National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Polymer Hybrid Photovoltaics for 

Inexpensive Electricity Generation

Final Technical Report 

1 September 2001 — 30 April 2006 

S.A. Carter 

University of California–Santa Cruz 

Santa Cruz, California 

Subcontract Report 

NREL/SR-520-40044 

July 2006 
Polymer Hybrid Photovoltaics for 

Inexpensive Electricity Generation

Final Technical Report 

1 September 2001 — 30 April 2006 

S.A. Carter 

University of California–Santa Cruz 

Santa Cruz, California 

NREL Technical Monitors: R. McConnell, F. Posey-Eddy 

Prepared under Subcontract No. ACQ-1-30619-03 

Subcontract Report 

NREL/SR-520-40044 

July 2006 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
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Table of Contents 

Project Objective:............................................................................................................................ 1 

Executive Summary of Project Results: ......................................................................................... 1 

High Conversion Efficiency in Polymer Photovoltaics.................................................................. 2 

Device Fabrication.................................................................................................................. 2 

Materials ................................................................................................................................. 2 

Device Optimization and Performance................................................................................... 3 

Tandem Devices...................................................................................................................... 4 

Temperature Dependence of Device Performance ................................................................. 4 

Numerical Simulations of Organic PV Devices ..................................................................... 6 

Additional Research: Steady State and Time Resolved Photoluminescence......................... 7 

Time Resolved Photoluminescence ........................................................................................ 7 

New Materials for Luminescent Solar Concentrators............................................................. 9 

Long Lifetime and Reliability With an Inexpensive Manufacturing Process............................... 10 

Inexpensive Manufacturing Process..................................................................................... 10 

Lifetime Properties of Device Structures.............................................................................. 11 

Papers in Scientific Journals......................................................................................................... 12 

Papers in Popular Press................................................................................................................. 12 

Ph.D. Thesis.................................................................................................................................. 12 

Presentations of results from this contract.................................................................................... 13 

List of Figures 

Figure 1: Left: The energy diagrams for two different device architectures studied.................... 3

Figure 2: Right: Typical IV curves for reproducible optimized devices utilizing the 

different device structures and materials shown in Figure 1......................................... 4

Figure 3: Left: Changes in M3EH-PPV mobility and γ with T .................................................... 5 

Figure 4: Top: Short circuit current (left) and power efficiency and Voc (right) as a function 

of blend fraction for polymer/polymer blends .............................................................. 6 

Figure 5: Left: Normalized PL for blend and layered films excited at 540 nm............................ 7

Figure 6: Left: Time-resolved PL data for polymer-polymer layers and blends (top) and for 

PCBM blends (bottom) ................................................................................................. 8 

Figure 7: Top: Normalized absorption (abs) and light emission (rad) for an organic laser 

dye (LDS821) and a light emitting polymer (Red PF), overlayed with Si’s EQE...... 10 

Figure 8: Device performance on (top) plastic & (bottom) of printed contact........................... 10

Figure 9: Shelf-life of PV cells ................................................................................................... 11

List of Tables 

Table 1: Summary of selected results from temperature-dependent studies ................................ 5 

Table 2: Summary of decay times of CN-ether-PPV and PCBM blends compared to 

neat film results............................................................................................................... 9 

iii
Project Objective: The project goal is to understand the operating mechanisms underlying the 

performance of polymer hybrid photovoltaics to enable the development of a photovoltaic with a 

maximum power conversion efficiency over cost ratio that is significantly greater than current PV 

technologies. Plastic or polymer-based photovoltaics can have significant cost advantages over 

conventional technologies in that they are compatible with liquid-based plastic processing and can 

be assembled onto plastic under atmospheric conditions (ambient temperature and pressure) using 

standard printing technologies, such as reel-to-reel and screen printing. Moreover, polymer-based 

PVs are light weight, flexible, and largely unbreakable which make shipping, installation and 

maintenance simpler. 

Executive Summary of Project Results: This final report summarizes the research completed 

by Sue Carter’s group in the physics department at the University of California, Santa Cruz. 

Major collaborators included J. Campbell Scott at IBM Almaden Research Center, Garry 

Rumbles at the National Renewable Energy Laboratories, Melissa Kreger at Add-vision, and H. 

Horhold at the University of Jena, Germany. This research comprises the PhD dissertations of 

Yuko Nakazawa and Stephanie Chasteen who were both supported under this contract. This 

contract also supported Melissa Kreger while she was postdoctoral researcher in Carter’s 

laboratory. In addition, several undergraduate students, including Tosan Ombegedho, Peter 

Journey-Kaler, Gareth Fortser, Charles Dearie, and Samuel Bergenher completed research on 

this contract. The contract resulted in six publications currently published or in press, and 

another two publications that are in the final process of submission. Furthermore, a numerical 

simulation program was developed (in collaboration with IBM) to fully simulate the performance 

of multicomponent polymer photovoltaic devices, and a manufacturing method was developed 

(in collaboration with Add-vision) to inexpensively manufacture larger area devices. 

The research completed under this contract included research into higher conversion efficiency 

polymer photovoltaics that maintained longer lifetime and reliability with an inexpensive 

manufacturing process. Studies were completed on novel PPV and polythiophene-based hole 

transporting polymers blended or layered with electron transporting polymers, such as CN-etherPPV, as well as TiOx, and C60-based nanoparticles. Blends with semiconducting nanoparticles 

were also studied; however, these experiments were not completed by the end of the contract. 

Several different polymer hole transport materials were developed and characterized in an 

attempt to improve charge transport and increase absorption across visible spectrum. The use of 

different electrode geometries and tandem structures to improve device efficiency was evaluated. 

Different TiOx, and C60-based morphologies were tested to understand their impact on charge 

dissociation and transportation. The completed characterization studies on the three main classes 

of photovoltaic devices included temperature dependence of PV device performance, 

dependence of device performance and degradation on operation conditions and device 

morphology, evaluation of the how the device performance depends on excited state dynamics 

through steady state and time resolved photoluminescence measurements (in collaboration with 

NREL), numerical simulations of device performance (in collaboration with IBM), and the 

viability of manufacturing the structures using a low cost printing process (in collaboration with 

Add-vision). 

The main outcome and conclusions of these results are: 

• Polymer/PCBM (i.e. C60) blends resulted, as expected, in the highest power conversion 

efficiency (~4%) of all device structures studied due to greater mobility of C60. Much greater 

1
power conversion efficiencies can be expected in this system through optimization of blend/layer 

morphology. 

• Exciton dissociation is insufficient for optimization of device performance. Improved exciton 

generation rate, through the presence of exciplexes, or layer thickness control, and charge 

transport pose more important limitations to performance. 

• Emissive interchain excitations may serve as useful sources of separable charge, whereas 

poorly emissive single-chain excitations are difficult to quench due to competing non-radiative 

rate constants. 

• For blended structures 

• In blends, the fill factor a, the maximum open-circuit voltage, Voc, is determined by HOMOdonor

and LUMOacceptor energy level offsets and further limited by the contact barrier formation. For 

layers, Voc is increased above this limit due to a substantial diffusion counter-current that is 

largely absent in blends.nd short-circuit current, JSC, are strongly a function of charge carrier 

mobility; however, in layers, both quantities saturate with increasing mobility and are more 

strongly determined by the exciton generation rate near an interface. 

• With currently developed materials, power efficiencies greater than 12% should be achievable 

in blends through graded (rather than homogeneous) morphologies or in layered structures with 

blended interfaces; however, clean layered structures are limited to significantly lower 

efficiencies without further improvements in absorption of photons across the solar spectrum. 

• Polymer-based photovoltaics, with metal-oxide (TiOx) interlayers, can be manufactured on 

plastic substrates with all layers being deposited and patterned using low cost liquid-based 

processing under ambient conditions; however, a reduction in power efficiency up to 50% can 

result from the lower work function and/or poorer conductivity resulting from the printable 

metals. Larger area device fabrication favors manufacturing methods such as slot coating and 

gravure over screen printing. 

• Polymer devices using metal-oxide (i.e. TiOx) interlayers are considerably more stable than 

structures based utilizing doped-polymer (i.e. PEDOT-PSS) interlayers. 

• Morphology (including thickness) is critical in all aspects of device performance, from 

deactivation of the excited state to charge transport. Poor control over morphology is the critical 

factor in limiting the power conversion efficiency in polymer-based photovoltaic devices. 

High Conversion Efficiency in Polymer Photovoltaics 

Device Fabrication 

Devices were fabricated inside an inert glove box atmosphere by spin casting the polymer hybrid 

solution onto a prepatterned ITO on glass or plastic substrate, that has been coated with the 

appropriate electron and or hole transporting layer, and then evaporating the top electrode. The 

ITO is either precoated with a titanium oxide sol-gel layer (sintered at 400 C) or a PEDOT-PSS 

layer, and the evaporated top electrode is either Au or Al, respectively. A flat band diagram for 

the two structures is shown in Fig. 1 (left side). J-V curves in the dark and under white-light 

conditions (100 mW/cm2

) were taken inside the glove box. Absorption and steady state PL 

measurements were taken using Varian and Perkin Elmer spectrometers. Additional steady state 

and time-resolved PL were taken at NREL. 

Materials 

For the hole transport layers, a variety of PPV and polythiophene materials were used with 

different mobilities, band gaps, and crystallinity. Liquid crystalline polymers were synthesized 

to improve charge mobility, but did not result in improved device performance. In addition, 

2
smaller bandgap polymers that incorporated polythiophene groups into the PPV backbone were 

synthesized in order to increase the absorption of solar light. This material resulted in similar 

device performance as their larger bandgap PPV-based counterparts. The overriding factor that 

dictated the final device performance was the mobility of the hole transport polymer and the 

choice of the electron transport material. Much of our results used M3EH-PPV (Fig. 1, top 

middle), a unique derivative of the much more commonly studied MEH-PPV, but with a 

significantly higher mobility due to increased crystallinity induced by the additional insoluble 

monomer unit. For the electron transporter, CN-ether-PPV, PCBM (Fig. 1, top middle), TiOx 

sol-gel and nanoparticles (Fig. 1, right), and PbS nanoparticles (not shown) were studied. 

 

Figure 1: Left: The energy 

diagrams for two different device 

architectures studied. In 

addition, LiF was studied as an 

interlayer between polymer and 

aluminum (Al). Middle-top: 

Structures of some materials 

used in the studies. Middlebottom: Relative energy levels of 

materials. Right: AFM images 

of TiOx surface made from 

smooth sol-gel layer and rougher 

nanoparticle layer.

Device Optimization and Performance 

In Figure 2 (right), typical current-voltage (J-V) curves for optimized polymer hybrid blend and 

layered devices are shown, revealing a more than 3x improvement in Jsc over neat M3EH-PPV 

films on TiO2 and 13x over M3EH-PPV on PEDOT. Polymer-PCBM blends result in a further 

4x enhancement. Optimized devices had film thickness ~ 30 ± 5 nm, except for polymer layers 

for which the best device has at thickness of 20 nm M3EH-PPV/50 nm CN-ether-PPV. Blend 

performance was optimal at 50% and 80% blend wt% for polymer and PCBM blends, 

respectively. TiOx devices showed consistently the highest fill factors, with values >60% being 

observed for some devices at lower light intensities. 

Figure 2 (left) shows changes in the short circuit current density, Jsc, and fill factor, FF, with 

blend wt%. For polymer blends, the fill factor is limited by poor transport in CN-ether-PPV due 

3
to the inherent properties of PPV polymers and the presence of the ether linkage that disrupts the 

conjugation. This effect results in a rapid decrease in the fill factor with increasing CN-etherPPV percentage that is offset, in part, by the greater exciton dissociation resulting from the 

blended hybrid structure. Both the fill factor and Jsc increase in PCBM as electron transport 

improves with increased PCBM percentage. The maximum power efficiency observed was 

typically ~3% for a 1:4 M3EH:PCBM blend. This result was lower than the state-of-art values 

mainly due to the smaller fill factors observed for M3EH blends compared to the regioregular 

polythiophene blends. We note that while regioregular polythiophene blends were also 

fabricated, the results are not included in this study because the materials were found to yield 

much less consistent results than the PPV-derivatives. Simulations (discussed below) combined 

with our experimental studies indicate that much higher power efficiencies (>10%) are 

obtainable for these class of materials by controlling and improving the blend morphology. 

1.6

1.2

0.8 Jsc (mA/cm 2

)

0 20 40 60 80

wt% CN-ether-PPV

40

36

32

28

 Jsc

FF (%)

FF

12

8

4

0

Jsc (mA/cm 2

)

0 20 40 60 80

wt% PCBM

36

32

28

FF (%)

 FF

 Jsc

A

B

Figure 2: Right: Typical IV curves for reproducible optimized 

devices utilizing the different device structures and materials 

shown in Figure 1. Left: Evolution of the short circuit current 

(Jsc) and fill factor (FF) with weight % for blends of M3EHPPV with (A) CN-ether-PPV and (B) PCBM 

Tandem Devices 

In general, device performance was found to be limited by a trade off between photon absorption 

and charge transport which is optimized by thicker or thinner films, respectively. In an attempt 

to overcome this limitation, tandem devices consisting of layers of two polymer PV cells were 

fabricated and studied. Here, challenges were encountered in trying to optimize device 

performance while maintaining the low cost fabrication by making every layer liquid 

processible. The liquid deposition of the second PV cell on top of the first PV cell was shown to 

detrimentally affect its performance, leading to an overall reduction in device performance as the 

first polymer PV cell was shown to contribute over 75% of the overall power efficiency. 

Temperature Dependence of Device Performance 

The temperature dependence of the polymer PV were studies from ~100 K to 400 K in order to 

understand how the device operation and stability were affected by temperature (Figure 3). 

Since the polymer mobility changes significantly with temperature T (Fig. 3, left) according the 

formula μ ~ μ*exp(-∆/kT)exp(γE1/2) where ∆ is the hopping activation energy γ is the fieldprefactor, and E is the electric field across device, these measurements also provide insight on 

how changes in the polymer’s mobility affects device performance. It was found that the short 

circuit current (Isc) is exponentially dependent on temperature with an activation ∆SC with small 

4
variance for a wide range of materials with similar device architectures. Similarly the 

temperature dependence of the open circuit voltage (Voc) was shown to be largely linear, with the 

maximum Voc determined by the difference in the HOMO level of the donor, LUMO level of the 

acceptor, binding energy ΦB (Fermi Level pinning) and ∆SC. The fill factor increases with 

increasing mobility as expected. It is noted that device failure tended to occur a few degrees 

below the glass transition temperature (~300 K to 350 K)) and was worse for devices containing 

LiF. Temperature-induced device failure was not observed for TiOx-based devices. The results 

of the temperature dependent studies for 3 different materials are given in Table 1. 

Table 1: Summary of selected results from temperature-dependent studies 

*

Maximum Voc = 1/e(HOMOdonor – LUMOacceptor-ΦB) - ∆SC where ∆o = HOMOdonor – LUMOacceptor 

Constituent polymer blends Anode ∆sc Voc

* ∆o-∆sc ∆o-∆sc−ΦΒ

Thio-M3-PPV:PCBM 

Al 

LiF/Al 

62 meV 

36 meV 

0.95 – 0.0012T (0.6 

sat) 

1.14 -0.0016T 

1.14 0.68 

M3EH-PPV:CN-ether-PPV 

Al 

LiF/Al 

76 meV 

70 meV 

(1.22 sat) 

1.65-0.0014T (1.36 

sat) 

1.63 

1.63 

1.27 

1.36 

P3HT:PCBM Al 

LiF/Al 

75 meV 

41 meV 

1.37 – 0.0014T 

0.0

0.50

1.0

1.5

150 200 250 300 350 400

Al electrode241 

Al electrode210

LiF/Al230

LiF/Al189

Voc (V)

Temperature (K)

PCBM-blend

CN-ether-PPV blend

0.0

10

20

30

40

50

150 200 250 300 350 400

PEDOT / PCBM blend / Al210

PEDOT / PCBM blend / LiF/Al189

Fill Factor (%)

Temperature (K)

 Isc(T) = Isc0 exp(− ∆sc

kBT

)

Figure 3: Left: Changes in M3EH-PPV 

mobility and γ with T. Middle: Tdependence of Jsc for PCBM blend 

(squares), CN-ether-PPV blend 

(triangles) and a TiOx layered devices 

(open diamonds), which is described 

by equation for Isc (left). Right: Tdependence of fill factor and Voc for 

blends with and without LiF interlayer. 

5
Numerical Simulations of Organic PV Devices 

A quasi-1D model was developed to model the performance of layered, blended, and partially 

blended organic PVs which is described in more detail in publications. For blends, a change 

between phases was allowed at every position throughout the device, and blending at layer 

interfaces was modeled using mass diffusion into the adjacent layer with 100% being a 

homogeneous blend. This model enables the determination of device performance as a function of 

the materials absorption (given index of refraction n & dispersion k), electrode’s work function, 

the organic HOMO and LUMO levels and mobility, the device thickness and material morphology, 

ambient temperature, and finally the light intensity and spectrum. 

As shown in Fig. 4 (top), the short circuit current and power efficiency is a strong function of the 

blend fraction and mobility, and the best performance is achieved for 67% blend fraction. Layered 

structures (0%) only obtain a power efficiency of <3% even for high mobility materials due to 

poor exciton dissociation through the bulk, and homogenous blends are limited to a maximum 

Figure 4: Top: Short circuit current (left) and power efficiency and Voc (right) as a function of blend fraction 

for polymer/polymer blends. Bottom: Comparison of simulations and experiment for layers (left) and 

exciton generation rate for two different layer thickness (right) with 25 nm/50 nm resulting in significantly 

more exciton generation at the polymer interfaces, and ultimately better performance. 

6
power efficiency of ~7.5% due to limits in both charge transport and Voc. The decrease in Voc with 

increasing blend fraction is due to a decrease in the role of the diffusion counter-current in blends. 

Higher power efficiencies of ~12% result from partially blended (i.e. graded) structures as these 

systems optimize exciton generation, dissociation and charge transport. Excellent qualitative 

agreement between experimental and simulation are obtained for both layers and blends (Fig. 5 left 

bottom), with quantitative differences due to mobility. Fig. 5 (right bottom) reveals that optical 

reflection/interference effects can result in large differences in exciton generate rate near interfaces 

and is a determining factor optimizing the performance of layers. 

Additional Research: Steady State and Time Resolved Photoluminescence 

The steady state PL was strongly quenched for all layers and blends, with values ranging from 

70% for polymer layers to 95% for PCBM/polymer blends. For polymer/polymer systems, the 

PL and time-resolved data indicate the presence of exciplex formation that can result in exciton 

scavenging and increased dissociation (Figure 5). For PCBM blends, exciplex formation is not 

observed; however, a large shift in PL spectrum occurs, due to an effective dilution in the 

M3EH-PPV at high PCBM wt%. 

Normalized PL

1.6 1.8 2.0 2.2 2.4 2.6 2.8eV

Energy (eV)

8

6

4

2

0

x109

700 600 500 450

Wavelength (nm)

600

400

200

0

x106

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

x109

 M3EH-PPV Film

 M3EH-PPV Solution

 20 wt% PCBM

 80 wt% PCBM

 

PL

1.6 2.0 2.4 2.8eV

3

2

2

1

1

5

0

x106

@ 400 nm

Figure 5: Left: Normalized PL for blend and layered films excited at 540 nm. ∆PL is a 

subtraction of the M3EH-PPV spectrum from that of the 80% blend, revealing exciplex 

formation in polymer blends. Right: Normalized PL for PCBM blends. 

Time Resolved Photoluminescence 

In the left part of Figure 6, the time-resolved PL data for polymer-polymer layers and blends 

(top) and for PCBM blends (bottom) are shown. In Figure 6 (right) and Table 2, the decay times 

for the pristine polymers as well as the blended and layered structures are summarized. All 

blends and layers show a very rapid decay time (<0.05 ns). For polymer blended structures, 

significant exciton quenching occurs in both the electron- and hole-transporting polymers due to 

the interpenetrating morphology; however, the lack of complete quenching limits the achievable 

short-circuit currents. In layered structures, decay times were longer than for blends; thus 

exciton dissociation and charge transfer is more efficient in blended than in layered devices. 

Both steady-state and time-resolved data indicate that charge transfer occurs to an intermediate 

exciplex state in blended and layered structures of CN-ether-PPV with M3EH-PPV. The 

exciplex is characterized by a decay component of ~ 2.0 ns which predominates at 700 nm 

emission – the location of a broad, featureless steady-state spectral feature attributed to exciplex 

emission. Formation of the exciplex aids device performance, as indicated by the predominance 

7
of the exciplex peak (and notable absence of M3EH-PPV emission) in steady-state spectroscopy 

on particularly efficient photovoltaic devices. The exciplex may thermally re-excite the M3EHPPV exciton, providing an additional route for charge separation, or dissociate directly under the 

influence of an electric field. 

Photoluminescence decay from PCBM aggregates is detected in all heterojunctions. Decay from 

the PCBM phase is characterized by a time constant of 1.0 - 1.9 ns (see Table 2). PCBM 

quenches the excited state more efficiently than CN-ether-PPV, but the dominant decay time of 

M3EH-PPV (0.20 ns) is only ~75% for both systems, indicating incomplete quenching. Contrary 

to the naïve assumption that more PCBM should result in increased charge transfer (shortening 

the lifetime), the rate of decay increases as the wt% of PCBM in the blend is decreased, counter 

to the enhanced device performance at high wt% of PCBM in the blend (Table 2). These results 

indicate the importance of charge transport over exciton dissociation , on device performance. 

1

10

100

1000 Normalized PL Intensity

0 2 4 6 8 10 12

Time (ns)

4 6 8 10 12 14 16

kps

12 14 16 18 20 22

kps

8 10 12 14 16 18 2

kps

6 8 10 12 14 16 1

kps

4 6 6 88 10 10 12 1412 16

kps

CN-ether-PPV

20 wt% CN-ether

50 wt% CN-ether

80 wt% CN-ether

M3EH-PPV

IRF

Layer

1

10

100

1000

Normalized PL Intensity

Time (ns)

500 1000 1500 2000 2500 3000 3500

ps

1500 2000 2500 3000 3500 4000 4500 5000

ps

1500 2000 2500 3000 3500 4000 4500 5000

ps

1:4

1:1

4:1

IRF

 M3EH-PPV

 M3EH-PPV:PCBM

0 1 2 3

Figure 6: Left: Time-resolved PL data for polymer-polymer layers and blends (top) and for 

PCBM blends (bottom). Note that all the layers and blends exhibit an initial faster decay than the 

neat M3EH-PPV film although the average decay time is longer (except for 20 wt% PCBM 

blend). The CN-ether-PPV decay components are readily quenched, but the ~0.45 ns M3EH-PPV 

decay remains for all material combinations (see Table 2). Right: Summary of the yield of the 

major decay components (M3EH-PPV, PCBM, and exciplex) as a function of the CN-ether-PPV 

wt % (top) and the PCBM wt % (bottom). 

8
Table 2: Summary of decay times of CN-ether-PPV and PCBM blends compared to neat film results. 

Film η τ1 (ns) τ2 (ns) τ3 (ns) τave

(ns) 

Q 

Pristine polymers 

M3EH-PPV 

0.04% 0.20 

(43%) 

0.45 

(53%) 

1.7 

(3%) 0.38 

CN-ether-PPV 

N/A 0.30 

(2%) 

4.3 

(26%) 

12.2 

(73%) 10.0 

CN-ether blends 50 wt% CN-ether 

w/ M3EH-PPV 

0.86% 0.05 

(51%) 

0.46 

(20%) 

2.0 

(22%) 1.0 

75% 

M3EH/CN-ether 

layer 

0.89% 0.05 

(39%) 

0.46 

(17%) 

2.0 

(27%) 1.8 

75% 

TiOx layer 

TiOx/M3EH-PPV 

0.5% 0.12 

(42%)

0.32 

(44%) 0.35 

0% 

PCBM blends 80 wt% PCBM w/ 

M3EH-PPV 

3% 0.05 

(36%) 

0.50 

(49%) 

1.9 

(16%) 0.6 

75% 

20 wt% PCBM w/ 

M3EH-PPV 

0.9% 0.02 

(95%) 

1.0 

(5%) 0.07 

90% 

New Materials for Luminescent Solar Concentrators 

 The use of Luminescent Solar Concentrators (LSC) to generate electricity from solar 

radiation was introduced around 30 years ago and was based on earlier work that used 

fluorescence radiation converters as an inexpensive method to detect scintillation light over large 

areas. A LSC uses a photoluminescent material to absorb the suns photons, remit these photons 

at longer wavelengths, waveguide the emitted photons down a large area waveguide structure, 

and then convert the longer wavelength photons to electricity using strips of silicon-based 

photodetectors. The advantage of this technology is that one can use very inexpensive materials, 

usually organic dyes imbedded in a polymer matrix, to absorb the solar light and concentrate it; 

therefore, a much smaller area of the expensive silicon PV is needed. The device performance, 

however, was found to be severely limited by self absorption of the emitted light, absorption 

over solar spectrum and the dye efficiency; the highest power efficiencies obtained on the a LSC 

was reported in 1985 to be 4% using a two-stack concentrator and a relatively expensive GaAs 

solar cells. Since then, quantum dots have been discussed as new LSC’s and one group has 

started field testing organic LSCs although the power efficiencies are still too low for viable 

commercialization. 

The power efficiency of a LSC cell is determined by the following formula: 

ηLSC = ηPL*ηabs*ηWG*ηSi

where ηPL is the edge photoluminescence efficiency (includes self absorption), ηabs is the fraction 

of the solar spectrum absorbed, ηWG is the fraction of light that is waveguided, and ηSi is the 

power efficiency of the silicon detector at the wavelength of the emitted light. To achieve a solar 

power efficiency grater than 10% using standard silicon detectors, the luminescent material 

should have a ηPL >90%, and a ηabs >67% with minimal self absorption. Our group at UCSC has 

identified a small molecule dye with sufficient Stokes-shift to minimize self-absorption and solar 

absorption, as shown in Figure 7; however, its ηPL is on the order of 20%. Conversely, a 

9
conjugated red-emitting polyfluorene material has also been identified that has a high ηPL

(>90%); however, the absorption across the visible and Stoke’s shift is too limited. The ηPL of 

the small molecule dye can be increased by using surface enhanced fluorescence effects and/or 

surfactants, and the absorption across the visible (and Stoke’s shift) for the semiconducting 

polymer-based system can be improved through the incorporation if IR-emitting organic 

chromophores or inorganic quantum dots. This research has now obtained funding from the 

PIER program administered by the CEC and the State of California. 

Long Lifetime and Reliability With an Inexpensive Manufacturing Process 

Inexpensive Manufacturing Process 

Devices were constructed on PET plastic substrates (Vitex) to 

compare with devices manufactured on glass (Figure 8, top). 

The power efficiency is reduced less than 10% from cells made 

on glass substrates. Methods were also investigated to make a 

fully printable solar cell under atmospheric conditions by 

eliminating the need for an evaporated top electrode. Because 

materials with work function similar to Al are not printable, full 

printability is only achievable with the TiOx layered structures. 

Two printable electrodes were studied: a solvent soluble silver 

paste and a water soluble conducting polymer, PEDOT-PSS. 

Working devices were fabricated using both of these top 

electrodes although the silver has a lower open circuit voltage 

due to its lower work function than PEDOT-PSS and Au 

(Figure 8, bottom). The power efficiency is normally reduced by 

a factor of 50% or more from those cells made with an 

evaporated top Au electrode. Large areas devices (>50 cm2

) 

were also fabricated using a complete printing process in 

collaboration with Add-vision. Using this process, the thin 

larger areas devices had difficulties with shorting and the thick 

devices suffered from low power efficiency. New methods to 

Figure 7: Top: Normalized 

absorption (abs) and light 

emission (rad) for an organic 

laser dye (LDS821) and a light 

emitting polymer (Red PF), 

overlayed with Si’s EQE. 

Bottom: Typical design for a 

LSC. 

0

0.2

0.4

0.5

0.7

-1 -0.75 -0.5 -0.25 0

Glass

FG 500 Plastic

J (mA/cm2

)

Voltage (V)

0

0.04

0.08

0.1

0.2

-0.8 -0.6 -0.4 -0.2 0

PeDOT

Ag paint

J (mA/cm2

) Fig. 8: Device performance on (top) 

plastic & (bottom) of printed contact. 

Voltage (V)

10
liquid deposit smooth larger area solar cells and to improve the 

overall power efficiency of these inexpensive devices are now 

being developed under an NSF STTR Phase I grant. 

Lifetime Properties of Device Structures 

Lifetime behavior of all three types of devices (polymer 

blends, TiOx layers, and PCBM blends) was monitored for 

changes in their IV-curves and quantum efficiency upon 

exposure to solar radiation. The blended devices were 

observed to degrade the fastest (typically <30 days), with 

PCBM blends being the most unstable, while the TiOx layers 

were stable for the duration of the test (>80 days). The rapid 

degradation of the blends appeared to be caused by UVinduced damage to the PEDOT-PSS and PPV polymers that 

was mitigated by including TiOx in the device structure. The 

rapid degradation of the PCBM blend devices were also 

observed for devices kept in a dark dry nitrogen atmosphere. 

The significantly short shelf life (7 days) of PCBM blend 

(Figure 9, top) was likely due to the chemical change in 

PCBM itself or a chemical reaction with PCBM. In those 7 

days, the Isc decreased by 60 % while the Voc remained 

constant. In contrast, the TiOx devices showed shelf life of 

over one year (Figure 9, bottom). The Voc and FF both 

increased while Isc remained constant. The largest increase in 

Voc occurred within the first 30-60 days, and eventually 

reached ~0.8 V. This is due to the evolution of Au-polymer 

interface. The interface between the soft gold and polymer 

surface heals over a period of time due to slow diffusion, 

resulting in a better ohmic contact. In contrast, Al and 

printed electrodes appear to form a good initial contact upon 

deposition. 

TiOx//Thio//Au 

PEDOT//PCBM:Thio//Al 

Fig. 9: Shelf-life of PV cells 

11
Papers in Scientific Journals 

1. Improving Power Efficiencies in Polymer - Polymer Blend Photovoltaics. A. J. Breeze; Z. 

Schlesinger; S.A. Carter; H. -H. Hoerhold; H. Tillmann, Solar Energy Mater. and Solar 

Cells 83, 263-271 (2004). 

2. Numerical Simulations of Layered and Blended Organic Photovoltaic Cells, J. O. Haerter, S.V 

Chasteen, S. A. Carter, J. C. Scott, Applied Physics Letters (86), 164101 (2005). 

3. Exciton Dynamics and Device Performance in Polythiophene Heterojunctions for 

Photovoltaics. S. V. Chasteen, S. A. Carter, G. Rumbles, Proc. of SPIE (5938), 59380J-1, 

2005. 

4. Blended versus Layered Structures in Polymer Photovoltaics. S. V. Chasteen, J. O. Haerter, G. 

Rumbles, C. Scott, S. A. Carter. J. of Applied Physics, March, 2006. 

5. The effect of ether-linkage upon photophysics and dynamics in PPV polymers: CN-PPV vs. 

CN-ether-PPV, S. V. Chasteen1, G. Rumbles2, S.A. Carter, J. of Chem. Phys. (in press). 

6. Towards optimization of device performance in conjugated polymer photovoltaics:

Charge generation and transport in poly (p-phenylene vinylene) polymer heterojunctions, S. 

V. Chasteen, V. Sholin, S. A. Carter and G. Rumbles (in final preparation). 

7. Temperature Dependence of Polymer Photovoltaic Devices, Y. K. Nakazawa and S. A. Carter, 

H. H. Hoerhold, Applied Physics Letters (in review). 

* Also Sue Carter was general editor of Materials Research Society Bulletin 2005 responsible for 

Theme Issue on Organic Photovoltaics 

Papers in Popular Press 

1. Bonnie Raitt powers solar benefit By STEPHANIE CHASTEEN, November 15, 2002, 

03local.htm, Search in: 2002 Archives 

http://www.santa-cruz.com/archive/2002/November/15/local/stories/03local.htm

2. Solar-energy research heats up By STEPHANIE CHASTEEN, November 10, 2002 

02local.htm, Search in: 2002 Archives 

http://www.santa-cruz.com/archive/2002/November/10/local/stories/02local.htm

3. Solar power still too expensive By STEPHANIE CHASTEEN, November 10, 2002 

06biz.htm, Search in: 2002 Archives 

http://www.santa-cruz.com/archive/2002/November/10/biz/stories/06biz.htm

Ph.D. Thesis 

1. Yuko Nakazawa, “Temperature Dependent Studies of Polymer Photovoltaics,” December 

2004. 

2. Stephanie Chasteen, “Excited State Dynamics in Polymer Photovoltaics,” December 2005. 

12
Presentations of results from this contract 

Invited Talks 

1.Melissa Kreger, SPIE symposia on Organic Photovoltaics, Seattle, July 2002. 

2. Melissa Kreger, American Chemical Society Meeting, Boston, August 2002 

3. Sue Carter, Colloquia at the University of California, Davis, Spring 2003. 

4. Sue Carter, Colloquia at the Naval Postgraduate School in Monterey, May 2004. 

5. Sue Carter, Hallgren Lecturer at General Electric, GE, May 2004. 

6. Sue Carter, Platts E Source Energy Conference, Colorado Springs, 2004 

7. Sue Carter, SPIE symposia on Organic Photovoltaics, August, 2004 

8. Sue Carter, American Chemical Society Meeting, San Diego, 2005. 

9. Sue Carter, Materials Research Society, Spring Meeting, San Francisco, 2006. 

Contributed Talks 

1. Melissa Kreger, Talk at MRS symposia on Organic Electronics, San Francisco, April 2002. 

2. Yuko Nakazawa, MRS symposia on Materials for Renewable Energy, December, 2004. 

3. Jan Haerter, American Physical Society, LA, March 2005. 

4. Stephanie Chasteen American Physical Society, LA, March 2005 

5. Yuko Nakazawa, American Physical Society, LA, March 2005. 

6. Sue Carter, IEEE World Conference on Photovoltaic Energy Conversion, May, 2006. 

Conference Posters 

1. Tosan Ombegeho, MRS symposia on Organic Electronics, San Francisco, April 2002. 

2. Yuko Nakazawa, MRS symposia on Organic Electronics, April 2002. 

3. Stephanie Chasteen, MRS symposia on Organic Electronics, April 2002. 

4. Yuko Nakazawa, SPIE symposia on Organic Photovoltaics, Seattle, July 2002. 

5. Stephanie Chasteen, SPIE symposia on Organic Photovoltaics, Seattle, July 2002. 

13
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1. REPORT DATE (DD-MM-YYYY)

July 2006 

2. REPORT TYPE

Subcontract Report 

3. DATES COVERED (From - To)

1 September 2001 – 30 April 2006 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Polymer Hybrid Photovoltaics for Inexpensive Electricity Generation: 

Final Technical Report, 1 September 2001 – 30 April 2006 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/SR-520-40044 

5e. TASK NUMBER

PVA60001 

6. AUTHOR(S)

S.A. Carter 

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University of California, Santa Cruz 

Santa Cruz, CA 95064 

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REPORT NUMBER

ACQ-1-30619-03 

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NREL 

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National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

NREL/SR-520-40044 

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

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13. SUPPLEMENTARY NOTES

NREL Technical Monitors: R. McConnell, F. Posey-Eddy 

14. ABSTRACT (Maximum 200 Words)

The project goal is to understand the operating mechanisms underlying the performance of polymer hybrid 

photovoltaics to enable the development of a photovoltaic with a maximum power conversion efficiency over cost 

ratio that is significantly greater than current PV technologies. Plastic or polymer-based photovoltaics can have 

significant cost advantages over conventional technologies in that they are compatible with liquid-based plastic 

processing and can be assembled onto plastic under atmospheric conditions (ambient temperature and pressure) 

using standard printing technologies, such as reel-to-reel and screen printing. Moreover, polymer-based PVs are 

lightweight, flexible, and largely unbreakable, which make shipping, installation, and maintenance simpler. 

Furthermore, a numerical simulation program was developed (in collaboration with IBM) to fully simulate the 

performance of multicomponent polymer photovoltaic devices, and a manufacturing method was developed (in 

collaboration with Add-vision) to inexpensively manufacture larger-area devices. 

15. SUBJECT TERMS

PV; polymer hybrid photovoltaics; cost; inexpensive manufacturing process; conversion efficiency; device; large-area; 

plastic; printing technologies; numerical simulation 

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