Transport & Morphology - Effect on Nongeminate Recombination in Organic Solar Cells (TEMET NOSCE)
Final Report Abstract
We have investigated the polymer-NFA system PBDB-T:ITIC in standard and inverted architecture with respect to morphology and recombination dynamics, in order to discover the link between an active layer‘s microstructure and charge carrier recombination, and its impact on the overall device performance. To vary the morphology, three different solvents, chlorobenzene, dichlorobenzene and chloroform, were used without additives. We applied grazing-incidence X-ray techniques to gain information on domain sizes and distances as well as the degree of domain crystallinity and crystal orientation. Steady-state electrical measurements as well as electro-optical pump–probe experiments on devices were used to gain insights into the dominating recombination and charge extraction processes. The isolated domain sizes in films made from solutions with different solvents were found to show small, mid-range and large domains. While small and mid-range domains were of comparable size for all solvents investigated, the large domains were found to be larger for a volatile solvent such as chloroform. The same yields for the inter-domain distance, where the largest distances were found in chloroformbased films. Also, the overall degree of PBDB-T crystallinity is found to be negatively affected by solvent volatility. Although ITIC crystallinity has been found to be largest for chloroform-based samples in standard architecture, the corresponding device is the least performing of all samples investigated, indicating ITIC crystallinity to not be a determining factor for device performance at all. JV measurements showed inverted devices made from DCB and CF solutions performing comparably well, mainly suffering from reduced short-circuit current compared to CB; this loss was probably caused by singlet exciton losses due to larger domain sizes. The same holds for standard architecture samples from DCB, whereas the device from CF suffers from a heavily reduced fill factor. We found this FF reduction to be caused by the transport resistance being almost one order of magnitude higher in the CF device compared to CF, which we see caused by smaller degree of PBDB-T crystallinity. Again, we point out that the loss mechanism of the voltage drop due to a high transport resistance, caused by a low active layer conductivity, is more prominent in its negative impact on the performance than has been formerly appreciated. As in both sample architectures the chlorobenzene-based devices exhibited the highest PCE, it can be concluded that the maximum domain size in conjunction with high PBDB-T domain crystallinity has the most positive impact on device performance. The importance of minimal transport resistance for achieving high device performance is further underlined by our investigation of devices with the active material PM6:Y6. Under thermal stress, these samples have been found to also show reduced fill factors. We found this to be again caused by an increased transport resistance being caused by an increased trap density. Our findings show that morphology optimization is necessary in order to overcome limitations by small charge carrier mobility, leading to an increased transport resistance even in state-of-the-art highperformance systems like PM6:Y6. This optimization should include limiting the size of the largest occurring domains as well as forcing domain crystallinity. Our results further indicate that, by maximizing crystallinity, the non-radiative contribution to the overall non-geminate recombination could be limited, promoting radiative recombination as the dominant mechanism.
Publications
- Charge Carrier Concentration Dependence of Encounter- Limited Bimolecular Recombination in Phase-Separated Organic Semiconductor Blends. Phys. Rev. B 93, 205204 (2016)
M. C. Heiber, T.-Q. Nguyen, and C. Deibel
(See online at https://doi.org/10.1103/PhysRevB.93.205204) - Impact of Tortuosity on Charge-Carrier Transport in Organic Bulk Heterojunction Blends. Phys. Rev. Applied 8, 054043 (2017)
M. C. Heiber, K. Kister, A. Baumann, V. Dyakonov, C. Deibel, and T.-Q. Nguyen
(See online at https://doi.org/10.1103/PhysRevApplied.8.054043) - Effect of solvent additives on the morphology and device performance of printed non-fullerene acceptor based organic solar cells. ACS Appl. Mater. Interfaces 11, 42313-42321 (2019)
K.S.Wienhold, V.Körstgens, S.Grott, X.Jiang, M.Schwartzkopf, S.V.Roth, P.Müller-Buschbaum
(See online at https://doi.org/10.1021/acsami.9b16784) - In situ printing: insights into the morphology formation and optical property evolution of slot-die coated active layers containing low band gap polymer donor and non-fullerene small molecule acceptor. Solar RRL 4, 2000086 (2020)
K.S.Wienhold, V.Körstgens, S.Grott, X.Jiang, M.Schwartzkopf, S.V.Roth, P.Müller-Buschbaum
(See online at https://doi.org/10.1002/solr.202000086) - In situ studies of solvent additive effects on the morphology development during printing of bulk heterojunction films for organic solar cells. Small Methods 4, 2000418 (2020)
D.Yang, S.Grott, X.Jiang, K.S.Wienhold, M.Schwartzkopf, S.V.Roth, P.Müller-Buschbaum
(See online at https://doi.org/10.1002/smtd.202000418) - Tailoring morphology compatibility and device stability by adding PBDTTPD-COOH as third component to fullerene-based polymer solar cells. ACS Appl. Energy Mater. 3, 2604-2613 (2020)
D.Yang, B.Cao, N.Saxena, N.Li, C.Bilko, S.Grott, W.Chen, X.Jiang, J.E.Heger, S.Bernstorff, P.Müller-Buschbaum
(See online at https://doi.org/10.1021/acsaem.9b02290) - Charge Photogeneration in Non-Fullerene Organic Solar Cells: Influence of Excess Energy and Electrostatic Interactions. Adv. Funct. Mater. 31, 2007479 (2021)
M. Saladina, P. Simón Marqués, A. Markina, S. Karuthedath, C. Wöpke, C. Göhler, Y. Chen, M. Allain, P. Blanchard, C. Cabanetos, D. Andrienko, F. Laquai, J. Gorenflot, and C. Deibel
(See online at https://doi.org/10.1002/adfm.202007479)
