Film Solar Cells
4.3.2 Ethanol-enriched bilayers
For the bilayer approach discussed in the following a 1900 nm thick ZnO bulk layer was deposited on a glass substrate before the addition of 20 sccm of ethanol during the deposition process introduces the cap-layer deposition. In Figure 4.8a a sketch of the bilayer stack is shown (bottom). The impact of the cap-layer to the surface morphology will be discussed later. The results in Figure 4.9 present measured data of
sheet resistance values gained from samples with cap-layer thicknesses varying between 0 and about 700 nm. It is interesting to note, that with increasing cap-layer thickness, no clear trend can be observed. In an approximation we conclude the sheet resistance of all measured samples is equal and about 36 Ω/sq, in average. Such behaviour can be expected although the ethanol-enriched layer possesses much higher resistivity, see Figure 4.4. For better understanding, a simplified equivalent circuit of resistors is shown in Figure 4.8b, at the top. Since we measured with a collinear four point probe configuration the sketched circuit between the probes S1 and S2 it can be assumed to be the same between S3 and S4.
Figure 4.8: a) Sketch and RMS surface roughness of ZnO layers grown without ethanol (top), with 10 sccm of ethanol (center), and of a bilayer con-sisting of a standard 1900 nm thick ZnO layer capped by a 200 nm thick ZnO film deposited with 20 sccm of ethanol (bottom). Layers have a equivalent thickness of 2 µm. b) shows a simplified equivalent circuit diagram of an ethanol-enriched bilayer stack to evaluate the sheet resistance results presented in Figure 4.9. Rc1 and Rc2 are assigned to the contact resistances at the interfaces. R2 is assigned to resistance current has to pass through the cap-layer in order to enter the ZnO bulk material. REthanol and RBulk are assigned to the volume resistance of cap-layer and ZnO bulk material.
The full simplified circuit, representing the measurement configuration, is shown in Figure 4.8b, at the bottom. Rc1 and Rc2 are assigned to the contact resistances between measurement probes on top of the surface and ethanol-enriched ZnO cap-layer and between cap-cap-layer and bulk ZnO material, respectively. R2 is assigned to the resistance the current has to run through in order to enter the ZnO bulk material.
REthanol and RBulk are assigned to the volume resistance of cap-layer and ZnO bulk material, respectively. Current is introduced into the circuit on the left hand side through S1 and is leaving through S2. According to the current a voltage will drop across Rc1, Rc2 and R2, which should increase linearly with thickness of the
ethanol-enriched cap-layer. From Figure 4.4 we know that the resistivity of the ethanol-ethanol-enriched layer, determining in first approximation R2 and especially REthanol, is about 4 to 5 orders of magnitude higher compared to the one of the ZnO bulk material. Since REthanol and RBulk are interconnected parallel, the lower RBulk will determine the overall resistance of the parallel circuit. The voltage needed to derive bilayer’s effective sheet resistance is measured between S3 and S4, with the circuit on the right hand side.
Hence, the current passing the circuit is very small, not least because of the typical high internal resistance of the voltmeter, connected to S3 and S4. As a result the voltage drop across Rc1, Rc2 and R2 can be neglected and the voltage measured is equal to the voltage drop across RBulk. The setup is not sensitive to R2, which is the explanation of the observed results in Figure 4.9. Even if R2 rises due to increasing thickness of the ethanol-enriched cap-layer the determined sheet resistances will remain constant.
Nevertheless, R2 of course might be important for future applications. When apply-ing ethanol-enriched bilayers as front electrode for thin film solar cells, R2 contributes to cell series resistance and lower the fill factor. This can be translated into losses in conversion efficiency. One would have to make a trade-off between possible beneficial effects of the adjusted surface morphology and an increased series resistance. But note, the bilayers discussed in the present work were deposited without the addition of an extrinsic doping source. For ZnO deposited with a LPCVD process, boron in form of diborane (B2H6) is typically used as extrinsic doping source. With boron the free electron concentration in the ZnO can be adjusted in the range of 1019 to 10201/cm3, which could solve the mentioned resistivity issues.
- 1 0 005 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0
1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5
Rsheet [ohm]
c a p - l a y e r t h i c k n e s s [ n m ]
Figure 4.9: Sheet resistance of ethanol-enriched bilayers as a function of cap-layer thickness. The ZnO bulk cap-layer was deposited without ethanol whereas for the cap-layer, in addition to the main precursors, 20 sccm of ethanol were used. The ZnO bulk layer had a thickness of 1900 nm.
The influence of the cap-layer on the surface morphology is sketched in Figure 4.8a (bottom) and exactly shown in Figure 4.10b. Even for bilayers the smoothening effect
of adding ethanol during deposition is observed. Figure 4.8a shows that the RMS surface roughness is slightly reduced from 76 to 72 nm for a 1900 nm thick standard ZnO layer capped by a 200 nm thick ZnO film enriched with ethanol during deposition.
Note that Figure 4.6, column A, shows that, with increasing cap-layer thickness, the haze is reduced from nearly 47 % for a 200 nm thick cap layer to 26 % for a 500 nm thick cap-layer.
Figure 4.10: SEM images of a) the surface of a standard 2 µm thick as-grown rough ZnO film and b) a bilayer consisting of a 1900 nm thick stan-dard ZnO film capped by a 200 nm thick ZnO layer deposited with ethanol.
4.4 Conclusion
The present work tested the suitability of adding ethanol during ZnO:B deposition to create a surface morphology that is more favorable for µc-Si:H growth. A strong impact on the electrical and optical properties of 2 µm thick ZnO layers deposited with ethanol as an additional precursor was found. It is proposed a growth mechanism in which oxygen incorporation on oxygen vacancies is mediated by ethanol. Although ethanol-enriched ZnO single layers might be suitable for good µc-Si:H growth, their high resistivity rules out their usage as electrodes for photovoltaic applications. In a two-step deposition process, a 200 nm thick ZnO cap layer on top of a 1900 nm thick as-grown rough ZnO film was deposited. A change in surface morphology was observed for these bilayers as well.
Furthermore, the sheet resistance of bilayer films was maintained at reasonable val-ues, independent of the thickness of the ethanol-enriched cap-layer. It was found that the used four point probe measurement was not sensitive to the resistive cap-layer. To solve resistivity issues, which might become a problem for the potential application of bilayers, additional doping of the underlying and cap layers still has to be studied.
Future work will have to be done to investigate the effects of parameters like pressure and temperature on morphology. Also, other types of precursors will be evaluated.
Finally, a complete solar cell, incorporating an optimized bilayer as the front electrode, will have to be tested. Work has been carried out by Ding et al. incorporating the
above presented approach into a micromorph solar cell [105]. Cell efficiencies above 12.2 % were achieved.