which is mainly determined by disordered regions and generally not representative for most excitons20,21. The observed incomplete charge generation in hybrid poly- mer/ZnO films makes them distinctly different from photoactive blends with fullerenes as the electron acceptor, where exciton quenching is generally quantitative, even at very low fullerene con- tents. This could be related to a higher miscibility for the molecular acceptors with the donor polymer, affording better dispersion of the acceptor (domains) inside the polymer matrix22,23. A sub-optimal morphology is one possible cause for incomplete charge carrier generation, demonstrated by PIA measurements. To examine the morphology of the P3HT/ZnO film, transmission
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ARTICLES NATURE MATERIALS^ DOI: 10.1038/NMAT
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Figure 5 | Exciton quenching efficiency cross-sections in P3HT/ZnO bulk heterojunctions. a – c , Quenching efficiency in horizontal cross-sections of bulk material of different thicknesses, 57 nm ( a ), 100 nm ( b ) and 167 nm ( c ). The white areas constitute ZnO domains. d – f , Quenching efficiency in the vicinity of electrodes (grey areas) for different layer thicknesses, 57 nm ( d ), 100 nm ( e ) and 167 nm ( f ).
electron microscopy (TEM) was used. The bright-field TEM image of a cross-section of a typical complete device (Fig. 3a) shows that the photoactive layer is much rougher than all layers underneath. In the image, ZnO shows up as the darker regions as a consequence of a higher electron density. Evidently, ZnO is not homogeneously dispersed inside the polymer matrix, especially at the interface with the PEDOT/PSS, where a layer of pure P3HT is present. This layer is at least 10 nm thick, causing excitons generated within the first few nanometres from the PEDOT–PSS interface not to reach a ZnO interface to create charges. Possibly, (part of) the triplets observed using PIA originate from this layer. This interface layer constitutes a larger fraction for thinner devices, which could explain the larger triplet signals in these samples. In absolute terms, however, the number of excitons that are expected to be lost in this P3HT interface layer is not large enough to completely account for the measured IQE losses, which exceed 50% for all devices. More detailed TEM analyses were carried out on freestanding films. Representative mixed films of different thicknesses (57, 100 and 167 nm) were spin-coated on PEDOT/PSS and then floated on water, leaving P3HT/ZnO films that were analysed by electron tomography^24. The snapshots of the reconstructed volume of these films are shown in Fig. 3b. A threshold was applied to the raw data as obtained by electron tomography in such a way that ZnO appears yellow and P3HT looks transparent. Ostensibly, a large difference between the three films is obtained, showing finer phase-separated domains with increasing layer thickness. In particular, the thinnest film has much larger domains, both for the ZnO and the P3HT. The presence of P3HT domains that are substantially larger than the exciton diffusion lengths in P3HT is consistent with the large triplet signals in PIA measurements and the low IQE in the thin devices.
To quantify the relevant morphological parameters, an extensive statistical analysis of the 3D-TEM images was carried out. Therefore, the original 3D-TEM data of the bulk of the film (about 60% of the total thickness) were binarized to decide which voxels (3D data points) are ZnO and which are P3HT. The results are summarized in Table 1. In the two thickest layers, the estimated ZnO volume fraction is close to the expected value of 19%, on the basis of the ratio of di- ethylzinc and P3HT in the spin-coating solution. The ZnO content in the thinnest layer is significantly lower at 13%. This is tentatively rationalized by a comparatively large fraction of the diethylzinc evaporating during spin-coating with the higher spin rate applied for this thin layer. The low ZnO content of the thinnest film partially accounts for the relatively large observed P3HT domains. Next, spherical contact distances, defined as the distance from a certain voxel of one material to the nearest voxel of the other mate- rial, were determined for these three films. As under operation, exci- tons are primarily generated inside P3HT, we focus on the distance distribution from P3HT to ZnO. Figure 4a shows the probability to find P3HT at a certain shortest distance to a ZnO domain. For the 100- and 167-nm-thick films, most P3HT lies well within a shortest distance of 10 nm from ZnO. On the other hand, the 57-nm-thick sample has a large amount of polymer at shortest distances as high as 25 nm from an interface with ZnO. This analysis substantiates that coarser phase separation is present in thinner layers. It is not exactly clear what governs the large morphology dif- ferences. During deposition several processes take place simulta- neously: evaporation of three different solvents and diethylzinc, adsorption of water, reaction of diethylzinc with water, evap- oration of condensation reaction products and gelation and/or
822 NATURE MATERIALS | VOL 8 | OCTOBER 2009 | www.nature.com/naturematerials
ARTICLES NATURE MATERIALS^ DOI: 10.1038/NMAT
Received 18 May 2009; accepted 20 August 2009; published online 13 September 2009
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Acknowledgements
The authors gratefully acknowledge D. Veldman for assistance with the PIA measurements and N. Lousberg for preparing a cross-section of a complete device by FIB processing. The research was supported by a TOP grant of the Chemical Sciences (CW) division of the Netherlands Organization for Scientific Research (NWO) and is part of the Joint Solar Programme (JSP). The JSP is co-financed by the Foundation for Fundamental Research on Matter (FOM), Chemical Sciences of NWO and the Foundation Shell Research. This work was further supported by the Deutsche Forschungsgemeinschaft under Priority Programme 1355 ‘Elementary Processes of Organic Photovoltaics’, Senter/Novem in the EOS project Zomer (EOSLT03026) and is part of the research program of the Dutch Polymer Institute (DPI, project 524).
Author contributions
S.O. made and characterized the devices, S.v.B. did the TEM measurements. R.T. carried out the statistical analysis. L.J.A.K. carried out the exciton diffusion calculations. J.G. did the optical modelling. M.W., J.L., V.S. and R.J. planned the research and supervised it. The article was written by S.O. and M.W.
Additional information
Supplementary information accompanies this paper on www.nature.com/naturematerials. Reprints and permissions information is available online at http://npg.nature.com/ reprintsandpermissions. Correspondence and requests for materials should be addressed to R.A.J.J.
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