Post-treatment method for photovoltaic cell using thermal treatment at temperature above glass transition temperature of electron donor

The invention relates to a method for the aftertreatment of a photovoltaic cell with a photoactive layer composed of two molecular components, namely an electron donor and an electron acceptor, in particular a conjugated polymer component and a fullerene component, and with two metallic electrodes provided on both sides of the photoactive layer, wherein the photovoltaic cell is subjected to a heat treatment above the glass transition temperature of the electron donor for a predetermined treatment period.

Plastics called conjugated plastics with an alternating sequence of single and double bonds have energy bands comparable to semiconductors in terms of electron energy, so that they can also be converted from a nonconductive to a metallically conductive state by doping. Examples of such conjugated plastics are polyphenylenes, polyvinylphenylenes, polythiophenes or polyanilines. However, the efficiency of the energy conversion of photovoltaic polymer cells made of a conjugated polymer is typically between 10 '³ and 10 '² %. In order to improve this efficiency, it is known (US Pat. No. 5,454,880 A) to build up the photoactive layer from two molecular components, one of which by a conjugated polymer as an electron donor and the other by a fullerene, in particular a Buckminster fullerene C₆o, are formed as an electron acceptor. The very fast electron movement induced by light at the interfaces between these components prevents further charge carrier recombination, which results in a corresponding charge separation. However, this effective charge separation only takes place in the interface area between the electron donor and the electron acceptor, so that the aim is to achieve as homogeneous a distribution as possible of the fullerene component, which is effective as an electron acceptor, in the polymer component forming the electron donor.

Since it has been shown that the electron mobility increases in a crystalline polymer matrix compared to an amorphous matrix, and crystal formation increases at a temperature above the glass transition temperature, it has already been proposed to subject the photovoltaic cells to an after-treatment with heat in order to increase the efficiency to be able to. The photovoltaic cells were exposed to a treatment temperature of 60 to 150 ° C. for a treatment period of 1 hour, with an upper limit for the efficiency of approx. 3% which could no longer be increased by optimizing the heat treatment.

The invention is therefore based on the object of designing a method for the aftertreatment of a photovoltaic cell of the type described at the outset in such a way that a further increase in the degree of efficiency is possible.

The invention solves the problem in that the heat treatment of the photovoltaic cell is carried out at least during a portion of the treatment period under the influence of an electric field which is excited by an excitation voltage applied to the electrodes of the photovoltaic cell whose open circuit voltage is greater.

The efficiency of the photovoltaic cell can be increased in a surprising manner by the influence of the electric field excited via the electrodes of the photovoltaic cell during the heat treatment. A possible explanation for this improvement in efficiency is seen in the fact that the electric field injects additional charge carriers into the photoactive layer via the electrodes. These additional charge carriers support the alignment of the polymer component in the direction of the applied electric field, which requires a corresponding mobility of the polymer molecules, which is ensured by heating the photovoltaic cell above the glass transition temperature of the polymer component. With the increased alignment of the polymer, its conductivity for charge carriers increases. In addition, the electrical contacts between the electrodes and the photoactive layer are lastingly improved, so that the series resistance within the photovoltaic cell is reduced. This reduction in the series resistance is also accompanied by an increase in the short-circuit current and the fill factor.

So that charge carriers can be injected into the photoactive polymer component via the electric field, the excitation voltage applied to the electrodes of the photovoltaic cell to excite the electric field must correspondingly exceed the open circuit voltage of the photovoltaic cell. In order to achieve a good effect, the excitation voltage should be Leer2

AT 41 1 305 Β exceed the running voltage by at least 1 V. For most applications, the conditions are particularly favorable if the excitation voltage is selected between 2.5 and 3 V. The upper limit of the excitation voltage is actually limited by the load capacity of the photovoltaic cell from the applied electric field. However, an increase in the excitation voltage over the specified range from 2.5 to 3 V generally does not result in an increase in the directivity on the photoactive polymer component.

The positive influence of the heat treatment on the tendency of the photoactive polymer component to crystallize decreases after a certain period of treatment so that the period of time during which the photovoltaic cell is subjected to a heat treatment under the influence of an electric field is advantageously limited. Treatment times between 2 and 8 minutes result in favorable conditions for the heat treatment, with an optimum in the range of a treatment time between 4 and 5 minutes.

The method according to the invention for aftertreatment of a photovoltaic cell is explained in more detail with the aid of the drawing. Show it

1 shows a photovoltaic cell to be subjected to an aftertreatment in a schematic section,

Fig. 2 characteristic curves which show the relationship between the voltage and the current density for photovoltaic cells basically the same structure, but without heat treatment, with heat treatment and with heat treatment under the influence of an electric field,

3 shows the charge yield related to the wavelength of the photo-excitation per incident light output again for photovoltaic cells with the same structure without and with heat treatment and heat treatment under the influence of an electric field and

4 shows the dependence of the achievable efficiency of photovoltaic cells on the duration of the heat treatment with and without the influence of an electric field.

According to FIG. 1, the photovoltaic cell consists of a transparent glass substrate 1 which is coated with an electrode 2 made of an indium tin oxide (ITO). On this electrode 2, which is generally covered with a smoothing layer made of a polymer made electrically conductive by doping, usually polyethylene dioxythiophene (PEDOT), a photoactive layer 3 made of two molecular components, namely a conjugated polymer component and a fullerene component, is applied. The photoactive layer 3 then carries the counter electrode 4 which, when ITO is used as the hole-collecting electrode 2, consists of an aluminum layer to form an electron-collecting electrode.

In the case of the exemplary embodiment, a polythiophene was used as the polymer component, which has pronounced crystallization properties as a prerequisite for good hole conductivity. The polythiophene used was a poly-3-hexylthiophene (P3HT) with a methanofullerene, namely [6.6] -phenyl-C₆Methyl i-butylate (PCBM), used as an electron acceptor. On the ITO electrode 2, which had a layer thickness of 125 nm, a layer of polyethylene dioxythiophene-polystyrene sulfonate (PEDOT-PSS) was applied in a thickness of about 50 nm before, after a drying time of about 45 min under a vacuum of 10 '¹ until 10² mbar the photoactive layer was applied in the form of a solution of 10 mg P3HT and 20 mg PCBM per ml of solvent. 1,2-dichlorobenzene was used as the solvent. After a drying time of approx. 45 min under a vacuum of 10 '¹ until 10'² mbar was first a layer of 0.6 nm lithium fluoride and then the aluminum electrode in a layer thickness of 70 nm in the same high vacuum step (10^-6 mbar) evaporated.

The photovoltaic cells produced in this way were subjected to an after-treatment by heat, in conjunction with an electric field. For this purpose, the photovoltaic cells were placed on a heating plate 5, the electrodes 2 and 4 being connected to an electrical voltage source 6. Between the 2.7 V applied electrodes 2 and 4, the photoactive layer 3 was exposed to the influence of an electric field excited by this excitation voltage as soon as the photoactive layer 3 was heated to a treatment temperature between 70 and 75 ° C, i.e. a temperature above Glass transition temperature of the polymer component. After a treatment period of 4 minutes, the aftertreatment was discontinued. The photovoltaic cells cooled on room 3

AT 411 305 B temperature. To illustrate the effects that can be achieved with the heating and the simultaneous excitation of an electric field, the characteristics shown in FIGS. 2 and 3 were measured for identically constructed photovoltaic cells, which on the one hand remained without post-treatment and on the other hand a heat treatment with and without the influence of an electric field have been exposed to the above conditions.

The characteristics according to FIG. 2 were obtained with exposure to white light (80 mW / cm^{1 2}) recorded. The characteristic curve a recorded for a photovoltaic cell without aftertreatment shows an open-circuit voltage of 300 mV and a current density for the short-circuit current of approx.

2.5 mA / cm² with a fill factor of 0.4. The efficiency of these photovoltaic cells can be given as approx. 0.4%. The characteristic curve b was recorded for a photovoltaic cell that was only subjected to an aftertreatment using heat. Compared to characteristic curve a, the open-circuit voltage increases to 500 mV and the density of the short-circuit current to approx.

7.5 mA / cm². The fill factor was determined to be 0.57. The efficiency of these photovoltaic cells is 2.5%. For photovoltaic cells that have been subjected to heat treatment under the influence of an electric field, the characteristic curve c gives an open circuit voltage of approx. 550 mV and a short circuit current density of approx. 8.5 mA / cm² at. With a fill factor of 0.6, the efficiency increases to 3.5%.

In FIG. 3, the charge yield per incident light power IPCE [%] = 124O.1_k / λ.Ι, can be read from the wavelength λ measured in nm for the photovoltaic cells to be compared. With l_k is in pA / cm² measured density of the short-circuit current and with l | those in W / m² Enter the measured light output in the above formula. It turns out that the quantum efficiency IPCE for photovoltaic cells without aftertreatment reaches a maximum of about 30% at a wavelength of 440 nm, as can be seen from the characteristic curve a. With a heat treatment without the influence of an electric field, the quantum efficiency IPCE increases by almost double with a simultaneous shift to a range of higher wavelengths, so that these wavelength ranges of solar radiation can be better used. The aftertreatment with heat under the influence of the electric field ensures a further slope in accordance with the characteristic curve c, which results in a quantum efficiency IPCE of 61%.

4 shows the efficiency of photovoltaic cells during a heat treatment with and without the influence of an electric field as a function of the treatment time. It can be seen immediately that the efficiency changes with the treatment time. For photovoltaic cells with a heat treatment without the influence of an electric field, a maximum efficiency is achieved with a treatment time in the region of 6 minutes. The effect of an electric field results in shorter treatment times in the order of about 4 minutes for the maximum efficiency.