Organic Solar Cells and Molecular Photovoltaics

.Photovoltaics

áTomás Torres Universidad Autónoma de Madrid

Solar Energy Utilization

H2O

O

CO2

sugar50 - 200 °C 500 - 3000 °C

O2

naturalphotosynthesis

5 space, water

heatingheat engines

electricity generationprocess heat

Solar ElectricSolar Fuel Solar Thermal

.001 TW PV$0.30/kWh w/o storage 1.4 TW solar fuel (biomass) 0.002 TW

1.5 TW electricity $0.03-$0.06/kWh (fossil)

11 TW fossil fuel (present use) 2 TW

space and waterheating

~ 14 TW additional energy by 2050heating

Solar Energy and Nanotechnologygy gy

Based on:Basic Research Needs for Solar Energy Utilization:

Nathan S. LewisGeorge L Argyros Professor of ChemistryGeorge L. Argyros Professor of ChemistryCalifornia Institute of Technologywith George Crabtree, Argonne NLArthur Nozik, NRELMike Wasiele ski No th este nMike Wasielewski, NorthwesternPaul Alivisatos, UC-Berkeley

Basic Research Needs for Solar Energy

• The Sun is a singular solution to our future energy needs- capacity dwarfs fossil, nuclear, wind . . .- sunlight delivers more energy in one hour

than the earth uses in one year- free of greenhouse gases and pollutants- secure from geo-political constraints

• Enormous gap between our tiny use g p yof solar energy and its immense potential- Incremental advances in today’s technology

will not bridge the gap- Conceptual breakthroughs are needed that come

only from high risk-high payoff basic research

• Interdisciplinary research is requiredphysics, chemistry, biology, materials, nanoscience

B d l d h ld l l l• Basic and applied science should couple seamlesslyhttp://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf US Department of Energy

lSolar Energy Spectrum

2• Power reaching earth 1.37 KW/m2

Nanoscience and Solar Energy

manipulation of photons, electrons, and molecules

N

artificialphotosynthesis

TiO2nanocrystals

adsorbeddyeg

glas

s

naturalphotosynthesis

dye

liquidelectrolyteco

nduc

ting

tran

spar

ent

elec

trod

e

quantum dot solar cells

photosynthesis

nanostructuredthermoelectrics

t

theory and modelingmulti-node computer clusters

density functional theory

nanoscale architecturestop down lithography

bottom up self-assembly

characterizationscanning probes

electrons neutrons x-rays density functional theory10 000 atom assemblies

bottom up self assemblymulti-scale integration

electrons, neutrons, x rayssmaller length and time scales

S l n is int disciplin n n sci nc Solar energy is interdisciplinary nanoscience

Solar cells are one of the most promising devices in search of sustainable renewable sources of energy. Although silicon cells based on solid-state p-n junction devices have dominated the field, new generations of

l l ll (PSC BHJSC) i- polymer solar cells (PSC, BHJSC) or organic photovoltaics (OPV) solar cells,

- oligomer (small molecule) solar cells (OSC) andoligomer (small molecule) solar cells (OSC) and - hybrid solar cells (Dye sensitized solar cells, DSSC)

are emerging.

Comparing the last ones and the inorganic silicon based cells in 3 categories i e efficiency lifetime/stability andcells in 3 categories, i.e. efficiency, lifetime/stability andcost, it is evident that the OPVs dominate in the low production cost and the inorganic dominate in the other 2production cost and the inorganic dominate in the other 2 areas with efficiencies of 10-25% and lifetimes up to 35 years. However, recent reports have described increase in the efficiency and lifetime (up to 10000 hours in accelerated tests) of the OPV.

From molecules to devices.From molecules to devices.

Figura 1. Diferentes tipos de dispositivos fotovoltaicos moleculares (a) dispositivos bi-capa o“single junction”, (b) dispositivos tipo Gratzel o semiconductor mesoporoso sensitivizado concolorante y (c) dispositivos orgánicos de hetero-unión masiva o “bulk heterojunction”.

Organic solar cells

L dDye-sensitized solar cells (DSSC)electrochemical cells

2I-

Load

TiO2

+-

11 13 5% (liquid) (9%)

R uN

N

C O O H

H O O C

NN

C O O H

H O O C

S C

S C

N

N

I2

Ru-complex

+- 11-13,5% (liquid) (9%)

Polymer solar cells (PSC, BHJSC) prepared from solution (low temperature) -

glass/ITOPEDOT/PSS

p +

Ru-complex

Oligomer (small molecule) solar cells (OSC)

metaln

glass/ITO

- 10% (6%)

Oligomer (small molecule) solar cells (OSC) through vacuum deposition (high temp.) p

n

metal

glass/ITO

+

- 10% (6%)

Challenges: 1. Increase of efficiency (electrode area!)2. Increase of stability

Challenges: 1. Increase of efficiency (electrode area!)2. Increase of stability2. Increase of stability3. Technology for large areas4. Low cost (< 1 €/kWp)

2. Increase of stability3. Technology for large areas4. Low cost (< 1 €/kWp)

Type of Solar Cells

inorganic pnjunction solar cell

photoelectrochemical solar cell

organic solar cellju ct o so a ce a so a ce

LUMOe–

e–

ECB

LUMO

LUMO

e– e–

e–

A

h+

EVB

HOMO

HOMO h+

h+

A–

n-type semiconduct

or

p-type semiconduct

orElectron acceptor

Hole acceptor

CathodeAnodeRedox

electrolyte

n-type semiconduct

orMetal

Solar Cells

Organic Compounds for Photovoltaic Applications

1. Strong absorption (light-harvesting, high extinction coefficients, broad coverage of solar spectrum) 2. HOMO/LUMO levels adjusted to the electrodes (rich redox chemistry, photoinduced electron transfer)3. Procesability (solution-processing or vacuum-technology)4. Packing in solid state, control of morphology, self-assembling properties 5 Excellent charge transport properties

6x1018

photon flux AM 1.5

5. Excellent charge transport properties

Soret or B Band

Q Band

3x1018

4x1018

5x1018

MDMO-PPV/PCBM 1/4

[n m

-2 s

-1 n

m-1]

50

100

[%]

integrated photon flux [%] absorbed photons [%]

Soret or B Band

400 600 800 1000 12000

1x1018

2x1018

phot

ons

Wavelength [nm]

0

Multi-parameter problem to solve

Organic solar cells (OSC)Important parameters for characterization of a solar cell

I (V)

illuminated

I (V)

dark

VOC V

LkTqV IeII )( 10

IL

VOC V

ISC(Vmp, Imp)

mpmp IVFF

Fill factorscocmpmp FFIVIV

Energy conversion efficiency

scoc IVFF

inin PP

Bilayer HeterojunctionsBilayer Heterojunctions.

Bulkheterojunctions (BHJSC) PolymerBulkheterojunctions (BHJSC), Polymersolar cells (PSC) or Plastic Solar Cells.

PSCs can be flexible and can easily be processed in What is a PSC or OPV?

PSCs can be flexible and can easily be processed in different shapes with patterns. PSCs are build up from different layers

1) substrate, 2) - transparent electrode, i.e. Indium Tin Oxide (ITO),

PEDOT PSS P l (3 4 th l di thi h ) - PEDOT:PSS, Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)

3) active layer (for example a conducting polymer and C60)

4) metal electrode (aluminium).

The PEDOT:PSS layer is a barrier layer and functions as a barrier layer which makes the ITO electrode smoother.

And how does the OPV work? The active layer consists of a donor (polymer) and an acceptor(fullerene). When the sun shines on the OPV the polymer in the active layer absorbs thephotons and an exciton (an electron and hole pair) is created. The electron is transferred tothe acceptor which results in a charge separation The charge separation needs to bethe acceptor which results in a charge separation. The charge separation needs to beefficient (i.e. it needs to be faster then the charge recombination) to ensure a highefficiency. When the charges have been separated the electrons moves to the metalelectrode and the holes to the ITO electrode and we have a current running in the device.

How does a solar cell work?How does a solar cell work?

(a) A solar cell requires a material that acts as a semiconductor, neither aconductor (like a copper wire) or an insulator (like a grant counter top). Ina solar cell, upon absorption of light electrons are promoted to theconduction band (CB) leaving behind holes in the valence band. One oft (1) th l t d h l k it t th ltwo processes can occur (1) the electrons and holes make it to the solarcell contacts and the energy is converted into electricity or (2) theelectrons and holes recombine insode the semiconductor to generate heat.

(b)The maximum power (current x voltage) is at P The open circuit(b)The maximum power (current x voltage) is at Pmax. The open circuitvolatage, Voc, is the maximum voltage obtained in the system and theshort curcuit current, Isc, in the largest current that can be obtained by thesystem

Quantum efficiency (QE) and Incident Photon to Charge Carrier EfficiencyPhoton to Charge Carrier Efficiency (IPCE)

External Quantum efficiency (EQE) and IncidentQ y ( Q )Photon to Charge Carrier Efficiency (IPCE)indicates the ratio of the number of photons

d l ll h b fincident on a solar cell to the number ofgenerated charge carriers. Specifically, EQE is ameasure of the external efficiency while IPCE ismeasure of the external efficiency, while IPCE isa measure of the internal efficiency; that is, thephotons reflected back from the surface of thepcell are not considered.

Both QE and IPCE measurements are of criticalimportance during the materials research and celldesign stages. This is because the spectralesponse of the sola cell sho ld be matched toresponse of the solar cell should be matched to

the spectral distribution of sunlight to ensurehighest efficiency in charge carrier

Quantum efficiency (QE) and Incident Photon to Charge Carrier Efficiency (IPCE) This ratio is obtained by measuring the

photocurrent spectrum of the photovoltaic device

to Charge Carrier Efficiency (IPCE)

photocurrent spectrum of the photovoltaic device under test and comparing it to the photocurrent spectrum of a calibrated photodetector, thereby removing the spectral characteristics of the test system.

Solar cell efficiency Is the ratio of the electrical output of a solar cell to

the incident energy in the form of sunlight. The energy conversion efficiency (η) of a solar cell is the percentage of the solar energy to which the cell is e posed that is con e ted into elect ical ene g This exposed that is converted into electrical energy.This is calculated by dividing a cell's power output (in watts) at its maximum power point (P ) by the input watts) at its maximum power point (Pm) by the input light (E, in W/m2) and the surface area of the solar cell (Ac in m2).

By convention, solar cell efficiencies are measured under standard test conditions (STC) unless stated

h f f dotherwise. STC specifies a temperature of 25 °C and an irradiance of 1000 W/m2 with an air mass 1.5 (AM1 5) spectrum These conditions correspond to a (AM1.5) spectrum. These conditions correspond to a clear day with sunlight incident upon a sun-facing 37°-tilted surface with the sun at an angle of 41.81°

Dye-sensitized nanocrystalline solar cells (DSSC)

.(DSSC)

Photoelectrochemical Processes in a DSSCloade

TiO2

D*/D

LUMO

DYE

i nanocrystalline I/I based

CB

LUMO

organic dye

a oc ysta eTiO2 film

I /I3 basedelectrolyte

electrolyte0

-0.5

e-

hνII3

D/D++0.5

h

VB

HOMO

TCOcoated

I3

TiO2

+0.25 hνB

external circuit

e- flowPlatinized TCO coated

PtV (Vs. SCE)

Primary photophysical and electrochemical processeselectrochemical processes

• The adsorbed dye molecule absorbs a photon and electrons are excited forming an excited state: [dye*]

• The excited state of the dye can be thought of as an electron-holeThe excited state of the dye can be thought of as an electron hole pair (exciton).

• The excited dye transfers an electron to the semiconducting TiO2 (electron injection) This separates the electron hole pair leaving(electron injection). This separates the electron-hole pair leaving the hole on the dye: [dye*+]. Charge must be rapidly separated to prevent back reaction.

• The hole is filled by an electron from an iodide ion. [2dye*+ + 3I- 2dye + I3

-]

El t t f d f th TiO t th FTO• Electrons are transferred from the TiO2 to the FTO.

• Electrons go to the counter-electrode after working at external load.

• I - is reduced at the counter electrode• I3- is reduced at the counter-electrode.

Primary photophysical processesp

TiO

Electron Injection process on TiO2 very fast ~ps

Electron Recombination with oxidized

S+/S*

0 8

TiO2E[V] vs SCE

~~ps ps

Electron Recombination with oxidized dye molecules or with the oxidized form of the electrolyte redox couple (I3- ions) occurs very slow ~ss--ms and ms and ~mmss--ss (dark

)-0.8

~~ nsns

current)

Electron Regeneration very fastThe reduction of the oxidized dye by the

0.0 e-

red/ox~~ msms--ss

The reduction of the oxidized dye by the redox electrolyte’s I- ions occurs in the range of ~nnss--ps ps

+0.8

red/ox~~ ss--msms

S /S+0.8 ~~ ss

All Electron kinetics just on time!Structure and Working Principles at Molecular Level.

Chemical Note

Triiodide (I3-) is the brown ionic species that 3

forms when elemental iodine (I2) is dissolved in water containing iodide (I-).

III 32 I II

The overall conversion efficiency

= Jsc * Voc * ff

(JSC), integral photocurrent densityA/ 2mA/cm2

(VOC), the open-circuit photovoltage( OC), p p g

(ff), the fill factor of the cell

N t lli I i S i d tNanocrystalline Inorganic Semiconductor

Titanium Dioxide (TiO2):Titanium Dioxide (TiO2):That Sounds Expensive

TiO2 provides whiteness and opacity to products such as paints, coatings, plastics, papers, inks, foods, and most papers, inks, foods, and most toothpastes.

TiO2 is used in sunscreens to block harmful UV B radiation from the sun. Small UV B radiation from the sun. Small particles (~ 10-1 m or 1/100th the thickness of a human hair) are dispersed in the sunscreen solutionsunscreen solution.

Porous TiO2 NetworkPorous TiO2 NetworkTo the left is a scanning electronmicroscope image of TiO in a DSSC Themicroscope image of TiO2 in a DSSC. Thescale bar is 60 nm. 10 nm particles arefused together to form a 10 m thick filmof porous TiOof porous TiO2.A paste of nanometer TiO2 particles andviscous organic compounds is spread onto transparent conductive glass (F dopedto transparent conductive glass (F dopedSnO2). The film is then heated in an ovento 450 °C burning off the organic pasteleaving behind a fused network of TiO2leaving behind a fused network of TiO2

particles.

10 mthick filmof TiO

Transparent Conductive Glassof TiO2

Typical electrolyte (M1)Typical electrolyte (M1)

C d f Composed of 0.6M M-methyl-N-butyl imidiazolium iodide,

0.04 M iodine, 0.025 M LiI,

0.05M guanidinium thiocyanate and

0 28 M t ti b t l idi 0.28 M tertiary butylpyridine

in 15/85 (v/v) mixture of valeronitrile and acetonitrile.

Spiro fluorene OMeTAD

Photosenzitizers

Light Harvesting MoleculesNear IR Dyesy

Some of the Most Successful Dyes

COOTBA

NN

Ru

NCSN

HOOC

11% 9,5%C6H13 C8H17O OC8H17

12.3 and 13 %

S

S

NN

NCS

HOOC

11% 9,5%

N NZn CO2HN

6 13 8 17 8 17

N N

RNC CO2H

JK-2COOTBA

N719 N N

C6H13 OC8H17C8H17O

N

N N

NZn CO2HN

6 13

Science, 2011, 629Nature Chem. 2014, 6, 242

R 11%

R = C6H13 YD2

Chem. Commun., 2010, 46, 7090-710

Requirements of the Sensitizers

The optimal sensitizer for the dye sensitized solar cell should be panchromatic, i.e. absorb visible light of

all colorsall colors.It must be firmly grafted to the semiconductor oxide

surface and inject electrons into the conduction band surface and inject electrons into the conduction band with a quantum yield of unity.

It should possess suitable ground- and excited state It should possess suitable ground and excited state redox properties (0.5 and -0.8 V vs.SCE)

It should exhibit thermal and photochemical stabilityp y

Recent Research TrendsRecent Research Trends

HigherHigherHigherHigherEfficiency !!Efficiency !!