Record 1.0 V open‐circuit voltage in wide band gap chalcopyrite solar cells

Abstract

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...

Wide‐gap (Ag,Cu)(In,Ga)Se2 solar cells with different buffer materials—A path to a better heterojunction

“…Comparison of some reported exceptional V OC values for chalcopyrite solar cells achieved by different groups and from this work (only samples with clear “spike” configuration excluding Ag80Ga85). “ZMO” stands for Zn‐Mg‐O buffer, “PDT” for applied alkali PDT, “mono” for monocrystalline absorber, and “p.a.” for post‐annealing.…”

“…However, so far, wide‐gap chalcopyrite solar cells cannot compete with their low band gap counterparts. An efficiency drop of more than 10% absolute is reported when going from the current record device with a band gap of E g ~1.1 eV (η = 23.35% for sulfurized CIGS; η = 22.6% for pure CIGS) to a solar cell with E g > 1.6 eV (η = 11.9%) . The most pronounced limitation for wide‐gap chalcopyrite solar cells is the low V OC relative to the band gap, which typically increases slower (often referred to as “ V OC saturation”) for E g > 1.3 eV (ie, [Ga]/([Ga] + [In]) ≡ GGI > 0.4 in CIGS) .…”

“…By comparing electrical characteristics of devices with a standard CdS layer to those of solar cells using an alternative Zn 1‐ y Sn y O z buffer that has a ~100 mV lower electron affinity, additional information about limiting processes can be acquired. Our previous results emphasized the great potential of high‐ E g ZTO buffer layers for CuGaSe 2 solar cells, leading to V OC values exceeding 1 V …”

Band Gap Tuning of Cu2ZnGeSxSe4-x Absorbers for Thin-Film Solar Cells

“…One possibility would be to replace the Mo back contact that is known to be thermodynamically instable [30] with other materials or add a suitable barrier layer between absorber and back contact. The absorber itself also needs to be optimized in terms of grain growth, and the use of a different buffer layer with proper band alignment might be especially promising, since an open-circuit voltage of more than 1 V was recently achieved with a high band gap CuGaSe 2 absorber using a Zn 1-x Sn x O y buffer layer [31].…”

Analysis of Failure Modes in Kesterite Solar Cells

“…12 . PV data and related bandgaps from References [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] . Bandgaps are extracted from EQE spectra.…”