Tandem solar cell structures require a high-performance wide band gap absorber as top cell. A possible candidate is CuGaSe 2 , with a fundamental band gap of 1.7 eV. However, a significant open-circuit voltage deficit is often reported for wide band gap chalcopyrite solar cells like CuGaSe 2 . In this paper, we show that the open-circuit voltage can be drastically improved in wide band gap p-Cu(In,Ga)Se 2 and p-CuGaSe 2 devices by improving the conduction band alignment to the n-type buffer layer. This is accomplished by using Zn 1−x Sn x O y , grown by atomic layer deposition, as a buffer layer. In this case, the conduction band level can be adapted to an almost perfect fit to the wide band gap Cu(In,Ga) It has been proven difficult to maintain a good device quality when the gallium content is increased. The main problem is that the opencircuit voltage (V oc ) does generally not scale with the band gap energy as predicted, and it tends to saturate for absorber band gap energies above roughly 1.3 eV. 7,8 This lack of performance in high gallium CIGS devices has been the subject of numerous studies. 7-11The dominating recombination paths, in CuGaSe 2 devices, have been assigned to tunneling enhanced recombination either in the space-charge region or at the interface. 12,13 A possible explanation is trap states formed by cation anti-site (In Cu /Ga Cu ) or anion vacancy (V Se ) defects that become deeper positioned within the band gap when the gallium content increases, and thereby form more effective recombination centers. 11 Furthermore, the difference between the Fermi level and the valence band energy at the absorber surface seems to remain constant around 0.8 eV, independently of gallium content. 13 Consequently, the Fermi level position at the absorber/buffer interface is closer to the middle of the band gap at high gallium contents, and no beneficial type inversion can be expected. Thus, the influence of recombination close to or at the interface appears to become more prominent when the CIGS band gap is widened. The bulk recombination rate is also expected to increase with a large number of these defects, but it has not In the first section of this study, the growth and material characterization of nongraded CIGS absorbers with varying gallium contents (0.3 ≤ GGI ≤ 1) deposited in a single-stage co-evaporation process are described. These absorbers are applied in the second section, where we use the temperature dependence of ALD grown ZTO buffer layers to show that the V oc deficit in wide band gap CIGS can be reduced by improving the absorber/buffer conduction band alignment.In the last section, we further investigate the effect of an improved band alignment by using CuGaSe 2 absorbers of higher material quality, evaporated in a 3-stage type process. 21-24It can be observed in the SEM images that the grain size is reduced with increased GGI. This general observation is often reported in the literature. The grain size reduction has been found to be influenced by a number of factors, such as film t...

The effect of absorber stoichiometry in (Ag,Cu)(In,Ga)Se 2 (ACIGS) solar cells with bandgaps (E g ) > 1.40 eV is studied on a large sample set. It is confirmed that moving away in composition from ternary AgGaSe 2 by simultaneous reduction in Ga and Ag content widens the chalcopyrite single-phase region and thereby reduces the amount of ordered vacancy compounds (OVCs). As a consequence, a distortion in currentÀvoltage characteristics, ascribed to OVCs at the back contact, can be successfully avoided. A clear anticorrelation between open-circuit voltage (V OC ) and short-circuit current density (J SC ) is detected with varying absorber stoichiometry, showing decreasing V OC and increasing J SC values for [I]/[III] > 0.9. Capacitance profiling reveals that the absorber doping gradually decreases toward stoichiometric composition, eventually leading to complete depletion. It is observed that only such fully depleted samples exhibit perfect carrier collection, evidencing a very low diffusion length in wide-gap ACIGS films. The results indicate that OVCs at the surface play a minor or passive role for device performance. Finally, a solar cell with V OC ¼ 0.916 V at E g ¼ 1.46 eV is measured, which is, to the best of our knowledge, the highest value reported for this bandgap to date.

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The effects of introducing a passivation layer at the rear of ultrathin Copper Indium Gallium di-Selenide Cu(In,Ga)Se2 (CIGS) solar cells is studied. Point contact structures have been created on 25 nm Al2O3 layer using e-beam lithography. Reference solar cells with ultrathin CIGS layers provide devices with average values of light to power conversion efficiency of 8.1 % while for passivated cells values reached 9.5 %. Electronic properties of passivated cells have been studied before, but the influence of growing the CIGS on Al2O3 with point contacts was still unknown from a structural and morphological point of view. Scanning Electron Microscopy, X-ray Diffraction and Raman spectroscopy measurements were performed. These measurements revealed no significant morphological or structural differences in the CIGS layer for the passivated samples compared with reference samples. These results are in agreement with the similar values of carrier density (~8x1016 cm-3) and depletion region (~160 nm) extracted using electrical measurements. A detailed comparison between both sample types in terms of current-voltage, external quantum efficiency and photoluminescence measurements show very different optoelectronic behaviour which is indicative of a successful passivation. SCAPS simulations are done to explain the observed results in view of passivation of the rear interface.

This study evaluates the potential of hydrogen-doped In 2 O 3 (IOH) as a transparent back contact material in (Ag y ,Cu 1-y )(In 1-x ,Ga x )Se 2 solar cells. It is found that the presence of Na promotes the creation of Ga 2 O 3 at the back contact during (Ag y ,Cu 1-y )(In 1-x ,Ga x )Se 2 growth. An excessive Ga 2 O 3 formation results in a Ga depletion, which extends deep into the absorber layer.Consequently, the beneficial back surface field is removed and a detrimental reversed electrical field establishes. However, for more moderate Ga 2 O 3 amounts (obtained with reduced Na supply), the back surface field can be preserved. Characterization of corresponding solar cells suggests the presence of an ohmic back contact, even at absorber deposition temperatures of 550°C. The best solar cell with an IOH back contact shows a fill factor of 74% and an efficiency contacts are further needed in bifacial devices, which can be semitransparent if desired (eg, for "solar windows"). The upper cells in a multijunction tandem structure need to be grown on TBCs as well to allow for light propagation to the bottom cell. An advantage of these approaches over a superstrate configuration is that the required buffer layer does not have to undergo the thermal stress during absorber formation. Up to now, there are no buffer materials found, which are chemically stable at high temperatures, which results in a significant efficiency drop compared to substrate-configured solar cells. 4,5 Nevertheless, there are some requirements to a TBC as well. After the thermal stress during absorber formation, it still needs to (1) create an ohmic contact to the absorber, (2) be highly transparent, and (3) exhibit a low sheet resistance (R SH ). In the case of CIGS-based solar cells, it also needs to exhibit a certain permeability for alkaline ions, if they are supposed to be supplied from the underlying glass substrate.In the past, different transparent conductive oxide (TCO) layers have been investigated as potential TBC materials for CIGS solar cells. In this study, we use (Ag,Cu)(In,Ga)Se 2 (ACIGS) as an absorber material, which is proven to result in large grain sizes 19 and highefficiency solar cells. 20,21 The silver was added intentionally, since V OC losses for higher absorber band gap energies are less pronounced in ACIGS compared to CIGS. 21 This is in particular interesting for top cells in a tandem configuration where the optimum band gap energy E G is about 1.6 eV (two junctions).It should be emphasized that this work is supposed to act as a first potential evaluation for IOH as a TBC. Hence, the cell structure was designed for maximized efficiencies at normal front side illumination, which means that the absorber layer is about 2 μm thick and has a band gap energy of about 1.2 eV. | MATERIALS AND METHODS | Sample fabrication and configurationsThe ACIGS solar cells investigated in this study were manufactured as a stack of soda lime glass (SLG)/back contact/ACIGS/CdS/i-ZnO/AZO.As a back contact either IOH or standard Mo (DC-sp...