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This study evaluates the effect of silver alloying, stoichiometry, and deposition temperature of wide‐gap (Ag,Cu)GaSe2 (ACGS) absorber films for solar cell applications. Devices using a standard CdS buffer exhibit a strong anticorrelation between the open‐circuit voltage (VOC) and short‐circuit current density (JSC), with VOC decreasing and JSC increasing toward stoichiometric absorber composition. Increasing the ACGS deposition temperature leads to larger grains and improved JSC, while VOC is not affected. By adding more silver to the absorber (maximum tested [Ag]/([Ag]+[Cu]) [AAC] = 0.4), the widening of the space charge region (SCR) significantly enhances carrier collection. Experimental quantum efficiency spectra can be accurately simulated when assuming a very low diffusion length and perfect collection in the SCR. The highest efficiency of 8.3% (without antireflection coating [ARC]) is reached for an absorber with AAC = 0.4 grown at 600 °C. Replacing CdS by a (Zn,Sn)O buffer with lower electron affinity strongly mitigates interface recombination. Moreover, the VOC–JSC anticorrelation is not evident anymore and the highest efficiency of 11.2% (11.6% w/ARC, VOC = 985 mV, JSC = 18.6 mA cm−2, fill factor = 61.0%) is reached for a close‐stoichiometric ACGS solar cell with AAC = 0.4 processed at 650 °C. [ABSTRACT FROM AUTHOR]

The Cu2ZnGeXSn1‐XS4 (CZGTS) thin‐film solar cells have a limited open‐circuit voltage (VOC) due to bulk and interface recombination. Since the standard CdS buffer layer gives a significant cliff‐like conduction band offset to CZGTS, alternative buffer layers are needed to reduce the interface recombination. This work compares the performance of wide bandgap Cu2ZnGeS4 (CZGS) solar cells fabricated with nontoxic ZnxSn1–xOy (ZTO) buffer layers grown by atomic layer deposition under different conditions. The VOC of the CZGS solar cell improved significantly to over 1 V by substituting CdS with ZTO. However, VOC is relatively insensitive to ZTO bandgap variations. The short‐circuit current is generally low but is improved with KCN etching of the CZGS absorber before deposition of the ZTO buffer layer. A possible explanation for the device behavior is the presence of an oxide interlayer for nonetched devices. [ABSTRACT FROM AUTHOR]

Tandem solar cell structures require a high-performance wide band gap absorber as top cell. A possible candidate is CuGaSe2, 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 CuGaSe2. In this paper, we show that the open-circuit voltage can be drastically improved in wide band gap p-Cu(In,Ga)Se2 and p-CuGaSe2 devices by improving the conduction band alignment to the n-type buffer layer. This is accomplished by using Zn1− xSn xO 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)Se2 and CuGaSe2 materials. With an improved buffer band alignment for CuGaSe2 absorbers, evaporated in a 3-stage type process, we show devices exhibiting open-circuit voltages up to 1017 mV, and efficiencies up to 11.9%. This is to the best of our knowledge the highest reported open-circuit voltage and efficiency for a CuGaSe2 device. Temperature-dependent current-voltage measurements show that the high open-circuit voltage is explained by reduced interface recombination, which makes it possible to separate the influence of absorber quality from interface recombination in future studies. [ABSTRACT FROM AUTHOR]

Cu2ZnSnS4(CZTS) thin‐film solar cell absorbers with different bandgaps can be produced by parameter variation during thermal treatments. Here, the effects of varied annealing time in a sulfur atmosphere and an ordering treatment of the absorber are compared. Chemical changes in the surface due to ordering are examined, and a downshift of the valence band edge is observed. With the goal to obtain different band alignments, these CZTS absorbers are combined with Zn1−xSnxOy (ZTO) or CdS buffer layers to produce complete devices. A high open circuit voltage of 809 mV is obtained for an ordered CZTS absorber with CdS buffer layer, while a 9.7% device is obtained utilizing a Cd free ZTO buffer layer. The best performing devices are produced with a very rapid 1 min sulfurization, resulting in very small grains. [ABSTRACT FROM AUTHOR]

This study reports on the beneficial effect of an absorber surface passivation by Al 2 O 3 on the performance of Cu(In, Ga)Se 2 (CIGSe) solar cells. Here the Al 2 O 3 layer is deposited by atomic layer deposition (ALD) subsequently to a CdS buffer layer. It is shown that a very thin film of about 1 nm efficiently reduces the interface recombination rate if the buffer layer is too thin to not fully cover the CIGSe absorber. An Al 2 O 3 thickness of 1 nm is sufficiently low to allow current transport via tunneling. Increasing the thickness to >1 nm leads to a detrimental blocking behavior due to an exponentially decreasing tunnel current. Losses in open circuit voltage ( V oc ) and fill factor ( FF ) when reducing the buffer layer thickness are significantly mitigated by implementing an optimized Al 2 O 3 layer. It is further shown, that the heat treatment during the ALD step results in an increase in short circuit current density ( J sc ) of about 2 mA/cm 2 . This observation is attributed to a widening of the space charge region in the CIGSe layer that in turn improves the collection probability of electrons. For not fully covering CdS layers the decrease in interface defect density by the passivation contributes as well, leading to a gain of about 5 mA/cm 2 for cells without a buffer. Finally, the leakage current of the solar cell devices could be reduced when applying the Al 2 O 3 layer on insufficiently covering CdS films, which proves the capability of mitigating the effect of shunts or bad diodes. [ABSTRACT FROM AUTHOR]

Thin film Cu(In,Ga)Se 2 solar cells with ALD-deposited Zn 1– x Sn x O y buffer layers are fabricated and the solar cell properties are investigated for varying ALD deposition temperatures in the range from 90 °C up to 180 °C. It is found that a process window exists between 105 °C and 135 °C, where high solar cell efficiency can be achieved. At lower ALD deposition temperatures the solar cell performance is mainly limited by low fill factor and at higher temperatures by low open circuit voltage. Numerical simulations and electrical characterization are used to relate the changes in solar cell performance as a function of ALD deposition temperature to changes in the conduction band energy level of the Zn 1– x Sn x O y buffer layer. The Zn 1– x Sn x O y films contain small ZnO or ZnO(Sn) crystallites (<10 nm), which may lead to quantum confinement effects influencing the optical band gap of the buffer layer. The ALD deposition temperature affects the size of these crystallites and it is concluded that most of the changes in the ZTO band gap occur in the conduction band level. [ABSTRACT FROM AUTHOR]