Pure-sulfide kesterite Cu 2 ZnSnS 4 (CZTS)-based thin-film solar cell has been emerging as a promising cost-effective thin-film photovoltaic (PV) technology, enjoying its Earth-abundant and ecofriendly constituents, thermodynamically stable structure, combined with the ideal bandgap perfectly matching with solar spectrum, and the compatibility with both rigid and flexible substrates. [1][2][3][4][5][6] These compelling features endow this PV technology huge potential for application of various scenes in the future, including wearable and portable PV power sources, building-integrated PVs (BIPVs) at curved building surfaces, and sustainable power sources for internet of things (IOT). [7,8] Moreover, pure-sulfide CZTS is also one of the most promising candidates as the top cell for silicon-based tandem solar cells, potentially triggering further technological evolution for largescale deployment of PV technologies. [9][10][11] Nevertheless, the current status of CZTS thin-film solar cells suffers from a much more open-circuit voltage (V OC ) loss than low-bandgap Cu 2 ZnSnS,Se 4 (CZTSSe) solar cells. [3,12,13] Besides the more severe bandgap/potential fluctuation and shorter photoluminescence (PL) decay time (related to real minority carrier lifetime), [14][15][16] the unfavorable "cliff"-like conduction band offset (CBO) at CZTS/CdS heterojunction interface is well believed to be a serious limiting factor to the V OC of CZTS solar cells. [17][18][19][20] To address this issue, alternative buffer materials with wide bandgap and a suitable conduction band edge have been screened, among which (Zn,Cd)S and (Zn,Sn)O are the most successful materials, allowing a great V OC boost up to 100 mV. [18,19] Nevertheless, the V OC of CZTS solar cells with a bandgap of 1.5 eV is still limited to be below 750 mV, far lower than that of the moderate CdTe solar cells with a similar recombination bandgap (1.45 eV) and low minority carrier lifetime (1À2 ns), let alone the "cliff"-like CBO at the CdTe/CdS interface. [21][22][23][24] Results of SunsÀV OC measurements for these cells configured with (Zn,Sn)O or (Zn,Cd)S buffer layer have revealed that J 02 (representing nonradiative recombination at the heterojunction region) is still 5 orders of magnitude larger than J 01 (representing nonradiative recombination in the quasineutral bulk region) (10 À7 A cm À2 for J 02 vs. 10 À12 A cm À2 for J 01 ). [18,19] This verifies that the V OC of pure-sulfide CZTS solar cells is still currently limited by nonradiative recombination in the heterojunction interface region, even though the unfavorable "cliff-like" CBO has been avoided, indicating that other important interface recombination mechanisms may persist and are yet to be properly recognized.
Transparent conductive Ga-doped ZnO (ZnO:Ga) was fabricated to serve as p-contacts of InGaN-based light-emitting diodes (LEDs) using molecular-beam epitaxy. As-grown ZnO:Ga films typically have resistivities of ¼ 2 À 4 Â 10 À4 Ácm, and over 80% transparency in the near UV and visible wavelength ranges. The current-voltage characteristics between as-grown ZnO:Ga contacts and p-GaN layers were ohmic. The brightness of LEDs fabricated with ZnO:Ga p-contacts was nearly double compared to LEDs with conventional Ni/Au p-contacts. We obtained the external efficiency as high as 20.8% in the case of the near UV LED. The forward voltage at 20 mA was found not to increase even after the lamp LED with ZnO:Ga were kept for 80 h in high humidity and high temperature environments.
We performed a comparative study to find out the reasons why it is necessary to prepare the pure sulfide chalcopyrite, Cu(InGa)S2 (CIGS) under a Cu‐deficient condition to improve solar cell performance. It has been shown that CIGS that was grown under Cu‐deficient condition exhibits large open‐circuit voltage (VOC) boosting compared with that the one grown under Cu‐excess condition. Thus, CIGS were prepared from Cu‐excess and Cu‐deficient metal precursor (MP) and characterized concerning the origins of the different VOC. We observed that CIGS prepared using Cu‐excess MP suffered large recombination at buffer/CIGS interface and in bulk, which resulted in serious limitation of the VOC. On the other hand, CIGS prepared from Cu‐deficient MP exhibited largely improved VOC with reduced recombination. We conducted defect analysis using the photoluminescence (PL) method, and CIGS prepared using Cu‐excess MP exhibited strong deep emissions. However, CIGS prepared from Cu‐deficient MP exhibited PL emission characteristics without deep level transition. This implies that a large number of deep level defects exist in the CIGS prepared using Cu‐excess MP, and this might be the reason for the large recombination limiting VOC of the device. Finally, we also present the reason for deep levels in the CIGS prepared using Cu‐excess MP.
The efficiency potential of chalcopyrite and kesterite solar cells including CIGSe (CuInGaSe2), CIGS (CuInGaS2), CZTS (Cu2ZnSnS4) and CZTSSe [Cu2ZnSn(S,Se )4] solar cells is discussed based on external radiative efficiency (ERE), open-circuit voltage loss, fill factor loss, non-radiative recombination and resistance loss. CIGSe cells achieve efficiency potential of 26.8% and 27.5% by improving the ERE from around 1% to 10% and 20%, respectively. CIGS and CZTS(Se) cells achieve the efficiency potential of 25% and 22%, respectively, by improvement in ERE from around 1 × 10−4% to 3%–5%. The effects of non-radiative recombination and resistance loss upon the properties of wide-bandgap CIGSe, CIGS and CZTS(Se) cells are discussed. In the case of wide-bandgap CIGSe cells, lattice mismatching between the buffer layer and CIGSe active layer and deep-level defects are thought to originate from non-radiative recombination loss. CIGS and CZTS(Se) cells are shown to have lower ERE and higher resistance loss compared to that of CIGSe cells.
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