Physical interpretation of Mott-Schottky analysis for organic solar cells Lauren Shum, Dr. Adrienne Stiff-Roberts Duke University Department of Electrical & Computer Engineering
What are organic solar cells? Organic materials are not generally considered to be conductive, but the interaction of pi-molecular orbitals in conjugated polymers or small organic molecules gives rise to band structures not unlike the conduction and valence bands more commonly found in semiconductors [1,2]. instead of http://pubs.sciepub.com/rse/2/3/2/ http://inhabitat.com/... http://www.alibaba.com/...
Why organic solar cells? Flexible Cheap Ease of Production [3] Versatile Products [4] Lowenergy Transparent Largearea
Problem & Motivation Mott-Schottky analysis is typical for inorganic solar cells. It tells you built-in voltage (Vbi). Vbi https://openi.nlm.nih.gov/... http://lampx.tugraz.at/~hadley/ss1/problems/ pndepletionapprox/q.php
Problem & Motivation Mott-Schottky analysis is typical for inorganic solar cells. It tells you built-in voltage (Vbi). Vbi https://openi.nlm.nih.gov/... http://lampx.tugraz.at/~hadley/ss1/problems/ pndepletionapprox/q.php
Problem & Motivation People used to use Mott-Schottky analysis to extract Vbi for OSCs. Except it wasn t an accurate measure for Vbi, the potential difference between the contact electrodes. So it was concluded that MS analysis is useless for OSCs.
Problem & Motivation However, the fact that the Mott-Schottky voltage (Vms) doesn t correspond to Vbi doesn t mean Vms does not embody physical meaning. Predicating usefulness entirely on Vbi is a vestige of inorganic device physics, some of which transferred usefully to organics, and some of which didn t.
Problem & Motivation If Vms means something physical, and if we can identify what physicality it embodies, it could mean a way of quantifying a new parameter for device design and optimization. Is Vms physical? If so, what does it mean?
Method Resonant-Infrared Matrix Assisted Pulsed Laser Evaporation (RIR-MAPLE): Polymer dissolved in solvent. Solution emulsified with water. Emulsion is frozen and evaporated onto substrate with laser resonant with OH bonds. Contacts Test Vms extraction for 5 solvents: MB, DMB, TMB, ODCB, TCB Because we know the solvent used in fabrication changes the physical features of the solar cell active layer. Active region Our devices
Method CV measurements for [-2, 2] voltage sweeps 1/C^2 plotted and Vms extracted at voltage intercept Notice, however, that there are 2 slopes that can be used for extraction.
Method Let the computer choose: Using standard deviation as metric for similarity between adjacent slopes, find first instance of decreasing stdev and cut off at first subsequent uptick. Including the slope between points 3 and 4 caused an increase in stdev; therefore the trendline should be drawn between points 1 and 3. Standard deviations of slopes
Results: Vms & solvent families
Results Average of extracted Vms by solvent: Solvent Mean Vms (V) Standard Deviation Sample size MB -0.55460 0.75020 2 DMB -3.83496 0.01698 2 (6)* TMB -4.1919-1 (5)* ODCB -1.70672 0.01726 3 TCB -1.37457 0.28478 3 * The number in parentheses is the number of total reads across all devices. The number beside it is the true sample size, the total number of working devices fabricated using the given solvent
Results Note that when the CV data is averaged before the extraction is performed, a distinct trend appears between the two solvent families. The methylbenzene groups have a lower Vms than the chlorobenzene groups. The solvent used in fabrication affects the surface roughness of the active region. This is a physical parameter. It suggests Vms might be physical, too. Extractions for averaged data 1/C^2 (1/F^2) V (V)
Results: Vms & solar cell parameters
Open shapes = measured: 1. MB, 2. DMB, 3. TMB, 4. ODCB, 5. TCB. Red dots = data from literature [5-7].
Trendlines indicate potential relationship between Vms and other parameters. Note discrepancy in Jsc.
Open shapes = measured: 1. MB, 2. DMB, 3. TMB, 4. ODCB, 5. TCB. Red dots = data from literature [5-7].
No linear trend Trendlines indicate potential relationship between Vms and other parameters. Note discrepancy in PCE.
Results Clearer view on Voc vs. Vms, literature data only. Note seemingly exponential upward trend. Counting MB (1) as an outlier (it had the largest standard deviation of all solvent groups), we also see this upward trend in our measured data. This consistent trend with Voc further suggests Vms physicality.
Results: Interesting inconsistencies All measured data yielded negative Vms. However, all literature reviewed to date reported positive Vms. Jsc and Voc both demonstrated a direct correlation with Vms in measured data, which may account for the direct correlation in measured PCE as well. However, literature data demonstrate a negative correlation between Jsc and Vms. Few papers reported solar cell modeling parameters Rs and Rsh. This is likely because this model is also a vestige of inorganic device physics. Newer models for organics include capacitors [8]. Capacitance models may be a route to understanding Vms, which is derived from a capacitance characterization.
Results: Other evidence in literature Mingebach et al. (2011) [9] Vms (Vcv) and Voc seem linearly varying near room temperature. Qi et al. (2015) [7] Vms changes with temperature. Mingebach et al. (2011) [9] Vms (Vcv) changes with active layer thickness.
Conclusion Vms is reflective of physical phenomena, because it varies with physical parameters like surface roughness, Voc, temperature, active layer thickness, and solvent family in fabrication. Future work Capacitance models for organic electronics may help explain how this physicality is embodied. Determining differences in procedure between our methods and those used in literature studies may explain negative Vms values as well as opposing trends in Jsc. This may further help us understand the physicality of Vms.
Citations 1. Burroughes, J. H., Bradley, D. D. C., Brown, A. R., Marks, R. N., MacKay, K., Friend, R. H., Burns, P. L. & Holmes, A. B. (1990). Light -emitting diodes based on conjugated polymers. N ature, 347, 539-541. 2. Pate, R., McCormick, R., Chen, L., Zhou, W., & Stiff -Roberts, A. (2011). RIR -MAPLE deposition of conjugated polymers for application to optoelectronic devices. Applied Physics A, 1 05, 555-563. 3. Günes, S., Neugebauer, H., & Sariciftci, N. S. (2007). Conjugated Polymer-Based Organic Solar Cells. Chem. Review, 107, 1324 1338. 4. Built-in voltage of organic bulk heterojuction p-i-n solar cells measured by electroabsorption spectroscopy. Siebert-Henze, E. and Lyssenko, V. G. and Fischer, J. and Tietze, M. and Brueckner, R. and Schwarze, M. and Vandewal, K. and Ray, D. and Riede, M. and Leo, K., AIP Advances, 4, 047134 (2014), DOI:http://dx.doi.org/10.1063/1.4873597 5. Kichartz, T. et al. (2012). Sensitivity of the Mott Schottky analysis in organic solar cells. J. Phys. C., 116, 7672 7680. 6. Garcia-Belmonte, G. et al. (2010). Simultaneous determination of carrier lifetime and electron density-of-states in P3HT:PCBM organic solar cells under illumination by impedance spectroscopy, Solar Energy Materials and Solar Cells, 94, 366-375. 7. Qi, B.Y., Zhou, Q., Wang, J.Z. (2015). Exploring the open-circuit voltage of organic solar cells under low temperature. Nature Scientific Reports, 5, 11363. 8. Perrier et al. (2012). Impedance spectrometry of optimized standard and inverted P3HT-PCBM organic solar cells. Solar Energy Materials and Solar Cells, 101, 210-216. 9. Mingebach, M., Deibel, C., & Dyakonov, V. (2011). Built-in potential and validity of Mott Schottky analysis in organic bulk heterojunction solar cells. Phys. Rev. B, 84, 153201.