Dear Tawheed,

From the functional point of view the transport layers must energetically match the active layer. In case of solar cells,For the etl the conduction band edge must be slightly lower than the conduction band edge of the active layers while its valence band edge must must be lower than the valence band edge of the active layer to reflect the holes to the htl. The hole transport  must have its valence band edge slightly slightly  higher than the valence band edge of the active layer to collect the holes from the active layer while its conduction band edge must be above the conduction band edge of the active layer to reflect the electrons and direct them to the etl.

From the conduction point of view the htl must possess high hole concentration to make it good hole conductor with low resistance, The etl must possess high electron concentration to conduct the electrons  with small voltage drop to the cathode. 

The mobility of electrons in the etl must be also high to decrease its resistance and so the mobility of holes in the htl must be also high to lower its resistance.

The grain size affects two parameters: the energy gap which  decreases with increasing grain size. In fact the grain size can be used to tune the bandgap of the material. The other factor is the mobility which suffer much from the grain size. It is so that the mobility is degraded much by the grain boundaries. So, the mobility decreases with the increase grain size. So, the good transport layer function function favors larger grain size so that the single crystal may be extreme best  structure.

In case of LEDS the energy matching conditions are the reverse of the solar cells. However the conduction properties remain the same.

Best wishes

Hi Towhid Hossain Chowdhury,

As Abdelhalim Zekry have mentioned in the aspect of electrical properties of carrier transport layers, you might have some benefits of using the transport layers in optical aspects:

+) The transport layers can be used as optical spacer for organic solar cells (OSC), OLED to place active layers at the maximum light intensity in OSC, OLED for improvement of absorption efficiency in OSC or out-coupling efficiency in OLED.

-) The carrier transport materials need to have high optical gap while maintain a good conductivity. This hinders the choice of suitable materials, especially organic materials that you might have problems with doping.

Best,

Thank you Professor Abdelhalim Zekry  and Nong Van Hoang for your valuable comments