Probing carrier lifetimes in photovoltaic materials using subsurface two-photon microscopy.

Samples Rhodamine 101 was diluted in ethanol to a concentration of 2 × 10 −4 M and placed in a cuvette. For the 2P measurement a 10× objective was used to allow for a greater working distance required by using a cuvette. The InGaN quantum well sample was grown using MOCVD on a C-plane sapphire substrate using a Thomas Swan 2 × 3 close-coupled showerhead reactor. Trimethylgallium, trimethylindium and ammonia where used as Ga, In and N precursors, respectively. Two sets of five 2.5 nm-thick InGaN quantum wells, with 7 nm GaN barriers were grown on top of a 2 μm-thick GaN buffer and separated by a 3 μm-thick GaN layer and capped by 300 nm-thick GaN layer. The emission wavelength of the QWs was tuned to ~430 nm and ~460 nm by changing the indium concentration through varying the growth temperature between 750 and 720°C, respectively. Bridgeman grown, CdTe (110) single crystals were obtained from MTI Corp. The crystals were polished to a mirror finish as received. The CdTe crystals were subsequently annealed with a piece of Cd shot in an evacuated (10 −5 Torr) quartz ampoule for 24 hours at 500°C and finally cooled for 5 hours. One-photon time-resolved photoluminescence Samples were excited with a pulsed laser diode, (PicoQuant LDH 485 nm, 100 ps FWHM pulse duration, 10 MHz) focused with a 10× NA = 0.25 objective, spectrally filtered using an Acton 2300i spectrometer and detected with a single-photon avalanche diode (Micro Photon Devices PDM series). A PicoQuant PicoHarp 300 time-correlated single-photon counting (TCSPC) system was used to record the timing data.

Two-photon time-resolved photoluminescence

A confocal microscope system was constructed using a Nikon Eclipse Ti-U microscope with a Nikon 100× NA = 0.95 NA objective and a three axis MadCityLabs NanoPDQ piezo nanopositioning sample stage. A Coherent Mira 900 Ti:Sapphire excitation laser (700–1000 nm tunable, 150 fs pulse duration, 80 MHz) is optically coupled to the input of the microscope. The laser was tuned to 830 nm for the InGaN quantum well and Rhodamine 101 samples and was tuned to 920 nm for the CdTe sample. Photoluminescence was collected through a 150 μm pinhole and into detection optics. For spectral collection we used and Acton 2300i spectrometer with 150 groves/mm grating and an Andor iXon electron-multiplied CCD. For time-resolved collection we first spectrally-filtered the PL using band pass filters and then collected with a single-photon avalanche diode (Micro Photon Devices PDM series). A PicoQuant PicoHarp 300 time-correlated single-photon counting (TCSPC) system was used to record the timing data. Lifetimes were fit to a biexponential decay using a constrained least-squared algorithm. A laser pulse energy of 4 pJ/pulse was used to excite the CdTe sample.

Cathodoluminescence microscopy

We performed CL measurements in a Zeiss Gemini Supra 55 scanning electron microscope at 15 kV. CL emission was collected via a multimode optical fiber positioned 300 μm away from the electron-beam location on the sample. CL was then spectrally filtered with an Acton 2300i spectrometer. For CL images collected in a narrow spectral bandwidth, the photons were collected by either a Hamamatsu 7360-01 photomultiplier tube or a Perkin-Elmer SPCM-AGR-15 silicon-avalanche photodiode (below and above 530 nm, respectively).