By knowing the desired properties of the optimum buffer layer material in CIGSe super-strate devices, it would be ideal to predict the perfect material by quantum mechanical computations [163]. However, the time frame of this work did not allow this approach.
An alternative option is to start with a good guess for a suitable material, like Ga2O3, and tweak its properties to these desired by alloying with other materials. In Sec. 8 amor-phous Ga2O3 was identified as a base material, which will be alloyed with other elements whose conduction band is made of s orbitals, like Zn, Sn or In. The spherical symmetry of these orbitals is beneficial, as it makes the delocalized electronic transport less sensitive to structural disorder compared to p or dorbitals [164].
A very useful strategy to do this sufficiently fast is to produce graded samples, which allow fast screening of the fabricated alloys. This can be achieved with combinatorial pulsed laser deposition (PLD, see Sec. 2.2), in which the co-deposition of different
materi-als, from spatially separated sources, allows to create very defined compositional gradients onto the substrate.
Experiment
Based on the considerations given in the beginning of this section, the base material was chosen to be Ga2O3, which is alloyed with TiO2, ZnO, SnO2, In2O3 and ZnSnO3 to change the electron affinity and charge carrier density. The alloys were deposited on a 5 cm x 5 cm large alkali-free glass substrate covered with GZO. Generally the layers were deposited with two gradients, the overall layer thickness in X-direction and the Ga2O3 content in Y-direction. The deposition temperature was set to 100 to achieve amorphous layers, and set to 500to achieve crystalline films. The oxygen partial pressure was set to 1.3e-5 mbar for the depositions at 1.3e-500 and increased to 6.5e-3 mbar for the depositions at 100to ensure the formation of transparent films. The doping with Sn and Ti is aiming to increase the charge carrier density and compensate potential acceptor states induced by Cu and Na during the subsequent CIGSe deposition. Additional materials with suitable electron affinities for CIGSe, like ZnMgO:Ga, SrTiO3and TiO2:Nb, were prepared by PLD on GZO substrates. All films were prepared together with the group ”Process Technology and Advanced Concepts” at the National Renewable Energy Laboratory (NREL).
The CIGSe layer were deposited by M. Contreas at NREL. The deposition process was a different three-stage process from the one explained in Sec. 2.1, due to different PVD system specifications. In the first stage evaporation of In-Ga-Se was set with a constant rate, followed by a Cu-In-Ga-Se at a constant rate, followed by In-Ga-Se in the 3rd stage.
The temperature was set to 450. An SEM image of the absorber can be seen in Fig. 10.8.
The process was not optimized and lead to poor absorber quality.
The back contacts were realized by thermal evaporation of 64 Au pixels (3x3 mm) distributed over the 5x5 cm CIGSe coated substrate. This allowed a mapping of the electrical properties of the graded compositions.
Results
The performance results of the solar cells with the different oxides as the buffer layer are shown in the table in Fig. 8.2. It should be noted, that only the best performing solar cell of the 64 solar cells on each sample is shown. The results originate from two CIGSe depositions, which lead to different absorber qualities, resulting in different values for the JSC but identical VOC values (due to Fermi-level pinning at the interface, will be discussed later). For both deposition runs, 10 nm thick amorphous Ga2O3 deposited at 100, leads to highest efficiencies. The following paragraph will shortly describe the performance results of the different alloys.
Similar as in the work of Nakada et al. [5] the pure i-ZnO leads to non-rectifying devices for the Cu/(In+Ga) ratio of about 0.85 used in this experiment. This was observed reproducibly on 128 solar cells on two substrates and not only for i-ZnO, but also for
(a) (b)
Material Gradient max. VOC max. PCE
ZnMgO:Ga Ga,Mg 0 0
i-ZnO d 0 0
ZnO:Ga - 0 0
SrTiO3 d 0 0
TiO2:Nb Nb, d 60 0.06
(Zn,Ga)Ox Ga, d 0 0
(Sn,Ga)Ox Ga, d 500 0.02
(Zn,Sn,Ga)Ox (am) Zn, Sn 580 0.6∗
(Ti,Ga)Ox Ga, d 500 0.9∗
(Zn,Sn,Ga)Ox (cr) Zn, Sn 450 1.2∗
(In,Ga)Ox:Sn Ga, d 420 2.5
Ga2ZnO4 Ga, d 500 2.6
Ga2O3 (am) d 500 3.5
Ga2O3:Sn (am) Sn, d 500 2.9
Ga2O3:Ti (am) Ti, d 580 1.6
Ga2O3 (cr) d 660 0.4∗
(c)
Figure 8.2: J −V curves of the best result of the 64 solar cells on each sample with a new buffer layer. CIGSe absorbers from a) the first CIGSe deposition run b) the second CIGSe deposition run. c) Overview of theJ −V curve parameters. The first group are Ga2O3-free layers, the second group are Ga2O3 alloy layers, the last group are pure Ga2O3layers.
* The efficiency values marked by the star are around two times lower compared to the others, due to a qualitatively worse CIGSe layer. However, the VOCvalues are comparable.
ZnMgO:Ga, ZnO:Ga and (Zn,Ga)Ox. Thus, it is unlikely, that shunting causes the non-rectifying behaviour. A possible explanation could be a tunnelling contact between these oxide layers and the CIGSe. Based on the device model described in Sec. 7.1, this requires a high electron density in the TCO and high acceptor density at the TCO/CIGSe interface.
The samples with buffer layers based on Ga2O3 rich alloys do show a rectifying be-haviour, and exhibit aVOC larger than zero. Ga2O3 is known to reduce the charge carrier density and reduce the electron affinity within alloys, which makes a tunnelling contact to CIGSe more unlikely. The highest VOC is achieved by the crystalline Ga2O3 layer, which suffers from a poor JSC though. The crystalline Ga2O3 and also the crystalline ZnSnGaOx films lead to poor adhesion of the CIGSe layer onto the substrate.
Alloying Ga2O3 with Ti leads to J−V curves similar to those with pure Ga2O3, but exhibit a slightly lower short circuit current. The same occurs when doping Ga2O3 with Ti in the concentration range below 1 at.%. The optical measurements, showed that alloying with Ti at 200 does not effect the band gap value, despite the fact, that the band gap for pure TiO2 is 3.2 eV. Pure TiO2 samples lead to non-rectifyingJ−V curves.
Alloying Ga2O3 with Zn leads to non-rectifying devices. Interestingly this is not the case for the buffer layer deposited from a single Ga2ZnO4 target, which leads to very similar J−V curves as the pure Ga2O3 layers, again with a slightly lower JSC. Alloying Ga2O3 with Sn leads to a strong electron barrier not allowing any current transport within the studied voltage range. Doping with Sn, in the concentration range below 1 at.% leads to a reduction of the photo-current. Alloying Ga2O3 with Sn and Zn also reduced the photo-current and for higher Ga concentration (above 50 at.%) a strong electron barrier develops. Similar behaviour was observed for the amorphous and crystalline ZnSnGaOx layers. Alloying Ga2O3 with In leads to a reduction of the VOC while the JSC remains the same as for the pure Ga2O3. The details are shown in Fig. 8.4d.
In summary, amorphous and undoped Ga2O3 was found to perform best as a buffer layer compared to the other oxides testes in this experiment. Doping and alloying Ga2O3 with Ti or Sn lead to lower JSC values. Alloying with In leads to lower VOC values. All materials not based on Ga2O3 lead to non-rectifying devices.
The positive result is, that the efficiency of 3.5 % was achieved without sodium addition and without optimisation of the CIGSe deposition process. The next section will therefore analyse the properties of the Ga2O3 layer and the corresponding solar cells to understand the limitations.