ENHANCEMENT OF OPTICAL ABSORPTION IN A-SI ...

28th European Photovoltaic Solar Energy Conference and Exhibition

ENHANCEMENT OF OPTICAL ABSORPTION IN A-SI:H FILMS BY SILVER NANOPARTICLE PLASMONIC INTERFACE Zaki M. Saleh1,2,*, Hisham Nasser1, Mete Gunoven1, Engin Ozkol1, Burju Altuntas1, Alpan Bek1, Rasit Turan1 Center for Solar Energy Research and Applications (GÜNAM), Middle East Technical University, Ankara, Turkey 2 Department of Physics, Arab American University-Jenin, Jenin, West Bank, Palestine

1

ABSTRACT: Plasmonic interfaces consisting of metal nanoparticles placed at the interface between two dielectric media can be designed to enhance absorption in the absorber layer and improve efficiency of PV devices. Several single and double plasmonic interfaces consisting of silver nanoparticles separated by dielectric spacer layers and placed at the interfaces of media with different thicknesses and dielectric constants have been integrated to hydrogenated amorphous silicon (a-Si:H) films to investigate light-scattering dependence on particle size, thickness and refractive index of the dielectric media. Even though the photocurrent under white light decreases due to damping losses, changes in the extinction spectra caused by the plasmonic interface suggest that light is preferentially scattered into the a-Si side of the interface due to the higher index of refraction (n = 4) for aSi, compared to the other materials used at the other side of the interface. The plasmonic interface design, broadening of the extinction spectra of the integrated structure and their impact on the photocurrent and consequently on the cell efficiency are discussed. Keywords: Silver nanoparticles, plasmonic interfaces, photocurrent, solar cells.

1. INTRODUCTION The photovoltaic industry has made great achievements in recent years but the progress in thin film solar cells has fallen below expectations. In addition to aobstacles involving atmospheric aging and light-induced metastabilities, the thickness of the absorber layer in these devices is not sufficient for optimal absorption of the solar spectrum. Textured surfaces, successfully used to reduce reflection and increase light trapping do not work well with thin film devices becasue the texture size is often larger than the film thickness [1]. Furthermore, low-energy photons pass through the cell structure essentially uninterrupted. The so-called up-conversion process in which more than one low-energy photons are converted into a higher energy one that can be utilized, has been tested but conclusive results have not been demonstrated.A Plasmonic interfaces consisting of metal nanoparticles placed at the interfaces between dielectric media of different thicknesses and dielectric constants have promising potential for enhancing optical trapping by the interface and preferential scattering of light into the absorber layer. This effect can best be detected by the enhancement of photocurrent in the absorber layer which is taken to indicate improvement of the efficiency in photovoltaic devices. Surface plasmon resonance (SPR) is the collective frequency of oscillating frequency of all electrons at the surfaces of metallic particles. Plasmonic resonance is characterized by a minimum in the transmittance spectrum or by a maximum in the optical extinction spectrum defined as the total number of photons absorbed or scattered by the interface. Plasmonic interfaces can be engineered to increase the electric field in the vicinity of the absorber layer (near field effect), or to preferentially scatter light into the absorber layer (farfield effect) to enhance the photocurrent [1]. For photovoltaic applications, we are interested in enhanced scattering and suppressed absorption by the plasmonic interface. Scattered light can be designed to be preferentially scattered into the absorber layer by selecting media with higher dielectric constant at the side of the absorber layer. Experimentally, extinction is calculated as 1 – T – R, where T is the transmittance and R is the reflectance [2].

Previous studies show that plasmonic interfaces integrated to the front interface of the absorber layer increase absorption losses [3]. Other studies predict that embedding interfaces within the absorber layer increases defects which act as recombination centers and reduce photocurrent [4]. It is therefore concluded that plasmonic interfaces are best placed at the rear of the absorber layer [5]. In this work, five sets of plasmonic interfaces consisting of Ag nanoparticles and various dielectric media are integrated to the back surface of device quality amorphous silicon (a-Si:H) films. Optical spectra are measured and correlated to changes in the spectral dependence photocurrent. Enhancement of photocurrent at any portion of the spectrum is taken as an indication of enhanced light trapping and is expected to cause improved photovoltaic efficiency in photovoltaic devices.

2. SAMPLES AND EXPERIMENT Single and double layered plasmonic interfaces were fabricated by the self-assembled dewetting technique in which a thin silver film of approximately 15 nm thickness is sputtered on the desired substrate and annealed at temperatures in the range of 200 C – 500 C in N2 environment for 60 minutes. This process leads to the formation of well-separated and nearly-spherical nanoparticles 50 – 100 nm in diameter. Single-layered interface were fabricated by sputtering 15-nm silver films on Corning glass substrate and dewetting them at 200 C – 500 C for 1 hour in N2 environment [6]. This process results in plasmonic interfaces with nanoparticles in the range of 50 – 100 nm in diameter. Double-layered interfaces were fabricated by repeating the above procedure at 400 C to form the lower layer with larger nanoparticle. A spacer layer of SiO2 or Si3N4 dielectric, 10 – 20 nm in thickness, is deposited on top and dewetted at lower 200 C to form the upper layer of smaller nanoparticles. The upper layer must be dewetted at temperatures below the lower layer so as not to alter the size of the nanoparticles in the lower layer. Finally, another spacer layer is deposited on top to electrically separate the interface from the subsequent amorphous silicon (a-Si:H) film used for optical and

371

28th European Photovoltaic Solar Energy Conference and Exhibition

electrical measurements. The optical spectra and SEM images are measured at selected fabrication stages to characterize the structural and optical properties of the interfaces. After the last layer of 250 nm a-Si:H film is deposited, the spectral dependence of photocurrent is measured using standard coplanar geometry at a bias of 10 volt. The sample is illuminated from the a-Si:H side by a halogen lamp through a monochromator and the wavelength is changed by 20 nm increments. Five plasmonic interfaces were fabricated using different nanoparticle size and media with different thicknesses and refractive indices to study their effects on the optical trapping and enhancements in photocurrent. These sets are described below where the thicknesses of Ag film is kept constant at 15 nm while that for a-Si:H is kept at 250 nm. The temperature indicated represents the dewetting temperature:

The surface plasmonic resonance (SPR) as characterized by the maximum in the Extinction spectra redshifts from 450 nm to 480 nm due to the increase in TD from 200 C to 400 C. On the other hand, SPR redshifts by 60 nm to 510 nm and broadens appreciably the into the red due to the addition of 20 nm SiO2 spacer layer. Fig. 2 shows the optical spectra (Transmittance, Reflectance and Extinction) of single plasmonic interface (PLS-B) dewetted at 400 C on Corning glass before subsequent layers are deposited. The SEM image of this structure is shown in Fig. 1(b). The spectra have well defined features compared to silver dewetted on other surfaces [6]. The spacer layer appears to broaden the Extinction peak into the red indicating more absorption and/or scattering by the interface in that spectral region. The interface integration to a-Si:H and presumably to a PV device can be chosen to prefer scattering over absorption in which case more light is scattered into the absorber layer.

Table I: Sample Sets PLS-A

Ag/200C

PLS-B

Ag/200C/20 nm SiO2/a-Si

PLS-C

Ag/400C/20 nm SiO2/a-Si

PLS-D

Ag/400C/20-nm SiO2/Ag/200C/10 nm SiO2 /a-Si

PLS-E

Ag/400C/10-nm SiO2/Ag/200C

3. RESULTS In Fig. 1, we show the Extinction spectra and SEM images of single layered plasmonic interfaces dewetted at 200 C and 400 C along with another interface dewetted at 200 C with a 20 nm SiO2 spacer layer on top. The SEM images demonstrate that the dewetting of a 15-nm sputtered Ag film on glass is successful in forming wellseparated and nearly spherical nanoparticles, although they exhibit broad size distribution. The mean size increases and becomes more spherical with increasing dewetting temperature (TD) from 200 C to 500 C. Above 500 C, the substrates begins to deform and interfere in the plasmonic properties.

Fig. 2: Optical spectra of single plasmonic interface (PLS-B) dewetted at 400 C on glass and covered by a 20-nm SiO2 spacer layer. Fig. 3 shows an SEM image of double plasmonic interface (PLS-D) consisting of Ag film dewetted at 400 C (lower layer) and at 200 C (upper layer) separated by a 20 nm spacer layer of SiO2. The image shows clear contrast between the upper and lower nanoparticles although size contrast between the two layers is not easy to capture. The structure with the SiO2 spacer appears to exhibit better contrast compared to other spacer layers such as Si3N4 (not shown).

PLS-A

1.0 m

PLS-B

(a) PLS-D

1.0 m

PLS-C

1.0 m

500 nm

Fig. 1: Extinction spectra and SEM images of single plasmonic interfaces before the a-Si:H layer: PLS-A (solid black line, 200 C anneal), PLS-B (dashed blue line, 400 C anneal), and PLS-C (dotted red line, 200 C anneal with 20-nm SiO2 spacer).

Fig. 3: SEM image of double plasmonic interface consisting of Ag nanoparticles dewetted at 400 C (lower layer) and an upper layer dewetted at 200 C separated by a 20-nm spacer layer of SiO2.

372

28th European Photovoltaic Solar Energy Conference and Exhibition

photocurrent in the integrated absorber layer. Work on this set was not continued because heavy parasitic losses were expected.

The double layer structure is clearly visible in the SEM image but the nanoparticles appearing as if they were suspended in air, may be misleading for particle counting. From our dewetting experiments on glass and conservation of mass arguments, we expect the nanoparticle density in the upper layer to be higher than what appears in the image. The reason for this is that nanoparticles from the second layer falling between nanoparticles in the lower layer do not give the same contrast as the ones standing on top of another nanoparticles. Nonetheless, this does not represent a major issue because the optical spectra obtained for these structures are distinctly different from those obtained for a single interface. Fig. 4 shows the optical spectra for the double plasmonic interface separated by a 20-nm SiO2 spacer layer (PLS-D) before the final spacer and a-Si:H layers are deposited. The optical extinction exhibit a broad peak that appears to consist of two overlapping components representing the two physically separated, but electrically and optically interacting layers. The position of the surface plasmon resonance is not expected to fall at the same position of the identical single layers because the two layers are optically interactive.

Fig. 5: Optical spectra of “DOUBLE” plasmonic interface (PLS-E) dewetted at 400 C (lower) and 200 C (upper) with a 10-nm SiO2 spacer layer. The spacer layer appears to be discontinuous causing the two layers to be connected. Next we consider the optical spectra for single and double interfaces integrated to a 200-nm device-quality amorphous silicon (a-Si:H) film to represent the absorber layer in a PV device. In Fig. 7 we show the Extinction spectra of a 250-nm thick a-Si:H film deposited on glass as the reference and on a single interface dewetted at 400 C (PLS-B). The interface clearly broadens the extinction spectrum of a-Si:H into the red. The wiggles in both spectra are due to interference effects between the incident light and light reflected off the a-Si:H back surface.

Fig. 4: Optical spectra of DOUBLE plasmonic interface (PLS-D) dewetted at 400 C and 200 C for the lower and upper Ag layers, respectively, on corning glass with a 20-nm SiO2 spacer layer in between, before the final spacer layer and a-Si:H are deposited. The thickness of the spacer layer separating the two plasmonic layers is critical in determining the optical and electrical properties of the double interface. In Fig. 5, we show the optical spectra for PLS-E which is a double plasmonic interface separated by a 10-nm SiO2 spacer layer compared to 20 nm for PLS-D. Reducing the spacer layer thickness to 10 nm reduces the extinction spectrum to one narrow peak centered at 370 nm, which is even lower than that of the single interface dewetted at 200 C shown in Fig. 1. This result suggests that the 10-nm spacer is probably discontinuous causing the two nanoparticle layers to physically connect and form one interface of connected and therefore larger nanoparticles with arbitrary shapes. Larger nanoparticles are known to shift the transmittance minima into the blue (Fig. 1). Such interfaces with arbitrary shapes lead to high absorption and low scattering cross-sections and therefore a reduced

Fig. 6: Optical Extinction versus wavelength of aSi:H deposited on: Corning glass (black solid), single plasmonic interface PLS-B (dashed blue). The broadening of the Extinction peak indicates increased absorption and/or scattering by the interface and does not automatically indicate higher photocurrent in that spectral region. Enhancement in photocurrent is expected only if scattering dominates the Extinction process. In Fig. 7 we plot the photocurrent and Extinction versus wavelength of incident light for a-Si:H codeposited on single interface PLS-B, and on glass as the

373

28th European Photovoltaic Solar Energy Conference and Exhibition

reference

reduced photocurrent, or increased scattering directing more light into the absorber layer and therefore enhanced photocurrent. The significant reduction in photocurrent across most of the visible region cannot be explained in terms of parasitic losses alone as was reported previously. We believe that the near field effect should be considered. The nanoparticles’ actual size for most interfaces used in this study is less than 100 nm which contributes to parasitic loss for increasing absorption and reducing scattering because the relative ratio of absorption to scattering cross-sections increases with decreasing actual particle size [7]. It is important to recall that the effective absorption and scattering cross-sections are significantly higher than their actual size and may in fact dominate the entire aSi:H layer thickness in our case. This means that the nanoparticles may act as defects in the absorber layer that limit the photocurrent. At 20-nm spacer layer thickness between the plasmonic interface and the absorber layer, Catchpole and Polman calculate the scattering cross section to be only 10 % of its value at infinity, while the absorption cross-section is significantly higher [7]. Since our spacer layer is about 20 nm, it is not surprising that most of the light is absorbed by the interface and little enhancement in photocurrent is observed. We therefore conclude that an optimization of spacer layer thickness is critically needed to measure real enhancement in photocurrent. The much lower photocurrent detected in a-Si:H with double layer interface (not reported here), suggests that the broadening of the Extinction peak observed for both SiO2 and Si3N4 spacers is most likely due to absorption rather than scattering. To obtain a real enhancement in photocurrent, it is best to focus on SiO2 spacers and increase the distance between the absorber layer and the interface to over 80 nm to increase the scattering cross-section and detect photocurrent enhancement.

Fig. 7: Spectral photocurrent (left axis) and optical Extinction (right axis) for a-Si:H co-deposited on single plasmonic interface PLS-B (Blue lines) and on Corning glass (Black lines). In both cases the film is illuminated from the a-Si:H side by halogen light through a monochromater in 20-nm wavelength increments. The measured photocurrents are normalized to unity at their maximum values. As indicated in Fig. 7, a relative though appreciable increase in photocurrent appears at the lower energy edge of the band gap (600 ~ 700 nm) and correlates well with the broadening of the extinction spectrum into that spectral region. For the double layers, plasmonic interface PLS-D with the SiO2 spacers between the two layers and below the a-Si:H did not make it for the last stage to measure the optical properties before Al electrodes were deposited and therefore a similar comparison was not possible in this study. The photocurrent for another double plasmonic interface using Si3N4 spacers, is dramatically reduced and correlating it to the extinction for a-Si:H codeposited on glass was not meaningful. Internal results suggest that Si3N4 exhibits significantly more parasitic losses compared to SiO2.

5. REFERENCES 1. S. Pillai, M. A. Green (2010), Plasmonics for photovoltaic applications. Solar Energy Materials & Solar Cells 94: 1481–1486 2. T.L. Temple, G.D.K. Mahanama, H.S. Reehal, D.M. Bagnall (2009), Solar Energy Materials & Solar Cells 93: 1978–1985 3. C. Ho, D. Yeh, V. Su, C. Yang, P. Yang, M. Pu, C. Kuan, I. Cheng, S. Lee (2012), J. Appl. Phys. 112: 023113 4. R Santbergen, R Liang, M Zeman (2010), Proc. 35th IEEE: 748. 5. C. Eminian, F.J. Haug, O. Cubero, X. Niquille, C. Ballif (2011), Progr. Photovolt. 19: 260. 6. H. Nasser, Z.M. Saleh, E. Özkol, M. Günoven, A. Bek, R. Turan (2013), Plasmonics 8: 1485 – 1492 7. K.R. Catchpole, A. Polman (2008), Appl. Phys. Lett. 93: 191113

4. DISCUSSION As shown in the SEM images of Fig. 1 and Fig. 4, single and double layered plasmonic interfaces have been fabricated successfully on glass substrates and integrated to a-Si:H to study their effect on light trapping through changes in the optical spectra and photocurrent in the aSi:H film. The single layer interfaces exhibit well-defined minima in Transmittance or single well-defined maxima in the extinction spectra that is red-shifted with increasing dewettng temparature and broadens into the red when SiO2 sacer layer is added. The double layered interfaces on the other hand exhibit broader peaks which appear to consist of two overlapping components representing the two optically-interactive layers. In both cases, these interfaces cause significant broadening to the extinction spectrum of a-Si:H that is skewed into the red. The extinction maxima of a-Si:H deposited on two of the interfaces show a clear broadening into the red, compared with a-Si:H on glass, indicating significant increase in scattering and/or absorption by the interface. Higher Extinction can indicate increased absorption meaning lower scattering into the absorber layer and a

374