Porous Metal Oxide Solar Cells

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Simple Approach to Hybrid Polymer/Porous Metal Oxide Solar Cells from Solution-Processed ZnO Nanocrystals Johann Boucle´,*,† Henry J. Snaith,‡ and Neil C. Greenham Optoelectronics Group, CaVendish Laboratory, Cambridge CB3 0HE, United Kingdom ReceiVed: September 30, 2009; ReVised Manuscript ReceiVed: January 15, 2010

This work is devoted to the development of hybrid bulk heterojunction solar cells based on porous zinc oxide (ZnO) electrodes and poly(3-hexylthiophene) (P3HT), using simple synthesis procedures and deposition techniques. Starting from ZnO nanocrystals with well-controlled properties, porous ZnO electrodes of suitable porosity are deposited by spin-coating, varying the main experimental parameters such as composition of the initial ZnO formulation and choice of the organic ligand. Significant charge transfer yields are observed in the corresponding solar cells, and the influence of processing conditions on device performance is investigated using conventional techniques as well as transient photovoltage/photocurrent decay measurements. The temperature used to sinter the ZnO electrode is found to be specifically crucial to ensure efficient charge transport in the device while avoiding a loss in interfacial area through nanocrystal coalescence. Using 8 × 13 nm ZnO nanorods, the best device exhibits a power conversion efficiency of 0.35% under 100 mW · cm-2 AM1.5G simulated solar emission. This strategy, using processing in air with simple deposition techniques, competes with related approaches based on nanostructured ZnO processed using more complex procedures. Moreover, device performance and photophysics are found to be greatly influenced by the morphology of the starting ZnO nanocrystals, illustrating that fine control of the inorganic component can effectively tune the performance of hybrid bulk heterojunction solar cells. 1. Introduction Organic photovoltaic devices have been widely investigated in the past decade as they show the promise of efficient solar energy conversion at low cost on flexible plastic substrates.1 The power conversion efficiencies achieved have been steadily rising,2 reducing the barrier to their commercial exploitation. However, the very short exciton diffusion lengths of conjugated polymers or molecules require the intimate intermixing of donor and acceptor materials at the nanoscale. In this context, the difficulty to finely control the phase segregation in organic active layers tends to limit the development of novel organic materials or device concepts toward more efficient plastic solar cells. Moreover, due to intrinsic degradation mechanisms occurring in π-conjugated organic materials,3 device lifetimes are still short, and careful encapsulation strategies have to be developed in practical working environments.4-6 One way to overcome some of these limitations is to replace one of the organic components by an inorganic material. Such hybrid devices benefit from inorganic nanostructured semiconductors with high mobilities that can also be synthesized from solution at low cost.7-9 With the benefit of well-controlled synthetic procedures, both nanocrystals of various morphologies and nanostructured inorganic materials have been successfully implemented as bulk heterojunction solar cells. Polymer/nanocrystal blends10 have already demonstrated respectable power conversion efficiencies up to nearly 3% for systems based on CdSe tetrapods,11 but the difficulty of properly balancing charge photogeneration and * Corresponding author. E-mail: [email protected]. † Current affiliation: XLIM UMR 6172, Universite´ de Limoges/CNRS, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France. ‡ Current affiliation: Department of Physics, Clarendon Laboratory, University of Oxford, OX1 3PU, United Kingdom.

charge carrier percolation after active layer formation remains a major issue. As an alternative approach, polymer/porous metal oxide structures, where a conjugated polymer is inserted into a porous inorganic network, offer several benefits compared with blend devices: electron transport is facilitated in the porous inorganic component, as the latter is usually processed before polymer infiltration; the polymer/metal oxide interface is mainly governed by the dimension of the nanostructured inorganic component, assuming a complete filling is achieved; the polymer/metal oxide interface can be chemically controlled using self-assembled molecular layers in order to assist charge separation or in order to block charge recombination across the donor-acceptor interface; and it is possible to grow vertically aligned inorganic rods or columns to assist charge transport and polymer infiltration. Such strategies have been employed over the past few years mainly using titanium dioxide (TiO2)12-14 or zinc oxide (ZnO)15-17 nanostructured electrodes, although some other porous inorganic materials such as silicon have been recently reported for hybrid devices.18,19 In order to efficiently photogenerate free charge carriers in the active layer, all excitons that are formed in the polymer have to reach an interface before recombining. This requirement constrains the optimum pore diameter to ideally less than 10 nm, leading to poor polymer infiltration. The corresponding reduction in the interfacial area between the donor and acceptor materials20 is responsible for the low power conversion efficiencies that have been reported for porous metal oxide/polymer geometry (from 0.2 to 1% under simulated solar conditions). To overcome this limitation, different strategies can be employed to better control the porous electrode features (porosity, pore dimensions, etc.) or the conjugated polymer wetting properties. A promising strategy is to incorporate an organic or organometallic dye at the polymer/metal oxide

10.1021/jp909376f  2010 American Chemical Society Published on Web 02/09/2010

ZnO-Based Hybrid Polymer/Metal Oxide Solar Cells interface in order to assist the processes of light harvesting, charge separation, and photocurrent generation.21 Efficient hybrid devices based on nanoporous TiO2 and poly(3-hexylthiophene) (P3HT) have indeed demonstrated power conversion efficiencies of 2.6% under AM1.5 solar illumination using an interfacial metal-free organic dye in conjunction with added ionic dopants.22 Regarding metal oxides, although TiO2 is associated with the best efficiencies among hybrid porous metal oxide/polymer devices, ZnO is also a material of primary interest due to a high mobility in the bulk and the possibility to easily synthesizesfrom solution and at low temperaturesnanostructures of various morphologies. Regarding ZnO nanocrystals, stable colloidal suspensions can be easily achieved at high concentrations in organic or polar solvents, which make them suitable for efficient polymer/nanocrystal blend solar cells.23,24 Alternatively, porous metal oxide/polymer devices have been implemented using ZnO nanorods vertically grown on substrates16,17,25 or using ZnO nanofibers15 and P3HT. However, although promising efficiencies up to 0.6% have been reported, unoptimized porous electrode morphology and limited polymer infiltration are still the main key factors to be improved toward more efficient energy conversion.26 In particular, efficient polymer wetting requires nanostructured electrodes with a feature size that enables capillary effects. In this context, we report a simple and reproducible approach for the development of ZnO/P3HT hybrid bulk heterojunctions. Starting from solution-processed ZnO nanoparticles or nanorods with well-controlled properties, mesoporous metal oxide electrodes are deposited and directly filled with P3HT by spincoating. The device photophysics is investigated through conventional optoelectrical measurements, as well as transient techniques, in order to study the main physical processes that control device performance. 2. Experimental Section Synthesis of ZnO Nanocrystals. Zinc oxide nanocrystals were synthesized by hydrolysis and condensation of zinc acetate dehydrate by potassium hydroxide in methanol, as previously reported.27,28 The synthesis conditions were adapted in order to control the morphology of the ZnO nanocrystals, from spherical particles to rods with various aspect ratios. A Zn2+:OH- ratio of 1:1.60 was used for all particles synthesized in this work. Nanorods were preferentially obtained in a two-step procedure, when a small amount of water is added in the precursor mixture,28 while large spherical particles can be synthesized without addition of water, in one (for small diameter particles) or two (for large particles) steps. After several final washing steps in methanol, the nanocrystals obtained are easily redispersed at high concentration (up to 150 mg/mL) in a chloroform:methanol solvent mixture (80:20 vol). The use of n-butylamine as ligand was found to be necessary to obtain transparent and stable colloidal suspensions from large ZnO nanocrystals (>15 nm), while no ligand is required to disperse smaller particles. All ZnO suspensions were found to be stable over at least several days. Preparation of the ZnO Paste. Porous ZnO films were deposited from a precursor ZnO paste composed of 80 mg/mL of ZnO particles in a chloroform:methanol solvent mixture (80:20). A controlled amount of poly(ethylene glycol) (PEG, Polymer Source Inc.) of specific molecular weight (10k) previously dissolved in chloroform at 80 mg/mL was then added to the suspension to reach the desired ZnO:PEG ratio (from

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Figure 1. Device structure and simplified energetic configuration of the hybrid ZnO/P3HT solar cells.

5:1 to 1:1). The resulting paste was homogenized for 5 min under magnetic stirring before being deposited by spin-coating. Fabrication of Hybrid Solar Cells. The hybrid solar cell consists of an inverted-type sandwich structure deposited on precleaned patterned ITO glass, as depicted in Figure 1. A 30-50 nm thick dense ZnO layer was initially deposited by chemical spray pyrolysis technique, as described in ref 29. This layer is referred as the hole-blocking layer (HBL) and prevents direct contact between the hole-accepting polymer and the ITO electron-collecting electrode in the device. The experimental deposition parameters were optimized to obtain the best rectifying behavior in bilayer (ITO/dense ZnO/P3HT/ Au) devices. Around 3-5 mL of the precursor solution (80 mg/ mL zinc acetate in methanol) was sprayed onto preheated ITO substrates (400 °C) using nitrogen as carrier gas. Subsequent annealing was then performed in air at 400 °C for 20 min. The porous ZnO layer was then deposited from the precursor ZnO:PEG paste in chloroform:methanol mixture by spin-coating at 2000 rpm in air. The corresponding film thickness is around 150-300 nm depending on the formulation parameters. A final sintering step in air at temperatures ranging from 150 to 540 °C was used in order to remove the organic fraction of the film and improve the ZnO particle connectivity and crystallinity. The resulting porous ZnO films were filled using poly(3-hexylthiophene) P3HT (Plextronics, regioregularity >99%, molecular weight 15-20k, polydispersity index 350 °C), significant ZnO nanocrystal coalescence is expected to be responsible for the saturation of device performance. However, much more rapid dark current onsets results in reduced open-circuit voltage at higher sintering temperatures. The charge recombination rate is found to be insensitive to sintering temperature at constant light intensity. Finally, the main device parameters have been investigated as a function of the shape and size of the ZnO nanocrystals used to deposit the porous ZnO films. Device performance, as well as recombination rate, is found to be strongly influenced by the dimensions and aspect ratio of the initial particles. This work illustrates how a fine control of the inorganic nanocrystal properties, through specific synthetic routes in solution, can be a way of tuning the performance of hybrid bulk heterojunction solar cells based on nanostructured metal oxides and conjugated polymers. Acknowledgment. The authors thank the European project NAIMO (Contract No. NMP-CT-2004-50035) for funding. J.B. gratefully acknowledges useful discussions with B. Sun and H. Wong regarding nanocrystals synthesis. Finally, A.-M. Petrozza and A. Abrusci are also acknowledged for their assistance with photoinduced absorption, as well as M. Colas (SPCTS, Limoges, France) regarding Raman scattering experiments. J.B. acknowledges J.-M. Nunzi for helpful discussions during manuscript revision. Supporting Information Available: Raman spectrum of a typical as-formed ZnO nanopowder; SEM image of a ZnO film deposited from a ZnO paste without PEG additive and of a

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