Origin of Photocurrent in Fullerene-Based Solar Cells

Abstract

Fullerene-based organic solar cells with only a minute amount of donor show a substantial photocurrent while maintaining a large open-circuit voltage. At low concentrations the donor is fully dispersed within the fullerene and no percolation pathways of holes toward the anode exist; this morphology is in contrast to bulk-heterojunction donor:acceptor blends where percolation pathways for both electrons and holes are present within their respective transport phases. Therefore, the question of how holes contribute to the photocurrent arises. Here we demonstrate that the photocurrent is readily explained by photogenerated holes transferring back to the fullerene matrix due to Coulomb repulsion and the fullerene acting as an ambipolar conductor for both electrons and holes. The two critical parameters controlling this process are the values of the highest occupied molecular orbital level difference between the donor and the acceptor and of the recombination strength; both are found to agree between experimental measurements and kinetic Monte Carlo simulations. We provide evidence that the highest occupied molecular orbital level difference between donor and acceptor is smaller in a dilute donor configuration. Successive percolation pathways toward the contactsthe reason for introducing the bulk-heterojunction configurationare not an absolute requirement to obtain substantial photocurrents in organic solar cells.

High‐Efficient Charge Generation in Single‐Donor‐Component‐Based p‐i‐n Structure Organic Solar Cells

“…This condition raises the intriguing question of what the exact hole‐transport mechanism at such low donor contents is. The typical molecule structures of fullerene‐based acceptor materials are found to transport electrons and holes when the highest occupied molecular orbital (HOMO)‐level difference between the donor and the acceptor is in the right range . Vandewal and coworkers consider that the holes do not travel in the C 60 phase; instead, they tunnel from donor to donor …”

“…However, such p‐i‐n device structures are seldom realized in single‐component layer‐based OSCs for their limited material properties. A pure fullerene‐based OSC was reported, which can work even without donor material, although PCE, especially J sc , is quite low . Other acceptor material‐based single‐layer OPV devices, such as subnaphthalocyanine chloride (SubNc) and subphthalocyanine chloride (SubPc), can effectively generate free charge carriers without any assistance from a P/N junction; the PCEs of single‐layer SubNc and SubPc devices are as high as 0.3% .…”

“…The typical molecule structures of fullerene-based acceptor materials are found to transport electrons and holes when the highest occupied molecular orbital (HOMO)-level difference between the donor and the acceptor is in the right range. [15] Vandewal and coworkers consider that the holes do not travel in the C 60 phase; instead, they tunnel from donor to donor. [11,16] P-i-n structure OPVs usually sandwich the light-harvesting layer between a metal-oxide photoanode and a hole-transport layer.…”

“…A pure fullerenebased OSC was reported, which can work even without donor material, although PCE, especially J sc , is quite low. [15] Other acceptor material-based single-layer OPV devices, such as subnaphthalocyanine chloride (SubNc) and subphthalocyanine chloride (SubPc), can effectively generate free charge carriers without any assistance from a P/N junction; the PCEs of single-layer SubNc and SubPc devices are as high as 0.3%. [18] These studies reveal that different from those "doublecable" polymer-based single-component OSCs, [19][20][21] fullerene, SubNc, and SubPc are special bipolar semiconductors, which can directly generate free carriers upon photoexcitation.…”

Nanotechnology in Materials and Medical Sciences

“…Quantum dots are used in optoelectronics (screens), photovoltaic cells, inks and paints for applications of type marking anti-counterfeiting. Fullerenes are used in sport materials, development of photovoltaic solar cells, used as hardening agents for the development of light weight materials and also several applications in health care sectors, used as excellent anti oxidants, anti viral agent, drug delivery and gene delivery, photo sensitizers in photo dynamic therapy and polymers with optical limiting properties (surface coating and phtotoconducting devices in particular for applications in artificial photosynthesis) [34,35]. Nanowires are used in the conductive layers of screens, solar cells and electronic devices [36].…”