In 2000, Peumans, Bulović and Forrest demonstrated ultrathin (about 100 Å) vacuum-deposited CuPc/PTCBI double-heterostructure organic photovoltaic cells with a bathocuproine exciton-blocking layer (EBL), monochromatic internal quantum efficiency up to 33%±4%, external power conversion efficiency of 2.4%±0.3% in a multiple-pass light-trapping geometry, and short-circuit current linear to ~17 suns. The paper left a coherent, still-unresolved set of issues with a single objective: turning the EBL-based ultrathin architecture into a genuinely efficient, high-intensity, scalable technology. The principal surviving open core is the first issue: the fitted exciton diffusion length LD≈30L_D\approx 30 Å is the main limiter of internal quantum efficiency, and growth or material modifications that reproducibly increase LDL_D in vacuum-deposited donor/acceptor bilayers — toward the projected internal quantum efficiency of ~80% and external efficiency of ~8% — were never demonstrated for this materials system.

Background

Organic photovoltaic (OPV) cells convert light into electricity in thin films of small conjugated molecules. Absorption in such films creates a tightly bound neutral excited state — a Frenkel (molecular) exciton — rather than free charge. The exciton must random-walk, i.e. diffuse, to an interface between an electron donor and an electron acceptor, where it dissociates into an electron and a hole. The mean distance an exciton travels before recombining is the exciton diffusion length LDL_D; in small-molecule films it is of order tens of Ångströms, much shorter than the optical absorption length (hundreds to thousands of Ångströms). A planar donor/acceptor bilayer — CuPc (copper phthalocyanine, the donor) over PTCBI (3,4,9,10-perylenetetracarboxylic-bis-benzimidazole, the acceptor) — therefore faces a tradeoff: thick films absorb light but excitons recombine before reaching the interface, while films thin enough for efficient exciton collection barely absorb. The archetype was the two-layer CuPc/perylene cell of Tang (1986) with about 1% power efficiency. A double heterostructure additionally confines the active layers between functional layers on both sides. The internal quantum efficiency, ηINT\eta_{\text{INT}}, is the ratio of collected electrons to photons absorbed in the photoactive layers; the external power conversion efficiency, ηP\eta_P, is the ratio of delivered electrical power to incident optical power under the standard AM1.5 solar spectrum (intensities are quoted in suns, one sun being standard solar intensity). Peumans, Bulović and Forrest (2000) inserted a wide-gap (∼3.5\sim 3.5 eV) exciton-blocking layer (EBL) of bathocuproine (BCP, a phenanthroline derivative) between the acceptor and the silver cathode, which blocks exciton quenching at the metal while permitting electron extraction. With total photoactive thicknesses of order 100 Å they measured ηINT\eta_{\text{INT}} up to 33%±4% monochromatically, and, in a simple light-trapping geometry (reflective Ag cathode plus reflective substrate with a small aperture, giving multiple near-normal passes), ηP=2.4%±0.3%\eta_P = 2.4\%\pm0.3\%, with linear current up to ~17 suns. Fitting ηINT\eta_{\text{INT}} versus thickness with a steady-state exciton diffusion model (uniform generation, dissociation only at the donor/acceptor interface) required LD≈30L_D \approx 30 Å, consistent with an independent (then unpublished) PTCBI value of 30±3 Å; by contrast the related material PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride, the parent compound of PTCBI) was reported to exhibit LD∼880L_D \sim 880 Å, motivating the projection that ηINT∼80%\eta_{\text{INT}} \sim 80\% and external efficiencies of order 8% would be attainable if similarly long diffusion lengths could be realized in the bilayer materials. Two further limitations were identified. First, the paper's optical model (absorbed-photon fraction computed from a multilayer optical calculation) did not include optical microcavity resonances — standing electromagnetic waves between the metal mirror and thin-film stack — nor parasitic absorption, and for the thinnest devices the modeled and measured ηINT\eta_{\text{INT}} disagreed for this reason. Second, the EBL itself was puzzling: BCP's large optical gap suggests an insulator, yet it transported electrons with low resistance, so its electron-transporting (conduction) level and the mechanism of electron transport were undetermined; the devices in fact used a PTCBI-doped BCP layer. The paper also proposed scaling the light-trapping geometry to large areas with reflective coatings and Winston-type nonimaging collectors (non-focusing reflective concentrators that accept light over a wide range of incident angles rather than forming an image), and noted that BCP recrystallization, suppressible by dopants, threatens device stability under sustained high-intensity operation.

Problem Statement

Within the scope of Peumans, Bulović and Forrest's ultrathin CuPc/PTCBI/BCP double-heterostructure photovoltaic cells, the open question is how to validate — or definitively retire — each design lever the paper identified as required for its projected performance, under one common objective: making EBL-based ultrathin organic double-heterostructure photovoltaics simultaneously efficient, operable at high optical intensity, and scalable. Concretely: (1) Identify and implement growth or material modifications that reproducibly increase the exciton diffusion length LDL_D (fitted at ≈30\approx 30 Å in 2000) in vacuum-deposited donor/acceptor bilayer films and determine whether this can realize the projected internal quantum efficiency of ~80% and external power conversion efficiency of order 8% in the planar architecture, with the tradeoffs between exciton transport, charge mobility, interface sharpness, and series resistance quantified. (2) Establish and validate a quantitative coupled optical-electrical model of the multilayer stack that includes microcavity resonances, spatially varying exciton generation, and parasitic absorption, sufficient to remove the model–experiment discrepancy observed for the thinnest devices and to guide thickness, mirror, and collector design. (3) Determine the electronic-energy-level alignment and electron-transporting (conduction) level of the BCP exciton-blocking layer and the mechanism by which it conducts electrons despite its ~3.5 eV optical gap, together with the implications for optimal EBL material choice and thickness. (4) Demonstrate the proposed large-area light-trapping schemes (reflective coatings and arrays of Winston-type nonimaging collectors — non-focusing reflective concentrators that accept light over a wide range of incident angles rather than forming an image) on scalable, including flexible, substrates, verifying that the multiple-pass efficiency gain (2.4%±0.3% demonstrated at small area) survives scale-up, and whether higher values obtain when combined with increased LDL_D. (5) Systematically characterize device stability under sustained high optical intensities, in particular the role of BCP recrystallization and the effectiveness of dopants (such as the PTCBI doping already used in the source devices) in suppressing it, and establish the limits of long-term operational robustness. The two levers that remain open are item (1), i.e. reproducible morphology/material control of the exciton diffusion length in vacuum-deposited donor/acceptor bilayers, carried through to whether the projected internal quantum efficiency of ~80% and external efficiency of order 8% are reachable in the planar architecture, and the stability questions of item (5). Items (2) and (3) are already resolved by prior work: quantitative coupled optical-electrical modeling of organic multilayer stacks, including microcavity resonances and spatially varying exciton generation, is available through the transfer-matrix treatment (References 3 and 5), and electron transport through the BCP exciton-blocking layer is attributed to cathode-induced defect states within its ~3.5 eV optical gap (References 4 and 13). Item (4), the large-area light-trapping scale-up, likewise lies outside the surviving open core.

References

  1. P. Peumans, V. Bulović, S. R. Forrest, "Efficient photon harvesting at high optical intensities in ultrathin organic double-heterostructure photovoltaic diodes", Appl. Phys. Lett. 76, 2650 (2000), DOI 10.1063/1.126433, https://doi.org/10.1063/1.126433
  2. C. W. Tang, "Two-layer organic photovoltaic cell", Appl. Phys. Lett. 48, 183 (1986), DOI 10.1063/1.96937, https://doi.org/10.1063/1.96937
  3. L. A. A. Pettersson, L. S. Roman, O. Inganäs, "Modeling photocurrent action spectra of photovoltaic devices based on organic thin films", J. Appl. Phys. 86, 487 (1999), DOI 10.1063/1.370757, https://doi.org/10.1063/1.370757
  4. P. Peumans, S. R. Forrest, "Very-high-efficiency double-heterostructure copper phthalocyanine/C60 photovoltaic cells", Appl. Phys. Lett. 79, 126 (2001), DOI 10.1063/1.1384001, https://doi.org/10.1063/1.1384001
  5. P. Peumans, A. Yakimov, S. R. Forrest, "Small molecular weight organic thin-film photodetectors and solar cells", J. Appl. Phys. 93, 3693 (2003), DOI 10.1063/1.1534621, https://doi.org/10.1063/1.1534621
  6. P. Peumans, S. Uchida, S. R. Forrest, "Efficient bulk heterojunction photovoltaic cells using small-molecular-weight organic thin films", Nature 425, 158 (2003), DOI 10.1038/nature01949, https://doi.org/10.1038/nature01949
  7. J. Xue, S. Uchida, B. P. Rand, S. R. Forrest, "Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions", Appl. Phys. Lett. 85, 5757 (2004), DOI 10.1063/1.1829776, https://doi.org/10.1063/1.1829776
  8. B. P. Rand, P. Peumans, S. R. Forrest, "Long-range absorption enhancement in organic tandem thin-film solar cells containing silver nanoclusters", J. Appl. Phys. 96, 7519 (2004), DOI 10.1063/1.1812589, https://doi.org/10.1063/1.1812589
  9. Y. Terao, H. Sasabe, C. Adachi, "Correlation of hole mobility, exciton diffusion length, and solar cell characteristics in phthalocyanine/fullerene organic solar cells", Appl. Phys. Lett. 90, 103515 (2007), DOI 10.1063/1.2711525, https://doi.org/10.1063/1.2711525
  10. X. Che, Y. Li, Y. Qu, S. R. Forrest, "High fabrication yield organic tandem photovoltaics combining vacuum- and solution-processed subcells with 15% efficiency", Nat. Energy 3, 422 (2018), DOI 10.1038/s41560-018-0134-z, https://doi.org/10.1038/s41560-018-0134-z
  11. O. V. Mikhnenko, M. Kuik, J. Lin, N. van der Kaap, T.-Q. Nguyen, P. W. M. Blom, "Trap-Limited Exciton Diffusion in Organic Semiconductors", Adv. Mater. 26, 1912 (2014), DOI 10.1002/adma.201304162, https://doi.org/10.1002/adma.201304162
  12. O. V. Mikhnenko, P. W. M. Blom, T.-Q. Nguyen, "Exciton diffusion in organic semiconductors", Energy Environ. Sci. 8, 1867 (2015), DOI 10.1039/c5ee00925a, https://doi.org/10.1039/c5ee00925a
  13. J. Lee, S. Park, Y. Lee, H. Kim, D. Shin, J. Jeong, K. Jeong, S. W. Cho, H. Lee, Y. Yi, "Electron transport mechanism of bathocuproine exciton blocking layer in organic photovoltaics", Phys. Chem. Chem. Phys. 18, 5444 (2016), DOI 10.1039/c5cp07099f, https://doi.org/10.1039/c5cp07099f
  14. B. R. Patil, M. Ahmadpour, G. Sherafatipour, T. Qamar, A. F. Fernández, K. Zojer, H.-G. Rubahn, M. Madsen, "Area dependent behavior of bathocuproine (BCP) as cathode interfacial layers in organic photovoltaic cells", Sci. Rep. 8, 12608 (2018), DOI 10.1038/s41598-018-30826-7, https://doi.org/10.1038/s41598-018-30826-7
  15. Y. Park, K. Vandewal, K. Leo, "Optical In-Coupling in Organic Solar Cells", Small Methods 2, 1800123 (2018), DOI 10.1002/SMTD.201800123, https://doi.org/10.1002/SMTD.201800123
  16. F. Jafari, B. R. Patil, F. Mohtaram, A. L. Fernandes Cauduro, H.-G. Rubahn, A. Behjat, M. Madsen, "Inverted organic solar cells with non-clustering bathocuproine (BCP) cathode interlayers obtained by fullerene doping", Sci. Rep. 9, 10422 (2019), DOI 10.1038/s41598-019-46854-w, https://doi.org/10.1038/s41598-019-46854-w
  17. X. Guan, S. Wang, W. Liu, D. Qin, D. Ban, "Determining the exciton diffusion length of copper phthalocyanine in operating planar-heterojunction organic solar cells", Eur. Phys. J. Appl. Phys. 89, 30201 (2020), DOI 10.1051/epjap/2020190322, https://doi.org/10.1051/epjap/2020190322
  18. Y. Zhang, P. Tong, S. Chen, Y. Liu, F. Dou, J. Guo, Y. Fu, X. Zhang, "Long exciton diffusion distance and efficient exciton dissociation enable high efficiency planar heterojunction non-fullerene organic solar cells", Mater. Today Phys. 46, 101495 (2024), DOI 10.1016/j.mtphys.2024.101495, https://doi.org/10.1016/j.mtphys.2024.101495
  19. K. Wang, F. Zhao, Y. Zhu, Y. He, Z. Liu, X. Han, Q. Ai, X. Shen, B. Li, J. Zhang, Y. Lin, C. Wang, D. He, "Prolonging the exciton diffusion length by manipulating molecular stacking enables pseudo-planar heterojunction organic solar cells to achieve over 19% efficiency", J. Mater. Chem. A 12, 12208 (2024), DOI 10.1039/d4ta00594e, https://doi.org/10.1039/d4ta00594e