Explaining the Fill‐Factor and Photocurrent Losses of Nonfullerene Acceptor‐Based Solar Cells by Probing the Long‐Range Charge Carrier Diffusion and Drift Lengths. Issue 22 (5th May 2021)

• Record Type: Journal Article
• Authors: Tokmoldin, Nurlan; Vollbrecht, Joachim; Hosseini, Seyed Mehrdad; Sun, Bowen; Perdigón‐Toro, Lorena; Woo, Han Young; Zou, Yingping; Neher, Dieter; Shoaee, Safa
• Is Part Of: Advanced energy materials. Volume 11:Issue 22(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-05-05
• Subjects: diffusion length -- drift length -- figure of merit -- lifetime‐mobility product -- steady‐state photoconductance; Energy harvesting -- Materials -- Periodicals; Energy conversion -- Materials -- Periodicals; Energy storage -- Materials -- Periodicals; Photovoltaics -- Periodicals; Fuel cells -- Periodicals; Thermoelectric materials -- Periodicals; 621.31
• DOI: 10.1002/aenm.202100804
• Languages: English
• ISSNs: 1614-6832

Abstract

Abstract: Organic solar cells (OSC) nowadays match their inorganic competitors in terms of current production but lag behind with regards to their open‐circuit voltage loss and fill‐factor, with state‐of‐the‐art OSCs rarely displaying fill‐factor of 80% and above. The fill‐factor of transport‐limited solar cells, including organic photovoltaic devices, is affected by material and device‐specific parameters, whose combination is represented in terms of the established figures of merit, such as θ and α. Herein, it is demonstrated that these figures of merit are closely related to the long‐range carrier drift and diffusion lengths. Further, a simple approach is presented to devise these characteristic lengths using steady‐state photoconductance measurements. This yields a straightforward way of determining θ and α in complete cells and under operating conditions. This approach is applied to a variety of photovoltaic devices—including the high efficiency nonfullerene acceptor blends—and show that the diffusion length of the free carriers provides a good correlation with the fill‐factor. It is, finally, concluded that most state‐of‐the‐art organic solar cells exhibit a sufficiently large drift length to guarantee efficient charge extraction at short circuit, but that they still suffer from too small diffusion lengths of photogenerated carriers limiting their fill factor. Abstract : An alternative pathway to the determination of organic solar cell fill‐factor figures of merit, θ and α, expressing them in terms of the effective carrier drift and diffusion lengths is presented. This simplifies their elucidation with the experimental results, covering a range of nonfullerene acceptor‐based devices, demonstrating a strong agreement both with the analytical equations and the drift‐diffusion simulations.