Unveiling microscopic carrier loss mechanisms in 12% efficient Cu2ZnSnSe4 solar cells

Abstract

Carrier loss mechanisms at microscopic regions is imperative for high-performance polycrystalline inorganic thin-film solar cells. Despite the progress on Kesterite, a promising environmental-benign and earth-abundant thin-film photovoltaic material, the microscopic carrier loss mechanisms and their impact on device performance remain unknown. Herein, we unveil these mechanisms in state-of-the-art Cu2ZnSnSe4 (CZTSe) solar cells using a framework that links microscopic-structural and optoelectronic characterizations with three-dimensional device simulations. The results indicate the CZTSe films have an encouraging intragrain minority carrier lifetime of >10 ns, a marginal radiative recombination loss through sub-band recombination and electrostatic potential fluctuation, whilst a large effective grain boundary recombination velocity of around 104 cm s-1 and a low net carrier density of ~1×1015 cm-3. We identify that severe grain boundary recombination and low net carrier density are the current limiting factors of device performance. The established framework can greatly advance the research of kesterite and other emerging photovoltaic materials.

Efficient Cd-Free Flexible CZTSSe Solar Cells with Quality Interfaces by Using the Zn1–xSnxO Buffer Layer

“…The open-circuit voltage-dependent temperature ( V OC - T ) characteristics of the flexible Z0 and Z20 devices are as shown in Figure f, which are obtained from the temperature-dependent J–V data (Figure S3). The activation energy ( E a ) is extracted from the extrapolated linear section of V OC - T at T = 0 K. , The E a values of the Z0 and Z20 devices are 0.503 and 0.670 eV, respectively. The difference between E g and E a (Δ­( E g – E a )) can estimate the recombination status of the devices.…”

Emergence of flexible kesterite solar cells: progress and perspectives

“…It has been unveiled that grain boundary recombination is dominating in high-e ciency pure-selenide CZTSe thin-lm solar cells. 27 Also it is highly likely that grain boundary recombination may play an important role in the carrier loss of both pure-sul de CZTS and S-Se mixed CZTSSe thin-lm solar cells. On the other hand, the ll factors (FF) of monograin kesterite solar cells are lower than those of the best thin-lm solar cells, which may arise from the increase of bulk resistance due to the too thick absorber compared to the limited hole conductance.…”

“…Though the e ciency of CZTSSe solar cells is still not competitive in commercialization, substantial progress on fundamental understanding of the materials and device loss mechanisms has been made in recent years, particularly, in the advanced characterization technologies for the determination of secondary phases and electronic band alignment, [119][120][121][122][123] the understanding of the killer defects in the bulk of CZTSSe and effective strategies to control the formation of these defects, 36, 124-126 the understanding of key role of isovalent cation and alkali doping/alloying, 127 the understanding of the carrier loss mechanisms in the bulk, at junction interfaces, and at grain boundaries, 27,29,128 and effective strategies to boost the carrier transport at front and back interfaces. 33,129 Based on our recent ndings, if these critical understandings and strategies can be properly integrated, for instance, the lateral grain size can be larger than 3 μm whilst high grain interior quality (electron lifetime and hole density) can be maintained and the junction interface can be well passivated by an epitaxial ZnSe nano-layer, a step-change to 15% e ciency is highly possible.…”

“…33,129 Based on our recent ndings, if these critical understandings and strategies can be properly integrated, for instance, the lateral grain size can be larger than 3 μm whilst high grain interior quality (electron lifetime and hole density) can be maintained and the junction interface can be well passivated by an epitaxial ZnSe nano-layer, a step-change to 15% e ciency is highly possible. 27 While further step-change in e ciency to 20% requires essential understanding of the grain boundaries of CZTSSe thin-lms and effective grain boundary passivation strategies. 27 To achieve this long-term goal, cation doping and/or alloying will also be important to get a high electron lifetime (50-100 ns) and suitable hole density (~5×10 16…”

“…26 The encouraging results in material and device characterization has shown that the inter-grain lifetime of CZTSSe in current state-of-the-art thin lm solar cells can be larger than 10 ns, and the e ciency of CZTSSe thin-lm solar cells can catch up CIGS and CdTe thin-lm solar cells if the grain boundaries of CZTSSe thin-lms can be passivated to a similar degree to that of CIGS and CdTe. 27 Furthermore, theoretically, if no bending applied, there should be no signi cant e ciency gap between the CZTSSe solar cells fabricated using exible and rigid substrates respectively when suitable exible substrates are available. Indeed, since the rst exible kesterite solar cell was made on polyimide (PI) substrate in 2010, the e ciency gap between exible and rigid solar cells has been greatly reduced in recent years, although there are relatively limited research groups working on exible kesterite solar cells.…”