For higher cell-to-module efficiency in Cu(In,Ga)Se<sub>2</sub> (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 cm<sup>2</sup> 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.
An ultrashort pulse laser system with precisely controlled output-timing and carrier-envelope phase (CEP) is reported.
Recently developed technology Ofl CEP control of a mode-locked laser not only introduced an optical frequency comh
in frequency domain hut also gave us a way to generate optical pulses whose oscillating electric field is under a fixed
phase relation with the intensity shape. Fortunately, recent advances on optical physics have also showed that sonic types
of light-matter interactions become sensitive to the field shape when the pulse approaches a few cycles in duration and
has a high peak intensity. Owing to those advances, field-controlled ultrashort pulse generation, based on
suh-femtosecond resolution timing-control and sub-radian CEP control of femtosecond lasers, becomes an attractive
challenge. Our final goal is to realize a shaped electric field within optical-cycle time scale br researches on light-matter
interaction and other future application.
CEP control Ofl a mode-locked Ti:sapphire laser is the first step of such a laser system. Trade-off between the
accuracy and robustness of the control, and the monitoring technique of CEP br amplilication, will he discussed.
Amplification of a CEP-controlled pulse, which is necessary for most of time-domain application, is successfully
performed by the CEP monitoring technique. Our chirped-pulse amplifier, that includes a grating-based
stretcher/compressor, has a potential to achieve higher-energy amplification of a fixed CEP pulse. Multichannel phase
control of spectrally divided ultrashort pulses is applied to dynamic control of pulse-timing and CEP of amplifled pulses.
Related results on short-pulse, sub-l3fs, generation by a chirped-pulse Ti:sapphire amplifier, and multicolor
phase-coherent pulse sources will be also discussed briefly, showing our on-going efforts to approach the final goal.
Recent progress on ultrashort pulse lasers has made it possible to control the pulse timing and optical phases from different cavities very precisely. By applying detection and control techniques of the carrier-envelope phase to multicolor pulse sources with synchronized pulse timing, we obtained phase-coherent pulse-trains in different wavelengths. The pulse-trains have sufficient stability to achieve Fourier synthesis among optical fields. One of our test sources was a femtosecond optical parametric oscillator (OPO). Using a specially designed OPO whose signal and idler are sub-harmonics of the pump frequency, we obtained long-term stabilization of the optical phase among those three waves. Since the OPO also generates sum-frequencies of the three waves, six-color coherent pulse-trains from 425nm to 2550nm are available. Another test source was a passively synchronized mode-locked laser with two kinds of gain media, Ti:sapphire and Cr:forsterite. Although it is harder to reduce phase-noise between the different color pulses than in the OPO example, we can expect shorter pulse duration and higher average power from this type of coupled laser. These coherent multicolor pulse sources will be applied not only to shorter-pulse generation by field summation, but also to some applications that include competing non-linear processes among multicolor pulses.