Shinho Kim

Improvement of voltage deficit of Ge-incorporated kesterite solar cell with 12.3% conversion efficiency

We demonstrate the improved efficiency of a Cu2Zn(Sn1− x Ge x )Se4 (CZTGSe) thin-film solar cell with a conversion efficiency of 12.3%; this cell exhibits a greatly improved open-circuit voltage (V OC) deficit of 0.583 V and a fill factor (FF) of 0.73 compared with previously reported CZTGSe cells. The V OC deficit was found to be improved through a reduced band tailing via the control of the Ge/(Sn + Se) ratio. In addition, the high FF was mainly induced by a reduced carrier recombination at the absorber/buffer interface and/or in the space charge region, whereas parasitic resistive effects on FF were very small.

Quantitative determination of optical and recombination losses in thin-film photovoltaic devices based on external quantum efficiency analysis

In developing photovoltaic devices with high efficiencies, quantitative determination of the carrier loss is crucial. In conventional solar-cell characterization techniques, however, photocurrent reduction originating from parasitic light absorption and carrier recombination within the light absorber cannot be assessed easily. Here, we develop a general analysis scheme in which the optical and recombination losses in submicron-textured solar cells are evaluated systematically from external quantum efficiency (EQE) spectra. In this method, the optical absorption in solar cells is first deduced by imposing the anti-reflection condition in the calculation of the absorptance spectrum, and the carrier extraction from the light absorber layer is then modeled by considering a carrier collection length from the absorber interface. Our analysis method is appropriate for a wide variety of photovoltaic devices, including kesterite solar cells [Cu2ZnSnSe4, Cu2ZnSnS4, and Cu2ZnSn(S,Se)4], zincblende CdTe solar cells, ...

Improvement of minority carrier lifetime and conversion efficiency by Na incorporation in Cu2ZnSnSe4 solar cells

The effect of Na incorporation in Cu2ZnSnSe4 (CZTSe) solar cells grown by the coevaporation method was investigated via photoluminescence (PL) and time-resolved PL (TRPL) measurements as well as photovoltaic properties. The TRPL decay curves showed a monotonic increase in CZTSe lifetime from 2 to 15 ns with increasing Na incorporation, which corresponds to the increase in the correction length estimated by quantum efficiency measurements. The TRPL decay curves included two decay components, fast and slow, which were discussed and concluded as originating from the recombination at the surface and bulk of CZTSe, respectively, which is also supported by TPRL measurements with various excitation wavelengths. The lifetime of CZTSe is limited by the surface-related nonradiative recombination compared to Cu(In,Ga)Se2 devices which are fabricated with the same device structure except for the absorber, and at present, it is concluded that the surface recombination of the CZTSe limits the cell performance. In addit...

Very small tail state formation in Cu2ZnGeSe4

We find that coevaporated Cu2ZnGeSe4 has an ideal bandgap for solar cells (1.39 ± 0.01 eV) and shows quite reduced tail state absorption with a very low Urbach energy of 28 meV, which is far smaller than those of more studied Cu2ZnSnSe4 and Cu2ZnSnS4. The small tail states in Cu2ZnGeSe4 are found to originate from almost perfect cation ordering, while unusual tail state generation occurs in the Sn-based quaternary compounds by extensive cation substitution. Quite remarkably, the crystal total energy derived from first-principles calculations reveals a unified rule for the cation disordering, confirming that the lighter group-IV element (i.e., Ge) is essential for eliminating the tail state generation induced by cation mixing.We find that coevaporated Cu2ZnGeSe4 has an ideal bandgap for solar cells (1.39 ± 0.01 eV) and shows quite reduced tail state absorption with a very low Urbach energy of 28 meV, which is far smaller than those of more studied Cu2ZnSnSe4 and Cu2ZnSnS4. The small tail states in Cu2ZnGeSe4 are found to originate from almost perfect cation ordering, while unusual tail state generation occurs in the Sn-based quaternary compounds by extensive cation substitution. Quite remarkably, the crystal total energy derived from first-principles calculations reveals a unified rule for the cation disordering, confirming that the lighter group-IV element (i.e., Ge) is essential for eliminating the tail state generation induced by cation mixing.