Photovoltaics of Polymer Heterojunctions

8/10/2019 Photovoltaics of Polymer Heterojunctions

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Innovation InPhotosensitive Devices

Nikita Obidin


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To my friends and family, for their support and love

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Foreword

The world of innovation has certainly gone through a less than magnificent

transformation in recent years. When we look at the defining inventions of each decade,

we are awed by our inability to perceive modernity in their absence. The advent of the

such inventions as the telegraph, the radio receiver, the personal automobile, and the

airplane has gone beyond simply the transformation of daily life, it has redefined the

dynamics of society. And now, as our world faces the greatest global challenges yet, it

seems that the well has run dry of ideas. But with every threat comes an opportunity,

and to every challenge arise a new generation of innovators to confront these changes

and dream of the future.

I was fortunate enough to have around me, individuals with a passion for intellect

and creativity. They taught me, amongst many things, to think differently and to innovate

in every aspect of my life. If it were not for their contribution, the many months of work in

this book would not have been a reality. From the very beginning they have been with

me. I know that you, the reader, are special, and that you have enormous potential. You

and I have a particular vision for the world. An energy independent and sustainable

world, in which resources are no longer the context of conflict, and where power is

freely available to all. My hope is that together, the people of the scientific community

can change the world, as they have before, and innovation and dreaming of the future

can be brought back into the forefront of our global discourse.

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Note on the text

This publication is written in a notebook style format and compiles research done mainly

at The University of Massachusetts Amherst as part of the DV Group in the Organic

Chemistry Department. The models presented in the Future Prospects section have

patents pending on them. All diagrams are provided courtesy of the DV Group.

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Sn (ii) catalyzed pi

conjugated pyrometallicpolymer synthesis inorganic photovoltaic

devices

University of Massachusetts Amherst

Nikita Obidin

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Backdrop for the solar revolution

In my day...

A professor in the physics department of some well regarded university began

every lecture with a bottle of hard liquor at the podium. By the end of each lecture, the

bottle would empty and the blackboard would be filled with Maxwells biography and

equations. Besides condoning excessive alcoholic consumption, his lectures also

promoted a sense of historical context and relevance. In other words, to fully

understand something, you have to understand from where it was eventually deduced.

Einsteins expansion of Newtons gravitational principles affected more than the science,

they redefined the man, from the common practitioner, to the rogue of the community.

Every mathematical derivation, every deduced principle, everything that ever was, came

from somewhere. Solar, being no exception to the rule, also has a history, albeit short in

comparison. Understanding the progression from the dream to the reality can give

insight into the contemporary potential of the Organic Solar Cell.

The first understanding of the potential to formulate mechanical power from

photons came in 1839 from Edmund Bequerel, a french physician who famed himself

on the study of the solar spectrum. At a time when energy derived from the sun and

electrical energy were seen to be fundamentally independent, Bequerel was the first to

propose that certain materials could produce small electrical currents when placed

under direct sunlight. Although his study remained available, it did not gain practical

viability until another engineer, attempting to stretch telegraph wires beneath the ocean

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Experimental Section

A conventional photovoltaic instrument contains layers of 4 metalloid structures,

typically (Ag, Pd, Ti, and Si), which create an electron hole as a path for electrons,

excited by photons to travel, generating a current3. Organic photovoltaic cells operate in

a different fashion: Typical bilayer panels contain two conducting electrodes surrounding

electron donor and acceptor layers, which have different ionization energies and

electron affinities, allowing for electrons, to excite from the donor to the acceptor

groups, creating a continuous current5. Since the early stages of development,

Fullerenes (C60), have been applied as electron

acceptors, while a variety of phthalocyanines (Ni, Cd),

have served as electron donors, however the use of

these metals has been discouraged by the high price7.

As an alternative, pi-conjugated polymers, with unusual

electrical and photoconductivity have been developed

to replace the metalloids. This group of polymers

generally consists of monomers with a high HOMO/ LUMO level, allowing for high

absorbance, as well as increased surface contact with the fullerene acceptor8. In one

particular case, the use of a benzo-thiophene core monomer demonstrated high UV and

IR absorbance8. Higher absorbing monomers will allow for increased OPV power

conversion efficiency.

The use of DPP and pyrrolo monomers has been the subject of research for the

DV group at The University of Massachusetts Amherst, led by Dr. Venkataraman of the

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Chemistry Department6. Their work has centered on addressing three main issues with

OPV at their current state: expensive metals comprising the donor layer, the inefficient

planar layers, which do not maximize contact area, and the unknown implications of

scaling up synthesis processes for industrial production. Current work has centered on

synthesizing a polymer with high UV and IR absorbance, and the arrangement of donor

and acceptor groups in a heterojunction pattern, that is, in an intertwined series of

layers, which maximize donor/ acceptor contact. Working off of aspects of a protocol,

two different versions of the donor polymer have been synthesized from a common

pyromellitic dianhydride core8

. The synthesis of monomers was conducted through

either the alkylation of the brominated pyrometallic anhydride with THF and hexylamine,

or through an iodination of the anhydride using iodine and dichloromethane6 . An

alternative was the bromination of the anhydride with Br2 and oleum refluxed for

approximately one week. The work-up of the monomer was confirmed with NMR/

MassSpec, and the pure product was extracted with column separation.

synthesis diagram of pyrometallic monomer

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Scheme 1. Monomer synthesis.

Alternative synthesis of pyrometallic monomer with pi conjugated aryl groups

The polymerization of both was performed with a similar core monomer using Ni(II)

catalyst and P(Ph3) complex, which produced an electron donor with high UV

absorbance and a low HOMO/ LUMO gap8. Continuing efforts were made by the DV

Group in using a Sn(II) complex, in addition to the P(Ph3)6.

NMR spectrum of monomer product prior to polymerization

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A typical donor polymer was used in the OPV, which achieved a conversion

efficiency of 1.8% and a quantum

efficiency of 25%.

A potential issue resulting from

the above synthesis protocol above

comprises the low percentage yields

of the polymer from the initial starting

material, app. 25% for the brominated

anhydride8

. Should the D-group show high power transfer efficiency, a scale-up of the

procedure could be designed to increase the yield, potentially with the use of stronger

catalysts or more effective solvents. Additionally, a pyrometallic anhydride could be

brominated with Br2 to bypass loss of mass through the reaction with AcOH6. A higher

yield would decrease the needed quantity of initial starting materials, further lowering

the cost of OPVs.

Another issue with conventional OPVs is the loss of overall operational efficiency

as a result of bilayer donor and acceptor group positioning. This design fails to fully

maximize the surface area contact

between the polymer and fullerene,

which allows fewer excited

electrons to jump between the

two layers5. The DV group has

proceeded with the development of

graded heterojunction arrangements, which maximize surface contact by gradually

Scheme 2. Polymer synthesis.

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altering the donor, such that the two form graded composition. Although literature

currently contained heterojunction OPVs, The DV group enhanced the design by adding

nanocrystalline particles to create an architecture that would increase D-A surface area

and the exciton (excited electron) transfer efficiency10. Additionally, the difference of

active surface thickness appeared to have a varying effect on the overall performance

of the OPV. Similar trials in literature, using inorganic D-groups, show power conversion

efficiencies of approximately 2.1%5, which have been mirrored by the DV lab using

polymer D-groups with power conversion efficiencies of approximately 1.9-2.3%6.

Despite the relatively low efficiency of OPV solar cells (2.1% versus a maximum

of 23-25% with crystalline Si cells) 9, OPV unique properties provide both market

viability and environmental benefits not available in conventional silicon PVs. Firstly, the

initial high cost of mining the metals found in Si panels is removed due to the lack of

metalloid compounds, (except for the electrodes) in every OPV cell; the production cost

of the polymer D-group and C60 A-group is considerably low in comparison5.

Additionally, OPVs exhibit flexibility and thinness, which allow them the potential to

cover large areas, or flexible surfaces such as films or fabrics, giving them potential

application in packaging, textiles, or recharging batteries in laptops and mobile phones4.

In a sense, the OPV market, if not able to compete with the efficiency of conventional

silicon PVs, will have the ability to provide solar cell solutions to a wider range of

industrial applications. Thirdly, OPV cells will decrease the negative environmental

impact associated with the manacturing of traditional silicon cells. Unlike crystalline

silicon, which must be formed at high temperatures (approximately 1600C9, all

assembly of OPVs occurs at temperatures between RT and 100C, lowering the

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necessary energy usage in the synthesis process6. Additionally, the metalloid PV

manufacturing process involves the use of hazardous inorganic chemicals, such as

trimethyl gallium, trimethyl aluminum, and trimethyl indium, in addition to hydride gases

including arsine and phosphine1. In comparison, the manufacturing process of OPVs do

not cause any significant health concerns, aside from the typical laboratory hazards of

solvent handling and proper disposal. The additional environmental and economic

benefits will be more explicitly described in the following sections. .

The Future Prospects

Among pi conjugated conducting polymers, special attention has been paid to the

absorbance properties of the aryl coupled pyrometallic anhydride, showing a high UV,

IR index absorbance. This is coupled with a high quantum efficiency, theoretically high

cap power conversion efficiency. Improvements to the material absorbance of the

incident light layers, in addition to a more efficient heterojunction of D-A groups, as well

as the full replacement of fullerene with a synthetic polymer as the primary acceptor.

However, the largest improvement over the typical OPV cell lies in the potential

for higher absorption in the donor layer. This can be achieved either through the

improvement of a polymer with a lower HOMO/LUMO band-gap or a better metalloid

cathode. Until present only translucent ITO metals have been considered. A great

amount of potential remains untapped in attempting an alternative structure. A quantum

dot layer would allow through confinement mechanics permits generation of up to seven

excitons for every 1 in a typical quantum layer. The mathematical basis for this

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phenomenon lies in the energy and wave function of a single carrier, which are solutions

for a semiconductor structure from the Schrodinger equation

( Hkp+Vc(r) )!(r) =E!(r)

However, quantum confinement is a far more useful property of a quantum dot cell

considering the potential for p-n junction tuning within the upper layer. The energy

potential eV bandgap within quantum confinement.

Typical multijunction cells are limited by the quantum efficiency of the metals

used in p-n junctions (GaAs, Ge, InGaP), however, due to the exciton potential of the

quantum dot layers, the power conversion efficiency of these new cells would be raised

from 31% to 42%, defying the effective efficiency barrier set by Shockley and Queisser

for a single threshold absorber. The following image demonstrates a schematic of a

Here kpH is the single band kp-Hamiltonian operator, &'&=)(*2

2

rmHkp "

!,

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potential model. Typical junctions with

properties listed in the table below mimic

this property.

Materi

al

Eg,

eV

a,

nm

absorption

(!= 0.8 "m), 1/

m

n, cm#/

(Vs)

$p,

s

Hardne

ss

(Mohs)

%, m/

K

S, m/s

c-Si 1.12 0.54 0.102 1400 1 7 2.6 0.160

InGaP 1.86 0.55 2 500 5 5.3 50

GaAs 1.4 0.57 0.9 8500 3 45 6 50

Ge 0.65 0.57 3 3900 1000 6 7 1000

InGaAs

1.2 0.59 30 1200 5.66 1001000

This tuning would intensify incident photon contact with the heterojunction, thereby

increasing not only power and quantum efficiency,

but also total power output. The usage of a gradient

nanofilm would allow multiple times the electron-hole

potential where the electrons are collected through

percolation and diffusion and serve as either a

junction or as a component in the donor polymer

layer, where the heterojunction was comprised

partially of the nanofilm. The internal electric field

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would fuel a higher generation of excitons and increase the internal efficiency of the cell.

Absorption was determined using two D/A structures in an OPV.

Assumes

where tis the measurement

time (in seconds), =incident optical power in

watts, = optical powerabsorbed in depletion layer,

also in watts.

With a fullerene based heterojunction having optimum absorption at ~550 nm, as in the

above example, a quantum dot layer could increase exciton yield, while maintaining the

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flexibility of conventional organic photovoltaic instruments. A conventional p-n

multijunction could also be used for this effect.

CeAs quantum dot synthesis and chromatographic separation of the pure product.

However, this remains only a portion of the reason that the quantum dot OPV has the

potential to become a viable solution to the economic and environmental obstacles

faced by the solar market. At present the power conversion efficiencies of solar

instruments are divided very clearly between organic and inorganic systems.

Although solar junction

s y s t e m s ( i n o r g a n i c

generally) have a clear

advantage in terms of

efficiency, this can mainly

be attributed to a high

incident absorption. By combining an inexpensive synthesis of quantum dot

semiconductor boundaries containing a similar tuning, the prototype has the potential

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for increased power conversion efficiency and low cost associated with donor/ acceptor

layers. This principle is explicated in the following two sections.

Economic Initiatives

This optimization could bring forward a new revolution in inexpensive solar

manufacturing. The economic interest in the solar market is divided into two main points

of argument; An alternative to the rising costs of inorganic based solar cells and the

addition of new functional possibilities that are not available in conventional

photovoltaics. This initiative adds viability to the argument for inexpensive quantum

hybrids of organic cells.

Currently, the market for silicon solar panels prices a typical home module at

approximately $18,000 to $40,000, depending on the scope and the geographical

location of the installation. However, this price will continue to rise if the availability of

silicon extract dwindles. If manufactured, quantum organic cells would decrease the

capital cost of installation and have virtually no operational cost. For geographical areas

with high average sunlight, such as those in Saharan and sub-Saharan Africa, the low

cost of OPVs would be favorable, given the added low GDP per capita in this area.

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Solar Energy as a whole has the potential to drop in operational cost over the next few

years which will only be compounded if the organic solar cell is made more efficient.

The added benefit of a more flexible process of installation, which does not include the

replacement of roofing fixtures should also drive down this value.

A lot of room to grow

Currently the energy market is

dominated by the petroleum,

coal, and natural gas sectors,

comprising 82% of total energy

production in the United States.

As solar and other renewables

comprise only 1% of the current

market, there is significant need

and potential for greater participation of solar technology in the energy market. Our

continued dependence on the use of non-renewable resources will have dire economic

liabilities in the coming years as worldwide reserves become exhausted.

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The

renewable alternative

energy market in 2009.

Although comprising of

only 1% of total energy use,

solar power has continued

to dominate alternative

energy

In addition, the

new potentials opened

up by the advent of an

organic solar cell would be very dramatic in terms of new market applications. Whereas

previous uses of solid state devises limited the installations to array panels, the new

modules could be liquid in state, allowing them to be applied to a variety of new areas.

From automobiles to buildings to textiles and computing, the new liquid state cells could

revolutionize how and where these cells are used. In an era of hybrid mechanisms, the

liquid solar cell is an indispensable addition to a synthesis of various fuels. Its thinness

and versatility as well as its stability all comprise factors, which provide it with the

potential to economic success on the market if properly manufactured and synthesized.

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Environmental Impact

The impact of

precious metal

d e p l e t i o n i s

dramatic. At the

current rates of

consumption and

deplet ion, the

most optimistic

estimates allot

only several hundred years to the time span of essential metals on Earth. With the

growing population estimated to continue an almost undeterred increase by a billion

individuals every 20 years, even a lessening dependence on metals will not halt their

depletion13. At the current state, only the innovation of non-metallic energy sources will

not be devastated by the current shortage in metals. In addition, the synthesis and

deposition of the metals after usage also contribute to their negative impact on the

worlds ecosystems. Silicons doping process in conventional cells occurs at 1600C and

requires a large amount of industrial energy as well as precise and inefficient

extraction9. Cadmium Telluride has the potential to form hazardous ions when it is

disposed after its expected lifespan14. While the two have questionable safety records,

the organic layers of the panels described in this paper are synthesized at RT - 100C, in

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standard lab conditions. Their derivatives are generally stable and do not produce

unstable or dangerous ions after their operational lifespan ends.

Conclusion

Nickel (II) Alkylated pyrometallic polymer acceptor with variating donor polymer/

quantum dot nanofilm surfactant heterojunction tuned to optimal absorbance with the

use of quantum dot tuning has the potential for wide ranging application due to high

optimal efficiency and low capital and operational cost as well as low environmental

impact. These benefits may transcend the typical and intrinsic assumptions that the

solar and energy fields are without fundamental benefit or innovation. Indeed, it is the

responsibility of those in the scientific community to vocalize the impact of the

groundbreaking research and development. Too often, developments with wide-

reaching potential and revolutionary applications are not available outside of the

Science based RSS feed. Innovation is not dead, it is simply fallen below the surface of

the national discussion, and this is unacceptable.

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1. Fthenakis, Vasilis, and Biays Bowerman. "ENVIRONMENTAL HEALTH AND SAFETY

(EHS) ISSUES IN III-V SOLAR CELL MANUFACTURING." Print.

2. "Dye-sensitized Solar Cell." Wikipedia, the Free Encyclopedia. Web. 03 Nov. 2011.

3. Mitra!inovi", Aleksandar M., and Torstein A. Utigard. "Refining Silicon for Solar Cell

Application by Copper Alloying." Silicon1.4 (2009): 239-48. Print.

4. "Organic Photovoltaics, Organic Solar Cell, Graded Heterojunction - Office for Technology

Commercialization, Express_license, University_of_Minnesota,

Technology_Marketing_Site." Technology Marketing Site : Office for Technology

Commercialization : OVPR : University of Minnesota - Office for Technology

Commercialization, Express_license, University_of_Minnesota,

Technology_Marketing_Site. University of Minnesota. Web. 03 Nov. 2011>.

5. "Organic Solar Cell." Wikipedia, the Free Encyclopedia. Web. 03 Nov. 2011.

6. Venkataraman, Dhandapani. Organic Photovoltaics. DV Group. University of Massachusetts

Amherst. Amherst. October 15, 2011. Discussion

7. Pandey, Richa, and Russell J. Holmes. "Organic Photovoltaic Cells Based on Continuously

Graded Donor--Acceptor Heterojunctions."IEEE Journal of Selected Topics in Quantum

Electronics(2010). Print.

8. Rhee, Tae Hyung, Taeyoung Choi, Eun Young Chung, and Dong Hack Suh. "Soluble and

Processable Poly(p-phenylene) with Pendant Imide Groups."Macromolecular Chemistry

and Physics202.6 (2001): 906-10. Print.

9. "Silicon Crystal Growing or Casting." Solar Buzz. NPD Group. Web. 3 Nov. 2011.

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10. Tu, Kun-Hua, Shao-Sian Li, Wei-Chih Li, Di-Yan Wang, Jer-Ren Yang, and Chun-Wei Chen.

"Solution Processable Nanocarbon Platform for Polymer Solar Cells."Energy &

Environmental Science4.9 (2011): 3521. Print.

11. "The Armo Trader." http://jerrykhachoyan.com/. N.p., n.d. Web. 26 June 2012.


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Nano Lett., Vol. 9, No. 10, 2009

analysis and cyclic voltammetry. Charge-discharge curves. This material is available free

of charge via the Internet at http://pubs.acs.org.

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LETTER doi:10.1038/nature11067

All-solid-state dye-sensitized solar cells withhigh efficiencyIn Chung1, Byunghong Lee2, Jiaqing He1, Robert P. H. Chang2 & Mercouri G. Kanatzidis1

Dye-sensitized solar cells based on titanium dioxide (TiO2) arepromising low-cost alternatives to conventional solid-state photo-

voltaic devices based on materials such as Si, CdTe andCuIn12xGaxSe2(refs 1, 2). Despite offering relatively high conver-sion efficiencies for solar energy, typical dye-sensitized solar cellssuffer from durability problems that result from their use oforganic liquid electrolytes containing the iodide/tri-iodide redoxcouple, which causes serious problems such as electrode corrosionand electrolyte leakage3. Replacements for iodine-based liquidelectrolytes have been extensively studied, but the efficiencies of

the resulting devices remain low39

. Here weshow that thesolution-processable p-type direct bandgap semiconductor CsSnI3 can beused for hole conduction in lieu of a liquid electrolyte. The result-ing solid-state dye-sensitized solar cells consist of CsSnI2.95F0.05doped with SnF2, nanoporous TiO2 and the dye N719, and showconversion efficiencies of up to 10.2 per cent (8.51 per cent with amask). With a bandgap of 1.3 electronvolts, CsSnI3 enhances

visible light absorption on the red side of the spectrum to out-perform the typical dye-sensitized solarcells in this spectral region.

Photovoltaics is a promising renewableenergy technology that con-verts sunlight to electricity, with broad potential to contribute signifi-cantly to solving the future energy problem that humanity faces. Todate, semiconductor solar cells dominate commercial markets, withcrystalline Si having an 80% share; the remaining 20% is mostly thin-film solar technology, such as CdTe and CuIn12xGaxSe2(ref. 10). Theformer is an indirect bandgap semiconductor typically requiring a300-mm-thick absorption layer, and material and processing costsare very high. The latter contains elements that are toxic and of lowabundance in the Earth. However CuIn12xGaxSe2forms the best per-forming thin-film solar devices, exhibiting an efficiency of,20%, butis more than 1.4times as expensiveas CdTe andamorphous Si. A low-cost and environmentally friendly alternative to these solid-statedevices is the dye-sensitized solar cell (DSC)1,2. It is inexpensive toprepare, and the light-weight thin-film structures are compatible withautomated manufacturing.

Conventional DSCs (Gratzel cells) consist of a self-assembledmonolayer of molecular dye at the interface between a mesoporouswide-bandgap semiconductor oxide and a liquid electrolyte1,2. Themost commonly used redox couple is iodide/tri-iodide (I/I3) in anorganic liquid electrolytehowever, it is highly corrosive, volatile andphotoreactive, interacting with common metallic components andsealing materials. Consequently, it adversely affects long-term per-formance and durability3. Alternative liquid electrolytes free of theI/I3 redox couple have been a long-term goal in this field 5,6,11. Theultimatesolutionswouldbe purely solid-state cells, giventhe inevitable

problems of any liquid electrolyte, such as leakage, heavy weightand complex chemistry. Efforts have focused on using solid-state organic or p-type conducting polymer hole-transport materials(HTMs), but their conversion efficiency remains modest. Solar cellsusing spiro-OMeTAD (refs 9, 12) and bis-EDOT (ref. 7) exhibitthe highest conversion efficiencies among organic and conduct-ing polymer materials of 6.08% and 6.1%, respectively. (TAD is

2,29,7,79-tetranis(N,N-di-p-methoxyphenyl-amine)9,99-spirobifluorene,and EDOT is 2,29-bis(3,4-ethylenedioxythiophene).) A generalproblem of solid HTMs in DSCs is poor filling of the nanoporousTiO2 layer; this interrupts the hole-conducting path between theHTM and the dye molecule adsorbed on TiO2(ref. 1). Despite manyanticipated advantages, inorganicHTMs are uncommon. CuI,CuSCNandNiO areexamples,but their mobilities arevery low. Theefficiencyof CuI-based DSCs was found to initially reach about 3%, but rapidlyphotodegraded1. Cells of CuSCN (ref. 13) and p-type NiO particles14

showed low efficiencies.

Here we report a new type of all-solid-state, inorganic solar cellsystem that consists of the p-type direct bandgap semicon-ductor CsSnI3and n-type nanoporous TiO2with the dye N719 (cis-diisothiocyanato-bis(2,29-bipyridyl-4,49-dicaboxylato) ruthenium(II)bis-(tetrabutylammonium)). We show that CsSnI3 is well fitted forthis purpose because of its energy gap of 1.3 eV and a remarkably

high hole mobility ofmh5585 cm2 V21 s21 at room temperature. We

foundthat CsSnI3 is soluble in polarorganicsolvents, suchas acetonitrile,N,N,-dimethylformamide and methoxyacetonitrile. Consequently,it issolution-processable and can be transferred into TiO2 pores at amolecular level to make intimate contacts with dye molecules andTiO2. We present results showing that doping of CsSnI3with F and

1Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA. 2Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 USA.

300 400 500 6000

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Figure 1| Crystal structure and optical and electrical transport propertiesof CsSnI3. a, Distorted three-dimensional perovskite structure of CsSnI3atroom temperature. Red polyhedron, [SnI6/2]

2; yellow sphere, Cs. b, Sharpabsorption edge at 1.3 eV of CsSnI3. A, absorption in units ofaS

21, where a isthe absorption coefficient and Sthe scattering coefficient.E, energy in eV. Eg,the value of the energy gap. c, A typical ingot of CsSnI3grown in a Bridgmanfurnace. d, Temperature dependence of electrical conductivity (s, filledsquares) and Seebeck coefficient (S, filled circles).

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SnF2dramatically improves the photocurrent density (JSC) and powerconversion efficiency (g).At anoptimummolarconcentrationof 5%Fand5%SnF2, thecellexhibitsthe highest efficiency sofar reportedforasolid-state solar cell equipped with a dye-sensitizer: g5 10.2% underthe standard air mass 1.5 (AM 1.5) irradiation (100mW cm22), and

g5 8.51% with a mask. The observed value is close to that of thehighest reported performance N719-dye-containing Gratzel cell(g


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rectangular area shown in Fig. 3a demonstrates excellent distributionof Cs, Sn and I atoms throughout the 10-mm-thick nanoporous TiO2(Fig. 3b). The cross-sectional back-scattering electron image shows nodiscernible contrast difference over the examinedarea, suggestingthatCsSnI3homogeneously spreadsover the entire TiO2area (Fig.3c).The

cross-sectional high-resolution transmission electron microscopy(HRTEM) image taken at the bottom part of the TiO2electrode indi-cates that CsSnI3effectively fills the TiO2pores, and crystallizes onthesurface ofthe TiO2 (Fig.3d). TheCsSnI3andTiO2phasesare observedtooverlapwitheachother,as shownin area A inFig.3d. Area B hastoohigha(hkl) indexto give clearlatticefringes. Area C inFig.3d indicatesTiO2. Figure 3e shows experimental electron diffraction patterns (left)in comparison with that of the theoretical TiO2 (right). The ringpatterns are assigned to anatase TiO2, and the spots (indicated bythe white arrows) between the rings of TiO2correspond to CsSnI3.

To efficientlyoperateDSCs,the dyesensitizerthat is adsorbed on thesurfaceof thenanoporous TiO2 transfers anelectron toTiO2 andisthenregenerated by an electrolyte or p-type conductor1,2. Consequently,intimate contact between the latter and dye sensitizer is essential forperfectcharge separation. However, homogeneousinfiltrationof p-typesolid HTMs throughout the n-type nanoporous materials is the mainchallenge for solid-state DSCs1,3. Organic polymer HTMs do not effi-ciently infiltrate the micrometre-thick nanoporous TiO

2, degrading

performance. When monomers are used for better penetration intothe pores, undesirable complex chemical reactions, such as photo-electrochemicalpolymerization, are required2,10. Thekey characteristicsof CsSnI3 in our solar cells is that (1) it is solution-processable, andthus permeates throughout the entire TiO2 structure, allowing facilecharge separation and hole removal, and (2) it exhibits very large holemobilities.

To compare the optical response of the CsSnI2.95F0.05cell with thatof a conventional Gratzel cell, the latter was prepared with N719 dyeandshowed,10% efficiency18. Theopticalabsorption spectrumof theCsSnI2.95F0.05-containing cell, obtained in transmission mode, revealsa well-defined edge at 789 nm, which is significantly red-shifted fromthat of the Gratzel cell18 with N719 dye, at 667 nm (Fig. 4a). Thisobservation indicates that our cell absorbs red and near-infrared lightmoreefficientlythan the Gratzelcell. Notethat lackof sunlightabsorp-tion in the red and near-infrared regions has been a challenge fortypical ruthenium-based dyes. Figure 4b compares the incident

photon-to-current conversion efficiency (IPCE) as a function ofexcitation wavelength for the CsSnI2.95F0.05cell with the Gratzel cell.The IPCE spectrum is a measure of the light response of photovoltaic

devices, which is directly related to the short-circuit current. In the5502670 nm spectral range, our cell produces a higher and broaderphotocurrent density in the external circuit under monochromaticillumination (per photon flux). Note that the upper limit of ourIPCE measurement setting is700 nm,resultingin a sharpdropbeyond

670nm.The pristine CsSnI3cell gave very good photocurrent densityvolt-

age (JV) characteristicsopen-circuit voltage (VOC), fill factor (FF),short circuit current density (JSC) and overall power conversion effi-ciency (g)as a solid-state solar cell: VOC5 0.638 V, FF566.1%,JSC5 8.82mAcm

22, g53.72% (g5VOC3JSC3FF) (Fig. 5). ToimproveJVcharacteristics, we studied the effect of fluorine dopingon CsSnI3. The optimum molar concentration of 5% fluorine dopinggave a remarkable increase in JSC, which reached 12.2mA cm

22,resulting in a 1.5-fold larger value ofg, 5.62%. Further improvementwas obtainedby introducing SnF2 intoCsSnI2.95F0.05.TheSnF2 dopingprocess was simple. The desired amounts of SnF2and CsSnI2.95F0.05powderswere added to polarorganicsolvents withstirring. Theresult-ing solutions were injected onto the nanoporous TiO2electrodes. TheCsSnI2.95F0.05sample doped with 2% SnF2provided a 29% and 21%increase inJSC(15.7mA cm

22) andg(6.81%), respectively, comparedto the CsSnI2.95F0.05 sample. The optimum molar concentration of

SnF2dopingin CsSnI2.95F0.05wasfoundto be5%. Forthe correspond-ingcell, theTiO2 nanoporous filmwas pre-treated by a fluorineplasmaetching processto increasethe sizeof thenanopores andnanochannelsas described in the literature18. This also possibly helps to reducesurface states and charged particle recombination18. The resulting cellshowed very good JV characteristics: JSC5 17.4mA cm

22;VOC50.730 V; FF5 72.9%, g5 9.28%.

To fully employ the photon flux absorbed, we applied two layers ofthe three-dimensional inverse photonic crystal ZnO (ref. 19) over thecounter electrode of the same cell. Each layer of the photonic crystalhad a different hole diametervalues of 375 nm and 410nm wereused. The corresponding cell exhibited JSC5 19.2mAcm

22,VOC50.732V, FF572.7% and g5 10.2%. When a maskwas appliedon the cell, g5 8.51% was observed (Supplementary Information).The observed efficiency is the highest among any kind of dye-sensitized solar cell free of liquid electrolyte, and is close to that of

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Figure 4| Optical response of the CsSnI2.95F0.05cell and a conventionalGratzel cell. a, Optical absorbance spectra of the devices consisting ofCsSnI2.95F0.05/N719 dye/TiO2(red line) and N719 dye/TiO2(liquid electrolytewas not added here) (black line).b, The IPCE spectrum as a function of thewavelength of monochromatic light that impinges on the CsSnI2.95F0.05cell(filled circles ) in comparison with that of the N719-dye-contain ing Gratzel cell(filled squares).

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CsSnI2.95F0.05

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Figure 5| Photocurrent densityvoltage (JV) characteristics of the solarcell devices under irradiation of 100mW cm2 simulated AM 1.5 sunlight.These solar cells incorporated CsSnI3and CsSnI2.95F0.05doped with SnF2.Fluorine and SnF2doping increase photocurrent density, resulting in animproved power conversion efficiency (g). Application of three-dimensionalZnO photonic crystal layers further enhances the photocurrent density, andgives thehighestvalue ofg (10.2%)forthecellofCsSnI2.95F0.05 dopedwith SnF2.

RESEARCH LETTER

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the high performance Gratzel cell (g< 11%). The samples with 10%SnF2resulted in a reduction in JSC (measured value, 13.6mA cm

22)and g (measured value, 5.46%).Compoundsof the typeCH3NH3PbX3(X5Br, I), which are isostructural with CsSnI3, have been used asvisible light sensitizers adsorbed on the TiO2surface for photovoltaic

cells. These cells, however, employed organic liquid electrolyte con-taining LiX/X2redox couples, and exhibited low efficiency20.

We have demonstrated the superior performance of the p-typeinorganic high-hole-mobility semiconductor CsSnI32xFx and itsability to replace the problematic organic liquid electrolytes in dye-sensitized solar cells (DSCs). We refer to these solid-state solar cells assolid-state DSCs (SSDSCs). Unlike liquid electrolytes with theircomplex chemistry, crystalline inorganic solids promise long-termstability, and also enable theoretical calculations based on electronicstructure and further improvement of their photovoltaic properties.The newsolar cells described hereare all-solid-state, inorganic systems.TheCsSnI32xFxcompounds consistof inexpensive, abundantelementsand are solution-processable at room temperature, allowing for low-costprocessing. Ournew cellreaches a conversionefficiencyof,10.2%(8.51% with a mask), and is the first example of an all solid-state dye-sensitized solar cellsystemthat mayeventually exceed theperformanceof a liquid electrolyte Gratzel cell . With further optimization and newdyes11, much higher efficiencies are likely. This work opens up the

possibility of semiconducting solid materials becoming state of theart and promoting much higher efficiencies than have been possiblewith conventional DSCs.

METHODS SUMMARY

Synthesisof CsSnI32xFx. PureCsSnI32xFx(0#x#1) compounds were achievedbyheatinga stoichiometric mixtureof CsI,SnI2andSnF2 inan evacuated Pyrex orfusedsilicatubeat 450uC for30 min,followed byquenchingto roomtemperature.The ground powders (,100350 mg) were dissolved/dispersed in anhydrouspolar organic solvents (1.5 ml): N,N-dimethylformamide, acetonitrile andmethoxyacetonitrile. For SnF2 doping, appropriate ratios of CsSnI2.95F0.05 andSnF2powders were stirred in the same organic solvents.TiO2electrode preparation and device assembly. TiO2electrode preparation, afluorine plasmaetchingprocess,and device assembly aredescribed inthe literature18.The solutions of CsSnI32xFx(with SnF2if necessary) were injected into the cell by amicropipette anddried.The ZnOphotoniccrystalswere preparedas describedin theliterature19. They were attached on the top of the counter-electrode if necessary.

Full Methodsand any associated references are available in the online version of

the paper at www.nature.com/nature.

Received 6 February; accepted 8 March 2012.

1. Hagfeldt,A., Boschloo,G., Sun, L. C., Kloo, L. & Pettersson, H. Dye-sensitized solarcells.Chem. Rev.110,65956663 (2010).

2 . G ratzel, M. Recent advancesin sensitized mesoscopic solar cells.Acc. Chem. Res.42,17881798 (2009).

3. Yanagida,S., Yu, Y. H. & Manseki, K. Iodine/iodide-free dye-sensitized solar cells.Acc. Chem. Res.42,18271838 (2009).

4. Koh,J. K., Kim,J., Kim, B., Kim,J. H. & Kim,E. Highlyefficient, iodine-freedye-sensitized solarcells withsolid-statesynthesisof conductingpolymers.Adv.Mater.23,16411646 (2011).

5. Daeneke, T.et al. High-efficiency dye-sensitized solar cells with ferrocene-basedelectrolytes. Nature Chem.3,211215 (2011).

6. Wang, M. K. et al. An organic redox electrolyte to rival triiodide/iodide in dye-sensitized solar cells.Nature Chem.2,385389(2010).

7 . Liu ,X.Z. etal. An efficient organic-dye-sensitized solarcell within situpolymerizedpoly(3,4-ethylenedioxythiophene) as a hole-transportingmaterial.Adv. Mater. 22,E150E155 (2010).

8. Jiang, K.J. et al.Photovoltaics based onhybridization of effective dye-sensitizedtitanium oxide and hole-conductive polymer P3HT.Adv. Funct. Mater. 19,24812485 (2009).

9. Bach, U. et al.Solid-state dye-sensitized mesoporous TiO2solar cells with highphoton-to-electronconversion efficiencies.Nature395, 583585 (1998).

10. Bisquert, J. Dilemmasof dye-sensitized solar cells.ChemPhysChem12,16331636 (2011).

11. Yella, A. et al.Porphyrin-sensitized solar cells with cobalt (II/III)-based redoxelectrolyte exceed 12 percent efficiency.Science334,629634 (2011).

12. Cai, N. et al. An organic D-p-A dye for record efficiency solid-statesensitizedheterojunction solar cells.Nano Lett.11,14521456(2011).

13. ORegan, B., Lenzmann,F., Muis, R. & Wienke, J. A solid-statedye-sensitizedsolarcellfabricated withpressure-treatedP25-TiO2 andCuSCN:analysisof porefillingand IV characteristics. Chem. Mater. 14,50235029(2002).

14. Bandara,J. & Weerasinghe,H. Solid-statedye-sensitizedsolarcellwithp-typeNiOas a hole collector.Sol. Energy Mater. Sol. Cells 85,385390 (2005).

15. Kroon, J. M.et al. Nanocrystalline dye-sensitized solar cells having maximumperformance. Prog. Photovolt. Res. Appl. 15,118(2007).

16. Shum, K. etal. Synthesis and characterizationof CsSnI3 thinfilms.Appl.Phys. Lett.96,221903(2010).

17. Gratzel, M. Photoelectrochemical cells.Nature414, 338344 (2001).18. Lee,B., Buchholz, D. B., Guo,P. J., Hwang, D.K. & Chang,R. P. H. Optimizing the

performance of a plastic dye-sensitized solar cell.J. Phys. Chem. C115,97879796 (2011).

19. Lee, B. et al. Materials, interfaces, and photon confinement in dye-sensitized solarcells.J. Phys. Chem. B114,1458214591(2010).

20. Kojima,A., Teshima,K., Shirai,Y. & Miyasaka,T. Organometalhalideperovskitesasvisible-light sensitizers for photovoltaic cells.J. Am. Chem. Soc.131,60506051(2009).

Supplementary Informationis linkedto the online version of the paper atwww.nature.com/nature.

Acknowledgements The authorsacknowledge supportfor thiscollaborativeresearch:NSF-DMR 0843962 for R.P.H.C.; DOE Energy FrontierResearch Center, ANSER,DE-SC0001059forB.H.L.,J.H.andM.G.K.;theInitiativeforEnergyandSustainability atNorthwestern(ISEN)for I.C.Devicetestingandmeasurementsweredonein theANSERFacilities and materials characterization was performed in the NSFMRSEC Facilities(DMR-1121262).

Author ContributionsI.C.and M.G.K. conceived and designed the experiments andprepared the manuscript. I.C. synthesizedmaterials. R.P.H.C. and B.L. designed andfabricatedthe solar cells, I.C. and B.L. performed measurements. J.H. collected TEMdata. I.C., B.L., R .P.H.C. and M.G.K. discussed the results and wrote the manuscript.

Author InformationReprints and permissions information is available atwww.nature.com/reprints. The authors declare no competing financial interests.Readers are welcome to commenton the online version of this article atwww.nature.com/nature. Correspondence and requestsfor m aterials should beaddressed to M.G.K. ([email protected]).

LETTER RESEARCH

Photovoltaics of Polymer Heterojunctions

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