@article{TokmoldinHosseiniRaoufietal.2020,
  author    = {Tokmoldin, Nurlan and Hosseini, Seyed Mehrdad and Raoufi, Meysam and Phuong, Le Quang and Sandberg, Oskar J. and Guan, Huilan and Zou, Yingping and Neher, Dieter and Shoaee, Safa},
  title     = {Extraordinarily long diffusion length in PM6:Y6 organic solar cells},
  series = {Journal of materials chemistry : A, materials for energy and sustainability},
  volume    = {8},
  journal   = {Journal of materials chemistry : A, materials for energy and sustainability},
  number    = {16},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {2050-7488},
  doi       = {10.1039/d0ta03016c},
  pages     = {7854 -- 7860},
  year      = {2020},
  abstract  = {The PM6:Y6 bulk-heterojunction (BHJ) blend system achieves high short-circuit current (J(SC)) values in thick photovoltaic junctions. Here we analyse these solar cells to understand the observed independence of the short-circuit current upon photoactive layer thickness. We employ a range of optoelectronic measurements and analyses, including Mott-Schottky analysis, CELIV, photoinduced absorption spectroscopy, mobility measurements and simulations, to conclude that, the invariant photocurrent for the devices with different active layer thicknesses is associated with the Y6's diffusion length exceeding 300 nm in case of a 300 nm thick cell. This is despite unintentional doping that occurs in PM6 and the associated space-charge effect, which is expected to be even more profound upon photogeneration. This extraordinarily long diffusion length - which is an order of magnitude larger than typical values for organics - dominates transport in the flat-band region of thick junctions. Our work suggests that the performance of the doped PM6:Y6 organic solar cells resembles that of inorganic devices with diffusion transport playing a pivotal role. Ultimately, this is expected to be a key requirement for the fabrication of efficient, high-photocurrent, thick organic solar cells.},
  language  = {en}
}
@article{ZeiskeSandbergZarrabietal.2022,
  author    = {Zeiske, Stefan and Sandberg, Oskar J. and Zarrabi, Nasim and Wolff, Christian Michael and Raoufi, Meysam and Pe{\~n}a-Camargo, Francisco and Gutierrez-Partida, Emilio and Meredith, Paul and Stolterfoht, Martin and Armin, Ardalan},
  title     = {Static disorder in lead halide perovskites},
  series = {The journal of physical chemistry letters},
  volume    = {13},
  journal   = {The journal of physical chemistry letters},
  number    = {31},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1948-7185},
  doi       = {10.1021/acs.jpclett.2c01652},
  pages     = {7280 -- 7285},
  year      = {2022},
  abstract  = {In crystalline and amorphous semiconductors, the temperature-dependent Urbach energy can be determined from the inverse slope of the logarithm of the absorption spectrum and reflects the static and dynamic energetic disorder. Using recent advances in the sensitivity of photocurrent spectroscopy methods, we elucidate the temperature-dependent Urbach energy in lead halide perovskites containing different numbers of cation components. We find Urbach energies at room temperature to be 13.0 +/- 1.0, 13.2 +/- 1.0, and 13.5 +/- 1.0 meV for single, double, and triple cation perovskite. Static, temperature-independent contributions to the Urbach energy are found to be as low as 5.1 ?+/- 0.5, 4.7 +/- 0.3, and 3.3 +/- 0.9 meV for the same systems. Our results suggest that, at a low temperature, the dominant static disorder in perovskites is derived from zero-point phonon energy rather than structural disorder. This is unusual for solution-processed semiconductors but broadens the potential application of perovskites further to quantum electronics and devices.},
  language  = {en}
}
@article{ZeiskeSandbergKurpiersetal.2022,
  author    = {Zeiske, Stefan and Sandberg, Oskar J. and Kurpiers, Jona and Shoaee, Safa and Meredith, Paul and Armin, Ardalan},
  title     = {Probing charge generation efficiency in thin-film solar cells by integral-mode transient charge extraction},
  series = {ACS photonics},
  volume    = {9},
  journal   = {ACS photonics},
  number    = {4},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {2330-4022},
  doi       = {10.1021/acsphotonics.1c01532},
  pages     = {1188 -- 1195},
  year      = {2022},
  abstract  = {The photogeneration of free charges in light-harvesting devices is a multistep process, which can be challenging to probe due to the complexity of contributing energetic states and the competitive character of different driving mechanisms. In this contribution, we advance a technique, integral-mode transient charge extraction (ITCE), to probe these processes in thin-film solar cells. ITCE combines capacitance measurements with the integral-mode time-of-flight method in the low intensity regime of sandwich-type thin-film devices and allows for the sensitive determination of photogenerated charge-carrier densities. We verify the theoretical framework of our method by drift-diffusion simulations and demonstrate the applicability of ITCE to organic and perovskite semiconductor-based thin-film solar cells. Furthermore, we examine the field dependence of charge generation efficiency and find our ITCE results to be in excellent agreement with those obtained via time-delayed collection field measurements conducted on the same devices.},
  language  = {en}
}
@article{SandbergKurpiersStolterfohtetal.2020,
  author    = {Sandberg, Oskar J. and Kurpiers, Jona and Stolterfoht, Martin and Neher, Dieter and Meredith, Paul and Shoaee, Safa and Armin, Ardalan},
  title     = {On the question of the need for a built-in potential in Perovskite solar cells},
  series = {Advanced materials interfaces},
  volume    = {7},
  journal   = {Advanced materials interfaces},
  number    = {10},
  publisher = {Wiley},
  address   = {Hoboken},
  issn      = {2196-7350},
  doi       = {10.1002/admi.202000041},
  pages     = {8},
  year      = {2020},
  abstract  = {Perovskite semiconductors as the active materials in efficient solar cells exhibit free carrier diffusion lengths on the order of microns at low illumination fluxes and many hundreds of nanometers under 1 sun conditions. These lengthscales are significantly larger than typical junction thicknesses, and thus the carrier transport and charge collection should be expected to be diffusion controlled. A consensus along these lines is emerging in the field. However, the question as to whether the built-in potential plays any role is still of matter of some conjecture. This important question using phase-sensitive photocurrent measurements and theoretical device simulations based upon the drift-diffusion framework is addressed. In particular, the role of the built-in electric field and charge-selective transport layers in state-of-the-art p-i-n perovskite solar cells comparing experimental findings and simulation predictions is probed. It is found that while charge collection in the junction does not require a drift field per se, a built-in potential is still needed to avoid the formation of reverse electric fields inside the active layer, and to ensure efficient extraction through the charge transport layers.},
  language  = {en}
}
@article{PhuongHosseiniSandbergetal.2020,
  author    = {Phuong, Le Quang and Hosseini, Seyed Mehrdad and Sandberg, Oskar J. and Zou, Yingping and Woo, Han Young and Neher, Dieter and Shoaee, Safa},
  title     = {Quantifying quasi-fermi level splitting and open-circuit voltage losses in highly efficient nonfullerene organic solar cells},
  series = {Solar RRL},
  volume    = {5},
  journal   = {Solar RRL},
  number    = {1},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {2367-198X},
  doi       = {10.1002/solr.202000649},
  pages     = {6},
  year      = {2020},
  abstract  = {The power conversion efficiency (PCE) of state-of-the-art organic solar cells is still limited by significant open-circuit voltage (V-OC) losses, partly due to the excitonic nature of organic materials and partly due to ill-designed architectures. Thus, quantifying different contributions of the V-OC losses is of importance to enable further improvements in the performance of organic solar cells. Herein, the spectroscopic and semiconductor device physics approaches are combined to identify and quantify losses from surface recombination and bulk recombination. Several state-of-the-art systems that demonstrate different V-OC losses in their performance are presented. By evaluating the quasi-Fermi level splitting (QFLS) and the V-OC as a function of the excitation fluence in nonfullerene-based PM6:Y6, PM6:Y11, and fullerene-based PPDT2FBT:PCBM devices with different architectures, the voltage losses due to different recombination processes occurring in the active layers, the transport layers, and at the interfaces are assessed. It is found that surface recombination at interfaces in the studied solar cells is negligible, and thus, suppressing the non-radiative recombination in the active layers is the key factor to enhance the PCE of these devices. This study provides a universal tool to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.},
  language  = {en}
}