Hybrid solar cell: Difference between revisions

In hybrid solar cells, an organic material is mixed with a high electron transport material to form the photoactive layer.<ref name="S1">{{cite journal|journal=MRS Bulletin|volume=30|doi=10.1557/mrs2005.2|year=2005|title=Organic–Based Photovoltaics|last1=Shaheen|first1=Sean E.|last2=Ginley|first2=David S.|last3=Jabbour|first3=Ghassan E.|pages=10–19}}</ref> The two materials are assembled together in a [[heterojunction]]-type photoactive layer, which can have a greater power conversion efficiency than a single material.<ref name="M1">{{cite journal|authorauthor1=Saunders, B.R.; |author2=Turner, M.L. |year=2008|title=Nanoparticle-polymer photovoltaic cells|journal=Advances in Colloid and Interface Science|volume=138|pages=1–23|doi=10.1016/j.cis.2007.09.001|pmid=17976501|issue=1}}</ref> One of the materials acts as the photon absorber and [[exciton]] donor. The other material facilitates exciton dissociation at the junction. Charge is transferred and then separated after an exciton created in the donor is delocalized on a donor-acceptor complex.<ref name="M2">{{cite journal|authorauthor1=Sariciftci, N.S.; |author2=Smilowitz, L.; |author3=Heeger, A.J.; and |author4=Wudl, F. |year=1993|title=Semiconducting polymers (as donors) and buckminsterfullerene (as acceptor): photoinduced electron transfer and heterojunction devices|journal=Synthetic Metals|volume=59|pages=333–352|doi=10.1016/0379-6779(93)91166-Y|issue=3}}</ref>

The acceptor material needs a suitable energy offset to the binding energy of the exciton to the absorber. Charge transfer is favorable if the following condition is satisfied:<ref name="M3">{{cite journal|authorauthor1=Ginger, D.S.; |author2=Greenham, N.C. |year=1999|title=Photoinduced electron transfer from conjugated polymers to CdSe nanocrystals|journal=Physical Review B|volume=59|pages=624–629|doi=10.1103/PhysRevB.59.10622|bibcode = 1999PhRvB..5910622G|issue=16 }}</ref>

where superscripts A and D refer to the acceptor and donor respectively, E_A is the electron affinity, and U the coulombic binding energy of the exciton on the donor. An energy diagram of the interface is shown in figure 1. In commonly used photovoltaic polymers such as MEH-PPV, the exciton binding energy ranges from 0.3 eV to 1.4 eV.<ref name="M4">{{cite journal|authorauthor1=Scheblykin, I.G.; |author2=Yartsev, A.; |author3=Pullertis, T.; |author4=Gulbinas, V.; |author5=Sundstrm, V. |year=2007|title=Excited State and Charge Photogeneration Dynamics in Conjugated Polymers|journal=J. Phys. Chem. B|volume=111|pages=6303–6321|doi=10.1021/jp068864f|pmid=17521181|issue=23}}</ref>

The average distance an exciton can diffuse through a material before annihilation by recombination is the exciton diffusion length. This is short in polymers, on the order of 5–10 nanometers.<ref name="M3" /> The time scale for radiative and non-radiative decay is from 1 picosecond to 1 nanosecond.<ref name="M5">{{cite journal|authorauthor1=Shaw, P.E.; |author2=Ruseckas, A.; |author3=Samuel, I.D.W |year=2008|title=Exciton Diffusion Measurements in Poly(3-hexylthiophene)|journal=Advanced Materials|volume=20|pages=3516–3520|doi=10.1002/adma.200800982|issue=18}}</ref> Excitons generated within this length close to an acceptor would contribute to the photocurrent.

[[Mesoporous material|Mesoporous films]] have been used for a relatively high-efficiency hybrid solar cell.<ref name="mesoporous">{{cite book|url=http://www.ct-si.org/publications/proceedings/procs/Cleantech2008/2/850 |title=Clean Technology 2008. Technical Proceedings of the 2008 Clean Technology Conference and Trade Show|chapter=Chapter 2: Renewables: Photovoltaics, Wind & Geothermal. Mesoporous TiO₂ thin-film for Dye-Sensitized Solar Cell (DSSC) Applicationv|isbn=|pages=113–116|authorauthor1=A. Vats, |author2=R. Shende, |author3=J. Swiatkiewicz, |author4=J. Puszynski }}</ref> The structure of mesoporous [[thin film solar cell]]s usually includes a porous inorganic that is saturated with organic surfactant. The organic absorbs light, and transfers electrons to the inorganic semiconductor (usually a transparent conducting oxide), which then transfers the electron to the electrode. Problems with these cells include their random ordering and the difficulty of controlling their nanoscale structure to promote charge conduction.

Researchers have been able to grow nanostructure-based solar cells that use ordered nanostructures like nanowires or nanotubes of inorganic surrounding by electron-donating organics utilizing self-organization processes. Ordered nanostructures offer the advantage of directed charge transport and controlled phase separation between donor and acceptor materials.<ref>{{cite journal|authorauthor1=Weickert, J.; |author2=Dunbar, R.B.; |author3=Wiedemann, W.; |author4=Hesse, H.C.; |author5=Schmidt-Mende, L. |year=2011|title=Nanostructured Organic and Hybrid Solar Cells|journal=Advanced Materials|volume=23|page=1810|doi=10.1002/201003991|doi_brokendate=2015-02-01}}</ref> The nanowire-based morphology offers reduced internal reflection, facile strain relaxation and increased defect tolerance. The ability to make single-crystalline nanowires on low-cost substrates such as aluminum foil and to relax strain in subsequent layers removes two more major cost hurdles associated with high-efficiency cells. There have been rapid increases in efficiencies of nanowire-based solar cells and they seem to be one of the most promising nanoscale solar hybrid technologies.<ref name="nanowire">{{cite journal|doi=10.1146/annurev-matsci-062910-100434|title=Nanowire Solar Cells|year=2011|last1=Garnett|first1=Erik C.|last2=Brongersma|first2=Mark L.|last3=Cui|first3=Yi|last4=McGehee|first4=Michael D.|journal=Annual Review of Materials Research|volume=41|pages=269–295}}</ref>

Nanoparticles are a class of semiconductor materials whose size in at least one dimension ranges from 1 to 100 nanometers, on the order of exciton wavelengths. This size control creates quantum confinement and allows for the tuning of optoelectronic properties, such as band gap and electron affinity. Nanoparticles also have a large surface area to volume ratio, which presents more area for charge transfer to occur.<ref name="M6">{{cite book|authorauthor1=Wu, M.H; |author2=Ueda, A.; |author3=Mu, R |chapter=Semiconductor Quantum Dot Based Nanocomposite Solar Cells|title=Organic Photovoltaics: Mechanisms, Materials, and Devices|year=2005|publisher=CRC Press|doi=10.1201/9781420026351.ch14|isbn=978-0-8247-5963-6}}</ref>