Yukiko Kamikawa-Shimizu

Highly Efficient Cu(In,Ga)Se2 Thin-Film Submodule Fabricated Using a Three-Stage Process

Using a three-stage process, a highly efficient, integrated chalcopyrite Cu(In,Ga)Se2 (CIGS) submodule was fabricated with a certified efficiency of 18.34% and an open circuit voltage of 2.963 V, a short circuit current of 29.05 mA, a fill factor of 0.762, and a designated area of 3.576 cm2. The diode properties and parasitic resistances of the submodule and a reference single cell containing a CIGS absorber layer identical to that in the submodule were determined using a distributed diode model. In addition, the fundamental loss mechanisms for the submodule were investigated.

Buried p-n junction formation in CuGaSe2 thin-film solar cells

CuGaSe2/CdS interfaces and the mechanism behind the hetero p-n junction formation were investigated using solar cell devices which demonstrated about a 10% energy conversion efficiency. It was found that the CuGaSe2/CdS interface could be described as a CuGaSe2/Cu-deficient Cu-Ga-Se layer (CDL)/CdS structure and the p-n junction was located at the CuGaSe2/CDL interface that was present 50–100 nm from the CDL/CdS interface. While the difficulty of the realization of equilibrium n-type CuGaSe2 has been generally recognized, this result suggests that CDL consisting of ordered vacancy compound phases such as CuGa3Se5 can play the role of an n-type material.

Cu(In,Ga)Se 2 Solar Cells With Amorphous Oxide Semiconducting Buffer Layers

A transparent amorphous oxide semiconductor (TAOS) layer for the suppression of interface recombination and enhancement of open-circuit voltage (V_oc) in Cu(In,Ga)Se₂ (CIGS) solar cells is demonstrated. A 60-nm-thick n-type a-In_2-2xGa_2xO₃ (x = 0.6, 0.7, 0.8, 0.9, 1)or a-Ga_2-2yAl_2yO₃ (y = 0.1, 0.2) layer was introduced between the CIGS film and the ZnO transparent front contact. The solar cell performance systematically varied as a function of x and y, and CIGS solar cells with a-In_2-2xGa_2xO₃ (x = 0.9, 1) buffer layers showed V_oc values comparable with those of a reference cell with standard i-ZnO/CdS buffer layers. Current density-voltage (J-V) curve behavior can be explained by conduction band discontinuity at the TAOS/CIGS interface and the carrier density of the TAOS layer.

Interface oxygen and heat sensitivity of Cu(In,Ga)Se2 and CuGaSe2 solar cells

Combined oxygen and heat exposure processes after p-CuGaSe2/n-CdS junction formation degrade CuGaSe2 solar cell efficiency, whereas such annealing processes can improve high In content Cu(In,Ga)Se2 device performance. This result is chiefly attributable to different interface structures consisting of oxygen-sensitive CuGaSe2 or relatively oxygen-insensitive Cu(In,Ga)Se2. To reduce CuGaSe2 interfacial recombination, reduction of the process temperature of the front contact layer deposition process is found to be the key. In this work, fill factor values exceeding 0.7 are reproducibly obtained from CuGaSe2 solar cells, though such high fill factor values have been very challenging to demonstrate to date using CuGaSe2 photoabsorber layers.

Fabrication of integrated CIGS modules using the in-line three-stage process

We have attempted an in-line three-stage process for the deposition of CIGS (CuInGaSe2) absorber layers. The allowed growth area of CIGS deposition apparatus is 30 × 30 cm2, and the compositional uniformity and distribution of cell performance have been investigated. Large-size grains and band-grading typical for the three-stage process have been observed. The composition ratios and the performance of small area cells (0.5 cm2) have shown good uniformity over the whole deposition area, with an average efficiency of 15.8 % without anti-reflection coating. By improving CIGS deposition process, efficiency of small cells enhanced up to 17.3 %. Minimodules with aperture area of 76.5 cm2 were also fabricated on 10 × 10 cm2 SLG substrates. Preliminary results showed efficiency as high as η = 14.2 % with anti-reflection coating.

Ultrafast laser scribing of transparent conductive oxides in Cu(In,Ga)Se2 solar cells via laser lift-off process: the control of laser-induced damage

For higher cell-to-module efficiency in Cu(In,Ga)Se2 (CIGS) thin-film solar cells, it is important to reduce the loss of active area due to integrated connection. The integrated connection contains three scribing processes: P1 (back contact insulation), P2 (electrical connection) and P3 (transparent conductive oxide, shortly TCO front contact insulation). In this work, we focused on ultrashort-pulse laser scribing (λ=1034 nm, Δτ=300 fs) of TCO via lift-off process as damage-less P3 scribing of CIGS thin-film solar cells. The lift-off of TCO was caused by laser ablation of only an upper part of CIGS light-absorbing layer. The dependence of lift-off behavior on the laser pulse energy and TCO film thickness has been investigated. It was observed that the lift-off of TCO formed a heat-affected zone (HAZ) with a thickness up to 250 nm beneath the trench bottom, where the CIGS experienced to melt. Further, thinner TCO film required lower laser energy threshold for the TCO lift-off, which is favorable to higher solar cell efficiency due to smaller HAZ. Using the TCO liftoff as P3, a submodule with an active area of approximately 3.5 cm2 made by all laser scribing exhibited the conversion efficiency of 11.6 %. After post-annealing at 85 °C for 15 h in vacuum for recovering laser-induced damages, the efficiency was successfully improved to 15.0 %, which is comparable to mechanically-scribed one.

Critical issues for high-efficiency low-cost CIGS solar cells and modules

High-efficiency CIGS solar cells and submodules have been developed at AIST. By developing the water-vapor assisted deposition technique, widegap CIGS solar cells with conversion efficiencies of over 18% with Voc=0.744V have been demonstrated. The conversion efficiencies of integrated submodules on 10x10cm2 sodalime glass substrates have been improved up to η=16.2% by using coevaporation process. These results indicate that the CIGS technologies are competitive with the current Si and CdTe technologies in terms of both cost and performance.

Progress in CIGS solar cell technologies

Cu(In1-xGax)Se2 (CIGS)-based solar cells have emerged as one of the most promising candidates for high-efficiency low-cost thin-film solar cells, and a significant improvement in solar cell performance has been reported with conversion efficiencies as high as eta=19.9%, though the efficiencies of commercial modules are limited to eta=11-12%. In Japan, the efficiency goal for 2030 is set to be eta=25% for small area cells and eta=22% for large-size modules, therefore improvement in conversion efficiencies for both cells and modules are required.