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...

Recently, Cu(In,Ga)Se 2 (CIGS) solar cells have achieved 21% world-record efficiency, partly due to the introduction of a postdeposition potassium treatment to improve the front interface of CIGS absorber layers. However, as high-efficiency CIGS solar cells essentially require long diffusion lengths, the highly recombinative rear of these devices also deserves attention. In this paper, an Al 2 O 3 rear surface passivation layer with nanosized local point contacts is studied to reduce recombination at the standard Mo/CIGS rear interface. First, passivation layers with well-controlled grids of nanosized point openings are established by use of electron beam lithography. Next, rear-passivated CIGS solar cells with 240-nm-thick absorber layers are fabricated as study devices. These cells show an increase in open-circuit voltage (+57 mV), short-circuit current (+3.8 mA/cm 2 ), and fill factor [9.5% (abs.)], compared with corresponding unpassivated reference cells, mainly due to improvements in rear surface passivation and rear internal reflection. Finally, solar cell capacitance simulator (SCAPS) modeling is used to calculate the effect of reduced back contact recombination on high-efficiency solar cells with standard absorber layer thickness. The modeling shows that up to 50-mV increase in open-circuit voltage is anticipated. Index Terms-Al 2 O 3 , Cu(In,Ga)Se 2 , electron beam lithography, local point contacts, nanosized openings, passivation layer, passivated emitter and rear cell (PERC), rear internal reflection, rear surface recombination velocity, Si. I. INTRODUCTIONO VER the past two years, CIGS solar cells have taken a sudden leap in world record efficiency of 1%, from around 20% to 21% [1]. Before 2013, CIGS solar cell efficiency improvements were mainly due to enhancements in absorber material quality, and cell efficiencies were lingering around 20% for a few years-as achieved by National Renewable Energy Laboratory (NREL) and the Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) [2]. However, in Manuscript

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Zn 1-x Sn x O y (ZTO) has yielded promising results as buffer material for the full sulfur Cu 2 ZnSnS 4 (CZTS), with efficiencies continuously surpassing its CdS-references. ZTO can be deposited by atomic layer deposition (ALD) enabling tuning of the conduction band position through choice of metal ratio or deposition temperature. Thus an optimization of the conduction band alignment between ZTO and CZTS can be achieved. The ZTO bandgap is generally larger than that of CdS and can therefore yield higher currents due to reduced losses in the short wavelength region. Another advantage is the possibility to omit the toxic Cd. In this study the ALD process temperature was varied from 105 to 165 °C. Current-blocked devices were obtained at 105 °C while the highest open-circuit voltage and device efficiency was achieved for 145 °C. The highest fill factor was seen at 165 °C. The best efficiency reached in this study was 9.0 %, which, to our knowledge, is the highest efficiency reported for Cd-free full-sulfur CZTS. We also show that the effect of heat needs to be taken into 2 account. The results indicate that part of the device improvement comes from heating the absorber, but that the benefit of using a ZTO-buffer is clear.

Previously, an innovative way to reduce rear interface recombination in Cu(In,Ga)(S,Se) 2 (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu 2 (Zn,Sn)(S,Se) 4 (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al 2 O 3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage (V O C ; +17%rel.), short-circuit current (J S C ; +5%rel.), and fill factor (FF; +9%rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32%rel. is obtained for the rear passivated cells.

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In this study, we investigate the influence of absorber thickness on Cu 2 ZnSnS 4 (CZTS) solar cells, ranging from 500 to 2000 nm, with nearly constant metallic composition. Despite the observed ZnS and SnS phases on the surface and backside of all absorber films, scanning electron microscopy, Raman scattering, and X-ray diffraction show no large variations in material quality for the different thicknesses. The open-circuit voltage (V oc ), short-circuit current and overall power conversion efficiency of the fabricated devices show an initial improvement as the absorber thickness increases but saturate when the thickness exceeds 750 nm. External quantum efficiency (EQE) measurements suggest that the current is mainly limited by collection losses. This can result from nonoptimal bulk quality of the CZTS absorber (including the presence of secondary phases), which is apparently further reduced for the thinnest devices. The observed saturation of V oc agrees with the expected influence from strong interface recombination. Finally, an effective collection depth of 750-1000 nm for the minority carriers generated in the absorber can be estimated from EQE, indicating that the proper absorber thickness for our device process is approximately 1000 nm. Performance could be improved for thicker films, if the collection depth can be increased.

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