MICROSTRUCTURED ZnO COATINGS FOR IMPROVED PERFORMANCE IN Cu(In, Ga)Se2 PHOTOVOLTAIC DEVICES

Abstract

A microstructured ZnO coating that improves the performance of Cu(In,Ga)Se.sub.2 (CIGS) photovoltaic (PV) devices via two mechanisms; it acts an antireflective layer with superior non-normal performance to thin film anti-reflective (AR) coatings, and it scatters a large fraction of incoming light at a large angle, resulting in absorption that is on average closer to the p-n junction.

Claims

1. A photovoltaic device, comprising: a p-type layer on a substrate; an n-type layer on the p-type layer, forming a p-n junction; a layer of ZnO on the n-type layer; a layer of aluminum-doped ZnO (AZO) on the ZnO; and a continuous microstructured ZnO topmost layer on the AZO, wherein the continuous microstructured ZnO topmost layer comprises antireflective surface structures on the AZO layer and scatters incoming light, increasing absorption of scattered light close to the p-n junction.

2. The photovoltaic device of claim 1, wherein the substrate comprises glass with a Mo bottom contact.

3. The photovoltaic device of claim 1, wherein the p-type layer is about 2 m thick.

4. The photovoltaic device of claim 1, wherein the n-type layer is about 50 m thick.

5. The photovoltaic device of claim 1, wherein the ZnO layer on the n-type layer is about 60 nm thick.

6. The photovoltaic device of claim 1, wherein the AZO layer is about 200 nm thick.

7. The photovoltaic device of claim 1, wherein the ZnO layer, the AZO layer, and the continuous microstructured ZnO topmost layer are deposited on the substrate at a substrate temperature of 200 C.

8. The photovoltaic device of claim 1, wherein the antireflective surface structures in the continuous microstructured ZnO topmost layer have a peak-to-peak height of about 500 nm or less.

9. The photovoltaic device of claim 1, wherein the p-type layer comprises Cu(In, Ga)Se.sub.2 (CIGS), CuInSe.sub.2 (CIS), CuGaSe.sub.2 (CGS), Cu.sub.2ZnSn(S,Se).sub.4 (CZTS), CdTe, amorphous Si, or any combination thereof.

10. The photovoltaic device of claim 1, where the n-type material is CdS.

11. A photovoltaic device, made by the method comprising: coating a substrate with a p-type layer; depositing an n-type layer on the p-type layer, forming a p-n junction; depositing a layer of ZnO on the n-type layer; depositing a layer of aluminum-doped ZnO (AZO) on the ZnO; depositing a continuous top layer of ZnO on the AZO; and etching the ZnO top layer to form a textured continuous ZnO topmost layer comprising subwavelength surface structures; wherein the textured continuous ZnO topmost layer comprises antireflective surface structures on the AZO layer and scatters incoming light, increasing absorption of scattered light close to the p-n junction.

12. The photovoltaic device of claim 11, wherein the substrate comprises glass with a Mo bottom contact.

13. The photovoltaic device of claim 11, wherein the p-type layer is about 2 m thick.

14. The photovoltaic device of claim 11, wherein the n-type layer is about 50 m thick.

15. The photovoltaic device of claim 11, wherein the ZnO layer on the n-type layer is about 60 nm thick.

16. The photovoltaic device of claim 11, wherein the AZO layer is about 200 nm thick.

17. The photovoltaic device of claim 11, wherein the ZnO layer, the AZO layer, and the continuous ZnO top layer are deposited on the substrate at a substrate temperature of 200 C.

18. The photovoltaic device of claim 1, wherein the antireflective surface structures in the textured continuous ZnO topmost layer have a peak-to-peak height of about 500 nm or less.

19. The photovoltaic device of claim 1, wherein the p-type layer comprises Cu(In, Ga)Se.sub.2 (CIGS), CuInSe.sub.2 (CIS), CuGaSe.sub.2 (CGS), Cu.sub.2ZnSn(S,Se).sub.4 (CZTS), CdTe, amorphous Si, or any combination thereof.

20. The photovoltaic device of claim 1, where the n-type material is CdS.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a schematic diagram of a CIGS device with a ZnO ARSS coating.

[0013] FIG. 2 shows SEM cross sections (top) and images taken at 20 from normal incidence of ZnO ARSS structures obtained with etch times varying from 0-30 s in 0.5% HCl solution.

[0014] FIG. 3 shows the peak-to-peak height of ARSS features, measured as a function of etch time.

[0015] FIG. 4 shows the change in J.sub.SC, compared before and after ARSS deposition, as a function of illumination angle.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention provides a new product that includes antireflective surface structures (ARSS) formed by chemical etching ZnO formed on top of CIGS devices. The ZnO structures improve PV device performance through a combination of two effects, the antireflective (AR) properties of the ARSS and the improvement resulting from large-angle scattering.

[0017] In one embodiment as depicted in FIG. 1, a soda lime glass (SLG) substrate 10 with a sputtered Mo bottom contact 12 was coated with 2 m of CIGS 14 via single-step evaporation. Approximately 50 nm of CdS 16 was then deposited by chemical bath deposition. For the top contact, 60 nm of ZnO 18 was deposited by reactive sputtering with flowing O.sub.2 in order to increase resistivity and transparency, and this was followed by a conductive 200 nm thick layer of aluminum-doped ZnO (AZO) 20 deposited by sputtering. Both ZnO and AZO were deposited at a substrate temperature of 200 C. Ni/Al grids, composed of 60 nm of Ni followed by 400 nm of aluminum, were deposited via electron beam evaporation. Samples were scribed by hand to obtain individual cells, each with an area of approximately 0.5 cm.sup.2. PV devices were then characterized prior to ARSS coating.

[0018] The contact pads for the grids 24 were protected with photoresist, and ZnO ARSS 22 were formed on top of the devices. An 870 nm thick layer of ZnO was sputtered on top of the complete CIGS device, again with flowing O.sub.2 and at a substrate temperature of 200 C. This layer was chemically etched in a 0.5% HCl solution at room temperature for 0-30 s resulting in a textured ZnO surface. The photoresist protecting the contact pads was removed, and devices were characterized.

[0019] Samples for cross sectional SEM analysis were obtained by mechanically breaking samples. Light J-V curves were obtained in a solar simulator under one sun, AM 1.5 G illumination calibrated using a Si reference cell. The setup was configured to allow for angular measurements of up to 60 from normal incidence. Dark current measurements were obtained with a Keithley 2400 SourceMeter in a darkened enclosure in order to evaluate diode properties of the devices.

[0020] Samples consisting of a ZnO/AZO electrode with ARSS coatings were etched for times varying from 0-30 s in dilute HCl. Samples were visibly hazy in transmission after etching. Spectroscopic measurements of etched ZnO films deposited on glass substrates showed an absolute decrease of 5% from 350-1200 nm in specular reflection.

[0021] SEM images showing cross sections and images taken at 20 from normal incidence are shown in FIG. 2. Prior to etching, the ZnO exhibited a small amount of surface roughness that increased rapidly with etch time. Features were subwavelength and consistent across the etched surface. For the 20 and 30 s etches, some features extended through the entire ZnO film but not the underlying electrode, indicating that the etch rate of AZO is smaller than that of ZnO. This is fortuitous in that the AZO layer acts as a barrier, preventing the HCl etch from damaging other layers of the device. This result was consistent with dark current measurements, made before and after ARSS deposition, showing that the diode properties of the junction were preserved.

[0022] The depth of ARSS features, measured peak-to-peak from cross-sectional SEM images, is shown in FIG. 3. Feature height initially increased with etch time, peaks at approximately 500 nm for an etch time of 15 s, and decreased gradually with further etching. The decrease in thickness resulted from etching of the tallest features while no ZnO remained to etch on the bottom. Feature height could potentially be increased further by using a thicker ZnO film.

[0023] Light J-V measurements were obtained for films with varying etch times for angles ranging from 060. The open circuit voltage, V.sub.OC, and fill factor (FF) were found to decrease slightly, by <10%, for all etch times and angles. This was attributes to the extra anneal that occurred during ZnO deposition. Further optimization of ZnO deposition parameters is expected to reduce this effect. The most pronounced change, however, was a dramatic increase in J.sub.SC. FIG. 4 shows J.sub.SC as a function of incident angle for varying etch times. A slight increase of approximately 5% was evident for the un-etched sample for all anglespotentially resulting from the extra anneal or from scattering caused by the intrinsic texture of the un-etched ZnO surface. The etched samples each exhibited a J.sub.SC increase of approximately 10% at normal incidence that further increased with incident angle. The J.sub.SC of the 20 s sample increased mostby 14% at 30 and 25% at 60.

[0024] As a result of the increase in J.sub.SC of the 20 s sample, its performance improved the most of all devices. Table 1 shows PV parameters for this sample. While the efficiency, increased only slightly, from 10.4% to 10.5% at normal incidence. It increased more significantly for non-normal incidence with a relative improvement of approximately 18% for 60 illumination. This is consistent with decreased surface reflection.

TABLE-US-00001 TABLE 1 Device Results for 20 s Etched Sample Condition V.sub.OC (mV) J.sub.SC (mA/cm.sup.2) FF (%) (%) 0 Before 520.3 30.9 64.6 10.4 0 After 514.7 33.8 60.1 10.5 30 Before 517.9 25.5 65.7 8.7 30 After 512.8 29.0 60.8 9.0 60 Before 503.5 13.3 66.7 4.5 60 After 503.5 16.5 64.0 5.3

[0025] It is significant to note that the increase in J.sub.SC at 60 is greater than the 17% Fresnel reflection expected at this angle. Thus, the AR properties of the ARSS alone are insufficient to explain the increased current. It is clear that the scattering properties of the coating, resulting in absorption closer to the junction, are necessary to fully explain the increase in J.sub.SC at large angles.

[0026] The PV absorber could be a different thin film PV absorber, such as CuInSe.sub.2 (CIS), CuGaSe.sub.2 (CGS), Cu.sub.2ZnSn(S,Se).sub.4 (CZTS), CdTe, amorphous Si, or organics.

[0027] A mask could be deposited on the ZnO prior to etching in order to affect the layer's post-etching morphology.

[0028] Deposition parameters for the ZnO film such as substrate temperature, partial pressure, and deposition power could be adjusted in order to affect the layer's post-etching morphology.

[0029] The oxygen content of the ZnO film could be varied by adjusting target composition or O.sub.2 flow during deposition in order to affect the layer's post-etching morphology.

[0030] The ZnO film could doped with an agent that affects grain formationresulting in changes in grain size, shape or orientationin order to affect the layer's post-etching morphology.

[0031] The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles a, an, the, or said, is not to be construed as limiting the element to the singular.