A tex­tile, non-tox­ic dye solar cell should make it pos­si­ble: Ener­gy can be gen­er­at­ed from the fab­ric of awnings, sun­shades, back­packs or tents, which can be used to charge or oper­ate small elec­tron­ic devices. Research is being car­ried out on this in the “SolarFlex” project at Biele­feld Uni­ver­si­ty of Applied Sciences.

Biele­feld (fhb). For­got to charge your smart­phone again? This may soon be pos­si­ble on the road! Whether on a bike, in a café or at a camp­site — the pos­si­bil­i­ties are end­less if you imag­ine solar cells in a tex­tile fab­ric: for exam­ple on a back­pack, in a para­sol or in the roof of a tent. Also in cri­sis sit­u­a­tions a pos­si­bil­i­ty to sup­ply one­self self-suf­fi­cient­ly with electricity.

Researchers at the Uni­ver­si­ty of Applied Sci­ences (FH) Biele­feld are devel­op­ing this new type of nat­ur­al dye solar cell, which for the first time is ful­ly inte­grat­ed into textiles.
Solar cell should be sus­tain­able and cost-effective

What sounds like a rev­o­lu­tion­ary high-tech inven­tion is based on nature and above all one thing: sus­tain­able. “The dye-sen­si­tized solar cell is non-tox­ic, and the clever selec­tion of com­po­nents should make it cost-effec­tive and easy to recy­cle lat­er on,” explains project leader Mar­ius Dot­ter, who is doing his doc­tor­ate on this top­ic at Biele­feld Uni­ver­si­ty of Applied Sci­ences in coop­er­a­tion with Biele­feld Uni­ver­si­ty. The project is still in its infan­cy at the moment, but the UAS is pick­ing up on a trend that could soon become part of our every­day lives.

Dye solar cells can already be used for stand-alone solutions

Prof. Dr. Andrea Ehrmann has been research­ing the top­ic at Biele­feld Uni­ver­si­ty of Applied Sci­ences for sev­er­al years and super­vis­es Mar­ius Dot­ter’s doc­tor­al the­sis. She sees dye solar cells as an inter­est­ing alter­na­tive to con­ven­tion­al solar cells: “Their pro­duc­tion does not require clean rooms and much less ener­gy than sil­i­con solar cells, for exam­ple. How­ev­er, their cur­rent effi­cien­cies are very low, espe­cial­ly when using inex­pen­sive, non-tox­ic mate­ri­als. They are not yet suf­fi­cient to make an essen­tial con­tri­bu­tion to the ener­gy tran­si­tion. How­ev­er, dye solar cells can already be used for stand-alone solu­tions, in tex­tile archi­tec­ture or on oth­er large sur­faces. This can be more sen­si­ble and envi­ron­men­tal­ly friend­ly than using con­ven­tion­al sil­i­con-based cells.”

“Grätzel cells”: Elec­tric­i­ty cir­cuit thanks to sunlight

Dye-sen­si­tized solar cells, also known among experts as Grätzel cells after their inven­tor Michael Grätzel, are based on the pho­to­elec­tric effect: They con­vert light into elec­tri­cal ener­gy. The dye absorbs light, there­by ‘releas­ing’ an elec­tron — an elec­tric cir­cuit is created.

The cells con­sist of two con­duc­tive elec­trodes, at least one of which must be trans­par­ent to allow light to enter the cell. Glass is usu­al­ly used as a car­ri­er mate­r­i­al for this pur­pose. The front elec­trode, which is exposed to sun­light, is coat­ed with a semi­con­duc­tor, typ­i­cal­ly tita­ni­um diox­ide. The dye is in turn deposit­ed on this. The dye mol­e­cules absorb the light, excit­ing elec­trons in the dye that ‘migrate’ to the con­duc­tion band of the semi­con­duc­tor. Through the semi­con­duc­tor and the front elec­trode, the elec­trons enter an exter­nal cir­cuit where they can release the ener­gy. The re-entry into the solar cell via the counter elec­trode is sup­port­ed by a plat­inum or graphite lay­er as cat­a­lyst. With the return of the elec­tron to the dye, the cir­cuit is closed. The dye can then absorb light again and con­vert it into energy.

The same prin­ci­ple is to be used at the FH Biele­feld — but with­out glass, but in tex­tiles and with exclu­sive­ly non-tox­ic materials.

Ener­gy from fruit tea

In the project at Biele­feld Uni­ver­si­ty of Applied Sci­ences, the experts are using the nat­ur­al dye antho­cyanin and the semi­con­duc­tor tita­ni­um diox­ide as light absorbers. Antho­cyanins can be eas­i­ly dis­solved from plants, almost like a tea. “We worked with wild berry tea for a while. In the mean­time, we use hibis­cus flow­ers and a mix of water and ethanol or the organ­ic sol­vent dimethyl sul­fox­ide as sol­vent,” explains Mar­ius Dot­ter. They use graphite as a cat­a­lyst. “All it takes for us is a lit­tle abra­sion from the pen­cil,” Dot­ter said. An iodine-potas­si­um iodide mix is used as the electrolyte.

Shap­ing the solar cell into a textile

But how do you get these com­po­nents for a dye solar cell in the right form on tex­tiles? And how is the cur­rent divert­ed from the tex­tile to charge a cell phone bat­tery, for exam­ple? Mar­ius Dot­ter explains: “First, we have a tex­tile car­ri­er lay­er. At first glance, it looks like a nor­mal tea tow­el from the kitchen. A clos­er look reveals sil­ver stripes: Met­al threads are woven here, which will lat­er con­duct the elec­trons.” The var­i­ous lay­ers are now to be applied to this cloth.

Elec­tro­spin­ning plant pro­duces fibres

How­ev­er, the indi­vid­ual com­po­nents are usu­al­ly present as dis­solved liq­uids or gels and are to be applied to the tex­tile sur­face. This is where the elec­tro­spin­ning machine comes into play: the device is locat­ed at the FH Biele­feld and belongs to the Tex­tile Tech­nolo­gies work­ing group. Elec­tro­spin­ning means that a solu­tion of the desired mix­ture, for exam­ple the dye togeth­er with tita­ni­um diox­ide and X‑PAN, is attract­ed by a high volt­age and attach­es to the sub­strate in ran­dom ori­en­ta­tion as fibers. In this way, the ‘kitchen tow­el’ is turned into a nanofi­bre fleece. Togeth­er with the gel elec­trolyte, which is print­ed on, all the com­po­nents in the cell are thus to be com­bined to form large-area tex­tile solar cell circuits.

“This is exact­ly the point where our inves­ti­ga­tions are cur­rent­ly under­way. We want to find out which com­bi­na­tion of mate­ri­als works best,” says the project man­ag­er. After the first tests in the elec­tro­spin­ning facil­i­ty, in which graphite, antho­cyanins and tita­ni­um diox­ide were applied, Mar­ius Dot­ter is already very sat­is­fied: “It does­n’t feel like tex­tile yet, but the prin­ci­ple works well. The next two years will show what an ide­al com­po­si­tion might look like.”

And how does the elec­tric­i­ty end up in the bat­tery? “The met­al threads in the cloth, which serves as a car­ri­er mate­r­i­al, are sup­posed to con­duct the cur­rent into a cable, to which you can then con­nect the bat­tery, for exam­ple,” says Dotter.
How much ener­gy the cells will pro­duce ulti­mate­ly depends on the sur­face area and num­ber of cells, as well as the irra­di­a­tion. “Our goal is to be able to charge three smart­phone bat­ter­ies with one square meter of pho­to­volta­ic tex­tile on an aver­age day in Ger­many. We will only be able to deter­mine more pre­cise val­ues towards the end of the project,” explains Dotter.

It is impor­tant to him that the mate­ri­als real­ly feel like tex­tiles that are expect­ed for the respec­tive use and that they are durable. And if the tent, back­pack or awning is too bad­ly dam­aged, the mate­r­i­al can be recy­cled. No tox­ic sub­stances are pro­duced in the process, as Dot­ter explains: “Our aim is that the mate­ri­als can be used with­out hes­i­ta­tion and also recycled.”