ITO-free flexible organic solar cells with printed current collecting grids

TL;DR: In this article, an alternative ITO-free transparent anode based on solution processed high conductive PEDOT:PSS in combination with a printed current collecting grid was developed.

About: This article is published in Solar Energy Materials and Solar Cells.The article was published on 2011-05-01 and is currently open access. It has received 338 citations till now. The article focuses on the topics: Indium tin oxide & Anode.

Summary

1. Introduction

• The biggest motivation for the development of organic solar cell technology is the low cost potential, based on the use of lowcost materials and substrates and the very high production speeds that can be reached by roll-to-roll printing and coating techniques [1–6].
• The second argument to omit ITO from OPV devices is mechanical flexibility.
• A lot of effort has been directed on the development of highly conductive polymeric materials such as poly(3,4-ethylenedioxythiophene):poly(4-styrene sulphonate) (PEDOT:PSS).
• Earlier, the authors have reported inkjet printed current collecting grids as a part of a composite anode in a conventional OPV device [18], where the grid is the first printed layer in the devices.
• The conductivity of the grid can be improved by increasing the line height, but the increase in topology of the grid might makes it impossible to overcoat the grid with the subsequent layers.

2. Experimental

• A series of organic solar cell devices were prepared on flexible PEN substrates.
• This silver paste is a blend of silver nano particles and a soluble silver complex.
• Low conductive PEDOT:PSS (Clevios P VP AI 4083 PEDOT:PSS from H.C. Starck) was used for the preparation of the ITO-based devices.
• The thicknesses of the films were measured using a Dektak profilometer.

3. Results and discussions

• Fig. 3(a) shows a picture of a typical part of a screen printed honeycomb current collecting grid, which is part of the composite anode.
• Lines pattern of the grid, which is more applicable for module design, has been compared with honeycomb structure.
• The 100 nm thick layer of high conductive PEDOT has a relatively high absorption in the visible region.
• Further improvement of the current density in ITO-free devices is possible by decreasing the shadow effect, by minimizing the line width in the grids and by increasing the transparency of the high conductive PEDOT and optimization of layer thicknesses.
• The results illustrate that the device with the line pattern, with a pitch size of 2 mm, has a higher fill factor than the honeycomb pattern, which has a pitch size of 5 mm.

4. Conclusions

• Organic solar cells free from ITO on flexible substrates were fabricated.
• The alternative anode is based on highly conductive PEDOT:PSS in combination with printed current collecting grids.
• This type of composite anode has a significantly lower sheet resistance in comparison with ITO, which makes larger area devices possible without substantial efficiency losses.
• All temperature treatments are compatible with flexible substrates and roll-to-roll processing.
• This work will ultimately contribute towards fully printed devices, which will provide low-cost roll-to-roll manufacturing.

Figures

Fig. 1. Schematic illustration of an ITO-based device and a device with a current collecting grid.

Fig. 2. A picture of the 2 2 cm2 device with current collecting grid, free from ITO.

Fig. 7. Transmittance of the device stacks: PEN/barrier/ITO with 40 nm LC-PEDOT:PSS versus PEN/barrier/with 100 nm HC-PEDOT:PSS.

Table 1 Characteristics of the photovoltaic devices.

Fig. 4. Schematic illustration of current collection grids: (a) printed on top and (b) embedded into the barrier. (c) Dektak profile of the surface with embedded grid.

Fig. 5. Conductive grids: (a) with honeycomb and (b) line patterns used for the 2 2 cm2 devices.

Fig. 3. (a) 50 microscope image and (b) a typical line profile of screen printed Inktec TEC-PA-010 ink.

Fig. 6. JV-curves of flexible OPV devices with an active area of 4 cm2 with different anodes: (a) ITO on glass substrate, (b) and (c) high conductive PEDOT:PSS with, respectively, a honeycomb and a line pattern current collecting grid.

Frequently asked questions

Q1. What are the contributions in "Ito-free flexible organic solar cells with printed current collecting grids" ?

The authors have developed an alternative ITO-free transparent anode, based on solution processed high conductive PEDOT: PSS in combination with a printed current collecting grid. Furthermore, as this composite anode is solution-processed, it is a step forward towards low-cost large area processing. 

Q2. What have the authors stated for future works in "Ito-free flexible organic solar cells with printed current collecting grids" ?

Future work will concentrate in maximizing the cell area without substantial efficiency losses using optimized grid structures. 

Q3. What is the biggest motivation for the development of organic solar cell technology?

The biggest motivation for the development of organic solar cell technology is the low cost potential, based on the use of lowcost materials and substrates and the very high production speeds that can be reached by roll-to-roll printing and coating techniques [1–6]. 

Q4. What is the main reason for omitting ITO from OPV devices?

indium-tin oxide (ITO), which is commonly used as a transparent electrode, is one of the main cost consuming elements in present photovoltaic devices [1,7]. 

Q5. What is the importance of the HC-PEDOT?

As the HC-PEDOT is still responsible for the current collection in the area between the grid lines, the high conductivity of this PEDOT is very important. 

Q6. How is the conductivity of a polymeric electrode improved?

Improving the conductivity of such a polymeric electrode is possible by combining it with a metal grid, which is either thermally evaporated through microstructured shadow masks [13,14] or patterned by a lithographic method [15,16]. 

Q7. What was the use of the PEDOT:PSS?

Low conductive PEDOT:PSS (Clevios P VP AI 4083 PEDOT:PSS from H.C. Starck) was used for the preparation of the ITO-based devices. 

Q8. What is the important argument for omitting ITO from OPV devices?

Screen printed silver grids [4] were demonstrated in a roll-to-roll processed inverted OPV device, where the grid is the last printed layer in the devices. 

Q9. How many layers were obtained by the spin coating?

The photoactive layers were obtained by spin coating of the blend with 4 wt% of the active materials at 1000 rpm for 30 s, which corresponds to a thickness of 220 nm. 

Q10. What is the fill factor of the ITO-based solar cell?

The low fill factor of the ITO-based device is dueto the high sheet resistance of the ITO, which on a foil substrate is typically about 60 O/&. 

Q11. How many Sms are used for a PEDOT?

organic photovoltaic devices with only a PEDOT:PSS electrode do not provide high efficiency for large area devices due to the limited conductivity of the PEDOT:PSS, which is typically up to 500 S m 1. 

Q12. What is the effect of the grids on the transmission of light?

as already mentioned above, the shadow effect of the grids contributes to the lower current density in the ITO-free devices. 

Q13. What is the effect of a conductive grid on the fill factor?

Introduction of a conductive grid with a sheet resistance of 1 O/& into the photovoltaic devices substantially improves the fill factor. 

Q14. What is the main reason to omit ITO from OPV devices?

The brittle ITO layer can be easily cracked, leading to a decrease in conductivity and as a result degradation of the device performance. 

Q15. How does the composite anode increase efficiency?

The experiments show that the replacement of ITO by a composite anode yields an efficiency increase by a factor of two for devices with an active area of 2 2 cm2. 

Q16. What is the effect of the fill factor on the solar cell?

Theoretical calculations [21] show that scaling up the size of solar cell devices is not possible without efficiency losses due to the decrease in fill factor. 

Q17. What is the main motivation for the development of organic solar cell technology?

A lot of effort has been directed on the development of highly conductive polymeric materials such as poly(3,4-ethylenedioxythiophene):poly(4-styrene sulphonate) (PEDOT:PSS). 

Q18. What is the effect of the increase in the line height on the grid?

The conductivity of the grid can be improved by increasing the line height, but the increase in topology of the grid might makes it impossible to overcoat the grid with the subsequent layers. 

Q19. What is the difference between the two types of anodes?

This type of composite anode has a significantly lower sheet resistance in comparison with ITO, which makes larger area devices possible without substantial efficiency losses. 

Q20. What is the difference between the two types of devices?

Further improvement of the current density in ITO-free devices is possible by decreasing the shadow effect, by minimizing the line width in the grids and by increasing the transparency of the high conductive PEDOT and optimization of layer thicknesses. 

Q21. What is the difference between the two structures?

this structure provides homogeneous current distribution in case of four bus-bar devices, which were used in this study.