Polymer-Fullerene Bulk Heterojunction Solar Cells: Theory and Examples

Polymer-fullerene bulk heterojunction (BHJ) solar cells are based on blends of semiconducting polymers and fullerene derivatives, such as PCBM. It is described as a bulk heterojunction as one of the layers is a blend of two materials with differing energy band gaps, forming a dispersed, interpenetrating network. Charge separation can occur at the interface between the materials, a process exploited in photovoltaics to harness the sun's energy. This page explores the relationship between semiconducting polymers and fullerene derivatives, focusing on how their combined properties are utilized to form active layers in polymer solar cells (PSCs).

Theory of Polymer-Fullerene Solar Cells

Polymer-fullerene solar cells are part of the third-generation of solar cells. This includes other organic photovoltaics such as hybrid polymer solar cells as well as other solar technologies such as perovskite solar cells. Polymer-fullerene solar cells contain, as mentioned above, bulk heterojunctions as the active layer.

A conventional bulk heterojunction polymer solar cell is typically comprised of:

Substrate | Anode | Hole Transport/Extraction Layer | Active Layer | Electron Transport/Extraction Layer | Cathode

The active layer is responsible for absorbing photons and generating charge carriers which then flow through the device. For polymer-fullerene solar cells; the polymer is the electron donor and the fullerene is the electron acceptor. The polymer is usually responsible for absorbing photons as comparatively fullerenes have weak absorption in the visible and near-IR spectral range. The bulk heterojunction consists of regions of polymer and fullerene with a healthy amount of interconnectivity. One of the most common combinations is the polymer P3HT as the donor and the fullerene derivative PCBM as the acceptor (see schematic below). The degree of crystallinity and morphology of the heterojunction is really influential on the overall efficiency of the resulting solar cell.

Charge Generation and Movement in Polymer-Fullerene Solar Cells

Excitons are generated through the absorption of photons by the semiconducting polymer. The electron-hole pair then diffuse towards the donor-acceptor (polymer-fullerene) interface, hopefully without recombining. At this interface the pair can dissociate and generate charge carrier species. The electron moves into the LUMO of the acceptor and the hole remains within the donor. The charge carriers must then reach their respective electrodes crossing the active layer/electrode interface.

Ideally the electron-hole pair separate and move towards their respective electrode with ease to produce a highly efficient solar cell. The factors that control this are summarized in the table below.

Absorption of photons is dependent on polymer properties which dictate the band gap energy as well as overall film thickness and absorption length. Excitons may be short lived due to the low dielectric constant of polymers. This value describes the permittivity, or how well electric charge moves through the polymer.

The energy difference between the LUMOs of the accepter and donor components must be well aligned in order to promote the separation of electron and hole. How well these charged species move through the active layer and further into the device is controlled by many factors including active layer morphology and work function of all materials.

Factors Controlling Polymer-Fullerene Solar Cell Performance

The efficiency (FF) of organic photovoltaics has been limited by the fact excitons can only diffuse < ~10 nm. This results in high levels of electron-hole recombination. These short diffusion lengths also have the knock on effect of needing thin active layers to restrict exciton recombination. Having thin active layers reduces the amount absorption as some light passes through the layer without being absorbed. This leads to reduced J_sc values.

V_oc is proportional to the energy gap between the LUMO level of fullerene (derivative) and HOMO of semiconducting polymer. The larger the energy difference, the higher the potential maximum voltage (V_oc). Typically, choosing a polymer with a deep HOMO will maximise the energy difference and contribute to a higher V_oc. Reducing the electron affinity of the fullerene component will also increase the V_oc.

Polymer Requirements

The semiconducting polymer donor is important as it plays two key roles within the solar cell. Firstly, it has to efficiently absorb photons and generate excitons. Secondly, it has to donate electrons to the fullerene acceptor and transport holes to the anode. The key requirements of the polymer donor are:

• Soluble and processible
• Miscible with fullerenes to form interpenetrating networks (bulk heterojunction)
• Efficient in harvesting a photo-induced current
• Low band gap
• Absorbs broadly on the solar spectrum
• High charge mobility
• Finely aligned HOMO and LUMO levels to fullerene derivative acceptor
• Deep HOMO levels to give a high photovoltage
• Large ionization potential
• High hole mobility

Low Band Gap Polymers

In order to fulfil the requirements for efficient polymer-fullerene solar cells, new polymers have been designed with a low band gap. Polymers have been optimized to decrease the band gap (E_g) and donor HOMO. This should in turn maximise the J_sc and V_oc.

Most low-bandgap polymers have electron donor and acceptor units arranged alternatively in the main chain. This arrangement of donor and acceptor units induces an intramolecular charge transport interaction resulting in the reduction in the HOMO-LUMO gap. A D-A polymer contains electron-rich (donor) and electron-deficient (acceptor) units. The HOMO and LUMO of the polymer can be tuned by independently varying the electron donating ability of the donor and electron affinity of the acceptor units. A weak donor decreases the HOMO and a strong acceptor decreases the band gap energy. As well as altering the electronic properties, changing the chemistry of the polymer units can also affect how the polymer interacts with the fullerene acceptor. Factors like polymer crystallinity and interconnectivity can be modified.

A limitation of D–A polymers is that they generally don't absorb photons across the entire light spectrum, particularly at longer wavelengths. Solar cells containing D–A polymers often rely on the fullerene acceptor (usually PC71BM) to absorb photons from the lower wavelength region. This provides complementary absorption and contributes to the photocurrent.

Another role the polymer must perform is hole transportation. Typically the polymers used have large amount of conjugation and π character. High hole mobility in polymers is strongly tied to its crystallinity, which increases π-π overlap. Crystallinity is strongly reliant on the processing of the active layer and its resulting morphology.

Popular Polymer Donors

The polymers listed have a range of characteristics and functionalities. The typical D-A alternating units contain different chemistries. A common functionality is fluorine, an electron withdrawing group that can decrease the polymers HOMO level. Long alkyl chains are also popular as they help influence the polymers processing, stability, and crystallinity.

P3HT

From $299

PTB7-Th, PCE10, PBDTTT-EFT

From $520

PTB7

From $520

PBDB-T (PCE12)

From $520

PCDTBT

From $338

Fullerene Requirements

Fullerenes are the acceptor component in polymer-fullerene bulk heterojunction solar cells. They are great electron acceptors as well as n-type semiconductors. The key requirements of fullerenes within solar cells are:

• High LUMO
• Small electron affinity
• Finely aligned HOMO and LUMO levels to polymer donor
• Soluble and processible
• Ideally also absorb photons and generate exciton - covering wavelengths missed by the polymer donor
• Excellent interconnectivity with the polymer donor

The Impact of Fullerene Derivatives

The classic fullerene C60 does not meet the requirements of an appropriate acceptor compound within a solar cell. It is has poor solubility and therefore is difficult to process to create a highly interconnected active layer. It also has poor absorption.

PCBM was designed as a soluble alternative to C60, with functional groups which allow it to be more easily solution processed. One drawback of PCBM is that it still has poor absorption in the visible region and high absorption lengths so does not contribute to the photocurrent (Jsc).

PC71BM combines the soluble butyric acid methyl ester functional group with the C70 fullerene. Due to the reduced symmetry of C70 it has a more broad spectral range and therefore can extend the absorption of the active layer to 380-500 nm. From the success of PC71BM, larger and more substituted fullerene derivatives have been designed to increase the light harvesting contribution from the fullerene acceptor.

The functionalization of fullerenes also effects the HOMO and LUMO levels of the molecule. The electron affinity of the fullerene component can be reduced through adding more functional substituents. This in turn impacts the Voc values of the resulting polymer-fullerene solar cells, particularly through an increase in the LUMO of the acceptor. The LUMO level can determine the appropriate selection of the fullerene acceptor within the solar cell system. The diagram below shows a comparison of various fullerene derivatives. The HOMO and LUMO values may vary depending on processing and the surrounding system.

Popular Fullerene Acceptors

The most common fullerenes to be used in polymer-fullerene bulk heterojunction solar cells are shown below. Most have functional groups which improve their processibility as well as their electronic properties.

PCBM

From $325

bis-PCBM (PC60BM Bisadduct)

From $494

PC70BM (C70-PCBM), PC71BM

From $481

ICBA (Indene-C60 Bisadduct)

Price $858

C70 Fullerene

From $338

Device Engineering

Optimizing active layer morphology

As mentioned above there are many aspect of a polymer-fullerene solar cell that are dependent on the morphology of the active layer:

• η_A - Photon absorption efficiency
• η_diff - Exciton diffusion efficiency
• η_diss - Exciton diffusion efficiency
• η_tr - Charge carrier transport efficiency
• J_sc - Short circuit density
• FF - Fill Factor

Points to consider are:

The relationship between active layer thickness and absorption length

• If the photoactive layer is too thin, not enough light is absorbed. Some light can pass through the layer and this reduces the number of charge carriers that are generated (reduced η_A and J_sc)
• If the layer is too thick, it may be able to absorb more light but it could increase the amount of recombination of electrons and holes before they manage to reach an electrode (reduced η_diff, η_tr and FF)

Phase segregation

• The active layer needs to have sufficient mixing of polymer and fullerene phases (maximise the interface) to ensure exciton dissociation (η_diss).
• The separate phases need a high degree of molecular ordering and connectivity to facilitate efficient charge transport (increase η_tr).
• Phase segregation also helps to avoid exciton recombination (η_diff).

Techniques for optimizing active layer morphology

• Choice of processing solvents
• Post-treatment
• thermal annealing – increases crystallinity of the active layer, improves charge transport and collection
• external electric field - influences phase separation, polymer orientation and offers control over morphology during processing
• Self-organizing materials with the capability to control active layer microstructure
• Additives (such as 1,8-octanedithiol, DIO) - control over phase separation, often reducing excess fullerene aggregation and enhancing polymer crystallinity

All of the above techniques will impact the overall morphology of the active layer. The aim is to produce an interconnected polymer-fullerene blend with the right balance of phase segregation and local ordering.

Interface Engineering

Interface engineering is another important factor in ensuring charge extraction efficiency (η_tr and η_cc). The hole/electron extraction/transport layers also play a vital role in harvesting photocurrent. The key properties of interface layers is their work function and surface energy. With work function influencing the flow of charge and surface energy influencing the morphology of the active layer. The architecture of device will influence which properties are more important and will help you select the appropriate interface materials.

The HTL and ETL facilitate selective charge transport by having appropriately aligned energy levels. This reduces the chance of electron-hole recombination thereby increasing the overall photocurrent. They can also act as protective layers, contributing to the long term stability of the polymer-fullerene solar cell.

PC₇₁BM

Browse now

Learn More

Organic Photovoltaics: An Introduction

Organic photovoltaics (OPVs) have become widely recognized for their many promising qualities. This page introduces the topic of OPVs, how they work and their development.

Read more...

Solar Cells: A Guide to Theory and Measurement

A solar cell is a device that converts light into electricity via the ‘photovoltaic effect’. They are also commonly called ‘photovoltaic cells’ after this phenomenon, and also to differentiate them from solar thermal devices. The photovoltaic effect is a process that occurs in some semiconducting materials, such as silicon.

Read more...