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Development of Semiconducting Polymers for Solar Energy Harvesting
微信签名大全Yongye Liang a ; Luping Yu a a
Department of Chemistry and James Franck Institute, The University of Chicago, Chicago, IL Online publication date: 06 November 2010
To cite this Article Liang, Yongye and Yu, Luping(2010) 'Development of Semiconducting Polymers for Solar Energy
Harvesting', Polymer Reviews, 50: 4, 454 — 473
To link to this Article: DOI: 10.1080/15583724.2010.515765URL: dx.doi/10.1080/15583724.2010.515765
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Polymer Reviews ,50:454–473,2010虾滑小吃
Copyright ©Taylor &Francis Group,LLC ISSN:1558-3724print /1558-3716online DOI:
10.1080/15583724.2010.515765
Development of Semiconducting Polymers
for Solar Energy Harvesting
YONGYE LIANG AND LUPING YU
Department of Chemistry and James Franck Institute,The University of Chicago,929E 57th Street,Chicago,IL 60637
Semiconducting polymer solar cells are an attracting class of devices for low-cost solar energy harvesting.The bulk hetero-junction structure bad on composite materials of miconducting polymer donor and fullerene acceptor is an effective form of active layers for polymer solar cells.So far,the limiting factors for widespread,practical applications in polymers solar cell is their low power conversion efficiency (PCE)and potential instability under light exposure.Thus new polymeric materials with desired properties and stability are crucial for improving the solar cell performance.Numerous conjugated polymers,such as poly[phenylene vinylene]s (PPVs)and polythiophenes,have been explored for this purpo,which lead to PCE as high as 5%.To improve the performance,low bandgap polymers and polymers with low lying HOMO energy levels have been the subject of recent focus.Efficiencies clo to 8%have been achieved in the polymer system compod of thieno[3,4-b]thiophene and benzodithiophene alter-nating units (PTB).The high efficien
cy is due to the synergistic combinations of desired properties in the polymer system through detailed fine-tuning of the polymer structure.The recent results reaffirmed the notion that better solar cell polymers could be further developed for vital applications in real devices.
Keywords miconducting polymer,solar cell,bulk hetero-junction,fine-tuning
1.Introduction
In the recognition of energy demand increa and fossil fuel depletion,as well as the envi-ronmental pollution from fuel combustion,the arch and utilization of clean and renewable energies is becoming one of the greatest challenges for our society.Solar energy is the largest renewable energy source,which can potentially provide about 124PW (PW =1015Watts)energy globally,more than 8000times of the total worldwide energy consumption in 2004(15TW,1TW =1012Watts).1The direct conversion of solar radiation into electricity,called photovoltaic,is a simple and clean way to harness such a vast energy source.The photovoltaic effect requires active miconducting materials,and currently inorganic solar cell bad on silicon is the dominant technology.It exhibits good performance in PCE and lifetime.However,the high production and installation cost of silicon solar cells limits its widespread u to provide a large fraction of our electricity.2,3New photovoltaic (PV)
Received May 7,2010;accepted August 5,2010.
Address correspondence to Professor Luping Yu,929E 57th Street,Chicago,IL 60637,USA.E-mail:lupingyu@uchicago.edu酸甜苦辣
454
东马塍
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Developing Semiconductor Polymers for Solar Energy Harvesting 455
systems exhibiting low cost and high efficiency for solar energy conversion are actively pursued worldwide.4
An alternative material is the miconducting polymers.Solar cells bad on organic miconductors enjoy veral unique advantages compared to their inorganic counterparts.5First of all,polymer miconductors are soluble in organic solvents and the composite active layer of solar cells can be fabricated from solution in a single step by a variety of simple techniques such as spin coating,inkjet printing,and roller casting,and all are cost-effective and easier than the inorganic ones.The physical properties of organic solar cells,such as absorption spectrum and charge transport property,can be t
ailored by chemical synthesis.Meanwhile,the thickness of active layer can be very thin (about 100nm)due to their high absorption coefficient within their absorption range so that organic solar cell can be mitransparent and the color can be tuned.Furthermore,the lightweight and mechanical flexibility of polymer materials enable some specific application of organic solar cells,such as in portable devices.
The donor/acceptor concept pioneered by Tang in 19866and bulk-heterojunction (BHJ)developed by Yu et al.in 19957led to the active pursuit of polymer solar cells.The solution procesd BHJ solar cells are easy to prepare and prent a high density of heterojunction interfaces that enables the efficient exciton dissociation and charge generation over the whole active layer,which favors high PCE.The two advantages establish the solution procesd BHJ solar cells as the most promising technology for organic solar cells.Gradual progress in the solar cell performance is underway bad on this architecture.PCE evolved from 1%in 1995,to 5%in 2005and clo to 8%most recently.8
Despite the envisioned advantages and recent technology advances,so far polymer solar cells are still inferior to inorganic counterparts in terms of PCE and life times.9Solar cells with PCE over 10%is necessary for widespread application.10Theoretical calculations have shown that the polymer/fullerene BHJ solar cell in a single layer configuration can achieve more than 10%in PCE.11
After an exhaustive effort in morphology control,electrode modification,the introduction of dielectric layers,and the utilization of the plasmonic effect,it is clear that the most important limiting factor for further improvement on the photovoltaic performance is the materials in active layers,especially the polymer donor material.12,13In this article,we will briefly review rearch progress in the development of new miconducting polymers for solar energy harvesting.
2.Materials for BHJ Organic Solar Cells
There are two components in the BHJ structure,the donor and the acceptor.The lection of the materials for both components is very important for solar cell performance.Photovoltaic properties of veral reprentative solar cell systems with different donor/acceptor are outlined in Table 1.Owing to their strong absorbing ability,good film-forming ability and unique electronic properties from the long pi-conjugated system,miconducting polymers are good candidates as donor component.A potential drawback is that conjugated polymers exhibit a broad distribution in molecular weight,which is easy to cau purification difficulty and poor reproducibility due to batch-to-batch difference.Small conjugated molecules offer the advantages of mono-dispersity and easy purification,so they are actively arched for solar cell applications.However,the film forming ability from solution and the charge mobility in the blend structure of small molecules are not as good as polymers.As a result,so
far,the PCE of the solution procesd BHJ solar cells using small molecules donors is much lower than the polymer counterpart.14,15
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456Y.Liang and L.Yu
Table 1
Photovolatic properties of reprentative BHJ solar cell systems with different donor/
西井幸人acceptor materials.Some structures are showed in Fig.1Donor Acceptor V oc Jsc FF PCE
Type
materials materials (V)(mA/cm2)(%)(%)Reference Polymer/Fulleride PTB7
PC 71BM 0.7414.5697.4074Small molecule/Squaraine 1PC 61BM 0.625.7351.2412b Fulleride
DH6TDPP PC 61BM 0.678.42452.3312a Polymer/Polymer P3HT F8TBT 1.153.6341.2014c Polymer/Inorganic P3HT
CdSe 0.75.7401.7017nanocrystals
MDMOPPV
ZnO
0.84
2.40
59
三国杀经典台词
1.60
18
Note:V oc =open-circuit voltage,Jsc =short-circuit current density,FF =fill factor.
Thus far,fullerene derivatives are found to be the best candidates as acceptor component due to their high electron affinity and superior electron mobility.Especially,the three-dimensional structure of fullerene offers unique packing ability in blend,which can form the electron transport channels efficie
ntly.The original fullerenes do not have enough solubility in organic solvents,so fullerene derivatives with solublizing groups are usually ud.16Introduction of the functional group usually improves the miscibility with the donor components,but with small impact on the electronic properties of fullerene.As a result,fullerene derivatives BHJ structure can provide the advantages of efficient exciton dissociation by intermixing the donor/acceptor phas well enough and efficient extraction of the parated charges from the interface by high electron mobility.The weakness of fullerene derivatives as acceptor materials in BHJ solar cells lies in their weak absorption in visible region (like C 60)and expensive materials cost (like C 70).N-type polymers as acceptor materials offer the advantages of low cost and high optical absorption.The absorption of the acceptor polymers can be tuned so that both the donor and the acceptor can cover complementary parts of the solar spectrum.However,the efficiency obtained from polymer acceptor solar cells is much lower than that from the fullerene derivatives.17–19It has been reported recently that the low efficiency in polymer/polymer BHJ is due to the limited transport of the parated charges.20To overcome this problem,polymers with high local electron mobility are needed.N-type miconducting inorganic nanocrystals were also ud as the acceptor components.Such nanocrystals provide high electron mobility and high optical absorption coefficient.21CdSe nanocrystals 22and ZnO 23nanocrystals have been ud to form polymer/inorganic hybrid BHJ solar cells.However,the achieved efficiencies
are rather low.Problems associated with the inorganic nanocrystals are their poor solubility in organic solvents and the lack of connectivity between nanocrystals due to the existence of stabilizing bulky ligands .
3.Conjugated Polymer/Fulleride bad BHJ Solar Cells世界最大的瀑布
The most effective composite structure in solution procesd BHJ organic solar cells is bad on conjugated polymer/fullerene BHJ structure,in which electron rich miconducting polymer plays as donor and fullerene plays as acceptor.PPV/PC 61BM and P3HT/PC 61BM (Fig.1)are two well-studied systems,which lead to the current understanding of the factors affecting the device performance of BHJ solar cells.
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Developing Semiconductor Polymers for Solar Energy Harvesting
457
Figure 1.Structures of widely ud conjugated polymers and fullerene derivative and some structures mentioned in Table 1for BHJ solar cells.
3.1PPV/PC 61BM BHJ Solar Cells
The first BHJ solar cell was reported by Yu et al.in 1995bad on MEH-PPV//[6,6]-phenyl-C 61-butyric acid methyl ester (PC 61BM)composite.7After veral years of optimization,the efficiency of devices bad on this system was still around 1%under AM 1.5illumination.In 2001,Shaheen ported that a 2.5%efficiency was obtained from a better process-able composite of poly[2-methoxy-5-(3 ,7 -dimethyl-octyloxy)-1,4-phenylene vinylene](MDMO-PPV)and PC 61BM by controlling the casting condition.24The active layer is prepared by spincoating MDMO-PPV/PCBM in 1:4weight ratio from chlorobenzene.The device has a V oc of 0.82V ,a Jsc of 5.25mA/cm 2,and an FF of 0.61,which leads to a PCE of 2.5%.There is about a three-fold enhancement in efficiency compared to the one spincoating from toluene.The performance difference is attributed to the change of active layer morphology.PCBM has better solubility in chlorobenzene than toluene,which increas the miscibility of the two components.A more homogenous and bicontinuous composite was obrved in the chlorobenzene-casting film compared to the toluene-casting film.Therefore,the heterojunction interface increas for excitons dissociation and more biocontinuous network forms for charge transport,which attributes to the performance enhancement.The performance of MDMO-PPV/PC 61BM system can be further optimized to about 3%by using LiF as interlayer.25Another performance improvement was achieved by substituting PC 61BM to PC 71BM.PC 71BM has better absorption in the visible region than PC 61BM,and as a result,the MDMO-PPV//[6,6]-phenyl-C 71-but
yric acid methyl ester (PC 71BM)system gives a larger Jsc,and about 3%in efficiency was achieved.26However,PPV materials can only harvest light below 600nm due to their relatively large bandgap (2.2eV)and they usually have low hole mobility,which limits the further improvement of such solar cells.
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