@article{AlqahtaniBabicsGorenflotetal.2018,
  author    = {Alqahtani, Obaid and Babics, Maxime and Gorenflot, Julien and Savikhin, Victoria and Ferron, Thomas and Balawi, Ahmed H. and Paulke, Andreas and Kan, Zhipeng and Pope, Michael and Clulow, Andrew J. and Wolf, Jannic and Burn, Paul L. and Gentle, Ian R. and Neher, Dieter and Toney, Michael F. and Laquai, Frederic and Beaujuge, Pierre M. and Collins, Brian A.},
  title     = {Mixed Domains Enhance Charge Generation and Extraction in Bulk-Heterojunction Solar Cells with Small-Molecule Donors},
  series = {Advanced energy materials},
  volume    = {8},
  journal   = {Advanced energy materials},
  number    = {19},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.201702941},
  pages     = {16},
  year      = {2018},
  abstract  = {The interplay between nanomorphology and efficiency of polymer-fullerene bulk-heterojunction (BHJ) solar cells has been the subject of intense research, but the generality of these concepts for small-molecule (SM) BHJs remains unclear. Here, the relation between performance; charge generation, recombination, and extraction dynamics; and nanomorphology achievable with two SM donors benzo[1,2-b:4,5-b]dithiophene-pyrido[3,4-b]-pyrazine BDT(PPTh2)(2), namely SM1 and SM2, differing by their side-chains, are examined as a function of solution additive composition. The results show that the additive 1,8-diiodooctane acts as a plasticizer in the blends, increases domain size, and promotes ordering/crystallinity. Surprisingly, the system with high domain purity (SM1) exhibits both poor exciton harvesting and severe charge trapping, alleviated only slightly with increased crystallinity. In contrast, the system consisting of mixed domains and lower crystallinity (SM2) shows both excellent exciton harvesting and low charge recombination losses. Importantly, the onset of large, pure crystallites in the latter (SM2) system reduces efficiency, pointing to possible differences in the ideal morphologies for SM-based BHJ solar cells compared with polymer-fullerene devices. In polymer-based systems, tie chains between pure polymer crystals establish a continuous charge transport network, whereas SM-based active layers may in some cases require mixed domains that enable both aggregation and charge percolation to the electrodes.},
  language  = {en}
}
@article{CaprioglioStolterfohtWolffetal.2019,
  author    = {Caprioglio, Pietro and Stolterfoht, Martin and Wolff, Christian Michael and Unold, Thomas and Rech, Bernd and Albrecht, Steve and Neher, Dieter},
  title     = {On the relation between the open-circuit voltage and quasi-fermi level splitting in efficient perovskite solar cells},
  series = {advanced energy materials},
  volume    = {9},
  journal   = {advanced energy materials},
  number    = {33},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1614-6832},
  doi       = {10.1002/aenm.201901631},
  pages     = {10},
  year      = {2019},
  abstract  = {Today's perovskite solar cells (PSCs) are limited mainly by their open-circuit voltage (VOC) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity-dependent measurements of the quasi-Fermi level splitting (QFLS) and of the VOC on the very same devices, including pin-type PSCs with efficiencies above 20\%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the VOC is generally lower than the QFLS, violating one main assumption of the Shockley-Queisser theory. This has far-reaching implications for the applicability of some well-established techniques, which use the VOC as a measure of the carrier densities in the absorber. By performing drift-diffusion simulations, the intensity dependence of the QFLS, the QFLS-VOC offset and the ideality factor are consistently explained by trap-assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the VOC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the VOC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS-VOC relation is of great importance.},
  language  = {en}
}
@misc{CaprioglioStolterfohtWolffetal.2019,
  author    = {Caprioglio, Pietro and Stolterfoht, Martin and Wolff, Christian Michael and Unold, Thomas and Rech, Bernd and Albrecht, Steve and Neher, Dieter},
  title     = {On the relation between the open-circuit voltage and quasi-Fermi level splitting in efficient perovskite solar cells},
  series = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe},
  number    = {774},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-43759},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-437595},
  pages     = {10},
  year      = {2019},
  abstract  = {Today's perovskite solar cells (PSCs) are limited mainly by their open-circuit voltage (VOC) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity-dependent measurements of the quasi-Fermi level splitting (QFLS) and of the VOC on the very same devices, including pin-type PSCs with efficiencies above 20\%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the VOC is generally lower than the QFLS, violating one main assumption of the Shockley-Queisser theory. This has far-reaching implications for the applicability of some well-established techniques, which use the VOC as a measure of the carrier densities in the absorber. By performing drift-diffusion simulations, the intensity dependence of the QFLS, the QFLS-VOC offset and the ideality factor are consistently explained by trap-assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the VOC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the VOC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS-VOC relation is of great importance.},
  language  = {en}
}
@article{ProctorKimNeheretal.2013,
  author    = {Proctor, Christopher M. and Kim, Chunki and Neher, Dieter and Thuc-Quyen Nguyen,},
  title     = {Nongeminate recombination and charge transport limitations in diketopyrrolopyrrole-based solution-processed small molecule solar cells},
  series = {Advanced functional materials},
  volume    = {23},
  journal   = {Advanced functional materials},
  number    = {28},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1616-301X},
  doi       = {10.1002/adfm.201202643},
  pages     = {3584 -- 3594},
  year      = {2013},
  abstract  = {Charge transport and nongeminate recombination are investigated in two solution-processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)-based donor molecules, mono-DPP and bis-DPP, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PCBM). While the bis-DPP system exhibits a high fill factor (62\%) the mono-DPP system suffers from pronounced voltage dependent losses, which limit both the fill factor (46\%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current-voltage characteristics indicate that the mono-DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most significant factor leading to the difference in fill factor is the comparatively poor hole transport properties in the mono-DPP system (2 x 10(-5) cm(2) V-1 s(-1) versus 34 x 10(-5) cm(2) V-1 s(-1)). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination.},
  language  = {en}
}