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Ashar A Z

Organic electronics

Efficiency limitations of Organic Solar Cells

Organic solar cells differ from inorganic counterpart in many ways because of their

operational principle. Processes happening in organic solar cells when it is exposed to light

can be summarized as

1)Light falling on PV cell generate electron - hole pair (exciton) which will havebinding energy of the order of few hundreds of meV and dont spontaneously

dissociate in to free charges.

2) These excitons formed move in the materials and has to be dissociated to getfree charges, and is done by some external field.

3)Separated charges move in polymer material. Since they dont offer acontinuous path, these charges have to hope between polymer chains

4)Free charges are collected at respective electrodes to provide externalcurrent.

Each of these processes has associated losses, which greatly reduces the efficiency of

organic solar cells. The first loss comes from light absorption. The number of generated

carriers depends on the number of absorbed photons in the active layer with energy greater

than the bandgap energy. Some of the photons get reflected off from the top layer and

some others may not get absorbed and some others get absorbed in the substrate. The

absorption of light is dependent on the material used in the organic cell. Absorption in the

near infrared (>650 nm) is still a challenge. Sensitized organic polymers are one of the

remedy for this.

Second barrier is the optimization between exciton diffusion length and the

thickness of organic layer. Since the exciton diffusion length is of the order of 10 nm, they

should meet the field separating electron pair before that. An effective solution is

distributed D-A interface which is materialized through bulk heterojunction, in which

excitons formed on donor meet acceptor, which is very close to the site where light formed

and e-h pair.

Third one is recombination losses of charge carriers. There should be enough and

effective path for the free charge carrier to reach the electrodes and to be collected,

otherwise these charge carriers will be trapped in other sites and wont contribute to current

flow. Mobility of the charge carriers is an important criteria and the disorderness of the

polymer chains greatly reduces it. Fourth one is the use of electrodes. The work functions of

electrodes using should be appropriate for the charge collection.

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The recent calculations of thermodynamic efficiency limit of organic solar cells (Noel

C. Giebink et al Phys.Rev B 83, 195326 (2011)), including free energy for transfer of charge

at interface, give a reduced efficiency than the theoretical limit of efficiency for single p-n

junction (Schokley-Queisser limit). Their calculations bring down maximum efficiency from

30 % as established by SQ limit to 22% for a 0.5 eV free energy for charge transfer at D-Ainterface.

The main processes affecting the efficiency are discussed below

Absorption losses

Contributions to this come from reflectance of layer, absorption spectrum of

polymer materials used and the exciton diffusion length. Every layer and interfaces will have

finite reflectance. Optical absorption in polymers happens between -* bands of the

organic polymer. Since organic polymer in general have band gap higher than 2 eV (620 nm),

it limit the optical absorption to 30% of the solar spectrum. The light absorbed dont directly

form free carriers, but they form excitons which are stable at room temperature and need

to be dissociated before recombining. The absorption coefficient of organic materials is high

(~ 105

cm-1

) and a 1 m is enough to absorb all the light, but the problem is with the

absorption of whole solar spectrum, the low exciton diffusion length and reduced mobility

due to random orientation of organic molecule. It was observed that ordering of organic

molecule can increase the mobility of carriers. The practice of introducing dye into organic

polymers is also adopted to improve the light absorption. The famous P3HT:PCBM blend

have increased efficiency when dye is added to it, because P3HT absorption is limited to

wavelength below 650 nm and it was not able to absorb a large amount of solar spectrum.

The addition of DPA at optimum concentration improved the photocurrent by a factor of 3-7

times.

Organic materials have high fluorescent yield and in order to have a charge

separation excitons formed should be dissociated into electron and hole within the exciton

life time. So we cannot increase length of the organic layer beyond few tens of nm.

Exciton dissociation and diffusion

In organic solar cell the light absorption form Frenkel type excitons, the electron and

hole remain localized on a few polymer repetition units or a molecule. Charge formed on

the molecule can interact with the polymer chain to form polaron and they can drift along

the chain of the polymer. Intrachain excitons formed can drift from one chain to another by

FRET.

Typical binding energies of excitons in organic materials is of the order of 100 meV

and these have to be dissociated in to free carriers using some energy, which is usually

provided by the interface between different materials. The excitons can be lost by

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recombination when the dissociation driving forces (temperature and electric field) are

small. The recombination of excitons reduces the photocurrent. In addition, the

recombination reduces the free charge concentration and reduces Fermi energy difference

between electrons and holes, resulting in a lower Voc, reducing the output power. Higher

the binding energy of excitons higher is the energy difference needed for dissociating them.This driving force is usually provided by the difference between LUMO of Donor and

Acceptor. In a D-A type organic solar cell Voc is limited by the gap between LUMO of

Acceptor and HOMO of Donor. The charge carrier coupling to polymer chain also

contributes to loss in Voc . This will also reduce the efficiency of solar cell. So it is important

to select polymers having appropriate energy levels with respect to the exciton energy, and

electron phonon coupling as small as possible so that the optical gap is as close as possible

to the exciton zero-phonon energy.

Charge transport

Charge transport in inorganic solar cell is band transport because of the long range

order present in them. But in general there is no long range order present in the organic

materials and this affect the mobility. In addition charge traps, losses at interfaces also

reduce the photocurrent. Hole mobility in organic materials is generally less than that of the

electron mobility. Charge carrier mobility should be kept high to get high charge extraction

and hence high FF.

The dissociated charge carriers are driven by external field supplied by asymmetric

electrodes placed on each end. The one carrying electron is connected with high workfunction metal and hole carrying organic region is generally connected with ITO.

Electrodes

Losses due to electrodes and electrode-organic layer are very important and they

seriously affect the Voc of organic solar cells. The organic material-electrode energies should

match to have an efficient collection of charge carriers