Limits on the Fill Factor in Organic Photovoltaics: Distinguishing

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Letter

Limits on Fill Factor in Organic Photovoltaics: Distinguishing Non-geminate and Geminate Recombination Mechanisms George F A Dibb, Fiona Carol Jamieson, Andrea Maurano, Jenny Nelson, and James R. Durrant J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz400140p • Publication Date (Web): 05 Feb 2013 Downloaded from http://pubs.acs.org on February 19, 2013

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The Journal of Physical Chemistry Letters

Limits on Fill Factor in Organic Photovoltaics: Distinguishing Nongeminate and Geminate Recombination Mechanisms George F.A. Dibb,a,b Fiona C. Jamieson,a Andrea Maurano,a Jenny Nelsonb* & James R. Durranta* Centre for Plastic Electronics and Departments of Chemistrya and Physicsb, Imperial College London, Exhibition Road, London, SW7 2AZ, UK. *Corresponding authors: Email: [email protected], [email protected]

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Abstract In this letter we present transient opto-electronic experimental studies of the recombination processes limiting the fill factor (FF) in three conjugated polymer:fullerene systems: poly(3hexylthiophene) (P3HT) and two lower bandgap polymers which exhibit lower FFs poly[2,6(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b’]dithiophene)-alt-4,7-(2,1,3benzothiadiazole) (PCPDTBT) and poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl2′,1′,3′-benzothiadiazole)) (APFO-3). Using transient absorption spectroscopy, charge extraction and transient photovoltage experiments, we show that lower FF observed for the PCPDTBT based device results from enhanced non-geminate recombination even at short circuit, In contrast we show that for APFO-3 devices, the FF is primarily limited by a voltage-dependent free charge generation which we assign to a geminate recombination process. ToC Figure

Key words Organic solar cells, recombination, fill factor, bulk heterojunction, non-geminate


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Manuscript Text Polymer:fullerene bulk heterojunction (BHJ) solar cells have recently achieved relatively high record power conversion efficiencies (PCE). 1 Improved efficiency has mainly been achieved through the use of low bandgap polymers in order to match the absorption spectrum of the material with the solar spectrum and so increase the short circuit current density (JSC),2,3 or by utilising materials with a higher ionisation potential and so increase the open circuit voltage (VOC). Whilst many such new materials do achieve higher JSC and/or VOC values, the resulting solar cell efficiency is often disappointing as the devices exhibit relatively modest FFs.4-6 Indeed even good organic solar cells exhibit considerably lower fill factors than the best inorganic devices; 0.6-0.7 compared to ~0.85.7 Various mechanisms have been proposed to explain the limited FF of organic solar cells, such as charge collection losses, field-dependent charge generation and diffusion limited transport, however there is no general consensus on this issue. 8-16 This paper is concerned with addressing the origins of the lower fill factors typically observed. We consider three representative material systems and address the origin of the low fill factor in each case. As a benchmark material system for our study we consider poly(3-hexylthiophene):[6,6]phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends as solar cells from this material have been widely studied, with reported device efficiencies of up to 5%.17 P3HT:PCBM devices often exhibit good fill factors, typically between 0.55 - 0.65,17,18 and transient optical and electrical techniques have shown the VOC and FF of reasonably optimised devices to be determined

by

non-geminate

recombination

with

voltage

independent

charge

generation.9,14,19-21 In P3HT devices, the non-geminate recombination flux around short circuit is very low, thus the J-V curve is very flat through short circuit conditions resulting in a high FF.19 The other two polymers studied herein are low bandgap polymers, poly[2,6-(4,43 ACS Paragon Plus Environment


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bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b’]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole) (PCPDTBT)

and

poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-

benzothiadiazole)) (APFO-3), which, when blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) have achieved device efficiencies exceeding 5% 4 and 3.5%6 respectively. However, whilst APFO-3:PC71BM devices achieve higher VOCs and PCPDTBT:PC71BM devices achieve increased JSC values compared to P3HT:PCBM devices, both these lower bandgap BHJ solar cells exhibit relatively modest FFs, which limit device performance.4-6,15 In particular both PCPDTBT and APFO-3 based devices show a slope in the J-V curves around short circuit which complicates analysis. As photon absorption is voltage independent, this slope must be caused by additional voltage dependent loss processes at short circuit; the origin of these losses remains a contentious topic, and has been attributed to many different effects.8-13,16 Voltage dependent losses at short circuit can be assigned to enhanced non-geminate recombination (due to space-charge limitations8,22 and/or poor charge transport9), or possibly a new loss pathway (such as a photoshunt or contact selectivity12,13). Another possible mechanism is a field-dependent charge generation, usually assigned to a voltage dependent geminate recombination process. 10,11,15,16,23 Studies have recently reported voltage dependent charge photogeneration in BHJ solar cells, 10,11 and discussed the impact of this upon the device FF. Transient absorption studies indicate that on the microsecond24 timescale the charge generation yield in representative P3HT:PCBM and PCPDTBT:PC71BM + octanedithiol (ODT) (where ODT is a processing additive) devices is independent of the voltage applied across the cell and thus the carrier generation is field independent.25,26 As such, there are significant variations in the literature over the factors limiting the FF of OPV devices made from these three donor polymers. In this study we address this issue by using both transient optical and opto-electronic measurements to


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quantify the impact of both geminate and non-geminate losses processes on the FF and VOC of solar cells employing these three material systems. Figure 1a shows J-V curves, in the light under simulated 100 mWcm-2 AM 1.5 illumination (solid lines) and in the dark (dashed lines), for PCPDTBT:PC71BM+ODT (purple), APFO3:PC71BM (orange) and P3HT:PCBM (green) devices studied here. Information on device fabrication and architecture are included in the experimental section and the parameters (JSC, VOC and FF) related to these J-V curves are detailed in the Supporting Information (SI). The J-Vs for PCPDTBT:PC71BM+ODT and APFO-3:PC71BM exhibit relatively low FFs of 0.40 and 0.48 respectively, compared to that for P3HT:PCBM, with these trends in FF being typical for devices made from these donor polymers. Both the low bandgap cells exhibit a non-negligible gradient at short circuit indicating that the observed JSC and FF are reduced by loss mechanisms at short circuit. The light intensity dependence of JSC (Figure 1b) deviates clearly from linearity only in the case of PCPDTBT. Whilst the nonlinearity is insufficient to determine the order of the recombination mechanism in solar cells, 27 it indicates that significant charge density dependent losses are occurring at short-circuit at high light intensity in this device,28 in contrast to the P3HT and APFO-3 devices. This is consistent with our previous measurement showing charge density is very low under short-circuit conditions in P3HT:PCBM cells and therefore non-geminate losses at short circuit are negligible.



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Figure 1: a) J-Vs under simulated AM 1.5 illumination of typical PCPDTBT:PC 71BM + ODT (purple), APFO-3:PC71BM (orange) and P3HT:PCBM (green) BHJ solar cells studied herein. b) Normalised linearity of short circuit current density, α, as a function of the light intensity.

In order to quantify the impact of non-geminate recombination upon the device J-V curves, the charge carrier densities, n, and lifetimes, τ(n), at different applied optical and electronic biases were measured using charge extraction and transient photovoltage techniques respectively. As shown previously10,19 the magnitude of the non-geminate recombination current density JNGR can be determined from  J NGR  ed  n  (n )  

Equation 1

where e is the elementary change and d is the device thickness. τ(n) encompasses all nongeminate recombination losses, within the blend layer and at the photoactive layer/electrode interfaces. The average electron density in the photoactive layer in excess of that in the dark under short circuit conditions. It is assumed that n = p as previously.29 As we have discussed previously, variations in τ(n) and therefore JNGR result from differences in charge carrier mobility and film microstructure, although such dependencies are not explicitly analysed in this paper.30


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Figure 2: The measured non-geminate recombination current density, JNGR, under applied bias, VAPP, for PCPDTBT, APFO-3 and P3HT devices. The data show non-geminate recombination at short-circuit for PCPDTBT:PC71BM + ODT solar cell is ~ 3 orders of magnitude larger than in P3HT:PCBM. Dashed vertical lines indicate the open circuit voltage for the respective cells.

Figure 2 shows the non-geminate recombination current density for all devices under applied bias across the J-V curve under 1 sun illumination. The charge density data used in these calculations have been corrected for charges lost to recombination during extraction, as well as for charges stored on the electrodes, whilst the applied voltages have been corrected for series resistance. The linear series resistance is obtained from the dark J-V curves and is attributed to components external to the photoactive layer. The dashed vertical lines in the figure indicate the respective open circuit voltage for each device. All three devices show non-geminate recombination losses at open circuit of similar magnitude to their respective 7 ACS Paragon Plus Environment


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JSC, indicating that the dominant loss pathway limiting VOC in all cases is non-geminate recombination (discussed below). However as the device voltages reduce towards short circuit, there is a qualitative difference in the behaviour of the P3HT and APFO-3 cells relative to the PCPDTBT device. For both the P3HT and APFO-3 devices JNGR reduces strongly with reducing cell bias. In contrast for the PCPDTBT device JNGR reduces only modestly (less than one order of magnitude) with reducing cell bias. As a consequence, the magnitude of JNGR at short-circuit in the PCPDTBT devices is much larger (>1 mAcm-2) than in the P3HT and APFO-3 devices, ~0.01 mAcm-2. This indicates that the JSC for the PCPDTBT device is limited by a non-geminate loss process. The high non-geminate recombination losses for the PCPDTBT at short circuit suggest that these non-geminate losses are likely to contribute to the lower FF observed for this device. However, the low JNGR in the APFO-3 device for applied voltages VAPP≤0.6V indicate that the non-geminate recombination cannot explain the low FFs for the APFO-3:PC71BM device. We therefore address the possibility of voltage dependent geminate recombination losses in this device.

Figure 3: (a) Polaron absorbance at 500ns as a function of voltage normalised relative to that at -4V for APFO-3 and PCPDTBT blends and (b) values of JGEN(V) calculated from measurements of non-geminate recombination and equation 2. 8 ACS Paragon Plus Environment

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Figure 3a shows the yield of charges measured by TAS over a range of applied voltages for this APFO-3 device, compared to the previously published data on PCPDTBT devices.25 Analogous P3HT:PCBM devices to those studied herein have been previously been shown to exhibit voltage independent polaron yields. 26 The kinetic data for the APFO-3 device and discussion of the calculation of the polaron yield is included in the SI. For the P3HT and PCPDTBT devices the yield of polarons is measured at a set time of 500ns to represent the relative yield of long lived charges. The data shown in Figure 3a shows that while charge photogeneration in PCPDTBT devices show relative little dependence upon applied voltage, for the APFO-3 device the yield of charge photogeneration reduces strongly as the device voltage is increased towards open circuit, indicative of field dependent geminate recombination losses. The second method to assay field dependent geminate recombination losses uses the assumption that the free charge generation current density JGEN(V) can be expressed as: J GEN (V )  J (V )  J NGR (n,V )

Equation 2

JGEN(V) can then be determined by subtracting the experimentally derived JNGR(n,V) (Figure 2) from the J-V (Figure 1). The resultant calculated JGEN(V) for all three cells are shown in Figure 3b, for voltages between short and open circuit. It is again apparent that the APFO-3 device shows a significantly larger voltage dependence of charge photogeneration than either the P3HT or PCPDTBT cells. Using equation 2, it is also possible to reconstruct the J-V curve of each device using the experimentally determined values of non-geminate recombination losses, and voltage dependence of charge photogeneration. As described above, in the P3HT and PCPDTBT devices we have determined that the field dependence of geminate recombination is negligible, thus we can approximate JGEN(V) with a constant value of JGEN measured in 9 ACS Paragon Plus Environment


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reverse bias. For the APFO-3, we model JGEN(V) as a linear function fitted to the data in Figure 3a and normalised so that J at 0V matched the experimental JSC.

Figure 4: Experimental J-V curves (solid lines) under simulated AM 1.5 illumination, compared against the calculated J-V data (points) for both PCPDTBT:PC71BM + ODT (purple), APFO-3:PC71BM (orange) and P3HT:PCBM (green) devices, as determined using in Equation 2. Dashed lines show the assumed form of the free charge generation which is voltage-dependent only in the case of APFO-3.

Figure 4 compares the experimentally measured J-V curves (solid lines) with the reconstructed J-V curve (points), for all three materials. The dashed lines indicate the free charge generation current density used for these reconstructions. The predicted FFs of the PCPDTBT and P3HT devices are 0.38 and 0.59 compared to their actual values of 0.40 and 0.53 respectively. This agreement is consistent with, for these devices, the impact of voltagedependent generation being relatively small (< 10 % change over the voltage range studied), with the device FFs being primarily determined by non-geminate recombination. The actual FF of the APFO-3 device measured here was 0.48; this can be compared to the predicted value without voltage dependent generation of 0.79 (see SI figure S4) and the value of the 10 ACS Paragon Plus Environment

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reconstruction in figure 4 taking in account the voltage dependence of charge generation of 0.58. The agreement between the measured and calculated J-V curves is not exact; this discrepancy most likely results from the simplicity of our analysis and experimental techniques, and in particular our assumption that τ depends only upon n and not V, and therefore neglects variations in the spatial distribution of charges between short and open circuit. Nevertheless our analysis is able to reproduce the observed trends in the J-V curves, supporting the validity of our analysis. For the PCPDTBT device, the low device FF can be explained primarily by non-geminate recombination losses, whereas for APFO-3, the low device fill factor appears be dominated by voltage dependent geminate recombination losses. Additionally we note that whilst it is clear that the FF of the APFO-3 device is limited by voltage dependent geminate recombination, its VOC is relatively insensitive to this loss process and is still dominated by non-geminate recombination. This can be understood as resulting from the sharp turn on of non-geminate losses as the charge density in the device increases when cell voltage approaches the band edges (see SI fig S2). The contribution of non-geminate recombination to reduction in the device FF depends on the relative timescales of charge collection and non-geminate recombination. The charge carrier lifetimes measured at the same equilibrium charge densities are similar for the APFO3 and PCPDTBT devices (see SI fig S1). Thus, the low FF of the PCPDTBT resulting from significant non-geminate recombination losses even at short circuit appears to be a consequence primarily of slower charge carrier collection in this device. Our charge extraction data indicates that even at short circuit, there is a significant charge density in the active layer of the device, in contrast to the P3HT and APFO-3 devices (see Figure S2 in Supporting Information), suggesting slower extraction. This slower extraction may result from inferior film microstructure or mobility impacting charge transport and collection. 11 ACS Paragon Plus Environment


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Our conclusion that our studied APFO-3 cell exhibits voltage dependent geminate recombination losses is in agreement with a previous transient optical study by Sundstrom et al.31,32 and modelling by Andersson et. al..33 Additionally our conclusion that for our studied PCPDTBT and P3HT devices, any geminate recombination losses, if present, are relatively voltage independent, and that device FF is primarily limited by non-geminate recombination losses is in agreement with several studies.30,34-38 This different behaviour is observed using the same experimental techniques for all the devices, thereby countering suggestions in the literature that differing conclusions regarding the importance of voltage dependent geminate recombination may derive from different studies employing different experimental techniques.15 We note that Neher and co-workers have recently employed time-delayed collection field measurements on a series of PCPDTBT based solar cells to demonstrate voltage dependent charge photogeneration, with the magnitude of this dependence varying with device processing conditions. 15,23 It is possible that the PCPDTBT device investigated in this letter exhibits a voltage dependent charge generation below the sensitivity of our experiments (

Limits on the Fill Factor in Organic Photovoltaics: Distinguishing

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