ARTICLE pubs.acs.org/JPCC
Direct Observation of Charge Generating Regions and Transport Pathways in Bulk Heterojunction Solar Cells with Asymmetric Electrodes Using near Field Photocurrent Microscopy Sabyasachi Mukhopadhyay, Srinidhi Ramachandra, and K. S. Narayan* Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560064, India
bS Supporting Information ABSTRACT: We present a versatile method to examine the donoracceptor based bulk heterojunction structures using a combination of structural, optical and optoelectronic contrast. The technique relies on current-contrast-optical scanning microscopy on asymmetric photovoltaic device structures with the electrodes extending in orthogonal directions to form a cross-type structure which provides a near-field access for the incident light beam. The method was used to follow changes with annealing and different ratios of the Si-PCPDTBT:PC71BM system, where the correlation between the changes in the morphology and charge carrier generation leading to photocurrent was clearly established. The general viewpoint of increasing heterogeneity between two components and continuous pathways in the entire photovoltaic layer upon thermal annealing are clearly evident from the high resolution optical and current contrast images. Fourier analysis of the images was used to extract the relevant length scales which prevail in these binary mixtures and quantify the changes upon thermal annealing.
1. INTRODUCTION Polymer solar cells consisting of a homogeneous blend of donor conjugated polymer (D) and acceptor molecules (A) forming bulk heterojunction structures have shown to be a promising route for realizing low-cost, large area photovoltaic elements.15 The blend-film morphology which is controlled by factors which include solvent evaporation,6 thermal annealing,79 and appropriate additives10,11 are known to play a key role in the photophysical12 and electrical transport properties.13,14 One of the important prerequisites for efficient exciton dissociation at the D:A interface is the phase separation between donor and acceptor in the blend at optimum length scales.2,1518 Probing of charge generation and transport in these mixed phase regions at nanoscale is expected to be informative and improves our general understanding of these systems. The evolution of local morphology as a function of processing conditions19 is typically followed by atomic force microscopy (AFM) based methods.17,20 A combination of local optical and electrical probes will be additionally valuable to track the effect of nanomorphology/ phase segregation on photogenerated charge transport. In this regard, transient electron force microscopy (trEFM) studies have been employed to probe the charging rates in PFB/F8BT blends,21 where it was reported that charging rates are maximum at the center of the topographically visible domains compared to the edges. Similarly, photoconducting-AFM (PC-AFM)21,22 has been utilized to map the microstructure in P3HT/PC71BM blends using an electrical tip with far-field as well as near-field illumination to probe local charge generation in a contact mode. It is natural to utilize high resolution (100 μm) from the counter electrode-perimeter (Al), in an asymmetric (cross-bar electrodes) device.25,26 The presence of this Iph arising from the periphery enables high resolution photocurrent contrast spatial imaging without the need for locally positioned electrodes. For light incident from the cathode side, the Iph decreases but persists over a distance of 100 μm from the Al periphery. The possibility of near field optical access to the active region then can provide us topography, transmission, fluorescence, and photocurrent imaging simultaneously. The Iph decay is relatively insignificant over scan areas of 10 10 μm, and the combination of these images from a given region in the binary-mixture film reveal the spatial Received: May 31, 2011 Revised: July 21, 2011 Published: July 27, 2011 17184
dx.doi.org/10.1021/jp205065q | J. Phys. Chem. C 2011, 115, 17184–17189
The Journal of Physical Chemistry C
Figure 1. Parts (a) and (b) are the schematic representations of the experimental setup (c) Photocurrent decay profile out-side the Al electrode.
coordinates of the active spots for absorption and carrier generation along with the carrier transport pathways. We present results from this combination of microscopy techniques obtained by studies on Si-PCPDTBT:PC71BM BHJ based devices (Figure 1). This D:A system which has a propensity to form sizable crystalline phases, where the processing conditions can control the grain size, is an ideal model system for the proposed scheme of microscopic technique.27,28 We study the evolution of morphology as a function of different blend ratios and the varying degree of thermal annealing treatment. Simultaneous mapping of topography, transmission NSOM contrast (T-NSOM) and near field short circuit photocurrent (NPC) of preannealed and postannealed devices demonstrate the nanoscale spatial evolution of local phase and effective photocarrier generation region. The utility of power spectral density (PSD) analysis in following the trends and extracting length scales is also demonstrated.
2. EXPERIMENTAL SECTION The photovoltaic devices used for this work were fabricated as follows: a film of PEDOT:PPS (Baytron P) was spin coated onto a precleaned Indium tin oxide (ITO) coated glass or coverslip and dried at 110ο C to get a thickness of about 30 nm. Then the active polymer blend (with D:A ∼1:1, 1:2 and 1:3) of thickness of about 200 nm was deposited by spin coating from chlorobenzene solution at 1200 rpm on top of the PEDOT:PSS film under ambient condition. For electrical characterization, photovoltaic devices were completed by vacuum evaporating (106 mbar) a 40 nm Al electrode using shadow metal mask. Lower evaporation rate (5 Å/sec) was used to ensure the uniform and good contact, and less void spaces in top electrode coating. IV characterization was carried out in airtight chamber (103 mbar) using xenon lamp with UV cutoff filter.
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Near field scanning measurements were performed with Nanonics Multiview 4000 system and the experimental setup shown in the Figure 1a. Images were collected with tapping mode configuration by raster scanning the NSOM probe near the Al electrode edge (∼10 μm) and chopping the laser beam at a frequency ∼1 kHz in combination with far-field collection with 60 objective, NA ≈ 0.95 and photomultiplier tube (PMT). The device current was always measured in short circuit conditions using a lock-in amplifier (SRS 830) with the output being fed into the auxiliary port of the Nanonics interface system which allows to covert analog short circuit current signal into a digital voltage signal that can be stored. Overlapping of photocurrent line profiles in both trace and retrace scan direction, indicates that the blend films did not exhibit any photodegradation due to the incident light beam. 2D Fourier transform was applied in the analysis of AFM, T-NSOM, and NPC images.29,30 The taper window function was utilized to reduce the edge effects and to minimize the spectral leakage on all of the images before calculating PSD. In the present analysis, the average 2D power spectral density was evaluated by using commercial WSxM software.31 The computation for the 2D average discrete PSD function can be express as follows: ! 1 N N 2 2 2πiΔLðkm x þ kn yÞ Imn e ðΔLÞ g PSD2D ðkx , kx Þ ¼ 2 f L m¼1 n¼1
∑ ∑
ð1Þ Here Imn represents the profile information contrast (height, current or transmitted light intensity) for (m, n)th pixel with scan surface area (L2) at sampling dimension ΔL (L/N) and ki represents the spatial frequency along i direction. Finally, average angular power spectral density was calculated which is represented as the radial frequencies k0 (= k/2π) to generate radial 1D PSD as follows: Z 1 2π PSDðk0 , θÞdθ ð2Þ PSD2D ðk0 Þ ¼ 2π 0 AFM and NSOM scans were carried out for different films at 15 15 μm, 10 10 μm, 5 5 μm area with 512 512 data points with ∼10 nm (5 μm/512) sampling rate. The minimum spatial frequency corresponds to kmin = 1/15 μm = 0.066 μm1 and spatial frequency resolution is limited by the scan sampling size kmax = 1/10 nm =100 μm1. Those form the lower and upper bandwidths of limitation of our PSD plots. Further, PSD spectra obtained upon Fourier analysis of images recorded upon illuminating different areas away from the Al electrode resulted in a similar PSD distribution indicating that the phase separation is quite consistent over a large device area.
3. RESULTS AND DISCUSSION Photovoltaic devices were fabricated on ITO coated coverslip (thickness ≈ 70 μm) to minimize wave-guiding and scattering effects upon local illumination. Simulation of beam propagation and evanescent wave interaction with blend film was examined using FDTD analysis which reveals the spatial resolution ∼80 nm (Supporting Information). The process-optimized devices (1:3, annealed at 110 °C for 15 min) showed the following characteristics: PCE ≈ 4.1%, JSC ≈ 18 mA cm2, FF ≈ 0.45 and VOC = 0.63 V at 1.3 Sun. AFM topography images of the region in the vicinity of the Al electrode (Figure 2a), depicts 17185
dx.doi.org/10.1021/jp205065q |J. Phys. Chem. C 2011, 115, 17184–17189
The Journal of Physical Chemistry C
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Figure 2. AFM and NSOM images of pre and post annealed 1:2 Si-PCPDTBT:PC71BM films. (a) Preannealed topography and (b) post annealed topography. Parts (c),(d) transmission contrast map for pre and post annealed film, respectively. Near field photocurrent (NPC) contrasts from same regions are shown in (e) and (f), respectively.
the coexistence of two different morphological length scales— one with mesoscale features (∼500 nm to 1 μm) and other with nanoscale features (
