Recovering a degraded solar cell

A method for recovering a degraded solar cell is disclosed. The method comprises radiating an ultraviolet (UV) light on the degraded solar cell for a period of time in a range from 30 seconds to 5 minutes.

TECHNICAL FIELD

The present disclosure generally relates to solar cells, and particularly, to methods for recovering degraded solar cells.

BACKGROUND

In recent years, the demand for sustainable and clean energy resources has led to a growth in the development of solar cells, which directly convert sun light into electricity. An efficient, stable, and low-cost photovoltaic technology may be used in industrial applications. Silicon photovoltaic cells which are first-generation solar cells are stable, with a long lifetime around 25 years and power conversion efficiencies (PCE) as high as 20%, but their fabrication process may be complex and expensive. Photovoltaic uptake has been growing to introduce sufficient alternatives to conventional solar cells, such as crystalline silicon solar cells. A low-cost and simple fabrication method for solar cells is a solution that provides roll-to-roll printing as a beneficial method for large scale production. Among different types of photovoltaics, polymer solar cells and perovskite solar cells have received extensive attention because of their potential for aiding in developing cheap, light weight, facile, and fast fabricated solar cells. Although these solar cells achieved reasonable PCEs, their short lifetimes due to degradation and lack of reproducibility still remain a challenge. There is, therefore, a need for a method for recovering the performance of degraded solar cells.

SUMMARY

According to one or more embodiments, the present disclosure describes a method for recovering a degraded solar cell. The method comprises radiating an ultraviolet (UV) light on the degraded solar cell. In an exemplary embodiment, the degraded solar cell may comprise a metal contact.

In an exemplary embodiment, radiating the UV light may comprise radiating the UV light on a side of the degraded solar cell with the metal contact.

According to one exemplary embodiment, radiating the UV light may comprise radiating the UV light by a UV light source with a power in a range of about 100 Watts to about 1000 Watts. The UV light source may be placed at a distance in a range of about 15 cm to 25 cm from the degraded solar cell.

According to an exemplary embodiment, radiating the UV light may comprise radiating the UV light for a period of time. The period of time may be in a range of about 30 seconds to about 5 minutes.

According to an exemplary embodiment, radiating the UV light on the degraded solar cell may comprise radiating the UV light on a degraded polymer solar cell. In other exemplary implementations, radiating the UV light on the degraded solar cell may comprise radiating the UV light on a degraded perovskite solar cell.

DETAILED DESCRIPTION

The following detailed description is presented to enable a person skilled in the art to make and use the methods and devices disclosed in exemplary embodiments of the present disclosure. For purposes of explanation, specific nomenclature is set forth to provide a thorough understanding of the present disclosure. However, it will be apparent to one skilled in the art that these specific details are not required to practice the disclosed exemplary embodiments. Descriptions of specific exemplary embodiments are provided only as representative examples. Various modifications to the exemplary implementations will be readily apparent to one skilled in the art, and the general principles defined herein may be applied to other implementations and applications without departing from the scope of the present disclosure. The present disclosure is not intended to be limited to the implementations shown, but is to be accorded the widest possible scope consistent with the principles and features disclosed herein.

Disclosed herein is a method for recovering and enhancing the performance of degraded solar cells, and particularly, third generation solar cells. The recovering process may comprise radiating an ultraviolet (UV) light on a solar cell for a limited time. The UV light may be radiated on a side of the solar cell on which a metal contact is located. The radiation time, power of the UV light source, and the distance of the source from the degraded solar cell may be selected in certain ranges. The method can be applied on both degraded and failed devices, i.e., devices with weak performance after fabrication.

FIG. 1illustrates a schematic representation of a system for implementing a method for recovering a degraded solar cell102, consistent with one or more exemplary embodiments of the present disclosure. Referring toFIG. 1, according to an exemplary embodiment, degraded solar cell102may include one of a degraded polymer solar cell and a degraded perovskite solar cell. In an exemplary embodiment, the method may include radiating a UV light104on degraded solar cell102. Degraded solar cell102may include a metal contact106. In an exemplary implementation, radiating UV light104may include radiating the UV light on a side of degraded solar cell102with metal contact106for a period of time in a range from about 30 seconds to about 5 minutes. In addition, radiating UV light104may include radiating UV light104by a UV light source108with a power in a range of about 100 Watts to about 1000 Watts. UV light source108may be placed at a distance110from degraded solar cell102. In an exemplary embodiment, the distance110may be in a range of about 15 cm to about 25 cm.

Example 1: Recovering a Degraded Perovskite Solar Cell

In this example, an exemplary degraded perovskite solar cell is recovered by an exemplary implementation of the disclosed method. To fabricate the exemplary perovskite solar cell, a plurality of etched fluorine doped tin oxide (FTO) coated glass substrates are first cleaned with deionized water, acetone, and isopropanol. After drying the substrates at a temperature of about 100° C. for about 30 minutes, they are coated with a compact layer of TiO2by spin coating a mild acidic solution of tetraisopropyl orthotitanate in ethanol followed by annealing at about 500° C. for about 30 min. The compact layer is treated by a TiCl4solution and annealed at about 500° C. for about 30 min. Then, the mesoporous TiO2layer is deposited by spin coating of a TiO2paste at about 5000 RPM for about 30 s, dried at about 70° C. for about 30 min, annealed at about 500° C., and finally is treated by TiCl4. A perovskite layer is deposited via a two-step process from a PbI2precursor solution (about 1M in an anhydrous DMF) and a CH3NH3I solution (about 7 mg/ml in an anhydrous IPA). Then, the surface morphology of the active layer is obtained by scanning electron microscopy (SEM) and its topography is obtained by atomic force microscopy (AFM).FIG. 2Ashows a SEM image of the perovskite active layer of the exemplary perovskite solar cell.FIG. 2Bshows a topography image of the perovskite active layer of the exemplary perovskite solar cell.

Next, P3HT is used as a hole transporting layer (HTL) that is spin coated from about 10 mg/ml solution of P3HT in chlorobenzene. Finally, an about 100 nm-thick Au layer is deposited on the top of the P3HT film by a thermal evaporator in a vacuum condition (about 10−5torr).FIG. 3Ashows a schematic of the fabricated exemplary perovskite solar cell.FIG. 3Bshows a SEM cross-section of the exemplary perovskite solar cell.FIG. 3Cshows the energy band diagrams of different components of the exemplary perovskite solar cell.

After absorbing incident photons by CH3NH3PbI3, excitons with a low binding energy are generated and dissociated into a free charge carrier. To extract the photocurrent, photogenerated holes and electrons are transported to the FTO and the Au contact.

The fabricated exemplary perovskite solar cell, hereinafter referred to as the fresh perovskite solar cell, is stored in the ambient condition (with a relative humidity of about 25-30%) for 30 days, to obtain a degraded perovskite solar cell.

Referring again toFIG. 1, to recover the performance of the degraded perovskite solar cell by an implementation of the disclosed method, an implementation of the UV light104may be radiated on the degraded perovskite solar cell by an implementation of the UV light source108(for example, a pressure Hg lamp) from the Au contact side of the degraded perovskite solar cell for about 4 min, to obtain a recovered perovskite solar cell.

Current-voltage (J-V) characteristics of the fresh perovskite solar cell is measured by an Iviumstat potentiostat under a calibrated AM 1.5 solar simulator at about 100 mW/cm2light intensity.FIG. 4shows first J-V characteristics402of the fresh perovskite solar cell, second J-V characteristics404of the degraded perovskite solar cell, and third J-V characteristics406of the recovered perovskite solar cell by the example implementation of the disclosed method. As shown inFIG. 4, the performance of the degraded perovskite solar cell significantly increases after the UV-treatment.FIG. 4shows that the performance of the recovered perovskite solar cell is about 20% higher than the performance of the fresh perovskite solar cell. In other words, the performance after recovery has even exceeded the initial performance of the fresh perovskite solar cell. The recovery effect of the UV-treatment may be attributed to interfaces modification in the degraded perovskite solar cell by dissociating adsorbed water molecules during the fabrication and storage of the perovskite solar cell.

The measured photovoltaic characteristics, including the short circuit current density (Jsc), the open circuit voltage (Voc), the fill factor (FF), and the power conversion efficiency (PCE) of each of the fresh perovskite solar cell, the degraded perovskite solar cell, and the recovered perovskite solar cell are reported in Table 1. As shown in Table 1, PCE decreases from about 10.5% for the fresh perovskite solar cell to about 6.33% for the degraded perovskite solar cell during the storage time.

Because of the possibility of the degradation of solar cell layers under the UV-treatment, The XRD diffraction peaks of the fabricated perovskite solar cell before and after UV-treatment are recorded by a Philips diffractometer (model: X′Pert MPD) equipped with a proportional Xe filled detector, a Cu tube (λ=1.54056 Å).FIG. 5Ashows the XRD pattern of the degraded perovskite solar cell (before the UV-treatment), andFIG. 5Bshows the XRD pattern of the recovered perovskite solar cell (after the UV-treatment). The perovskite layer is the main crystalline layer that has sharp peaks in the XRD patterns. There is no sign of the presence of PbI2peaks in the XRD pattern ofFIG. 5B. Therefore, it is clear that no decomposition happens in the perovskite layer during the UV-light irradiation since the perovskite layer decomposition leads to the formation of PbI2.

Example 2: Recovering a Degraded HTM-Free Solar Cell

In this example, an exemplary HTM-free solar cell is fabricated according to the procedure explained in Example 1, except that no HTM is used in the exemplary HTM-free solar cell, to show that the effectiveness of the disclosed method works for structures with various different solar cell components.

After obtaining a fresh HTM-free solar cell, it is kept in the ambient condition for 7 days to obtain a degraded HTM-free solar cell. Similar to Example 1, the degraded HTM-free solar cell is recovered by an example implementation of the disclosed method. A UV-light is radiated on the Au side of the degraded HTM-free solar cell to obtain a recovered HTM-free solar cell. Table 2 shows photovoltaic characteristics of the fresh HTM-free solar cell, the degraded HTM-free solar cell, and the recovered HTM-free solar cell, under a 1-sun illumination. As shown in Table 2, PCE of the fresh HTM-free solar cell is about 2.06% that reduces to 1.4% after the 7 day storage, for the degraded HTM-free solar cell, and is then enhanced to about 3.2% after recovery by the UV-treatment, for the recovered HTM-free solar cell.

Example 3: Recovering a Degraded Solar Cell by UV Radiations with Different Powers

In this example, the exemplary degraded perovskite solar cell described in Example 1 is recovered by two example implementations of the disclosed method. In a first exemplary implementation, a UV-light source with power of about 400 Watts is used for radiating a UV light on the degraded solar cell. In a second exemplary implementation, a UV-light source with a power of about 1000 W is used for radiating a UV light on the degraded solar cell. The photovoltaic characteristics of the fresh perovskite solar cell, the degraded perovskite solar cell, and the recovered perovskite solar cell for each of the example implementations are presented in Table 3.

Example 4: Recovering a Degraded Polymer Solar Cell

In this example, an exemplary degraded polymer solar cell is recovered by an exemplary implementation of the disclosed method. To fabricate the exemplary polymer solar cell, an indium tin oxide (ITO) substrate is etched and cleaned with deionized water, acetone, hexane, and isopropanol. A ZnO layer is deposited on ITO via a hydrothermal method by using a zinc acetate and KOH. The ZnO layer serves as an electron transporting layer (ETL). Next, a solution of PCBM and P3HT (a 1:1 weight ratio, about 30 mg/ml in dichlorobenzene) is spin-coated on the ZnO layer with about 1500 RPM for about 50 s. Next, the coated substrate is heated at about 140° C. for removing the residual solvents and annealing an active layer of the exemplary polymer photovoltaic cell under an inert gas. Finally, an about 100 nm thick Ag layer is deposited on the top of the film by using a thermal evaporator in a vacuum condition (<10−5torr).

The active layer is sandwiched between two contacts and involves a blend of a conjugated polymer (P3HT) as a donor and a PCBM as an acceptor that form a nanoscale interpenetration network. When incident photons from a transparent contact are absorbed by the active layer, excitons are generated in conjugated polymers. Since the dielectric constants of conjugated polymers are low, there is a strong Coulomb attraction force between photoexcited electrons and holes which reduces the probability of the exciton dissociation under the ambient condition. Therefore, an acceptor material with a higher electron affinity is required for dissociation of excitons into free charge carriers. In a normal structure, excitons are dissociated at the donor-acceptor interface, holes are transferred throughout the highest occupied molecular orbitals (HOMO) of the polymer and collected at the anode, and electrons are transferred from the LUMO of the donor to the LUMO of the acceptor, transported to the cathode, and collected at the cathode.

FIG. 6Ashows a SEM image of the active layer of an exemplary fabricated polymer solar cell, hereinafter referred to as the fresh polymer solar cell.FIG. 6Bshows a topography image of the active layer. The fresh polymer solar cell is stored in the ambient condition for about 3 weeks to obtain a degraded polymer solar cell. Similar to Example 1, the degraded polymer solar cell is recovered by an implementation of the disclosed method, and a recovered polymer solar cell is obtained. Table 3 shows the photovoltaic characteristics of the fresh polymer solar cell, the degraded polymer solar cell, and the recovered polymer solar cell, under a 1-sun illumination.

TABLE 4Photovoltaic characteristics of the fresh, degraded,and recovered polymer solar cells.VOCJSCFFPCEDevice(V)(mA/cm2)(%)(%)Fresh polymer solar cell0.4213.30.341.8Degraded polymer solar cell0.419.450.31.16Recovered polymer solar cell0.41513.970.341.97