Hybrid solar cell: Difference between revisions

'''Hybrid [[solar cell]]scells''' combine advantages of both [[Organic semiconductor|organic]] and inorganic [[semiconductor]]s. Hybrid [[photovoltaics]] have organic materials that consist of [[conjugated polymers]] that absorb light as the donor and transport [[Electron hole|holes]].<ref name="S2">{{cite journal|journal=MRS Bulletin|volume=30|pages=41–44|year=2005|title=Hybrid Organic–Nanocrystal Solar Cells|doi=10.1557/mrs2005.8|last1=Milliron|first1=Delia J.|last2=Gur|first2=Ilan|last3=Alivisatos|first3=A. Paul|s2cid=137370366 }}</ref> Inorganic materials in hybrid cells are used as the acceptor and [[electron]] transporter in the structure. The hybrid photovoltaic devices have a potential for not only low-cost by [[Roll-to-roll processing|roll-to-roll]] processing but also for scalable [[solar power]] conversion.

[[Solar cells]] are devices that convert sunlight into electricity by the [[photovoltaic effect]]. Electrons in a solar cell absorb photon energy in sunlight which excites them to the [[conduction band]] from the [[valence band]]. This generates a [[exciton|hole-electron pair]], which is separated by a potential barrier (such as a [[p-n junction]]), and induces a current. [[Organic solar cells]] use organic materials in their active layers. Molecular, polymer, and hybrid organic photovoltaics are the main kinds of organic photovoltaic devices currently studied.

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=1010–19|doi-access=free}}</ref> The two materials are assembled together in a [[heterojunction]]-type photoactive layer, which can have a greater power [[Energy conversion efficiency|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 exitonexciton created in the donor is delocalized on a donor-acceptor complex.<ref>{{Cite book|title=Organic and hybrid solar cells : an introduction|last=Lukas|first=Schmidt-Mende|others=Weickert, Jonas|isbn=9783110283204|location=Berlin|oclc=950902053|date = 2016-05-24}}</ref><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|bibcode=2008AdM....20.3516S |s2cid=136557509 }}</ref> Excitons generated within this length close to an acceptor would contribute to the photocurrent.

[[File:Heterojunction mike.JPG|thumb|Figure 2. Two different structures of heterojunctions, a) phase separated bi-layer and b) bulk heterojunction. The bulk heterojunction allows for more interfacial contact between the two phases, which is beneficial for the [[nanoparticle]]-polymer compound as it provides more surface area for charge transfer.]]

[[Mesoporous material|Mesoporous films]] have been used for a relatively high-efficiency hybrid solar cell.<ref name="mesoporous">{{cite book|chapter-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|pageissue=181016 |pages=1810–28 |doi=10.1002/adma.201003991 |doi_brokendatepmid=201421509826 |bibcode=2011AdM....23.1810W |s2cid=16507122 |url=http://nbn-03resolving.de/urn:nbn:de:bsz:352-12179678 }}</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|author3-link=Yi Cui (scientist)|author4-link=Michael D. McGehee|last4=McGehee|first4=Michael D.|journal=[[Annual Review of Materials Research]]|volume=41|pages=269269–295|bibcode=2011AnRMS..41..269G}}</ref>

Nanoparticles[[Nanoparticle]]s 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>

* Nanoparticles with size near their Bohr radius can generate two excitons when struck by a sufficiently energetic photon.<ref name="dx">{{cite journal|last=Saunders|first=Brian R.|title=Hybrid polymer/nanoparticle solar cells: Preparation, principles and challenges|journal=Journal of Colloid and Interface Science|date=March 2012|volume=369|issue=1|pages=1–15|doi=10.1016/j.jcis.2011.12.016|pmid=22209577|bibcode=2012JCIS..369....1S}}</ref>

The highest demonstrated efficiency is 3.2%, based upon a PCPDTBT polymer donor and CdSe nanoparticle acceptor. The device exhibited a short circuit current of 10.1 mA·cm⁻², an open circuit voltage of .68 V, and a fill factor of .51.<ref>{{cite journal|last=Dayal|first=Smita|coauthorsauthor2=Nikos Kopidakis , Dana C. Olson , David S. Ginley and Garry Rumbles|title=Photovoltaic Devices with a Low Band Gap Polymer and CdSe Nanostructures Exceeding 3% Efficiency|journal=Nano Letters|year=2010|volume=10|issue=1|pages=239–242|doi=10.1021/nl903406s|pmid=20000623|bibcode = 2010NanoL..10..239D |last3=Olson|first3=Dana C.|last4=Ginley|first4=David S.|last5=Rumbles|first5=Garry}}</ref>

[[Carbon nanotube]]s (CNTs) have high electron conductivity, high thermal conductivity, robustness, and flexibility. Field emission displays (FED), strain sensors, and field effect transistors (FET) using CNTs have been demonstrated.<ref name="H1">{{cite journal|journal=Nano lettersLetters|year=2002|volume=2|pages=1191–1195|title=Enhanced Electron Field Emission in B-doped Carbon Nanotubes|doi=10.1021/nl0256457|bibcode = 2002NanoL...2.1191C|issue=11|last1=Charlier|first1=J.-C.|last2=Terrones|first2=M.|last3=Baxendale|first3=M.|last4=Meunier|first4=V.|last5=Zacharia|first5=T.|last6=Rupesinghe|first6=N. L.|last7=Hsu|first7=W. K.|last8=Grobert|first8=N.|last9=Terrones|first9=H. |last10=Amaratunga|first10=G. A. J.|display-authors=8}}</ref><ref name="H2">{{cite journal|journal=Nanotechnology|volume=15|year=2004|pages=379–382|doi=10.1088/0957-4484/15/3/026|title=Nanotube film based on single-wall carbon nanotubes for strain sensing|bibcode = 2004Nanot..15..379D|issue=3|last1=Dharap|first1=Prasad|last2=Li|first2=Zhiling|last3=Nagarajaiah|first3=Satish|last4=Barrera|first4=E V |s2cid=250909198 }}</ref><ref name="H3">{{cite journal|journal=Nature|volume=393|year=1998|pages=49–62|doi=10.1038/29954|bibcode = 1998Natur.393...49T|issue=6680|last1=Dekker|first1=Cees|title=Room-temperature transistor based on a single carbon nanotube|last2=Tans|first2=Sander J.|last3=Verschueren|first3=Alwin R. M.|s2cid=4403144 }}</ref> Each application shows the potential of CNTs for nanoscale devices and for flexible electronics applications. Photovoltaic applications have also been explored for this material.

To increase the photovoltaic efficiency, electron-accepting impurities must be added to the photoactive region. By incorporating CNTs into the polymer, dissociation of the exciton pair can be accomplished by the CNT matrix. The high surface area (~1600 m²/g) <ref name="H4">{{cite journal|journal=Chemical Physics Letters|year=2002|volume=365|page=69|title=Pore structure of raw and purified HiPco single-walled carbon nanotubes|doi=10.1016/S0009-2614(02)01420-3|bibcode = 2002CPL...365...69C|last1=Cinke |first1=Martin |last2=Li |first2=Jing |last3=Chen |first3=Bin |author-link3=Bin Chen |last4=Cassell |first4=Alan |last5=Delzeit |first5=Lance |last6=Han |first6=Jie |last7=Meyyappan |first7=M |year=2002 |title=Pore structure of raw and purified HiPco single-walled carbon nanotubes |journal=Chemical Physics Letters |volume=365 |issue=1–2 |pages=69–74 |bibcode=2002CPL...365...69C |doi=10.1016/S0009-2614(02)01420-3|s2cid=96046204 }}</ref> of CNTs offers a good opportunity for exciton dissociation. The separated carriers within the polymer-CNT matrix are transported by the percolation pathways of adjacent CNTs, providing the means for high carrier mobility and efficient charge transfer. The factors of performance of CNT-polymer hybrid photovoltaics are low compared to those of inorganic photovoltaics. SWNT in P3OT semiconductor polymer demonstrated open circuit voltage (V_oc) of below 0.94 V, with short circuit current (I_sc) of 0.12 mA/cm².<ref name="H4"/>

CNT may be used as a photovoltaic device not only as an add-in material to increase carrier transport, but also as the photoactive layer itself.<ref name="H6">{{cite journal|journal=Small|year=2008|volume=4|pages=1313–1318|doi=10.1002/smll.200701309|pmid=18702123|title=Nanowelded carbon-nanotube-based solar microcells|issue=9|last1=Chen|first1=Changxin|last2=Lu|first2=Yang|last3=Kong|first3=Eric S.|last4=Zhang|first4=Yafei|last5=Lee|first5=Shuit-Tong}}</ref> The semiconducting single walled CNT (SWCNT) is a potentially attractive material for photovoltaic applications for the unique structural and electrical properties. SWCNT has high electric conductivity (100 times that of copper) and shows ballistic carrier transport, greatly decreasing carrier recombination.<ref name="H7">{{cite book|title=Topics in Applied Physics|volume=80|publisher=Springer|author=Dresselhaus, M. S.|author-link=Mildred Dresselhaus|isbn=978-3-540-72864-31|year=2008}}</ref> The bandgap of the SWCNT is inversely proportional to the tube diameter,<ref name="H7"/> which means that SWCNT may show multiple direct bandgaps matching the solar spectrum.

Several challenges must be addressed for CNT to be used in photovoltaic applications. CNT degrades overtimeover time in an oxygen-rich environment. The passivation layer required to prevent CNT oxidation may reduce the optical transparency of the electrode region and lower the photovoltaic efficiency.

TiO₂ nanoparticles are synthesized in several tens of nanometer scales (~100&nbsp;nm). In order to make a photovoltaic cell, molecular sensitizers (dye molecules) are attached to the titania surface. The dye-absorbed titania is finally enclosed by a liquid electrolyte. This type of dye-sensitized solar cell is also known as a Grätzel cell.<ref name="K1">{{cite journal|author=O’Regan, B. and Grätzel, M.|journal=Nature|volume=353|pages=737–740|year=1991|doi=10.1038/353737a0|title=A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films|bibcode = 1991Natur.353..737O|issue=6346|s2cid=4340159 }}</ref> Dye-sensitized solar cell has a disadvantage of a short diffusion length. Recently, [[supermolecular]] or multifunctional sensitizers have been investigated so as to enhance carrier diffusion length.<ref name="K2">{{cite journal|author=Jacques-e Moser|journal=Nature materialsMaterials|volume=4|pages=723–724|year=2005|doi=10.1038/nmat1504|pmid=16195761|title=Solar cells: later rather than sooner|issue=10|bibcode = 2005NatMa...4..723M |s2cid=1508500 }}</ref> For example, a dye [[chromophore]] has been modified by the addition of secondary electron donors. Minority carriers (holes in this case) diffuse to the attached electron donors to recombine. Therefore, electron-hole recombination is retarded by the physical separation between the dye–cation moiety and the TiO₂ surface, as shown in Fig. 5. Finally, this process raises the carrier diffusion length, resulting in the increase of carrier lifetime.

[[Mesoporous material]]s contain pores with diameters between 2 and 50&nbsp;nm. A dye-sensitized mesoporous film of TiO₂ can be used for making photovoltaic cells and this solar cell is called a ‘solid-state dye sensitized solar cell’. The pores in mesoporous TiO₂ thin film are filled with a solid hole-conducting material such as p-type semiconductors or organic hole conducting material. Replacing the liquid electrolyte in Grätzel’sGrätzel's cells with a solid charge-transport material can be beneficial. The process of electron-hole generation and recombination is the same as Grätzel cells. Electrons are injected from photoexcited dye into the conduction band of titania and holes are transported by a solid charge transport electrolyte to an electrode. Many organic materials have been tested to obtain a high solar-to-energy conversion efficiency in dye synthesized solar cells based on mesoporous titania thin film.<ref name="K3">{{cite journal|journal=Adv. Mater.|volume= 18|pages=2579–2582|doi=10.1002/adma.200502023|title=All-Solid-State Dye-Sensitized Nanoporous TiO2 Hybrid Solar Cells with High Energy-Conversion Efficiency|year=2006|issue=19|last1=Lancelle-Beltran|first1=E.|last2=Prené|first2=P.|last3=Boscher|first3=C.|last4=Belleville|first4=P.|last5=Buvat|first5=P.|last6=Sanchez|first6=C.|bibcode= 2006AdM....18.2579L|s2cid= 138764511}}</ref>

In 2008, scientists were able to create a nanostructured lamellar structure that provides an ideal design for bulk heterojunction solar cells.<ref name="K4">{{cite journal|journal=Nature Materials|volume= 8|pages= 68–75|year=2009|doi=10.1038/nmat2336|pmid=19060890|title=A synergistic assembly of nanoscale lamellar photoconductor hybrids|issue=1|bibcode = 2009NatMa...8...68S|last1=Sofos|first1=Marina|last2=Goldberger|first2=Joshua|last3=Stone|first3=David A.|last4=Allen|first4=Jonathan E.|last5=Ma|first5=Qing|last6=Herman|first6=David J.|last7=Tsai|first7=Wei-Wen|last8=Lauhon|first8=Lincoln J.|last9=Stupp|first9=Samuel I. |display-authors= 9}}</ref> The observed structure is composed of ZnO and small, conducting organic molecules, which co-assemble into alternating layers of organic and inorganic components. This highly organized structure, which is stabilized by π-π stacking between the organic molecules, allows for conducting pathways in both the organic and inorganic layers. The thicknesses of the layers (about 1–3&nbsp;nm) are well within the exciton diffusion length, which ideally minimizes recombination among charge carriers. This structure also maximizes the interface between the inorganic ZnO and the organic molecules, which enables a high chromophore loading density within the structure. Due to the choice of materials, this system is non-toxic and environmentally friendly, unlike many other systems which use lead or cadmium.