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Solar & Alternative Energy

A speedier way to evaluate organic photovoltaics

Time-resolved microwave conductivity promises to be a faster, easier evaluation method for organic solar cells that could help engineers screen optoelectronics without first building them into a device.
12 December 2011, SPIE Newsroom. DOI: 10.1117/2.1201111.003967

Researchers of organic photovoltaic cells (OPVcs) focus great attention on producing a low-cost, bendable, large-area device that can be fabricated via a roll-to-roll process.1, 2 The great flexibility inherent in organic materials makes them promising candidates for such applications. But their widespread use has been hindered by low power conversion efficiencies (PCEs) and instability over long operation times in comparison with their inorganic counterparts. PCE in particular is considerably affected by many factors, such as the photoabsorption spectrum (optical band gap), the highest occupied and lowest unoccupied molecular orbitals of the electron donor and acceptor molecules, exciton formation and diffusion, and charge separation and transport. The complicated interplay of parameters intrinsic to the material or device (e.g., electrode work function, interface barrier, and buffer layer) and extrinsic (e.g., impurities) makes evaluation and optimization of an OPVc quite time-consuming.

There are potential alternatives to building a new device each time a researcher needs to characterize a new OPVc. Several non-contact techniques have been proposed, such as transient absorption spectroscopy,3 terahertz time-domain spectroscopy,4 and flash-photolysis time-resolved microwave conductivity (FP-TRMC).5 But transient absorption spectroscopy can detect only the density of charge carriers in an OPVc, not their velocity. And terahertz time-domain spectroscopy uses wavelengths too short to detect subtle photoconductivity signals in OPVcs.

Figure 1. Left: Organic photovoltaic cell (OPVc) device with current-voltage (J-V) curve. Center: Chemical structures of methanofullerene (PCBM) (bottom) and poly(3-hexylthiophene) (P3HT), where S is sulfur, R indicates a hydrocarbon, and n/2 means the structure shown is half of the polymer's repeated unit (top). Right: Flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurement with transient photoconductivity signal.

Figure 2. Correlation between device efficiency (PCE) and FP-TRMC maximum (φΣμmax) in P3HT:PCBM = 1:1. oDCB: Ortho-dichlorobenzene. CB: Chlorobenzene. CF: Chloroform. (Reproduced with permission of Wiley-Vch.5)

Figure 3. Impurity effect in P3HT:PCBM =1:1 films. (a) J-V curves of the devices. (b) FP-TRMC transients. (Reproduced with permission of Wiley-Vch.5)

FP-TRMC uses microwaves to excite charge carriers (electrons and ‘holes,’ e.g., spaces where electrons might fill in) in an OPVc. When the charge carriers vibrate, they discharge some of the microwave energy. That loss can be detected in the microwave signal and reveals both the density and velocity of the charge carriers. It is sensitive enough to detect even subtle signals. We tested FP-TRMC and found a good correlation between the photovoltaic performance of an OPVc and the dynamics of transient photoconductivity (see Figure 1).

We examined a typical bulk heterojunction (BHJ) film consisting of poly(3-hexylthiophene) (P3HT) and methanofullerene (PCBM) blended in a 1:1 ratio. As seen in Figure 2, the transient photoconductivity maxima of the BHJ film (horizontal axis, φΣμmax, where φ is the photo charge carrier generation efficiency and Σμ is the sum of the hole and electron mobilities) were drastically changed by the thermal process and solvent. Notice that the PCE of the corresponding OPVc devices (vertical axis) showed a good correlation with φΣμmax. This clearly demonstrates that the electrode-less FP-TRMC technique is an extremely versatile evaluation tool of BHJ layer and process optimization.

We also note that FP-TRMC transients are relatively unaffected by chemical impurities. Figure 3(a) shows the current-voltage curves of the solar cells in the presence of 0, 0.2, 1, and 5wt% Pd complex, tetrakis(triphenylphosphine)palladium(0), also written as Pd(PPh3)4. It is a typical catalyst of Suzuki polycondensation. Upon increasing Pd concentration, PCE decreased to about the half of the original value. However, FP-TRMC transients shown in Figure 3(b) were not significantly changed. Vigorous purification of synthesized polymers is indispensable for improving device performance. In contrast, FP-TRMC experiments do not need such laborious purification, leading to the stable and speedy evaluation of BHJ films.

We found the minimum charge carrier mobility to be 0.22cm2V−1s−1 in P3HT:PCBM=1 : 1 film along with 3.26% PCE. The mobilities proved by FP-TRMC were found to scale with the PCE, suggesting that local charge carrier motion has an impact on overall device performance. We expect the good correlation between FP-TRMC and PCE will make FP-TRMC an easy screening method for surveying the potential of optoelectronic materials. We hope to apply this technique in the future to low-bandgap materials being developed all over the world.

Akinori Saeki
Osaka University
Yamadaoka, Japan

Akinori Saeki received a DEng in applied chemistry from Osaka University (2007), and held the position of assistant professor at the Institute of Scientific and Industrial Research, Osaka University, from 2003 to 2009. He has been an assistant professor (tenure track) of the Graduate School of Engineering at Osaka University since 2010.

1. J. Peet, A. J. Heeger, G. C. Bazan, ‘Plastic’ solar cells: self-assembly of bulk heterojunction nanomaterials by spontaneous phase separation, Acc. Chem. Res. 42, no. 11, pp. 1700-1708, 2009. doi:10.1021/ar900065j
2. Y. Liang, L. Yu, A new class of semiconducting polymers for bulk heterojunction solar cells with exceptionally high performance, Acc. Chem. Res. 43, no. 9, pp. 1227-1236, 2010. doi:10.1021/ar1000296
3. T. M. Clarke, A. Ballantyne, S. Shoaee, Y. W. Soon, W. Duffy, M. Heeney, I. McCulloch, J. Nelson, J. R. Durrant, Analysis of charge photogeneration as a key determinant of photocurrent density in polymer:fullerene solar cells, Adv. Mater. 22, no. 46, pp. 5287-5291, 2010. doi:10.1002/adma.201002357
4. P. Parkinson, J. Lloyd-Hughes, M. B. Johnston, L. M. Herz, Efficient generation of charges via below-gap photoexcitation of polymer-fullerene blend films investigated by terahertz spectroscopy, Phys. Rev. B 78, no. 11, pp. 115321, 2008. doi:10.1103/PhysRevB.78.115321
5. A. Saeki, M. Tsuji, S. Seki, Direct evaluation of intrinsic optoelectronic performance of organic photovoltaic cells with minimizing impurity and degradation effects, Adv. Energy Mater. 1, no. 4, pp. 661-669, 2011. doi:10.1002/aenm.201100143