High-efficient n-i-p thin-film silicon solar cells

Abstract

In this thesis we present results of the development of n-i-p thin-film silicon solar cells on randomly textured substrates, aiming for highly efficient micromorph solar cells (i.e., solar cells based on a ?c-Si:H bottom cell and a-Si:H top cell). For the efficiency of n-i-p thin-film silicon solar cells the interfaces between different layers are very important. In this thesis the influence of some important interfaces in the n-i-p configuration solar cells on the solar-cell performance has been studied. The results are presented in a structure that follows the sequence of making an n-i-p solar cell, starting with the textured substrate (Chapter 3), then the development of the ?c-Si:H bottom cell (Chapter 4), the optimization of a-Si:H top cell (Chapter 5), and finally the characterization of the i-p interface of the top cell (Chapter 6). In Chapter 3 the development of textured ZnO:Al (AZO) is presented. Obtaining textured transparent conductive oxide (TCO) using sputtering of AZO layers and subsequent wet etching has already been published in literature and is used as a reference in this thesis. The relation between the etched AZO surface morphology, and the sputtering and wet etching conditions are discussed. The surface of wet-etched AZO films is V-shaped. A high root mean square roughness (?RMS) is achieved with this process and the textured AZO presents high haze in reflection with most of the reflected light located in high angles. Therefore, the n-i-p ?c-Si:H solar cells deposited on this type of substrate have a very high short-circuit current density (Jsc). However, a low open-circuit voltage (Voc) is obtained, because of the relatively high density of defective regions within the ?c-Si:H layers. These defective regions primarily appear in the valleys of the rough V-shaped textured surface. In Chapter 3 we also present a novel way of glass texturing using AZO as sacrificial layer, which we have called the AZO induced texturing process (ZIT process). For this process a AZO layer is deposited onto the glass substrate by sputtering. Then this AZO covered glass is exposed to an etching solution, which contains HF and HNO3. Due to the anisotropic structure of the AZO film the etching of this layer is not homogeneous. Where the AZO etches faster, the glass underneath is exposed to HF sooner. Once some parts of the glass are exposed to the etching solution, the HF starts to etch the glass. When the AZO layer is etched away, the glass texturing process finishes and a rough glass surface is obtained. Therefore, the crucial parameters for this process are: AZO layer thickness and its sputtering temperature, and the composition of the etching solution (in particular the HNO3 to HF concentration ratio). With this process, the ?RMS of the textured glass surface can be tuned between 20 nm and 500 nm. The textured glass surface is U-shaped, on top of which a layer of ?c-Si:H can be deposited with a low density of defective regions as is shown in Chapter 4. In Chapter 3 we also present the development of modulated surface textures (MST) consisting of both large (>10 ?m) and smaller features. The purpose of MST is to obtain a high Jsc, whilst maintaining a high Voc and FF for the ?c-Si:H solar cells. For the MST substrates the large features (> 10 ?m) on the glass were obtained using the so-called ITO induced texturing (IIT) process. Smaller features on top of this IIT structure were obtained by aluminium induced texturing (AIT) process, the ZIT process, or by wet-etching of sputtered AZO. Compared to single textures all the MST substrates have even higher haze in reflection, with a larger fraction of light reflected to higher angles. In Chapter 4 we show results of ?c-Si:H solar cells deposited on the textured substrates developed in Chapter 3. For a 1.2-?m thick ?c-Si:H n-i-p solar cell deposited on the ‘reference’ back reflector (BR) of wet-etched AZO covered with Ag a high short-circuit current density of 24.4 mA/cm2 was obtained. When varying the morphology of the substrates, the Voc and FF of solar cells were not affected for thinner cells, but a rapid drop in Voc and FF was found for thicker intrinsic layers. Using textured glass as substrate obtained with the ZIT process and covered with a Ag layer, high efficiency n-i-p ?c-Si:H solar cells were obtained. We studied the dependence of the n-i-p ?c-Si:H solar-cell performance on the Rs value (defined as Rs = ?RMS/lc, with lc the correlation length) of the textured glass substrates. We found that with increasing Rs the Jsc of the cells increases. The Voc for thin cells decreases, whereas for 3-?m thick solar cells the effect of Rs on the Voc is less obvious. The best-performing solar cell deposited on ZIT textured glass showed high Voc and FF in comparison to cells reported in literature and deposited on state-of-the-art textures, but this gain in electrical performance was counterbalanced by a slightly lower Jsc. This resulted in a conversion efficiency of 10.64% (active area, one best cell, 3.0 ?m thick i-layer) with Voc = 0.533 V, FF = 0.727 and Jsc = 27.47 mA/cm2. Finally, ?c-Si:H n-i-p solar cells were deposited on MST substrates. For these solar cells we found a higher red response in the external quantum efficiency than the reference cell that was fabricated on IIT glass. However, compared to the best ZIT cell this MST cell shows a relatively low red response leading to a Jsc of 26.36 mA/cm2. We think that this lower red response is due to the flattening of the rough surface of the glass when preparing the MST substrate with ZIT process (MST-ZIT). On the other hand, we found from the scanning electron microscopy images that the solar cell deposited on a MST-ZIT process has a very low density of defective regions in the bulk ?c-Si:H layer and gives the highest FF of 0.748 for a 3-?m thick cell. In addition, the Voc remains nearly the same as for the best ZIT cell. These lead to a cell efficiency of 10.49%, which is slightly lower than the efficiency of the best ZIT cell. For a-Si:H solar cells, the recombination rate is highest in the i-p interface region due to the high defect density in this region. As a result the solar-cell performance is suppressed. In Chapter 5 we show results of the effect of H2-plasma treatment before p-layer deposition on the solar-cell performance of n-i-p a-Si:H solar cells. Following this treatment all external solar-cell parameters (i.e., the Voc, FF and Jsc) increase significantly. Dark current-voltage (J-V) characteristics of the solar cells suggest that the current recombination at the i-p interface is lower for H2-plasma treated cells than for a cell without treatment. The FTIR measurement shows that the a-Si:H layer contains a higher hydrogen content after H2-plasma treatment. This a-Si:H layer with higher hydrogen content is around 20 nm thick. Compared to the bulk a-Si:H material this layer has a higher activation energy and lower conductivity. In order to understand the mechanism of the solar-cell performance improvement by the H2-plasma treatment of the i-p interface, we carried out capacitance-frequency (C-f) measurements on n-i-p a-Si:H solar cells. These measurements are presented and discussed in Chapter 6. By applying and further developing the theory of Walter et al.1 on C-f measurements, we have been able to study the energy defect distribution of the i-p interface region. We found that during the capacitance measurement, virtually only the defects near the n-i and i-p interface region contribute to the measured capacitance when a forward bias is applied. When extracting the defect density of states in the i-p interface region from C-f measurements, we find that after a H2-plasma treatment this defect density is decreased and peaks at a deeper energy position. Combining the dark J-V results of these solar cells, the C-f measurements indicate that the lower recombination rate in the i-p region after the H2-plasma treatment is mainly due to the decrease of the defect density in the i-p interface region. Finally, in Chapter 7 of this thesis we present the results of an n-i-p micromorph solar cell made using the results of the development presented in the earlier chapters. This micromorph solar cell has an efficiency of 12.2%. The open-circuit voltage is 1.382 V, the fill factor is 0.72, and the short-circuit current density for the top a-Si:H cell is 12.2 mA/cm2 and 13.1 mA/cm2 for the bottom ?c-Si:H cell. The main conclusions of this thesis and a general outlook for further enhance the efficiency of thin-film silicon solar cell are also presented in Chapter 7.