Passivation ability of graphene oxide demonstrated by two-different-metal solar cells
© Hsu et al.; licensee Springer. 2014
Received: 27 June 2014
Accepted: 10 December 2014
Published: 23 December 2014
The study on graphene oxide (GO) grows rapidly in recent years. We find that graphene oxide could act as the passivation material in photovoltaic applications. Graphene oxide has been applied on Si two-different-metal solar cells. The suitable introduction of graphene oxide could result in obvious enhancement on the efficiency. The simple chemical process to deposit graphene oxide makes low thermal budget, large-area deposition, and fast production of surface passivation possible. The different procedures to incorporate graphene oxide in Si two-different-metal solar cells are compared, and 21% enhancement on the efficiency is possible with a suitable deposition method.
Energy from solar cells has been thought as the possible alternative to the traditional fuel energy. In order to compete with the traditional energy, increase on the efficiency of solar cells in a cost-effective way is important. For a solar module with an efficiency of 20%, 1% improvement on efficiency can correspond to 5% reduction in cost. Surface structures[1–3] and passivation[4–7] can be utilized to improve the efficiency. Passivation of bare Si surfaces can be easily achieved with hydrogen termination, alkylation, and so on, but the effect may deteriorate in a certain time. Passivation by dielectric films, such as SiO2, SiNx, and Al2O3 could overcome the stability issue. The high-quality SiO2 is common oxide for surface passivation of Si solar cells. Al2O3 prepared by atomic layer deposition is also used due to its promising ability of passivation for Si, especially for the p-type Si. Since various oxide materials have been used for passivation of solar cells, we would like to investigate the effect of graphene oxide (GO) as the passivation layer. GO is broadly studied after the developing of graphene in recent years[9–11]. The above mentioned oxide passivation films are almost demonstrated in chambers, and GO deposited in chemical solution may be a much simpler method. For the photovoltaic applications, GO has been adopted in organic solar cells as the hole transport layer. We will apply GO to Si solar cells with the purpose of surface passivation. The different procedures to incorporate GO in Si two-different-metal solar cells are compared. To the best of our knowledge, GO has not been utilized on the applications of solar cell passivation. The chemical solution method makes the low thermal budget, large-area deposition, and fast production possible.
Results and discussion
The passivation effect of GO is supposed due to the field effect passivation contributed by the negative fixed charge in GO as verified in ref.. In ref., the capacitance verse voltage relation of GO and control metal-insulator-semiconductor (MIS) capacitors were compared. The curves of MIS capacitors with GO coated shifted to the positive-bias direction, and it meant that extra negative charge existed in GO. Such a dielectric GO film with negative fixed charge could be used to passivate solar cells, especially for the p-side. For our two-different-metal solar cells, GO can be coated on the rear side of the p-type Si substrate. Without GO being coated, many of the photo-generated electron-hole pairs may easily recombine at the rear surface due to the termination of the periodic Si structure. With GO coated, minority carriers (electrons) can be repelled from the surface. Since recombination should only occur between an electron and a hole, the repulsion of electron from the surface could contribute to the decrease of recombination. More electrons can be collected by the Al electrode successfully, and hence, more holes can be collected by the Au electrode. That is why SiGb1 and SiGb2 show the better performance as compared with the ConSi.
Silicon nitride (SiNx) is a common passivation film for solar cells. We have also prepared a two-different-metal solar cell with SiNx on the rear side for comparison. First, we demonstrated another control two-different-metal solar cell, ConSi2, and its IV characteristic under AM 1.5 G illumination was obtained as shown in the inset of Figure 2. Then, the native oxide on the rear side of ConSi2 was removed by buffered oxide etch (BOE). SiNx was subsequently deposited on the rear side by the sputter. Its IV characteristic is also shown in the inset of Figure 2 as the curve of ‘ConSi2 with SiNx’. It can be found that the performance of ‘ConSi2 with SiNx’ is even worse than ConSi2. One reason for the degradation may be due to the un-optimized facility for passivation. The sputter SiNx might have poor quality as compared with the commercial PECVD SiNx. The other reason is that SiNx with positive fixed charge is more suitable for passivation of n-Si substrates instead of p-Si substrates in our case.
Because the best GO cell, SiGb1, has been immersed in the GO suspension for 40 min, it may be suspected that the performance enhancement is due to the more oxidation in water (in GO suspension) but not GO deposition. We prepared two extra control samples. One was immersed in DI water, and the other was rear-side down floating on the water surface of GO suspension to have the similar immersion condition but avoid GO deposition. These two control samples after immersion did not show better cell performance than the results before immersion (not shown here), indicating that the improvement was indeed only due to the GO passivation on the surface.
GO is first-time proven to have the ability to enhance the performance of a solar cell by surface passivation due to its negative fixed charge. GO provides the potential on low-cost and large-area passivation. In the current stage, the simple two-different-metal structure is adopted as the beginning. Further optimization on deposition conditions and light transmission is deserved. More efforts should be made to incorporate the benefit of GO in commercial Si pn solar cells.
This work is supported by National Science Council of R.O.C. under contract no. NSC 101-2221-E-259-023-MY3.
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