If the conduction band Ec and valence band Ev both of semiconducting materials lies below to the Ec and Ev absorber it will work as ETM. Opposite for HTM. For metal, if WF of metal lies below and close to conduction band then ETM or above to valence band (close) for HTM.

An OLED requires holes to enter from one contact and electrons from the other contact so they can radiatively recombine in the active area. A solar cells requires holes to be collected at one contact and electrons at the other contact so that you get a net voltage and current in the external circuit.

A good textbook for understanding this is Peter Würfel's "Physics of Solar Cells". You can also have a look at this paper: http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6960066

thank you

1). for solar cell also we need of HTL &ETL ?

2). while fabrication of solar cell how to place the n type & p type material? either like side by side or by stack like structure ? plz specify the reason too

thank u sir

may i know any std book to understand this concept in a deep manner from basics !!

Dear Vimal

There is also energy mismatch between Emitter and the electrodes (Anode and cathode). So there is a need to cover that gap. Secondly, organic materials are better hole carriers and poor electron carriers, so holes move faster as compared to electrons in organic materials. So holes can reach cathode before electrons can reach emissive layer hence recombination will not be possible. As you can check in any paper on OLED, HTL is always very thicker than ETL so that holes and electrons can reach emmisive layer in equal proportion. Introductory part of the attached thesis can be useful for understanding.

thank u sir

thank u  ling

Hi, vimal

Electron transport layer (ETL) is a layer which has high electron mobility and high electron affinity. Thus in this layer holes are blocked and cannot go through while electrons can flow through. Hole transport layer (HTL) do the opposite of (ETL) behavior it blocks electrons to fllow through .

Materials of these characteristics  are used to optimize both the performance and the stability of the devices.

Best wishes and this be helpful

This is an interesting question and therefore i tried to give an answer to it .

The solar cell must be built to build an internal electric field inside the material

and efficiently extract the electrons and holes generated inside the active layer which serves as a light absorber. So the role of the ETL and HTL accordingly

must increase the built in voltage as possible, and have proper interface energy levels with the active layer such that they extract the electrons and holes towards the metal electrodes.

So, they must be built to promote the extraction of holes by the HTL and of the electrons by the ETL. This is called a collection function.

In the LED they must act in a reverse manner where the ETL must inject electrons in the active layer and the HTL must inject holes in the same layer.

So they must be heavily doped and have energy levels to enable them performing as injectors. This means that the HTL valence level must lie deeper than that of the active material. The HTL must have a conduction band edge that lies higher than that of the active layer. For optimum operation the ETL must reject the holes to the active layer and the HTL must reject the electrons active layer such they do nor leak out. It is said that the ETL and the HTL must inject electrons and holes to flood them with electrons and holes which will recombine radiatively and emit light.

One has to notice that both ETL and HTL must be heavily doped in both cases.

In case of the solar cells to reduce the series resistance and for the LED to affect high injection efficiency.

These are the requirements on the materials of ETL and HTL to work properly as solar cell or as a LED.

Best wishes